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InP(110)
surface science ELSEVIER
Surface Science 331-333 (1995) 528-533
Evidence for surface derelaxation induced by metals on III-V compound semiconductors: Cs/InP(ll0) Th. Chass6 a, G. Neuhold b, j.j. Paggel b, K. Horn a,, a
Institute of Physical and Theoretical Chemistry, University of Leipzig, D-04103 Leipzig, Germany b Fritz-Haber-lnstitute, Faradayweg 4-6, D-14195 Berlin, Germany Received 22 September 1994; accepted for publication l December 1994
Abstract High resolution In 4d core and valence level photoemission spectra from InP(110) show that a new peak is induced by Cs adsorption, and that the surface core level (SCL) peak shifts towards larger separations from the bulk line with Cs coverage. While the presence of the Cs-induced peak is related to charge transfer processes on the basis of spectra which show emission above the valence band maximum, the increase in SCL is interpreted, by recourse to tight-binding calculations, as due to a lifting of the bond angle rotation-relaxation of the clean surface. These findings provide a consistent interpretation of changes in the electronic structure of the surface upon alkali metal deposition, and a basis for a valid model function for investigations of Fermi level movement through core level line shape analysis. Keywords: Angle resolved photoemission; Chemisorption; Low index single crystal surfaces; Metal-semiconductor interfaces; Schottky barrier; Soft X-ray photoelectron spectroscopy; Surface relaxation and reconstruction
1. Introduction Investigations of the influence of metal overlayers on the structure of semiconductor substrates are important for the characterization of metal-semiconductor contacts and their charge transport barriers. The structure at the interface can influence the height of the barrier; structural changes that accompany interface formation may therefore be of direct relevance for the determination of trends in barrier heights. Studies of the (110) surface of I I I - V semiconductors have been particularly important in this respect. The clean surfaces do not exhibit band bending, since the anion- and cation-derived dan-
* Corresponding author.
gling bond surface states are moved out of the fundamental gap by the bond-angle rotation-relaxation [1]. The absence of states in the gap thus permits a determination of the influence of metals on the Fermi level position as a function of coverage, which is not possible for many other semiconductor surfaces. A large body of data on metal interaction with these I I I - V surfaces has been collected, and core level photoemission data in particular have been used in order to examine the formation of the Schottky barrier from the lowest metal coverages onwards [2]. The reliability of such studies depends, among other things, on a correct description of the core level line shape. Subtle changes in line shape, which can be determined in high resolution experiments, can then be related to chemical reactions and charge transfer processes between the metal overlayer and
0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
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Th. Chass~ et al. /Surface Science 331-333 (1995) 528-533
the semiconductor and, possibly, to structural changes of the substrate. The correct description of the core level line shape is important in a very direct sense in studies of Fermi level movement in the early stages of Schottky barrier formation, since the bulk core level contribution, which is the basis for the measurement of the Fermi level position, is usually derived from a line shape analysis, and hence its precision depends on the validity of the model function. Here we present a careful examination of the In 4d line shape with coverage showing that, upon Cs deposition, the separation between bulk and surface core levels is affected by metal deposition, and a new peak occurs at lower binding energy. The latter observation is interpreted as due to charge rearrangement in the surface, while the first is assigned to structural changes, i.e. the lifting of the reconstruction. InP is a well suited substrate for the purpose of detailed core level examination because of the large spin-orbit splitting of the 4d peak, such that small shifts a n d / o r new components may be distinguished more readily than in GaAs, for example. Our data for C s / I n P ( l l 0 ) are augmented by results for N a / G a P ( l l 0 ) and Ag/GaAs(ll0), suggesting that the present results and discussions concern a widespread phenomenon, and are not simply related to chemical processes in one particular system.
2. Experimental Experiments were carried out at the BESSY (Berliner Speicherring-Gesellschaft flir Synchrotronstrahlung mbH) storage ring beam line TGM 6, using a hemispherical electron energy analyzer (HA 50 by VSW Ltd. GB), with an overall instrumental resolution of about 80 meV in the photon energy range of 40 to 60 eV. Samples were n-doped (concentration 1 x 1018 cm -3) InP (from MCP Ltd., GB) which were cleaved in the (110) direction in situ. Cs was deposited from well outgassed getter sources (SAES Getters. S.A., Italy), at a base pressure of 7 × 10 -11 mbar, with a pressure rise to about 1 X 10 -1° mbar, mostly due to the alkali metal itself, during deposition. All spectra were recorded at a substrate temperature of about 120 K.
529
3. Results and discussion The influence of Cs on the In 4d core level spectrum and the valence band maximum (VBM) region are shown in Fig. 1. The bottom spectrum is from the clean surface, exhibiting the well-known bulk and surface core level lines with a shift of - 0 . 3 1 + 0.01 eV, i.e. towards lower kinetic energies, in agreement with previous studies [3,4]. Deposition of Cs, with coverages measured by the work function change, induces two changes: (a) the surface component moves towards lower kinetic energy, clearly detected by the broadening top of the peak doublet, and (b) a new peak emerges on the high kinetic energy side (see fourth and fifth spectrum from bottom). In an interpretation similar to that for N a / G a P ( l l 0 ) [5], we assign the peak on the high kinetic energy side to those indium surface atoms which interact indirectly with a Cs atom. The shift of the SCL peak with coverage shows that, beyond the interaction with the surface atom to which the Cs atom is bonded, Cs deposition also leads to a interaction which affects the clean regions of the surface. It should be noted at this point that the phosphorus 2p level does not lend itself to a similar analysis since the surface core level and the alkali-induced feature are both on the same, low binding energy side of the bulk peak [6], whereas they are well separated on the high and low binding energy side of the peak for the cation peak. In order to explain the Cs-induced changes in the In 4d core level line shape, the model of alkali metal-semiconductor bonding through charge transfer will be briefly discussed. The reconstructed surface causes the anion dangling bond state to be fully occupied; the corresponding cation state is shifted into the conduction band. Charge transfer from the alkali metal s level can only occur into this latter level, which is lowered in energy in this process. A peak due to this level should then appear in the region around the valence band maximum. The valence band spectra in Fig. lb indeed show a peak located about 0.4 eV above the clearly visible valence band maximum (VBM), which increases with deposition up to t9 ~ 0.3. Note that the nominal coverage has been scaled by taking the work function minimum as 19 ~ 0.5, in accordance with intensity attenuation and with our previous work on
Th. Chass£ et aL / Surface Science 331-333 (1995) 528-533
530
-2
1 0 relative binding energy (eV)
Cs/InP(11 O) T = 120K
valence band maximum region
(b)
O 1.20 1.00 t-
0.80 0.45 0.30
C s / G a P ( l l 0 ) [6]. We emphasize that the Cs surface atom density is 4.34 X 1014 cm -2 in bulk bcc Cs. A coverage corresponding to a complete Cs layer is about a half monolayer, which is 8.8 X 1014 cm -2 for InP(ll0) from the number density of substrate atoms in the (110) plane. In this manner coverages of 19 ~ 0.5 correspond to one Cs atom per two surface unit cells of InP(110). In the coverage regime beyond the work function minimum emission intensity near the Fermi level appears, signalling the beginning metallization of the overlayer [6]. This point is just reached in the top spectrum where a faint feature at E F can be distinguished. The fact that the new peak is clearly split off from the valence band precludes any interpretation in terms of a bulk state. We have previously argued [5,7] that this state is due to the previously empty cation dangling bond state; this interpretation is compatible with the experimental data, and is supported by the similarity of the evolution of the peak in the band gap with coverage with recent calculations of the density of states in this region [8], which for low coverages confirms our interpretation. Another interpretation could be that the new peak arises from the filled anion dangling bond surface state, which is shifted back into the band gap by an adsorbate-induced derelaxation of the surface. This would account for the decrease in intensity in the region just below the VBM, where the anion surface state is located [9]. In the relaxed geometry, this state exhibits a strong dispersion with a total band width of about 0.7 eV [9]. The Cs-induced feature, on the other hand, has a dispersion of less than 100 meV for kit along the [1-10] azimuth, i.e. along the rows in the (110) surface. For Na/GaAs(110), theoretical predictions show that this state has a very localized character [10]. It is obviously dangerous to compare peak dispersions in such different structural environments, such that for the moment this assignment remains unresolved.
0.15
0.00 I
43
I
I
I
VBM 44 45 kinetic energy (eV)
Fig. 1. (a) Set of In 4d core level spectra recorded for different Cs depositions as indicated. Note the shift in the surface core level emission at lower kinetic energies as indicated by the arrows. (b) Spectra of the valence band maximum region of InP(ll0) for different coverages of Cs as indicated. Cs induces a new peak located about 0.4 eV above the VBM; at coverages beyond one monolayer, emission near the Fermi edge appears as seen in the top spectrum.
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Th. Chass~ et al./ Surface Science 331-333 (1995) 528-533
In order to derive quantitative data about the interaction between Cs and InP(ll0), we turn to a more detailed analysis of the core level data. These were derived from a line shape analysis based on the Levenberg-Marquardt algorithm [11], in which the peaks were modeled by three doublets, i.e. the bulk peak, the surface component, and the charge transfer peak at higher kinetic energy. The free parameters were peak separation, surface peak and Cs-induced peak Gaussian width, and branching ratio; all other parameters such as lifetime broadening, spin-orbit splitting, third-order polynomial background etc. were derived from the clean surface spectrum (and transferred to the new lines where necessary). An example of such line shape analyses is shown in Fig. 2a. From the line shape analysis of an entire set of spectra, the shift of the SCL peak with respect to the bulk line as a function of coverage f9 is derived as shown in Fig. 2b. The SCL intensity is found to decrease linearly with deposition, while that of the Cs-induced peak increases, demonstrating the connection between the two peaks. At higher coverages (~9> 0.5) the SCL peak interferes with Cs loss features on the intense main peaks such that a measurement of its intensity is difficult. Coverages here relate to the work function minimum which is reached at a coverage of 0.5, and is attributed to the completion of half a Cs layer [5]. The shift of the SCL peak is approximately linear with coverage, starting from 0.30 eV and rising to 0.48 eV at 6)= 0.3. At the highest coverages the SCL shift is probably less reliable since the intensity is rather low, and the peak width may well be affected by its proximity to the bulk line, and may also be related to barrier height inhomogeneities which have been shown to influence the line width [12]. The surface photovoltage effects observed at low temperatures are expected to smooth out these inhomogeneities. The observed shift in the SCL peak with metal deposition is not restricted to the C s / I n P ( l l 0 ) system; in fact, it seems to be a widespread phenomenon. Upon reexamination of core level photoemission results for A g / G a A s ( l l 0 ) [13] and N a / G a P ( l l 0 ) [5], it was found that in these systems the SCL also exhibits systematic shifts with metal coverage; these are included in the compilation in Fig. 2b. A g / G a A s ( l l 0 ) is of particular interest in
Cs/InP(110) T = 120K
(a)
~
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-
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25 26 27 28 Kinetic energy (eV)
29
30
SCL shift as a function of metal coverage
0 Cs/InP(110) Ag/GaAs(110) • Na/GaP(110)
(b)
•
-0.5 >
~E - 0 . 4 _1 O cO
-0.3
-0.2
I
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0.0
0.1
0.2
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metal coverage (ML)
Fig. 2. (a) Example of a In 4d line shape analysis in order to obtain quantitative data for SCL shift energies. The residuum is given below the data and the model function. (b) Separation between bulk-derived and SCL peaks for Cs/lnP(ll0), Na/GaP(ll0), and Ag/GaAs(ll0) as a function of metal overlayer coverage, showing the common trend of an increase with coverage.
this context. The alkali metal atoms induce a high kinetic energy component in the substrate core level [5-7,14], while Ag does not. Moreover, the peak above the VBM observed in the systems mentioned above, interpreted as due to the cation-derived surface state filled as a consequence of alkali metal adsorption, is conspicuously absent for A g / G a -
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Th. Chass~ et al. // Surface Science 331-333 (1995) 528-533
As(ll0). These differences may be taken as an indication for a more covalent bonding between Ag and the GaAs substrate. In order to relate the changes in SCL peak position, the interpretation of SCLs needs to be briefly outlined. Core level shifts on semiconductor surfaces are generally interpreted in terms of initial state effects [15], even though there are cases such as Si(100) in which a specific valence level structure causes final state screening to have an effect [16,17]. The surface core level shifts on III-V semiconductor surfaces have been the subject of many experimental and theoretical studies [18-22]. For the clean surface, the major part of SCL shifts can be traced to the surface Madelung energy which is different in the surface from the bulk [3,18,23], without invoking any redistribution of charge between cation and anion at the surface. The influence of surface relaxation has also been dealt with, and there are conflicting predictions of its influence on the SCLs. The Madelung energy calculations yield an increase of the SCL with surface relaxation [3,18]. In a tight-binding calculation based on a local charge neutrality approximation, on the other hand, Priester et al. [19] found that the separation between the surface-shifted components actually increases when going from the relaxed to the ideal geometry, by 200 meV on average. A similar result was also arrived at in self-consistent tight-binding calculations by RodriguezHernandez and Munoz [21], where the average increase for the eight III-V compounds studied was 360 meV, and the increase in average cation-anion SCL separation was found to be 24% in a calculation with charge neutrality approximation, and 41% in the self-consistent calculation. As suggested by the calculated charge contour plots [8,10], the charge transfer process between the alkali metal atom and the substrate proceeds in a localized manner, such that adjacent surface atoms (and it is these, which still give rise to a SCL, that we are considering here) are not directly affected. The tight-binding calculations then suggest an interpretation of the shift in SCL with coverage which is caused by a lifting of the surface relaxation by metal deposition. Evidence for an unrelaxation of the GaAs(ll0) surface upon metal deposition has been derived from a number of different experiments [2426]. A detailed LEED study of K / G a A s ( l l 0 ) by
Ventrice and DiNardo [27] concludes from a kinematical analysis of I / V data that the surface becomes unrelaxed for K deposition. Total energy calculations by Hebenstreit et al. for N a / G a A s ( l l 0 ) also find that the Ga and As atoms underneath the Na atoms assume almost their bulk positions. We may therefore conclude that unrelaxation upon alkali metal deposition is a common occurrence. The above observation of an increase in SCL shift is then readily interpreted in terms of an unrelaxation by the surface surrounding a Cs atom on InP (and, likewise, the other surfaces for which data are given in Fig. 2b) by comparison with the tight-binding calculations of SCL shifts. The situation is complicated, of course, by the charge transfer that the adatoms induce. While the unrelaxation can explain the coverage-dependent shift of the SCL by comparison with the tight-binding calculations, one may speculate that it is partly related to surface atoms which, while not directly involved in the bonding interaction with an adatom (which would extinguish their contribution to the SCL peak), are in neighboring sites and may participate in the charge rearrangement caused by the deposition of metal atoms. However, the fact that a shift of the SCL peak is also induced by Ag atoms in Ag/GaAs(ll0), where charge transfer can be ruled out, renders this explanation unlikely. In this context it is interesting to note that the width of the SCL peak also exhibits an increase as Cs is deposited, increasing by about 40% up to half monolayer coverage. One may speculate that the unrelaxation of the surface is a local process, such that contributions from unit cells which are affected by the alkali metal atoms to a varying degree may overlap, giving rise to the apparent broadening as the coverage of Cs increases such that the free areas become successively smaller. In a similar manner, the shift with coverage can be explained. Consider an In surface atom not directly affected by the adsorbed Cs atoms. With increasing Cs coverage the number of Cs atoms in the vicinity of this In site will increase, such that the average distance to the Cs adsorption site will decrease. The increasing number of Cs atoms, which enhances the interaction with the In site, will then lead to a gradual shift of the SCL with coverage. In summary, a large shift in the surface core level peak in III-V semiconductor surfaces has been found upon metal deposition. This is interpreted in terms of
Th. Chass~ et al. / Surface Science 331-333 (1995) 528-533
a lifting of the b o n d angle r o t a t i o n - r e l a x a t i o n o f the clean surface induced by the metal adatoms, a process that has long b e e n suggested in the literature. Apart f r o m the general interest in structural changes resulting f r o m metal o v e r l a y e r formation, this observation is particularly important in cases w h e r e a line shape analysis o f core level p h o t o e m i s s i o n spectra is used for the determination o f F e r m i level positions, w h e r e a valid m o d e l function for the core level e m i s s i o n is required.
Acknowledgements This w o r k was supported by the Bundesministerium • r F o r s c h u n g und T e c h n o l o g i e under grant 05 5 E B F X B .
References [1] A. Huijser, J. v. Laar and T.L.v. Rooy, Surf. Sci. 62 (1977) 472. [2] I.P. Batra, in: Metallization and Metal-Semiconductor Interfaces, NATO Advanced Study Institute, Vol. B 195 (Plenum, New York, 1989). [3] W.G. Wilke, V. Hinkel, W. Theis and K. Horn, Phys. Rev. B 40 (1989) 9824. [4] A.B. McLean, Surf. Sci. 220 (1989) L671. [5] D.A. Evans, G.J. Lapeyre and K. Horn, Phys. Rev. B 48 (1993) 1939. [6] T. Chasse, J.J. Paggel, G. Neuhold, W. Theis and K. Horn, Surf. Sci. 307-309 (1994) 295.
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[7] D.A. Evans, G.J. Lapeyre and K. Horn, J. Vac. Sci. Technol. B 11 (1993) 1492. [8] M. Heineraann and M. Scheffier, Phys. Rev. B 49 (1994) 5516. [9] L. Sorba, V. Hinkel, H.U. Middelmann and K. Horn, Phys. Rev. B 36 (1987) 8075. [10] J. Hebenstreit, M. Heineraann and M. Scheffier, Phys. Rev. Lett. 67 (1991) 1031. [11] W. Theis, PhD Thesis, FU Berlin (1992). [12] R. Ciraino, A. Gearante, K. Horn and M. Pedio, Phys. Rev. Lett. (1994). [13] D.A. Evans and K. Horn, J. Electron Spectrosc. Relat. Phenora. 62 (1993) 59. [14] D.A. Evans and K. Horn, to be published. [15] F.J. Hirapsel, B.S. Meyerson, F.R. McFeely, J.F. Morar, A. Taleb-Ibrahirai and J.A. Yarmoff, in: Photoeraission and Absorption Spectroscopy of Solids and Interfaces with Synchrotron Radiation, Eds. M. Campagna and R. Rosei (NorthHolland, Amsterdam, 1990) p. 203. [16] E. Landemark, C.J. Karlsson, Y.-C. Chao and R.I.G. Uhrberg, Phys. Rev. Lett. 69 (1992) 1588. [17] E. Pehlke and M. Scheffier, Phys. Rev. Lett. 71 (1993) 2338. [18] J.W. Davenport, R.E. Watson, M.L. Perlrnan and T.K. Sham, Solid State Coraraun. 40 (1981) 999. [19] C. Priester, G. Allan and M. Lannoo, Phys. Rev. Lett. 58 (1987) 1989. [20] W. M6nch, Solid State Coraraun. 58 (1986) 215. [21] P. Rodriguez-Hemandez and A. Munoz, J. Electron Spetrosc. Relat. Phenom. 52 (1990) 155. [22] W. M6nch, Semiconductor Surfaces and Interfaces (Springer, Berlin, 1993). [23] V. Hinkel, L. Sorba and K. Horn, Surf, Sci. 194 (1988) 597. [24] G.D, Waddill, I.M. Vitorairov, C.M. Aldao and J.H. Weaver, Phys. Rev. Lett. 62 (1989) 1568. [25] P.S. Mangat, K.M. Choudhary, D. Kilday and G. Margaritondo, J. Vac. Sci, Technol. B 8 (1990) 995. [26] T. Guo, R.E. Atkinson and W.K. Ford, Phys. Rev. B 41 (1990) 5138. [27] C.A, Ventrice and N.J. DiNardo, Phys. Rev. B 43 (1991) 14.
Surface Science 331-333 (1995) 528-533
Evidence for surface derelaxation induced by metals on III-V compound semiconductors: Cs/InP(ll0) Th. Chass6 a, G. Neuhold b, j.j. Paggel b, K. Horn a,, a
Institute of Physical and Theoretical Chemistry, University of Leipzig, D-04103 Leipzig, Germany b Fritz-Haber-lnstitute, Faradayweg 4-6, D-14195 Berlin, Germany Received 22 September 1994; accepted for publication l December 1994
Abstract High resolution In 4d core and valence level photoemission spectra from InP(110) show that a new peak is induced by Cs adsorption, and that the surface core level (SCL) peak shifts towards larger separations from the bulk line with Cs coverage. While the presence of the Cs-induced peak is related to charge transfer processes on the basis of spectra which show emission above the valence band maximum, the increase in SCL is interpreted, by recourse to tight-binding calculations, as due to a lifting of the bond angle rotation-relaxation of the clean surface. These findings provide a consistent interpretation of changes in the electronic structure of the surface upon alkali metal deposition, and a basis for a valid model function for investigations of Fermi level movement through core level line shape analysis. Keywords: Angle resolved photoemission; Chemisorption; Low index single crystal surfaces; Metal-semiconductor interfaces; Schottky barrier; Soft X-ray photoelectron spectroscopy; Surface relaxation and reconstruction
1. Introduction Investigations of the influence of metal overlayers on the structure of semiconductor substrates are important for the characterization of metal-semiconductor contacts and their charge transport barriers. The structure at the interface can influence the height of the barrier; structural changes that accompany interface formation may therefore be of direct relevance for the determination of trends in barrier heights. Studies of the (110) surface of I I I - V semiconductors have been particularly important in this respect. The clean surfaces do not exhibit band bending, since the anion- and cation-derived dan-
* Corresponding author.
gling bond surface states are moved out of the fundamental gap by the bond-angle rotation-relaxation [1]. The absence of states in the gap thus permits a determination of the influence of metals on the Fermi level position as a function of coverage, which is not possible for many other semiconductor surfaces. A large body of data on metal interaction with these I I I - V surfaces has been collected, and core level photoemission data in particular have been used in order to examine the formation of the Schottky barrier from the lowest metal coverages onwards [2]. The reliability of such studies depends, among other things, on a correct description of the core level line shape. Subtle changes in line shape, which can be determined in high resolution experiments, can then be related to chemical reactions and charge transfer processes between the metal overlayer and
0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0039-6028(95)00280-4
Th. Chass~ et al. /Surface Science 331-333 (1995) 528-533
the semiconductor and, possibly, to structural changes of the substrate. The correct description of the core level line shape is important in a very direct sense in studies of Fermi level movement in the early stages of Schottky barrier formation, since the bulk core level contribution, which is the basis for the measurement of the Fermi level position, is usually derived from a line shape analysis, and hence its precision depends on the validity of the model function. Here we present a careful examination of the In 4d line shape with coverage showing that, upon Cs deposition, the separation between bulk and surface core levels is affected by metal deposition, and a new peak occurs at lower binding energy. The latter observation is interpreted as due to charge rearrangement in the surface, while the first is assigned to structural changes, i.e. the lifting of the reconstruction. InP is a well suited substrate for the purpose of detailed core level examination because of the large spin-orbit splitting of the 4d peak, such that small shifts a n d / o r new components may be distinguished more readily than in GaAs, for example. Our data for C s / I n P ( l l 0 ) are augmented by results for N a / G a P ( l l 0 ) and Ag/GaAs(ll0), suggesting that the present results and discussions concern a widespread phenomenon, and are not simply related to chemical processes in one particular system.
2. Experimental Experiments were carried out at the BESSY (Berliner Speicherring-Gesellschaft flir Synchrotronstrahlung mbH) storage ring beam line TGM 6, using a hemispherical electron energy analyzer (HA 50 by VSW Ltd. GB), with an overall instrumental resolution of about 80 meV in the photon energy range of 40 to 60 eV. Samples were n-doped (concentration 1 x 1018 cm -3) InP (from MCP Ltd., GB) which were cleaved in the (110) direction in situ. Cs was deposited from well outgassed getter sources (SAES Getters. S.A., Italy), at a base pressure of 7 × 10 -11 mbar, with a pressure rise to about 1 X 10 -1° mbar, mostly due to the alkali metal itself, during deposition. All spectra were recorded at a substrate temperature of about 120 K.
529
3. Results and discussion The influence of Cs on the In 4d core level spectrum and the valence band maximum (VBM) region are shown in Fig. 1. The bottom spectrum is from the clean surface, exhibiting the well-known bulk and surface core level lines with a shift of - 0 . 3 1 + 0.01 eV, i.e. towards lower kinetic energies, in agreement with previous studies [3,4]. Deposition of Cs, with coverages measured by the work function change, induces two changes: (a) the surface component moves towards lower kinetic energy, clearly detected by the broadening top of the peak doublet, and (b) a new peak emerges on the high kinetic energy side (see fourth and fifth spectrum from bottom). In an interpretation similar to that for N a / G a P ( l l 0 ) [5], we assign the peak on the high kinetic energy side to those indium surface atoms which interact indirectly with a Cs atom. The shift of the SCL peak with coverage shows that, beyond the interaction with the surface atom to which the Cs atom is bonded, Cs deposition also leads to a interaction which affects the clean regions of the surface. It should be noted at this point that the phosphorus 2p level does not lend itself to a similar analysis since the surface core level and the alkali-induced feature are both on the same, low binding energy side of the bulk peak [6], whereas they are well separated on the high and low binding energy side of the peak for the cation peak. In order to explain the Cs-induced changes in the In 4d core level line shape, the model of alkali metal-semiconductor bonding through charge transfer will be briefly discussed. The reconstructed surface causes the anion dangling bond state to be fully occupied; the corresponding cation state is shifted into the conduction band. Charge transfer from the alkali metal s level can only occur into this latter level, which is lowered in energy in this process. A peak due to this level should then appear in the region around the valence band maximum. The valence band spectra in Fig. lb indeed show a peak located about 0.4 eV above the clearly visible valence band maximum (VBM), which increases with deposition up to t9 ~ 0.3. Note that the nominal coverage has been scaled by taking the work function minimum as 19 ~ 0.5, in accordance with intensity attenuation and with our previous work on
Th. Chass£ et aL / Surface Science 331-333 (1995) 528-533
530
-2
1 0 relative binding energy (eV)
Cs/InP(11 O) T = 120K
valence band maximum region
(b)
O 1.20 1.00 t-
0.80 0.45 0.30
C s / G a P ( l l 0 ) [6]. We emphasize that the Cs surface atom density is 4.34 X 1014 cm -2 in bulk bcc Cs. A coverage corresponding to a complete Cs layer is about a half monolayer, which is 8.8 X 1014 cm -2 for InP(ll0) from the number density of substrate atoms in the (110) plane. In this manner coverages of 19 ~ 0.5 correspond to one Cs atom per two surface unit cells of InP(110). In the coverage regime beyond the work function minimum emission intensity near the Fermi level appears, signalling the beginning metallization of the overlayer [6]. This point is just reached in the top spectrum where a faint feature at E F can be distinguished. The fact that the new peak is clearly split off from the valence band precludes any interpretation in terms of a bulk state. We have previously argued [5,7] that this state is due to the previously empty cation dangling bond state; this interpretation is compatible with the experimental data, and is supported by the similarity of the evolution of the peak in the band gap with coverage with recent calculations of the density of states in this region [8], which for low coverages confirms our interpretation. Another interpretation could be that the new peak arises from the filled anion dangling bond surface state, which is shifted back into the band gap by an adsorbate-induced derelaxation of the surface. This would account for the decrease in intensity in the region just below the VBM, where the anion surface state is located [9]. In the relaxed geometry, this state exhibits a strong dispersion with a total band width of about 0.7 eV [9]. The Cs-induced feature, on the other hand, has a dispersion of less than 100 meV for kit along the [1-10] azimuth, i.e. along the rows in the (110) surface. For Na/GaAs(110), theoretical predictions show that this state has a very localized character [10]. It is obviously dangerous to compare peak dispersions in such different structural environments, such that for the moment this assignment remains unresolved.
0.15
0.00 I
43
I
I
I
VBM 44 45 kinetic energy (eV)
Fig. 1. (a) Set of In 4d core level spectra recorded for different Cs depositions as indicated. Note the shift in the surface core level emission at lower kinetic energies as indicated by the arrows. (b) Spectra of the valence band maximum region of InP(ll0) for different coverages of Cs as indicated. Cs induces a new peak located about 0.4 eV above the VBM; at coverages beyond one monolayer, emission near the Fermi edge appears as seen in the top spectrum.
531
Th. Chass~ et al./ Surface Science 331-333 (1995) 528-533
In order to derive quantitative data about the interaction between Cs and InP(ll0), we turn to a more detailed analysis of the core level data. These were derived from a line shape analysis based on the Levenberg-Marquardt algorithm [11], in which the peaks were modeled by three doublets, i.e. the bulk peak, the surface component, and the charge transfer peak at higher kinetic energy. The free parameters were peak separation, surface peak and Cs-induced peak Gaussian width, and branching ratio; all other parameters such as lifetime broadening, spin-orbit splitting, third-order polynomial background etc. were derived from the clean surface spectrum (and transferred to the new lines where necessary). An example of such line shape analyses is shown in Fig. 2a. From the line shape analysis of an entire set of spectra, the shift of the SCL peak with respect to the bulk line as a function of coverage f9 is derived as shown in Fig. 2b. The SCL intensity is found to decrease linearly with deposition, while that of the Cs-induced peak increases, demonstrating the connection between the two peaks. At higher coverages (~9> 0.5) the SCL peak interferes with Cs loss features on the intense main peaks such that a measurement of its intensity is difficult. Coverages here relate to the work function minimum which is reached at a coverage of 0.5, and is attributed to the completion of half a Cs layer [5]. The shift of the SCL peak is approximately linear with coverage, starting from 0.30 eV and rising to 0.48 eV at 6)= 0.3. At the highest coverages the SCL shift is probably less reliable since the intensity is rather low, and the peak width may well be affected by its proximity to the bulk line, and may also be related to barrier height inhomogeneities which have been shown to influence the line width [12]. The surface photovoltage effects observed at low temperatures are expected to smooth out these inhomogeneities. The observed shift in the SCL peak with metal deposition is not restricted to the C s / I n P ( l l 0 ) system; in fact, it seems to be a widespread phenomenon. Upon reexamination of core level photoemission results for A g / G a A s ( l l 0 ) [13] and N a / G a P ( l l 0 ) [5], it was found that in these systems the SCL also exhibits systematic shifts with metal coverage; these are included in the compilation in Fig. 2b. A g / G a A s ( l l 0 ) is of particular interest in
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Fig. 2. (a) Example of a In 4d line shape analysis in order to obtain quantitative data for SCL shift energies. The residuum is given below the data and the model function. (b) Separation between bulk-derived and SCL peaks for Cs/lnP(ll0), Na/GaP(ll0), and Ag/GaAs(ll0) as a function of metal overlayer coverage, showing the common trend of an increase with coverage.
this context. The alkali metal atoms induce a high kinetic energy component in the substrate core level [5-7,14], while Ag does not. Moreover, the peak above the VBM observed in the systems mentioned above, interpreted as due to the cation-derived surface state filled as a consequence of alkali metal adsorption, is conspicuously absent for A g / G a -
532
Th. Chass~ et al. // Surface Science 331-333 (1995) 528-533
As(ll0). These differences may be taken as an indication for a more covalent bonding between Ag and the GaAs substrate. In order to relate the changes in SCL peak position, the interpretation of SCLs needs to be briefly outlined. Core level shifts on semiconductor surfaces are generally interpreted in terms of initial state effects [15], even though there are cases such as Si(100) in which a specific valence level structure causes final state screening to have an effect [16,17]. The surface core level shifts on III-V semiconductor surfaces have been the subject of many experimental and theoretical studies [18-22]. For the clean surface, the major part of SCL shifts can be traced to the surface Madelung energy which is different in the surface from the bulk [3,18,23], without invoking any redistribution of charge between cation and anion at the surface. The influence of surface relaxation has also been dealt with, and there are conflicting predictions of its influence on the SCLs. The Madelung energy calculations yield an increase of the SCL with surface relaxation [3,18]. In a tight-binding calculation based on a local charge neutrality approximation, on the other hand, Priester et al. [19] found that the separation between the surface-shifted components actually increases when going from the relaxed to the ideal geometry, by 200 meV on average. A similar result was also arrived at in self-consistent tight-binding calculations by RodriguezHernandez and Munoz [21], where the average increase for the eight III-V compounds studied was 360 meV, and the increase in average cation-anion SCL separation was found to be 24% in a calculation with charge neutrality approximation, and 41% in the self-consistent calculation. As suggested by the calculated charge contour plots [8,10], the charge transfer process between the alkali metal atom and the substrate proceeds in a localized manner, such that adjacent surface atoms (and it is these, which still give rise to a SCL, that we are considering here) are not directly affected. The tight-binding calculations then suggest an interpretation of the shift in SCL with coverage which is caused by a lifting of the surface relaxation by metal deposition. Evidence for an unrelaxation of the GaAs(ll0) surface upon metal deposition has been derived from a number of different experiments [2426]. A detailed LEED study of K / G a A s ( l l 0 ) by
Ventrice and DiNardo [27] concludes from a kinematical analysis of I / V data that the surface becomes unrelaxed for K deposition. Total energy calculations by Hebenstreit et al. for N a / G a A s ( l l 0 ) also find that the Ga and As atoms underneath the Na atoms assume almost their bulk positions. We may therefore conclude that unrelaxation upon alkali metal deposition is a common occurrence. The above observation of an increase in SCL shift is then readily interpreted in terms of an unrelaxation by the surface surrounding a Cs atom on InP (and, likewise, the other surfaces for which data are given in Fig. 2b) by comparison with the tight-binding calculations of SCL shifts. The situation is complicated, of course, by the charge transfer that the adatoms induce. While the unrelaxation can explain the coverage-dependent shift of the SCL by comparison with the tight-binding calculations, one may speculate that it is partly related to surface atoms which, while not directly involved in the bonding interaction with an adatom (which would extinguish their contribution to the SCL peak), are in neighboring sites and may participate in the charge rearrangement caused by the deposition of metal atoms. However, the fact that a shift of the SCL peak is also induced by Ag atoms in Ag/GaAs(ll0), where charge transfer can be ruled out, renders this explanation unlikely. In this context it is interesting to note that the width of the SCL peak also exhibits an increase as Cs is deposited, increasing by about 40% up to half monolayer coverage. One may speculate that the unrelaxation of the surface is a local process, such that contributions from unit cells which are affected by the alkali metal atoms to a varying degree may overlap, giving rise to the apparent broadening as the coverage of Cs increases such that the free areas become successively smaller. In a similar manner, the shift with coverage can be explained. Consider an In surface atom not directly affected by the adsorbed Cs atoms. With increasing Cs coverage the number of Cs atoms in the vicinity of this In site will increase, such that the average distance to the Cs adsorption site will decrease. The increasing number of Cs atoms, which enhances the interaction with the In site, will then lead to a gradual shift of the SCL with coverage. In summary, a large shift in the surface core level peak in III-V semiconductor surfaces has been found upon metal deposition. This is interpreted in terms of
Th. Chass~ et al. / Surface Science 331-333 (1995) 528-533
a lifting of the b o n d angle r o t a t i o n - r e l a x a t i o n o f the clean surface induced by the metal adatoms, a process that has long b e e n suggested in the literature. Apart f r o m the general interest in structural changes resulting f r o m metal o v e r l a y e r formation, this observation is particularly important in cases w h e r e a line shape analysis o f core level p h o t o e m i s s i o n spectra is used for the determination o f F e r m i level positions, w h e r e a valid m o d e l function for the core level e m i s s i o n is required.
Acknowledgements This w o r k was supported by the Bundesministerium • r F o r s c h u n g und T e c h n o l o g i e under grant 05 5 E B F X B .
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