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The adsorption of H2S on InP (110) and GaP (110)
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The adsorption of H$3 on InP ( 110) and GaP
( 110)
E. DudziP? a, R. Whittle a, C. Miiller a, I.T. McGovern a, C. Nowak b, A. M&l A. Hempelmann b, D.R.T. Zahn b, A. Cafolla c, W. Braun d
b,
a Department of Pure and Applied Physics, Trinity College, Dublin 2, Ireland b Technische UniversitiitBerlin, Berlin, Germany ’ Dublin City University, Dublin, Ireland d BESSY, Berlin, Germany (Received 20 August 1993)
Abstract The adsorption of H$ on the InP ( 110) and GaP ( 110 1 surfaces is studied with core-level soft X-ray photoelectron spectroscopy (SXPS) and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) using synchrotron radiation and low energy electron diffraction (LEED). Similar effects are observed on both substrates. At dosages of the order of 5 langmuir every second spot in the clean 1 x 1 LEED pattern is extinguished. The sulphur 2p core level shows two components, a dominant, low-exposure component which scales with a satellite on the phosphorus 2p level and a lesser, high-exposure component that scales with a satellite on the indium 4d (gallium 3d) level. Possible adsorption geometries consistent with both the LEED pattern and the photoemission data are discussed.
1. Introduction Sulphur or sulphur containing compounds can be used for passivating semiconductor surfaces [ 1,2]. Exposure to hydrogen sulphide provides a simple method of introducing sulphur onto a surface. Although the ( 110) surface is not of prime technological interest, the fact that it can be easily and reproducibly prepared makes it an ideal substrate for initial study [ 3,4]. There have been several previous studies of the adsorption of HzS on the ( 110) surface of III-V semiconductors [ 12,13 1. Montgomery, Hughes and Williams [ 5,6] studied the adsorption of HZS on InP ( 110) at room temperature by He I angle-resolved photoelectron spectroscopy. They proposed that the adsorption was mostly * Corresponding
author.
molecular and reported that the LEED pattern changed on adsorption. The HzS was found to desorb easily upon heating the sample leaving the surface nominally clean. Ranke and coworkers [7-91 studied the adsorption of H# on several surfaces, including the ( 1 lo), of a cylindrical GaAs sample. At low temperature ( 150 K) a satellite was observed on the As 3d core level and features in the valenceband spectra were interpreted as the spectral lines of HzS molecularly adsorbed at the arsenic surface atoms. In the present study the room temperature adsorption of HzS on ( 110) surfaces of InP and GaP is studied by valence and core photoelectron spectroscopy using synchrotron radiation, employing higher photon energies to excite the deeper lying phosphorus and sulphur 2p core levels.
0039-6028/94/$07.00 @ 1994 Elsevier Science B.V. All rights reserved XSDZ0039-6028(93)E0811-8
224
E. Dudzik et al. /Surface Science 307-309 (1994) 223-22’
2. Experiment The experiments were performed at the Berlin electron synchrotron BESSY at the toroidal grating monochromator TGM II. The electron analyser was an ADES 400 (VG Ltd); the combined resolution was typically 250 meV for grating 2 used for the gallium and indium core levels, and about 450 meV for the “high energy” grating 1 used for the phosphorus and sulphur core levels. The samples - n-type InP and n-type GaP were cleaved in ultrahigh vacuum (pressure < 5 x lo-lo Torr) and afterwards dosed with HzS (Messer Griesheim, purity 99.6%). The dosing was increased step by step from 0.5 to 10 000 L. At each dosage step low energy electron diffraction was done and spectra of the P 2p, S 2p, In 4d (or Ga 3d) core levels were recorded as well as valence band spectra at the main symmetry points of the surface Brillouin zone. The core level spectra were fitted using leastsquares analysis. The background was fitted with a third order polynomial. The fits were done assuming equal Lorentzian and Gaussian linewidths and also equal spin-orbit splitting for all components.
3. Results and discussion The adsorption of HzS was found to be very similar on GaP and InP, so that the results for both can be discussed together. The only difference is that the adsorption appears to be faster on GaP; the change in the LEED pattern, for example, which occurs after dosing with about 5 L on GaP, appears only after dosing with 50 L on InP. The spectral changes also show this behaviour. 3. I. LEED observation The 1 x 1 LEED pattern of the clean surface changes to a pattern where every other spot is extinguished. This is the change reported by Montgomery et al. [ 61. The surface unit cell appears to have decreased in size by a factor l/v’% This could be due either to the adsorp-
tion of two sulphur-atoms per surface unit cell or to the adsorption of one sulphur atom which is centred in the 1 x 1 mesh of phosphorus atoms, treating phosphorus and sulphur atoms as near-equivalent scatterers. 3.2. Photoelectron spectra The core level fits for the clean surfaces agree well with the literature [ lo,11 1. All the substrate core levels show a larger Gaussian broadening after dosing, possibly due to uneven Fermi level pinning at the surface. Fig. 1 shows the changes in the phosphorus 2p and sulphur 2p core levels for progressive dosages of HlS on GaP ( 110). The P 2p core level shows a third component in addition to bulk and surface from the earliest dosages while the surface peak is correspondingly reduced in height. The new feature is shifted by about 0.8 eV towards higher binding energies relative to the bulk and saturates at about 5 L dosage. The sulphur 2p core level shows a single component at low dosages. The growth of this component (S-I) correlates well with that of the third phosphorus 2p component. At higher dosages (about 5 L), however, a second feature (S-II) develops at 0.7 eV lower binding energy relative to the first component. It saturates at about 50% of the S-I intensity. Assuming diffraction effects are not large and correcting for the photoionisation cross sections, the estimated ratio of sulphur atoms to surface phosphorus atoms is approximately 1 : 1. Fig. 2 shows the changes in the In 4d and Ga 3d core spectra. At low dosages the surface peak of In 4d is strongly reduced in height. At higher dosages, however, a third component appears shifted by 0.64 eV towards higher binding energies (for Ga 3d the changes begin at 1 L). The new component increases slowly with dosage and the surface component is not extinguished. The new component correlates well with the second sulphur component S-II. The relative heights of the two sulphur components S-I and S-II depend very differently on time, heating, irradiation with zero order synchrotron light and LEED. At low dosages (when
E. Dudzik et al. /Surface Science 307-309 (1994) 223-227
225
.._ 1000 Langmuir
21
28
29
30
31
32
33
34
kinetic energy in CV
34
3s
In 4d
36 37 38 39 kinetic energy in eV
40
/=+j 1, 5 Langmuir
Ca 3d
i
’
’
/
1 Langmuir
33 24
25
26 27 28 29 30 kinetic energy in L-V
31
Fig. 1. (a) Changes in the phosphorus 2p level; a sulphur related satellite appears at 1 L. The spectra were taken at fiw = 160 eV. (b) The sulphur 2p level at various dosages, showing two components. hw = 190 eV.
34 3.5 36 kinetic energy in eV
37
Fig. 2. Changes in the (a) In4d and the (b) Ga3d core levels after various dosages. Both core levels show a satellite at higher binding energies after dosage while the surface peak decreases in height. All spectra were taken at hw = 55 eV.
226
E. Dudzik et al. /Surface Science 307-309 (1994) 223-227
S-II is not yet present) S-I is not very stable and desorbs very easily overnight or after heating to 50°C. This desorption restores the clean surface; the surface component in the core levels and the valence band surface state come back. S-II, however, is stable against all these factors and can even increase after heating. Moreover the presence of S-II seems to stabilise S-I on the surface. Fig. 3 shows the effect of annealing a high dosage surface to 200°C; S-I and the phosphorus 2p satellite are correspondingly reduced but not totally removed. Annealing at higher temperatures to remove S-II (heating to about SOO’C) does not restore the clean surface (i.e. surface core shifts) and no LEED pattern is observed. The valence band spectra are strongly modified after adsorption. On dosing the surface state decreases in height and three new features are observed which have previously been attributed to the three ionisation levels of the HIS molecule. Another possible assignment is that of partially dissociated HS, as discussed for the adsorption of water vapour [ 141. These spectra will be discussed in more detail elsewhere [ 15 1. 3.3. Other results Some InP surfaces showed a modified adsorption where the second sulphur component S-II was present from the earliest exposure; this component remained at its initial height for all subsequent dosages while S-I increased with dosage in the usual way. The third In 4d component was also observed, in parallel with S-II. This is probably due to cleavage quality and suggests that the high dosage creation of S-II is connected with defect site adsorption.
4. Conclusions Two sulphur adsorption complexes labelled SI and S-II are observed in these studies. From the valence band data and their desorption behaviour it seems reasonable to assign S-I to adsorbed HzS or possibly HS. This complex seems to be associated with phosphorus atom sites. The minority S-II seems to be associated with indium
Fig. 3. The third phosphorus 2p component is diminished after an anneal to 200°C; at the same time the S-I component is reduced in height.
atom sites. The nature of this complex is unclear. The presence of two complexes provides a straightforward explanation for the new LEED pattern in terms of two sulphur atoms per surface unit cell, although S-II is not present in the
E. Dudzik et al. /Surface Science 307-309 (1994) 223-227
same amount as S-I. The alternative is that S-I sulphur atoms and the phosphorus atoms combine to create the new diffraction structure. The new LEED pattern is only observed when there is a significant amount of S-II present. But this may reflect the increased stability of S-I to the LEED electron beam in the presence of S-II. Future measurements will focus on these questions.
Acknowledgements E.D., R.W. and I.T.McG. would like to thank the Large Scale Installations Plan and the Science and Technology Cooperation GermanyIreland for funding. We would also like to thank Dr. W. Ranke (FHI Berlin) for the loan of his H# dosing apparatus and Dr. M. Bridge (TCD) and Professor N. Richardson (IRC Liverpool) for their advice on LEED.
References [ 1] E. Kaxiras, Phys. Rev. B 43 ( 1991) 6824.
227
[ 21 H. Sugahara, M. Oshima, H. Oigawa and Y. Nannichi, Surf. Sci. 242 (1991) 335. [ 31 I.T. McGovern, R. Whittle, D.R.T. Zahn, C. Miiller, C. Nowak, A. Cafolla and W. Braun, J. Phys. Condens. Matter 3 ( 1991) 367. [4] R. Whittle, LT. McGovern, D.R.T. Zahn, C. Miiller, C. Nowak, A. Cafolla and W. Braun, Appl. Surf. Sci. 56-58 (1992) 218. [ 51 G.J. Hughes, T.P. Humphreys and R.H. Williams, Vacuum 31 (1981) 539. [6] V. Montgomery, R.H. Williams and G.P. Srivastava, J. Phys. C: Solid State Phys. 14 (1981) L191. [7] H.J. Kuhr, W. Ranke and J. Finster, Surf. Sci. 178 (1986) 171. [8] W. Ranke, J. Finster and H.J. Kuhr, Surf. Sci. 187 (1987) 112. [9] W. Ranke, H.J. Kuhr and J. Finster, Surf. Sci. 192 (1987) 81. [lo] W.G. Wilke, V. Hinkel, W. Theis and K. Horn, Phys. Rev. B 40 (1989) 9824. [ 111 A.B. McLean and R. Ludeke, Phys. Rev. B 39 (1989) 6223. [ 121 R.W.M. Kwok and W.M. Lau, J. Vat. Sci. Technol. A 10 (1992) 2515. [ 13 ] A. J. Nelson, S. Frigo and R. Rosenberg, J. Appl. Phys. 71 (1992) 6086. [ 141 K. Fives, R. McGrath, C. Stephens, LT. McGovern, R. Cimino, D.S.-L. Law and G. Thornton, J. Phys. Condens. Matter 1 (1989) SB247. [ 151 E. Dudzik et al., to be published.
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,.,. .A. .::; ... .3:.:.:.>,.: .:,,. ‘:‘I:.. ::::::. ::::I :.I.. .:. :.:.:,:,:, ..:: ...:. :,:,..:.,.,..:.: :). :G::: xc: :0’ ,:,: ,...,((,. ‘.‘.‘.‘. ‘.““,.:.:.:,“::.:,:,~::‘:::ii:::j::::: .:.:_ ~::::::... ....’ ‘....... ..................... :.,.,.: Surface Science 307-309 (1994) 223-227
The adsorption of H$3 on InP ( 110) and GaP
( 110)
E. DudziP? a, R. Whittle a, C. Miiller a, I.T. McGovern a, C. Nowak b, A. M&l A. Hempelmann b, D.R.T. Zahn b, A. Cafolla c, W. Braun d
b,
a Department of Pure and Applied Physics, Trinity College, Dublin 2, Ireland b Technische UniversitiitBerlin, Berlin, Germany ’ Dublin City University, Dublin, Ireland d BESSY, Berlin, Germany (Received 20 August 1993)
Abstract The adsorption of H$ on the InP ( 110) and GaP ( 110 1 surfaces is studied with core-level soft X-ray photoelectron spectroscopy (SXPS) and angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) using synchrotron radiation and low energy electron diffraction (LEED). Similar effects are observed on both substrates. At dosages of the order of 5 langmuir every second spot in the clean 1 x 1 LEED pattern is extinguished. The sulphur 2p core level shows two components, a dominant, low-exposure component which scales with a satellite on the phosphorus 2p level and a lesser, high-exposure component that scales with a satellite on the indium 4d (gallium 3d) level. Possible adsorption geometries consistent with both the LEED pattern and the photoemission data are discussed.
1. Introduction Sulphur or sulphur containing compounds can be used for passivating semiconductor surfaces [ 1,2]. Exposure to hydrogen sulphide provides a simple method of introducing sulphur onto a surface. Although the ( 110) surface is not of prime technological interest, the fact that it can be easily and reproducibly prepared makes it an ideal substrate for initial study [ 3,4]. There have been several previous studies of the adsorption of HzS on the ( 110) surface of III-V semiconductors [ 12,13 1. Montgomery, Hughes and Williams [ 5,6] studied the adsorption of HZS on InP ( 110) at room temperature by He I angle-resolved photoelectron spectroscopy. They proposed that the adsorption was mostly * Corresponding
author.
molecular and reported that the LEED pattern changed on adsorption. The HzS was found to desorb easily upon heating the sample leaving the surface nominally clean. Ranke and coworkers [7-91 studied the adsorption of H# on several surfaces, including the ( 1 lo), of a cylindrical GaAs sample. At low temperature ( 150 K) a satellite was observed on the As 3d core level and features in the valenceband spectra were interpreted as the spectral lines of HzS molecularly adsorbed at the arsenic surface atoms. In the present study the room temperature adsorption of HzS on ( 110) surfaces of InP and GaP is studied by valence and core photoelectron spectroscopy using synchrotron radiation, employing higher photon energies to excite the deeper lying phosphorus and sulphur 2p core levels.
0039-6028/94/$07.00 @ 1994 Elsevier Science B.V. All rights reserved XSDZ0039-6028(93)E0811-8
224
E. Dudzik et al. /Surface Science 307-309 (1994) 223-22’
2. Experiment The experiments were performed at the Berlin electron synchrotron BESSY at the toroidal grating monochromator TGM II. The electron analyser was an ADES 400 (VG Ltd); the combined resolution was typically 250 meV for grating 2 used for the gallium and indium core levels, and about 450 meV for the “high energy” grating 1 used for the phosphorus and sulphur core levels. The samples - n-type InP and n-type GaP were cleaved in ultrahigh vacuum (pressure < 5 x lo-lo Torr) and afterwards dosed with HzS (Messer Griesheim, purity 99.6%). The dosing was increased step by step from 0.5 to 10 000 L. At each dosage step low energy electron diffraction was done and spectra of the P 2p, S 2p, In 4d (or Ga 3d) core levels were recorded as well as valence band spectra at the main symmetry points of the surface Brillouin zone. The core level spectra were fitted using leastsquares analysis. The background was fitted with a third order polynomial. The fits were done assuming equal Lorentzian and Gaussian linewidths and also equal spin-orbit splitting for all components.
3. Results and discussion The adsorption of HzS was found to be very similar on GaP and InP, so that the results for both can be discussed together. The only difference is that the adsorption appears to be faster on GaP; the change in the LEED pattern, for example, which occurs after dosing with about 5 L on GaP, appears only after dosing with 50 L on InP. The spectral changes also show this behaviour. 3. I. LEED observation The 1 x 1 LEED pattern of the clean surface changes to a pattern where every other spot is extinguished. This is the change reported by Montgomery et al. [ 61. The surface unit cell appears to have decreased in size by a factor l/v’% This could be due either to the adsorp-
tion of two sulphur-atoms per surface unit cell or to the adsorption of one sulphur atom which is centred in the 1 x 1 mesh of phosphorus atoms, treating phosphorus and sulphur atoms as near-equivalent scatterers. 3.2. Photoelectron spectra The core level fits for the clean surfaces agree well with the literature [ lo,11 1. All the substrate core levels show a larger Gaussian broadening after dosing, possibly due to uneven Fermi level pinning at the surface. Fig. 1 shows the changes in the phosphorus 2p and sulphur 2p core levels for progressive dosages of HlS on GaP ( 110). The P 2p core level shows a third component in addition to bulk and surface from the earliest dosages while the surface peak is correspondingly reduced in height. The new feature is shifted by about 0.8 eV towards higher binding energies relative to the bulk and saturates at about 5 L dosage. The sulphur 2p core level shows a single component at low dosages. The growth of this component (S-I) correlates well with that of the third phosphorus 2p component. At higher dosages (about 5 L), however, a second feature (S-II) develops at 0.7 eV lower binding energy relative to the first component. It saturates at about 50% of the S-I intensity. Assuming diffraction effects are not large and correcting for the photoionisation cross sections, the estimated ratio of sulphur atoms to surface phosphorus atoms is approximately 1 : 1. Fig. 2 shows the changes in the In 4d and Ga 3d core spectra. At low dosages the surface peak of In 4d is strongly reduced in height. At higher dosages, however, a third component appears shifted by 0.64 eV towards higher binding energies (for Ga 3d the changes begin at 1 L). The new component increases slowly with dosage and the surface component is not extinguished. The new component correlates well with the second sulphur component S-II. The relative heights of the two sulphur components S-I and S-II depend very differently on time, heating, irradiation with zero order synchrotron light and LEED. At low dosages (when
E. Dudzik et al. /Surface Science 307-309 (1994) 223-227
225
.._ 1000 Langmuir
21
28
29
30
31
32
33
34
kinetic energy in CV
34
3s
In 4d
36 37 38 39 kinetic energy in eV
40
/=+j 1, 5 Langmuir
Ca 3d
i
’
’
/
1 Langmuir
33 24
25
26 27 28 29 30 kinetic energy in L-V
31
Fig. 1. (a) Changes in the phosphorus 2p level; a sulphur related satellite appears at 1 L. The spectra were taken at fiw = 160 eV. (b) The sulphur 2p level at various dosages, showing two components. hw = 190 eV.
34 3.5 36 kinetic energy in eV
37
Fig. 2. Changes in the (a) In4d and the (b) Ga3d core levels after various dosages. Both core levels show a satellite at higher binding energies after dosage while the surface peak decreases in height. All spectra were taken at hw = 55 eV.
226
E. Dudzik et al. /Surface Science 307-309 (1994) 223-227
S-II is not yet present) S-I is not very stable and desorbs very easily overnight or after heating to 50°C. This desorption restores the clean surface; the surface component in the core levels and the valence band surface state come back. S-II, however, is stable against all these factors and can even increase after heating. Moreover the presence of S-II seems to stabilise S-I on the surface. Fig. 3 shows the effect of annealing a high dosage surface to 200°C; S-I and the phosphorus 2p satellite are correspondingly reduced but not totally removed. Annealing at higher temperatures to remove S-II (heating to about SOO’C) does not restore the clean surface (i.e. surface core shifts) and no LEED pattern is observed. The valence band spectra are strongly modified after adsorption. On dosing the surface state decreases in height and three new features are observed which have previously been attributed to the three ionisation levels of the HIS molecule. Another possible assignment is that of partially dissociated HS, as discussed for the adsorption of water vapour [ 141. These spectra will be discussed in more detail elsewhere [ 15 1. 3.3. Other results Some InP surfaces showed a modified adsorption where the second sulphur component S-II was present from the earliest exposure; this component remained at its initial height for all subsequent dosages while S-I increased with dosage in the usual way. The third In 4d component was also observed, in parallel with S-II. This is probably due to cleavage quality and suggests that the high dosage creation of S-II is connected with defect site adsorption.
4. Conclusions Two sulphur adsorption complexes labelled SI and S-II are observed in these studies. From the valence band data and their desorption behaviour it seems reasonable to assign S-I to adsorbed HzS or possibly HS. This complex seems to be associated with phosphorus atom sites. The minority S-II seems to be associated with indium
Fig. 3. The third phosphorus 2p component is diminished after an anneal to 200°C; at the same time the S-I component is reduced in height.
atom sites. The nature of this complex is unclear. The presence of two complexes provides a straightforward explanation for the new LEED pattern in terms of two sulphur atoms per surface unit cell, although S-II is not present in the
E. Dudzik et al. /Surface Science 307-309 (1994) 223-227
same amount as S-I. The alternative is that S-I sulphur atoms and the phosphorus atoms combine to create the new diffraction structure. The new LEED pattern is only observed when there is a significant amount of S-II present. But this may reflect the increased stability of S-I to the LEED electron beam in the presence of S-II. Future measurements will focus on these questions.
Acknowledgements E.D., R.W. and I.T.McG. would like to thank the Large Scale Installations Plan and the Science and Technology Cooperation GermanyIreland for funding. We would also like to thank Dr. W. Ranke (FHI Berlin) for the loan of his H# dosing apparatus and Dr. M. Bridge (TCD) and Professor N. Richardson (IRC Liverpool) for their advice on LEED.
References [ 1] E. Kaxiras, Phys. Rev. B 43 ( 1991) 6824.
227
[ 21 H. Sugahara, M. Oshima, H. Oigawa and Y. Nannichi, Surf. Sci. 242 (1991) 335. [ 31 I.T. McGovern, R. Whittle, D.R.T. Zahn, C. Miiller, C. Nowak, A. Cafolla and W. Braun, J. Phys. Condens. Matter 3 ( 1991) 367. [4] R. Whittle, LT. McGovern, D.R.T. Zahn, C. Miiller, C. Nowak, A. Cafolla and W. Braun, Appl. Surf. Sci. 56-58 (1992) 218. [ 51 G.J. Hughes, T.P. Humphreys and R.H. Williams, Vacuum 31 (1981) 539. [6] V. Montgomery, R.H. Williams and G.P. Srivastava, J. Phys. C: Solid State Phys. 14 (1981) L191. [7] H.J. Kuhr, W. Ranke and J. Finster, Surf. Sci. 178 (1986) 171. [8] W. Ranke, J. Finster and H.J. Kuhr, Surf. Sci. 187 (1987) 112. [9] W. Ranke, H.J. Kuhr and J. Finster, Surf. Sci. 192 (1987) 81. [lo] W.G. Wilke, V. Hinkel, W. Theis and K. Horn, Phys. Rev. B 40 (1989) 9824. [ 111 A.B. McLean and R. Ludeke, Phys. Rev. B 39 (1989) 6223. [ 121 R.W.M. Kwok and W.M. Lau, J. Vat. Sci. Technol. A 10 (1992) 2515. [ 13 ] A. J. Nelson, S. Frigo and R. Rosenberg, J. Appl. Phys. 71 (1992) 6086. [ 141 K. Fives, R. McGrath, C. Stephens, LT. McGovern, R. Cimino, D.S.-L. Law and G. Thornton, J. Phys. Condens. Matter 1 (1989) SB247. [ 151 E. Dudzik et al., to be published.