Surface electronic structure of GaAs(311)A studied by angle-resolved photoelectron spectroscopy

surface s c i e n c e ELSEVIER

Surface Science 366 (1996) 121-128

Surface electronic structure of GaAs(311)A studied by angle-resolved photoelectron spectroscopy L.O. Olsson a, M. Bj6rkqvist b, j. Kanski "'*, L. Ilver a, P.O. Nilsson a a Department of Physics, Chalmers University of Technology, S-412 96 GOteborg, Sweden b Material Physics, Department of Physics, The Royal Institute of Technology, S-I00 44 Stockholm, Sweden

Received 27 January 1996;accepted for publication 4 March 1996

Abstract

The valence and core electronic structure of the sputtered and annealed 1 × 1 periodic GaAs(311)A surface has been studied by angle-resolved photoelectron spectroscopy. Five surface bands are identified and their dispersions along high symmetry lines in the surface Brillouin zone are mapped out. While electron-counting indicates a metallic surface, the experiment show no evidence for partly occupied surface bands. Analysis of the core level spectra reveals one As and two Ga surface shifted components. The results are discussed in terms of surface geometry models. Keywords: Angle resolved photoemission; Gallium arsenide; Low index single crystal surfaces; Surface electronic phenomena; Surface relaxation and reconstruction

1. Introduction

The (311 )A surface of GaAs has recently received considerable attention because of the attractive properties of heterostructures grown along this direction [ 1-8 ]. For instance, the (311)A surface allows for growth of high quality p-type material by using Si as a dopant, thereby eliminating m a n y of the problems encountered with other acceptors that have to be used for (100) surfaces [1,2]. Since also n-type doping can be achieved, this opens the possibility of device manufacturing by all Si doping [2]. The optical and vibrational properties of heterostructures grown in the (311) direction has also been a subject of interest [ 3 - 7 ] . To reduce

* Corresponding author. Fax: +46 31 16 51 76; e-mail: f [email protected]

the dimensionality within the superlattice layers a method has been developed to fabricate quantumwires in GaAs(311)A/A1As heterostructures by molecular beam epitaxy (MBE) [3,4]. This method takes advantage of an inherent ability of the GaAs(311)A surface to reconstruct, under specific conditions, and form an array of prolonged nanometer-scale facets [9,10]. It has, however, also been shown that GaAs/A1As heterostructures along (311)A can be grown with fiat interfaces [5,8]. Although all of the above mentioned features are influenced by the detailed atomic and electronic structure of the GaAs(311)A surface, there are still very few studies of these properties [ 9 - 1 2 ] . Investigations of high index surfaces also provide an opportunity to test the fundamental principles behind most reconstructions on low-index surfaces, such as reduction of the number of dangling bonds

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L.O. OIsson et al./Surface Science 366 (1996) 121-128

and the general tendency to form semiconducting surfaces by filling anion dangling bonds and leaving cation dangling bonds empty [10,13]. The structural properties of the nano-scale facets which can be grown on the (311)A surface by MBE have been studied by high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) [9,10]. This As-rich surface was found to exhibit an 8 × 1 reconstruction, with a 3.4 ,~ corrugation [10]. The GaAs(311)A surface can also be prepared by sputtering and annealing in which case a 1 × 1 periodic surface is produced [11,12]. The atomic geometry of an ideal cut forming this surface is shown in Fig. 1. The surface bilayer contains in this case two-fold and three-fold coordinated Ga-atoms, CI and C2, respectively. Another possible ideal cut of this surface contains a surface bilayer with two-fold coordinated As- and threefold coordinated Ga-atoms, which in Fig. 1 can be visualised by removing C1, thus leaving A 1 and C2 at the surface. These geometries will be referred to as Ga- and As-terminated, respectively. It is also evident from Fig. 1 that there are two inequivalent (311) faces of the zincblende crystal with As and Ga surface sites interchanged. They are distin-

guished by A and B, referring to the type of (111) plane (i.e. Ga- (A) or As- (B) termination) that lies closest in angle. Also the (311)B surface can be prepared with a 1 × 1 periodicity [ 11 ]. This surface has been studied by angle-resolved photoelectron spectroscopy (ARPES) [14]. The relaxed 1 x 1 geometry of the As-terminated GaAs(311)A and the corresponding Ga-terminated (311)B surface have been analyzed within the empirical tight binding formalism with respect to energy minimization and surface band structure [ 15]. According to this study both surface configurations are metallic. It was also found that dimerization of the As atoms would be energetically favourable for the As-terminated (311)A. The 1×1 surfaces have experimentally been studied by low energy electron diffraction (LEED), according to which the geometries of (311)A and (311)B cannot be understood merely by interchanging Ga and As surface atoms [11]. From LEED and empirical total-energy tight binding calculations, it was found that the GaAs(311)A-1 × 1 surface is stabilized by a Ga-terminated geometry [ 12]. Two distinct structures were considered with the C1 Ga-atom either in a two-fold coordinated "bridge" position, as in

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Fig. 1. Side and top views of an ideally terminated (311)A surface together with the corresponding SBZ. The surface unit cell is marked by a dashed line.

L.O. Olsson et al./Surface Science 366 (1996) 121 128

Fig. 1, or with the C1 atom relaxed downwards and bonding to Cz, i.e. in a "hollow-type" site. Although the hollow site geometry was favoured, since this was the only surface geometry which by energy-minimization was found to give a semiconducting surface, the bridge site could not be excluded. In the present study the surface atomic and electronic structure of sputtered and annealed GaAs(311 )A has been studied by means of ARPES of valence and core electronic states. Surface bands are identified, and by comparing these with the existing surface band calculations, information is obtained about the atomic geometry. Analysis of core level data provide support for the same surface geometry as suggested by valence band data.

2. Experimental The photoemission experiments were made using a rare gas resonance lamp in a Vacuum Generators ADES 400 system at Chalmers and synchrotron radiation in a modified photoelectron spectrometer, Vacuum Science Workshop, at the MAX-lab storage ring in Lund, Sweden. Both analysis systems were equipped with hemispherical electron energy analyzers (angular resolution 2°), LEED, and sample treatment facilities. The base pressure in the analysis chambers was ~1 x 10 -1° Torr. Throughout the experiments the light incidence angle was kept at 45 ° and the energy resolution was better than 0.1 eV. The detection angle was changed by rotating the energy analyzer in the plane of incidence and the electrons were detected on the opposite side of the surface normal relative to the incoming photons. The azimuthal orientation was adjusted by turning the sample around an axis coinciding with the surface normal and was checked by LEED. The sample material was taken from a polished semi-insulating GaAs(311)A wafer which was treated in vacuum by repeated cycles of 500 eV Ar + sputtering and annealing at approximately 550°C. The surface quality was monitored by LEED, which after the treatment showed sharp 1 x 1 spots with low background intensity, and, as shown below, valence band emission with well-

123

defined dispersive peaks. All published spectra were measured at room temperature. Spectra were also recorded with the sample at elevated temperatures to check for any surface photovoltage related shift. No sign of such effects were found.

3. Results and analysis Valence band spectra were recorded for different photon energies along the FM', FK, and FK azimuths in the surface Brillouin zone (SBZ). Two such sets of spectra, along the FK azimuth, are shown in Fig. 2. The data were analyzed by condensing the peak positions into "structure plots", i.e. the binding energy vs. the surface projection of the crystal momentum of the detected electrons, hklL. For a smooth surface, the latter quantity is conserved in the photo-emission process and can be calculated from kll(A-~)=0.512 sin 0, where Ekin is the kinetic energy and 0 is the emission angle (relative to the surface normal) of the photoelectron. The structure plots of the spectral series in Fig. 2 are shown in Fig. 3. To identify contributions in spectra from bulk state emission, theoretical structure plots of direct bulk interband transitions were generated. These were calculated with a scheme using initial bands obtained within the LCAO formalism and final bands in the crystal approximated with broadened free-electron parabolas in an inner potential of 9 eV relative to the valence band maximum (VBM) [ 16]. Such theoretical structure plots are included in Fig. 3. By comparing theory with experiment the strong features dispersing from below 4 eV binding energy at F towards 1 eV at ~l are identified as bulk contributions. This assignment is supported by the observation of a corresponding feature in spectra from GaAs(31 1)B [ 14]. Another bulk related structure is observed at ~7.2 eV, reflecting the high initial bulk density of states (DOS) at the bottom of the third band which in combination with weak transitions into life-time broadened final states give rise to peaks in the spectra. Before assigning the remaining peaks to surface bands the photon energy dependence and dispersion within the SBZ of these structures were

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L.O. Olsson et al./Surface Science 366 (1996) 121-128

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checked. In a structure plot surface bands must be non-dispersive with k± i.e. stationary with changing photon energy, and display the periodicity of the SBZ along kll. Fig. 4 shows the peaks along FK which are identified as surface induced. They appear to form five bands which are denoted S1-$5, with the same notations used in Figs. 2 and 5. Also included in Fig. 4 is the projection of LCAO bulk bands onto the (311) SBZ. Theory and experiment were aligned at VBM, determined as discussed below. The S1 and $5 bands are seen

only as weak shoulders in the spectra over limited parts of the SBZ but can still clearly be assigned as surface since they appear in gaps of the projection of bulk bands. Band $2 is seen over most of the SBZ for both photon energies as it disperses downwards from ~" towards K and up again along the zone boundary towards 1(,I.Along the FM (and the equivalent FM') azimuth, $2 is almost constant in binding energy. In the FK azimuth band $3 can only be clearly identified between the r" and I~ points, dispersing downwards with a bandwidth of at least 1.2 eV. In the FM and FM' azimuths $3 disperses downwards with 0.3 eV bandwidth. To form a band the K and 19I points must be connected in the FK azimuth. This is, however, not seen in the spectra, probably due to overlapping bulk features. The assignment of $4 as a surface state is less clear. In all spectra along FM and FM' and in most 16.8 eV spectra along FK, it is one of the dominant peaks. In 21.2 eV spectra, on the other hand, it is only detected occasionally. To check this assignment further, spectra were recorded with various parameter sets probing the same symmetry point in different SBZs. Peaks assigned to $4 are, as expected for surface induced features, seen at constant energy, see Fig. 5. We note also that no equivalent structures are reported in spectra from (311 )B [ 14 ]. Fig. 5 also shows that peaks identified as $2 and $3 are also found at constant energy at equivalent SBZ points, confirming their surface origin. Core level spectra of the Ga and As 3d levels measured under surface sensitive conditions are shown in Fig. 6. Spectra were analyzed in a curvefitting procedure to determine the number of constituting components, their binding energies, and relative intensities. This fitting involved Gaussian and Lorentzian broadened spin-orbit split peaks on linear backgrounds. The Lorentzian widths and spin-orbit separations were kept fixed in the numerical routine while the Gaussian broadening and branching ratio was allowed to vary between different spectra but were equal for different components in each spectrum. For the As 3d spectra it is clear from the spectral shape that they are composed of at least two components. This is confirmed in the numerical fits for several different spectra. From the systematic intensity variations

L.O. Olsson et al./Surface Science 366 (1996) 121 128

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with varying probing depth it could be concluded that the low-energy component (B) reflects bulk atomic sites while the high-energy component originates from surface atoms. Also the Ga 3d spectral shape reveals the existence of two major components. But in order to obtain consistent fits for all Ga spectra, with an additional constraint concerning the energy separation of 21.84 eV between the As 3ds/2-Ga 3d5/2 bulk levels [17], a third component had to be included on the low-energy side. This gave fits of the same quality as for the As spectra. The bulk component (B) was found to be located inbetween the two surface components ($1 and $2). It is worthwhile to mention that the similarly prepared l x 1 surface of InAs(311)A shows core levels with the same qualitative deconvolution [-18]. Considering that the surfaces of different III-V semiconductors are known to behave similarly, this is taken as strong support for the present fits with respect to the number of components and their identifications• Having identified the bulk core level components, we can determine the position of VBM relative to the Fermi level (Ev) by utilizing the

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4. Discussion From electron counting principles both the Gaand As-terminated geometries are expected to be metallic [-12,15]. On the As-terminated surface there should be a surface band with a fractional occupancy of 5/8, and the metallic surface band on the Ga-terminated surface should have an occupancy of 1/8. In the present study, however, there is no sign of any metallic surface band, since no emission is detected in the fundamental bandgap in any spectrum. Although a missing observation

74

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Kinetic energy (eV) Fig. 6. Normal emission core level spectra excited with 100 eV photons. Spectra are shown as dots together with curves of the decomposed components, the total fit, and the residue. The fitting parameters for the As3d (Ga3d) core level are: dEGauss =0.50 eV (0.36eV), AELo , =0.16 eV (0.18eV), AE,_o=0.68eV (0.45eV) and branching ratio=l.56 (1.48)• EF was located at 95.36 eV, determined by photoemission from a Mo foil in contact with the sample• cannot be taken as a firm proof of the lack of any such band (one could e.g. argue that the excitation probability is very weak), we note that occupied states above VBM should generally be easy to detect above the intrinsically zero intensity level. This surface therefore exemplifies a case where electron-counting can not be directly applied. It should be pointed out at this point that the electron counting principles are strictly applicable only under certain symmetry conditions [ 12,19]. As the (311) surface does not fulfill these conditions, our findings are not unexpected. From the surface Fermi level position it is,

L. O. Olsson et al. /Surface Science 366 (1996) 121 128

nevertheless, possible to draw conclusions about the surface band structure in the bandgap region, and therefore also about the surface geometry. The band structure calculations for an As-terminated geometry predict the existence of a partly occupied surface state, located at 0.80-1.04eV [12] or at 1.00-1.17eV [15] above VBM. For obvious reasons these results are not compatible with the pinning position determined from the present data (0.37eV above VBM). Furthermore, the As-terminated geometry was predicted to be unstable with respect to As-dimerization along [011] [15], which should result in a 2 x 1 reconstructed surface. This is not observed. From these contradictions we conclude that the As-terminated geometry is ruled out. The calculated surface band structures of the Ga-terminated geometries are both consistent with the observed surface Fermi level position. With the C~-atom in bridge site, a metallic surface band is located at 0.4-0.5eV above VBM [12]. As already noted, no such band is observed in the spectra. If the surface is allowed to relax into a "hollow site" geometry the surface is predicted to be semiconducting [12]. In this latter configuration the topmost occupied band was located 0.5 to 0.8 eV below VBM. This band is derived from the Ga-As bond between C1 and A~ (see Fig. 1) and corresponds possibly to the S1 band. Since inequivalent surface atomic sites are reflected in spectra via different binding energies of core electronic states, the validity of a surface geometry can be tested by comparing the number of deconvoluted components and the number of surface sites. Assuming that all atoms in the Ga-terminated geometry with coordinations differing from the bulk give rise to different surface shifted components, the Ga core level should, as observed, have two such components. In order to achieve an empty dangling bond orbital on the C~ atom (within the hollow site geometry) [ 12], a charge transfer is needed which increases the initial state contribution to the binding energy. This suggests that the more intense S~ peak corresponds to the C1 site in the first atomic layer and that $2 originates from C2 atomic sites in the second atomic bilayer. Applying the argument about coordination to

127

As atoms within the Ga-terminated geometry, the As core level should be fitted by a single component (bulk). However, the Al-atom is situated in the topmost atomic bilayer of the surface, so for this atom there is reason to expect a lower Madelung binding energy than for the bulk atoms. With this tentative argument it is reasonable that also As has a surface shifted component ($1), reflecting the A 1 atomic site.

5. Summary The valence band spectra of GaAs(311)A were found to contain five surface bands in addition to contributions from bulk states. The latter could be identified via a calculational scheme simulating direct bulk interband transitions, The corresponding bulk related structures were also observed in spectra from GaAs(311)B, while none of the surface bands identified on (311)A were seen on (311)B [14]. The As and Ga 3d core levels were found to consist of two and three components, respectively. By comparing these experimental results with model based calculations [12,15], a model with the topmost Ga in a three-fold hollow site was found to be favored.

Acknowledgements We would like to thank the staff at MAX-lab for technical assistance. The Swedish Natural Science Research Council is acknowledged for financial support.

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