Hydrogen chemisorption on III–V semiconductor surfaces

Surface Science Reports 51 (2003) 1–149

Hydrogen chemisorption on III–V semiconductor surfaces S. Nannaronea,b,*, M. Pedioc a

Dipartimento di Ingegneria dei Materiali e dell’Ambiente, Universita` di Modena, Via Vignolese 905/A, I-41100 Modena, Italy b Istituto Nazionale di Fisica della Materia, UdR di Modena, Modena, Italy c TASC-Istituto Nazionale di Fisica della Materia, Area Science Park, S.S. 14, Km 163.5, Basovizza, I-34012 Trieste, Italy Received in final form 10 April 2003

Abstract A review of the experimental data and theoretical results dealing with the fundamental aspects of the hydrogenation of III–V semiconductor surfaces is presented. The collected material covers the production available in the specialised literature in this field over the last 30 years. The whole body of surface science experimental and theoretical tools were exploited. According to the investigated physical properties the paper is divided into six sections including experimental, surface atomic geometry, vibrational properties, electronic properties and adsorption and desorption. The most extended part of the collected material deals with the (1 0 0) and (1 1 0) GaAs surfaces, InP surfaces were extensively studied as well, though at a reduced extent. As a general result the hydrogenation of GaAs can be taken as a case study and the consequent picture of general validity in the interpretation of the hydrogenation of all the III–V surfaces. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Semiconductors III–V compounds; Chemisorption; Hydrogen

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 2. Experimental methods . . . . . . . . . . . . . . . . 3. Atomic geometry . . . . . . . . . . . . . . . . . . . 3.1. Hydrogen exposure of (1 0 0) surfaces 3.1.1. GaAs(1 0 0) surfaces . . . . . . . 3.1.2. InP(1 0 0) surface . . . . . . . . . 3.2. The (1 1 0) surfaces . . . . . . . . . . . . . 3.2.1. GaAs(1 1 0) surface. . . . . . . .

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*

Corresponding author. E-mail address: [email protected] (S. Nannarone). 0167-5729/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-5729(03)00014-1

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3.2.2. InP(1 1 0) surface . . . . . . . . . . . . . 3.2.3. Other (1 1 0) surfaces . . . . . . . . . . 3.3. (1 1 1) and (1¯ 1¯ 1¯ ) surfaces . . . . . . . . . . . . 3.3.1. GaAs(1 1 1) surface. . . . . . . . . . . . 3.3.2. InP(1 1 1) surface . . . . . . . . . . . . . 3.3.3. InSb(1 1 1) and InSb(1¯ 1¯ 1¯ ) . . . . . . 4. Vibrational properties . . . . . . . . . . . . . . . . . . . . 4.1. GaAs(1 0 0) surface . . . . . . . . . . . . . . . . . 4.2. InP(1 0 0) surface. . . . . . . . . . . . . . . . . . . 4.3. GaP(1 0 0) surface . . . . . . . . . . . . . . . . . . 4.4. GaAs(1 1 0) surface . . . . . . . . . . . . . . . . . 4.5. InP(1 1 0) surface. . . . . . . . . . . . . . . . . . . 4.6. GaP(1 1 0) surface . . . . . . . . . . . . . . . . . . 4.6.1. (1 1 1) and (1¯ 1¯ 1¯ ) surfaces . . . . . . 5. Electronic properties . . . . . . . . . . . . . . . . . . . . . 5.1. (1 0 0) surface . . . . . . . . . . . . . . . . . . . . . 5.1.1. GaAs(1 0 0) surface—valence states 5.1.2. GaAs(1 0 0) surface—core states . . 5.1.3. InP(1 0 0) surface—valence states . . 5.1.4. InP(1 0 0) surface—core states . . . . 5.2. (1 1 0) surfaces . . . . . . . . . . . . . . . . . . . . 5.2.1. GaAs(1 1 0) surface—valence states 5.2.2. GaAs(1 1 0) surface—core states . . 5.2.3. InP(1 1 0) surface—valence states . . 5.2.4. InP(1 1 0) surface—core states . . . . 5.2.5. GaP(1 1 0) surface . . . . . . . . . . . . 5.2.6. InSb(1 1 0) surface—valence states . 5.2.7. InSb(1 1 0) surface—core states . . . 5.3. GaAs(1 1 1) surface . . . . . . . . . . . . . . . . . 6. Adsorption and desorption . . . . . . . . . . . . . . . . . 6.1. Adsorption and sticking. . . . . . . . . . . . . . . 6.2. Thermal absorption and desorption . . . . . . . 6.3. Electron stimulated desorption . . . . . . . . . . 6.4. Photon stimulated desorption . . . . . . . . . . . 7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction Hydrogenation of semiconductor is a wide and important topic of technology and basic physics and chemistry of bulk materials and their surfaces. The interaction of hydrogen with semiconductor surfaces is a key aspect in a number of technological processes including crystal growth, crystal defects and bulk impurities passivation, procedures of surface cleaning and etching. The study of the

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hydrogenation of semiconductor surfaces stimulated in the last 30 years considerable activity aiming at the optimisation of these processes. This review deals with the basic properties of the hydrogenated III–V semiconductor surfaces. This work summarises experimental data and theoretical results appeared in papers published approximately in the last 30 years. The whole body of surface experimental and calculation techniques have been employed producing a significative understanding of the physics at the hydrogenated surfaces. The surface properties include atomic geometry, vibrational structure, electronic properties and absorption and desorption properties from both experimental and theoretical point of view. GaAs(1 0 0) and GaAs(1 1 0) were the most studied surfaces being the former the surface of most prominent technological interest and the exposed surface in the crystal growth in molecular beam epitaxy (MBE) apparatuses and the latter that of the cleavage face of the zincblend structure. They are followed, in the order, by the InP(1 0 0) and InP(1 1 0) surfaces being, also in this case, the (1 0 0) surface produced in the MBE apparatuses and the (1 1 0) commonly obtained by cleavage. Comparative less data are available in the literature on the other III–V semiconductors or surfaces including all the (1 1 1) and ð 1 1 1Þ surfaces and those of GaP, GaSb and InSb materials. A general picture of the properties of the hydrogenated surfaces can be derived by this comprehensive view of data. The hydrogenation of GaAs, both (1 0 0) and (1 1 0) surfaces, plays the role of a useful case study whose results seem extensible to surface hydrogenation of the whole III– V semiconductor family. An ordered surface is obtained for coverages not exceeding the monolayer, surface relaxation is removed leading to a substrate surface with an atomic geometry close to the ideally terminated crystal. A surface compound is consequently formed showing its peculiar electronic and vibrational structure. After the monolayer formation surface disruption takes place with continuous change of electronic and vibrational properties on one hand and stoichiometry, local atomic geometry and morphology on the other one. Beside the effort of singling out the general trend of the physics of the hydrogenation of the III–V semiconductor surfaces this paper aims to collect the whole body of available experimental data and theoretical results of the relevant surface phases formed at different stages of hydrogenation of the different III–V semiconductor surfaces. The review is organised as follows. In Section 2 the experimental aspects related with the preparation of hydrogenated III–V semiconductor surfaces are reviewed. Section 3 is devoted to experimental and theoretical results regarding the surface atomic geometry of hydrogenated III–V semiconductor surfaces while Section 4 deals with the presentation of vibrational properties. In Section 5—the most extended—the electronic properties are reviewed and, eventually, desorption and adsorption information are collected in Section 6. The material in the different sections is organised per different surfaces for ease of consultation.

2. Experimental methods In this chapter the experimental methods to obtain hydrogenated III–V semiconductor surfaces are reviewed. The variety of the experimental methods is due to the fact that molecular hydrogen does not stick on III–V surfaces and does not dissociate at room temperature and consequently an exposure to atomic hydrogen is required.

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An intrinsic difficulty is the evaluation of the H/H2 ratio achieved by different methods. To this end a number of spectroscopic fingerprint features have been used to estimate the number of H atoms deposited on the system. Almost all the surface sensitive spectroscopies were used to investigate these systems, including ultraviolet photoemission spectroscopy (UPS) in both angle integrated (AI) and angle resolved (AR) modes, electron energy loss spectroscopy (EELS), optical spectroscopy mainly by reflectance differential spectroscopy (RDS), photoelectron diffraction (PED), grazing incidence X-ray diffraction (GIXRD), scanning tunnelling microscopy (STM) and thermal desorption spectroscopy (TDS). The techniques routinely used to prepare atomically clean semiconductor surfaces were employed to prepare clean substrates1 to be exposed to atomic hydrogen. They include cleavage, ion bombardment and annealing (IBA) and decapping. Cleavage is the most simple and straightforward method to obtain atomically clean and ordered surface of a crystal. The main drawback of this technique is its intrinsic limitation to preferential cleavage faces of crystals, the (1 1 0) in the case of III–V semiconductors. Mirror like surfaces with low step density can be obtained both by single or doublet notches and wedges techniques. Doublet notches and wedges technique is based on a couple of notches of 308 and 608, respectively. A 308 wedge is generally used in single wedge mode. Typical surface areas are several mm2; 5  5 mm2 is a commonly used dimension. Generally the (1 1 0) surfaces of most of the III–V semiconductor do not have surface electronic states in the gap and present flat bands condition (GaP(1 1 0) exception). For the III–V surfaces a wellestablished relation between the density of defects and Fermi level position was found [2]. The IBA of mirror polished and chemically etched surfaces routinely produce clean surfaces. This method is not limited to a particular surface. Typically used gases include Ne, Ar or Xe with current of the order of mA with an energy ranging from hundreds to several keV. III–V surfaces prepared by IBA show specific imperfections. During an IBA treatment, arsenic is preferentially sputtered with the consequent production of small gallium droplets and steps. In comparison cleaved surfaces, though not perfect because of the above mentioned presence of punctual and extended defects, are more expected to exhibit a surface stoichiometry close to the ideal one. However small segregates of arsenic have been reported also for cleaved surfaces, see f.i. [3]. A further possibility of producing clean III–V surfaces is offered by decapping in UHV MBE grown and As capped surfaces. The performance of the decapping technique has been tested in recent works resulting a reliable method for the preparation of well defined (1 0 0) surfaces [4,5]. Le Lay et al. [4], by using high resolution core level spectroscopy, studied the ð4  2Þ-cð8  2Þ surface prepared by UHV decapping MBE grown surfaces. Comparing with the results obtained on the GaAs(1 1 0) surface two surface components for the Ga 3d emission line were found and ascribed to two inequivalent Ga dimers of the ð4  2Þ-cð8  2Þ unit cell. Instead only one component was necessary for the As line. The Fermi level was found at 0.7 and 0.55 eV above the valence band maximum on n and p type samples, regardless of the surface As composition. Resch et al. [5] heated stepwise MBE grown GaAs(1 0 0) capped samples to temperatures of at least 545 8C and followed the surface properties evolution with RAS flanked by low energy electron diffraction (LEED) and AES. They were able to reproduce all the previously observed surface reconstructions of the (1 0 0) surface. 1

See for instance [1], and references therein.

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Common surface contaminants, carbon and oxygen, can be removed by atomic hydrogen exposure at much lower temperatures than by conventional thermal cleaning under arsenic pressure. This is a very useful mechanism when the process is applied to the fabrication of devices requiring clean and abrupt hetero-interfaces or doping profiles. In this field hot filament production and hydrogen plasma are used. Landesman et al. [6] reported successful hydrogen cleaning of GaAs(1 0 0) wafers by using 10 s of exposure to a hydrogen plasma in a multipolar plasma source [7]. After this exposure the oxide contribution in XPS completely disappeared and the oxygen and carbon contribution have been reduced to a minimum. In this kind of source the multipolar plasma is created by a discharge generated by a heated tungsten filament in a UHV chamber. The electrons are confined simply by the positioning of permanent magnets (multipolar) around the UHV chamber. The specific properties of this plasma are the low pressure (between 103 and 102 Torr H2), the low ionic temperatures (<1 eV) preventing ion bombardment effects on the surface, the high density (1010 cm3) and the possibility to tune the working parameters to change the plasma composition. By using an H2 pressure of 7  103 Torr, filament bias 75 V and a discharge current of 100 mA a plasma density of 1010 cm3 (with composition H  1011 cm3, H3 þ  1010 cm3 and Hþ  107 cm3) was typically obtained. Moreover by turning off the filament bias the composition of discharge is changed, containing mainly excited hydrogen atoms but very little ionic species, because all the reactions which produce ions have an energy threshold of at least equal to ionisation potential (13.6 eV) of the hydrogen atom. Semiconductor surfaces, III–V included, do not react with hydrogen in molecular state at room temperature. This fact is due to the kind of dependence of the interaction energy on the surface distance, which does not lead to the molecule dissociation. Similar behaviour was observed with completely different materials like Al and Be [8]. At variance, spontaneous dissociation was observed on transition metal surfaces, like Ni and Pt while any spontaneous dissociation was reported for noble metal surfaces (like Cu, Ag and Au). A general explanation is not available yet, though the local filling of metal d shell, the position of d-band centre and the effect of surface states on the hydrogen dissociation barrier are the aspects known to play a major role [9]. In Fig. 1 an experimental example by EELS of reaction absence upon H2 exposure is given for GaAs(1 1 0). Similar evidences were earlier reported also by Hagstrum et al. [10] by ion neutralisation studies and by Gregory and Spicer [11] by UPS. Moreover high resolution electron energy loss (HREEL) experiments by Lu¨ th and Matz did not observe any energy loss feature in the loss spectrum of hydrogenated GaAs(1 1 0) in the range near 4400 cm1 loss range, where the stretching vibration of H2 is expected, ascribable to the presence of molecular hydrogen [12]. Consequently predissociation of the H2 molecule is required in order to form H-surface chemical bonds. To this end a number of methods have been envisaged both for research and device fabrication purposes. The most simple and commonly used one is based on the presence of a hot W surface in the UHV chamber in presence of a base pressure of H2. Since Langmuir [13] it is known that H2 is partly dissociated in presence of a hot tungsten filament. An extended study of reflection and dissociation of H2 on tungsten is reported in an early paper [14] showing that the dissociation probability saturates at ’0.3 at 2000 K, as shown in Fig. 2, where the experimental dissociation probability of H2 versus W temperature is reported. An evolution of this method was reported in [15]. A hot tungsten filament was placed in an effusion H2 molecular pipe. The effused gas impinged onto a pyrex plate cooled to liquid nitrogen temperature acting as a reflector for hydrogen atom. This method permitted an efficient exposure to atomic hydrogen of a sample surface not in line of sight reducing possible metal contamination. A factor of

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Fig. 1. EEL of GaAs(1 1 0) taken in second derivative mode at primary beam energy Ep ¼ 100 eV: (a) fresh cleaved surface, (b) after 104 L of molecular hydrogen, (c) after 4200 L of molecular hydrogen in presence of a hot filament [9].

four in the exposure was lost with respect to the line of sight geometry while the use of a beam doser increased the source efficiency of a factor of ’50. Absence of excited hydrogen was a further advantage of this method. The plot of the efficiency of atomic hydrogen production obtained with this apparatus on the tungsten as a function of the tungsten temperature is shown in Fig. 3. Sugaya and Kawabe used an apparatus to expose surfaces to atomic hydrogen made of a simple cracking cell [16]. In their apparatus, atomic hydrogen was produced by fluxing molecular hydrogen through a boron nitride tube with an inner a hot W filament. Saturation in the cracking efficiency was reached at about 2000 K of filament temperature. A schematic diagram of the H2 cracking cell is shown in Fig. 4. An upgrading of this method was presented by Bischler and Bertel [17]. The experimental solution consists in dosing the molecular hydrogen through a tungsten capillary of 0.6 mm of inner diameter

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Fig. 2. Probability of dissociation of hydrogen molecule as a function of tungsten surface temperature. The solid lines represent extreme values obtained during the course of the experiments [14].

Fig. 3.

Efficiency of atomic hydrogen production on a tungsten filament as a function of tungsten temperature [15].

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Fig. 4.

Schematic diagram of a molecular hydrogen cracking cell [16].

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Fig. 5. Schematic drawing of an atomic hydrogen source based on a tungsten capillary heated up by electron bombardment at ’1800 K [17].

(ensuring also some degree of focalisation) taken at ’1800 K by electron bombardment. Its schematic drawing is reported in Fig. 5. Under this conditions a dissociation rate of ’45% was reported with an intensity at the sample of ’1014 atoms s1 (of the order of a typical density of surface atom 8:8  1014 cm2 atoms cm2 for GaAs(1 1 0)). Comparable exposures with the W filament method were obtained with doses a factor ’1000 higher. The use of this kind of sources was tested for metals. Any result has been reported yet about its use with semiconductors. Gas crackers to be used in MBE apparatuses are also commercially available and they are routinely used, e.g. for low temperature cleaning of InP and GaAs wafers or for low dislocation MBE growth of GaAs and other III–V semiconductors. Another method of exposure stems from a beam of atomic hydrogen [18]. In this experimental set up a thermal effusion beam of atomic hydrogen was produced by radio frequency dissociation in a pyrex flask with a 0.3 mm diameter hole. The effusion beam was obtained by using a skimmer and two differential pumping stages. The atomic hydrogen intensity was ’1014 hydrogen atoms s1 at a 5  5 mm2 surface placed ’1000 mm from the source. Some results of the use of this beam of atomic hydrogen are shown in Fig. 6 in the case of exposure of GaAs(1 1 0) [19]. The flow rate of atomic hydrogen at the surface can be measured by a suitable mass spectrometer, though some care must be paid to the inner dissociation of H2 in the spectrometer itself which could introduce some systematic errors. Mechanical beam chopping could further reduce systematic errors arising from background pressure or allow beam intensity measurement. By using mechanical chopping and lock-in detection techniques absolute beam intensity measurements can be performed by inserting an ion gauge along the beam path. Another possibility for absolute calibration is offered by the exothermic recombination of hydrogen on a heated Pt surface (in the range of 150–400 8C). In fact in this conditions hydrogen atoms will recombine to form molecular hydrogen. The recombination reaction is exothermic releasing 435.94 kJ/mol and causing a temperature rise at the Pt detector. The subsequent resistance change, in a way completely similar to Pirani vacuum gauges,

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Fig. 6. Effect of exposing GaAs(1 1 0) to atomic hydrogen molecular beam. Top: HREELS in the region of Ga–H and As–H stretching frequencies. Exposure times are indicated. Bottom: Ga–H (squares) and As–H (dots) loss amplitudes as a function of exposure time (lower scale) and total number of hydrogen atoms (upper scale) [19].

allows to measure the rate of impinging atoms. The amount of H2 dissociating at the detector is assumed to be negligible because the relatively low Pt temperature does not produce significative H2 cracking. The production of atomic hydrogen was also monitored by measuring the increase of the surface conductivity of a preannealed ZnO crystal.2 An estimate of hydrogen flux at the sample obtained with the W filament was given by M’hamedi et al. [20]. They used a hot W ribbon, 1.5 mm wide, held in front of the sample surface, at 2

See experimental of Ref. [12].

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Fig. 7. EELS of GaAs(1 1 0) at different hydrogenation (exposure is in Langmuir of molecular hydrogen in presence of a hot filament) in the region of Ga 3d surface exciton excitation; the loss is indicated by the arrow [9].

a distance of about 60 mm, with an impinging direction of about 458. The filament temperature was about 1750 8C. In this condition the measured surface temperature did not raise more than 40 8C during the exposures which took about 200 s each. In this condition, by using the dissociation probability of [21], the H flux reaching the sample was estimated in 1011 atoms cm2 L1 of H2. As mentioned before atomic hydrogen can be produced by plasma discharge [6,7]. The same method was used for surface hydrogenation. Spectroscopy offers the additional possibility of measuring (a posteriori) the hydrogen flux at the surface measuring the evolution of coverage through the evolution of some surface feature versus hydrogen exposure. The quenching of Ga 3d surface exciton loss was carefully followed by Antonangeli et al. [9] by second derivative EELS. This transition, cation related, is present in all the Ga containing III–V semiconductors and can be used as monitor of hydrogen sticking. In this way complete quenching of Ga 3d loss is associated to the complete saturation of Ga dangling bond and/or to the completion of 1 monolayer (ML) of coverage. A typical evolution of EELS is reported in Fig. 7. Small amount of chemisorbed hydrogen can be easily removed by mild (’300 8C) thermal annealings as shown for the case of GaAs(1 1 0) in Fig. 8.

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Fig. 8. Second derivative EELS of clean GaAs(1 1 0) surface versus annealing cycles (following the arrows): (a) clean GaAs(1 1 0), (e) EEL spectrum at the Ga 3d surface exciton quenching, (b)–(d) after subsequent annealing cycles (temperature and time, in minutes are indicated). The features of the clean surface are labelled as IV or IS according to their volume or surface nature, respectively (exposure is in Langmuir of molecular hydrogen in presence of a hot filament) [9].

As shown in Fig. 8 the EEL spectrum of the clean surface is recovered after the annealing cycles. Similar evidences were obtained by UPS by Gregory and Spicer (see also Section 5 and [11]). The integrated intensity of the hydrogen induced peak in photoemission yield spectroscopy (PYS) was used by M’hamedi et al. to monitor coverage and to derive the sticking coefficient (see Section 6 and [20]). Grizzi and co-workers monitored the coverage by following the integrated intensity of the hydrogen direct recoil (HDR) in ion scattering spectroscopy with time of flight (ISS–TOF) experiments (see Section 6 and [62]).

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3. Atomic geometry The most commonly used surface structural techniques were used to investigate the surface structure of the hydrogenated III–V semiconductor surfaces. They include mainly LEED, PED and GIXRD. Electron and photon scattering are at the basis of these techniques with the consequence that, because of the negligible scattering cross-section offered by hydrogen, the direct insights come from the hydrogen effects on the substrate structure and not from the hydrogen bond itself. Additional insights come from infrared (IR) spectroscopy as well as from the comparison of experimental spectra with theoretical results. The available data on surface structure modifications induced by the exposure to hydrogen are restricted to low Miller index surfaces, including the (1 0 0), (1 1 0), (1 1 1) and ð1 1 1Þ surfaces. In the modelling of the structure a frequent reference is made to the structure of the clean substrate. The ideal truncated bulk structures of the two polar (1 0 0), (1 1 1) planes and the non-polar (1 1 0) one are reported. Generally these surfaces present relaxation or reconstruction. A comprehensive account of structures can be found in [21,22]. A summary is shown in Table 1. The three (1 0 0), (1 1 0) and (1 1 1) clean GaAs surfaces result to be the most widely studied and understood. The commonly accepted models of (1 0 0), (1 1 0) and (1 1 1) III–V surfaces [21,22] are reported in Figs. 9, 10 and 11, respectively. Table 1 Experimental structures of clean III–V semiconductors surfaces Crystal surface

LEED

Comments

Reference

GaAs(1 0 0) GaAs(1 0 0) GaAs(1 0 0) GaAs(1 0 0) GaAs(1 0 0) GaAs(1 0 0) GaAs(1 1 1) GaAs(1 1 1) GaAsð1 1 1Þ GaAsð1 1 1Þ GaAsð1 1 1Þ GaAs(1 1 0) InP(1 0 0) InP(1 0 0) InP(1 0 0) InP(1 0 0) InP(1 0 0) InP(1 1 0) InP(1 1 1) InPð 1 1 1Þ GaSb(1 1 1) GaSbð1 1 1Þ InSb(1 1 1) InSbð 1 1 1Þ InAs(1 1 1) InAsð1 1 1Þ

46 16 cð8  2Þ cð2  8Þ cð4  4Þ 11 22 19  19  23:4 33 22 19  19  23:4 11 41 42 s-(2  4) d-(2  4) 21 ð1  1Þ 22 11 22 33 22 33 22 33

8/8 As per Ga in unit cell 7/8 As per Ga in unit cell 4/8 As per Ga in unit cell 2.5/6 As per Ga in unit cell 7.5/26 As per Ga in unit cell As terminated No/As stabilised Ga stabilised (1 1 0) facet at 600 8C As stabilised Ga stabilised 1/1 As per Ga in unit cell Diffuse streaks P rich P dimers In–P dimers – 1/1 P per In in unit cell In terminated P terminated Ga terminated Sb terminated In terminated Sb terminated In terminated As terminated

[21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [21] [23] [23] [23] [23] [23] [21] [24] [24] [21] [21] [21] [21] [21] [21]

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Fig. 9.

Models of GaAs(1 0 0)-cð4  4Þ, GaAs(1 0 0)-(2  4) and GaAs(1 0 0)-ð4  2Þ surfaces [27].

In the general framework of interaction, including chemisorption and dissociation stages, in the initial chemisorption stage the interaction between H and III–V semiconductor surfaces gives rise to surfaces preserving a high order with the hydrogens bound to surface atoms through existing dangling bonds or after the breakdown of anion–anion or cation–cation dimers. Consequently the reconstruction or relaxation of the clean surface (see below) results affected by the charge rearrangement during reaction and/or bond breaking. In the following stage of dissociation, occurring at higher exposures, a stronger surface perturbation leading to defect formation and surface disruption takes place. These two stages are named slightly differently after different authors. In some cases the completion of the first stage was reported as saturation (e.g. [25]) or monolayer formation (e.g. [9]) or chemisorption (e.g. [26]), while dissociation was used for the second stage (e.g. [26]). The defects induced by the hydrogen exposure and occurring mainly in the dissociation stage are predominantly anion vacancies and metallic cation islands. Desorption of volatile compounds like

Fig. 10. Models (side and top view) for the ideal non-reconstructed (left) and the relaxed (right) surface [21,22].

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pffiffiffiffi pffiffiffiffi pffiffiffiffiffi pffiffiffiffiffi Fig. 11. Models of the GaAs(1 1 1)-ð 19  19ÞR23.48 and GaAs(1 1 1)-ð 3  3ÞR308 relaxed surfaces [22].

arsine and phosphine is the main mechanism of defects formation occurring in the second step. The desorption of cation-hydrogen compounds desorption is less favoured because of their lower volatility. These effects are at the base of etching processes used in electronic devices technology not at issue in this review. In this section the information are organised according to the surface orientation and grouped by compounds. A summary of the structural properties of the hydrogenated III–V semiconductor surfaces is reported in Table 2. 3.1. Hydrogen exposure of (1 0 0) surfaces Along the [1 0 0] direction of the zincblend structure alternate planes of cations and anions stack alternately. They are indicated as polar surfaces. In practice the surface composition can vary, according to different preparation methods and recipes, resulting in different average surface composition or stoichiometry continuously ranging from ‘‘anion rich’’ to ‘‘cation rich’’ surfaces. Table 2 Summary of the structural properties of the hydrogenated III–V semiconductor surfaces Crystal surface

Low hydrogen exposure

Hydrogen exposure (103 L)

Hydrogen exposure (104 L)

Reference

GaAs(1 0 0)-cð4  4Þ GaAs(1 0 0)-2  4 GaAs(1 0 0)-4  2 GaAs(1 0 0)-cð8  2Þ GaAs(1 1 0) InP(1 0 0)-4  2 InP(1 1 0)

11 14 42 41 11 41 ð1  1Þ

12 11 41 11 11 41 ð1  1Þ

11 11 11 12 1  1 some spot missing 11 1  1 some spot missing

[27] [27] [27] [28] [20] [29] [30]

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The atoms are equally spaced on the (1 0 0) surface of ideally bulk truncated crystal where every atom has two backbonds and two dangling bonds. Examples of the most common reconstructions are shown in Fig. 9 for GaAs(1 0 0). It is important to stress that the cð2  8Þ; ð4  6Þ; ð4  1Þ and cð8  2Þ reconstructions occur in this given order as a function of decreasing As surface concentration. Moreover cð4  4Þ is the As richest reconstruction terminated by As dimers chemisorbed to an underlying As layer, while the ð2  4Þ is formed by As dimers. The 1  6 is also reported as 2  6 and 4  6 according to the degree of surface disorder [31]. 3.1.1. GaAs(1 0 0) surfaces Bringans and Bacharach [32,33] carried out combined LEED and UPS studies on the hydrogen interaction with the MBE grown surfaces ð4  6Þ, Ga-rich, and cð4  4Þ, As-rich. They found a saturation of the hydrogen coverage with the exposure. At saturation exposure both surfaces, independently of starting reconstruction, showed a ð1  1Þ LEED patterns together with a similar As/Ga core level photoemission intensity ratio as shown in Fig. 12. Together with the ð1  1Þ spots some faint and diffuse spots at (1/2, 1) were observed in a way similar to the clean surface having a surface composition between the cð4  4Þ and cð2  8Þ. The annealing of hydrogenated surface always reverted to a cð2  8Þ regardless of whether the exposure started on a cð4  4Þ or

Fig. 12. Dependence of As/Ga 3d core level photoemission intensity ratio as a function of hydrogen exposure (in Langmuir of molecular hydrogen in presence of a hot filament) for (1 0 0) and ð 1 1 1Þ GaAs surfaces for different surface initial reconstructions [32].

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ð4  6Þ surfaces. Combining these two evidences the authors concluded in favour of an As-rich hydrogenated surface. High resolution electron energy loss spectroscopy (HREELS) was performed on hydrogen exposed cð2  8Þ, ð4  1Þ or cð8  2Þ reconstructed surfaces [34]. Clean surfaces were obtained by decapping As covered MBE grown surfaces. Loss spectra were taken in the region of As–H and Ga–H stretching vibrations. Surface concentration of Ga and As was derived by monitoring the loss intensities of As–H and Ga–H stretching vibrations, respectively. Exposing the cð2  8Þ surface a wide compositional range was found with a substantial amount of Ga atoms exposed. The exposure of the cð8  2Þ surface gave no As–H signal allowing to conclude in favour of the absence of As dimers on the clean surface. By using similar approach and mainly stemming from HREELS Schaefer and co-workers [35–37] published several papers presenting a wide phenomenology containing information on local geometry, morphology and etching. Starting from a cð4  4Þ surface dangling bond saturation was observed leading to a ð1  1Þ hydrogen-terminated As monolayer. Further exposure to hydrogen causes breaking of As backbonds with consequent arsine evolution. In this manner etching of the first arsenic layer and part of the second As layer was shown to be possible [29]. On IBA cleaned surfaces different Ga-rich reconstructions can be generated, namely 1  1, 1  6, 4  1, 4  6 in the order of increasing annealing temperature. The vibrational measurements for the low exposure regime at room temperature showed that atomic hydrogen adsorbs at both Ga and As sites. It was shown that the As–H and Ga–H stretching frequencies ratio is specific of each reconstruction [38]. In particular the arsenic hydride concentration gradually decreases while finally a strong 1  2 reconstruction was obtained independently of the initial reconstruction. In Fig. 13 the different LEED patterns obtained starting from an As-rich GaAs(1 0 0)-cð8  2Þ after different hydrogen exposures are shown as reported by Schaefer [28]. On the right side the model supported by LEED and mainly by HREELS data are shown. These models are in agreement with the intensity ratio of the As–H to Ga–H monohydrides, with the frequency shift of modes with exposure and with the appearance of a GaH2 dihydride stretching vibration. The breaking of the As dimers provokes the desorption of As presumably in the form of AsHx molecules. The ð1  1Þ reconstruction obtained by 103 L of exposure in total was explained in terms of gallium surface diffusion and/or Ga hydrides formation. The further decrease of Ga–H2 stretching intensity was ascribed to Ga–Ga dimer formation allowed by the interaction of hydrogen dimers with subsequent H2 desorption. In the same experiment, the influence of substrate temperature in the As–H to Ga–H intensity ratio was studied. At a substrate temperature of 200 K the As–H to Ga–H signal ratio is less dependent on the hydrogenation indicating a higher stability at reduced temperature of As hydride [38]. The same authors followed the dependence of the As–H to Ga–H intensity ratio for the 1  1, 4  6 and cð8  2Þ Ga-rich reconstructions as a function of hydrogen exposure. Data are reported in Fig. 30. The inspection to the figure shows that the Ga–H loss has a monotonic increase while the As–H one, after an initial increase, undergoes to a monotonic reduction. In these experiments a correlation of As loss from surface, presumably as arsine like molecules, and hydrogen exposure was found by simultaneous observation of specific electron energy losses and desorption [36]. Reflection high energy electron diffraction (RHEED) changes upon hydrogen exposure were reported in [39]. The pattern upon hydrogen exposure reverted to a 1  1 structure with a strong diffuse background compared with the 2  4 reconstructed surface. A fact that was assumed as the evidence of partial disorder.

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Fig. 13. LEED patterns (left) and suggested model structures (right) for GaAs(1 0 0)-cð8  2Þ before (a) and after (b–d) hydrogen exposures: (b) 102, (c) 103, (d) 104 L. The exposures are in Langmuir of molecular hydrogen in presence of a hot filament. The 4  2 units with missing Ga dimers, shown in the right hand part of (a), form the cð8  2Þ structure [28].

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Evidences useful in geometry determination were also obtained by optical spectroscopies including infrared spectroscopy [23,40] and RAS [27,41–43] (for further aspects related with the vibrational and electronic properties contained in those papers the reader is addressed to Sections 4 and 5, respectively). Joseph et al. [40] by infrared spectroscopy of the hydrogen exposed cð2  8Þ As-rich did not observe optical absorption due to Ga–H, a fact that was assumed as the evidence of absence of Ga termination. They performed a relative measure of the absorbance of As–H by comparing the spectra with one obtained for CO adsorbed on Pt film using internal reflection. The ratio of integrated absorption intensity for As–H (2140 cm1) relative to CO adsorbed on Pt (2040 cm1) is 1–190. The same authors found a shift of As–H vibration with exposure related to a structural change and a high coverage shift from 2110 to 2140 cm1 attributed to AsH2 formation. Hicks and co-workers [23,44] presented detailed vibrational information obtained by multiple internal reflections IR absorption of the GaAs(1 0 0)-cð2  8Þ and GaAs(1 0 0)-(1  6) in the 2200  1200 cm1 region. Measurements were also taken with light polarised normal and parallel to the plane of incidence (parallel and near normal to the surface, respectively) [23]. The different exposure stages were monitored by LEED. At saturation coverage a 1  1 pattern was obtained. The bands observed on the cð2  8Þ and ð2  6Þ surfaces are due to molecular aggregates present on the surface including arsenic hydrides, terminal gallium hydrides and bridging gallium hydrides. The latter is an evidence of bridge-bonded hydrogen. Spectra taken with linear polarised light show that As– H and Ga–H bonds are oriented along the [1 1 0] and [1 1 0] axis, respectively in agreement with a clean surface structure composed of As and Ga dimers with bonds in the [1 1 0] and [1 1 0] directions. For the cð2  8Þ and ð2  6Þ they proposed insertion of H atom across the As–As and As– Ga bonds of As dimers to form arsenic monohydrides and dihydrides following the scheme of Fig. 14. Spectra and frequencies assignment are reviewed in Section 4. In the models there are four As–H chemical environments: arsenic dihydrides, coupled monohydrides, monohydrides (Ga2)AsH in the top layer and monohydrides in the second layer (Ga3)AsH. The former three species are produced by adsorption on As dimers, while the latter is generated at second layer As atoms. Terminal Ga hydrides are formed by absorption of H atoms onto second layer Ga atoms and onto Ga dimers, while the bridging Ga hydride is formed only by adsorption onto Ga dimers. The hydrogenated (2  4) A and B surfaces exhibit the same distribution of As and terminal Ga hydrides. However, these structures differ greatly from the hydrogenated ð2  6Þ which contains bridging Ga hydrides as well as terminal Ga hydrides on Ga dimers. Moreover the Ga monohydride band can be decomposed into three subbands occurring at 1325, 1460 and 1600 cm1, respectively, and assigned to asymmetric stretching. By using the valence force theory and by using the symmetric stretching frequencies of the similar compounds observed in the gas-phase (out of the instrument transmission) the three features correspond to the angles of 988, 1058, 1138, ˚ —to a Ga–Ga distance of 2.55, 2.70 and respectively (corresponding—with a H–Ga distance of 1.7 A ˚ 2.85 A, respectively). Hicks compared density functional theory (DFT) calculations with IR reflectance vibrational spectra [45] for the H interacting both with the As-rich (2  4)-GaAs(0 0 1) and the Ga-rich (4  2)GaAs(0 0 1) systems. They found that the clusters Ga5As4H11,13, Ga4As5H11,13 and Ga7As8H19, shown in Fig. 37 are good representations of the As- and Ga-rich reconstructions. STM measurements were performed on the As-rich GaAs(0 0 1)-2  4 surface by Kuball et al. [46]. STM images were systematically correlated with LEED patterns. The clean surface presents As dimers along the ½1  1 0 direction on top of a Ga layer. STM results show that exposure to atomic hydrogen leads

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Fig. 14. (a) Models of the cð2  8Þ and ð2  6Þ GaAs(1 0 0) reconstructions, the cð2  8Þ reconstruction model can be obtained from the (2  4) by staggering the unit cells on each side of the vacancy row; (b) models of possible reaction scheme of hydrogen on GaAs(1 0 0)-cð2  8Þ and ð2  6Þ [23].

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Fig. 15. Scanning tunnelling microscope images of the GaAs(0 0 1)-2  4 surface ½400 —  400 — after (a) 100 L [LEED: ð1  4Þ], (b) 400 L [LEED: ð1  4Þ], (c) 1000 L [LEED: ð1  1Þ], (d) 105 [LEED: ð1  1Þ] hydrogen exposure (in Langmuir of molecular hydrogen in presence of a hot filament). Large white areas correspond to higher terraces [46].

to a degradation of the surface. Three steps of surface modification due to hydrogen were identified: (I) the appearance of a pronounced contrast variation within the As dimer rows; (II) the appearance of depression within the As dimer rows; (III) the creation of a disordered (rough) surface. The disappearance of the 2 LEED pattern was a common feature of the three steps and it was ascribed to the progressive disruption of the As rows made up of package of As dimers followed by two missing dimers. The disappearance of the 2 periodicity could also indicate an increase of disorder due to breaking of As dimers, as shown in the model of Fig. 14, due to the reaction of hydrogen with As dangling bonds. The structure evolution observed by STM is summarised in Fig. 15(a) and (b). After 100 L rows exhibiting local contrast variations due to punctual defects were observed. The hydrogen adsorption sites (white) are marked with rectangles and the As desorption sites (black) are marked with circles. The white elongated spots visible in Fig. 15(b) were interpreted as an enlargement of the valence charge as found by theoretical calculations [47] (see also Section 5) in the vicinity of the As–H bond. At very ˚ range. At this stage high exposures, a disordered surface develops with height variations in the 10–20 A of hydrogen exposure, the surface proves to be very reactive with the STM tip indicating weak bonds with surface species. STM investigation of the ð2  1Þ reconstructed surface was reported in [43]. Results show a very poor degree of ordering; short rows separated by 1.2 nm are observed suggesting an inhomogeneous surface. The periodicity of 1.2 nm was guessed to be due to remaining As atoms from the topmost As layer, i.e. the surface etching was not layer by layer. 3.1.2. InP(1 0 0) surface The 4  2 reconstructed surface is the most studied clean (1 0 0) surface of InP. In–H and P–H bonds were identified by HREELS by Hou et al. [24] on an 4  2 reconstructed (IBA prepared) surface.

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The work was mainly motivated by the search by hydrogen exposure of P adatoms and P dangling bonds together with In atoms on the 4  2 surface and consequently the paper contains only indirect information on the hydrogenated surface. Further structural information about the hydrogenated surface of InP(1 0 0)-4  2 have been obtained through a combined HREELS and LEED studies [29]. The clean surface was prepared by IBA resulting in In-rich. The behaviour of the ratio of In–H and P–H stretching modes intensities and of LEED pattern allowed the authors to conclude that, at low exposures, only passivation of dangling bonds occurs, being the LEED pattern preserved and the P–H/In–H intensity ratio of stretching vibrations ðR ¼ 0:2Þ that of an In-rich surface. Additional hydrogen exposure gave first a 4  1 and then a 1  1 pattern, with an increasing background up to a stage where no LEED pattern could be observed. For these high hydrogen exposures (above 1000 LH) the decreasing of the In–H intensity was ascribed to the formation of In metallic droplets, confirmed by XPS in the In 3d core level emission. Scanning electron microscopy (SEM) analysis evidenced metal islands of medium size about 0.2 mm in diameter. The formation of a metallic edge observed by photoemission yield spectroscopy (PYS) [48], after a strong hydrogen exposure, was ascribed to the same phenomenon. Similar phenomenon was observed on the (1 1 0) surfaces of both GaAs and InP. The reader is addressed to the corresponding paragraphs in this sections and to Section 5 for further discussion. Metallic droplets formation which was reported as a dissociation stage of the surface is ascribed to volatile phosphine formation with subsequent agglomeration of metal. Similar evidences were obtained by Allinger and Schaefer [49] in the etching of InP(1 0 0) wafers by hydrogen or methane. In both cases the exposure resulted in a preferential loss of phosphorus compared to indium. Simultaneously metallic indium droplets were developed. Hicks’s group used infrared adsorption and STM, in analogy to the case of GaAs(1 0 0), on the InP(1 0 0) with the main target of obtaining the reconstruction models of the clean surface [50] shown in Fig. 16(a). An inspection to the figure shows that the ð2  1Þ and s-(2  4) reconstructions are characterised by P dimers while the d-(2  4) one shows In–P heterodimers. In Fig. 16(b), the structural models of the hydrogenated surfaces are shown. They were obtained stemming from the values of the observed IR bands reported in Section 4 (see Fig. 40 and related discussion) and from the comparison with the experimental and calculated vibrational modes of molecular clusters containing In–H and P–H bonds. It was assumed that all the dangling bonds present in the different reconstructions are saturated and the number of electrons involved in bond formation is conserved. The models shown in Fig. 16(b) indicate the relative amounts of adsorbed H atoms that bonds to phosphorous sites, terminal indium sites and bridged indium sites, in agreement with the infrared spectra intensity of the vibrational bands. 3.2. The (1 1 0) surfaces The (1 1 0) surface is non-polar containing an equal concentration of cation and anion atoms in equally spaced layer and it is the cleavage plane of the zincblend structure. Each atom has two bonds to neighbours in the plane and one bond to an atom in the plane above and below. Thus the ideal bulk truncated surface has one dangling bond for each surface atoms. The real surface is relaxed and is shown in Fig. 10. 3.2.1. GaAs(1 1 0) surface Cleavage and IBA are the most common ways to obtain clean GaAs(1 1 0) surfaces. Lu¨ th and Matz [12] obtained by HREELS the first experimental evidence that hydrogen chemisorption occurs both on

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Fig. 16. Models of the (a) clean and (b) hydrogen-terminated InP(1 0 0) reconstructions. Upper panels refer to ð2  1Þ, middle panels to s-(2  4) and the lower panels to d-(2  4). Filled and open circles represent P and In atoms, respectively [50].

As and Ga. Two losses (see also Section 4 and related discussion) were identified at 1890 and 1380 cm1 (and at 2150 and 1660 cm1 with D) due to the stretching vibrations of As–H(D) and Ga– H(D), respectively. Not-preferential chemisorption is also in agreement with the predictions of total energy calculations performed by self-consistent pseudopotential [51]. In fact—see below—a total energy difference of only of 0.09 eV per atom was found for the two configuration of preferential chemisorption consisting in 0.5 ML of H chemisorbed on Ga and 0.5 ML of H chemisorbed on As, respectively. The LEED pattern preserves its ð1  1Þ periodicity upon moderate hydrogen exposure [20]. The comparison between clean and hydrogenated patterns are shown in Fig. 17. The spot intensity as a function of hydrogen is shown in Fig. 17(d) where the average intensity is compared with the background behaviour. All the intensities showed an initial reduction, followed by a plateau with a further intensity reduction at the onset of the background increase. These observations were correlated with a change of structure. In particular the (10) spot revealed a large decrease in intensity, which can be related with the surface de-relaxation. I–V profiles confirmed this hypothesis. The preservation of stoichiometry and chemisorption on both Ga and As atoms in the moderate exposure range is also confirmed by UPS in Ga 3d and As 3d core levels region [52]. Data will be presented and discussed in Section 5. For surface morphology purposes it is important to stress that at 1 ML of coverage some left-over emission typical of the clean surface is still present. This was ascribed to the presence of some density of surface atoms not reacted with hydrogen. Moreover such a component is still present also at very high exposures. The line width of As–H component in the moderate exposure regime is twice as large as the analogous component in the high exposure regime. This allowed to guess the presence of not-equivalent hydrogen bonding sites with As in the low exposure range. A PED experiment was performed to study the local geometry of the hydrogenated surface at ’1/4 ML coverage [53]. In Fig. 18 a polar plot (points) of the measured photoemission intensity of the surface shifted component of the As 3d core (at higher kinetic energy with respect to the bulk component) is

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Fig. 17. LEED patterns of GaAs(1 1 0) at a primary beam energy of 56 eV at normal incidence: (a) after cleavage; (b) after interaction with 104 L of hydrogen (molecular hydrogen in presence of a hot filament); (c) schematic diagram of the LEED pattern; (d) average spot intensity and background intensity versus hydrogen exposure [20].

reported as a function of the polar angle measured from the surface normal. Because of low kinetic energy of electrons, data were analysed in a renormalised multiple scattering framework. Continuos line represents calculation while data presented in the insets refer to bond length change with respect to volume (right) and buckling angle change with respect to the clean surface (left). Best fit to experiment was obtained for 58 of buckling angle, to be compared with the ideal surface buckling angle of ’25– 298 [21], and an average bond length coinciding with bulk one. A GIXRD experiment was performed on an MBE grown GaAs(1 1 0) surface with 1 ML H coverage at surface exciton quenching [54]. Results are reported in Fig. 19. Because the relaxed surface does not show fractional diffraction peaks the model analysis was carried out on odd integer peaks. Fit results indicated an 80% coverage together with counter relaxation with a negative buckling angle of 58 indicating a small amount of counter relaxation. This result is dramatised by the vanishing into noise of the (3, 4) reflex, as expected for an ideal surface structure. It is important to stress that the presence of 20% of uncovered surface is in agreement with indication coming from photoemission (core level and valence band, see above and the discussion in Section 5) and that a counter relaxation of 58 was predicted by theoretical calculations [55] (see below and Section 5). Wright et al. [51] reported theoretical results obtained by total energy calculations performed by selfconsistent pseudopotential. Two 0.5 ML coverage configurations were studied corresponding to chemisorption at As and Ga sites, respectively. They found that total energies for the 0.5 ML were

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Fig. 18. PED of GaAs(1 1 0) with 1 ML of hydrogen coverage. Simulated (solid line) and experimental (full points, the dashed line is a guide for the eye) polar scan of the As 3d surface shifted photoemission component. Top left inset: R factor as a function of buckling angle. Top right inset: R factor as a function of surface bond length [53].

differing by only 0.09 eV with the Ga–H bonding having the lower energy. The low energy difference does not allow to conclude in favour of Ga or As bonding. For both the 0.5 ML configurations and for the 1 ML case only small deviations were found from the ideal positions. ˚ and the magnitude of the forces on the atoms are not higher The largest of these deviations is 0.2 A 9 than 10 N. The difference between the 0.5 ML values and the 1 ML is only on the third digit as a consequence of the low interaction between As–H and Ga–H bonds as shown also, see Section 5, by the electronic energy levels and electron charge densities. Theoretical calculations of chemisorption sites in [51,55–57] give information about the length of Ga–H and As–H bonds otherwise almost hidden to diffraction techniques, because of the negligible ˚ for scattering efficiency of hydrogen for both electrons and photons. Wright et al. [51] found 1.61 A ˚ ˚ for Ga–H and 1.57 A for As–H while Bertoni et al. [55] (depending on cutoff) found 1.624–1.549 A ˚ for As–H. These values must be compared with the bond length of Ga–H and Ga–H and 1.571–1.534 A ˚ in hydride molecules, respectively. As–H of 1.59 and 1.52 A After hydrogen deposition Pulci et al. [57] found by calculation a counter relaxation with the As and Ga atoms close to the ideal terminated surface positions. The buckling between Ga and As resulted very ˚ ) the one between hydrogen atoms results slightly larger (0.24 A ˚ ). The Ga–H bond length small (0.18 A ˚ and the As–H is 1.54 A ˚ . For the 1 ML case the values of 1.61 and 1.57 A ˚ were found for the is 1.57 A As–H and Ga–H bond lengths, respectively.

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Fig. 19. GIXRD for the GaAs(1 1 0). Azimuthal F scan around the Bragg position of the (2, 1) (upper left panel) and (3, 4) (lower left panel) diffraction peaks of the clean substrate. In the right panels the same scans for the hydrogenated surface are shown [54].

In [55] the atomic geometry of the electronic structure (to be presented in Section 5) at 1 ML coverage of chemisorbed hydrogen was derived in the scheme of DFT and by using a norm-conserving ˚ , respectively pseudopotential. By assuming H–As and H–Ga bond lengths equal to 1.52 and 1.59 A [58], the total energy of the two chemisorption geometries reported in Fig. 20 (upper panel) were calculated together with the most stable configuration according to the minimal residual forces in the Hellman–Feynmann scheme. The de-relaxed configuration showed an energy gain of 0.73 eV per surface atom, a value considerably higher than the 0.3 eV per atom found for the energy gain of the clean surface reverting from ideal to relaxed geometry. The force analysis reported in Fig. 20 (lower panel) performed around the minimum energy configuration is in favour of a counter relaxation of about 58. As discussed in Section 5, by comparing photoemission results with the theoretical electronic level spectrum obtained by Manghi et al. [59], by a fully self-consistent pseudopotential, it was possible to select between different surface atomic geometry and conclude in favour of de-relaxation of surface induced by hydrogen.

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Fig. 20. Calculation of Hellman–Feynmann forces for GaAs(1 1 0). Upper: sketch of two of the trial chemisorption geometries used at the initial stage of calculation. Lower: atomic positions on a (1 1 0) plane perpendicular to the surface. The Z ¼ 0 plane is the central (1 1 0) plane of the supercell: (A) case of substrate atoms in the ideal positions, (B) positions for the optimised geometries. Thick and thin large circles represent As and Ga atoms, respectively. Small circles represent H atoms. The scale of force is the same in both pictures with Fmax ¼ 1:6  102 Ry/a0 [55].

Grizzi and co-workers [60] used ion scattering spectroscopy–time of flight (ISS–TOF) to obtain detailed information on the atomic structure of the hydrogenated surface of GaAs(1 1 0). The same group by previous ISS–TOF experiments carried on the clean surface gave the As–Ga first interlayer spacing and the spacings between the first and second As layers and between first and second Ga layers in close agreement with the generally accepted picture [61]. An ISS–TOF for 6 keV Neþ result for the hydrogenated surface is shown in Fig. 21. The two features derived from quasi-single collisions with As (IBS(As)) and with Ga (IBS(Ga)). Their intensity ratio is strongly dependent on the incidence angle, a fact that proves the surface sensitivity of the chosen scattering geometry (for numerical values of scattering angles see the caption). In fact the dependence of the spectra on the incidence angle and on the hydrogen exposure is understood in terms of shadowing and blocking effects on a relaxed surface where the As atoms occupy positions higher than the Ga ones. Since the first Ga layer is below the first As layer, its contribution to the backscattered signal at low incident angles is weak (the Ga atoms are inside the shadow cone

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Fig. 21. ISS on hydrogenated GaAs(1 1 0). TOF spectra for 6 keV Neþ ions acquired at the f ¼ 61:4 azimuth (is measured from the ½1 1 0 direction), y ¼ 5:5 (a), y ¼ 8:3 (b), and at three different exposures (in Langmuir of molecular hydrogen in presence of a hot filament) [60].

produced by the As atoms). In order to observe the Ga contribution (Fig. 21(b)), the incident angle must be increased beyond 78. After the hydrogen exposure the As contribution decreases while that of Ga remains almost constant. This variation indicates a de-relaxation process, that is, the Ga and As atoms recover the position of the bulk terminated surface, with both Ga and As layers at a similar height. For projectiles hitting a clean (relaxed) surface with suitable angles (see below) only the first layer of As atoms will be accessible, while for the unrelaxed surface the short interatomic distance will preclude the backscattering from the As layer as well. Thus the backscattered intensity from As atoms, IBS(As) is representative of the fraction of the surface remaining relaxed after the exposure to hydrogen. Some typical results of the quantitative analysis of shadowing scattering regions, based on a computer code are reported in Fig. 22. Results were interpreted by using the shadowing diagrams reported in Fig. 22 for relaxed and bulk terminated surface. The comparison of the two groups of diagrams shows that the critical angle for focusing onto AsI moves up to yc ¼ 10:3 which causes the strong decrease observed in IBS(As) at both incidence directions (Fig. 21). For the GaI contribution as Ga atoms move up (As down) the critical angle to see the GaI layer decreases to yc ¼ 4:3 . Additional information was derived from the azimuthal dependence of TOF–ISS data, in particular about the direction of the hydrogen bonds. The relatively low intensity of the first layer focussing, observed along the f ¼ 72 and f ¼ 61:4 , and the rise of IBS(Ga) seen at ðf; yÞ ¼ ð72 ; 10:6 Þ hydrogen atoms adsorbed along the dangling bonds. From data analysis the authors concluded that at 1000 and 2000 L the difference in vertical position was ˚ as expected for bulk terminated surface. At these exposures there is a reduced to DðZÞ ¼ 0  0:08 A fraction of the surface that remains relaxed. On the other hand at 5000 L, when most of the surface has ˚. been relaxed data is consistent with a small counter relaxation with a buckling angle $ ¼ 3  0:08 A In the same framework of experiments the same group performed an analysis of the dependence of the intensity IBS(As) of the quasi-single collisions of Neþ with surface As atoms on hydrogen exposure to measure the fraction of de-relaxed surface as a function of coverage [62]. To this end ISS–TOF spectra at fixed incidence ðf; yÞ ¼ ð63:7 ; 6:3 Þ were collected as a function of the hydrogen dose.

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Fig. 22. ISS on hydrogenated GaAs(1 1 0). (a) Shadowing regions calculated for 6 keV Neþ backscattering from a relaxed GaAs(1 1 0) surface. AsI: first, AsII: second layer of As atoms; GaI: first, GaII: second layer of Ga atoms. The crosses indicate the direction of measurement. Vertical dashed lines indicate the span of angular scans. Thick and thin lines are the shadowing regions produced by As and Ga atoms, respectively; (b) is similar to (a) but for a bulk terminated surface [60].

As also explained above in this scattering conditions the short interatomic distance between an arsenic and a gallium atom will preclude the backscattering from the As layer. The result for IBS(As) are reported in Fig. 23 showing different regimes. In Fig. 24 the function 1  IBS (As) (proportional to the fraction of unrelaxed surface) is plotted versus the normalized intensity N (H) (proportional to the coverage). The figure shows the existence of three different of direct recoil IDR linear regions indicating that in each region the number of surface atoms moving to unrelaxed positions per adsorbed H is constant. Close to the 1 ML coverage (determined from the integrated intensity of IDR(H), see also Section 6) a large fraction of the surface atoms remains relaxed as in the clean surface.

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Fig. 23. ISS on hydrogenated GaAs(1 1 0). Quasi-single backscattering intensity from As atoms on the GaAs(1 1 0) surface versus molecular hydrogen exposure in presence of a hot filament (f; y ¼ 63:7 , 6.38) for 6 keV Neþ ions. IBS is normalised to the clean value of the clean surface. The thin lines are drawn to guide the eye [62].

Fig. 24. ISS on hydrogenated GaAs(1 1 0). 1  IBS (As), proportional to the fraction of unrelaxed surface is plotted as a N function of IDR ðHÞ, proportional to the H coverage. The straight lines are the result of least-squares fits [62].

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Fig. 25. Peak-to-peak amplitudes ratio of the Ga M3M4M4 and As M4VV Auger lines for GaAs(1 1 0) as a function of hydrogen exposure (in Langmuir of molecular hydrogen in presence of a hot filament): (a) from [20]; (b) from [3] together with the plot of the surface 3d exciton behaviour (triangles) relative to the clean surface value is reported. The lines are for the guide of the eyes [3].

This result is similar with the results obtained by core level photoemission of [52,63]. Finally ISS–TOF scans versus incident angle and versus azimuthal angle give information on the depth of chemisorption and on the surface order, respectively. The results, obtained with 1 ML of coverage, showed that the majority of hydrogen atoms are located above the substrate top layer and not buried in the subsurface layer. In addition the azimuthal scans did not show any significant angular dependence indicating a not ordered surface. This last result is at variance with the results reported by GIXRD of [54] and reviewed above. Some measurements of the peak-to-peak amplitudes of the two As M4VV (at 32 eV) and Ga M3M4M4 (at 50 eV) Auger lines upon hydrogen exposure of the GaAs(1 1 0) surface were reported by Sebenne and co-workers [20] and Mo¨ nch and co-workers [3]. Any energy shift and any new feature of the line shape were reported. In Fig. 25(a) the results obtained by Sebenne and co-workers are shown. Both curves are flat and parallel, revealing within the error bar, the absence of surface stoichiometry alteration up to about 5  103 L where, in this experiment, dissociation started. The two different slopes of Ga and As amplitudes are the evidence of stoichiometry alteration with As desorption while data show that the only presence of a hydrogen overlayer in the chemisorption region ðL < 3  105 Þ does not affect Auger lines intensities. Beside this effect, which seems to be the dominant one, also at the light of core level ratio measured by Santoni et al. [52] and Sorba et al. [63], Sebenne and co-workers [20] pointed out that some intensity reduction of the valence–valence As core line could come from

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valence band modifications due to reaction with hydrogen. In Fig. 25(b) the results obtained by Mo¨ nch and co-workers are reported. It is still an open question if the de-relaxation is restricted to chemisorption sites or it involves a wider region. From PYS experiments at partial coverage [20] some electronic states were observed to show up in the gap, induced by hydrogen, with an evaluated density of 5  1012 cm2. Such a low density is almost hidden to UPS. These states have been tentatively assumed as states of the clean surface brought back into the gap by de-relaxation reverting the electronic properties to those of the clean surface [64]. On the other side the surface exciton absorption of Ga 3d, which has an empty surface state as final state (though through an excitonic interaction), does not shift in energy as a function of hydrogenation [9]. For exposures much higher than the monolayer formation dissociation effects play the most important role. Changes in surface stoichiometry were evidenced by looking at surface sensitive As and Ga Auger electrons [3,20,65,66] and by photoemission from Ga 3d and As ratio at the minimum of escape depth [63]. Core level results are reported in Fig. 26 indicating a Ga enrichment at the surface as a function of hydrogen exposure.

Fig. 26. Core level photoemission for hydrogenated GaAs(1 1 0). (a) Core level UPS in the region of Ga and As 3d excitation; (b) Ga(3d)/As(3d) ratio as a function of hydrogen exposure (in Langmuir of molecular hydrogen in presence of hot filament) [63].

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By PYS important contributions to the identification of reaction pattern and formed species during the dissociation were given [26]. This class of results will be reviewed in Section 5. For present purposes it is worth mentioning that metal island formation was induced by the gradual lowering of low energy onset of total emission observed at increasing exposures. An ionisation edge of 4.30 eV was observed which corresponds to the metallic edge of Ga [67]. The desorption of Ga at lower temperature reported in Fig. 136 (see Section 6) was put into relation [26] with the desorption of Ga from metal islands. As will be shown below similar mechanism operates in InP but with higher efficiency. This may be the consequence of the different properties of metals or reflect the easiness by which metal clusters can be formed in the semiconductor matrix, implying a change in metal–metal distance is 39% for Ga in GaAs and 22% for In in InP. In the same experiments the presence of an AsH3 like molecule was inferred from the presence of an unusually strong absorption band (named ‘‘black hole’’ after the author of [26]) occurring at ’5.5 eV of photon energy and from the lowering of ionisation threshold with hydrogenation [20] to be related—see also Section 5—with the lowering of work function with exposure [3,9]. The latter being ascribed to the dipole formation at surface due to such polar molecule and the former to absorption optical band of AsH3 occurring in the isolated molecule at 5.57 eV [68,69]. This strong absorption band is no longer observable after heating the surface at 350 8C [20] indicating desorption of the previously formed AsH3 like molecule. Similar effects were observed in the case of bombardment with H2 þ for which also a metallic threshold was observed [70]. The H2 þ beam kinetic energy was also sufficient to make the arsine absorption band to disappear likely because of AsH3 like molecules desorption resulting in a layer by layer etching of the surface. Evidence of defect formation at surface was claimed on the basis on a huge photoemission structure present in the valence band photoemission spectrum [25]. Present data do not allow to exclude, as suggested in [26], that instead this feature could come from arsine. The GaAs(1 1 0) surface shows a strong anisotropy evidenced by EELS [71] and, as discussed in Section 5, almost cancelled by hydrogenation. 3.2.2. InP(1 1 0) surface The InP(1 1 0) surface can be obtained both by cleavage and IBA showing a ð1  1Þ relaxed surface [21,72]. HREELS (to be presented more extensively in Section 4) was exploited by measuring the absorption frequencies of H–P and H–In to derive surface stoichiometry and active absorption sites. Dubois and Schwartz [73] found that the relative intensity of both H–In and H–P modes were very sensitive to surface preparation by IBA. In order to determine the active sites of absorption at both cleaved and IBA prepared surfaces the initial uptake of hydrogen was investigated by HREELS by Schaefer et al. [74]. Results are shown in Fig. 47 (see Section 4), H–P and H–In modes are indicated. Absorption at the In site is dominant in the case of cleaved surface, while the situation is reversed in the case of IBA prepared one. The higher intensity of the H–P stretching mode on sputtered surface was ascribed to the presence of broken bonds on the sputtered surface owing to the fact that the surface stoichiometry, as derived from AES and XPS, was substantially the same. An additional hydrogen exposure strongly affected the intensity of the H–P stretching. This fact was interpreted as H-induced desorption of phosphine. Schaefer et al. [74] gave an explanation for the uptake of hydrogen at the In site on the cleaved surface. In fact in the outward relaxation of anion and in the inward relaxation of cation a charge transfer occurs from cation to anion. In this situation the hydrogen easily makes a bond with almost empty In dangling

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bond donating charge to bond formation. This mechanism should contribute to surface de-relaxation. Observed gap states by PYS [66] at moderate exposures could be interpreted as P related dangling bonds, of the uncovered surface brought back into the gap by de-relaxation, also if—see below—a more significative contribution coming from metal islands seems, at least at high exposures, more reasonably. On the other side UPS data from P 2p and In 4d core levels indicate—see Section 5—chemisorption on both anion and cation in close analogy with the case of GaAs(1 1 0). At moderate exposure (103 L in this experiment) [66] a ð1  1Þ LEED pattern, reported in Fig. 27 is still visible. In this figure the behaviour of average intensity and of background are also shown.

Fig. 27. LEED patterns from the hydrogenated InP(1 1 0) surface: (a) and (b) clean surface with 40 and 60 eV of primary beam energy, respectively; (c) and (d) same primary energies bur after 104 L of molecular hydrogen in presence of hot filament; (e) schematic of the LEED pattern; (f) average intensities of diffraction peaks: (1 2), (1 3), (0 2), (0 0), (0 2), (0 3), (1 2) and (1 3) (squares) and (0 1), (1 0) and (1 0) filled dots [66].

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Fig. 28. Core level photoemission for hydrogenated InP(1 1 0) in the region of In 4d with hn ¼ 65 eV from In 4d core level in InP(1 1 0) at various interaction stages with hydrogen. Exposures are given in Langmuir of molecular hydrogen in presence of a hot filament. Each spectrum is normalised to the highest signal [30].

As mentioned before in PYS experiments (to be discussed also in Section 5) a shift in the ionisation threshold in the direction of lower binding energy was found [26,66], together with the variation of the work function for n and p type samples. The lowering of ionisation energy was assumed as an indication of In metal islands formation in agreement with the difference in photoelectric threshold difference value between InP and In metal. Another direct evidence of the formation of In metal is given by the evolution of In 4d photoemission line shape [30,75]. This is shown in Fig. 28. In fact after 6  103 Langmuir of hydrogen, which corresponded to an exposure slightly higher than the dissociation onset, a shoulder on the high kinetic side (lower binding energy) was observed. The shoulder reverts into a 4d spin-orbit doublet at higher exposures associated to emission from In metal island. M’hamedi et al. [66] followed the variation of the P L23VV (120 eV) and In M4M45N45 (401 eV) Auger lines as a function of the hydrogen dose. Any line shape change was reported. The peak-to-peak amplitudes are reported in Fig. 29. The inspection to the figure shows that, qualitatively, the P to In intensity ratio is almost constant up to about 103 L decreasing at higher coverage indicating a P loss. In analogy with the case of GaAs(1 1 0) the presence of hydrogenated species of group V element was ascertained by the appearance of the strong absorption band (black hole, after [26]) in PYS

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Fig. 29. Auger spectroscopy of hydrogenated InP(1 1 0) surface. Evolution of the peak-to-peak amplitudes of P L23VV (120 eV, squares) and In M4M45N45 (401 eV, dots) Auger lines, as a function of the hydrogen dose in Langmuir of molecular hydrogen in presence of a hot filament [66].

experiments [26,66,76]. In these case the observed intense and broad band is assigned to phosphine like molecular aggregates on the surface. In fact the feature develops from two structures occurring at about 5.3 eV of photon energy. The former position is reproducible within 0.03 eV and coincides with the value of 5.33 eV of the first excitation of PH3 [69]. The InP(1 1 0) surface shows a strong anisotropy which, in close agreement with the case of GaAs(1 1 0) is drastically removed by hydrogen as was shown by HREEL in [77]. Data are presented in Section 5. Chemisorption on both sites must be concluded from, at variance with conclusions drawn by HREELS, from UPS experiments in the region of excitation of In 4d and P 4p as reported in [78,75]. In fact in analogy with the case of GaAs(1 1 0) one new component, beside that of surface and bulk, shows up in the deconvolution of both In 4d and P 2p. Both components are shifted of ’600 meV in the direction of higher binding energy with respect to bulk component. Data will be also discussed in Section 5. For morphology information it is also important that a third component, present only in the case of In 4d, at lower binding energy shows up and is associated to the presence of In in metal islands. Also in analogy with the case of GaAs(1 1 0) the surface component is needed in the deconvolution at almost any stage of the exposure. A PED study of the hydrogenated surface was performed [78] on a surface after a moderate exposure to hydrogen. The analysis was carried on both on the clean surface component and on the hydrogen related one. This allowed to derive information on local geometry around the two different sites. The authors found a tendency of the surface to de-relaxation. This was found almost complete at the hydrogenated site and more reduced buckling angle (208) at the not reacted sites. 3.2.3. Other (1 1 0) surfaces Only a few data are available on the surface structure and morphology of other (1 1 0) surfaces of III–V compounds. Hydrogenation of InSb(1 1 0) was studied stemming from core level emission in the core level region [79]. The exposure was about 3  107 L. The Sb 4d lines show little change with hydrogen adsorption except for a slight broadening while the In 4d line shape was affected. On this basis the authors concluded that hydrogen chemisorbs only on In in InSb. The deconvolution in terms of Lorentzian components resulted in the presence, beside surface and bulk, of a third component ascribed to the

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presence of hydrogen. At variance with the case of GaAs(1 1 0) and InP(1 1 0) its binding energy was lowered with respect to bulk component. This could induce to an alternate scheme of explanation where the shifted component is due to metallic In, in close analogy with the case of InP at high exposures as discussed above. Hydrogenation of InSb(1 1 0) has been studied by Hinkel et al. [79] by UPS in the emission of In and Sb 4d core level. Results are shown in Section 5. The estimated exposure was about 3  107 L. The Sb 4d lines show little change with hydrogen adsorption except for a slight broadening of the peaks. The In 4d spectrum, on the other hand, exhibits a strong modification upon hydrogen adsorption. The deconvolution procedure results into the presence of a third component, ‘‘H’’ of Fig. 128 (Section 5), ascribed to H bond to In. The absence of modification of the Sb line shape leads the authors of [79] to the conclusion that H on InSb(1 1 0) binds with In only. There are some analogy with the case of early stages of chemisorption on InP(1 1 0) reported by Schaefer et al. [74] discussed above. The binding energy of the H component is lower than both bulk and surface components of the clean surface. This is at variance with the case of GaAs(1 1 0):H studied by Sorba et al. [63]. Moreover Proix [30], as discussed above, related In cluster formation to the appearance of a low binding energy In metal component which can be put into relation with the H component of Fig. 128 (Section 5). More work seems to be needed to elucidate this point. The chemisorption sites on GaP(1 1 0) for the early stages of the hydrogenation have been identified by Chen et al. [80] by HREELS, LEED and XPS. The HREELS results are reported in Fig. 48 (Section 4). They found two modes at 233 and 291 meV ascribed to Ga–H and P–H stretching frequency, respectively. The two features were reported to saturate in intensity between 500 and 100 L in their experimental conditions. The ð1  1Þ LEED pattern was still visible at saturation coverage though with some increase in background but unaffected spot sharpness. Indicating surface order degradation but in an incoherent manner. In analogy with GaAs(1 1 0) and InP(1 1 0) the surface anisotropy was cancelled by hydrogen exposure as observed by azimuthal resolved EELS [81]. The two modes at 233 and 291 meV are due to Ga–H and P–H stretches. They are reported to saturate in intensity between 500 and 1000 L while their energy position and intensity ratio remains unchanged. 3.3. (1 1 1) and (1¯ 1¯ 1¯ ) surfaces The {1 1 1} surface are polar. There exists two types of surfaces: the (1 1 1)Ga surface and the  ð1 1 1ÞAs. The former ideally contains only Ga atoms in the first layer while the latter only As atoms. In the [1 1 1] direction the atomic layers are not equally spaced. The closely spaced layers are connected by three bonds per atom as opposed to one bond between the amply spaced layers. Thus the surface atoms has a single dangling bond. The commonly accepted structure models of the (1 1 1) reconstructed surfaces are shown in Fig. 11. 3.3.1. GaAs(1 1 1) surface The most widely studied is the Ga terminated 2  2 reconstructed surface, GaAs(1 1 1)-2  2. LEED studies have been reported by Tong et al. [82]. A vacancy-buckling model has been proposed by Tong et al. [83] and by Chadi [84]. In their model one-forth of the Ga atoms are equidistantly removed from the Ga terminated surface, leaving behind some amount of Ga vacancies. The Ga atoms move inwards,

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while the three As atoms in the vicinity of the vacancy move outwards. Orbital rehybridisation enables dangling bonds of the As surface atoms to form stable occupied states which are, as generally regarded, difficult to bond any further. Hou et al. [24] studied this surface by LEED and HREELS in the low and high exposure regime. For low exposures the (2  2) LEED pattern was preserved although the intensity of fractional-order spots was somewhat reduced. At higher exposures, the background was enhanced with the ultimate appearance of a ð1  1Þ pattern. On the other hand HREELS results (see also Section 4) show that at low exposures hydrogen atoms are adsorbed only at Ga dangling bonds while at higher exposures also bonding to As was observed. An annealing at 260 8C removed all hydrogen related losses and brought the reappearance of fractional LEED spots, though with a reduced intensity. Further annealing at 400–500 8C led the reformation of the original (2  2) pattern. The authors suggested a bonding mechanism based on charge redistribution between Ga and As surface bonds. In their scheme charge redistribution at the Ga site reacted with hydrogen involves also As–As bonds at surface allowing a subsequent bonding with hydrogen. They observed also the work function to increase from 4.21 to 4.58 eV in the initial reaction with only Ga. At higher exposures work function decreased to 4.47 eV. This was assumed as the formation of a negative and positive dipole, respectively, in the two regimes which can be explained by electronegativity or charge transfer arguments. The same authors also studied the ð 1 1 1Þ face, ideally only As terminated. However the HREELS results showed losses corresponding both to Ga–H stretching and As–H stretching. The presence of Ga–H stretching was assumed as an evidence of the presence of [1 1 0] oriented facets where As and Ga dangling bond coexist. In this case the work function decreased from the 4.75 eV value of the clean surface down to 4.43 eV upon 1000 L. This can be ascribed to the fact that the surface dipole layer originating from As–H bonds dominates the work function change due to hydrogenation. 3.3.2. InP(1 1 1) surface A behaviour of this surface very similar to that of GaAs(1 1 1) was found by the same group as above [24]. The HREELS of H adsorbed on the (2  2) reconstructed surface showed the presence of both In– H and P–H bonds (see also Section 4). Since the 2  2 is prepared by annealing in the P atmosphere the authors guessed that on the original In terminated surface there exist some undesorbed P atom contributing to the P–H stretching. The peak intensity was found to depend on annealing cycles. On the other hand no In–H was found on the ð 1 1 1Þ-1  1 indicating that the surface is completely terminated by P. 3.3.3. InSb(1 1 1) and InSb(1¯ 1¯ 1¯ ) Data obtained by HREELS (see also Section 4) and RHEED on both (1 1 1) and ð1 1 1Þ surfaces of InSb were reported by Hernandez-Calderon [85]. After cleaning by IBA the surfaces presented a (2  2) and ð3  3Þ reconstruction, respectively. After the hydrogen dosage (750 L for (1 1 1) and 1400 L for ð1 1 1Þ, respectively) the RHEED pattern indicated a reduction in the intensity of the reconstruction features, being weaker for the ð1 1 1Þ-ð3  3Þ surface. After a dose of ’3000 L a diffuse RHEED pattern of the relaxed surface was observed. The 216 and 231 meV losses were observed on the (1 1 1) and ð1 1 1Þ surfaces, respectively. They were interpreted as stretching modes of hydrogen bond with In and Sb, respectively.

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4. Vibrational properties Vibrational spectroscopy of the III–V hydrogenated surfaces deals with the vibrations of the H-cation and H-anion bonds at the surface. The stretching of the bond is the mode most frequently studied, also because of experimental aspects. In fact according to the small mass of hydrogen the frequency of the bond stretching results about 2000 cm1 (about 250 meV) and consequently it occurs in a range of relatively easy access for infrared absorption and also for EELS by using moderate resolution. The available data not always come from dedicated vibrational studies. A good bunch of data come as by product from studies of surface structure and morphology where the stretching frequencies of hydrogen bonds are used to monitor the surface anion and cation density at the surface, mainly in the case of (1 0 0) and (1 1 1) MBE grown surfaces. The infrared spectroscopy permits, in comparison with HREELS, to achieve a better resolution. Its sensitivity can, easily by a factor of the order of 10, be enhanced by the use of multiple internal reflections (attenuated total reflection, ATR) and it provides the determination of mode polarisation by the use of polarised light. Consequently the fine structure of the cation and anion hydrides absorption bands are detectable. On the other hand HREELS provides a higher surface sensitivity and gives access simultaneously to a wider range of excitations, including electronic transitions and plasmons. Beside stretching modes some data are also available for bending modes of Ga–H and As–H bonds but only in the case of the GaAs(1 1 0) surface. The HREELS in the region of interest for the vibrational spectroscopy of III–V surfaces are characterised by the presence of the Fuchs–Kliever surface phonon occurring at about 30–50 meV [73,86]. This mode is independent from the crystal surface orientation [73]. In HREEL spectroscopy this excitation often couples with vibrational and plasma modes [87]. This last coupled mode, whose energy depends on carrier density in the space charge layers is generally close to that of the Fuchs–Kliever phonon and causes the appearance of two modes whose energy occurs in the 80–800 cm1 (10–100 meV) range, as shown in [86]. Because generally hydrogen exposures affects band bending, and through it the conduction band plasmon energy, attention must be paid to the consequent shift of coupled mode or of its combination with vibrational excitations [86]. In assigning the observed features both in infrared absorption and in EELS it is of use the comparison with the values obtained from molecular spectroscopy of hydrides. To this purpose in Table 3 the vibrational frequencies of III and V hydrides are reported. The table includes symmetric and asymmetric stretching modes and bending modes. 4.1. GaAs(1 0 0) surface The vibrational HREELS of the hydrogenated Ga-rich GaAs(1 0 0)-ð4  6Þ, cð8  2Þ and ð1  1Þ surfaces measured by Schaefer et al. [28,38] are reported in Fig. 30. The observed loss features, apart from the 310 cm1 (36 meV) loss due to Fuchs–Kliever phonon, were assigned to 1860 cm1 (216 eV) and 2115 cm1 (245.9 eV) for Ga–H and As–H stretching modes, respectively. The observation of the As–H mode was partially hindered by the superposition of the Ga–H þ surface phonon occurring at ’2152 cm1 (’267 meV). Moreover the Ga–H mode was found to shift with hydrogen exposure in concomitance with surface reconstruction changes (see also Section 3) as shown in Fig. 31.

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Table 3 Vibrational frequencies of III–V hydrides Compounds

Frequencies (cm1)

Reference

Arsines AsH3 Et2AsH EtAsH2 tBuAsH2 GaAs(1 1 0):As–H GaAs(1 0 0):As–H

2123, 2116 2080 2107–2086 2126–2088 2150 2000–2150

[98] [44] [44] [44] [44] [44]

Gallane adducts Me3NGaHMe2 Me3NGaH3 [Me2NGaH2]2 Me3PGaH3 GaAs(1 1 0):Ga–H GaAs(1 0 0):Ga–H

1840–1770 1848–1839, 1790–1782 1911, 1907, 1901, 1870 1832, 1808 1890 1880–1840

[44] [44] [44] [44] [44] [44]

H-bridged gallanes Ga2H6 gas [GaH3]n solid [Me2GaH]2 gas [Me2GaH]n solid [Et2GaH] N2 matrix [Et2GaH]n solid a-GaAs:H InP(1 0 0):P–H InP(1 0 0):In–Hterminal InP(1 0 0):In–Hbridging InP(1 0 0):In–Hsmall bridging PH3 SbH3

1993, 1976, 1273, 1202 1978, 1705, 950 1290, 1185 1705, 950 1234, 1162 1657–1640 1460 2339, 2311, 2213 1660 1350 1150 2323 1891

[44] [44] [44] [44] [44] [44] [44] [50] [50] [50] [50] [98] [98]

The authors ascribed the increase in stretching frequency to dihydride formation, in analogy with what happens in Si(1 0 0) [88] and in GaAs(1 1 1) [89]. This picture is supported by the observation of a 710 cm1 (88 meV) loss tentatively assigned to GaH2 in analogy with the situation of amorphous hydrogenated GaAs [90]. Further information come from the As–H/Ga–H loss intensity ratio; the effect of structure changes and the effect of substrate temperature are shown in Fig. 32. Fig. 33 shows the HREELS of hydrogenated As-rich GaAs(1 0 0)-cð4  4Þ surface of [29]. Being the ð4  4Þ reconstructed surface As terminated only the As–H mode is visible at the beginning of exposure. As also discussed in Section 3 the progressive hydrogenation of the ð4  4Þ Asrich surface produces a surface with an increasing surface Ga concentration with a consequent increase of the Ga–H mode intensity. A parallel conversion from ð4  4Þ to ð1  1Þ takes place. In the right panel losses due to conduction band plasmon and Fuchs–Kliever phonon are visible. In this case the presence of the conduction band plasmon loss (which depends on hydrogenation through hydrogen induced band bending change) hinders the observation of low energy vibrations as in the case of GaH2 complexes modes.

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Fig. 30. High resolution EELS of hydrogenated (exposures are given in Langmuir of molecular hydrogen in presence of a hot filament) GaAs(1 0 0) surfaces. Left: spectra of gradually exposed (a) ð1  1Þ, (b) 4  6, and (c) ð8  2Þ reconstructions after hydrogen exposure of 4 L. The hatched area corresponds to the contribution of the GaH þ phonon combination loss. Centre: spectra of gradually exposed surfaces ð4  6Þ at different exposures. The hatched area corresponds to the contribution of the GaH þ phonon combination loss. Right: spectra of gradually exposed surfaces after (a) hydrogen exposure of 4  104 L, (b) subsequent annealing at 400 8C, (c) additional annealing at 500 8C. Curves (b)–(d) were obtained after 10 L at room temperature. The intensity ratio, R (see text) is reported [38].

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Fig. 31. Ga–H stretching mode energy as a function of hydrogen exposure for the GaAs(1 0 0) surface. The different LEED patterns are indicated (exposures are given in Langmuir of molecular hydrogen in presence of a hot filament) [28].

Fig. 32. Analysis of the stretching frequencies of GaAs(1 0 0). (Left) Influence of structure. (I) As–H intensity; (II) Ga–H intensity; (III) intensity ratio as a function of hydrogen exposure for ð1  1Þ, ð4  6Þ and cð8  2Þ reconstructions. (Right) Influence of substrate temperature. (I) As–H intensity; (II) Ga–H intensity; (III) ð8  2Þ surface: intensity ratio, for two different substrate temperatures, as a function of hydrogen exposure (exposures are given in Langmuir of molecular hydrogen in presence of a hot filament) [28].

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Fig. 33. HREEL of hydrogenated GaAs(1 0 0)-cð4  4Þ. (Left) HREELS as a function of hydrogen exposure; (right) HREELS in the region of conduction band plasmon and Fuchs-Kliewer phonon (exposures are given in Langmuir of molecular hydrogen in presence of a hot filament) [29].

In Fig. 34 the infrared multiple internal reflection spectra of hydrogen adsorbed on GaAs(1 0 0)cð2  8Þ at increasing hydrogen exposure are reported. The infrared absorption was mainly used to surface characterisation purposes even if some spectroscopic data were made available from this experiment. Initial exposure produces one peak at 2100 cm1 (260.4 meV) due to As hydride species. With prolonged exposure the 2100 cm1 disappears and a second one appears at 2140 cm1 (265.4 meV) due to arsenic monohydride. The authors compared also the absorbance of the As–H vibration with that of CO absorbed on a thin film of Pt (2040 cm1, 252.9 meV) obtaining a 1/190 ratio in favour of CO/Pt. Multiple internal reflection were also used by Hicks and co-workers [23,44,91] in the region of stretching frequencies of As and Ga hydrides for the cð2  8Þ and the (1  6) reconstructions (the 1  6 is also referred in the literature as 2  6 and 4  6 according to the degree of disorder [31]). Results are shown in Fig. 35(a) and (b) for the cð2  8Þ and (1  6) reconstruction, respectively. Spectra were taken at different exposures, saturation was assumed as 1 ML coverage and the intermediate spectra are labelled accordingly. The infrared reflectance spectra of both surface reconstructions present three groups of features in the 1950–2150 cm1 (242–266.6 eV), 1700–1950 cm1 (210.8–241.8 eV) and 1200–1700 cm1 (148.8– 210.8 eV). Each group shows a fine structure. These groups have been attributed to As hydrides, terminal Ga hydrides and to bridging Ga hydrides, respectively by comparing with the frequencies observed for molecular compounds with As–H and Ga–H bonds (see also Table 3).

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Fig. 34. Multiple internal reflection spectra of hydrogen adsorbed on GaAs(1 0 0) after an exposure of (a) 7 L, (b) 14 L, and (c) 50 L (exposures are given in Langmuir of molecular hydrogen in presence of a hot filament) [40].

The same spectra were also obtained for deuterium exposed surfaces obtaining the same overall behaviour [44] with a frequency shift ratio of 1.385 [91]. Similar non-harmonic behaviour was observed on GaAs(1 1 0) [12] where the H and D frequencies on the As and on the Ga site are related by factors 1.37 and 1.30. Because of the bond directionality infrared reflectance spectra presents a strong dependence on the s (electric field parallel to the surface) or p (electric field normal to the surface) light incidence. Results are shown in Fig. 36 for the hydrogen adsorbed on GaAs(1 0 0)-cð2  8Þ and GaAs(1 0 0)-ð2  6Þ. Maxima of absorption are due to the parallelism between the electric field of the light and bond direction. Consequently this polarisation dependence implies that the As–H bonds orient along the [1  1 1] direction, parallel to the As dimer bonds. By contrast the Ga–H bonds orient along the [1 1 0], perpendicular to the As dimer bonds. The assignment of all the spectral features was carried out either comparing with the observed molecular frequencies (see Table 3) either stemming from the models for substrate reconstructions together with bond character (see also Section 5). The comparison between the two panels of Fig. 35 shows that substantially the same number of fine structure features occurs and

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Fig. 35. Multiple internal reflection spectra of hydrogen adsorbed on GaAs(1 0 0) at different hydrogen coverage. Spectra were taken after different exposures to molecular hydrogen in presence of a hot filament, saturation was assumed as 1 ML coverage and the intermediate spectra are labelled accordingly with that: (a) GaAs-cð2  8Þ; (b) GaAs-(1  6) [23].

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Fig. 36. Absolute change in reflectance of polarised IR light due to hydrogen adsorption on the GaAs(1 0 0)-cð2  8Þ and GaAs(1 0 0)-ð2  6Þ surfaces oriented along the [1 1 0] and [1 1 0] axes: s-polarisation (dashed lines) and p polarisation (solid lines). The inserts describe the surface lattice and light electric field orientation [44].

at the same energy but with different relative intensities due to different Ga and As atom density on the two cð2  8Þ and (1  6) surfaces. As far as the first As band is concerned (1950–2150 cm1) the first peak at 2140 cm1 has been attributed to a As–H bond in a dihydride configuration at a step or at a vacancy. The 2050 cm1, 2020 cm1 and 1995 cm1 bands were assigned to hydrogen bounded to As dimers and producing an isolated monohydride or a coupled monohydride in the first layer. The peak at 2110 cm1 is due to As in the second layer bounded with H. The second group (1700–1950 cm1) in the cð2  8Þ reconstruction is formed by two sharp peaks, while in the (1  6) case there are at least four features at 1750, 1835, 1855 and 1870 cm1. At saturation coverage these peaks overlap to give a broad band. These peaks were attributed to H–Ga bonds in the second layer in a terminal Ga hydride configuration. The last group (1200–1700 cm1) was assigned to bridging gallium hydrides. The clear broadening were related either to the simultaneous presence of different bridging groups and to the coupling with phonons. The three broad features at 1325 cm1, 1460 cm1 and 1600 cm1 were assigned by means of the valence force theory to correspond to the angles of 988, 1058, 1138, respectively (corresponding— ˚ —to a Ga–Ga distance of 2.55, 2.70 and 2.85 A ˚ , respectively). with a H–Ga distance of 1.7 A The comparison between infrared reflectance spectra of H interacting with surfaces with those obtained for gas-phase molecules is not straightforward, because the structures formed on surfaces do not have exact molecular analogous in the gas. In order to make a detailed assignment of the stretching modes for each of the bands in the infrared spectra, Hicks and co-workers [45] performed ab initio cluster calculations based on density functional theory for H interacting both with the As-rich (2  4)GaAs(0 0 1) and the Ga-rich ð4  2Þ-GaAs(0 0 1) systems. They found that the calculations based on the three clusters with structures Ga5As4H11,13, Ga4As5H11,13 and Ga7As8H19 (shown in Fig. 37) provide an accurate model of H adsorption on the Ga and As dimers exposed on the GaAs(0 0 1) surface.

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Fig. 37. Ball and stick models of the optimised clusters, showing arsenic–hydrogen bonds (1–3) and gallium–hydrogen bonds (4–6). The As, Ga and H atoms are the large grey, large black and small with spheres, respectively [45].

The formation of monohydrides and dihydrides and its implication with surface reconstruction were reported in Section 3. The most striking difference between cð2  8Þ and ð2  6Þ spectra is the presence of the broad band between 1750 cm1 (216.9 meV) and 1200 cm1 (148.8 meV) for H:ð2  6Þ and below 1250 cm1 (155 meV) for the D:ð2  6Þ. This features were attributed to the asymmetric stretching vibration of a bridging gallium hydride as discussed in Section 3 and shown in [23]. In the case of amorphous GaAs the bridging hydride gives rise to a broad band exactly at the same frequency as observed here for H:GaAs(1 0 0)-ð2  6Þ. Moreover the broad band centred at 1460 cm1 (181 meV) has two shoulders at 1600 cm1 (198.4 meV) and 1325 cm1 (164.3 meV) attributed to two distinct species. These results allow, stemming from an extrapolation of the vibrational data for the gallanes, to correlate the 1600 cm1, 1460 cm1 and 1325 cm1 peaks with bond angles obtaining the values of 1158, 1058 and 1008, respectively. Moreover by assuming a constant Ga–H bond length in the Ga–H–Ga hydride these angles yield Ga–Ga distances of 2:85  0:1, ˚ , respectively, suggesting that Ga dimer bond length vary significantly on 2:70  0:1 and 2:60  0:1 A the ð2  6Þ surface.

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Fig. 38. HREELS of clean (lower trace) and hydrogen (deuterium) exposed (upper two traces) for InP(1 0 0). Exposures are given in Langmuir of molecular hydrogen ion presence of a hot filament [73].

4.2. InP(1 0 0) surface Fig. 38 shows the HREELS of a hydrogen and deuterium exposed surface reported in [73]. The surface showed a ð4  1Þ-458 LEED pattern. The 1694 cm1 (210 meV), 2036 cm1 (252.4 meV) and 2300 cm1 (285.2 meV) features were assigned the first to In–H and the other two to P–H stretches, respectively. The 1195 cm1 (148.2 meV) and 1600–1650 cm1 (198.4–204.5 meV) bands are the corresponding stretches with deuterium. Shown in Fig. 39 are the HREELS data reported in [92] about the ð4  2Þ In-rich surface. The two In–H and P–H stretches are visible. Their behaviour with temperature gives structural and morphological information as reported in Section 3. Fig. 40 shows the internal multiple reflectance spectra as reported by Li et al. [50]. Clean and hydrogenated surface models of the InP(1 0 0) surface were derived from these data. They are reported in Section 3. The s (electric field parallel to the surface) light incidence probes dipole moments parallel to the [ 1 1 0] crystal axis while the p (electric field normal to the surface) light incidence probes dipole moments with component along the surface [1 0 0] surface normal. The overall observed frequencies can be grouped into three main bands occurring between 2350 cm1 and 2200 cm1 (291.4–272.8 eV), 1750 cm1 and 1600 cm1 (217–198.4 eV), and 1600 and 1000 cm1 (198.4–124 eV), respectively. By comparison with the stretching vibrations of known gas-phase molecules (see also Table 3) the high frequency bands were assigned to phosphorous–hydrogen bonds while the bands in the medium and low frequency ranges to indium hydrides. Intensity changes (absolute and relative) of the features are due to relative direction between P–H and In–H bonds. The absence of the In–H frequencies for the ð2  1Þ reconstructed surface for s incidence and their negligible contribution for p incidence light is in agreement with the absence of In–H bonds along the ½1 1 0 direction. The comparison with the results obtained for cluster model calculation allows to assign the three P–H stretches to phosphorous di-hydrogen bonds and to coupled phosphorous mono-hydrogen bonds. The two low frequency bands at 1350 cm1 (167.4 eV) and 1150 cm1 (142.6 eV) were assigned to the asymmetric stretch of bridging indium hydride. In the former case the stretching mode corresponds to an isolated In–H–In structure while, in the latter case, two bridge bonds are coupled together to form an In–H–In–H–In structure.

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Fig. 39. HREELS of hydrogen exposed InP(1 0 0)-4  2. Left: spectra as a function of temperature after 100 L of hydrogen. Right: idem as left but after 1000 L. The 212 meV (1710 cm1) and 285 meV (2299 cm1) losses were assigned to In–H and P–H vibrations. Exposures are given in Langmuir of molecular hydrogen ion presence of a hot filament [92].

4.3. GaP(1 0 0) surface Dubois and Schwartz [73] reported that the studies of vibrational properties of H and D adsorption on this surface was precluded by the relatively low elastic peak intensity in conjunction with a further reduction in signal by a factor from 8 to 10 following hydrogen chemisorption. 4.4. GaAs(1 1 0) surface On the GaAs(1 1 0) surface hydrogen sticks on both Ga and As sites. The available data come from HREELS and some theoretical calculations and deal with stretching vibration and bending vibration of the Ga–H and As–H bonds. Theoretical works include the calculation of surface phonon dispersion and of the bending vibration frequencies of the Ga–H and As–H bonds. Fritsch et al. [56] calculated the wave vector dispersion of the phonon energies range for the hydrogenated GaAs(1 1 0) substrate. The results are shown in Fig. 41. The surface-localised phonon modes of the substrate are indicated by solid lines in the part (a) of the figure, and in part (b) the surface

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Fig. 40. Multiple internal IR reflectivity of InP(1 0 0); s incidence of light (upper) and p incidence of light (lower) reflectance spectra of adsorbed H on the ð2  1Þ, s-(2  4) and d-(2  4) InP(1 0 0) reconstructed surfaces [50].

Fig. 41. (a) Phonon dispersion of the clean GaAs(1 1 0) surface; (b) theoretical phonon dispersion of the GaAs(1 1 0) surface covered by 1 ML of hydrogen. The surface projected bulk bands are represented by the shaded area. Surface-localised phonon modes of the substrate are indicated by solid lines. The bond stretching and bond bending modes of the hydrogen coverage are not represented. The irreducible wedge of the surface Brillouin zone is shown in the inset [56].

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phonon dispersion of the clean GaAs(1 1 0) surface is shown for comparison. In the hydrogenated surface the branch of the surface optical phonon is shifted downwards into the bulk optical bulk bands. It mixes strongly with bulk states and can not be identified as a surface state. The bond stretching and bond bending modes of the hydrogen coverage are not represented. By density functional theory four bending modes of Ga–H and As–H modes were calculated obtaining frequencies values between 500 cm1 and 517 cm1 (62 meV and 64.1 meV) for Ga–H and between 523 cm1 and 554 cm1 (64.9 meV and 68.7 meV) for As–H bending modes, respectively. In addition by molecular dynamics the same modes were obtained at 470 cm1 and 540 cm1 (58.3 meV and 67 meV) [93]. The stretching vibrations were studied experimentally by the Lu¨ th’s and Mo¨ nch’s groups by HREELS. The HREELS data of the (1 1 0) cleaved surfaces obtained by Lu¨ th and Matz [12] are shown in Fig. 42. Apart from Fuchs–Kliever phonon and its overtones the two losses at 1890 cm1 (234.3 meV) and 2150 cm1 (266.6 meV) are associated to Ga–H stretching vibration and to As–H stretching vibration, respectively. The 1380 cm1 (171.1 meV) and 1660 cm1 (205.9 meV) are the corresponding modes with deuterium. Similar values were obtained by Dubois and Schwartz [73] on an IBA cleaned surface. Any loss in the region of H2 stretching, at ’4400 cm1 (545.5 meV) was found. By inserting in the electron energy loss cross-section calculated in [94] Lu¨ th and Matz derived the absolute value of the effective dynamic ionic charge e of the adsorbed H atoms obtaining eGaH ¼ 0:042e and eAsH ¼ 0:038e, where e is the electron charge. The integrated intensities of the two H losses of Fig. 42 are shown in Fig. 134 (Section 6) as a function of the annealing temperature.

Fig. 42. Electron energy loss spectra measured on cleaved GaAs(1 1 0) after adsorption of hydrogen (full line) and deuterium (dashed line). The two losses at 1890 cm1 (234.3 meV) and 2150 cm1 (266.6 meV) are associated to Ga–H stretching vibration and As–H stretching vibration, respectively. The 1380 cm1 (171.1 meV) and 1660 cm1 (205.9 meV) are the corresponding modes with deuterium. The exposure was 1000 L of molecular hydrogen in presence of a hot filament [12].

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Fig. 43. Morse potential Epðr  r0 Þ for hydrogen adsorbed on As and Ga sites. ED is the dissociation energy. The first two Hvibrational levels E0 and E1 and the experimental transitions (arrows) are also indicated. The potentials are referred to the same equilibrium position r0 [12].

The ratios pffiffiffi of the As–H with As–D stretching energies and Ga–H with Ga–D stretching energies deviate from 2 demonstrating the presence of anharmonic terms in the force potential. By using a Morse potential, as reported in Fig. 43 the authors calculated the anharmonicity and the dissociation energy at both As and Ga sites. They obtained for the anharmonicities 105 cm1 (84.7 meV) and 338 cm1 (27.3 meV) for the Ga–H and As–H bond, respectively. While for the dissociation energies the value of 1:3  0:3 eV and 0:73  0:1 eV for the Ga and As site, respectively, was obtained. The bonding energies result lower than those of the corresponding free molecules (’2.5 eV) though the estimated values derived from the experiment are similar to those of other hydrogen adsorption systems. ˚ longer than the Moreover from the repulsive part of the potential the As–H bond length results 0.15 A Ga–H bond according to the difference in covalent radii. Finally the difference in dissociation energy between H–Ga and H–As bonds is contrasted with the similar desorption temperature from Ga and As sites as shown by the results of Fig. 134, as discussed in Section 6. However the desorption process, as shown by the results of Mokwa et al. [95] discussed in Section 6, can be more involved. In fact hydrogen is desorbed as a molecule implying a surface diffusion and subsequent recombination reaction before desorption. Mo¨ nch and co-workers [96], also by HREELS, investigated the vibrational properties of hydrogen exposed GaAs(1 1 0) surfaces. Thanks to a higher resolution (about 3 meV) they investigated both the stretching and bending vibrational ranges of Ga–H and As–H bonds. The experimental spectra are shown in Fig. 44(a) and (b). The values of the stretching vibrations, indicated by nGa–H and nAs–H are shown as a function of the exposure. In Fig. 44(a) the effect of exposure to deuterium is also reported. The energy values are very close to those obtained by Lu¨ th and co-workers though the higher resolution shows that the H–Ga vibrational band, nGa–H in Fig. 44(b), reveals distinct variations of the loss lineshape with the exposure. Any loss line-shape variation of the As–H, nAs–H in the figure was instead observed. As shown in Fig. 45 the line can be deconvoluted into two main components A and B, respectively. Because the As–H frequency did not vary the authors suggested that Ga–H dipole–dipole interaction could be responsible of frequency shift. The absence of similar effect on the As–H mode can

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Fig. 44. (a) HREELS recorded with H-covered GaAs(1 1 0) surfaces after two exposures in Langmuir of molecular hydrogen in presence of a hot filament; (b) HREELS of hydrogen covered GaAs(1 1 0) surfaces at different exposures in Langmuir of molecular hydrogen in presence of a hot filament. FK indicates the Fuchs–Kliever phonon loss and its replica. The labels n and d indicate stretching and bending vibrations, respectively. Loss structures X are attributed to scissor vibrations of dihydride groups [96].

be due to reduced dipole moment of the As–H bond because of the almost vanishing electronegativity offfiffiffi the hydrogen exposed surface, occurs at difference between As and H. The dH loss in Fig. 44 p 515 cm1 (63.9 meV) and it scales approximately as the 2 passing to the deuterium exposed one. Its energy is close to the energy of the calculated bending vibrations of the As–H and Ga–H bonds, see above, and was assigned to this kind of mode (experimental resolution did not allow to distinguish between As–H and Ga–H modes). The dH 0 loss was assigned to a combination loss of dH and a Fuchs–Kliever phonon. The XH loss has an energy close to the energy of the scissor vibrations observed in molecules possessing a As–H2 group whose scissor modes occur at 750 cm1 (93 meV) and 529.8 cm1 (65.7 meV) and was assigned to this mode. This assignment is also supported by the results shown in Fig. 44. In fact the XH and dH features appear in different exposure ranges. The dH ones are already clearly detected after an exposure of only 180 L while XH is not observed until doses are larger than 500 L. Finally on the basis of these results the authors proposed a picture of the surface morphology consisting of ordered islands of chemisorbed hydrogen with the terraces and edges characterised by different Ga–H stretching frequencies occurring at 1860 cm1 (230.6 meV) and 1830 cm1 (226.9 meV), respectively.

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Fig. 45. Evolution of the energy loss features of the Ga–H (nGa–H) and As–H (nAs–H) stretching modes for the GaAs(1 1 0) surface as a function of the exposure in Langmuir of molecular hydrogen in presence of a hot filament for the GaAs(1 1 0) surface [96].

4.5. InP(1 1 0) surface Fritsch et al. [56] calculated the wave vector dispersion of the phonon energies of the hydrogenated InP(1 1 0) substrate. The results are shown in Fig. 46. The surface-localised phonon modes of the substrate are indicated by solid lines. The bond stretching and bond bending modes of the hydrogen coverage are not represented. Schaefer et al. [74] measured by HREEL the stretching energies of the In–H and P–H bonds for a cleaved and IBA prepared InP(1 1 0) surfaces. The two losses at 1710 cm1 (212 meV) and 2928 cm1 (363 meV) losses are associated to In–H and P–H stretches, respectively (Fig. 47). The hydrogen uptake at the In site (see also Section 3) is dominant on the cleaved surface while on the IBA cleaned surface the situation is reversed though with different ratio. 4.6. GaP(1 1 0) surface Chen et al. [80] measured by HREELS the energies and the intensities of the stretching modes of the Ga–H and Ga–P bonds for the cleaved and sputtered surface of GaP(1 1 0). The spectra shown in Fig. 48(a) and (b) are those of cleaved and sputtered surface of GaP(1 1 0) [80]. The 1872 cm1 (233 meV) and 2299 cm1 (285 meV) losses associated to Ga–H and P–H, respectively were observed. The adsorption on the cleaved surface after a light sputtering shows an enhancement of the H–P relative stretch intensity. The GaP results are contrasted to those obtained for GaAs and InP where on the cleaved surface the cation (Ga, In) stretch dominate. The loss at 48 meV is the Fuchs–Kliever phonon. 4.6.1. (1 1 1) and (1¯ 1¯ 1¯ ) surfaces HREELS of GaAs(1 1 1) and GaAs( 1 1 1) surfaces were measured by Hou et al. [24]. They are shown in Fig. 49. According to the different surface stoichiometry the ratio of Ga–H stretch intensity to As–H

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Fig. 46. Phonon dispersion of the InP(1 1 0) surface covered by 1 ML of hydrogen. The surface projected bulk bands are represented by the shaded area. Surface-localised phonon modes of the substrate are indicated by solid lines. The dotted lines indicate two surface-localised gap modes of the clean InP(1 1 0) surface obtained at a cutoff energy of 10 Ry. The diamonds indicate the corresponding phonon modes obtained for the ideal, unrelaxed InP(1 1 0) surface. The irreducible wedge of the surface Brillouin zone is shown in the inset [56].

Fig. 47. Left: HREELS of cleaved InP(1 1 0), taken before and after exposure to hydrogen. The exposure is given in Langmuir of molecular hydrogen in presence of a hot filament. The inset (g) shows HREELS after 6000 L at room temperature and subsequent cooling to 120 K. Right: HREELS of slightly sputtered surface as a function of hydrogen exposure in Langmuir [74].

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Fig. 48. HREELS of GaP(1 1 0) surface as a function of hydrogen exposure in Langmuir of molecular hydrogen in presence of a hot filament: (a) hydrogenation of the cleaved surface; (b) hydrogenation of the cleaved and sputtered surface [80].

stretch intensity depends on the surface orientation. It is in favour of Ga–H stretch on the Ga-rich (1 1 1) surface while in favour of As–H on the (1 1 1) As-rich surface. HREELS of the hydrogenated InP(1 1 1)-(2  2) and InP( 1 1 1)-ð1  1Þ surfaces were measured by Hou et al. [24] and shown in Fig. 50. The losses at 209 meV (1685 cm1) and 280 meV (2258 cm1) represent the excitation of the In–H and P–H stretching, respectively. The loss feature at 42 meV is the Fuchs–Kliever phonon of InP. The In–H stretch is visible on the (1 1 1)-(2  2) In-rich surface, while the opposite occurs for the ( 1 1 1)-ð1  1Þ P rich surface where, according to the surface stoichiometry, only the P–H mode is detected. HREELS of InSb(1 1 1)-(2  2) and InSb(1 1 1)-ð3  3Þ were published by HernandezCalderon [85]. The stretches at 1740 cm1 (216 meV) and 1863 cm1 (231 meV) corresponding to In–H and Sb–H were observed on the (1 1 1)-(2  2) and on the (1 1 1)-ð3  3Þ, respectively.

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Fig. 49. HREELS for hydrogen adsorbed on (a) GaAs(1 1 1) and (b) GaAs( 1 1 1). Features a and b are the Ga–H and As–H stretching vibrations, respectively, b and c their combination with the Fuchs–Kliever phonon. The hydrogen exposure was for both surfaces 10 L of molecular hydrogen in presence of a hot filament [24].

The 1740 cm1 loss is close to the value of 1710 cm1 found by Dubois and Schwartz [73] in their measurements on InP while the 1860 cm1 assigned to Sb–H is very close to the stretching frequencies of SbH3 (1890 cm1). The author attempted an ordering of the force constants, K, of hydrogen chemisorbed on III–V semiconductors by using a simple harmonic model. The result is shown in Fig. 51. It can be observed that hydrogen adsorption on elements of the V group (P, As and Sb) gives force constant very close to those of the corresponding hydride while the force constant for adsorption on group-IV and group-III elements results lower than the relative hydride. This could be interpreted as an increase of the H–substrate atom distance, in relation to the interatomic separation in the hydride compound, for hydrogen adsorbed in substrate atoms with lower electronegativity. The unusual order

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Fig. 50. HREELS for hydrogen adsorbed on (a) InP(1 1 1)-(2  2) and (b) InP( 1 1 1)-ð1  1Þ. Features a and b are the In–H and P–H stretching vibrations, respectively, b, c and d their combination with the Fuchs–Kliever phonon. The hydrogen exposure was for both surfaces 100 L of molecular hydrogen in presence of a hot filament [24].

Fig. 51. Force constant in harmonic approximation of H adsorbed on Si and III–V compounds. Empty circles correspond to IV–H4 and V–H3 hydride molecules, full square H–InSb [85].

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GaH > AlH > InH of the force constants predicted by the simple model has been observed in molecular species containing those elements [97].

5. Electronic properties The data on the electronic states of the hydrogenated III–V semiconductor surfaces regard the surface valence band and conduction band states and together with anion and cation surface core states. Valence and conduction states are formed by the interaction of the 3s and 3p states of the third group cation and fifth group anion starting from the 3s13p3 and 3s13p5 atomic configuration, respectively, in interaction with the 1s orbital of hydrogen. Available data on core level deal with shallow core states including the M4,5 (3d) levels of Ga, As and In and Sb N4,5 (4d) levels. Valence electronic states of hydrogenated semiconductor surfaces were studied experimentally mainly by electron spectroscopies, including UPS (both in AI and AR mode), EELS and AES. In addition optical spectroscopies like differential reflection spectroscopies (DRS) and reflection anisotropy spectroscopy (RAS) were used. Some contact potential data are also available. The whole of available data gives access to the electronic density of states, energy k-dispersion of electronic bands, binding energies and their shift with hydrogenation, Fermi level position at surface and band bending. From the theoretical side, charge densities and electronic bands together with density of states are available. Methods of calculations include tight binding, self-consistent pseudopotential and density functional theory. Effective, even if indirect, insights into the surface atomic geometry were also obtained by comparing band structure calculations performed at different surface geometry with the experiment. For core level states the study stemmed from photoemission and Auger concentrating on line shape analysis together with subcomponents binding energy determination and cation to anion emission ratio, respectively. A summary and a schematic description of the theoretical and experimental techniques are given in Section 1. Following the organisation of previous sections the results are arranged per different surfaces, (1 0 0), (1 1 0) and (1 1 1) in the order, with the additional inner distinction between valence and core level states. 5.1. (1 0 0) surface 5.1.1. GaAs(1 0 0) surface—valence states Bringans and Bacharach [32] studied by AR-UPS the modifications introduced by hydrogen exposure at saturation on the GaAs(1 0 0)-1  6 and GaAs(1 0 0)-cð4  4Þ, Ga- and As-rich surfaces, respectively. Photons in the 16–26 eV range were used. The final spectra were indistinguishable within experimental uncertainty. Both surfaces showed a 1  1 reconstruction demonstrating at high exposure the formation of a surface compound independent on the initial reconstruction. The same authors from core level analysis—see below—concluded in favour of an As-rich surface with hydrogen bond to As. It is worth mentioning that other authors arrived—see below—to different conclusions

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Fig. 52. AR photoemission spectra for GaAs(1 0 0)-1  6 and for the same surface after exposure to 106 L of molecular hydrogen in presence of a hot filament. The spectra were taken at the K point of the surface Brillouin zone (emission angle of 37.58, photon energy of 19 eV) for (a) 458 of incidence angle and (b) 08 of incidence angle [32].

reporting instead the evidence of chemisorption on both cation and anion with As depletion at high hydrogen exposure. Experimental results obtained by Bringans and Bacharach are summarised in Figs. 52–54. Spectra of Fig. 52 were taken for a mixed sþp (electric field normal and electric field parallel to the plane of incidence) of light and pure s (electric field normal to the plane of incidence) incidence condition. The spectra, which correspond to the K point (corner) of the surface Brillouin zone, show with hydrogenation a new feature at about 5 eV below the valence band and elsewhere undergo intensity changes. The fact that the hydrogen induced peak near 5 eV of binding energy is much weaker for the s light demonstrates that it represents a bond that has a large component directed normal to the surface. Band dispersions are reported in Fig. 53 and compared with the positions calculated for transitions from purely bulk states. The calculations assumed direct transitions from initial states to a free electron final state. The inner potential was 7.5 eV. The initial states were calculated along the nonsymmetry lines using the local pseudopotential approach. Apart from the non-dispersing peak at about 6.7 eV, which is seen on all GaAs surfaces and assigned to a maximum of volume states, the features in

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 Fig. 53. Valence band photoemission peak dispersion along the GK direction of the surface Brillouin zone (see the inset) 6 for hydrogenated GaAs(1 0 0) after an exposure of 10 L of molecular hydrogen in presence of a hot filament (squares) and compared calculated for bulk derived features (continuous lines) [32].

the spectra were assigned either to direct bulk transitions or to surface states. The hydrogen induced state observed at about 5 eV—Fig. 52—is present at the G point at 5.1 eV to disperse upwards to a maximum at 4.3 eV of binding energy and down to 4.9 eV at K. The other feature at 1.6 eV at K was assigned to a surface state on the base of the absence of a corresponding state in the volume band and the occurrence of a gap at this energy in the projected bulk band structure. The dispersions of the surface related peaks are summarised in Fig. 54. The clean 1  6 and cð4  4Þ surfaces are reported for comparison. As well as the strong surface state near the top of the valence band for the clean surfaces, a surface related state is seen near the edges of the zone in the region 2.7–4 eV. A synchrotron radiation photoemission experiment was carried out by Larsen and Pollmann [39] on the As terminated GaAs(1 0 0)-2  4 surface. Two different kinds of procedures for hydrogen exposures, namely plasma etching and dissociation by hot filament, were used. In analogy with previous results of Bringans and Bacharach a 1  1 surface structure was observed after hydrogen exposure while the Ga to As core level intensity ratio (see also Section 3) versus hydrogen exposure demonstrated the occurrence of As depletion.

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 and GK  Fig. 54. Comparison of valence band AR photoemission surface peaks dispersion along the J0 G directions of the surface Brillouin zone for the clean GaAs(1 0 0)-1  6 and GaAs(1 0 0)-cð4  4Þ and the hydrogenated surface. The spectra were all taken at a photon energy of 22 eV [32].

Valence band spectra taken at normal emission (lower panel) and at a polar angle of 128 with 29 eV of photon energy are shown in Fig. 55. The hydrogen chemisorption has a dramatic effect on the GaAs(0 0 1) valence band spectra. The clean surface most pronounced spectral features, due to surface state emission (S2), together with the emission from bulk valence states (X3) [39] are mostly lost, while a new feature appears at Ei  7:7 eV in the heteropolar gap between the energies of X3 (6.7 eV) and X1 (10.7 eV) critical points. The spectra obtained by molecular hydrogen in presence of a hot filament are comparable with the spectra obtained at increasing plasma exposure but the clean surface feature are still present though depressed in intensity. The emission from the heteropolar gap is considerably high. The spectra for the 106 L (molecular hydrogen in presence of a hot filament) and the plasma-exposed surface (b) both display less GaAs valence band structure than the plasma-exposed surface (a) for which the hydrogen induced gap state is more prominent but the clean surface features are still present though depressed in intensity. The emission from the heteropolar gap is considerably high. The surface disorder induced by hydrogen was supposed to be at the basis of this effect. The presence of surface disorder induced by hydrogen was already shown for this surface by LEED (see Section 3) and by Richter et al. by RAS [99]. Fig. 56 shows a number of valence emission spectra (normal emission) in the 21–50 eV photon energy range. A hydrogen induced feature is present at about 8 eV. Its non-dispersive character confirms its surface state character while the intensity emission with photon energy indicates a decrease of the emission cross section by lowering the photon energy. This last point can be an explanation of the

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Fig. 55. Photoemission spectra of valence band taken at normal emission (lower panel) and at a polar angle of 128 (upper panel) for the GaAs(1 0 0)-2  4 reconstructed surface before and after a 106 L exposure to molecular hydrogen in presence of a hot filament. The dotted line curves are spectra for the 2  4 surface after two different hydrogen plasma exposures. Light polarisation was mixed s–p, angular resolution 28 and energy resolution 0.2–0.3 eV [39].

non-observation of this hydrogen induced peak by Bringans and Bacharach [32] who used a 19 eV photon. The dispersion of the bulk transition peak is shown by the dotted line in Fig. 56. By using molecular orbital considerations the authors concluded that the state localised in the heteropolar gap can be ascribed to both chemisorption on Ga or As surface atoms. To this end they used the scheme reported in Fig. 57. Stemming from calculations for hydrogen chemisorption on Si and Ge and from hydrogenated amorphous silicon they assumed that the dangling bond energy, EDB, which is close to EVBM, is several eV’s higher than the H 1s level of energy EH (the on site energy). Due to the intercation term, VDB;H , these two levels are shifted into new levels E as indicated in Fig. 57. In a simple two level picture these energies are given by qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 1 E ¼ 12 ðEDB  EH Þ  4 ðEDB  EH Þ þ VDB;H : To obtain a quantitative estimate of the binding energies involved the authors proceeded as follows. For the case of an As dangling bond the interaction term can be estimated by using a tight binding method

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Fig. 56. Photoemission spectra of valence band taken at normal emission as a function of the photon energy for the GaAs(1 0 0)-2  4 reconstructed surface after 106 L of exposure to molecular hydrogen in presence of a hot filament [39].

and by fitting the tight binding molecular calculations for AsH3 to the corresponding self-consistent 2 was found to be appropriate while the Hartree–Fock molecular results. The 3.5 eV value for VDB;H 1 eV value for the As dangling bond was assumed. Moreover being difficult to estimate the on site energy of hydrogen, it can be estimated by the above equation inserting the value 7.7 eV for E. In this way the value EH ¼ 5:9 eV was found which corresponds to a screening induced upward shift of 2.3 eV. A similar interaction was supposed for the case of hydrogen bonded to Ga dangling bond with an energy of the empty Ga dangling bond of 2–3 eV. In this way EH  6:3 eV was found. Since one would expect less screening of the hydrogen for the case of bonding to an empty Ga dangling bond than to a filled As dangling bond, the authors concluded that the observed hydrogen induced gap state can be ascribed to bonding of hydrogen to both As and Ga dangling bonds. Carette et al. [100] investigated the hydrogen chemisorption on (2  4)-As stabilised GaAs(0 0 1) surfaces exposed to an H2 plasma by photoemission and by RHEED. The experimental results were discussed on the basis of self-consistent tight binding calculations. The experimental and theoretical results are reported in Figs. 58–60, respectively. The coverages were evaluated assuming a 1 to 1 ratio between the ion density and the dissociated H density. In this way the short plasma exposure (plasma 2 of Fig. 56) corresponds to 0.04 ML and the

Fig. 57. Two level model indicating the bonding of a hydrogen atom to a surface dangling bond (left hand side) and the corresponding energy level diagram (right hand side) [39].

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Fig. 58. Photoemission spectra of valence bands at 29 eV of photon energy, of GaAs(0 0 1) clean surface and exposed to a  point and (b) at the K point of the surface Brillouin zone short (plasma 2) and long (plasma 4 and 5) H2 plasma (a) at the G [100].

long exposure (plasma 4 and 5 of Fig. 56) to 0.5 ML. The RHEED diffraction patterns showed a gradual change towards a 1  1 reconstruction, even though a slight reminiscence of the fourfold periodicity was still present for all the exposures.  (a) resemble very closely those shown in Fig. 55. A well defined The experimental spectra at G feature, Ha is present in the heteropolar gap at about 7.7 eV while the As dangling bond state together with the bulk states near the top of the valence band are greatly reduces after the exposures. Another level, Hb is visible at the K point at about 5 eV. This state was put into relation with the state observed by Bringans and Bacharach [32] (Fig. 52) at the K point. All the features result smoothed at higher exposures. At variance with the results of Bringans and Bacharach from the analysis of core level emission the authors found a depletion of As at the surface with a Ga to As surface density ratio ranging from 1 to 1.3. In the calculation of the local density of states the crystalline structure was considered an ideal free (1 0 0) surface with two dangling bonds per atom saturated, respectively, by two hydrogen atoms to form a dihydride surface. Figs. 59 and 60 show the results for 1 ML of H on the As and Ga terminated surfaces, respectively. The comparison with the experimental results allows to assign the state Ha and Hb to the Ga–H and to As–H bonds, respectively. More generally from these results it was concluded that the hydrogen interaction with the GaAs(1 0 0)-(2  4) As terminated surface breaks the As–As dimers and leads to the formation of two Ga–H or As–H bonds per Ga and As surface atom, respectively.

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Fig. 59. Theoretical calculation of the electron density of states for hydrogen on an GaAs(0 0 1) As terminated surface: (a) local density of states on the H atoms; (b) local density of states on the As surface atoms [100].

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Fig. 60. Theoretical calculation of the electron density of states for hydrogen on an GaAs(0 0 1) Ga terminated surface local density of states on the (a) H atoms and (b) Ga surface atoms [100].

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Information on the effect of hydrogen on the electronic properties of cð4  4Þ, (2  4) and ð4  2Þ GaAs(1 0 0) surfaces were obtained by Richter and co-workers [27,41–43] by RAS and spectroscopic ellipsometry [41] in correlation with the use of AES, HREELS, LEED and STM. Surfaces were prepared by As decapping and subsequent annealing (cð4  4Þ Tannealing ¼ 350 8C, (2  4) Tannealing ¼ 420450 8C, ð4  2Þ Tannealing ¼ 540550 8C). The structural results derived from these experiments are reported in Section 3; here the focus is on the results related with the electronic properties. RAS experiments consists in measuring the optical reflectivity with polarised light with the electric light vector directed along two different surface directions and taking the relative difference of reflectivity along interesting and physically different directions. On GaAs(1 0 0) the two directions ½0  1 1 and ½0 1 1 correspond to the directions of As–As or Ga–Ga dimers. Light absorption and reflectivity are higher (lower) when the electric field is parallel (normal) to the dimer bonds. This fact makes the RAS spectra sensitive to changes of surface reconstruction or dimerisation. The local character explains the different sensitivity to structural changes with respect to LEED, sensitive to long range order. The summary of the RAS data for the hydrogenation of the cð4  4Þ, (2  4) and ð4  2Þ surfaces are reported in Fig. 61(a). The As dimers contribute to the signal at 2.6 and 2.9 eV. The sign changes depend on the relative direction between the light electric field and dimer bond. Consequently (also refer to Fig. 9) the signal is positive in the (2  4) and negative in the cð4  4Þ reconstruction. Similarly the occurrence of the 2 eV and 2.2 eV structures is related to the Ga dimers on the Ga-rich ð4  2Þ reconstruction. Quantitative analysis of RAS data is carried on by using a number of functions built through the surface layer thickness d and the substrate and surface dielectric functions. These functions including ‘‘surface excess function’’ and ‘‘pseudo dielectric function’’ are built in order to single out the properties, anisotropy included, of the surface layer dielectric function. For further details see the relative references. Fig. 61(b) shows the imaginary part of the surface dielectric function anisotropy (SDA) for the ð4  2Þ, ð2  4Þ and cð4  4Þ surfaces are derived from the data of Fig. 61(a). The two different sets of data (related, respectively, to real and imaginary parts of the surface dielectric function) present features at the same energy position and with the same sign (positive maxima and negative minima) as expected from the real and imaginary part of the response function. The respectively observed LEED patterns are also indicated (for further details the reader is addressed to Section 3). Spectroscopic ellipsometry which monitors the complex dielectric response of the sample was used in conjunction with RAS and LEED to investigate the hydrogen effect on the GaAs(1 0 0)-cð4  4Þ surface [41]. In Fig. 62 the measured pseudodielectric functions of the arsenic-capped, cð4  4Þ clean GaAs(1 0 0) surface, after exposure to 104 and 105 L hydrogen, are shown. Hydrogen exposure increases the imaginary part of the pseudodielectric function in the low energy region and reduces its amplitude in the 4.4–4.9 eV region. The surface excess function for the GaAs(1 0 0)-ð4  2Þ is reported in Fig. 63 as a function of hydrogen exposure for the [0 1 1] and [1  1 1] directions, parallel and normal to As-As dimers. For low hydrogen exposure (up to 400 L of molecular hydrogen in presence of a hot filament) the surface excess function shows beside an increase at the bulk critical points (2.91 eV, 3.14 eV, 4.44 eV and 4.96 eV) two minima increasing with hydrogenation, between 2.0 eV and 2.8 eV for the [0 1 1] and between 3.5 eV and 4.4 eV for the ½1  1 1 direction, respectively, are assigned to the disappearance of the

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Fig. 61. (a) Real part of the RA Reðr½0 1 1  r½0 1 1 Þ=R at different atomic hydrogen exposure for (left to right) cð4  4Þ, (2  4) and ð4  2Þ GaAs(1 0 0) surfaces. The corresponding periodicity of the LEED pattern is given [99]. (b) Imaginary part of the SDA for the same reconstruction of part (a), exposure are given in Langmuir of molecular hydrogen in presence of a hot filament [43].

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Fig. 62. Experimental imaginary part of the pseudodielectric functions of the arsenic-capped, clean GaAs-cð4  4Þ surface, after exposure to 104 and 105 L of molecular hydrogen in presence of a hot filament [27].

electronic transitions of the surface As dimer states. Finally the final spectral line-shape at high exposures was assigned to a hydrogen etched phase characterised by a rough surface layer with voids and GaAs islands revealing the onset of an inhomogeneous surface etching at high exposures. The RAS taken on GaAs(1 0 0)-(2  4) is shown in Fig. 64. The positive feature at 2.9 eV was attributed to electronic transition between occupied As dimer states and unoccupied lone pair states in agreement with the orientation of dimers along the ½1 1 1 . The disappearance of this feature between 0 and 400 L and the corresponding evolution of LEED pattern from the initial (2  4) to the ð1  1Þ through the ð1  4Þ was attributed to the breaking of surface As dimers and the removal of As from the surface. The residual peak at 2.9 eV is due to bulk contribution. HREELS [23] support this interpretation showing, see Section 4, an increase of the As–H vibration up to 1000 L. For high exposures the anisotropy in RAS spectra increases again at bulk critical points. This was attributed to surface roughening. AES measurements and the above cited HREELS data show, in the same exposure range, an increase of the Ga to As signal and of the Ga–H to As–H loss ratio, respectively, further supporting this picture.

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Fig. 63. Imaginary part of the surface excess function, determined from the difference between the pseudodielectric function of clean GaAs(1 0 0)-cð4  4Þ surface and that measured after various hydrogen exposures (given in Langmuir of molecular hydrogen in the presence of a hot filament), as indicated at the spectra [27].

The RAS taken on GaAs(1 0 0)-ð4  2Þ is shown in Fig. 65. The spectrum is dominated by a maximum at 3.3 eV and a minimum at 2.3 eV attributed to electronic transitions between occupied Ga dimer states and unoccupied lone pair states. The feature at 3.3 eV was attributed to absorption due to As dimers, in analogy with (2  4) reconstruction. The presence of As dimers on this reconstruction was explained by As dimers rows at surface steps, as found by STM [46], and supported by the disappearance of this feature with exposure faster than the Ga dimer disappearing at 400 L. Signs reflect the Ga and As dimer directions with respect to the light electric field. The joint evolution of RAS and LEED, in strict analogy with the case of (2  4) and cð4  4Þ reconstructions reported above can be explained by the breaking of dimers and subsequent H–Ga saturated surface formation. For exposures higher than 300 L the LEED pattern changes to ð1  1Þ and the corresponding RAS shows features at the bulk critical points. This was taken as an evidence of surface roughening induced by surface etching

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Fig. 64. Reflectance anisotropy for n ¼ 1  1016 cm3 doped n type GaAs(1 0 0)-(2  4) exposed to various amounts of hydrogen given in Langmuir of molecular hydrogen in presence of a hot filament. The observed LEED pattern is given for each hydrogen exposure [99].

Fig. 65. Reflectance anisotropy for n ¼ 1  1016 cm3 doped n type GaAs(1 0 0)-ð4  2Þ exposed to various amounts of hydrogen. The observed LEED pattern is given for each hydrogen exposure. The exposure is given in Langmuir of molecular hydrogen in presence of a hot filament [99].

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of the surface. The combination of this result with the results obtained by AES leads to the picture of a final surface rich in Ga saturated with hydrogen through Ga–H bonds. Finally the RAS spectra of the doped and undoped samples develop in a very similar fashion apart from a structure in the 2.9–3.1 eV range more pronounced for the doped sample. This effect is due to the variation of space charge region electric field due to hydrogen induced band bending changes. Landesman et al. [6] by following the binding energy evolution of Ga 3d, As 3d, Ga 2p and As 2p core levels obtained information on band bending evolution of UHV de-oxidised GaAs(1 0 0) wafers. For p type samples (Cd, 35  1018 cm3) they found almost no displacement of Fermi level during de-oxidation from its position at 0.4–0.5 eV above valence band maximum. On the opposite the 3d core level for the n type samples (Se, 6  1017 cm3) displays an increase in kinetic energy of 200 meV interpreted as a corresponding lowering of the surface Fermi level position. The band bending increased during de-oxidation. An exposure to pure H2 produced almost no additional change for n type samples, while an increase of 150–200 meV was observed on kinetic energy for 3d core levels. This increase was ascribed to the Fermi level going towards the valence band top due to band bending lowering. Finally there are a number of information from infrared spectroscopy about bonding properties of H with Ga and As on the GaAs(1 0 0)-cð2  8Þ and (1  6) [23,44,91]. The aspects related to geometry and vibrational properties are presented in Sections 3 and 4, respectively. In fact the models of the hydrogenated surfaces constructed from the infrared data, imply the presence of H bonded to As and Ga through arsenic dihydrides, arsenic coupled monohydrides, monohydrides (Ga2)AsH in the top layer and monohydrides in the second layer (Ga3)AsH (see also Fig. 14 of Section 3 and related discussion). In particular hydrogen bond with As in a dihydride configuration (2140 cm1) is independent on reconstruction and has been associated to the bond at steps and vacancies. H on As dimers (1950– 2070 cm1) produce isolated or coupled monohydrides interacting through an H bond as shown by the band broadening and consistent with the polarisation dependence [44]. 5.1.2. GaAs(1 0 0) surface—core states Bacharach and Bringans [32,33] studied the photoemission from Ga 3d and As 3d core levels of the GaAs(1 0 0) surface correlating it with the atomic geometry observed by LEED (see Section 3) and the electronic properties of valence band (see above). Core level line shape and binding energy modifications of the Ga 3d and As 3d emission of the clean GaAs(1 0 0)-2  6 surface are shown in Fig. 66. Data treatment is reported in the figure caption. Beside the lower binding energy shifts of Ga and As hydrogenated core levels a remarkable line shape changes are observable. The As line shape is the most affected with an extra spectral weight appearing on the high-binding energy side. 5.1.3. InP(1 0 0) surface—valence states The hydrogen interaction with the InP(1 0 0)-4  2 In-rich surface has been studied by Woll et al. [101]. The evolution of the electronic states changes induced by thermally activated hydrogen is shown in Fig. 67. In the low exposure regime the peak at 1 eV, which is attributed to a P dangling bond is decreased drastically in intensity since the first hydrogen exposure of 100 L of molecular hydrogen in presence of a hot filament up to an exposure of 1000 L. With an exposure of 104 L this feature disappears and a distinct Fermi edge is observable. The peaks at 2 eV and 3.8 eV identified as bulk band emission in the clean spectrum and the one at 6.8 eV as a density state transition show a shift of 0.2 eV towards higher

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Fig. 66. Ga and As core level photoemission spectra of the GaAs(1 0 0)-4  6 surface measured with a photon energy of 130 eV for the clean and hydrogenated surface after an exposure of 106 L of molecular hydrogen in presence of a hot filament. All spectra have been normalised to the same peak height and the levels of the hydrogen covered surface have been shifted to lower binding energies by 0.12 and 0.03 eV for Ga and As 3d, respectively [33].

binding energies up to an exposure of 1000 L and for an exposure of 106 L cannot be identified any more. The work function obtained from the onset of secondary electron emission as a function of the hydrogen exposure, shows two stages: up to a hydrogen exposure of 103 L decreases from 4.23 eV to 4.12 eV and after 104 L there is a sudden increase. With further hydrogen exposure the work function value of the metallic In reference sample is reached.

Fig. 67. Valence band photoemission spectra of the InP(1 0 0)-4  2 surface with the hydrogen exposure as a parameter recorded with He I radiation (21.2 eV). The dashed lines show the spectra enlarged by a factor of 10 in the vicinity of the Fermi level. Energies are referred to the Fermi level, exposures are given in Langmuir of molecular hydrogen in presence of a hot filament [101].

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Fig. 68. Photoemission spectra of the In 4d levels of the InP(1 0 0)-4  2 surface with the hydrogen exposure taken with He II radiation (40.8 eV). Circles are the experimental data, the solid lines the least-squares fit, dash-dotted lines indicate the decomposition into two doublets. The trace below each curve gives the residuum enlarged by the indicated factor. Exposures are given in Langmuir of molecular hydrogen in presence of a hot filament. Binding energies are referred to the Fermi level [101].

5.1.4. InP(1 0 0) surface—core states The hydrogen interaction with the InP(1 0 0)-4  2 In-rich surface has been studied by Woll et al. [101]. Core level spectra of the In 4d level excited by He II radiation are shown in Fig. 68. The In 4d level at the clean surface exhibits a doublet at 17.5/18.4 eV as well as a shoulder at 16.8 eV. Hydrogen exposure up to 103 L of molecular hydrogen in presence of a hot filament causes no dramatic change of the spectral line shape, while after 104 L the shoulder at 16.8 eV is visible as a distinct peak. The intensity of this peak is increasing with H2 dose while those of the doublet at 17.5/18.4 eV is decreasing. In the In 4d spectra the comparison with the metallic In reference sample shows the dominant metallic character of the surface after heavy hydrogen doses, 106 L. The starting of the transition from In in a dimer configuration into that of metallic In is observed after 400 L H2.

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Fig. 69. Photoemission spectra of (a) the In 3d and (b) the P 2p levels for the clean InP(1 0 0)-4  2 surface and the hydrogen treated surface (106 L of molecular hydrogen in presence of a hot filament) [101].

Simultaneously a strong phosphorous depletion can be observed in XPS as shown in Fig. 69 where the In 3d and P 2p lines are displayed for the clean and hydrogen treated surface. The shift of the In 3d line to lower binding energies and the appearance of the bulk plasmon (11.5 eV) of metallic In excited by photoelectrons from the In 3d level still indicate a metallisation of the sample. 5.2. (1 1 0) surfaces 5.2.1. GaAs(1 1 0) surface—valence states The hydrogenated GaAs(1 1 0) surface results to be the most studied hydrogenated III–V surface. The results of H chemisorption on GaAs(1 1 0) are presented starting from the models derived by theoretical calculations. Though this choice does not follow the chronological evolution it simplifies the presentation of data. Manghi et al. calculated [59], by using a self-consistent pseudopotential method the energies of the electronic states at the high symmetry points of the surface Brillouin zone. The results are shown in Fig. 70. The two structures refer to the two sketched geometries reported in the figure. Geometry A corresponds to 1 ML of hydrogen chemisorbed on both cation and anion in the ideal configuration along the dangling bonds directions. In geometry B hydrogen atoms are bonded to both elements in on-top geometry; substrate atoms are in the same configuration of the relaxed surface, given by a rotation–relaxation model [102]. In the calculations the As–H and H–Ga bond lengths ˚ , respectively were taken equal to those measured in the hydride molecules given by 1.52 and 1.59 A [58]. The energy spectrum for 1 ML of hydrogen on GaAs(1 1 0) for the A and B geometries are shown in Fig. 70. Model B features hydrogen induced states from 1 eV above the valence band top to 12 eV below it. All these states are filled states. Some of them have a counterpart in the results for model A, but at different energies, in particular the state H5 occurring at a few eV below the valence band top in model A. Model A, unlike model B, does not give any electronic state in the absolute gap. Experimental data allows to discriminate between model A and B, for the related discussion, see below. Fig. 71 shows the charge density contour plots [59] of the states of the geometry B of Fig. 70 at points X and M. States H1, H2 and H4 arise from hydrogen atoms sitting on top of As sites, while states

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Fig. 70. Energy spectrum for 1 ML of atomic hydrogen on GaAs(1 1 0) for A and B geometries sketched in the inserts (A: 1 ML of hydrogen chemisorbed on both atoms in the ideal configuration along the dangling bonds directions; B: hydrogens bonded to both anion and cation sites in on-top geometry). The states localised on the H atoms and on the Ga and As atoms of the first substrate plane are plotted at the high symmetry points of the two-dimensional Brillouin zone over the projected bulk band structure (dashed region) and are labelled by Hi, Ci and Ai, respectively. Energies are referred to the valence band top [59].

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Fig. 71. Charge density contours of H-induced states for hydrogen on GaAs(1 1 0) according to geometry B of Fig. 70. They are plotted along a ½1 1 0 plane passing either through As or through Ga surface atoms. The atomic sites are indicated according to Fig. 70. The contours are spaced by 0.4 electrons per bulk unit cell [59].

H3 and H5 come from hydrogens on top of Ga sites. Moreover state H1 arises from a combination of the hydrogen orbital with the s-like surface state A2 at 9 eV and the state H2 has a significant contribution from Ga s-like state, C2, localised in the second plane in the clean spectrum. State H4 which is the most pronounced H-induced feature arises from dangling bond arsenic state combined with the hydrogen orbital. Similarly H3 comes from Ga s-like surface state C1 combined with hydrogen orbital in a way similar to A1 in the H1 state. State H5 arises from bonding with Ga dangling bond states. In the clean spectrum this state runs slightly above the bottom of the conduction band. Interaction with hydrogens leads to a lowering of this state by 2–4 eV. No empty state was found in the proximity of the conduction

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band bottom suggesting that any antibonding combination of Ga dangling bonds states with hydrogen is placed at very high energies inside the conduction band. Similar results were found for the same geometry on GaP as reported below. Here it is worth anticipating that for chemisorption in geometry shown in Fig. 124(b) (0.5 ML on anions) except for C2 all other Ga-derived surface states are not significantly modified by hydrogen. This demonstrate that energy spectrum is basically the combination of two different spectra, one due to chemisorption on anions sites and the other one due to chemisorption on cations sites. This is possible because hydrogens on different surface atoms are very far apart and do not interact appreciably. Finally states A4 and A6, being planar states having contribution from planar anion orbitals are not affected by hydrogen chemisorption. Pulci et al. [57] performed a calculation of the electronic properties within the first principles density functional theory in the local density approximation. Results for clean and hydrogenated surfaces are shown in Fig. 72. The cation band C3 is related to the Ga empty dangling bond. The band A5 is instead related to the filled dangling bond of As. After hydrogen deposition the dangling bond are saturated and the corresponding electronic states are pulled apart and shifted away from the gap region. Also the surface bands A2, C2 and A3 disappear after hydrogenation and two new surface bands appear, one in the stomach gap at about 5.0 eV and another about 11 eV below the top of the valence band peak resonances are also visible both in the valence and conduction bands. In agreement with the results of Manghi et al. for geometry A of Fig. 70 and those of Bertoni et al. [55] the gap region becomes rid of surface states and H related states.

Fig. 72. Calculated density functional theory—local density approximation band structure for the GaAs(1 1 0) (a) clean and (b) hydrogenated surface. Large dots represent surface states, small dots bulk states [57].

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Fig. 73. Electron density plot for GaAs(1 1 0) in the plane containing: (a) Ga–H bond for Ga–H bonding configuration at 0.5 monolayer; (b) As–H bond at 0.5 ML; (c) Ga–H at 1 ML; and (d) As–H bond at 1 ML [51].

Wright et al. [51] calculated the equilibrium configuration of atoms, the electronic charge densities and the Fermi level position for 0.5 and 1 ML coverages. They used a self-consistent pseudopotential method with slab geometry, in the framework of a Hellman–Feynmann forces scheme. The results, specifically related with the equilibrium atomic geometries, have been reported in Section 3. In Fig. 73 the contour plots of the total electron densities in planes containing the Ga–H and As–H bonds are shown. The bonding nature is covalent. The results of the calculation of the charge density contour plots are reported in Fig. 73 and in Fig. 74 for the total surface charge and for the state at the Fermi level, respectively. The point of maximum of the electron density for the Ga–H bond is slightly closer to the hydrogen atom than for the As–H bond. This reflects the difference in electronegativities of Ga and As atoms.

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Fig. 74. The charge density for GaAs(1 1 0) of the state at the Fermi level for the: (a) Ga–H bonding configuration at 0.5 ML; (b) As–H bonding configuration at 0.5 ML [51].

Moreover, in analogy with the results of Manghi et al. [59] the electron density plots of both Ga–H and As–H are not significantly affected by the occupation of the neighbour atom. In fact density contour plots of 0.5 and 1 ML do not show significant differences demonstrating a negligible interaction between bonds. The energies of the Ga–H and As–H bonding states are in the range of 4.7 to 2.8 eV with respect to EF with a maximum density at 3.5 eV below it. The overlayer bonding energy resulted to be for the 1 ML case 6.2 eV per unit surface cell or 3.1 eV per hydrogen atom. For the 0.5 ML case resulted 2.7 and 2.6 eV per hydrogen atom for the Ga–H and As–H bond, respectively. These values are of the order of typical bonding energies for As–H and Ga–H bonds in free molecules. Different Fermi level positions were found for 1 and 0.5 ML coverage for Ga–H and As–H bonding, respectively. For 1 ML the Fermi level was found at the valence band maximum while for 0.5 ML was found at 0.16 eV above the valence band maximum for Ga–H bonding and at 1.3 eV for As–H bonding. Bertoni et al. [103] by using a density functional approach with a norm-conserving pseudopotential also obtained information on surface structure and charge density. The charge density contour plots calculated by Bertoni et al. [55,103,104] are shown in Figs. 75 and 76. The relevant results for surface structure modification (relaxation removal with the further slight counter relaxation of 58 of the first plane of the substrate) are reported in Section 3. The calculations also provided vibrational frequencies of the surface reported in Section 4. The overall charge density features a maximum along the bond direction closer to hydrogen in the Ga–H bond case. The individual contribution from different states were identified. In Fig. 76 some of them located at the M point of the surface Brillouin zone

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Fig. 75. Valence electron density in the ð1 1 0Þ plane passing for GaAs(1 1 0) through two different chains connecting the atomic positions. The two panels show chains containing H–As and H–Ga bonds, respectively. Contour plot spacing is three electron/bulk cell [104].

are reported. The electronic states which contribute to the charge piling up along the H–As-bond comes from the As s-like bands (at 10.6 eV with respect to the valence band maximum) and from the stomach gap states (at 4.5 eV) as shown in Fig. 76. The contribution to the charge localised on the H–Ga bond comes primarily from many weak surface resonances located between 5 and 1 eV. Two of them are shown in Fig. 76(c) and (d). The four states, (a), (b), (c) and (d) of Fig. 76, can be put into relation with the states H1, H4, H5 and A6, respectively. In a quantitative comparison the difference in surface relaxation between the Manghi et al. and Bertoni et al. results should be taken into account. Beyer et al. [105] used tight binding scattering method to obtain the local density of states for H adsorption on anion and cation surface sites obtaining insights into the electronic modifications accompanying the interaction of hydrogen with the GaAs(1 1 0) surface [105]. They evaluated the perturbation to the local density of states induced by the chemisorption of H on the Ga and As sites. The local density of states was calculated as a function of the hydrogen on site energy, EH, the H–substrate interaction by taking the Vss and Vsp interaction potentials as input variables allowing to pass from the physisorption (weak interaction) to chemisorption (strong interaction) regime. The dependence of the local density of states on the strength of the surface bond are shown in Fig. 77 while the local density of states changes at different on site level position EH are shown in Fig. 78. The results of AR-UPS taken at 21 eV of photon energy by a discharge lamp at 1 ML coverage by Plesanovas et al. [106] are shown in Figs. 79 and 80(a) and (b). The coverage was determined, see Section 2 for details, by following the EEL intensity of the Ga 3d surface exciton [9]. In the following the states of the clean surface are labelled according to the paper by Alves et al. [107] where the peaks labelled by A and C refer to states localised on the anion and cation sites, respectively.

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Fig. 76. Square modulus of the wave functions of occupied electronic states having a relevant localisation at the surface for GaAs(1 1 0). The states calculated at the M point of the two-dimensional Brillouin zone are plotted along the ½1  1 0 plane. They have the following energies (referred to the top of the bulk valence band): (a) Ea ¼ 10:65 eV; (b) Eb ¼ 4:46 eV; (c) Ec ¼ 3:40 eV and (d) Ed ¼ 1:20 eV [104].

The structure in the range 3.7–4 eV of binding energy along the XM and MX0 lines was assigned to H4 (see Fig. 69). The absence of emission due to hydrogen induced states from the top of the valence  as shown in Fig. 78, the emission from A5, A4 band indicates the absence of surface relaxation. In G, 0 and A2 is suppressed, in X the emission from A5 is reduced to a tail at the very onset, in M the line shape is not affected, within the experimental error, near the onset and a new structure shows up at about 2.5 eV of binding energy assigned to the state H5 (originating from a H bond to the surface through a bond with Ga) according to model A of Fig. 70. This has been verified experimentally by different techniques as reviewed in Section 3 and confirmed by the theoretical results cited above [51,104].

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Fig. 77. Calculated local density of states for the hydrogenated GaAs(1 1 0) surface at the adsorbed atom (full lines) and at the neighbouring Ga substrate atom (dashed lines) for H adsorbed on an As site. With the H on site level fixed at EH ¼ 4 eV, the ss and sp H–Ga interactions have been varied to monitor the transition from physisorption (weak interaction) to chemisorption (strong interaction) [105].

In X0 the emission is reduced at the valence band offset. The shoulder at the onset in X0 is assigned to A6. The feature at about 2.8 eVof binding energy upward dispersing along the XM line was assigned to H5 not excluding the possibility of contribution coming from A1, a clean surface state originating from  emission is related to the in plane bonds likely less affected by hydrogen. The 6.8 eV feature in the G state H2 arising from hydrogen atoms sitting on top of As sites. Moreover some traces of A5 are present.

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Fig. 78. Characteristic changes in the calculated local density of states for the hydrogenated GaAs(1 1 0) surface at the H atom (full lines) and at the neighbouring As atom (dashed lines) for H chemisorbed on a Ga site. Here the LDOS of states changes for fixed ss and sp interactions (strong) and varying on site level positions EH from 0 to 10 eV in 2 eV steps. Depending on EH, the H-LDOS may be more typical for physisorption (see EH ¼ 0 eV) or for chemisorption (see EH ¼ 10 eV), although the interaction matrix elements are held fixed [105].

From this kind of assignment clearly results that hydrogen, since the initial stages is chemisorbed both on Ga and As sites. This is in agreement with a number of experimental results reviewed in present paper and also with the results of Wright et al. [51], see before, who obtained a very small difference in total energy calculations between chemisorption on Ga or As site. Mo¨ nch and co-workers [108] measured AR-UPS and the ionisation energy and the Ga 3d surface exciton intensity by EELS of the cleaved GaAs(1 1 0) surface as a function of H2 exposure at 140 K (Fig. 81). They confirmed the simultaneous saturation of the Ga (from the exciton intensity) and As (decreasing of the surface valence states close to the valence band top) dangling bonds also at low temperatures. The decreasing of the ionisation energy (DI ¼ 1:2 eV at saturation coverage) with the hydrogen exposure was found to be linear with the Ga 3d surface exciton. Using the surface-molecule

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Fig. 79. (a) AR photoemission spectra excited by He I (21.2 eV) for the hydrogenated GaAs(1 1 0) at 1 ML coverage,  along the X0 M and MX symmetry lines of the surface Brillouin zone (shown in the referred to the top of valence band in G, insert). Bold spectra correspond to high symmetry points. Lines connecting peaks are for the guide of the eye. Values of F and y angles are reported for each curve. Labelling of the features is made according to [59]; (b) experimental peak dispersion for H:GaAs(1 1 0) in the surface Brillouin zone at Ga 3d surface exciton quenching (1 ML coverage), squares and filled dots. Theoretical states are indicated by triangles [106].

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 Fig. 80. Comparison of AR photoemission spectra excited by He I (21.2 eV), referred to the top of valence band at the G point, for clean GaAs(1 1 0) (dashed line) and H:GaAs(1 1 0) at 1 ML coverage (solid line) in the high symmetry points of the  and (b) X. The labelling of clean and hydrogenated features is made according to [59,107], surface Brillouin zone: (a) G respectively, from [106].

approximation for the dipole moment formed by the adsorption of H with surface atoms, i.e. the nearest-neighbour interaction between adatom and surface atoms of the substrate only, the experimental results are interpreted in terms of formation of Ga–H and As–H surface dipoles. The authors evaluated the DI ¼ 0:8 eV and the different contributions that leads to the variation of the ionisation energy: while the adatom-induced surface dipoles are oriented such as to increase the ionisation energy, due to the ionicity of the GaAs, the untilting and countertilting of the Ga–As chains reduces the ionisation energy. The latter two contributions overcompensate the effect of the Ga–H and As–H dipoles.

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Fig. 81. Energy distribution curve of photoemitted electrons excited by He I (21.2 eV) radiation from a cleaved GaAs(1 1 0) surface and after successive exposure to atomic hydrogen in Langmuir of molecular hydrogen at 140 K [108].

Angular integrated photoemission spectra were obtained in a number of different experiments performed by higher photon energy (in the 60–90 eV range). Their analysis takes to the same picture for the surface atomic geometry and electronic structure. The results are shown in Figs. 82–85. In Fig. 82(a) the result of an AI UPS study carried out by Santoni et al. [52] at different hydrogen coverage is reported. The Ga 3d surface exciton quenching, as measured by partial yield was obtained between 6600 and 8800 L. Consequently the corresponding EDCs (see Section 3 and [9]) are representative of 1 ML coverage. The difference curves obtained by subtracting from the hydrogenated spectra the clean initial one are reported in Fig. 82(b). The difference maxima indicate the showing up of new states, the negative minima state depletion. The two minima of the difference curves at 10–12 and 0–3 eV are located in correspondence of Asderived s-like and As-derived dangling bond surface states, respectively. The maxima correspond to new Ga–H hydride features and H-induced localised states in the heteropolar gap. Similar photoemission results were obtained by Sorba et al. [63] and Astaldi et al. [25]. They are reported in Figs. 83 and 84. On the basis of core level intensities (see the following paragraph on GaAs(1 1 0) core states) the results of Sorba et al. are representative of high hydrogen coverages. Similar results are not available for the experiment by Astaldi et al. though the comparison the valence band line shape takes to the conclusion that they are representative of low exposures.

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Fig. 82. (a) Valence band emission EDCs for hydrogenated GaAs(1 1 0) with a 63 eV photon energy for atomic hydrogen exposures on GaAs(1 1 0) ranging from 0 L (bottom) to 72 400 L (top) of molecular hydrogen in presence of a hot filament. The zero of binding energy scale is referred to the valence band maximum. The Ga 3d surface exciton quenching was obtained between 6600 and 8800 L. Symbols in the lower (upper) scale indicate the average positions of electronic states of clean (hydrogenated) surface. (b) Difference curves obtained by subtracting from the H-exposed GaAs(1 1 0) valence band emission EDCs of [63] and Fig. 83 the clean valence band spectrum: (*) 2200 L, (&) 4400 L, (^) 6600 L, (!) 8800 L, (þ) 72 400 L. The Ga 3d surface exciton quenching was obtained between 6600 and 8800 L as indicated by the arrow. Exposure are given in L of molecular hydrogen in presence of a hot filament. Symbols in the lower (upper) scale indicate the average positions of electronic states of clean (hydrogenated) surface [52].

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Fig. 83. (a) Valence band photoemission at 60 eV of photon energy for hydrogenated GaAs(1 1 0). All the spectra were aligned to valence band maximum which corresponds to the zero of the binding energy scale. The spectra are shown after background subtraction and normalisation to monochromator throughput. Curves 1, 2, 3 and 4 correspond to 0  103 , 10  103 , 20  103 and 40  103 L of molecular hydrogen in presence of a hot filament, respectively. (b) Difference curves obtained from energy distribution curves 2, 3 and 4 in (a) after subtraction of the clean GaAs emission (energy distribution curve 1 in (a)) [63].

In Fig. 85 the valence band photoemission spectra obtained by Landesman et al. are reported [6]. For the clean surface the sharp structure labelled S near the valence band top emission reasonably assigned to emission from As p-like surface state (reasonably A5 of [107] and see also Fig. 79) resonates at about 80 eV of photon energy (similar behaviour was reported for Ge(1 1 1), see [6] and references therein). In the hydrogenated surface (upper panel) the surface feature S has almost

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Fig. 84. Difference curves of valence band photoemission spectra between hydrogenated and clean GaAs(1 1 0) taken with (a) hn ¼ 60 eV and (b) hn ¼ 90 eV after 54 000 L of hydrogen exposure are given in Langmuir of molecular hydrogen in presence of a hot filament. Prior of subtraction background was subtracted and spectra normalised to storage ring current [25].

completely disappeared but a shoulder is still detectable at 80 eV of photon energy, where the maximum of resonance occurs, which shows that some of the surface atoms are still in the relaxed surface configuration. This is also in agreement with the evidence coming from core level spectra (see below). Any subtraction of the clean valence band was attempted by the authors but clear hydrogen induced features are visible in the 4–6 eV range at 68 eV of photon energy. These features must be correlated with the states induced by hydrogen in the stomach gap observed by Astaldi et al. [25], Sorba et al. [63] and Santoni et al. [52], and derived by calculation by Manghi et al. [59] and labelled as H5, H4 and H3 in Fig. 71. In Figs. 86 and 87 the results by Gregory and Spicer are summarised. Data were normalised to electron yield at the used photon energy corrected for the GaAs reflectivity at that energy [109]. The energy distribution curves were derived through an analogic derivation of the total photoyield signal [110]. The inspection to the figure shows that a carefully comparison of the emission from the very top region of the valence band indicated an enhancement of the emission in the 0.2–0.3 eV region of binding energies. This fact bears analogy with the results obtained by PYS by Sebenne [111] by using photons in the 4–6.3 eV photon energy range where a similar feature was observed and ascribed to clean surface states modified by hydrogen chemisorption. A similar feature is not as evident at higher photon energy likely because of an optical excitation matrix element effect. This is confirmed by comparing the photoemission line shape at the top of valence band of Fig. 87—which show

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Fig. 85. Lower: valence band photoemission at three photon energies (68, 80 and 90 eV) of cleaved, p type, of hydrogenated GaAs(1 1 0) surface. The feature labelled S is the surface state emission. Upper: valence band photoemission at three photon energies (68, 80 and 90 eV) of plasma hydrogenated GaAs(1 1 0) surface. The feature labelled H is an hydrogen induced state emission. Hydrogen pressure 7  103 Torr, filament bias: 75 V, discharge current: 100 mA, duration 1 min [6].

progressive decreasing with increasing photon energy of the feature associated to surface emission— with those of Santoni et al. [52], Sorba et al. [63] and Astaldi et al. [25] obtained using higher photon energies. Pasquali et al. [112] used the high surface sensitivity of metastable de-excitation spectroscopy (MDS) to get insights into the electronic states and their charge distribution of the 1 ML hydrogen covered GaAs(1 1 0) surface. Results are shown in Fig. 88. In the case of clean and hydrogenated GaAs because of the relative value of helium metastable level and empty semiconductor states the helium atom deexcites through a resonant ionisation plus an Auger neutralisation (the so called RI þ AN process) producing an experimental spectrum with a good approximation proportional to the convolution of the density of states. Consequently the observed features were assigned comparing the curve (c) of Fig. 88 with the energy spectrum of electronic states and their charge density of [59] also shown in Figs. 70 and 71, respectively. On this basis the features F2–F5 of the deconvoluted spectrum were assigned to the H5–H2 states. The authors also made a comparison with the difference curves of Fig. 82. The state H5 which is only a shoulder in UPS, because of its charge distribution is a well defined peak in MDS, while H4 occurs at an energy 1 eV lower with respect to UPS. This discrepancy was ascribed to an interplay between k-dispersion of state and distribution in energy of surface charge. The negative dip due to the disappearance of C2 is replaced by the peak of H3, while H2 occurs at 7 eV in good agreement with UPS and with the absence of dispersion of this state. As far as F1 is involved the authors assigned this feature to A5. Two states, A5 of the clean surface and A6 of the hydrogenated one were considered. A5 being a

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Fig. 86. Photoemission energy distribution curves for clean and hydrogenated GaAs(1 1 0) taken with 10.2 eV of photon energy. The Fermi level positions are indicated, spectra are aligned to the emission from the top of valence band. Sample was p type, 1:5  1017 Zn doped. Exposures are in L of molecular hydrogen in presence of a hot filament [11].

p-As dangling bond with its charge protruding into vacuum and dominating the high kinetic energy region of the MD spectrum [113] and of the photoemission spectrum of the clean surface. The A6, which is an in plane p-orbital, localised at the As site not corresponding to an As–H bond (giving some contribution to the photoemission spectrum of the hydrogenated surface) whose charge is localised—on ˚ below that of A5 is not expected to contribute significantly to the helium the average—some tenths of A de-excitation with respect to all other states much protruding into vacuum. This assignment is in agreement with the picture of surface where some degree of dangling bonds are always present at any degree of hydrogenation. EEL spectroscopy was applied to the study of the electronic properties of the hydrogenated GaAs(1 1 0) surface. A second derivative EELS experiment was carried out by Antonangeli et al. [9] in the region of the excitation of the valence band and Ga 3d core level (the letter to be discussed below). The results are reported in Figs. 1, 8 and 9. Antonangeli et al. [9] on the basis of the loss intensity dependence on hydrogen exposure and desorption were able to assign the 8.4 eV, 10.2 eV and 12.5 eV loss features to electronic transitions involving hydrogen induced states. Hydrogenation as can be observed in Fig. 9 beside these well defined losses induces also a broadening accompanied by a shoulder in the 6 eV loss region. A phenomenological attempt to identify the initial and final states of the transitions responsible of these features was proposed by Antonangeli et al. [9] and Nannarone et al. [114].

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Fig. 87. Photoemission energy distribution curves for the hydrogenated GaAs(1 1 0), after 4  107 L of exposure to molecular hydrogen in presence of a hot filament, referred to the top of valence band, taken with 10.2, 11.1 and 11.6 eV of photon energy. Sample was the same of Fig. 86 [11].

The simulation of the dielectric function of the hydrogenated layer was based on a three layer model (vacuum–surface–bulk, see f.i. [115,116] and references therein). The dielectric function of the hydrogenated layer was described by two oscillators with energies oT1 ¼ 4:8 eV and oT2 ¼ 13:0 eV, respectively. In this way the features of the experimental loss function were reproduced allowing the assignment of the initial and final states of the electronic transitions. The results of the model are shown in Figs. 89 and 90. The three peaks in the 5–6 eV region are the features of the surface loss function related with the interface plasmons associated with the oscillator of energy oT1 ¼ 4:8 eV, while the 9 eV and 13 eV losses are taken into account by the second oscillator of the surface layer at oT2 ¼ 13:0 eV. In Fig. 90 the connection between UPS and EELS results is made. Theoretical density of states is also reported. The 4.8 eV transition connects the p-As and Gaderived states to the empty states of the conduction band, while the 13 eV transition connects the higher binding energy s-As states with the same final band of empty states. The 5 eV transition can be put into relation, see below, with the strong minimum in photoelectric yield at about this energy in hydrogenated III–V surfaces [28]. The azimuthal dependence of the EELS of the clean and hydrogenated GaAs(1 1 0) was studied by Nannarone et al. [71]. The results are reported in Fig. 91. Similar results were obtained for GaP(1 1 0) and InP(1 1 0) and are reported below. According to these results the (1 1 0) clean surfaces of III–V compounds show different electron energy loss properties according to the direction of the exchanged

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Fig. 88. MDS results of 1 ML of hydrogen on GaAs(1 1 0): (a) experimental spectrum (points) and spline fit (continuous line) versus ejected electron kinetic energy; (b) first derivative of spectrum of (a); (c) deconvolution of spectrum versus binding energy [112].

momentum. The ½1  1 0 direction, f ¼ 0 of Fig. 91 is the zig-zag atomic chains direction characterising the III–V (1 1 0) surfaces, f ¼ 90 is the direction orthogonal to them. The richer structure of features along the f ¼ 0 indicates that the excitation is favoured with wave vector exchange along the zig-zag chains. The hydrogenation reduces the overall degree of anisotropy, it affects mainly in intensity the shape of the spectra at f ¼ 0 , though, in analogy with the clean surface spectrum, still it remains the more structured one. The optical properties of the GaAs(1 1 0) hydrogenated surface were studied by Chiaradia et al. [117]. In Fig. 92 the differential reflectivity (DR) of the hydrogenated surface is reported (open squares). The result of the same experiment for GaAs(1 1 0) exposed to oxygen is reported (dashed curve). This spectrum was downward shifted for ease of comparison with that of the hydrogenated surface. The comparison indicates that hydrogen, at variance with oxygen, introduces a feature in the reflectivity due to an electronic excitation at 2 eV. A similar assignment could be done for the feature at 3.8 eV.

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Fig. 89. EELS (second derivative) results and simulation for hydrogenated GaAs(1 1 0): (a) experimental EEL spectrum of clean surface; (b) electron energy loss spectrum of H-covered surface; (c) calculated surface loss function for H-covered GaAs; (d) surface loss function of GaAs calculated from literature data [114].

Kuball et al. [118] used spectroscopic ellipe`sometry, RAS and differential reflectivity in the study of the hydrogenated surfaces of GaAs(1 1 0). In Fig. 93 the reflectivities taken with the light electric field along the [0 0 1] and [1  1 0] directions, respectively, are shown and compared with the changes of the surface dielectric function, derived from spectroscopic ellipsometry for light polarised along the same directions. The most prominent surface feature, indicates by ds, was associated with an optical transition in the hydrogenated layer. This feature can be related to the surface optical transition around 2.8 eV obtained in the calculation [119] and by EELS [71]. The two other features were associated to the perturbation induced by hydrogen on the bulk states wave functions near the surface in correspondence of the critical points of the density of states E1 and E1 þ D1 [119]. The feature ds is present only for light polarisation along the ½1  1 0 direction, i.e. along the zig-zag chains, in agreement with EEL results discussed above. Similar results were obtained for the GaAs(1 1 0) surface hydrogenated with 1000 L of exposure. The results

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Fig. 90. Connection scheme, for the hydrogenated GaAs(1 1 0), between photoemission, theoretical results and electron energy loss dielectric model. The shaded area indicates the energy region of empty states in conduction band and Eg is the energy gap, the oT1’s indicate the oscillator energies of the dielectric model (see text) and the arrows indicate the experimental maxima of photoemission [114].

are reported in Figs. 94 and 95. From these results the imaginary part of the surface excess function were derived. The results are shown in Fig. 96. Manghi et al. [119] calculated the optical properties of GaAs(1 1 0) and GaP(1 1 0) (see below) surfaces by means of self-consistent local density calculations. Beside the information about the optical properties of the hydrogenated layer, this paper contains important quantitative findings about the optical properties of seminfinite crystals. In particular it was found that a significative contribution to transitions between bulk states is present in the reflectance anisotropy and differential reflectivity as a consequence of the perturbation of bulk states due to crystal termination. In this framework the differential reflectivity of GaAs(1 1 0) and GaP(1 1 0) (see below) was calculated between clean and hydrogen covered surface for light polarisation along the ½1 1 0 and [0 0 1] directions (directions parallel and normal to the zig-zag chains, respectively). Calculations were carried out for the two geometries of Fig. 70, corresponding to on top chemisorption on the relaxed surface and chemisorption along the ideal dangling bond directions on the unrelaxed surface. The results are shown in Fig. 97. Different chemisorption geometries produce different differential reflectivity spectra. In both cases, however, the contribution of transitions involving bulk states perturbed by the crystal truncation is of the order of 50% at all the energies. In Fig. 97(a) the negative valley present around 2.2 eV was ascribed to transitions from G15 to X3 bulk states (giving a higher contribution in the hydrogenated

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Fig. 91. Electron energy loss spectra for clean (top) and hydrogen covered (bottom) GaAs(1 1 0) taken at 08 and 908 of azimuth, respectively, with respect to the ½1 1 0 direction [71].

surface). A fact possible because of the removal of the kz 0 ¼ kz selection rule due to chemisorption bond on the surface. In fact this feature is strongly reduced when the hydrogen atoms are arranged along the dangling bond direction (Fig. 97(b)) suggesting that in this case the potential change is not sufficiently abrupt to allow a significative breaking down of the kz 0 ¼ kz selection rule. These results must be compared with the experimental differential reflectivity results of Chiaradia et al. [117] of Fig. 92 obtained by unpolarised light. The negative contribution at about 2 eV followed by a positive band in the 3 eV region is shown by the experiment. A detailed comparison with experiment performed with polarised light would be interesting to obtain indications for the structural determination of the hydrogenated surface. A study based on surface photovoltage (SPV) was carried out by Lu¨ th and co-workers [120]. For photon energies below the band gap the method can give information about the surface states in the gap [121] (the work on hydrogenation was carried out in parallel to the study of GaAs(1 1 0) interaction with oxygen, water and hydrogen sulphide). The results, obtained at 100 K are shown in Fig. 98. An exposure of 108 L produces a well developed shoulder for photon energy lower than the band gap energy with an onset at 1:38  0:02 eV which was interpreted as due to an optical transition connecting bulk states with surface states. Because the SPV signal does not allow to distinguish between a transition from (or to) surface states

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Fig. 92.

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DR for the hydrogenated GaAs(1 1 0) (open squares) compared with that of the oxidised one (dashed curve) [117].

Fig. 93. (a) Imaginary part of the change of surface dielectric function for semi-insulating GaAs(1 1 0) exposed to 100 L hydrogen in presence of a hot filament, as derived from ellipsometric data. (b) Evaluated DR for the hydrogenated GaAs(1 1 0). Dashed line corresponds to light polarised along the ½1  1 0 direction, solid line corresponds to light polarised along the [0 0 1] direction [118].

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 before (full line) and after 1000 L of exposure Fig. 94. Reflection anisotropy of the cleaved GaAs(1 1 0) ðR½0 0 1  R½1 1 0 Þ=R to molecular hydrogen in presence of a hot filament exposure (dashed line) [118].

to (or from) volume states, both a transition from a full surface state 0.13 eV above the valence band top (the energy gap at 100 K is 1.51 eV) to conduction band states and a transition from bulk valence states to empty surface states 0.13 eV below the conduction band bottom can give account of the observed shoulder. A number of valuable information, on a wide range of exposures, on the electronic states localised in the band gap or near the valence band top were obtained by Sebenne and co-workers [20,48] stemming from PYS. Partial yield experiments were flanked by LEED (see also Section 3), EELS and AES. The wide exposure range used allowed to study both the chemisorption and dissociation regime.

Fig. 95. Imaginary part of the hydrogen induced change in the anisotropy of the surface dielectric function of hydrogenated (1000 L of molecular hydrogen in presence of a hot filament) GaAs(1 1 0): (a) semi-insulating and (b) n ¼ 4:7  1018 cm3 [118].

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Fig. 96. Imaginary part of the surface excess function of hydrogenated GaAs(1 1 0) for semi-insulating bulk for several hydrogen exposures (in Langmuir of molecular hydrogen in presence of a hot filament): (a) light polarisation along the [0 0 1]; (b) light polarisation along the ½1 1 0 [118].

Photoemission yield spectra (PYS) obtained at different hydrogen exposures are shown in Fig. 99 for n (1:6  1017 cm3) and p (1:6  1018 cm3) type samples. The emission onset at about 4 eV corresponds to emission from the bottom of conduction band. Moreover, because of the value of the highest photon energy used in this experiments (6.3 eV) and the value of the ionisation threshold of GaAs(1 1 0) (5.5 eV) an energy region of the order of 0.8 eV of the full states distribution near the valence band top was investigated. The emission from the fundamental energy gap (4–5.2 eV) is due to emission from defect states; its intensity gives an indirect measure of the PYS sensitivity.

Fig. 97. DR of GaAs(1 1 0) calculated as difference between the clean and hydrogen covered surface reflectance for light polarised along the x (parallel to the ½1 1 0 , solid line) and y (parallel to the [0 0 1], dashed line) for (a) hydrogens in on top sites on the relaxed surface, geometry B of Fig. 70 and (b) hydrogens chemisorbed along the dangling bond directions, geometry A of Fig. 70 [119].

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Fig. 98. SPV spectra of clean and hydrogenated GaAs(1 1 0) (n type, 2  1017 cm3) taken at 100 K with 13 Hz of chopping frequency. Hydrogen doses are given in Langmuir of molecular hydrogen in presence of a hot filament. The phases used in the lock-in detection for particular spectral range are indicated [120].

A small quantity of hydrogen induces important changes in the yield curve followed by a smooth evolution taking place up to 104 L. At larger exposures important changes occur with a large increase of the yield at high photon energies, correlated with an important shift of the curves to lower photon energies, and the onset of a new phenomenon which leads to the vanishing of the photoemission signal over a narrow range of photon energy around 5.5 eV (this phenomenon was reported for the first time by Proix et al. [26] for H:GaAs(1 1 0) and named by these authors ‘‘black hole’’). Its origin is still under debate, the related discussion is summarized below (see also Section 3.2.1). In this experiment the exposure of about 104 L can be taken as the end of the ordered chemisorption of hydrogen on the GaAs(1 1 0) as demonstrated by the achievement of a 1  1 LEED pattern and by the almost unaffected Ga to As peak-to-peak Auger signal ratio. The evolution of the effective density of full states, derived by taking the first derivative with respect of photon energy of the PY signal is shown in Fig. 100(b). The results show the presence of an hydrogen induced state at about 0.12 eV below the top of valence band increasing with the hydrogen dose. This evidence might be in contrast with the absence of hydrogen induced states reported by Santoni et al. [52] and by Astaldi et al. [25] in the same energy region by UPS by using a photon energy of about 60 eV. On the other side Gregory and Spicer [11], by using a photon energy ranging between 10.2 and 11.6 eV, found a shoulder due to hydrogen induced states at about 0.2 eV of binding energy. Optical excitation matrix element effects could be responsible of this kind of different line shape. At our knowledge any investigation was carried out in the case of the hydrogenation of the III–V surfaces. The feature observed by PYS could be connected with the A6 state obtained by theoretical calculation by Manghi et al. [59] which shows up as a shoulder in the AR-UPS by Plesanovas et al. [106] (see also Fig. 79) taken with a 21 eV photon energy.

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Fig. 99. Photoemission yield spectra, Y(E), in the 4–6.3 eV photon energy range of GaAs(1 1 0), exposed to molecular hydrogen in presence of a hot filament (exposures are in L) for (a) n type (1:6  1017 cm3) and (b) p type (1:6  1018 cm3) bulk doping at different hydrogen exposures. Both semi-log and linear plots are given: (a) Curves: A, after cleavage; B, 1 L; C, 10 L; D, 102 L; E, 103 L; F, 104 L; G, 3  104 L; H, 105 L; I, 3  105 L. (b) Curves: A, after cleavage; B, 10 L; C, 102 L; D, 103 L; E, 104 L [20].

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Fig. 100. Effective density of GaAs(1 1 0) exposed to molecular hydrogen in presence of a hot filament (exposures are in Langmuir, L) as derived from partial yield data: (a) effective density of states, N (E), versus photon energy for the curves (a) of Fig. 99. Curves: A, after cleavage; B, 1 L; C, 10 L; D, 102 L; E, 103 L; F, 104 L; G, 3  104 L; (b) effective density of filled hydrogen induced states Ns(E) versus binding energy at different hydrogen exposures derived by subtracting the spectrum of the clean surface from the hydrogenated ones. The zero of the binding energy is taken at the top of the valence band [20].

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Fig. 101. Evolution of GaAs(1 1 0) photoemission yield spectra as a function of exposure to atomic hydrogen (upper) and to a beam of molecular hydrogen ions (lower) of about 80 eV of energy for an n type sample. The work function of metallic Ga is indicated by jGa. The arrows point to the dip observed at an intermediate stage between curves 2 and 3. The dose of hydrogen ions are given (upper) in Langmuir and (lower) in monolayer (ML) [30].

Moreover the hydrogen induced state of Fig. 100(b) shows some evolution with hydrogenation. The authors suggested that this evolution could be mainly due to the removal of relaxation which takes the hydrogen induced states out of the fundamental gap. An additional comment is deserved by the vanishing of emission, though not completely understood, shown by the curve I of Fig. 99(a). Similar effect was reported by the same group [70] when the surface was also exposed to a beam H2 þ of about 80 eV of energy. Results are shown in Fig. 101 together with those related to the exposure to atomic hydrogen (already shown in Fig. 100(a)). Moreover such a phenomenon was observed also by hydrogenation of InP(1 0 0) and InP(1 1 0) (see paragraphs 3.1.2 and 5.2.3, respectively). It was not observed on heavily hydrogenated GaP surfaces. The strong reduction of emission is a clear property of the dissociated surface: it occurs after 104 L of exposure where Ga to As Auger signal ratio indicates an altered stoichiometry and the LEED pattern is typical of a disordered surface [20]. The inspection to Figs. 99 and 101 shows that the band where emission vanishes is centered at about 5.5 eV and has a width of a few tenth of eV. For this reason it was put into relation with the presence of a molecule, containing an element of the fifth group, of the AsHx kind in the specific case of GaAs. Moreover at these exposures the sample work function value is close to that of metallic Ga suggesting, at dissociation, the separation of Ga from As with the consequent formation of Ga metallic drops or islands and molecules of AsHx kind.

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Fig. 102. Photoemission yield versus photon energy for different surfaces (indicated) after various treatments: (a) H:InP(1 1 0) annealed in ultra high vacuum; (b) H:GaAs(1 1 0) annealed in ultra high vacuum; (c) H:InP(1 0 0) taken at room temperature in ultrahigh vacuum; (d) GaAs(1 1 0) bombarded with H2 þ ions. Curves 1 (dotted) and 2 (full) refer to the dissociated state before the treatment and to the state after the treatment, respectively [30].

However the phenomenon appears too intense if the optical cross-section of gaseous AsH3 and the overlayer thickness are considered; a simple model would give an absorption coefficient as high as 108 cm2. The presence of a transition occurring at 4.8 eV—though observed in the chemisorption regime— related to states involving As-p orbitals observed in EELS by Antonangeli et al. [9] and Astaldi et al. [25] is not in contradiction with the above picture. The stability of the emission vanishing against annealing cycles was checked by Cherchour et al. [70]. Results are reported in Fig. 102 showing the disappearance of the emission vanishing which was explained as due to AsH3 release. The inspection to the figure shows that the quenching at about 5 eV is unstable for all the investigated systems. Proix et al. [48] showed that the yield quenching is also affected by contamination. Contaminants included oxygen, carbon and sulphur. Results are summarised in Fig. 103. The contaminated GaAs(1 1 0) did not show this phenomenon. The work functions jn and jp of the n and p samples, respectively, and the ionisation edge F were derived from the PYS data by fitting the Fermi distribution at the threshold and by the GaAs valence band contribution, respectively. They are reported in Fig. 104. The ionisation edge was not determined beyond 3  104 L because at this stage the ‘‘black hole’’ phenomenon takes place corresponding also to the beginning of the dissociation stage. The results show that an upward (downward) band bending takes place for n (p) sample. The band bending stabilises at about 102 L of exposure at about 0.5 eV above the valence band top. This value which is independent

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Fig. 103. Photoemission yield spectra versus photon energy of hydrogenated (upper) and contaminated (lower) clean surfaces of (left to right) GaAs(1 1 0) with atomic hydrogen, GaAs(1 1 0) with molecular hydrogen ions and InP(1 1 0) with atomic hydrogen [48].

from the bulk doping can be taken as a measure of the neutrality level [122] of the GaAs(1 1 0) hydrogenated surface with about 1 ML of coverage in a chemisorption regime. Consequently a net electron transfer from bulk (surface) to surface (bulk) should occur for n (p) type bulk (a space charge region of complementary sign will build up). For exposures higher than 5  104 L, independently of bulk doping, the work function begins to lower. Similar results were obtained by Santoni et al. [52] by photoemission and Bartels et al. [3] by contact potential difference as reported in Fig. 105. Bartels et al. [3] followed the dependence of the contact potential difference by using a Kelvin probe. The results are reported in Fig. 106. As can be seen by an inspection to the figure the contact potential (i.e. the GaAs hydrogenated sample work function minus the W work function) is reduced for p (1:4  1017 cm3) samples and increased for n (2:6  1017 cm3) samples (the difference of about 1 eV being with good approximation equal to the Fermi level difference in the bulk of the samples). These findings indicate that depletion layers are formed for both kinds of doping in total analogy with the above reported results. At exposure of about 102 L curves merge and stay constant up to about 104 L, above this value the increase in contact potential difference indicate an increase of work function independently of bulk doping at variance with what observed by Sebenne and co-workers [20] and Santoni et al. [52]. No explanation is available at present for this discrepancy. Pinning of the Fermi level at about 0.4–0.5 eV above valence band top was also reported by Gregory and Spicer [11] by UPS and Landesman et al. [6]. The observed direction of work function change cannot be simply explained on the basis of the formation of a surface dipole due to charge transfer from Ga and As surface atoms toward chemisorbed

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Fig. 104. Work function j, ionisation energy F(Evs) and conduction band bottom Ecs (electron affinity) of GaAs(1 1 0) as a function of hydrogen exposure obtained by fitting photoemission yield data. The subscripts n and p refer to n and p sample, respectively [20].

hydrogens according to the Pauling electronegativity scale (Ga 1.6, As 2 and H 2.1). This is also in agreement with the theoretical charge plots shown in Figs. 71–76 revealing a predominant covalent character of the hydrogen bond. However, as shown below, simple electronegativity arguments allow to take into account the direction of energy shifts of Ga and As 3d surface core level components.

Fig. 105. Work function as a function of hydrogen exposure (given in Langmuir of molecular hydrogen in presence of a hot filament) for GaAs(1 1 0) obtained by combining photoemission data from degenerate p type sample [52].

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Fig. 106. Contact potential difference between hydrogenated GaAs(1 1 0) and a tungsten reference electrode as a function of hydrogen exposure, in Langmuir of molecular hydrogen in presence of a hot filament, for p (2:6  1017 cm3) and n (1:4  1017 cm3) type samples [3].

5.2.2. GaAs(1 1 0) surface—core states Data available include EEL study of Ga 3d excitation and photoemission study of both Ga and As 3d core levels as a function of hydrogenation. As far as EELS is concerned the study was focused mainly on the Ga 3d excitation because of the presence of the surface exciton. The core level photoemission data deal mainly with the photoemission line shape analysis to identify the binding energy shifts of surface atoms bonded with hydrogen. As shown in Section 3 through the core level study insights on the surface stoichiometry and morphology can be achieved. Here the accent is on the electronic properties of Ga 3d and As 3d level. The existence of a Ga 3d surface exciton was discovered by Lapeyre and Anderson [123] by absorption spectroscopy. Because of excitonic interaction the transition energy on the clean surface is lowered. As shown by van Laar et al. [124] by EELS a similar situation occurs beside GaAs(1 1 0) also for the excitation from cations in the other III–V (1 1 0) surfaces. Beside hydrogen this transition is affected by the exposure to other gases, like oxygen [3]. In Figs. 8 and 9 the evolution of the energy loss feature due to the excitation of the Ga 3d core level is shown as a function of hydrogen exposure and annealing temperature. Its loss intensity strongly depends on the hydrogen exposure and its vanishing was related to the saturation of Ga surface atom dangling bonds during the formation of 1 ML of hydrogen [123–125]. In this sense it was used to monitor the surface coverage. Its line shape and energy position in EELS were not affected by hydrogen coverage, a fact in agreement with the local nature of this surface excitation interpreted as a Frenkel type exciton [123]. The behaviour of the exciton intensity was correlated with the Ga and As Auger signal ratio by Bartels et al. [3]. The results are reported in Section 3 and in Fig. 25. Moreover Mo¨ nch and co-workers measured the intensity of Ga 3d exciton at 140 K as a function of the hydrogen exposure. The results are reported in Fig. 107. From [108] it is possible to relate the Ga 3d surface exciton intensity with the increasing of the states in the heteropolar gap in the valence band spectra. The spectra for energy loss higher than the exciton present features due to the excitation into empty final states of the conduction band at surface. As an inspection to Fig. 8 shows, these final states result much less perturbed with respect to the excitonic transition.

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Fig. 107. Correlation for GaAs(1 1 0) taken at 140 K of (a) surface Ga 3d exciton intensity and (b) ionisation energy as a function of hydrogen exposure (in Langmuir of molecular hydrogen in presence of a hot filament) [108].

As far as photoemission from Ga 3d and As 3d is concerned results were reported by Santoni et al. [52], Sorba et al. [63] and Landesman et al. [6]. Santoni et al. [52] followed in detail the evolution of Ga 3d and As 3d photoemission line shape by using synchrotron radiation, in order to maximise the surface sensitivity, during the monolayer formation. They monitored the coverage by looking at the Ga 3d surface exciton absorption intensity. The core level study was conducted in parallel with valence band study by photoemission, reviewed above (see Fig. 82 and related discussion). The experimental results together with their analysis are shown in Figs. 108 and 109. The Ga 3d surface exciton was found to be quenched in this experiment between 6600 and 8800 L. The variation of the surface shifted and hydrogen induced core components as a function of exposure indicates that hydrogen chemisorption involves the formation of both Ga–H and As–H bonds during the monolayer formation (without affecting the surface stoichiometry, see Section 3). Any preferential bonding was also observed at higher exposures. These results were

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Fig. 108. Ga 3d core level emission spectra for GaAs(1 1 0) at 55 eV of photon energy (dots) for the clean and H-exposed surface. The solid line superimposed to the experimental data is the result of the least-squares fitting procedure. The spin-orbit split bulk, surface and H-induced subcomponents resulting from the spectral decomposition are also shown. The zero of the binding energy scale corresponds to the peak position of the bulk related Ga 3d5/2 spin-orbit split subcomponent, exposure are given in Langmuir of molecular hydrogen in presence of a hot filament [52].

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Fig. 109. As 3d core level emission spectra of GaAs(1 1 0) at 77 eV of photon energy (dots) for the clean and H-exposed surface. The solid line superimposed to the experimental data is the result of the least-squares fitting procedure. The spin-orbit split bulk, surface and H-induced subcomponents resulting from the spectral decomposition are also shown. The zero of the binding energy scale corresponds to the peak position of the bulk related As 3d5/2 spin-orbit split subcomponent, exposure are given in Langmuir of molecular hydrogen in presence of a hot filament [52].

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achieved on the basis of a line shape deconvolution in terms of Gaussian convoluted Lorentian functions describing the single spin-orbit split subcomponent corresponding to bulk, surface and hydrogenated atoms emission, respectively. It is important to notice that the component related to both Ga and As surface atoms of the clean spectra do not vanish after hydrogenation and their intensity is reduced by only 1/3; a fact not yet completely understood deserving further investigation. The deconvolution results are shown in Figs. 108 and 109. They indicate the appearance of hydrogen induced doublets on both set of spectra. For Ga emission the binding energy of the 5/2 component of the hydrogen induced doublet ranges from þ0.76 to þ0.78 eV and its intensity increases gradually from about 5% to 15% of bulk emission at 8800 L (surface exciton quenching). The 5/2 component of the surface related doublet appears at a relative binding energy ranging from þ0.27 eV to þ0.29 eV and its intensity decreases sharply (by 43%) upon exposure to 2200 L, but remains relatively constant upon further hydrogenation. At the coverage of 6600–8800 L (surface exciton quenching) the residual Ga 3d surface related emission is 41–45% of the initial surface intensity, suggesting that a substantial fraction of surface atoms are still in an environment similar to that of the clean surface. For As emission the 5/2 component of the hydrogen induced doublet appears at a relative binding energy ranging from þ0.48 eV to þ0.51 eV at all coverages and its intensity increases gradually from about 8% of the bulk As 3d emission to about 30% at 8000 L. The same component of the surface related doublet appears at a relative binding energy ranging from 0.37 eV to 0.39 eV. Its intensity decreases sharply (by 30%) upon exposure to 2000 L, but remains relatively constant upon further exposure. At the coverage of 6600–8800 L (surface exciton quenching), the residual As 3d surface related emission is 60–61% of the initial surface intensity, suggesting that a substantial fraction of the As dangling bonds has not been saturated by hydrogen. The overall line width of the hydrogen induced As 3d doublet is almost twice that observed by Sorba et al. [63] at high coverage (see below). This suggests that the presence of the several inequivalent H–As chemisorption sites at the surface. In this way the sharpening of the line shape observed by Sorba et al. [63], and the somewhat decreased chemical shift (see below) is likely to be the result of the desorption (see Section 3) from the surface of the As atoms with highest hydrogen coordination. The direction of the chemical shifts would suggest a fraction of ionic character in the bond with Ga and As surface atoms to atomic hydrogen, in agreement with simple electronegativity arguments (H 2.1, As 2, and Ga 1.6). However the absence of the hydrogen core level emission hinder the evaluation of the amount of charge transferred to or from hydrogen. Consequently it is not possible to monitor the chemisorption induced variation in electrostatic potential around the chemisorbed atom. An analogous experiment was carried out by Sorba et al. [63] starting from the exposure of 104 L, approximately above the surface exciton quenching (both set of results can be correlated taking into account that the Santoni’s exposures were less effective for a factor 1.3–1.7 [52]). Also in this case the core level study was carried out in parallel with valence band spectroscopy as reviewed in Section 5.2.1. The analysis of core level line shape as a function of hydrogen coverage and photoelectron escape depth showed that in the high exposure regime hydrogen is bonded to both Ga and As atoms. Moreover, as discussed in Section 3, the decomposition in terms of bulk and surface components demonstrate that hydrogen adsorption is accompanied by preferential etching of As, surface roughening and subsequent large variation of surface stoichiometry. The experimental data together with the result of the decomposition analysis are shown in Figs. 110 and 111. Also in this case a component at the same

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Fig. 110. High resolution photoelectron energy distribution curves at hn ¼ 80 eV for the Ga 3d core emission from GaAs(1 1 0) at increasing atomic hydrogen exposure. The solid circles are the experimental points ad solid lines the fit result. Experimental curves are normalised to the monochromator throughput and are given in relative units. The zero of the binding energy scale corresponds to the position of the bulk Ga 3d5/2 level, exposure are given in Langmuir of molecular hydrogen in presence of a hot filament [63].

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Fig. 111. High resolution photoelectron energy distribution curves at hn ¼ 100 eV for the As 3d core emission from GaAs(1 1 0) at increasing atomic hydrogen exposure. The solid circles are the experimental points ad solid lines the fit result. Experimental curves are normalised to the monochromator throughput and are given in relative units. The zero of the binding energy scale corresponds to the position of the bulk As 3d5/2 level, exposures are given in Langmuir of molecular hydrogen in presence of a hot filament [63].

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binding energy of the surface component of the clean spectra is present up to the final exposure with a significative intensity though for both core levels the Lorentzian width shows an increase of the order of 30%. A fact not completely clarified requiring further investigation. The binding energy EH of the hydrogen induced feature on the Ga 3d emission increases slightly with hydrogen exposure from 0.80 eV to 0.98 eV with a corresponding increase in Lorentzian width from 0.30 eV to 0.42 eV. A fact that can be explained by the presence of unresolved high-binding energy components due to inequivalent absorption sites. The overall intensity of the surface components (hydrogen induced plus surface related ones) of Ga 3d emission increases with hydrogen exposure but appears to saturate at the highest coverage explored. On the other side for the As 3d emission the reduction of the clean surface doublet is accompanied by the emergence of the hydrogen induced component which in this case is not shifting with exposure. Moreover at variance with the Ga 3d hydrogen induced component, whose line width is increasing with exposure, the corresponding As line width remains constant with hydrogenation. A fact that can be put into relation with the desorption of As avoiding the formation of inequivalent sites like in the case of Ga 3d. Similar results were reported by Landesman et al. [6] on GaAs(1 1 0) surfaces exposed to H2 plasmas at room temperature, see also Section 2. The valence band results are reported above in this section, see Fig. 85 and related comments. The core level results are reported in Fig. 112. The poorer signal-tonoise ratio in this experiment did not permitted the observation of the hydrogen induced component in the As 3d core level emission. 5.2.3. InP(1 1 0) surface—valence states The evolution of the electronic properties of the cleaved InP(1 1 0) surface was studied on a wide exposure range by using PYS, AES, EELS [66,76,126] and UPS [75]. In analogy with the study carried out on GaAs(1 1 0) two different interaction regimes were identified corresponding to chemisorption and dissociation. The data concerning the effect of hydrogen on atomic geometry and morphology are presented in Section 3. PYS results are shown in Fig. 113 for n (1:4  1016 cm3) and p (1:1  1016 cm3) samples. The inspection to the figure shows that PYS is affected since 1 L of exposure with a smooth evolution up to about 3  103 L. At 104 L the threshold becomes suddenly steeper and for higher exposures (106 L) the phenomenon of the ‘‘black hole’’ shows up. A fact that, in analogy with the case of the hydrogenation of GaAs(1 1 0) ascribed to the formation of metallic In and PHx molecule, as discussed below.

Fig. 112. As 3d and Ga 3d photoemission spectra for GaAs(1 1 0) exposed to H2 plasma. Plasma conditions: hydrogen pressure, 7  103 Torr; filament bias, 75 V; discharge current, 100 mA; duration, 1 min; [6].

S. Nannarone, M. Pedio / Surface Science Reports 51 (2003) 1–149 Fig. 113. Photoemission yield spectra of InP(1 1 0) taken after different hydrogen dose indicated in Langmuir. Left panel: n sample (1:4  1018 ) cm3, except for curve 2. Right panel: p sample (1:1  1016 ) cm3 [66].

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Fig. 114. Evolution of the effective density with hydrogen exposure of filled hydrogen induced states for InP(1 1 0) at different hydrogen exposures in Langmuir of molecular hydrogen in presence of a hot filament. The energy is referred to the valence band top at the surface [66].

From data of Fig. 113 the effective density of filled surface states, Ns ðEÞ, can be obtained. The result is shown in Fig. 114. The results reveal the growth of a band of surface states which saturates at about 3  103 L and centred at 0.06 eV below the valence band top with a full width half maximum of 0.26 eV, slightly increasing with the hydrogen dose. Further insights into valence states induced by hydrogen was obtained at different photon energy (by using a synchrotron source) by UPS at normal emission [75]. The results are summarised in Fig. 115. The appearance of a sharp metallic edge at the Fermi level as shown in Fig. 115(b) for curve 4 corresponding to 6  105 L H2, at 38 eV and 50 eV of photon energy was taken as an evidence of the formation of metallic In. In order to enhance the hydrogen induced changes difference curves have been taken. They are shown in Fig. 116. The energy position of the features do not show significant dependence on the photon energy. Important changes of the valence states are detectable after hydrogen exposure and assigned to bonding of hydrogen to both anion and cation surface atoms. Fig. 116(a)–(c), dealing with the chemisorption stage, the negative dip near valence band top was attributed to the removal of the p-like surface state localised on the anion site. The features at about 4 and 6 eV were assigned to hydrogen

S. Nannarone, M. Pedio / Surface Science Reports 51 (2003) 1–149 Fig. 115. Normal emission valence band photoemission spectra for cleaved InP(1 1 0) at photon energy of (a) at 21 eV of photon energy: curve 1, after cleavage; curves 2–4, after 5  102, 1  103 and 3  103 L of molecular hydrogen in presence of a hot filament, respectively; (b) 38 eV and (c) 50 eV: curve 1, after cleavage; curves 2–4, after 2  103 ; 6  103 and 6  105 L of thermally excited molecular hydrogen, respectively. EF is the Fermi level. The energy is referred to Evs, the valence band edge at the surface. Each spectrum is normalised to the highest signal. Any normalisation was made to the incoming flux [75]. 119

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Fig. 116. Photoemission spectra difference curves for InP(1 1 0) at 21, 38 and 50 eV photon energies taken between the spectra of hydrogenated surface (in Langmuir of molecular hydrogen in presence of a hot filament, L) and the as cleaved one: (a) gives the difference between curve 2, after 5  102 L, and curve 1, after cleavage, of Fig. 115; (b) and (c) are the differences between curve 2, after 2  103 L, and curve 1, after cleavage, of Fig. 113, for hn ¼ 38 and 50 eV, respectively; (d) and (e) are the differences between curve 4, after 6  105 L, and curve 2, after 2  103 L, of Fig. 115, for hn ¼ 38 and 50 eV, respectively. The zero of the energy is referred to the valence band top [75].

induced states due to chemisorption on P and In sites, respectively. The former involving p-like P dangling bond the latter s-like In dangling bonds. This assignment, in absence of calculations was made in analogy with the case of H/GaAs(1 1 0) system. The negative deep and the positive feature showing up in the 10 eV binding energy region were assigned to the removal of the P derived s-like surface state and to the growth of an hydrogen induced state at a slightly higher binding energy, respectively. At higher exposures, curve 4 of Fig. 115(b) and (c), the spectrum is characterised by the feature at about 6 eV of binding energy together with a contribution at about 10.5 eV in the heteropolar gap. Following the scheme applied to the discussion of the highly hydrogenated GaAs(1 1 0) these observation were related to removal of P and to the increasing presence of hydrogen at the In sites. Beside this picture the authors alternative explanation for the 6 eV feature was proposed based on the assumption of the presence of a PHx species at the surface. This explanation stems from the fact that the phosphine

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Fig. 117. Work function (j) and ionisation energy (F) variation, and of InP(1 1 0) as a function of the dose in Langmuir molecular hydrogen in presence of a hot filament: jn (n type samples, dots) and jp (p type samples, triangles), F open symbols [66].

Fig. 118. Fermi level position as a function of hydrogen exposure in Langmuir of molecular hydrogen in presence of a hot filament for InP(1 1 0). Filled and open dots refer to p and n sample, respectively [127].

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molecule has an ionisation energy of 10.06 eV close to sum (11.07 eV) of the observed binding energy (6 eV) plus the ionisation energy of the InP(1 1 0). The exposure to hydrogen affects the work function pinning the Fermi level at about 0.9 eV above the valence band top at the beginning of the chemisorption stage. By increasing the exposure a parallel

Fig. 119. Electron energy loss spectra taken with the plane of incidence (defined by the wave vector of the incident electron and the surface normal) containing the ½1 1 1 , F ¼ 0 , and normal to this direction F ¼ 90 . The hydrogen dose is given in Langmuir of molecular hydrogen in presence of a hot filament [77].

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variation of ionisation energy and work function was observed up to the dissociation stage where the work function undergoes a decrease. The work function and ionisation threshold variation with the hydrogen dose obtained by M’hamedi et al. [66] are shown in Fig. 117. Comparable data on surface Fermi level pinning were obtained by Hou et al. [127] by combining CPD and UPS. Data are shown in Fig. 118 for both n (1:1  1017 and 7:9  1016 cm3) and p (1  1017 cm3) dopings with the sample taken at 170 K. An experimental study of the azimuthal dependence of the electron energy loss spectrum was reported in [77] in the 1–6 eV energy loss region for the cleaved surface of InP(1 1 0). Placing the plane of incidence (defined by the wave vector of the incident electron and the surface normal) parallel to the ½1  1 1 surface direction (i.e. the direction of the zig-zag chain of the surface atoms), F ¼ 0 , and normal to this direction, F ¼ 90 , a strong anisotropy was detected on the clean surface, in close analogy with the case of GaAs(1 1 0), and ascribed to the anisotropic excitation between surface states (Fig. 119). Like in the case of GaAs(1 1 0) the hydrogenation reduces the overall degree of anisotropy though some differences are still detectable in the two directions.

Fig. 120. Photoemission yield spectra for InP(1 1 0); n type, 1  1018 cm3 at different hydrogen exposures beyond 3  103 L. The hydrogen doses are given in Langmuir of molecular hydrogen in presence of a hot filament [76].

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Fig. 121. Photoemission yield spectra for InP(1 1 0) of a p type sample 1:1  1016 cm3, exposed to 5  106 L of molecular hydrogen in presence of a hot filament as a function of annealing temperature [76].

As anticipated, in close analogy with the case of GaAs(1 1 0), the highly hydrogenated InP(1 1 0) shows the phenomenon of the vanishing of photoemission yield (the so-called black hole) reported in detail in Fig. 120 [76]. The explanation follows the same pattern of GaAs(1 1 0), implying dissociation with concomitant formation of In metallic drop or islands and PHx like molecules. In Fig. 121 the effect of annealing on the PYS line shape is reported. The experiment consisted in annealing at the indicated temperatures an heavily (5  106 L) hydrogen exposed surface. As can be seen the yield suppression gradually decreases with increasing the annealing temperature eventually displaying at 550 8C a line shape very similar to that corresponding to the end of the adsorption stage. Upon re-exposing the surface to the same hydrogen dose the yield suppression again shows up. 5.2.4. InP(1 1 0) surface—core states A study of the evolution of the photoemission line shape of the In 4d and P 2p was reported for the InP(1 1 0) [30,128] for a wide range of exposures (1006  105 L H2). The results are shown in

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Fig. 122. In 4d core level emission spectra of InP(1 1 0) at eV of photon energy (dots) for the clean and hydrogenated surface. Exposures are in Langmuir of molecular hydrogen in presence of a hot filament. The solid line superimposed to the experimental data is the result of the least-squares fitting procedure [30,128].

Figs. 122 and 123. For low H2 exposure, in the chemisorption stage, the In 4d and P 2p core levels can be decomposed with three doublets, with the hydrogen induced components lying at lower kinetic energies in close analogy with the case of the hydrogenated GaAs(1 1 0). After higher exposure (above 500 L H2 a new peak grows up at higher kinetic energies. This peak intensity increases with the exposure, up to 6  105 L. The fit analysis shows that the new feature has a Doniac-Sunjic line shape, indicating the presence of metallic In on the surface at the decomposition stage. It is worth noticing that the onset of In droplets formation is favoured by the quality of the cleavage: the formation of metallic In is less favoured for poor cleavages, where it is inhibited up to approximately 5  105 L.

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Fig. 123. P 2p core level emission spectra of InP(1 1 0) at eV of photon energy (dots) for the clean and hydrogenated surface. Exposures are in Langmuir of molecular hydrogen in presence of a hot filament. The solid line superimposed to the experimental data is the result of the least-squares fitting procedure [128].

5.2.5. GaP(1 1 0) surface Some theoretical and experimental results are available for the valence states of hydrogenated GaP(1 1 0). In conjunction with the self-consistent pseudopotential calculations of the electronic structure of 1 ML of hydrogen chemisorbed on GaAs(1 1 0), the electronic structure of 0.5 ML of

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Fig. 124. Calculated energy spectrum for 0.5 ML of atomic hydrogen chemisorbed on GaP(1 1 0) surface according to ontop geometry (upper panel). The surface or H-induced states are plotted at the high symmetry points of the twodimensional Brillouin zone over the projected bulk band structure (dashed region). They are labelled by Hi, Ci and Ai depending on their main localisation on hydrogen, Ga and P atoms, respectively. Energies are referred to the valence band maximum [59].

hydrogen chemisorbed on the anion sites was studied [59]. In the calculations an on top chemisorption ˚ [58] as measured in the hydride site was assumed while the P–H bond length was taken equal to 1.43 A molecules. The results are reported in Fig. 124. Except for C2 all other Ga-derived surface states are not significantly modified by hydrogen deposition on anion sites. The state H6 is an empty state, where both P and Ga orbitals are mixed with hydrogen. Although it shows a maximum amplitude at the hydrogen plane, it is not a very well localised feature. It is left unmodified by the chemisorption on anion sites, but when hydrogens are deposited on Ga sites, it is moved at higher energies by nearly 0.5 eV as shown in Fig. 70 in the case of 1 ML on GaAs(1 1 0). In fact (see also the discussion of GaAs(1 1 0) results) the spectrum for chemisorption of 1 ML in model B of Fig. 70 is close to a superposition of two almost independent set of states for chemisorption on anion and on cation, respectively. Manghi et al. [119] calculated the differential reflectivity of hydrogenation of GaP(1 1 0) surface. The results are shown in Fig. 125 and compared with the measured values for light polarisation along the ½1  1 0 (x in the figure) and [0 0 1] (y in the figure) directions (directions parallel and normal to the zig-zag chains, respectively). The azimuthal dependence of the EELS of the hydrogenated GaP(1 1 0) was reported in [81]. The results are reported in Fig. 126. Similar results were obtained for GaAs(1 1 0) and InP(1 1 0) and are reported above. The ½1  1 0 direction, F ¼ 0 of Fig. 126 is the zig-zag atomic chains direction characterising the III–V (1 1 0) surfaces, F ¼ 90 is the direction orthogonal to them.

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Fig. 125. DR of GaP(1 1 0) calculated as difference between the calculated clean and hydrogen covered surface reflectance for light polarised along ½1 1 0 (x solid line) and [0 0 1] (y dashed line) for geometry shown in Fig. 124 [119] compared with the measured values for light polarisation along the (x in the figure) and (y in the figure) directions (directions parallel and normal to the zig-zag chains, respectively).

The more intense spectrum along the F ¼ 0 indicates that the excitation is favoured with wave vector exchange along the zig-zag chains. The hydrogenation reduces the overall degree of anisotropy affecting mainly the two broad features at about 3.5 eV and 5.6 eV of the F ¼ 0 spectrum. 5.2.6. InSb(1 1 0) surface—valence states At our knowledge any spectroscopic study of valence states was performed on the hydrogenation of this (1 1 0) surface. The change of work function with hydrogenation was reported by Hinkel et al. [79] for an n type bulk (n  1  1017 cm3) and for a high exposure range, between 20 000 and 200 000 L of molecular hydrogen in presence of a hot tungsten filament. The result is shown in Fig. 127.

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Fig. 126. Electron energy loss spectra for clean (top) and hydrogen covered (bottom) GaP(1 1 0) taken with the plane of incidence along the F ¼ 0 and F ¼ 90 azimuths, respectively, with respect to the ½1  1 0 direction. Exposures are in Langmuir of molecular hydrogen in presence of a hot filament [81].

Hydrogen adsorption causes a decrease of the work function. If compared with the behaviour of the work function of GaAs(1 1 0) reported in Figs. 104–106 a work function lowering of an n type sample would imply exposures corresponding to the dissociation regime. 5.2.7. InSb(1 1 0) surface—core states The photoemission line shape variation of In 4d and Sb 4d core levels as a function of hydrogenation was studied by Hinkel et al. [79]. The results are shown in Fig. 128 for an exposure

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Fig. 127. Work function change for the InSb(1 1 0) surface of n type bulk (n  1  1017 cm3) as a function of hydrogen exposure (in Langmuir of molecular hydrogen in presence of a hot filament) recorded by the shift of the cutoff of secondary spectra [79].

of 2  105 L of molecular hydrogen in presence of a hot filament. The In line shape shows major changes mainly due to the filling of the valley between the two spin-orbit split components and a broadening. The Sb line shape shows negligible change with hydrogen adsorption except for a slight broadening. The results of the analysis carried out by using as fitting function three Lorentian spin-split components convoluted with a Gaussian are shown in Fig. 129 by the continuos curves. For the Sb 4d line shape two components are still sufficient to reproduce the experimental line shape. For In a third component shows up associated with the hydrogen induced component. The hydrogen induced component in the fit shows lower binding energy with respect to the clean surface component of 0.72 eV at the highest exposure. Moreover In surface component is found to decrease in intensity with hydrogen coverage, while the hydrogen induced peak exhibits a concomitant increase. This behaviour is summarised in the plot of peak area ratios versus hydrogen exposure in Fig. 129. Moreover the In surface component binding energy increases to 0.46 eV with respect to bulk (i.e. almost a factor 2) while its line width increases. This behaviour is different from that of GaAs(1 1 0). 5.3. GaAs(1 1 1) surface Bacharach and Bringans some results on the dependence of the line shape and binding pffiffiffiffiffi[33]preported ffiffiffiffiffi energy of the Ga-rich 19  19 R (23.48) and of the As-rich 2  2 reconstructed surfaces as a function of the hydrogenation. The line shape evolution was similar to that observed for the GaAs(1 0 0)-4  6. Core level intensity ratio [32] is reviewed in Section 3, see Fig. 12 and related comments.

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Fig. 128. Photoemission (dots) for the In 4d (upper) and Sb 4d (lower) core levels of cleaved InSb(1 1 0) after 2  105 L of molecular hydrogen in presence of a hot filament taken with 75 eV of photon energy. The Lorentian components and the fitting function (continuous lines) are shown; (b), (s) and (h) refer to bulk, surface and hydrogen components, respectively [79].

6. Adsorption and desorption In this section the data regarding the dynamics of sticking, absorption and desorption of hydrogen on III–V semiconductor surfaces are reviewed. The whole of the available data is by far more reduced with respect to those of structural, vibrational and electronic properties. The material is organised by topics, starting from adsorption and sticking followed by thermal adsorption and desorption, electron stimulated desorption (ESD) and photon stimulated desorption in the order.

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Fig. 129. Peak height ratio for InP(1 1 0) of: (dots) surface to bulk and (squares) hydrogen to bulk Lorentian components as a function of hydrogen exposure in Langmuir of molecular hydrogen in presence of a hot filament [79].

6.1. Adsorption and sticking A dependence of the sticking coefficient of atomic hydrogen on the surface coverage was reported by M’hamedi et al. [20] for GaAs(1 1 0) by using PYS data. The result is reported in Fig. 100. From these data the dependence of the sticking coefficient S on the coverage Y was derived. In fact by assuming the density of hydrogens stuck on the surface, n(H), proportional to the density of hydrogen induced state, Ns(E) (see Fig. 100(b)). The coverage Y is given by the ratio nðHÞ=n0 , where n0 is the surface atom density (equal to 8:8  1014 cm2 for GaAs(1 1 0)). The change, dn, of the density of surface atoms not reacted with hydrogen between t and t þ dt is given by dn ¼ SðyÞgp dt where SðyÞ is the sticking probability and gp(t) the number of atoms striking the surface per unit time and unit area (gp(t) is a coefficient depending on the efficiency for atomic H production proportional to the hydrogen molecular pressure, gp ðtÞ ¼ gp ð0Þ p(t), gp ð0Þ ¼ 9:6  1010 cm2 L1 for this experimental conditions) obtaining the relation dy ¼ SðyÞ

g0 pðtÞ dt n0

(6.1)

from which S(y) can be derived. The result is shown in Fig. 130. The result was compared with a case of the coverage dependent sticking coefficient, SðyÞ ¼ 1  y, and with the case of coverage independent, unitary, sticking coefficient. This last model implies that a H atom striking an occupied site sticks on the surface, it does not desorb but can migrate over the surface until a vacant site is found. The authors concluded in favour of a better agreement with the constant sticking coefficient case.

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Fig. 130. Dependence of the area under the surface feature induced by hydrogen, Ns(E), of GaAs(1 1 0), obtained by partial yield spectroscopy, as a function of hydrogen exposure (in Langmuir of molecular hydrogen in presence of a hot filament). The broken line refers to a coverage dependent model of the sticking coefficient and the continuos line to a constant sticking coefficient one (see also text) [20].

This fact might indicate that the hydrogens interacting with the surface should have enough surface mobility to migrate toward the vacant site before absorption. As reviewed in Section 3 Grizzi and coworkers gave by ISS-TOF detailed insights into the structure of the hydrogenated GaAs(1 1 0) surface and on the de-relaxation mechanism. In the same framework of activity the group studied the hydrogen absorption and desorption kinetics. Under suitable scattering condition (see caption for details) the TOF spectrum of an hydrogenated GaAs(1 1 0) surface shows the direct recoil peaks of hydrogen, As and Ga whose intensities, IDR(H), IDR(As) and IDR(Ga) are quantitatively related to the density of surface H, As and Ga atoms [62]. The results are shown in Fig. 131 (for experimental details see caption). The hydrogen direct recoil (HDR) peak intensity, IDR ðHÞ, was used to measure the surface coverage according to the relation YðHÞ ¼

nðHÞ ¼ aIDR ðHÞ n0

Fig. 131. ISS of an hydrogenated (102 L of molecular hydrogen in presence of a hot filament) GaAs(1 1 0) by TOF with 6 keV Arþ at incident angle y ¼ 20 , azimuthal angle f ¼ 90 (measured from the ½1  1 0 direction) and scattering angle d ¼ 42:3 . The scattering conditions ensure equal exposure of surface Ga and As atoms without shadowing and focussing effects. As and Ga direct recoil, the argon, carbon (contamination) and the HDR peaks are visible, in the order, from high to low TOF [62].

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with IDR normalised to the IDR ðAs þ GaÞ of the clean surface and with a given by aðHÞ ¼

sðGaAsÞBðGaAsÞ sðHÞBðHÞ

where s and B are the differential cross-sections for direct recoil and the detector efficiency, respectively. On this ground the spectrum of Fig. 131 corresponds to about 1 ML of coverage and to a concentration of 0.05 of carbon. It is important to stress that, within this scheme, this is a quantitative determination of the monolayer coverage. The sticking coefficient was derived from the results reported in Fig. 132 taking the first derivative of IDR(H), shown in the top inset and inserted in the above expression. Above 1 ML coverage, indicated by the arrow, the sticking coefficient becomes almost constant approaching a steady stage beyond 2500 L N (H)) saturation value ranged from 1.2 to 1.8). (IDR The experimental sticking coefficient was compared with three models. The simplest one, in analogy with of M’hamedi et al. [20] (see above), considered S constant obtaining a linear dependence of Y on the exposure. This choice for S is not satisfactory for the low exposure part. The second model considered assumes a dependence on coverage given by S ¼ S0 ð1  Y=YS Þ, with S0 the sticking coefficient at zero coverage and YS the saturation coverage. This choice gives an exponential growth with the exposure, Y ¼ YS ð1  expfL½ðS0 g0 Þ=ðYSn0 ÞgÞ, shown by the dotted line in Fig. 132.

Fig. 132. HDR intensity as a function of the H2 exposure for 6 keV Neþ (&) and Arþ (*) along ðf; yÞ ¼ ð90 ; 20 Þ N normalised to IDR ðAs þ GaÞ for the clean surface (IDR ). Bottom inset, expanded view of the low exposure region. Top inset, N N derivative of IDR (H), proportional to the sticking coefficient S, versus IDR (H). The arrow indicates the estimated position for 1 ML coverage as determined from the integrated intensity of the IDR(H). The dashed and dotted lines are fits of the curve for Ne projectiles with two models of S (see text). The thin line is to guide the eye. The error bars are mainly due to uncertainties in the background subtraction [62].

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Finally a third model was compared assuming that in order to be adsorbed on the surface the H atom must have an impact energy exceeding an activation barrier Ea increasing linearly with Y. In this case the sticking coefficient is given by S ¼ S0 exp½ðEa =KTÞðS0 g0 =n0 ÞL þ 1 . This model follows closer the curve of Fig. 132 at low exposures, dashed line in the figure. At variance with conclusion drawn by M’hamedi et al. a coverage dependent sticking coefficient better reproduces the experimental data. A complete coverage should imply complete saturation of dangling bonds. Actually in the same experiment, as reported in Section 3, a surface exposed to 400 L of hydrogen (i.e. above the completion of 1 ML) still present an important fraction of relaxed surface. 6.2. Thermal absorption and desorption Qi et al. [23] reported thermal absorption and desorption experimental results on the As-rich GaAs(1 0 0)-cð2  8Þ and on the Ga-rich GaAs(1 0 0)-(1  6) reconstructed surfaces following the behaviour of the infrared bands corresponding to different As–H and Ga–H bonds. Desorption results are shown in Fig. 133(a) for the cð2  8Þ and (b) for the (1  6). According to the assignments reported in Section 3 the first group of features in Fig. 133(a) consists of bands between 2050 cm1 and 2140 cm1 due to adsorption on As atoms or dimers located at steps or defects. The second group at 2110 cm1 and 2080 cm1 is assigned to As atoms in the second layer while the third group at 1975 cm1 and 1985 cm1 and 2020 cm1 is assigned to As dimers. The Ga–H peaks between 1835 cm1 and 1875 cm1 are due to H adsorption on second layer Ga atoms. The three broad peaks between 1200 cm1 and 1700 cm1 of Fig. 133(b) are due to bridging Ga hydrides. All the vibrational bands disappear over a similar temperature range during heating, indicating that the energetics of desorption do not vary greatly to one site to the other, an inspection to Fig. 133(a) shows that the different rates at which the four groups of infrared bands disappear during heating indicate that the hydrides responsible for these features exhibits different desorption kinetics. Based on the trends observed in the infrared spectra, the desorption rates from these sites increase as follows: As dimers and second layer Ga atoms  second layer As atoms < As atoms at steps and defects. At variance the results obtained for (1  6) there appear to be separate groups of peaks exhibiting different trends with temperature. The results of adsorption kinetics are summarised in Fig. 133(a) and (b). The results of Fig. 133(a) show that the hydrogenation of Ga and As sites follow a Langmuir adsorption isotherm (solid lines, fitting curve) given by AðtÞ ¼ As½1  eat , where t is the time, As is the area of the band at saturation and the a coefficient is given by a ¼ ð1=4ÞvS0CH/[L], where v is the mean molecular speed, S0 the sticking probability at zero coverage, CH the hydrogen atoms concentration above the surface and [L] is the density of the adsorption sites. The fact that hydrogenation is a highly exothermic process is in agreement with the possibility of fitting with a Langmuir isotherm. Different values of the absorption constant were obtained for the different chemisorption sites, ranging from 0.5 s1 for the As dimer sites to 0.05 s1 for the second layer Ga sites. By an analysis of data of Fig. 133(b) the authors evaluated a heat of hydrogen adsorption of about 9 kcal/mol (0.4 eV) for both the cð2  8Þ and (1  6)-GaAs(0 0 1) surfaces. This heat of adsorption results much smaller of the Si(0 0 1) one (75 kcal/mol; 3.3 eV) indicating a lower passivation efficiency of H for the GaAs(0 0 1) surfaces with respect to the silicon.

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Fig. 133. (a) Absolute change in reflectance spectra due to adsorbed hydrogen on cð2  8Þ GaAs(1 0 0) at temperatures from 303 to 473 K. (b) Absolute change in reflectance spectra due to adsorbed hydrogen on (1  6) GaAs(1 0 0) at temperatures from 303 to 473 K [23].

Desorption of hydrogen from hydrogenated surfaces was monitored by EELS for the GaAs(1 1 0) surface. In Fig. 8 of Section 2 the effect of subsequent annealing cycles (temperature and time are indicated) is reported [9]. Lu¨ th and Matz [12] followed the decrease of the integrated intensities of both the As–H and Ga–H vibrational losses as a function of the annealing temperature. The results are reported in Fig. 134. The results show an almost equal desorption rate of hydrogens from Ga and As sites. A fact in agreement

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Fig. 134. Integrated intensity of the two Ga–H and As–H vibrational electron energy losses on a hydrogenated surface of GaAs(1 1 0) versus annealing temperature [12].

with the similarity in Ga–H and As–H bond strength in molecules and with the close similarity of chemisorption energy on Ga and As sites. Mokwa et al. [95] reported a detailed study of hydrogen desorption from the hydrogenated surface of GaAs(1 1 0) by temperature programmed desorption (TPD) in the 250–900 K temperature range (the sample was kept at 250 K during absorption to avoid—see also Fig. 136 and related discussion— desorption of arsenic hydrides at room temperature). The chemisorbed hydrogen was completely removed in the 250–670 K heating temperature range. Hydrogen is mainly desorbed as H2 and, at a reduced extent, as arsenic hydrides. In the low exposure range AsH was the only observed contribution (see Fig. 136); for higher hydrogen exposures peaks at m=e ¼ 78, 77 and 76 were also detected and identified with AsH3, AsH2 and AsH. Because hydrogen is desorbed as molecule and, as shown f.i. by the above results of Fig. 134, the hydrogens are released by the Ga and As sites at the same rate H–H pairing must occur prior desorption. Heating at temperatures higher than 670 K resulted in Ga and As2 sublimation. Ga sublimation was observed to start at 670 K for prior hydrogenated surfaces. Instead on cleaved surfaces As2 sublimation started at 750 K and the Ga one slightly above 810 K. These results are summarised in Figs. 135–137 (for further details on different temperature cycles see also the captions). Further information were derived from these data. The desorption peak of molecular hydrogen shifts 2 y versus 1/Tmax, where Tmax to lower temperature with increasing exposure (coverage). A plot of ln Tmax is the maximum temperature, shows a constant slope. For a second-order process an activation energy of 12 kcal/mol (0.5 eV) was derived. Further an activation energy for constant coverage (0.02 ML) was determined by varying the heating rate: 16 kcal/mol (0.7 eV). The maximum of H2 desorption in Fig. 135 shifts to lower temperature with increasing hydrogen pressures during absorption. High pressures during absorption lead to an additional desorption maximum at about 500 K. A fact that can be explained by hydrogen desorbing from different surface atomic sites.

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Fig. 135. H2 desorption curves after exposure to different amounts of atomic hydrogen at 250 K. The exposure are given in Langmuir of molecular hydrogen in presence of a hot tungsten wire. Curve 1, 3  103 L; curve 2, 9  103 L; curve 3, 5  104 L; curve 4, 5  105 L; curve 6, 9  105 L and curve 7, 3  107 L [94].

Fig. 136. Programmed thermal desorption spectra after adsorption of atomic hydrogen at 250 K (3  107 L of hydrogen in presence of a hot filament). Curves A are obtained during adsorption–desorption cycles below 670 K. Curves C1 are obtained during adsorption–desorption cycles up to 970 K. Then curves C2 are obtained without prior hydrogen adsorption (no sensitivity corrections) [94].

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Fig. 137. Evaporation rates of GaAs(1 1 0). The last H2 desorption cycle with temperature up to 670 K is followed by a linear heating from 250 to 970 K, curves B1. Then in a second heating the curves B2 are obtained: (a) evaporation rates of As2; (b) evaporation rates of Ga [94].

The integrated desorption flux of curve 7 of Fig. 135 yielded a coverage of about 3  1014 cm2 hydrogen atoms, to be compared to the number of Ga atoms in the surface (4:4  1014 cm2) indicating that half monolayer of hydrogens from the As sites is desorbed as arsenic hydrides the second half, from Ga sites, as molecular hydrogen. A comparative study of desorption from hydrogenated (deuterated) Ga-rich GaAs(1 0 0)-ð4  6Þ surface and As-rich GaAs(1 0 0)-cð2  8Þ surfaces of D2 by TPD in the 100–900 K temperature range was reported by Creighton [129]. TDP spectra for the deuterated GaAs(1 0 0)-ð4  6Þ surface are shown in Fig. 138. The most prominent feature in the TPD spectra is that ranging between 510 and 480 K which saturates (indicative of a surface process) at 300 L of D2 exposure. The peak temperature of this state shifts down from 510 to 480 K as the coverage is increased. This behaviour, along with the geometrical shape of the peak, was ascribed to second-order desorption kinetics (it is worth mentioning that the authors attributed the two features at about 260 and 800 K to artefacts due to desorption from the sample holder).

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Fig. 138. TPD spectra from GaAs(1 0 0)-ð4  6Þ. Curves (a)–(g) for atomic deuterium exposures; 7.5, 15, 30, 60, 90, 150, 300 L of D2, respectively at 150–160 K. Curve (h) is for a 600 L molecular D2 exposure in presence of a hot filament scaled by a factor of 100 [129].

The comparison between Ga-rich and As-rich surfaces is reported in Fig. 139 for an exposure of 330 L D2. There are two noticeable differences between the D2 TPD results on the two reconstructions. For the Ga-rich surface more D2 desorption was observed at the 480 K peak than does the arsenicrich surface. The difference is even larger at lower doses indicating that the 480 K peak is due to recombinative desorption of deuterium bonded to surface gallium rather than surface arsenic. This results does not necessarily imply that deuterium preferentially bonds to the surface gallium atoms, but it does imply that deuterium bonded to gallium is responsible for the 480 K D2 desorption state. Deuterium bonded to As has also the arsine desorption channel available (see also the discussion below). At higher exposures (a few 100 L) however a more dramatic cause arises because surface arsenic is depleted thus opening up more Ga sites for hydrogen adsorption.

Fig. 139. D2 TPD from Ga-rich GaAs(1 0 0)-ð4  6Þ and As-rich GaAs(1 0 0)-cð2  8Þ following 330 L D2 exposure at 290 K in presence of a hot filament [129].

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Fig. 140. (Left): Arsine TPD (AsDþ, m=e ¼ 77) from GaAs(1 0 0)-ð4  6Þ for 90, 150, 300 and 600 LD2 exposures at about 150 K. (Right): Arsine TPD (AsDþ, m=e ¼ 77) from GaAs(1 0 0)-cð2  8Þ, curves (a)–(c) and GaAs(1 0 0)-ð4  6Þ, curve (d). Exposure are 330 (exposure made at 160 K), 330 (exposure made at 290 K), 90 and 330 (exposure made at 290 K) LD2, respectively. Arrows indicate maximum temperature attained during dosing [129].

The second difference in D2 TPD is that the As-rich reconstruction exhibits a high-temperature shoulder at 660 K. This desorption state appears to saturate for exposures lower than 100 L. Since this state only appears on the As-rich reconstruction, it is most likely due to recombinative desorption of deuterium bonded to surface As.

Fig. 141. Intensity of the direct recoil peak with surface hydrogens, IDR(H), in ISS with TOF for GaAs(1 1 0) surface acquired with 6 keV Arþ at ðf; yÞ ¼ ð67:4 ; 12 Þ versus the surface temperature after the two initial hydrogen exposures indicated (in Langmuir of molecular hydrogen) [62].

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The desorption of arsine is reported in Fig. 140(a) and (b) for the gallium-rich GaAs(1 0 0)-ð4  6Þ and for the arsenic-rich GaAs(1 0 0)-cð2  8Þ, respectively. Arsine desorption can occur even on the Ga-rich reconstruction as shown in Fig. 140. In fact even at relatively low exposures an arsine peak at about 330 K is observable. At higher exposures a low temperatures desorption peak appears at about 180 K and it was assigned to physisorbed arsine. Assuming a first-order kinetic process with a typical pre-exponential factor of 1013 s1 an activation energy of 20 kcal/mol (0.87 eV) should follow for the 330 K peak (similar to the value found for desorption from trimethylarsenic and consistent with the bond strengths of donor–acceptor bonds in III–V adducts). Thus arsine desorbing at 330 K can be described as weakly chemisorbed through a donor–acceptor type bond to the surface gallium atom. The arsenic-rich surface (Fig. 140(b)), also exhibits a chemisorbed arsine desorption peak around 340 K. The exposure were made at 290 and 160 K. There is typically 4–5 times more arsine desorbing from the arsenic-rich surface than from the gallium-rich surface under nominally the same conditions. No evidence of desorption of gallium hydrides (GaH3) was found in the experiments. Apparently the surface GaHx species decompose efficiently by molecular hydrogen formation. Gayone et al. [62] derived from ISS–TOF data information about the desorption kinetic by following the temperature dependence of the surface HDR peak intensity, IDR(H). The results are shown in Fig. 141.

Fig. 142. (a) ESD mass spectrum of negative ions from hydrogenated GaAs(1 0 0), under 300 eV electron bombardment; (b) positive ion mass spectrum from the same sample under 400 eV electron bombardment. The inset shows Gaþ and Asþ peak at 75 amu [131].

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6.3. Electron stimulated desorption Corallo et al. [130] used ESD measurements to study the interaction between hydrogen (deuterium) and GaAs(1 0 0) surface. They verified that molecular deuterium does not adsorb at significant rates at room temperature on the (1 0 0) surface, but it does adsorb when the sample is subjected to radiant heating in the range 200–400 8C with a hot tungsten filament. Energy analysis of the desorbing ions was performed by selecting a delay time appropriate to the ion of interest and scanning the kinetic energy with the CMA. It indicates that at least one and possibly two adsorbed states for H and D are present on the GaAs(1 0 0), but the nature of the adsorption states is not known. Heating the sample at 500 8C reduced the amount of adsorbed deuterium. Petravic and Williams [131] studied the positive and negative hydrogen ions ESD from hydrogenated GaAs(1 0 0) surfaces as a function of the impact electron energy in the 5–500 eV energy range. The negative and positive ions mass spectrum is shown in Fig. 142. The negative ions desorption is dominated by hydrogen, while oxygen, carbon and fluorine show up with a yield two order of magnitude lower. Positive ions yield is still dominated by hydrogen, with an intensity comparable to that of negative ions, and is also accompanied by the presence of oxygen and fluorine with the addition, with respect to the negative ions yield, of Ga and As ions. The dependence of the H and Hþ desorption as a function of the electron impact energy is reported in Fig. 143. The negative ion spectrum appears more structured. The correlation of the features with the binding energies of core levels, as shown in figure was assumed as an indication that H desorption

Fig. 143. Desorption yield of H and Hþ from hydrogenated GaAs(1 1 0) as a function of electron beam energy. Binding energies of some arsenic and gallium core levels are indicated by arrows: (1) As 3d, (2) Ga 3p, (3) As 3p, (4) Ga 3s, (5) As 3s. Inset shows the low energy region on an expanded scale [131].

144

Fig. 144.

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Photon stimulated desorption positive ion mass spectrum from a hydrogenated GaAs(1 1 0) surface [132].

arises primarily from excitation and breakup of surface hydrogen bonds suggesting a resonant like process for this mechanism. 6.4. Photon stimulated desorption Petravic et al. [132] studied the photon stimulated desorption of Hþ from hydrogenated GaAs(1 1 0) and GaAs(1 0 0) surfaces as a function of photon energy. The positive ion mass spectrum from a hydrogenated GaAs(1 1 0) surfaces is reported in Fig. 144 showing the desorption of Hþ and the presence of fluorine due to the high ionisation probability of the latter.

Fig. 145. (a) Normalised photon stimulated desorption yield from a hydrogenated GaAs(1 0 0) surface as a function of photon energy around As 3d core level binding energy; (b) desorption yield of Hþ around Ga 3p core level binding energy [132].

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The dependence of hydrogen ion desorption on the photon energy was studied in the 40–120 eV range in order to include the excitation of As 3d and Ga 3p core levels. The results for positive hydrogen ion desorption are shown in Fig. 145. For the GaAs(1 0 0) two peaks due to the resonant excitation of the As 3d core level show up. No resonance peaks due to Ga 3p excitation were detected. The situation is different for the GaAs(1 1 0) surface where resonance features of the desorption of Hþ were observed for both As 3d and Ga 2p core levels a fact which was assumed as the evidence that desorption occurs through a direct surface excitation of the As–H and Ga–H surface bonds, the latter missing on the As terminated GaAs(1 0 0) surface.

7. Conclusions A review of the experimental data and theoretical results of the physical properties of the hydrogenated surfaces of III–V semiconductor compounds was presented. The matter was organised per properties, including atomic geometry and surface morphology, vibrational properties, electronic properties and absorption and desorption. A section is devoted to each physical property. A brief section is devoted to the experimental methods of surface preparation and exposure to hydrogen. Subsections are divided according with different surface planes, namely (1 0 0), (1 1 0) and (1 1 1). The subsections are, moreover, divided per compound including GaAs, InP, GaP, GaSb and InSb. The (1 0 0) and (1 1 0) surfaces of GaAs result the most studied: the former is the surface of the MBE grown materials, the latter the cleavage one easily to be obtained, stable and suitable for case studies. Considerable work was also devoted to the (1 0 0) and (1 1 0) surfaces of InP for which the reasons of interest are the same as GaAs. From the theoretical and experimental sides all the tools traditionally used in theoretical and experimental surface physics were used. The experimental techniques include photoemission in the valence band and core level region for electronic and structural properties, optical techniques including differential reflectivity, ellipsometry and reflectance anisotropy spectroscopy, ion scattering and electron energy loss spectroscopy. Theoretical means include tight binding, self-consistent pseudopotential and density functional theory, molecular dynamics and local density approximation. The whole of available data, and more specifically for H:GaAs(1 0 0), H:GaAs(1 1 0), H:InP(1 0 0) and H:InP(1 1 0) present a good level of reliability and allow an almost quantitative description of the properties of these surfaces. The whole study is yet partially lacking in systematic, a quantitative approach to measure the doses of hydrogenation is still missing though its use would result in a great help in the direction of the quantitative correlation of surface properties with hydrogen doses. Some general conclusions can be drawn, derived mainly from the study of GaAs and InP surfaces but believed to be of general validity. The hydrogenation produces a surface compound peculiar of each surface whose properties depend on the hydrogen dose. Very mild exposures, producing one monolayer coverage i.e. about one hydrogen per surface atom, result in ordered surfaces. The general tendency is the removal of the relaxation of the clean surface structure with, in same cases, an evidence of same small counter relaxation. Electronic and vibrational properties show solid state effects like band dispersion and charge delocalisation, though with some degree of locality, preserving some memory of H-anion and H-cation

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molecular bonds. The chemisorption is not preferential, hydrogens stick both on anions and cations. High exposures produce surface dissociation and subsequent disruption of surface through the alteration of surface stoichiometry and morphology. The intrinsic mechanism at the base of disruption is related to the difference of volatility of hydrogenated compounds with anions and cations. Compounds with anions, e.g. arsine or phosphine, desorb more easily giving place to a surface richer in cations. By increasing the cations density through desorption give place to cations coalescence with the formation of drops or metallic islands e.g. of Ga or In. Acronyms AES CMA DFT DR EELS ESD GIXRD IBA IR ISS-TOF HDR HREELS LDA LEED MBE MD MDS PED PYS RAS RHEED SDA SPV UPS SDA STM TPD

Auger Electron Spectroscopy Cylindrical Mirror Analyser Density Functional Theory Differential Reflectivity Electron Energy Loss Spectroscopy Electron Stimulated Desorption Grazing Incidence X-ray Diffraction Ion Bombardment and Annealing Infrared Ion Scattering Spectroscopy with Time Of Flight Hydrogen Direct Recoil High Resolution Electron Energy Loss Spectroscopy Local Density Approximation Low Energy Electron Diffraction Molecular Beam Epitaxy Molecular Dynamics Metastable De-excitation Spectroscopy Photoelectron Diffraction Photoemission Yield Spectroscopy Reflectivity Anisotropy Spectroscopy Reflection High Energy Electron Diffraction Surface Dielectric Anisotropy Surface Photo Voltage (AI or AR) Ultra Violet Spectroscopy (Angular Integrated or Angular Resolved) Surface Dielectric function Anisotropy Scanning Tunneling Microscopy Thermal Programmed Desorption

Acknowledgements The present material was collected during more than a decade of work on this subject. The authors are indebted to several people. It is a pleasure to acknowledge L. Sorba, C. Astaldi, A. Ruocco, L. Pasquali, A. Plesanovas, M.G. Betti, S.D’Addato and A.M. Prandini for their invaluable help. Moreover

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the fruitful and stimulating discussions with C. Calandra, F. Manghi, A. Franciosi, M. Sauvage-Simkin, A. Santoni, J.A. Schaefer, G.J. Lapeyre, F. Proix and C.A. Sebenne are gratefully acknowledged.

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