Electronic Structure of Adsorbates on Semiconductors

CHAPTER 12

Electronic Structure of Adsorbates on Semiconductors R. MIRANDA and E.G. MICHEL Departamento de Ffsica de la Materia Condensada and Instituto Universitario de Ciencia de Materiales "Nicolds Cabrera" Universidad Aut6noma de Madrid Madrid Snain

9 2000 Elsevier Science B.V. All rights reserved

Handbook of Surface Science Volume 2, edited by K. Horn and M. Scheffler

Contents 12.1. Introduction

.................................................

865

12.2. Adsorption of hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

867

12.2.1. Adsorption of hydrogen on S i ( l l l ) 7 x 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

867

12.2.2. Adsorption of hydrogen on Si(100)2 x 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

872

12.2.3. Adsorption of hydrogen on III-V semiconductors

........................

12.3. Oxidation of silicon surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

874 875

12.3.1. Oxidation o f S i ( l l l ) 7 x 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

875

12.3.2. Oxidation of Si(100)2 x 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

877

12.3.3. Oxidation of III-V semiconductors

................................

12.4. Adsorption of other species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4.1. Adsorption of water 12.4.2. Nitridation

........................................

............................................. .............................................

886

12.6.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.6.3. Group IV + halogens

886 .......................

.......................................

12.6.4. Group III-V compounds + halogens

882 883

12.6. Adsorption of halogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.6.2. Adsorption-site symmetry and polarization effects

879 880

12.4.3. Adsorption of other elements and compounds . . . . . . . . . . . . . . . . . . . . . . . . . . 12.5. Surface passivation

878 879

...............................

12.7. Adsorption of C60 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

887 888 892 892

12.8. Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

893

References

893

.....................................................

864

12.1. Introduction

A general overview of the electronic structure of adsorbates on semiconductors is not easy to construct. Ideally, one would like to determine experimentally and understand theoretically the trends of adsorption kinetics, saturation coverages, adsorption sites, bond lengths, charge transfer, type of bonding and electronic states, since all these topics are interwoven. Reviews on several of these aspects are available (Williams et al., 1980; M6nch, 1990). Here, we restrict ourselves to a tutorial description of the electronic structure of some model adsorbate/semiconductor systems. The most widely used experimental techniques to gain information on the occupied and empty electronic states are ultraviolet photoemission spectroscopy (UPS) (Himpsel, 1983) and inverse photoemission spectroscopy (IPS) (Himpsel, 1990), respectively. By measuring energy and angular distributions of photoemitted electrons, one obtains density of states and electronic band dispersions E(k). Using large photon energies, core-level excitations and the corresponding chemical information, are also accessible (Htifner, 1996). The invention of the scanning tunneling microscope (STM) and its development as an spectroscopic tool (Stroscio and Kaiser, 1993) has allowed investigators to probe the spatial distribution of electronic properties at the atomic level. Spatial mapping of the distribution of electronic states with the STM is possible by recording I-V curves at selected positions on the surface, obtaining the differential conductivity dI/dV and normalizing to I/V. The resulting quantity, (dI/dV)/(I/V), is proportional to the local density of states (Stroscio and Kaiser, 1993). A look at the literature suffices to realize that for a number of years the experimental data for nominally identical systems, e.g., H/Si(111) or O2/GaAs(110), have been contradictory and a systematic picture was absent. There are good reasons for this controversy. First, electronic structure and adsorption geometry are strongly interdependent. Most semiconductor surfaces reconstruct and the corresponding atomic structure has been elucidated only recently. Second, the spectroscopic techniques available, such as photoemission spectroscopies (PES) or inverse photoemission (IPS) average over 1012-1014 atomic sites on the surface. Surface homogeneity at comparable scales is then very important to analyze the electronic spectra. However, it is difficult to prepare in a repetitive fashion semiconductor surfaces which are clean and homogeneous over these lateral scales, unless one has experimental techniques able to characterize the surface at the required level. Luckily, the situation started to change in recent years. STM topographs can characterize the cleanliness and perfection of a surface at a scale not previously achievable by standard techniques like Auger electron spectroscopy (AES) and low energy electron diffraction (LEED), while recording local spectroscopic information. In this way, connection to averaging spectroscopies can be achieved. An example of the importance of a combined characterization of "clean" semiconductor surfaces is given in Figs. 12.1 and 12.2. Following a common method to prepare a 7 x 7 reconstructed Si(111) surface, the sample was

865

866

R. Miranda and E.G. Michel

Fig. 12.1. STM images of a flashed S i ( l l l ) 7 • 7 surface (left, 2700 • 2700 ,~2, right 10200 x 10200 ~2). The white protrusions are SiC clusters.

Fig. 12.2. LEED patterns corresponding to the surface shown in Fig. 12.1. Left, 30 eV, right, 60 eV.

cleaned by flashing out the native oxide in UHV conditions. The LEED pattern is reproduced in Fig. 12.2. It corresponds to a good quality 7 x 7 reconstruction. The AES spectrum indicates that the sample was clean within the sensitivity of AES, that is, the C/Si ratio of the peaks was 1/ 150, just at the level of noise of the electron spectrometer. The large scale STM images reproduced in Fig. 12.1, however show that 15% of the surface is covered by SiC clusters 30 ,~-high, with a density of 6.5 x 10 l~ cm -2. Zooming in between the clusters shows that the 7 x 7 reconstruction is present in these regions (Vazquez de Parga, 1992). It is quite obvious that extracting information on adsorption onto this Si(111)-7 x 7 surface from averaging techniques may result difficult. In recent years the combination of PES, IPS and topographic and spectroscopic STM has started to yield a consistent picture of adsorption on some semiconductor surfaces. The summary of our present knowledge is the following: the chemical reactivity of semiconductor surfaces is determined by the dangling bonds, i.e., the surface states. Adsorbates

Electronic structure o f adsorbates on semiconductors

867

form chemical bonds with semiconductor surfaces in an attempt to saturate the dangling bonds. This process is accompanied by local deformations of bond angles that modify the strain energy of the surface. Stable structures result when the strain energy is overcompensated by the energy gain resulting from the saturation of the dangling bonds. In general, simple electron counting arguments are enough to find a reasonable starting point for the adsorption site and resulting surface electronic structure. In the case of homopolar semiconductors, as silicon or germanium, the truncated sp3-bonds give rise to surface states in the band gap between valence and conduction band. Their dispersion depends on the orbital symmetry at the surface, and they are strongly affected by defects and adsorbates. Thus, the adsorption of hydrogen or the formation of a SiO2/Si interface completely eliminate the electronic states from the band gap. This process takes place through formation of strong bonds and displacement of the corresponding dangling bonds into the valence band. Both the intrinsic electronic surface states of the clean surface and the molecular orbitals of the adsorbate are involved in this process. A similar behavior is found for adsorption on many heteropolar-covalent semiconductors, as GaAs, InP, etc. These materials also exhibit surface states (not always in the bulk band gap in this case). The surface states can be classified in a simplified model as coming from anion or from cation sp3-orbitals (M6nch, 1990). Their exact location in energy depends on the local geometry, giving rise in some cases to strong energy dispersions with parallel momentum that is detected by angular resolved photoelectron spectroscopy (ARPES) (Himpsel, 1983). In the following we shall concentrate in some selected examples which mostly represent old, but still relevant, problems.

12.2. Adsorption of hydrogen Semiconductor surfaces, in particular Si, are inert with respect to exposures to H2 molecules (Pretzer and Hagstrum, 1966), which have a large dissociation energy of 4.52 eV. Hydrogen atoms, on the other hand are easily adsorbed on Si and the bonding strength is 3.9 eV (notice that a Si-Si bond amounts to 2.32 eV). Accordingly, hydrogenation is usually achieved by exposing the sample to H2 pressures in the presence of hot tungsten filaments, which dissociate H2 into H atoms. The filaments are placed close to the sample and the intensity passing through them is carefully controlled. Since, in most cases, the actual rate of arrival of H atoms to the surface is not known, the exposure is usually given in Langmuirs (1 L = 1 x 10 -6 Torr s) of H2, but one has to keep in mind that actual H exposures in different laboratories can be difficult to compare. The use of sources specially designed to produce atomic hydrogen is also possible, and in this case a more accurate control of the exposure can be achieved.

12.2.1. Adsorption of hydrogen on S i ( l l l ) 7 x 7 The Si(111) 7 x 7 reconstructed surface is a prototype (Schlier and Farnsworth, 1959). Its metallic character and atomic structure, given by the dimer-adatom-stacking fault model (Takayanagi et al., 1985), are widely accepted. The electronic structure is described in detail elsewhere in this book. Three occupied surface states, S1, $2 and $3 are detected in

R. Miranda and E.G. Michel

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Binding Energy [eV] Fig. 12.3. Angle-integrated ultraviolet photoemission spectra of clean Si(111) 7 x 7 and a hydrogen-saturated Si(111):H "7 x I" surface. The difference spectrum shows the lost surface state emission and the peaks due to Si-H bonds. The spectra have been taken with a photon energy of 21.2 eV.

angle-integrated PES (Himpsel and Fauster, 1984; Uhrberg et al., 1985; Chrost et al., 1995) (see Fig. 12.3) and IPS data (Himpsel and Fauster, 1984; Fauster and Himpsel, 1983). ARUPS studies show that the bands corresponding to S 1 and $2 are quite flat, indicating that these features are highly localized in real space and atomiclike, as expected for isolated dangling bonds. STM has identified the spatial location of these surface states (Hamers et al., 1986). The state S 1, producing the metallic edge at the Fermi level and with a maximum at - 0 . 3 eV corresponds to a half-filled dangling bond state located at the adatoms that form part of the 7 x 7 reconstruction. The state $2, appearing as a well defined peak at - 0 . 9 eV, is the filled dangling bond state situated on the rest atoms, while $3, the filled surface state at - 1 . 8 eV, is related to the backbonds between the Si adatoms and the three Si atoms directly underneath (Hamers et al., 1986). The intensity of the state $1 is a good test of the quality of the surface reconstruction in Si(111). Saturation exposure of Si(111) 7 x 7 to atomic hydrogen produces the Si(111):H 7 x 1 structure with a coverage of 1.25 ML (Culbertson et al., 1982). The effect of saturation hydrogenation on the valence band spectrum of Si(111) 7 x 7 is illustrated also in Fig. 12.3. The features associated to the surface states $1, $2 and $3 do not appear in the spectrum of the saturated surface. In fact, the formation of covalent Si-H bonds completely removes the surface states of Si(111) 7 x 7 (Chrost et al., 1995; Ibach and Rowe, 1974; Sakurai and Hagstrum, 1975; Eastman et al., 1979). In the difference spectra, two hydrogen-induced peaks appear at - 5 . 4 5 and - 6 . 8 eV with respect to the Fermi level. Hydrogen is the simplest adsorbate with only one s-electron. Calculations for the monohydride Si(111):H 1 x 1 surface (Schltiter and Cohen, 1978) have been performed quite long ago and indicate that the state at - 5 . 4 5 eV is the Si-H bonding state (the corresponding antibonding state ap-

Electronic structure o f adsorbates on semiconductors

869

Fig. 12.4. STM topographs of the surface following a low coverage exposure to H atoms. The area is 200 • 200 ,~2, recorded at + 1 V (left) and -+-3 V (right). From Boland (1991b).

pears at +3.5 eV, i.e., above the Fermi level). The feature at - 6 . 8 eV corresponds to the bulk Si sp-band emission (at - 7 . 4 eV in the clean crystal), enhanced and shifted to lower binding energy. The sequence of the hydrogenation reaction can be summarized as follows: the adsorption initiates at the corner-hole dangling bonds, as first shown by infrared reflection spectroscopy (Chabal, 1983), then the dangling bonds of adatoms start to become saturated with hydrogen. The backbonds of the Si atoms are under strong tensile stress and are, thus, also easily attacked by hydrogen. Accordingly, surface states S1 and $3 disappear first from the UPS spectrum. After removal of the Si adatoms, the restatoms are saturated and the $2 states vanish. This sequence of events has been visualized by STM. Figure 12.4 shows STM images recorded after submonolayer exposure to hydrogen (Boland, 199 l b). The images show the disappearance of both occupied and empty adatom dangling bonds, as indicated by some adatoms turning dark during exposure (Tokumoto et al., 1990; Sakurai et al., 1990; Sakurai and Hagstrum, 1976; Boland, 1993). It turns out that the Si adatoms remain at their original T4 site for these low hydrogen exposures. Only the dangling bond $1 has been removed from the energy window imaged by the STM due to Si-H bond formation. The "lost" Si adatom reappears in the images at sample bias voltages above 2.2 eV, which image the empty states. The reappearance of the reacted adatoms is due to the broadening of the antibonding Si-H state induced by hybridization with conduction band states. Figure 12.5 reproduces scanning tunneling spectroscopy data recorded at selected sites on the 7 • 7 unit cell before and after hydrogen exposure (Boland,1991b; Sakurai et al., 1990; Sakurai and Hagstrum, 1976). For the clean surface, occupied surface states are visible as peaks for negative sample bias, while empty dangling bonds are detected at positive sample bias. The spectrum obtained over the dark adatoms does not show the filled and empty dangling bonds characteristic of the clean surface. Furthermore, the spectrum recorded over the restatoms (only visible as saddle points in the filled state images) proves that some occupied restatom dangling bonds ($2 state in Fig. 12.3) have been also eliminated at this early stage of the hydrogenation reaction. The empty dangling bonds at 0.5 eV

870

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Fig. 12.5. I - V curves recorded over specific sites of the surface: (a) unreacted adatom, (b) reacted adatom, (c) unreacted rest-atom, (d) reacted rest-atom. On the saturated surface (e) and (f) were recorded over an isolated adatom and a restlayer atom, respectively. From Boland (1991b).

Fig. 12.6. STM topograph of the saturated surfacerecorded at a samplebias of +2 V. The area shown corresponds to 230 x 130 ~2. From Boland (1991b).

above the Fermi level have been observed to disappear during hydrogenation by means of IPS (Bouzidi et al., 1992) and ELS (Eastman et al., 1979), while the removal of the occupied dangling bonds was shown before by means of UPS (Ibach and Rowe, 1974; Sakurai and Hagstrum, 1975; Eastman et al., 1979). At this stage of the reaction the surface coverage is 0.4 ML and the reaction with the adatoms has led to limited etching of the Si adatoms and binding to the dangling bonds of the next layer. The room temperature (RT) saturated surface is imaged in Fig. 12.6 (Boland, 1991b). Many adatoms have been removed from their original sites, allowing the underlying (intact) layer of restatoms to be visible. The adatoms have rebonded to form clusters. Some of the

871

Electronic structure o f adsorbates on semiconductors

H/Si(111 ) 7 x 7 4.75 -

~ 4.70 E .9 "5 4.65 C ~ 4.60 0

4.55 4.50 0

I 500 e x p o s u r e (L)

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Fig. 12.7. Photoelectric work function changes corresponding to the process of adsorption of H on Si(111)7 x 7. From Chrost et al. (1995).

Si adatoms are etched away from the surface since Sill3 and Sill4 desorption occurs during hydrogen exposure at RT (Boland, 1993). Obviously, the surface is far from an ideally Hterminated Si(111). Thus, detailed comparison with theory is difficult. The corresponding work function changes are reproduced in Fig. 12.7 (Chrost et al., 1995). The clean Si(111)7 x 7 surface has the Fermi level pinned at the surface and a work function of 4.58 -4- 0.01 eV. Exposure to 50 L of hydrogen reduces slightly the work function, while further exposures up to 350 L increase it by 150 meV. Notice that the work function change is not saturated at 350 L exposure. It rather continues with a slow increase up to the largest exposure employed (1100 L). For exposures up to 100 L, no changes are detected in the LEED pattern, which is still 7 x 7. At larger exposures the LEED pattern shows the disappearance of the fractional order spots, except those connecting the 1 x 1 spots. This is referred to as the "7 x 1" or "7 x 7" structure (Sakurai and Hagstrum, 1975). ARUPS measurements indicate that the monohydride phase is formed on the Si(111):H 7 x 1 surface (Karlsson et al., 1990). The evolution of the work function can be understood easily: H is more electronegative than Si. H-induced surface dipoles will then have the negative charge outwards, producing a work function increase. The initial decrease is probably due to the elimination of the band bending. The thermal desorption process of atomic hydrogen chemisorbed at RT on Si(111) represents an additional test of the capability of UPS to quantify the density of surface states. In effect, STM images prove that an undisturbed 7 x 7 surface is not fully recovered after complete desorption of hydrogen at 750 K, since limited etching of the Si adatom layer leading to Sill3 and Sill4 desorption occurs during hydrogen exposure at RT (Boland, 1993). The effect of annealing the H-saturated RT surface to increasing temperatures is shown in Fig. 12.8 (Boland, 1993). The intensity of the H-related emission at - 5 . 4 eV increases at 520 K before any desorption of hydrogen is detected. This might be related to some reordering of the adlayer. The desorption is noticeable at 650 K (Chrost et al., 1995) where the surface states are detected again and the Si-H peak disappears. Hydrogen

872

R. Miranda and E.G. Michel

hv = 21.22eV Desorption of Hydrogen oo r E~

_d L------=

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Binding Energy [eV] Fig. 12.8. Angle-integrated UPS spectra taken during desorption of hydrogen from the Si(111) surface. Note the reappearance of the surface states above 650 K. The intensity of S 1, however, is smaller than in the case of Fig. 12.3.

desorbs completely at 750 K leaving behind a residually disrupted Si(111) 7 x 7 surface, as judged from the reduced intensity of the adatom surface state S 1. STM, LEED and UPS show that a perfect 7 x 7 surface is obtained only after annealing to 900 K, which allows for recreation of an ordered layer of Si adatoms by surface diffusion. 12.2.2. Adsorption of hydrogen on Si(lO0)2 x 1 In contrast to Si(111) 7 x 7, both UPS and STS studies show that the clean Si(100) 2 • 1 surface is semiconducting. The reconstruction consists in tilted Si-Si dimers forming chains on the surface. The STS spectrum of the clean surface reproduced in Fig. 12.9 (Boland, 199 l a) shows an occupied state at - 0 . 9 eV and the unoccupied counterpart at 0.5 eV above the Fermi level. The states are the bonding and antibonding combinations derived from the Jr interaction of the dimer dangling bonds. At low exposures, hydrogen atoms react at RT with the dangling bonds of Si(100) 2 x 1 yielding the 2 x 1 monohydride surface shown schematically in Fig. 12.10, where the dimer structure still exists (Sakurai et al., 1990; Sakurai and Hagstrum, 1976). The dimers, however, are no longer tilted but they become symmetric. This removes the dimer-related surface dipole and then changes the ionization energy of the surface. This is the reason behind the observation illustrated in Fig. 12.11 of a work function decrease during hydrogenation of Si(100) 2 • 1 (Oura et al., 1990; Koke and M6nch, 1980). Because of the larger electronegativity of H (2.2) with respect to Si (1.9), H-induced dipoles will exist with the negative charge outwards. Considering the surface density of dipoles, the work function is expected to increase by +0.8 eV. The reported negative work function change of - 0 . 4 eV reflects the difference between the decrease in the ionization energy (1.14 eV) produced by

873

Electronic structure of adsorbates on semiconductors

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Si(001):H-2 x 1

Si(001):H-1 x 1

(a)

(b)

Fig. 12.10. Schematic representation of a monohydride (a) and dihydride (b) Si(100) surfaces.

the symmetrization of the dimers and the increase due to the Si-H surface dipole. A similar observation has been reported for H/GaAs(110), another surface where tilted dimers exist (Koke and M6nch, 1980). The initial stages of the reaction are illustrated by the STM image of Fig. 12.9. In addition to unreacted dimer rows, bright, ball-like spots and dark dimers are seen. The corresponding STS spectra for the different reacted sites are displayed in Fig. 12.9 also. The bright features reveal two states at +0.5 and - 0 . 5 eV due to unpaired dangling bonds that remain after reaction of the dimer with one hydrogen atom. The dark spots present an empty state at 4-1.2 eV, assigned to dimers reacted with two H atoms. For larger exposures of hydrogen, the uptake reaches a coverage of 1 ML and the LEED pattern changes to 1 x 1 (Sakurai et al., 1990; Sakurai and Hagstrum, 1976). The LEED analysis shows the surface to be bulk-like. The dimer bonds are broken and the two dangling bonds per surface atom are saturated with hydrogen. The resulting dihydride structure is also depicted in Fig. 12.10.

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12.2.3. Adsorption of hydrogen on III-V semiconductors We consider now the adsorption of hydrogen on III-V semiconductors, and in particular on GaAs(110), the best studied example. Cleaved GaAs (110) surfaces are 1 x 1 reconstructed by forming chains of Ga-As dimers tilted away from the horizontal. The empty dangling bonds are localized in the protruding Ga atoms, while the occupied dangling bonds are located on the As atoms. Both kinds of surface states are energetically placed outside the bulk band gap and the bands are fiat up to the surface. In the first interaction step, atomic hydrogen is chemisorbed on both Ga and As atoms (del Pennino et al., 1993), surface acceptors are induced in the bulk band gap and the GaAs(110) surface derelaxes, with final positions close to those of the bulk terminated surface (Ruocco et al., 1995). The occupied As dangling bonds disappear from the UPS spectra, the Ga empty dangling bonds are also saturated during hydrogenation. The Ga-H

Electronic structure of adsorbates on semiconductors

875

and As-H bonding levels are well below the valence band edge and doubly occupied. The antibonding As-H level is the surface acceptor level. The ionization energy decreases by 1.2 eV. In a second stage, the substrate is dissociated into metallic Ga and AsH3 molecules, and the surface is disordered (Sorba et al., 1990). Thus, the initial adsorption stage does not differ much from the phenomenology of Si surfaces, but in the second stage the Ga-As bonds are broken to a much larger extent than in Si. In recent years a number of experiments on hydrogen adsorption on GaAs(100) have also been carried out (Pahlke et al., 1996). Atomic hydrogen was expected to modify the surface in a simple way by saturating the dangling bonds, but the interaction turned out to be complex and is not yet fully understood (Kawabe, 1995). Nevertheless, the practical use of H as a surfactant for the growth of GaAs (Pahlke et al., 1996) and InAs on GaAs(100) (Yong et al., 1995) ensures further future work on this system.

12.3. Oxidation of silicon surfaces The formation and properties of the SiO2-Si interface is one of the most investigated subjects in surface science (Engel, 1993). This is because silicon device technology is largely based on the unique ability to fabricate SiO2/Si interfaces with electrically active interface states densities of less than 1 defect per 104-105 interfacial bonds (Balk, 1988). Due to this reason it has been long desired to control the oxidation process at atomic level. This goal is crucial to obtain oxide layers atomically flat with controlled thickness (Balk, 1988). Many experimental and theoretical works have been devoted to the understanding of the reaction process. For a detailed account on the literature on this subject we refer the reader to Engel (1993). One of the most debated topics are the atomistic processes involved in the initial stages of the reaction. The first step of the reaction is dissociation of the oxygen molecule. This is also the rate limiting step in most models. The existence of a molecular precursor is now established, but it is not yet clear which are the first steps of atomic oxygen chemisorption (Weldon et al., 1997). This issue is now of high interest since the device dimensions have decreased so much that one oxide monolayer represents about 1/3 of the whole gate oxide (Weldon et al., 1997). A lot of information has been obtained by measuring the valence band of partially oxidized silicon, and comparing it to theoretical calculations (H6fer et al., 1989b; Dujardin et al., 1994). The Si 2p-core level has also provided a lot of information of the oxidation process (Hollinger and Himpsel, 1983; Mascaraque et al., 1997). As shown in Fig. 12.12, Si atoms bonded to one, two, three or four O atoms exhibit Si 2p-core level with binding energy shifts of ~0.95 eV per O atom (Mascaraque et al., 1997). First-principles investigations support this image (Pasquarello et al., 1996). 12.3.1. Oxidation of S i ( l l l ) 7 x 7 Many studies were devoted in the past to the investigation of the initial stages of oxygen reaction with Si(111)7 x 7, and in particular to the existence of a molecular precursor for the oxidation reaction. Some years ago it was found using high resolution electron energy

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BINDING ENERGY (eV) Fig. 12.12. Lower panel: deconvolution of the Si 2p-core level from a clean surface in different components. Upper panel: deconvolution of the Si 2p-core level after exposure to 0.7 L oxygen (from Mascaraque et al., 1997).

loss spectroscopy (HREELS) that at the initial stage of adsorption, most of the oxygen is dissociated, and occupies on-top sites (Si-O) and bridge sites (Si-O-Si) (Edamoto et al., 1985). X-ray photoemission spectroscopy (XPS) results at low temperature have provided evidence for a molecular precursor state (H6fer et al., 1989a, b; Morgen et al., 1989). The negatively charged, superoxide-like, bridging molecule is formed by a process of"harpooning" whereby electrons from the adatom dangling bonds tunnel to the 2re* affinity level of the neutral oxygen molecule. Incident molecules probe the surface on a physisorbed precursor until they get trapped by harpooning at sites of high density of occupied states at the Fermi level, i.e., at corner-adatom sites. The molecular chemisorbed species has a lifetime of 14 min at 300 K and 60 min at 150 K. The molecular or atomic nature of adsorbed oxygen is reflected in the peaks appearing in the valence band. For atomic oxygen there is a single major peak in the UPS spectrum due to re-bonded O Px, Py at about - 6 eV. Molecularly chemisorbed oxygen is characterized by two peaks with O 2pz character at - 11 and - 3 . 5 eV (Schubert et al., 1993). STM images such as displayed in Fig. 12.13 have shown the existence of two different reaction stages, which appeared as "bright" or "dark" sites in the pictures (Avouris, 1990;

Electronic structure o f adsorbates on semiconductors

877

Fig. 12.13. Upper panel: 70 x 70 ~2 STM topograph of a 0.15 L 0 2 exposed Si(111)7 x 7 surface. Lower panel: UPS spectrum of a 0. 3L exposed Si(111)7 x 7 surface. From Dujardin et al. (1994).

Avouris and Wolkow, 1989; Avouris and Lyo, 1991; Avouris et al., 1991; Neddermeyer, 1996). Detailed tight-binding slab calculations (Schubert et al., 1993; Lyo and Avouris, 1991) identified the initial "bright" site with an oxygen atom inserted in one of the adatom backbonds, while the "dark" site was explained by one oxygen atom on-top and a second one in backbond position. More recently, it has been found that the molecular precursor has a much longer lifetime (Dujardin et al., 1994; Sakamoto et al., 1996), ranging between 400 min and 1500 min, depending on exposure and sample. The STM and valence band photoemission results shown in Fig. 12.13 (Dujardin et al., 1994, 1996) supported that the "bright" STM site corresponds to molecular oxygen, in particular the peak at - 3 . 8 eV below the Fermi energy is a molecular band of O 2pz character and that these molecular sites react very efficiently with further oxygen molecules (and residual H20) to produce the "dark" site, that was associated to various states of dissociated oxygen (Dujardin et al., 1994, 1996). The shorter lifetimes reported before were attributed to residual H20, which might explain the well-known effect of even trace amounts of H20 on accelerating the thermal oxidation of Si by oxygen. 12.3.2. Oxidation of Si(lO0)2 x 1

High-resolution electron loss spectroscopy (HREELS) (Schaefer and G6pel, 1985) results suggested that oxygen atoms are inserted in the Si backbonds, bridging between the top and second layer Si atoms, followed by further adsorption of oxygen at the same Si atom. Surface-extended X-ray-absorption fine-structure (SEXAFS) (Incoccia et al., 1987) supported the existence of bridging sites between the two top layers, but also between dimers in the top Si layer. STM has been used also to analyze the oxidation process at room temperature (RT) and has provided a wealth of information on the reaction process (Avouris, 1990; Avouris and Wolkow, 1989; Avouris and Lyo, 1991; Avouris et al., 1991; Kliese et al., 1992; Udagawa et al., 1992). However, a conclusive assignment of the observed phenomena to precise adsorption or reaction events has not yet been reached. Again the main change after exposing the surface to oxygen is the appearance of bright spots in the STM pictures. Their identification, however, is controversial. While Avouris (Avouris,

878

R. Miranda and E.G. Michel

1990; Avouris and Wolkow, 1989; Avouris and Lyo, 1991; Avouris et al., 1991) identified the spots as ejected Si dimers, Kliese et al. (1992) suggested that they are a weakly bound oxygen species, either atomic or molecular. They found also small protrusions at the earliest stages of oxidation, frequently bridging between two neighboring dimer rows. These protrusions induced local buckling. From a theoretical point of view this interface has recently been studied by Miyamoto (1992) who found three metastable sites, the most stable being an oxygen atom inserted in the dimer bond. Other sites found were the on-dimer and the backbond site. At variance with these results, Uchiyama and Tsukada (1997) have proposed that the most stable site is the backbond on the lower dimer atom, followed by backbond of the upper dimer atom, and finally an oxygen atom bridging between the two Si atoms of a dimer. The thermodynamics of the oxygen insertion and migration has been calculated by Stefanov and Raghavachari (1997). They found that the dimer bond is the initial target of O entry and an oxygen-inserted dimer was proposed as the most likely structure at low temperature. At higher temperatures, an asymmetrically oxidized dimer unit with three oxygen atoms inserted into the Si-Si bonds at the same Si was the dominant feature.

12.3.3. Oxidation of lII-V semiconductors The interaction between oxygen and III-V semiconductors has been investigated for many years. We shall review next the most studied substrate, GaAs. The adsorption of oxygen on cleaved GaAs(110) induces surface acceptors and proceeds via a rather slow kinetics. At room temperature, an exposure of 1 mbar is needed to get 1 ML coverage (Ranke and Jacobi, 1979; Kraus et al., 1989). The sticking probabilities are in the range of 10 -5 to 10 -6 (Bartels and M6nch, 1990; Alonso et al., 1985; Ranke et al., 1982), much smaller than on Si(111)2 x 1. Oxygen is adsorbed dissociatively above 60 K, as evidenced by the formation of Ga- and As-oxides. The mechanisms proposed do not differ much from those of silicon surfaces, but the existence of two chemically different atoms provides with two distinct targets for oxygen atoms. Ga20 is desorbed at temperatures around 800 K (Ranke and Jacobi, 1979; Landgren et al., 1984; Tone et al., 1992). The oxidation process depends critically on the experimental conditions. For instance, the presence of excited oxygen molecules in the gas enhances very much the oxidation rate (Kraus et al., 1989). Illumination or X-ray irradiation of the surface may influence both the oxidation rate and the chemical properties of the oxide (Bartels and M6nch, 1990; Bartels et al., 1983; Hiratani et al., 1990; Taneya et al., 1990; Anderson et al., 1990; Seo et al., 1990). This high sensitivity to the experimental conditions has hindered the elucidation of the oxidation process. In recent studies (Verheij et al., 1995a, b), it was found that oxygen adsorbs in molecular form on GaAs(100) below ~370 K. Annealing above 425 K gave rise to an irreversible disordering process concomitant with the dissociation of the incident oxygen molecules (Verheij, 1997). Adsorption on GaAs(100) above 470 K gives rise to direct oxygen dissociation. It was proposed that oxygen adsorbs initially in molecular form. Then, a reaction of oxygen molecules and surface atoms (stimulated by temperature, photons or electrons) produces defects, where incident oxygen molecules are rapidly dissociated.

Electronic structure of adsorbates on semiconductors

879

12.4. Adsorption of other species 12.4.1. Adsorption o f water

Studies on H20 adsorption on silicon and germanium were started in the 60's. Early work has been reviewed by Meyer and Spaarnay (1975). In brief, early gas volumetric studies of H20 uptake by crushed germanium demonstrated an initial fast adsorption of which the saturation point was labeled monolayer coverage. Later studies showed that it in fact corresponded to somewhat less than one monolayer (Meyer and Spaarnay, 1975). The adsorption was deemed dissociative, and this was supported by the observation of the H2 evolution upon annealing. Henzler and Topler (1973) performed extensive LEED and Auger studies on the adsorption of H20 on Ge(111) surface. A few percent of saturation coverage removed the LEED superstructures (2 x 1 or c(2 x 8)). Later studies (Sinharov and Henzler, 1975) revealed that for low coverage, only molecular H20 was desorbed from the 2 x 1 surface, compared with H20 and H2 from the c(2 • 8) surface, but it could not be concluded from the data whether the mechanism was dissociative or non-dissociative. For high coverages on the c(2 x 8), the mechanism was dissociative. Fujiwara and co-workers (Fujiwara and Ogata, 1979; Fujiwara, 1981) have interpreted their photoemission, Auger and EELS results in the non-dissociative model (for Si(111)7 x 7), with a single state of adsorption up to 850 ~ The photoemission spectra of the saturated surface were interpreted in terms of the molecular orbitals of H20. Other authors have pointed out the difficulty to distinguish in UPS molecular orbitals from a mixture of OH and H dissociatively adsorbed (Buchel and Ltith, 1979). STM results (Avouris and Lyo, 1991) are consistent with a dominant dissociative chemisorption on the adatoms. The hydroxyl groups tend to form islands on the surface. Electron energy loss spectroscopy (EELS) (Nishijima et al., 1986; Ibach et al., 1982; Kobayashi et a1.,1983; Sch~ifer et al., 1984), infrared spectroscopy (Chabal and Christmann, 1984), and valence band photoemission (Poncey et al., 1995) have supported also dissociative chemisorption. We may note that the number of studies devoted to the adsorption of water on highsymmetry surfaces is small. Conflicting results were reported concerning the adsorption on Si(100). HREELS (Ibach et al., 1982) and ellipsometric studies (Meyer and Spaarnay, 1975) conclude that water adsorbs dissociatively on Si(100) at RT, while other UPS studies (Schmeisser, 1984; Schmeisser et al., 1983) favored molecular adsorption at RT. It was claimed that water was monomerically physisorbed at 100 K on Si(100), without hydrogen bonding between water molecules for coverages below 0.5 ML, at variance with adsorption on metallic substrates. The UPS spectra show the emission peaks, attributed to water molecular orbitals (b2, al, bl). The width of the two peaks at higher binding energies give information on the water-water interaction. The surface state emission is quenched after 0.5 L exposure, and a strong work function decrease takes place (-0.95 eV). The conflicting results on the nature of adsorption at RT (or after warming) have been solved in favor of a predominantly dissociative adsorption at RT, forming Sill and SiOH (Thiel and Madey, 1987; Waltenberg and Yates, 1995; Struck et al., 1997; Ranke, 1996). STM images show that the two dangling bonds are occupied simultaneously as the result of each chemisorption event (Andersohn and K6hler, 1993), and that these occupied sites can be resolved into atomic features of different intensities (Chander et al., 1993b). Dissociative

880

R. Miranda and E.G. Michel

chemisorption represents the favored process from a thermodynamic point of view, but involves an activation energy. Thus, also molecular chemisorption can be observed. Adsorption at low temperature takes place molecularly. Upon warming to RT the spectrum changes to indicate dissociation in Si-H and Si-OH species. Further annealing leaves only O chemisorbed. Weldon et al. (1997) have found that the Si-Si dimer bond is the target for the initial insertion of oxygen into the Si(100)2 • 1 surface, following H20 exposure and annealing. They reported also the observation of HSi-SiH, HSi-O-SiH, and HSi-OSi(O)H dimers. The adsorption of H20 has been investigated on III-V (110) surfaces as well (Buchel and Ltith, 1979). Two distinct molecular phases were observed. For adsorption at 180 K a physisorption-like state is observed. For room temperature adsorption, a shift in the orbitals suggests that the molecule is chemisorbed with the bl oxygen lone pair next to the surface, thus causing band bending of the n-GaAs sample. This behavior is analogous to the Si(100) case, the only difference being that further adsorption at RT causes a change in the UPS spectrum so that it looks similar to the spectrum obtained at 180 K. A concomitant shift of the work function by 1 eV was detected, suggesting the following interpretation: a reordering of H20 molecules takes place, so that the dipole field is directed towards the surface. The two adsorption phases on GaAs are also seen in surface photovoltage measurements. By contrast Ranke et al. (1987) reported that SH2 adsorption on GaAs(110) is molecular. Dudzik et al. (1995) proposed a dissociative model at high coverages. 12.4.2. Nitridation

The nitridation of silicon surfaces has been extensively studied for more than thirty years. The growth of silicon nitride (SiNx (x ~< 4/3)) is important from the point of view of electronic applications. Thermal nitridation with N2 gas was primarily used, but it was found that a too high temperature (>~ 1200 ~ was required to obtain a continuous film (Ito et al., 1978a). More reactive species, such as NH3 (Ito et al., 1978b; Mrarka et al., 1981) and plasma-excited NH3 (Ito et al., 1981) were used instead of N2, resulting in a better quality silicon nitride film. More recently, atomic N has also been employed (Tabe and Yamamoto, 1997). The adsorption of NH3 on Si(111) 7 x 7 results in dissociation of the molecules even at temperatures as low as 100 K (Kubler, 1987; Boszo, 1988), giving rise to NH2 + H. The most reactive sites correspond to the rest atoms of the 7 • 7 reconstruction (Lyo and Avouris, 1991). Figure 12.14 (Avouris et al., 1990) reproduces UPS spectra during annealing of an adsorbed layer from 90 K to 1000 K. At 350 K, the spectrum shows peaks characteristics of dissociated NH3. In fact, H, NH2, and NH species are detected at the surface. The STM image of Fig. 12.15 (Avouris et al., 1990) shows a real space image of the adsorption process. Most of the adatoms, except the corner hole adatoms at the edge of the 7 x 7 unit cell, have disappeared. Deposition of B on Si(111) 7 x 7 and annealing results in a ~/3 x ~/3R30 ~ surface which is semiconducting according to UPS and STM (Lyo et al., 1989). In order to reduce the strain energy of the 7 x 7 reconstructed surface, B atoms occupy substitutional sites underneath the Si adatoms in T4 positions (Kaxiras et al., 1990). In this way the transfer of electrons from the Si surface adatoms to the B atom empties the dangling bonds and the surface is semiconducting. Adsorption of NH3 onto this passivated surface results

881

Electronic structure of adsorbates on semiconductors

Si(111) - 7X7 UPS: ~

= 40.8 eV

z

a:i v

=,,..

z I,.i.I I

z 0 (,/3 f./3 ~E ILl I . ,

"r" t't

0

2

4

6

8

10

12

14

BINDING ENERGY(eV) Fig. 12.14. He II valence photoemission of the surfaces produced by the interaction of clean Si(111)7 x 7 with 2 L NH3 at 90 K, followed by annealing at the indicated temperatures. From Avouris et al. (1990).

Fig. 12.15. Left: STM topograph of a Si(111)7 x 7 exposed to 5 L ofNH 3 at 300 K. Inset: topograph of a clean Si surface. Right: STM topograph of a B/Si(111)-~/3 x ~/-3R30 ~ surface exposed to ~400 L of NH 3 at 300 K. From Avouris et al. (1990).

now in bonding of the molecule via donation of the lone pair at N atoms to the empty dangling bond of the surface (Avouris et al., 1990). This is demonstrated in Fig. 12.16 where the UPS spectra show only reversible molecular adsorption. The STM image of Fig. 12.15

882

R. Miranda and E.G. Michel

I

i

I

I

I

I

i

!

B/Si(111)-43xV3

-U

_~~ 0

CLEAN

I

I

I

,,,I

2

4

6

8

I

10

I ,

12

l

14

BINDING ENERGY(eV) Fig. 12.16. He II valence photoemission of the surfaces produced by the interaction of B/Si(111)-V/3 • x/-3R30~ with 5 L NH 3 at 90 K, followed by annealing at the indicated temperatures. From Avouris et al. (1990).

proves that the surface has not reacted at RT to an exposure of 400 L of NH3. Note that the STM visualizes in this case the Si adatoms on top of the B layer. At variance with silicon surfaces, the adsorption of ammonia on clean germanium surfaces is molecular (Ranke, 1995). The adsorption properties change with the substrate orientation due to the different orbital distribution in each case. A strong interaction was reported (Ranke, 1995) with the dangling bonds of the Ge(100)2 x 1 dimers, that were found to be the preferential adsorption sites.

12.4.3. Adsorption of other elements and compounds Due to the growing interest of II-VI semiconductors, the adsorption of S on Si surfaces has been investigated as a first step to a I]-VI/Si epitaxy (Kaxiras, 1991; Moriarty et al., 1993b; Metzner et al., 1997). As on metallic substrates, S is adsorbed in a well ordered fashion giving rise to new surface reconstructions (Metzner et al., 1997). It seems that a competition between substrate dangling bond filling and S-S interactions gives rise to a complex chemisorption behavior on Si(111) (Metzner et al., 1997). The adsorption of S on Si(100)2 x 1 has been studied in several works (Moriarty et al., 1993a; Papageorgopoulos and Kamaratos, 1996). While Moriarty et al. (1993a) proposed that the substrate 2 x 1 reconstruction is preserved after S adsorption, Papageorgopoulos and Kamaratos (1996) suggested that the surface reverted to 1 x 1 after adequate dosing.

Electronic structure of adsorbates on semiconductors

883

SH2 adsorption on Si and Ge has been studied only in few cases. Gas volumetric studies on crushed germanium favor dissociative adsorption, while energy loss studies indicate molecular adsorption (in the case of Si(111)7 x 7). Several superstructures have been reported after annealing the SH2 overlayer on Ge(111), and this is in fact a method to deposit the chalcogen on the surface. Adsorption of SH2 on the (110) surfaces of III-V semiconductors has been reviewed recently (Dudzik et al., 1995). Adsorption on InP, GaP and GaAs does not differ much, except for the adsorption temperatures needed. The adsorption of SH2 is dissociative. The surface anion receives a proton, while the S atoms remains bonded to the surface cation. Several other adsorbates have been investigated due to their importance in the growth of in situ doped Si thin films as well as optoelectronic materials on Si. One relevant example is the adsorption of phosphine PH3, that has been analyzed using STM (Boszo and Avouris, 1991) and molecular beam techniques (Maity et al., 1995). The adsorption on Si(111)7 x 7 affects the 7 x 7 reconstruction, giving rise to a P-substituted 1 x 1 structure. This effect is interesting because the 1 x 1 structure is much more reactive, and gives rise to a kind of "autocatalytic" effect (Maity et al., 1995). The adsorption of many other molecules mainly on Si substrates has been investigated as well. In general, the surface dangling bonds act as reaction active sites, and are actively involved in the chemisorption process. In some cases, Si-Si bonds can be broken (in particular, intra dimer bonds) (Widdra et al., 1995).

12.5. Surface passivation The modification of the dangling bonds present on clean semiconductor surfaces produces important differences in their adsorption behavior. The basic rule explaining the behavior of adsorbates on semiconductor surfaces is that adsorbates saturate dangling bonds. Provided that the adsorbate-substrate bond is strong enough, suitable adsorbates may passivate the surface against reaction with other gases such as oxygen. This simple idea has been used in recent years to produce many different passivation methods. Furthermore, an adequate passivating layer is able to eliminate the surface states completely. Further epitaxial growth would produce layers free of interface states, which would exhibit improved electronic properties (Saiz-Pardo et al., 1996). An inert Si surface is extremely important as substrate for MBE or CVD applications, since a flat passivated surface may allow the formation of devices with atomically abrupt interfaces of macroscopic dimensions. A complete passivation of Si(111) surfaces can be obtained by dipping the samples in aqueous HF solutions (Higashi et al., 1990; Jakob et al., 1991; Dumas and Chabal, 1991). For basic solutions, the resulting surface is hydrogenterminated and shows a 1 x 1 LEED pattern of outstanding quality. The surface is strain free and homogeneous over a fairly large lateral scale. The nature of the 1 x 1 phase can be studied analyzing the Si 2p-core level. In the (7 x 7) reconstruction, the existence of several inequivalent atoms gives rise to many different surface components in the core level, which makes a direct assignment difficult. On the contrary, the line shape observed for the 1 x 1 phase is much simpler (Hricovini et al., 1993; Karlsson et al., 1994), with only two dominant components that can be attributed to surface and bulk atoms. Thus the 1 x 1 surface is simply the bulk terminated surface with

884

R. Miranda and E.G. Michel

ta')

0

~ - ~

,,

'

I l i

"~

!! i!!i!l

Ili!i~!!il!:i. .(i,lr

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10

8

6 4 2 Energybelow EF(eV) (a)

0

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~ K

!!!II,

=

r

(b)

Fig. 12.17. Left: representative photoemission spectra of the Si(111) 1 • 1 surface obtained at hv = 17.5 eV as a function of polar emission angle. Right: surface bands calculated using the LDA (full lines) and the GW (open circles) methods. The experimental points are represented by solid black squares, and the background shows the projection of the bulk Si(111) bands. The energy scale is in eV and the zero is at the top of the valence band. From Hricovini et al. (1993).

all the dangling bonds saturated by hydrogen atoms. The ionization energy of Si(111):H 1 x 1 is 6.6 eV, i.e., 1.3 eV larger than the value for clean Si(111) 7 x 7. The hydrogen atoms adsorb on top with a bond length of 1.48 A and because of their small size the mutual depolarization of Si-H dipoles can safely be ignored. The calculated value of the change in the ionization energy amounts to + 1.1 eV, close to the experimental one. Angle-resolved photoemission spectra of such a samples are shown in Fig. 12.17 (Hricovini et al., 1993; Karlsson et al., 1994). The corresponding surface bands are plotted in Fig. 12.17 together with quasiparticle energy dispersions calculated using a self-energy approach based in the G W method described elsewhere in this volume. The agreement between experiment and theory is excellent. Since there are no dangling bonds available, hydrogen-terminated Si surfaces are extremely passive against reaction with molecular oxygen, as proven by the uptake data shown in Fig. 12.18. The initial sticking coefficient is strongly reduced with respect to clean Si(111): irrespective of surface orientation, 1025 02 molecules per cm 2 result in no oxygen uptake. Although hydrogen is the most widely used adsorbate for passivation purposes, several other elements may act as passivants as well. Adsorption of chlorine on Si(111) 7 x 7 at 850 K is capable of removing the Si adatoms leaving the underlying restatom layer intact (Villarrubia and Boland, 1989). Figure 12.19 shows the resulting surface. Chlorinated surfaces are basically unreactive towards oxygen exposure, even at 630 ~ or towards water adsorption (Wise et al., 1996; Klyachko et al., 1997).

885

Electronic structure of adsorbates on semiconductors

9

02:Si(111):H-1 x 1

I

II `

iI

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iI

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[]

(l.) > 0 0

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o

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j

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. . . . . .

I

I

I

1024

1026

1028

oxygen

exposure

1030

(cm "2)

Fig. 12.18. Reactivity vs. molecular oxygen of Si(111)- and Si(100):H-1 x 1 surfaces: V data from Thornton and Williams (1989), [] data from Gr~if et al. (1990), O data from Stockhausen et al. (1992).

Fig. 12.19. STM topograph of a Si(111) surface after a saturation exposure to chlorine and anneal cycle. The area corresponds to 89 x 96 ~2 at 3 V. From Villarrubia and Boland (1989).

Hydrogen passivated Si surfaces exhibit several interesting properties, based on the lack of surface states pinning the Fermi level and the drastically reduced surface energy. The possibility of growing metallic films on top of these interfaces has been tested recently in several studies. Surface passivation influences the mode of growth, inducing island formation (Nishiyama et al., 1996; Shen et al., 1997; Copel and Tromp, 1994; Murano and Ueda, 1996; Naitoh et al., 1996), or the growth of epitaxial films (Horn-von Hoegen and Golla, 1996; Ohba et al., 1997; Ababou et al., 1995). In principle, the formation of a Schottky barrier without surface state pinning could be studied also by this method. Nevertheless, and depending on the nature of the metal, partial reaction with the substrate can be of importance. Thus, while alkali metals behave in an almost ideal way (Grupp and Taleb-Ibrahimi, 1998a), Au destroys partially the passivating layer (Grupp and Taleb-Ibrahimi, 1998b). Surface passivation by adsorbates plays a major role in the preparation of surfaces of III-V semiconductors. For instance, good quality GaAs(100) surfaces are usually prepared

886

R. Miranda and E.G. Michel

by in situ MBE growth. In most cases, not much surface analytic equipment is available in MBE chambers. Thus, GaAs is usually passivated using an arsenic cap, that allows even ambient transfer (Resch-Esser et al., 1996).

12.6. Adsorption of halogens 12.6.1. Introduction

The chemisorptive properties and electronic structure of halogens on solids have been a subject of interest since the mid-seventies, as a field inserted in the more general goal of understanding the gas-solid reactions, and the formation of solid interfaces (King and Woodruff, 1984). In the case of semiconducting substrates, most of the works have been devoted towards understanding the oxidation reaction, since it plays a crucial role in the determination of the properties of many microelectronics components. Nevertheless, the heterogeneous nature of oxide interfaces (Hollinger and Himpsel, 1983) complicated the task of finding the answers to the fundamental physical questions open: the role played by the detailed surface geometry in the reaction process, the factors determining the oxide growth mode, and the geometric and electronic properties of the grown oxide itself. Thus, halogen atoms (which usually form well-ordered overlayers on semiconductors) appeared as adequate model systems to study the chemisorption of highly electronegative elements on semiconducting substrates. In fact, the nominal monovalency of halogens, and the strong periodic trends expected, make them ideal probes for surface studies. On the other hand, halogen-semiconductor interfaces revealed themselves as a field of great technological interest. For instance, F or C1 adsorption plays a key role in many technologically important processes, such as reactive ion etching and chemical vapor deposition. Good examples are the usual method to produce epitaxial Si layers in gas-phase epitaxy, where silicon is deposited through the destruction of chlorsilanes on the substrate surface, or the selective silicon etch in a plasma reactor (through reaction with CF4 or NF3) (Oehrlein, 1992; Flammt and Donnelly, 1981). The initial lack of interest can be certainly attributed to the corrosive nature of these elements, which may difficult of even prevent some types of studies. During the eighties a large number of fundamental works was devoted to the adsorption of fluorine, chlorine, bromine and iodine on silicon and GaAs substrates, studied as the initial step of the etching process. The nature of these interfaces is also relevant to understand passivated surfaces, a topic which has deserved widespread attention in the last few years. Since halogens are among the most reactive elements of the periodic table, once a semiconducting surface has reacted with a layer of halogens, the surface remains in many cases passivated against reaction with any other element, which obviously opens many ways to novel applications. The nature of the chemical bonding of the atoms of interest at an interface is governed by the electronic configuration near the surface, and also by the atomic positions of both adatoms and substrate atoms (symmetry and interatomic distances). These two types of properties, geometric and electronic, and deeply related, since changing the position of an adatom changes its chemical bonding, with a corresponding change in electronic configuration, and vice versa. Generally speaking, structural techniques such as LEED (Jona et

Electronic structure of adsorbates on semiconductors

887

al., 1982), SEXAFS (Citrin et al., 1983; Citrin, 1987), XSW (Funke and Materlik, 1987) or more recently STM (Villarrubia and Boland, 1989), are employed to obtain information on the geometric structure of surfaces. In the case of halogen/semiconductor surfaces, these techniques have been applied recently as well, but a significant part of the information was obtained by photoemission (see below), making use of the sensitivity of the electronic cross section of the different orbitals to polarization effects. Thus, halogen/semiconductor interfaces are a nice example of how photoemission can be employed to obtain geometric information, i.e., how deeply electronic and geometric structure are interconnected, a circumstance very often undervalued.

12.6.2. Adsorption-site symmetry and polarization effects Two high-symmetry sites are possible for a halogen atom to adsorb on a semiconductor of (111) orientation (Schltiter and Cohen, 1978) (see Fig. 12.20): the so-called onefold covalent site (on top) and the threefold ionic site (either H3 or T4). Taking the z axis perpendicular to the surface plane, in the on top site, a bond of cr type is formed between the partially filled substrate dangling-bond (belonging to the sp 3 hybridization of Si atoms), and the halogen Pz orbital. The bonding splits off the Pz orbital, so that Px and py orbitals behave at first approach as lone-pairs, and do not participate in the chemical bond. In principle, at least two photoemission peaks should be observed, one for the Pz orbital, and one for Px and py orbitals (Schltiter and Cohen, 1978). On the other hand, in the adsorption geometry of the threefold site, the adatom sits equidistant to three substrate atoms. Thus, three Si dangling bonds (from sp 3 hybridization) are involved in the bonding. In this geometry, the overlapping of Pz orbital with the dangling bonds is lesser than in the on top site, and thus the corresponding photoemission peak should appear at smaller binding energy than in the on top case. In conclusion, we may expect the appearance of two peaks in the photoemission spectra: one corresponding to the Pz orbital and one corresponding to the Px and py orbitals. The appearance of the Pz orbital at a greater (smaller) binding energy would indicate the occupation of the onefold (threefold) site. Given the two peaks, the problem is to assign

Fig. 12.20. Side and top views of a T4 (right) and on top (left) site on a Si(111) surface.

888

R. Miranda and E.G. Michel

them to the corresponding orbitals. This can be easily done provided the symmetry of the peaks can be experimentally determined, which is easy to do using polarization selectionrule effects (Plummer and Eberhardt, 1982). When polarized light is used, if it is normally incident to the surface (i.e., its electrical vector is parallel to the surface), then excitation from pz-like orbitals is forbidden (assuming plane-wave final states). Conversely, when the electrical vector has some component perpendicular to the plane of the surface, excitation from pz-like orbitals can occur. For some examples see next section.

12.6.3. Group IV + halogens The interaction of halogens with group IV semiconductors has been extensively studied because of the technological importance of dry etching processes (Winters and Coburn, 1992), but also because its fundamental interest (Farrel, 1984; Williams, 1984). At room temperature, F2 etches, while the other gasses (C12, Br2 and I2) saturate the surface (Villarrubia and Boland, 1989; Citrin, 1987; Funke and Materlik, 1987; Engstrom et al., 1988; Seel and Bagus, 1983, 1984; Schnell et al., 1985; Whitman et a1.,1990; Boland and Villarrubia, 1990a, b; Michel et al., 1991). The simplest situation is found when C1 is deposited on the cleaved Si(111)2 x 1 surface. Information coming from different experimental techniques and theoretical calculations exist since several years (Rowe et al., 1977; Larsen et al., 1978; Pandey et al., 1977; Mednick and Lin, 1978; SchRiter et al., 1978). A 1 x 1 structure is formed, with one C1 atom per Si atom on the surface. Using the polarization selection-rule, the adsorption site has been determined by SchRiter et al. (1978). Using s- and p-polarized synchrotron light, they could observe the effects explained above (Fig. 12.21). Two major peaks are observed experimentally in the ultraviolet photoemission. With s-polarized light, the peak at larger binding energy (7-8 eV from the valence band maximum) decreased in intensity compared with p-polarization, the behavior predicted for a one-fold site. The result was further supported by comparing the experimental spectra with theoretical calculations. Angle-resolved studies by Larsen et al. (1978) have further supported this assignment. Keeping the polarization degree fixed, the angular dependencies of the photoemission peak intensity and energy position were measured. The observed dispersions of Cl-induced surface energy bands were in good agreement with calculations based on the one-fold model. A more detailed study on the features observed in the photoemission spectra can also be performed (Pandey et al., 1977). In particular, a set of features is observed in the region of 2 eV below the valence-band maximum. These features are partly due to p-like backbonding states of Si atoms (slightly perturbed by the adsorbate), and o'-bonding states corresponding to bonding between C1 Pz and Si s-orbitals. In the region of 4-6 eV below the valence-band maximum, C1 Px and py orbitals are seen to split due to C1-C1 interactions, once the 1 x 1 structure is completed. As already mentioned, a variety of theoretical calculations have been performed for the system, including pseudopotential calculations (Schlfiter and Cohen, 1978; Larsen et al., 1978; Schlfiter et al., 1978), and semiempirical and first-principles tight-binding or LCAO calculations (Seel and Bagus, 1983, 1984; Pandey et al., 1977; Mednick and Lin, 1978). The adsorption of C1 atoms on Si(111)7 x 7 gives rise to a weakening of the 7 x 7 LEED spots. At saturation coverage, only the 7th order spots nearby the 1st order spots remain (the

889

Electronic structure of adsorbates on semiconductors

B

2 p-pol.

22

s-pol.

1 t2

I

I 8

1

I 4

I

I O'Er

ENERGY(eV)

-t2

-8 -4 ENERGY {IN)

0,Ev

Fig. 12.21. Left: calculated density of states for a Si(1 l l) surface covered with a C1 layer in top and three-fold sites. Right: experimental photoemission spectra of cleaved Si(111)-C1 employing s- and p-polarized photons of energies 22, 25, and 28 eV. From Schlfiter et al. (1978).

so-called "7 x 1" pattern, similar to the pattern observed after hydrogen exposure) (Pandey et al., 1977). The pattern can be reproduced by 1 x 1 islands separated by the troughs commonly observed between the 7 x 7 unit cells. This hypothesis has been confirmed by STM results (Fig. 12.19) (Villarrubia and Boland, 1989). The photoemission spectra are

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R. Miranda and E.G. Michel

very similar to those obtained in the case of the 2 • 1 surface, although the polarization effects are much smaller. When C1 atoms are deposited on Si(111)7 x 7, the reconstruction is destroyed, and significant mass transport takes place, as has been beautifully shown by STM work (Villarrubia and Boland, 1989). The whole process is temperature activated, and annealing or deposition at elevated temperature is necessary to get a completely-covered, well-ordered Si(111)1 x 1 surface, formed by a layer of C1 atoms saturating the dangling bonds of the substrate. Additional information exists coming from LEED (Rowe et al., 1977; Pandey et al., 1977), SEXAFS (Citrin et al., 1983; Citrin, 1987), and theoretical calculations. All these studies have stressed the importance of annealing processes in order to improve the ordering of the halogen atoms on the surface, which might otherwise present different adsorption geometries. STM results have shown the importance of effects of this type (Villarrubia and Boland, 1989). Theoretical studies for these system have favored covalent bonding. The results of Rowe et al. (1977) indicated the possible presence on the surface of other adsorption sites in addition to the on top site. Schnell et al. (1985) have studied the chemisorption of C1 on Si(111)7 x 7 analyzing also the Si 2p-core level. Their work clearly shows the appearance at RT of Si atoms with multiple C1 atoms bonded to them. After annealing at moderate temperatures, only monochloride species are observed. In this case, the polarization selection rules were employed to study the symmetry of the bands formed after a well-ordered C1 layer had been produced. The results favored adsorption at on top sites, in agreement with SEXAFS results. The two-dimensional dispersion E (k) was directly mapped, together with the band dispersion of the chlorine induced states. The main result was the observation of the non-dispersing a- and s-bands, in contrast with the re bands (formed by the non-bonding C1 bonds). The results were compared with both pseudopotential calculations by Larsen et al. (1978) for this surface and self-consistent LCAO calculations by Batra (1979) for a free C1 layer, to demonstrate the effect of the adsorbate-substrate interaction on the band structure. The agreement with the experiments was reasonably good, although several effects were observed. In particular, the theoretical binding energies are larger than the experimental values. The adsorbate-surface interaction releases the degeneracy at the K point of the free C1 monolayer. Generally speaking, the dispersions observed support a 1 x 1 surface Brillouin zone. Studies involving the Ge(111) surface are less frequent (Michel et al., 199 l; G6thelid et al., 1997). In spite of the similarity of both surfaces, Schltiter and Cohen have shown that the adsorption site in this case is threefold ionic (Schltiter and Cohen, 1978). In this case, upon going from p- to s-symmetry, only an asymmetrical narrowing of the main C1induced peak was observed, which is compatible with threefold adsorption geometry. At variance with the Si(111) surface, no strongly-localized, a-like states were observed in this case. The adsorption of halogens on Si(100)2 x 1 has also been studied. In these case, several adsorption sites of different symmetry are possible. The surface is formed by pairs (dimers) of Si atoms, with dangling bonds pointing towards a direction at ---54~ from the surface. Rowe et al. (1977) studied this surface using LEED and photoemission polarization effects. The 2 • 1 reconstruction was still present after C1 adsorption. No polarization effects were observed in the angle-integrated peaks, but changes were detected in the angle-resolved normal-emission spectra. To explain the observation, a model including some bonding character for Px- and py-orbitals, and mixing with pz-orbitals, was successful. The degree

Electronic structure of adsorbates on semiconductors

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of mixing is consistent with a bonding geometry where C1-Si bonds are not perpendicular to the surface, suggesting that the bond direction is close to the initial dangling bond direction. This results have been supported by ESDIAD (Yates et al., 1993), XSW (Etel/aniemi et al., 1991), electron stimulated desorption (Simpson and Yarmoff, 1996), and SEXAFS (Thornton et al., 1989) measurements. In fact, if a systematic analysis of the bond direction is done, it is easy to observe that the bond angle increases as the halogen atomic size does, supporting that halogen-halogen repulsion is the driving force behind the bond angle tilting. Johansson et al. (1990) have performed extensive angle-resolved photoemission investigations on C1:Si(100)2 x 1. The interpretation of the observed bands was difficult, even after detailed theoretical calculations (Krtiger and Pollmann, 1993; Craig and Smith, 1992). Halogen atoms (excluding F) can hardly penetrate the surface at room temperature, since the penetration barrier is high (Seel and Bagus, 1983, 1984; Bagus, 1985). This explains the saturation behavior during adsorption. Once saturation is reached, a passivating halogen layer is formed on the surface. Such halogen-covered surfaces have been reported to be stable against exposure to atmospheric pressure (specially for water-free environments) during periods of the order of days. As the protective layer can be removed by annealing, such passivated surfaces are interesting for different applications. In contrast, fluorine atoms are able to penetrate the surface, break silicon-silicon bonds, and etch the surface, even at room temperature. Thus, the electronic structure of F/Si(111) surfaces is less straightforward, because in this case a reaction layer coexists with chemisorbed F atoms. The F/Si(100) interface has been studied by Engstrom et al. (1989), and F/Si(111) by Lo et al. (1993). They found that the reaction process is characterized by four different regimes. In the first step, fluorination and etching of the 7 x 7 reconstruction takes place. In the second step, a quasiequilibrium reaction layer is formed (~ 1 ML thick), which acts as a passivating layer. The etching reaction proceeds through a third step, consisting in the disordering of the substrate and formation of a deeper reaction layer. Finally, a steady-state etching is reached when the reaction layer is completed. Interestingly, the etching process proceeds through the formation of defects which facilitate the amorphisation of Si. Amorphous Si is much easily etched than crystalline Si (whose Si-Si bonds cannot be broken by F atoms). Core-level X-ray photoelectron spectroscopy provides with a detailed information on how the reaction layer is built up when the Si 2p peak is detected with high surface sensitivity (Lo et al., 1993). Generally speaking, all halogens behave in a similar way, although the heaviest elements (Br and I) need higher fluxes and/or surface temperatures to attain analogous surface ordering. In this case, mixed layers containing Si atoms bonded to different numbers of halogen atoms are frequently observed by X-ray photoemission spectroscopy (XPS). The valence band presents non-dispersing features, which is generally an indication of worse ordering. Annealing promotes the formation of smoother interfaces, but in the case of heavy halogens the whole process may be prevented because halogen desorption appears at rather low temperatures. Thus, deposition at elevated temperatures is needed in order to achieve well-ordered layers. The role of temperature and surface defects has been investigated using the STM (Chander et al., 1993a; Rioux et al., 1994, 1995).

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12.6. 4. Group III- V compounds + halogens The (110) surfaces of III-V compounds, such as GaAs, provide interesting substrates for chemisorption. In the bulk, a (110) layer contains both cations and anions in equal proportions. Therefore, as a first approach, bonding of halogen adatoms to either or both the anion or the cation substrate atoms must be considered. On the basis of electronegativity, one may argue that the cation should be the preferred binding site. Alternatively, steric considerations would possibly favor the anion as the preferred site, since the actual GaAs(110) surface is relaxed in a such a way that the anion atoms are outside, and cation atoms inside the surface. Margaritondo et al. (1979, 1981) have performed detailed studies on the electronic structure of C1 adsorbed on GaAs, GaSb, and InSb. Angle-integrated photoemission results were compared to semiempirical calculations for the local density of states for several adsorption geometries. Three Cl-induced features were observed in the spectra, which approximately resemble those observed in the Si(111) case. Nevertheless, for all IIIV surfaces, no significant polarization effects were observed. Margaritondo et al. (1979, 1981) considered several adsorption geometries and surface relaxations, allowing also the bond length to change. The best agreement was found for a reconstructed surface such that the anion atoms are relaxed outwards (as in the clean GaAs case) from the bulk position, and C1 atoms are bonded to the anions, with a bond length contracted by 5-10% with respect to the sum of atomic radii. Both the intensity comparison and the absolute values of the observed binding energies were considered to account for the model. The polarization effects expected for some peaks could not be observed due to the weakness of the emissions under the available experimental conditions. Photoemission results by Troost et al. (1987) suggested that C1 bonds also to Ga atoms. More recent results supported adsorption on both Ga and As atoms (Gu et al., 1992), in agreement with other findings for F (McLean et al., 1989). Patrin and Weaver's (1993) STM work has recently shown how GaAs is etched by Br and C1. The authors have found different regimes depending on the halogen flux and the substrate temperature, which in fact parallel the behavior of multilayer growth. In particular, an overlayer of 1 x 1 symmetry is formed initially, with halogen features localized within the rectangle formed by four As atoms and on top of As atoms. Halogen atoms tend to coalesce and form chains, and when the flux is increased, multilayer etching takes place. A higher temperature facilitates surface ordering, and etch through step retreat is observed.

12.7. Adsorption of C60 Since the discovery of fullerenes (Kroto et al., 1985), their outstanding electronic properties have received widespread attention (Hebard et al., 1991; Rosseinsky et al., 1991; Holczer et al., 1991). The growth of well ordered layers of C60 on Au(111) (Wilson et al., 1990) made it possible the investigation of mono- and multilayers of C60 of high crystalline quality. The interaction of the molecule with the substrate was supposed to be of van der Waals type. Later on, the adsorption of C60 was studied on many metals and semiconductors (Si, GaAs, GaSe, GeS). The main conclusion was that there are significant electronic (Modesti et al., 1993) and vibrational changes (Suto et al., 1997) in the molecule as a consequence

Electronic structure of adsorbates on semiconductors

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of the adsorption process. This suggests that the nature of the bond should be more complex, and thus it was proposed that the bond is ionic (Suto et al., 1997). The formation of well-ordered layers has made it possible the use of angle-resolved photoemission to study the surface electronic bands (Gerstenblum et al., 1994). There is an overall broadening of the C60 characteristic structures when adsorbed on a semiconductor. It has been attributed to a symmetry reduction after adsorption that releases in part the levels degeneracy (Gerstenblum et al., 1994). The analysis and understanding of the complex phenomenology of this system is a topic of current research (Rudolf et al., 1997).

12.8. Conclusions and outlook

The electronic structure of adsorbate covered semiconductors is a fascinating field of current research. While the most relevant features of simple adsorbates have already been elucidated, the behavior of more complex molecules is investigated nowadays. This research is expected to continue in the future in view of its importance from both applied (microelectronics) and fundamental points of view (self-organization, organic thin films, etc.).

Acknowledgments This work was supported by DGES (Spain) under Grant. No. PB97-0031.

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