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Surface states on gallium phosphide
Volume 63A, number 3
PHYSICS LETTERS
14 November 1977
•SURFACE STATES ON GALLIUM PHOSPHIDE D. NORMAN1
Department of Physics, University of Leicester, Leicester LEI 7RH, England
I.T. McGOVERN School of Physical Sciences, New University of Ulster, Coleraine, Co. Londonderry BT52 iSA, Northern Ireland and
C. NORRIS Department of Physics, University of Leicester, Leicester LEI 7RH, England Received 27 July 1977
Ultraviolet photoelectron spectroscopy shows that the surface Fermi level of clean cleaved GaP (10) is pinned 1.50 eV above the valence band maximum by a band of empty surface states. Synchrotron radiation-excited photoemission partial yield spectroscopy supports this conclusion.
Although the electronic structure at the cleavage planes of 111—V semiconductors has been extensively studied the results have often conflicted. In contrast to earlier observations [1,2] recent measurements indicate that for most materials surface states do not extend into the bulk band gap [3—5].GaP appears to be an exception. Contact potential difference and photo-threshold measurements indicate that the Fermi level is pinned on n-type GaP [6] and photoemission partial yield measurements show a band of unfilled states lying below the conduction band minimum [3]. It is possible that discrepancies between observations are a consequence of specimen dependence [4]. We report here low energy photoemission (UPS) and new partial yield measurements which give support to the conclusions of the more recent measurements. UPS energy distribution curves (edc’s) were obtamed with a hemipsherical retarding field analyser and using light (7.4 ~ hw ~ 11.6 eV) provided by a hydrogen discharge source dispersed by a 1 metre normal incidence grating monochromator. The sample area was isolated by a LiF window. Photoemission partial yield measurements were performed using radiation emitted 1
Now at Department of Physics, University of Warwick, Coventry CV4 7AL, England.
384
from NINA, the Science Research Council’s 5 GeV electron synchrotron at Daresbury Laboratory, Cheshire, and monochromated by a Wadsworth-type instrument [7]. The experimental arrangement is described in detail elsewhere [8]. n-type GaP specimens of approximate dimensions 30 X 10 X 3 mm oriented with their long axis along (110) were supplied by the Plessey Co. Ltd. They cleaved easily to reveal (110) surfaces with no visible steps or irregularities. The excess donor concentration was about 6 X 1015 cm~3 and hence the bulk Fermi level was 0.24 eV below the conduction band minimum (EC) and 2.00 eV above the valence band maximum (Ev). Some photoemission edc’s are shown in fig. 1. All the structure is readily attributable to transitions between bulk states. Adsorption of oxygen did not preferentially remove any feature. Although most theoretical models [e.g., 9] give a band of surface states within ‘~-‘leV of EV, no structure corresponding to emission from filled states was observed. Such states may be spread throughout the energy range of the valence band, contributing no sharp structure to the edc’s, or they may be strongly mixed with bulk states. Angularly resolved photoemission may reveal filled surface states [10]. From the spectra the upper F points in the conduction band were determined to lie ‘-‘94 and ‘~10.4eV above Ev; transitions between
Volume 63A, number 3
I
PHYSICS LETTERS
14 November 1977
~ 1: \ /
~~6eV
~
9.7eV
/
I 5x1Ô~
ø~
19
21
23
7.7eV
0-
1i~_EB(d5f
2)
5
—4 —3 —2 —1 0 Intiat-state energy (eV)
Fig. 1. Photoelectron energy distribution curves of clean GaP (110).
levels near F are seen as a weak shoulder at the top of the edc’s for several photon energies in the range 9.4—11.3 eV. The valence band maximum is thus readily found from these spectra. The surface Fermi level, determined from a freshly evaporated gold film, was found to be pinned 1.50 ±0.1 eV above Ev giving a band-bending voltage (Vbb) of 0.5 ±0.1. Controlled adsorption of oxygen up to 108 Langmuirs did not change this value, A photoemission partial yield spectrum is given in fig. 2. The binding energy of the Ga 3d512 level, relative to Ev, was determined to be 18.4 ±0.1 eV with the 3d3~2level deeper by 0.5 eV. The doublet feature, fitted by two Gaussians of 0.5 eV FWHM at 11w = 19.7 and 20.2 eV, is attributed to transitions from the spin-orbit split Ga 3d levels to empty surface states. The initial states in the transition are indicated. The surface state is thus found to be 1.3 ±0.1 eV above Ev, in agreement with previous measurements [3]. We note that the ratio of the contributions from the 3d5~2and 3d3~2core levels is observed to be 0.6 1, almost the reverse of the ration 6 : 4 expected from
Fig. 2. GaP (110) partial yield spectrum (solid line) after subtraction of a smoothly varying background. The surface peak at hw
=
20.2 eV has a magnitude —12% above background.
the degeneracies of the levels in a single-particle excitation picture, suggesting that many body effects are involved [11]. It seems likely that the simple interpretation of a band of empty surface states with a peak 0.94 ±0.1 belowEC will be disturbed by excitonic effects [12]. Values of<0.1 eV [13] and 0.8 eV [14] have been suggested for the bulk core level to conduction band excitonic binding energy in GaP and it is expected that excitonic energies for the surface will be larger than for the bulk because of the localisation of the surface states and the diminished dielectric screening at the surface. However, as pointed out previously [3], a very large excitonic binding energy is required for GaP to remove the apparent surface state position, as determined from partial yield spectroscopy, completely from the bulk band gap. This work provides further evidence for the location of empty surface states within the bulk band gap. It remains unclear why GaP appear different from other Ill—V compounds. Further work, in particular angularly resolved photoemission and parital yield experiments with excitation from the 2p levels, may help elucidate the nature of the electronic structure of this 385
Volume 63A, number 3
PHYSICS LETTERS
apparently anomalous Ill—V semiconductor surface. The supply of GaP crystals by the Allen Clark Research Centre of the Plessey Co. Ltd and the financial support of the Science Research Council are gratefully acknowledged.
References [1] e.g. J.H. Dinan, L.K. Gaibraith and T.E. Fischer, Surf. Sci. 26 (1971) 587; P.E. Gregory, WE. Spicer, Phys. Rev. B13 (1976) 725; W.E. Spicer et aL, J. Vac. Sci. Technol. 13 (1976) 233. [2] D.E. Eastman and J.L. Freeouf, Phys. Rev. Lett. 34 (1975) 1624. [3] W. Gudat and D.E. Eastman, J. Vac. Sci. Technol. 13 (1976) 831. [4] W.E. Spicer et al., J. Vac. Sci. Technol. 13 (1976) 780.
386
14 November 1977
[5] A. Huijser and J. van Laar, Surf. Sci. 52 (1975) 202. 161 A. Huijser, J van Laar and T.L. van Rooy, Surf. Sci. 62 (1977) 472. (1977) 259. [8] R.H. Williams, I.T. McGovern, RB. Murray and M. Howells, Phys. Stat. Sol. 73 (1976) 307. [9] J.D. Joannopoulos and M.L. Cohen, Phys. Rev. BlO (1974) 5075; J.R. Chelikowsky and M.L. Cohen, Phys. Rev. Bi 3 (1976) 826; C. Calandra and G. Santoro, J. Phys. C9 (1976) L51. [10] J.A. Knapp and G.J. Lapeyre, J. Vac. Sd. Technol. 13 (1976) 757. [11] J.L. Freeouf, Phys. Rev. Lett. 36 (1976) 1095. [121 G.J. Lapeyre and J. Anderson, Phys. Rev. Lett. 35 (1975) 117. [13] D.E. Aspnes, C.G. Olson and D.W. Lynch, Phys. Rev. B12 (1975) 2527. [14] P. Thiry et al., Solid State Comm. 20(1976)1107.
[7] M. Howells, C. Norris and G.P. Williams, J. Phys. ElO
PHYSICS LETTERS
14 November 1977
•SURFACE STATES ON GALLIUM PHOSPHIDE D. NORMAN1
Department of Physics, University of Leicester, Leicester LEI 7RH, England
I.T. McGOVERN School of Physical Sciences, New University of Ulster, Coleraine, Co. Londonderry BT52 iSA, Northern Ireland and
C. NORRIS Department of Physics, University of Leicester, Leicester LEI 7RH, England Received 27 July 1977
Ultraviolet photoelectron spectroscopy shows that the surface Fermi level of clean cleaved GaP (10) is pinned 1.50 eV above the valence band maximum by a band of empty surface states. Synchrotron radiation-excited photoemission partial yield spectroscopy supports this conclusion.
Although the electronic structure at the cleavage planes of 111—V semiconductors has been extensively studied the results have often conflicted. In contrast to earlier observations [1,2] recent measurements indicate that for most materials surface states do not extend into the bulk band gap [3—5].GaP appears to be an exception. Contact potential difference and photo-threshold measurements indicate that the Fermi level is pinned on n-type GaP [6] and photoemission partial yield measurements show a band of unfilled states lying below the conduction band minimum [3]. It is possible that discrepancies between observations are a consequence of specimen dependence [4]. We report here low energy photoemission (UPS) and new partial yield measurements which give support to the conclusions of the more recent measurements. UPS energy distribution curves (edc’s) were obtamed with a hemipsherical retarding field analyser and using light (7.4 ~ hw ~ 11.6 eV) provided by a hydrogen discharge source dispersed by a 1 metre normal incidence grating monochromator. The sample area was isolated by a LiF window. Photoemission partial yield measurements were performed using radiation emitted 1
Now at Department of Physics, University of Warwick, Coventry CV4 7AL, England.
384
from NINA, the Science Research Council’s 5 GeV electron synchrotron at Daresbury Laboratory, Cheshire, and monochromated by a Wadsworth-type instrument [7]. The experimental arrangement is described in detail elsewhere [8]. n-type GaP specimens of approximate dimensions 30 X 10 X 3 mm oriented with their long axis along (110) were supplied by the Plessey Co. Ltd. They cleaved easily to reveal (110) surfaces with no visible steps or irregularities. The excess donor concentration was about 6 X 1015 cm~3 and hence the bulk Fermi level was 0.24 eV below the conduction band minimum (EC) and 2.00 eV above the valence band maximum (Ev). Some photoemission edc’s are shown in fig. 1. All the structure is readily attributable to transitions between bulk states. Adsorption of oxygen did not preferentially remove any feature. Although most theoretical models [e.g., 9] give a band of surface states within ‘~-‘leV of EV, no structure corresponding to emission from filled states was observed. Such states may be spread throughout the energy range of the valence band, contributing no sharp structure to the edc’s, or they may be strongly mixed with bulk states. Angularly resolved photoemission may reveal filled surface states [10]. From the spectra the upper F points in the conduction band were determined to lie ‘-‘94 and ‘~10.4eV above Ev; transitions between
Volume 63A, number 3
I
PHYSICS LETTERS
14 November 1977
~ 1: \ /
~~6eV
~
9.7eV
/
I 5x1Ô~
ø~
19
21
23
7.7eV
0-
1i~_EB(d5f
2)
5
—4 —3 —2 —1 0 Intiat-state energy (eV)
Fig. 1. Photoelectron energy distribution curves of clean GaP (110).
levels near F are seen as a weak shoulder at the top of the edc’s for several photon energies in the range 9.4—11.3 eV. The valence band maximum is thus readily found from these spectra. The surface Fermi level, determined from a freshly evaporated gold film, was found to be pinned 1.50 ±0.1 eV above Ev giving a band-bending voltage (Vbb) of 0.5 ±0.1. Controlled adsorption of oxygen up to 108 Langmuirs did not change this value, A photoemission partial yield spectrum is given in fig. 2. The binding energy of the Ga 3d512 level, relative to Ev, was determined to be 18.4 ±0.1 eV with the 3d3~2level deeper by 0.5 eV. The doublet feature, fitted by two Gaussians of 0.5 eV FWHM at 11w = 19.7 and 20.2 eV, is attributed to transitions from the spin-orbit split Ga 3d levels to empty surface states. The initial states in the transition are indicated. The surface state is thus found to be 1.3 ±0.1 eV above Ev, in agreement with previous measurements [3]. We note that the ratio of the contributions from the 3d5~2and 3d3~2core levels is observed to be 0.6 1, almost the reverse of the ration 6 : 4 expected from
Fig. 2. GaP (110) partial yield spectrum (solid line) after subtraction of a smoothly varying background. The surface peak at hw
=
20.2 eV has a magnitude —12% above background.
the degeneracies of the levels in a single-particle excitation picture, suggesting that many body effects are involved [11]. It seems likely that the simple interpretation of a band of empty surface states with a peak 0.94 ±0.1 belowEC will be disturbed by excitonic effects [12]. Values of<0.1 eV [13] and 0.8 eV [14] have been suggested for the bulk core level to conduction band excitonic binding energy in GaP and it is expected that excitonic energies for the surface will be larger than for the bulk because of the localisation of the surface states and the diminished dielectric screening at the surface. However, as pointed out previously [3], a very large excitonic binding energy is required for GaP to remove the apparent surface state position, as determined from partial yield spectroscopy, completely from the bulk band gap. This work provides further evidence for the location of empty surface states within the bulk band gap. It remains unclear why GaP appear different from other Ill—V compounds. Further work, in particular angularly resolved photoemission and parital yield experiments with excitation from the 2p levels, may help elucidate the nature of the electronic structure of this 385
Volume 63A, number 3
PHYSICS LETTERS
apparently anomalous Ill—V semiconductor surface. The supply of GaP crystals by the Allen Clark Research Centre of the Plessey Co. Ltd and the financial support of the Science Research Council are gratefully acknowledged.
References [1] e.g. J.H. Dinan, L.K. Gaibraith and T.E. Fischer, Surf. Sci. 26 (1971) 587; P.E. Gregory, WE. Spicer, Phys. Rev. B13 (1976) 725; W.E. Spicer et aL, J. Vac. Sci. Technol. 13 (1976) 233. [2] D.E. Eastman and J.L. Freeouf, Phys. Rev. Lett. 34 (1975) 1624. [3] W. Gudat and D.E. Eastman, J. Vac. Sci. Technol. 13 (1976) 831. [4] W.E. Spicer et al., J. Vac. Sci. Technol. 13 (1976) 780.
386
14 November 1977
[5] A. Huijser and J. van Laar, Surf. Sci. 52 (1975) 202. 161 A. Huijser, J van Laar and T.L. van Rooy, Surf. Sci. 62 (1977) 472. (1977) 259. [8] R.H. Williams, I.T. McGovern, RB. Murray and M. Howells, Phys. Stat. Sol. 73 (1976) 307. [9] J.D. Joannopoulos and M.L. Cohen, Phys. Rev. BlO (1974) 5075; J.R. Chelikowsky and M.L. Cohen, Phys. Rev. Bi 3 (1976) 826; C. Calandra and G. Santoro, J. Phys. C9 (1976) L51. [10] J.A. Knapp and G.J. Lapeyre, J. Vac. Sd. Technol. 13 (1976) 757. [11] J.L. Freeouf, Phys. Rev. Lett. 36 (1976) 1095. [121 G.J. Lapeyre and J. Anderson, Phys. Rev. Lett. 35 (1975) 117. [13] D.E. Aspnes, C.G. Olson and D.W. Lynch, Phys. Rev. B12 (1975) 2527. [14] P. Thiry et al., Solid State Comm. 20(1976)1107.
[7] M. Howells, C. Norris and G.P. Williams, J. Phys. ElO