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A photoemission study of the (2 × 2) reconstructions of GaAs{111} surfaces
Journal of Electron Spectroscopyand Related Phenomena 72 (1995) 65-69
A photoemission study of the (2 × 2) reconstructions of GaAs{lll} surfaces J. M. C. Thornton a, P. Weightman a, D. A. Woolfb, and C. J. Dunscombe b aIRC in Surface Science, University of Liverpool, Liverpool, L69 3BX, U. K. bDepartment of Physics, U.W.C.C.P.O. Box 913, Cardiff, CF2 3YB, U. I~ The {111} surfaces of GaAs have been examined using soft X-ray photoemission (SXPS) following the thermal desorption of a protective As cap. A change in As and Ga 3d core-level lineshape has been correlated with a known phase transition from an As-trimer (2 × 2) reconstruction to a Gavacancy (2 × 2) with increasing temperature. The photoemission from the Ga-vacancy surface is found to be very similar to the cleaved (110) surface, on which a rehybridization occurs resulting in the separation of empty and filled surface orbitals on to Ga and As atoms respectively. The ( l l l ) B surface also yields a (2 x 2) reconstruction after de-capping, with the As-rich nature of the Astrimer reconstruction causing the Ga emission to be entirely bulk-like. Two surface components are found in the As 3d emission, which are attributed to trimer and rest-atom features on the surface 1. INTRODUCTION The {111} surfaces of GaAs have been largely neglected until recently due to the difficulty of their preparation and the dominating interest in the (110) surface prepared by cleavage. Such surfaces, in the ideal case, yield non-polar surfaces free from electronic states in the band gap. This has offered a useful platform from which to study the developm e n t of gap-states and Schottky barriers with the deposition of metal overlayers. A new interest in the {111} surfaces has arisen recently following the attention paid to them by the epitaxial growth community. Strained heteroepitaxial structures have been shown to exhibit interesting optical properties when grown on these surfaces, largely as a result of the absence of inversion symmetry about the polar interface. The surface reconstructions of the ( l l l ) A and ( l l l ) B orientations bear little similarity, with large differences in growth mode [1] and doping properties [21 also pointing to the kinetic behaviour and dopant incorporation mechanisms being markedly different between the two.
Both the ( l l l ) A and ( l l l ) B exhibit (2 x 2) reconstructions, with the ( l l l ) B transforming into the (1 x 1)LT, (~/19 x ~119) and (1 x 1)HT at higher temperatures [2]. By comparison, the (111)A surface remains as a (2 x 2) over a vast range of temperatures and fluxes, though recent scanning tunnelling microscopy (STM) studies have shown that at least two different (2 x 2) structures exist [3] which are stabilized by As-trimers and Ga-vacancies accordingly. Surface sensitive photoemission experiments have been performed on the various reconstructions found on GaAs{111} surfaces using synchrotron radiation. All the samples were prepared in the same way and so bear direct comparison. This avoids the uncertainty prevalent in the comparison of similar investigations, where a range of surface preparation methods have been used [4, 5]. It has been possible to characterize the reconstructions found on these surfaces through a detailed analysis of the core-level lineshapes, which has also shed light on the nature of the phase transitions between them. By comparison with earlier STM studies, it has also been possible to associate individual components in the photoemission data with specific surface environments.
0368-2048/95 $09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0368-2048(94)02337-9
66 2. EXPERIMENTAL The samples used for this study were grown by molecular beam epitaxy (MBE) on Zn-doped (P~1019 cm"3) on-axis GaAs(lll)A and Si-doped (N--10 Is cm"3) GaAs(lll)B substrates. GaAs buffer layers of 0. l~m thickness were grown, which were doped p- and n-type with Si ((lll)A and B respectively) to a concentration of ~1018 cm"3. The samples were then capped with As which allowed them to be transported between laboratories in air. Decapping was achieved by heating to 315°C in UHV, at which point the As desorbed as seen using a mass spectrometer. Subsequent anneals were performed with increasing temperature to drive off the excess As from the surfaces, which resulted in the different reconstructions being observed using low energy electron diffraction (LEED). A (2 x 2) LEED pattern was always seen immediately following a de-capping, regardless of orientation. The photoemission experiments were performed at the synchrotron radiation source (SRS) Daresbury laboratory, U. K. using the grazing incidence monochromator on beamline 6.1. This provided a photon flux in the range 50-170 eV, with measurements made using 70 eV and 95 eV radiation. A double pass cylindrical mirror analyser (CMA) was used to measure the photocurrent, resulting in a total instrumental resolution of 150 meV and 200 meV at photon energies of 70 and 95 eV respectively. The samples were bonded to Mo holders with In, with both the (lll)A and (lll)B samples mounted alongside each other in order that a direct comparison could be made. 3. ANALYSIS The surface structure of semiconductors can be very complicated, with the result that core-levels measured with surface sensitive photoemission can be the sum of several components arising from the range of surface environments. It is therefore especially important that great care is taken when
deconvolution of the data is made in order to avoid a non-physical interpretation being arrived at. Further complications can include phonon broadening, surface roughness and inhomogeneous pinning of the Fermi level, all of which broaden the measured photoemission peaks. These factors combine in the case of GaAs particularly, such that their contribution to the overall resolution dominates those of the monochromator and electron analyser. When fitting the core-level data, therefore, well established values were used for the Lorentzian components to the lineshape (0.155 eV and 0.170 eV for the Ga and As 3d levels respectively) [6]. The Gaussian component required proved to be much larger than the instrumental resolution, and was found to be significantly different between Ga and As peaks measured under the same conditions. This is thought to reflect differences in phonon broadening, since the bulk peaks behave in this way as well as the surface components. Throughout the fitting procedure, a consistent policy was adhered to so that the simplest fit be achieved, using the minimum of components, and keeping as many parameters constant as possible. To this end, fixed spin-orbit splittings and branching ratios were used and an emphasis on relative energy differences between bulk and surface components, and that between the Ga and As bulk peaks. The most surface sensitive spectra were collected using 95 eV radiation, and the consistency of fits between the two sets of data obtained with different photon energies is another indicator of the quality of fit. The energy shifts of the surface core level shifts (SCLS) were found to remain constant between the two datasets, and their intensity ratio with respect to the bulk emission behaved as expected from escape depth considerations. The energy difference between the Ga and As 3d bulk components was kept fixed at 21.95 eV, which was determined from the (lll)B spectra collected at the more bulksensitive photon energy of 70 eV. It is important to keep this a fixed parameter if meaningful fits are to be made from the essentially featureless Ga 3d peaks. Cubic or quadratic
67 backgrounds were subtracted simultaneous with the fitting procedure, which employed both Levenberg-Marquadt and conjugate gradients minimization techniques. 4. RESULTS The Ga and As 3d core-levels from the GaAs(111)A surface are shown as a function of annealing t e m p e r a t u r e in Fig. 1. They were obtained using the more bulk-sensitive 70 eV radiation, which delivered slightly improved resolution, as well as significant surface to bulk intensity ratios. Consider first the As 3d peaks, which j u s t after de-capping can only (2~)
_
I
3tS'C I
3SO'C
21.0" z/.0." il.o " d.0 " il.0. " i~0 43.0" ~.0. " ;d.o " 41.0." ii.0. " 4~ K.E. / eV
Figure 1. As and Ga 3d core-level spectra as a function of t e m p e r a t u r e from theGaAs(111)A(2 × 2)-reconstructions. fitted with at least two SCLS components in addition to the bulk ($1, 82). They bear close similarity to the (001) surface following a similar preparation [6], in which a low binding energy (LBE) component was attributed to As-dimers in the (2 × 4) structure, or threefold cooordinated sites for As in the (4 × 2). Another LBE component is also seen on the (110) surface, so it appears that a variety of threefold coordinated sites for As on a GaAs surface result in a LBE shifted component on
the As 3d peak. The high binding energy (HBE) component is attributed to excess As, in agreement with previous studies. As the annealing temperature increases, it is clear that the bulk-like excess As desorbs after only a small increment, leaving the As 3d emission best fitted by just the bulk and and one SCLS (S1). This then remains the case up to 400°C. By keeping the energy difference between the bulk Ga and As peaks to 21.95 eV, it is evident that j u s t after de-capping at 315°C there is a shifted component to LBE on the Ga 3d peak (S1, Fig. 1). With a small increase in temperature up to 350°C, a HBE component (S 2) is needed to obtain a good fit. Higher temperatures result in further diminution of S 1, and the increase in intensity of $2. This transition in the component required to fit the Ga 3d peakshape is entirely consistent with a recent STM study, which shows a transition from (2 × 2)-trimer to a (2 × 2)-vacancy reconstruction. The higher anneal temperature peakshapes closely resemble those of the (110) surface. This is not unexpected since the STM showed that a relaxation occurs, in which the surface bilayer collapses, and a rehybridization in a manner very close to the (110) takes place. We therefore attribute the SCLS's on the Ga and As 3d core-levels to be associated with the surface Ga atoms, and the As atoms surrounding the Ga-vacancy respectively. The similarity of the core-level emission between the two orientations is not so surprising, since the local bonding and electronic structure are remarkably close. The fact that the Ga 3d is not completely bulk-like, as found for the ( l l l ) B surface, reveals that there is a surface Ga environment. Furthermore, that only one component is required to low binding energy to fit the lowest temperature peak is indicative that the Ga-vacancy structure is not existent at this stage. The S 1 component diminishes with increasing anneal temperature, and so appears intimately linked with the As-trimer (2 × 2) structure seen by STM. We therefore assign the S 1 component to the Ga rest-atom in the (2 × 2)-trimer reconstruction observed
68 under similar conditions. The conclusion from the Ga 3d emission that the Ga-vacancy structure is not present at 315°C determines that the component S 1 in the As 3d emission is not due to t h a t structure either. The most likely conclusion is t h a t the S1 SCLS is due to a threefold coordinated As in an As-trimer a r r a n g e m e n t at low temperatures, which then transforms into threefold coordinated As around a Ga vacancy at higher temperatures. The intensity of these features would be expected to be constant since both structures have the same n u m b e r of surface As atoms per (2 x 2) unit cell. It appears at first to be a remarkable coincidence that both environments should produce the same SCLS on the As 3d emission, though if we consider the similarity in peakshape from the (110), (001)-(2 x 4), and (001)-(4 x 2) surfaces, it is not so unusual. The ideal G a A s ( l l l ) B surface is terminated by a half-bilayer of As, and so it can be expected to find an As-rich surface, especially after a thermal de-capping of As. From STM studies [3,4], we might expect to find spectra consistent with As-trimers and As rest-atoms from a (2 x 2) reconstructed surface. As and Ga 3d core-levels obtained with 70 eV radiation are shown as a function of anneal temperature in Fig. 2. The first thing to notice is the complete lack of surface components on the Ga 3d peaks over the entire temperature range. This confirms that the top two layers of the (2 x 2) reconstructed surface are composed entirely of As, with no surface environment for Ga. It is clear from this alone that the (Ill)B-(2 x 2)-trimer surface is quite distinct from the (Ill)A-(2 x 2)-trimer structure, which displayed a large SCLS on the Ga 3d peak. The As 3d peaks are much more complex, requiring two SCLS peaks to fit the data. These are similar to Cai et a]. [7] though the data shown here is angle integrated and of a higher resolution. This is probably more a function of sample quality than instrumental resolution. Following the results from the associated STM study of identically prepared samples [3], we assign the HBE component to
(2x2)
B :~O'C
[(~2)
A
=
,i
21,0 22.0 23.0 24.0 ~,0 26.043,0 44.0 45.0 46,0 47.0 4&O K.E,/eV
Figure 2. As and Ga 3d core-level spectra as a function of anneal temperature from the GaAs(lll)B-(2 x 2-As trimer reconstruction. bulk-like As-trimers (since they bond to an As second layer on the ( l l l ) B surface) and the LBE component to the As rest-atom (i.e. the surface As atoms not bonded to a trimer). The highest temperature peaks (at 425°C) are from the (1 × 1)LT structure, which is intermediate between the (2 x 2) and the (~]19 x ~]19) structures. It is clear from the As 3d peak that the feature attributed to As trimers has dramatically reduced in intensity, as one might expect as As desorbs at higher temperatures. The desorption of As is also reflected in the Ga 3d emission, which now displays a large SCLS due to Ga being being revealed at the surface. The detailed structure of this surface is not yet fully understood, though clearly it is much less As-rich than the (2 x 2)-trimer reconstruction. Another means of comparing the (IlI)A(2 x 2) and (Ill)B-(2 x 2) surfaces is to consider the valence band emission. Angleresolved studies have been reported which show a large difference between the (Ill)Avacancy and ( l l l ) B - t r i m e r surfaces [8], notably in the top-most band. Valence bands meas-
69 ured using 95 eV radiation for the (Ill)A-(2 x 2)-Ga-vacancy and (111)B-(2 x 2)-As-trimer reconstructions are shown in Fig. 3. They have been normalized to the highest and lowest energy background levels, and so the intensities of the band features can be compared.
(II1)B
.=
an angle-resolved study, it is clear that the electronic structure of these surfaces is strikingly different. 5. CONCLUSIONS We have thermally de-capped protective As layers from both the (111)A and (111)B surfaces of GaAs, and examined the resulting reconstructions using surface-sensitive photoemission. It has been shown that a transition takes place in the As and Ga 3d core-level peakshapes of the (Ill)A-(2 x 2) reconstructed surface, confirming an observed phase transition from STM experiments. The less As-rich Ga-vacancy phase is found to exhibit both core-level lineshapes and valence band emission similar to the (110) surface, due to similar bond rehybridization. The photoemission from the (lll)B surface is entirely in agreement with an As-trimer model, with no surface Ga present, and surface states localized on the As-trimer adstructures. REFERENCES
75.0
I
I
l
77.0
79.0
81.0
I
I
83.0 85.0 K.E. / eV
I
1
I
87.0
89.0
91.0
Figure 3. Valence band spectra from the GaAs (111)A-(2 x 2)-Ga vacancy and (111)B-(2 x 2) As-trimer reconstructions. The lower spectrum is from the (111)A surface, which bears a close resemblance to the emission seen from the cleaved (110) surface. This is entirely consistent with the corelevel data as well as the associated STM results [3]. The upper spectum, however, has a much higher intensity in the region up to 5 eV down from the valence band maximum. In particular, one sharp peak is dominant, which is indicative of highly localized states which can be expected to be associated with the Astrimer adstructure. Even without the sensio tivity to specific surface states enjoyed with
1. A. M. Dabiran, P. I. Cohen, J. E. Angelo and W. W. Gerberich, Thin Solid Films 231 (1993) 1. 2. D. A. Woolf, D. I. Westwood and R. H. Williams, Semicond. Sci. Technol. 8 (1993) 1075. 3. J. M. C. Thornton, M. D. Jackson, P. Unsworth, P. Weightman and D. A. Woolf, Surf. Sci. In-press. 4. D. I~ Beigelsen, R. D. Bringans, J. E. Northrup and L. -E. Swartz, Phys. Rev. Lett. 65 (1990) 452. 5. I~ W. Haberern and M. D. Pashley, Phys. Rev. B 41 (1990) 3226. 6. G. Le Lay, G. Mao, A. Kahn and G. Margaritondo, Phys. Rev. B 43 (1991) 14301. 7. Y. Q. Cai, R. C. G. Leckey, J. D. Riley, R. Denecke, J. Faul and L. Ley, J. Electron Spectrosc. Relat. Phenom. 61 (1993) 275. 8. R. D. Bringans and R. Z. Bachrach, Phys. Rev. Lett. 53 (1984) 1954.
A photoemission study of the (2 × 2) reconstructions of GaAs{lll} surfaces J. M. C. Thornton a, P. Weightman a, D. A. Woolfb, and C. J. Dunscombe b aIRC in Surface Science, University of Liverpool, Liverpool, L69 3BX, U. K. bDepartment of Physics, U.W.C.C.P.O. Box 913, Cardiff, CF2 3YB, U. I~ The {111} surfaces of GaAs have been examined using soft X-ray photoemission (SXPS) following the thermal desorption of a protective As cap. A change in As and Ga 3d core-level lineshape has been correlated with a known phase transition from an As-trimer (2 × 2) reconstruction to a Gavacancy (2 × 2) with increasing temperature. The photoemission from the Ga-vacancy surface is found to be very similar to the cleaved (110) surface, on which a rehybridization occurs resulting in the separation of empty and filled surface orbitals on to Ga and As atoms respectively. The ( l l l ) B surface also yields a (2 x 2) reconstruction after de-capping, with the As-rich nature of the Astrimer reconstruction causing the Ga emission to be entirely bulk-like. Two surface components are found in the As 3d emission, which are attributed to trimer and rest-atom features on the surface 1. INTRODUCTION The {111} surfaces of GaAs have been largely neglected until recently due to the difficulty of their preparation and the dominating interest in the (110) surface prepared by cleavage. Such surfaces, in the ideal case, yield non-polar surfaces free from electronic states in the band gap. This has offered a useful platform from which to study the developm e n t of gap-states and Schottky barriers with the deposition of metal overlayers. A new interest in the {111} surfaces has arisen recently following the attention paid to them by the epitaxial growth community. Strained heteroepitaxial structures have been shown to exhibit interesting optical properties when grown on these surfaces, largely as a result of the absence of inversion symmetry about the polar interface. The surface reconstructions of the ( l l l ) A and ( l l l ) B orientations bear little similarity, with large differences in growth mode [1] and doping properties [21 also pointing to the kinetic behaviour and dopant incorporation mechanisms being markedly different between the two.
Both the ( l l l ) A and ( l l l ) B exhibit (2 x 2) reconstructions, with the ( l l l ) B transforming into the (1 x 1)LT, (~/19 x ~119) and (1 x 1)HT at higher temperatures [2]. By comparison, the (111)A surface remains as a (2 x 2) over a vast range of temperatures and fluxes, though recent scanning tunnelling microscopy (STM) studies have shown that at least two different (2 x 2) structures exist [3] which are stabilized by As-trimers and Ga-vacancies accordingly. Surface sensitive photoemission experiments have been performed on the various reconstructions found on GaAs{111} surfaces using synchrotron radiation. All the samples were prepared in the same way and so bear direct comparison. This avoids the uncertainty prevalent in the comparison of similar investigations, where a range of surface preparation methods have been used [4, 5]. It has been possible to characterize the reconstructions found on these surfaces through a detailed analysis of the core-level lineshapes, which has also shed light on the nature of the phase transitions between them. By comparison with earlier STM studies, it has also been possible to associate individual components in the photoemission data with specific surface environments.
0368-2048/95 $09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0368-2048(94)02337-9
66 2. EXPERIMENTAL The samples used for this study were grown by molecular beam epitaxy (MBE) on Zn-doped (P~1019 cm"3) on-axis GaAs(lll)A and Si-doped (N--10 Is cm"3) GaAs(lll)B substrates. GaAs buffer layers of 0. l~m thickness were grown, which were doped p- and n-type with Si ((lll)A and B respectively) to a concentration of ~1018 cm"3. The samples were then capped with As which allowed them to be transported between laboratories in air. Decapping was achieved by heating to 315°C in UHV, at which point the As desorbed as seen using a mass spectrometer. Subsequent anneals were performed with increasing temperature to drive off the excess As from the surfaces, which resulted in the different reconstructions being observed using low energy electron diffraction (LEED). A (2 x 2) LEED pattern was always seen immediately following a de-capping, regardless of orientation. The photoemission experiments were performed at the synchrotron radiation source (SRS) Daresbury laboratory, U. K. using the grazing incidence monochromator on beamline 6.1. This provided a photon flux in the range 50-170 eV, with measurements made using 70 eV and 95 eV radiation. A double pass cylindrical mirror analyser (CMA) was used to measure the photocurrent, resulting in a total instrumental resolution of 150 meV and 200 meV at photon energies of 70 and 95 eV respectively. The samples were bonded to Mo holders with In, with both the (lll)A and (lll)B samples mounted alongside each other in order that a direct comparison could be made. 3. ANALYSIS The surface structure of semiconductors can be very complicated, with the result that core-levels measured with surface sensitive photoemission can be the sum of several components arising from the range of surface environments. It is therefore especially important that great care is taken when
deconvolution of the data is made in order to avoid a non-physical interpretation being arrived at. Further complications can include phonon broadening, surface roughness and inhomogeneous pinning of the Fermi level, all of which broaden the measured photoemission peaks. These factors combine in the case of GaAs particularly, such that their contribution to the overall resolution dominates those of the monochromator and electron analyser. When fitting the core-level data, therefore, well established values were used for the Lorentzian components to the lineshape (0.155 eV and 0.170 eV for the Ga and As 3d levels respectively) [6]. The Gaussian component required proved to be much larger than the instrumental resolution, and was found to be significantly different between Ga and As peaks measured under the same conditions. This is thought to reflect differences in phonon broadening, since the bulk peaks behave in this way as well as the surface components. Throughout the fitting procedure, a consistent policy was adhered to so that the simplest fit be achieved, using the minimum of components, and keeping as many parameters constant as possible. To this end, fixed spin-orbit splittings and branching ratios were used and an emphasis on relative energy differences between bulk and surface components, and that between the Ga and As bulk peaks. The most surface sensitive spectra were collected using 95 eV radiation, and the consistency of fits between the two sets of data obtained with different photon energies is another indicator of the quality of fit. The energy shifts of the surface core level shifts (SCLS) were found to remain constant between the two datasets, and their intensity ratio with respect to the bulk emission behaved as expected from escape depth considerations. The energy difference between the Ga and As 3d bulk components was kept fixed at 21.95 eV, which was determined from the (lll)B spectra collected at the more bulksensitive photon energy of 70 eV. It is important to keep this a fixed parameter if meaningful fits are to be made from the essentially featureless Ga 3d peaks. Cubic or quadratic
67 backgrounds were subtracted simultaneous with the fitting procedure, which employed both Levenberg-Marquadt and conjugate gradients minimization techniques. 4. RESULTS The Ga and As 3d core-levels from the GaAs(111)A surface are shown as a function of annealing t e m p e r a t u r e in Fig. 1. They were obtained using the more bulk-sensitive 70 eV radiation, which delivered slightly improved resolution, as well as significant surface to bulk intensity ratios. Consider first the As 3d peaks, which j u s t after de-capping can only (2~)
_
I
3tS'C I
3SO'C
21.0" z/.0." il.o " d.0 " il.0. " i~0 43.0" ~.0. " ;d.o " 41.0." ii.0. " 4~ K.E. / eV
Figure 1. As and Ga 3d core-level spectra as a function of t e m p e r a t u r e from theGaAs(111)A(2 × 2)-reconstructions. fitted with at least two SCLS components in addition to the bulk ($1, 82). They bear close similarity to the (001) surface following a similar preparation [6], in which a low binding energy (LBE) component was attributed to As-dimers in the (2 × 4) structure, or threefold cooordinated sites for As in the (4 × 2). Another LBE component is also seen on the (110) surface, so it appears that a variety of threefold coordinated sites for As on a GaAs surface result in a LBE shifted component on
the As 3d peak. The high binding energy (HBE) component is attributed to excess As, in agreement with previous studies. As the annealing temperature increases, it is clear that the bulk-like excess As desorbs after only a small increment, leaving the As 3d emission best fitted by just the bulk and and one SCLS (S1). This then remains the case up to 400°C. By keeping the energy difference between the bulk Ga and As peaks to 21.95 eV, it is evident that j u s t after de-capping at 315°C there is a shifted component to LBE on the Ga 3d peak (S1, Fig. 1). With a small increase in temperature up to 350°C, a HBE component (S 2) is needed to obtain a good fit. Higher temperatures result in further diminution of S 1, and the increase in intensity of $2. This transition in the component required to fit the Ga 3d peakshape is entirely consistent with a recent STM study, which shows a transition from (2 × 2)-trimer to a (2 × 2)-vacancy reconstruction. The higher anneal temperature peakshapes closely resemble those of the (110) surface. This is not unexpected since the STM showed that a relaxation occurs, in which the surface bilayer collapses, and a rehybridization in a manner very close to the (110) takes place. We therefore attribute the SCLS's on the Ga and As 3d core-levels to be associated with the surface Ga atoms, and the As atoms surrounding the Ga-vacancy respectively. The similarity of the core-level emission between the two orientations is not so surprising, since the local bonding and electronic structure are remarkably close. The fact that the Ga 3d is not completely bulk-like, as found for the ( l l l ) B surface, reveals that there is a surface Ga environment. Furthermore, that only one component is required to low binding energy to fit the lowest temperature peak is indicative that the Ga-vacancy structure is not existent at this stage. The S 1 component diminishes with increasing anneal temperature, and so appears intimately linked with the As-trimer (2 × 2) structure seen by STM. We therefore assign the S 1 component to the Ga rest-atom in the (2 × 2)-trimer reconstruction observed
68 under similar conditions. The conclusion from the Ga 3d emission that the Ga-vacancy structure is not present at 315°C determines that the component S 1 in the As 3d emission is not due to t h a t structure either. The most likely conclusion is t h a t the S1 SCLS is due to a threefold coordinated As in an As-trimer a r r a n g e m e n t at low temperatures, which then transforms into threefold coordinated As around a Ga vacancy at higher temperatures. The intensity of these features would be expected to be constant since both structures have the same n u m b e r of surface As atoms per (2 x 2) unit cell. It appears at first to be a remarkable coincidence that both environments should produce the same SCLS on the As 3d emission, though if we consider the similarity in peakshape from the (110), (001)-(2 x 4), and (001)-(4 x 2) surfaces, it is not so unusual. The ideal G a A s ( l l l ) B surface is terminated by a half-bilayer of As, and so it can be expected to find an As-rich surface, especially after a thermal de-capping of As. From STM studies [3,4], we might expect to find spectra consistent with As-trimers and As rest-atoms from a (2 x 2) reconstructed surface. As and Ga 3d core-levels obtained with 70 eV radiation are shown as a function of anneal temperature in Fig. 2. The first thing to notice is the complete lack of surface components on the Ga 3d peaks over the entire temperature range. This confirms that the top two layers of the (2 x 2) reconstructed surface are composed entirely of As, with no surface environment for Ga. It is clear from this alone that the (Ill)B-(2 x 2)-trimer surface is quite distinct from the (Ill)A-(2 x 2)-trimer structure, which displayed a large SCLS on the Ga 3d peak. The As 3d peaks are much more complex, requiring two SCLS peaks to fit the data. These are similar to Cai et a]. [7] though the data shown here is angle integrated and of a higher resolution. This is probably more a function of sample quality than instrumental resolution. Following the results from the associated STM study of identically prepared samples [3], we assign the HBE component to
(2x2)
B :~O'C
[(~2)
A
=
,i
21,0 22.0 23.0 24.0 ~,0 26.043,0 44.0 45.0 46,0 47.0 4&O K.E,/eV
Figure 2. As and Ga 3d core-level spectra as a function of anneal temperature from the GaAs(lll)B-(2 x 2-As trimer reconstruction. bulk-like As-trimers (since they bond to an As second layer on the ( l l l ) B surface) and the LBE component to the As rest-atom (i.e. the surface As atoms not bonded to a trimer). The highest temperature peaks (at 425°C) are from the (1 × 1)LT structure, which is intermediate between the (2 x 2) and the (~]19 x ~]19) structures. It is clear from the As 3d peak that the feature attributed to As trimers has dramatically reduced in intensity, as one might expect as As desorbs at higher temperatures. The desorption of As is also reflected in the Ga 3d emission, which now displays a large SCLS due to Ga being being revealed at the surface. The detailed structure of this surface is not yet fully understood, though clearly it is much less As-rich than the (2 x 2)-trimer reconstruction. Another means of comparing the (IlI)A(2 x 2) and (Ill)B-(2 x 2) surfaces is to consider the valence band emission. Angleresolved studies have been reported which show a large difference between the (Ill)Avacancy and ( l l l ) B - t r i m e r surfaces [8], notably in the top-most band. Valence bands meas-
69 ured using 95 eV radiation for the (Ill)A-(2 x 2)-Ga-vacancy and (111)B-(2 x 2)-As-trimer reconstructions are shown in Fig. 3. They have been normalized to the highest and lowest energy background levels, and so the intensities of the band features can be compared.
(II1)B
.=
an angle-resolved study, it is clear that the electronic structure of these surfaces is strikingly different. 5. CONCLUSIONS We have thermally de-capped protective As layers from both the (111)A and (111)B surfaces of GaAs, and examined the resulting reconstructions using surface-sensitive photoemission. It has been shown that a transition takes place in the As and Ga 3d core-level peakshapes of the (Ill)A-(2 x 2) reconstructed surface, confirming an observed phase transition from STM experiments. The less As-rich Ga-vacancy phase is found to exhibit both core-level lineshapes and valence band emission similar to the (110) surface, due to similar bond rehybridization. The photoemission from the (lll)B surface is entirely in agreement with an As-trimer model, with no surface Ga present, and surface states localized on the As-trimer adstructures. REFERENCES
75.0
I
I
l
77.0
79.0
81.0
I
I
83.0 85.0 K.E. / eV
I
1
I
87.0
89.0
91.0
Figure 3. Valence band spectra from the GaAs (111)A-(2 x 2)-Ga vacancy and (111)B-(2 x 2) As-trimer reconstructions. The lower spectrum is from the (111)A surface, which bears a close resemblance to the emission seen from the cleaved (110) surface. This is entirely consistent with the corelevel data as well as the associated STM results [3]. The upper spectum, however, has a much higher intensity in the region up to 5 eV down from the valence band maximum. In particular, one sharp peak is dominant, which is indicative of highly localized states which can be expected to be associated with the Astrimer adstructure. Even without the sensio tivity to specific surface states enjoyed with
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