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Investigation of intrinsic unoccupied surface states at GaP(110) by inverse photoemission
Surface science 133 (1983) 9-14 North-Holland Publishing Company
9
INVESTIGATION OF INTRINSIC UNOCCUPIED AT GaP(110) BY INVERSE PHOTOEMISSION
SURFACE
STATES
D. STRAUB, V. DOSE and W. ALTMANN Physikalisches
Instiiut der Universitsit, Am Hubland, D-8700
Received 5 May 1983; accepted for publication
Wiirzburg, Fed. Rep. of Germany
1 July 1983
GaP is the only III-V semiconductor with intrinsic unoccupied electronic states in the bulk band gap. We have employed inverse photoemission or isochromat spectroscopy to measure the density of these unoccupied states. Spectra were taken at quantum energies of 73 and 9.7 eV. The experimental data were analyzed by comparing the measured spectra at both quantum energies with a calculated bulk density of states. The analysis lead to three empty surface states at 2.2, 4.0 and about 7.5 eV above valence band maximum (VBM). The sensitivity of emission features at these energies to oxygen exposure and argon ion damage provides experimental evidence for their surface nature.
1. Introduction
The electronic structure of III-V compound semiconductors continues to receive considerable attention. A variety of experimental methods has been employed to attack this problem. For occupied electronic states these include core level and valence band photoelectron spectroscopy in the ultraviolet and the X-ray region. Much less research has been devoted to empty electronic states mainly employing electron energy loss and partial yield techniques. Concerning empty electronic states research has focused on the existence and energetic position of surface states relative to the bulk band gap. This is of interest not only in technological connections but is also intimately related to surface reconstruction and relaxation models. For GaAs( 110) for example, empty electronic surface states have been predicted for an ideally terminated surface while surface relaxation moves these states into the conduction band. This has been verified experimentally only recently by isochromat measurements [ 1,2]. Partial yield and energy loss techniques, both of which involve core hole production, suffer from final state excitor& shifts and have lead to erroneous positioning of surface states in the gap of GaAs( 110). For GaP(ll0) the situation is slightly different. Regardless of the type of surface reconstruction theory predicts empty electronic surface states in the 2.26 eV wide bulk band gap [3-51. Partial yield data by Gudat and Eastman 0039-6028/83/0000-0000/$03.00
0 1983 North-Holland
[6], and Norman et al. [7] identify an empty surface state at GaP(ll0) near midgap, Even if we consider the latest estimate for the excitonic binding energy at GaP(110) of 0.8 eV [8] this state remains in the bulk band gap. Work function in combination with photoemission measurements provide another access to the energetic position of surface states. From measurements on both p- and n-type GaP(ll0) samples it was concluded that a band bending of 0.5 eV occurs on vacuum cleaved n-type GaPfllO) samples while bands are practically flat on p-type GaP(I 10). This leads to an estimate for the lower limit of the energetic position of the empty surface state of 1.7 eV above VBM (valence band ma~mum) f9- 1l]. Fermi Ievel pinning may occur via intrinsic or defect induced extrinsic surface states Depending on the doping a density of states as low as 10’2/cm2 is sufficient to pin the Fermi level, It is therefore impossible to draw conclusions on the position of intrinsic surface states which are expected to have a density of the order of 1015/cm2 from work function and ionization energy measurements only. As pointed out earlier [ 1,2], inverse photoemission spectroscopy [ 12,131 is a suitable technique to reveal the energetic position of such intrinsic surface states since its sensitivity is limited to a density of about 0.1 to 0.01 states per eV per atom and since it does not suffer from excitor& energy shifts.
2. Experimentat
The experiment was carried out in an ion pumped UHV system at a pressure of 2 x lo-” mbar during data acquisition. The system is equipped with isochromat spectrometers for quantum energies of 9.7 and 73 eV, argon ion gun, quadrupole residual gas analyzer, Kelvin vibrating capacitor, LEED, and retarding field Auger facilities, The 9.7 eV isochromat spectrometer uses the energy selective properties of a Geiger-Mtiller counter with CaF, entrance window and He-I, filhng gas. The mean detection energy is 9.74 eV with a rms deviation of f0.35 eV 1141. Spectra with this spectrometer were taken at 100 meV energy increments with a total charge of 1 mC per measured point, The 73 eV spectrometer is of a conceptionally different type [ 151.It employs the quasi discontinuity of X-ray absorption at the Al L,,, edge which, when combined with potential modulation and lock-in techniques, makes up for a pseudomonochromator with a pass energy of 73 eV. Spectra with this spectrometer were taken at 200 meV energy increments with a modulation of 0.3 V,,, and 40 mC accumulated charge per data point. The reason for employing two spectrometers is the largely different electron penetration depths associated with the two operating energies. A lightly Te-doped n-type GaP(I 10) sample (n = 1.5 X 10’7/cm3) cleaved under atmospheric pressure was used for this experiment. Before each measurement the sample was prepared by ion ~mbardment and annealing until
D. Straub et al. / Intrinsic unoccupied surface states at GaP(ll0)
11
no contaminants could be detected by Auger analysis and a sharp LEED pattern with low background intensity was obtained. Isochromat spectroscopy refers energies to the Fermi level of the sample. The electronic states on the other hand are conveniently referred to the VBM. It is therefore of great importance to determine the position of the Fermi level with respect to the band edges. Work function measurements were carried out to this end on GaP( 110) and the well known GaAs( 110) [ 161 as a reference. Using the ionization energies determined by Huijser et al. [lo] we arrive at a Fermi level pinning at 1.11 eV above VBM. Note that this value differs from the previous quotation for an UHV cleaved sample. It is, however, in splendid agreement with results of Huijser et al. [lo], who report a Fermi level pinning at midgap due to surface defects. Such defects are inevitably created on samples prepared by ion etching and annealing. Consistently, neither surface damage of a freshly prepared sample by argon ion bombardment until the LEED pattern was hardly visible nor oxygen exposure up to 4.5 x lo3 L appeared to move the Fermi level. A similar insensitivity of the Fermi level position to oxygen exposure was observed by Normal et al. [7].
3. Results and discussion The solid circles in fig. 1 display isochromat spectra from clean GaP( 110) at quantum energies of 9.7 and 73 eV. The structural features in both spectra are by and large identical. Note that considerable emission tails into the region below CBM (conduction band minimum), i.e. into the band gap, in both spectra. As expected, this band gap emission is more intense for the 73 eV data due to enhanced surface sensitivity.
0
2
L (E-E
6 VBM)JeV
8
0
2
L
6
6
(E-E,,,lleV
Fig. 1. Solid circles are isochromat spectra from a clean well ordered GaP( 110) sample recorded at quantum energies of 9.7 and 73 eV respectively. The long-dashed curve is the bulk density of
states. The short dashes represent the surface contribution to the total emission (full line).
12
D. Straub et al. / Intrinsic unoccupied surface states at GaP(I 10)
In order to interpret the spectra in terms of surface and volume contributions to the overall emission, we compare the experimental data to a theoretical bulk density of states calculation shown as the long dashed curves. Theoretical results are from a very recent selfconsistent pseudopotential calculation [17]. They have been convoluted with the instrument response functions of the two spectrometers in order to account for the finite experimental resolution. Major discrepancies in structure between theory and experiment occur mainly in three regions, namely at CBM and at 4 and 7.5 eV above VBM where experimental emission maxima coincide with relative minima of the bulk density of states. This leads to the following procedure. A hypothetical surface density of states was constructed from three Gaussians of roughly twice the instrumental width with initial positions at 2, 4 and 7 eV, and arbitrary amplitudes. The total density of states consisting of the rigorously calculated bulk and the hypothetical surface contributions was then used to determine the amplitudes and positions of the Gaussians via a least squares fit to the experimental data. As a constraint, the positions had to be the same in the 9.7 and-73 eV data. The result of this procedure is shown as the short-dashed curve in fig. 1 for the surface density of states and the solid curve for the overall fit. The energetic positions of the surface states are 2.2,4.0 and 7.5 eV respectively. As expected, we find an enhanced surface contribution to the total emission in the 73 eV data with particularly strong enhancement of the 2.2 eV surface peak. The surface state at CBM is believed to be related to the exciton observed in partial yield spectroscopy. Since the energy of the VBM was determined independently in our experiment, it is possible to derive a new estimate for the excitonic binding energy at GaP(llO). From partial yield measurements the surface state called B; is placed at 1.3 eV above VBM. Our analysis rather indicates 2.2 eV above VBM which leads to an excitonic shift of 0.9 eV, in splendid agreement with a recent estimate by Eastman et al. of 0.8 eV [S]. Partial yield spectroscopy reveals only the lowest empty surface state and gives no indication of the states at 4 and 7.5 eV observed in this work. Several sets of data are available on electron energy loss at GaP( 110) surfaces. Van Laar et al. report two empty surface states from an analysis of EELS data at 2.2 and 8.2 eV above VBM [9]. Their spectra contain a 10.8 eV loss also which could be attributed to a transition from the B, occupied surface state at 6.5 eV below VBM to our 4 eV surface state. Though Van Laar et al. consider this possibility, they prefer to interpret the 10.8 eV loss to be due to a surface plasmon. Very recent energy loss measurements by Gant and Month [ 181have revealed a loss at 21.8 eV. Assuming an excitonic shift of 0.8- 1.0 eV, this loss could be due to a transition from the Ga(3d) core level to the 4 eV surface state observed in this work. Theoretical work on empty surface states at GaP(110) has so far been focused onto the lowest state at CBM. Its energy has been predicted to be
D. Straub et al. / Intrinsic unoccupied surface states at GaP(I10)
13
1.9-2.15 eV regardless of the rotation angle describing the surface relaxation [3]. Calculations for higher energies would be interesting since higher lying states are expected to depend sensitively, in position and amplitude, on the nature of the relaxation. Our analysis of bremsstrahlung isochromats from GaP( 110) in terms of bulk and surface emission relied heavily on theoretical data on the bulk density of states. Two other means of identification of surface features have been used too. The GaP sample was exposed to 4.5 x IO3 L of oxygen in order to saturate dangling bond states. From AES the oxygen coverage could be estimated to be less than 0.1 ML. These data were collected for 9.7 eV quantum energy. The difference spectrum of oxygen exposed minus clean is displayed in fig. 2. It shows a single peak at CBM indicating that oxygen adsorption quenches only this state. In accord with previous partial yield interpretations, this state is therefore assigned to derive from the gallium dangling bond. Oxygen adsorption has no influence on the states at 4 and 7.5 eV. As a second diagnostic tool, we exposed the well prepared GaP( 110) surface to gentle argon ion bombardment such that the LEED spots of the ordered surface were still visible. The difference of spectra from the well prepared and the damaged surface is also displayed in fig. 2. Three prominent features show up. While argon ion bombardment leads to enhanced emission at CBM we get attenuations of the 4 and 6.5 eV isochromat features. The enhancement at CBM is attributed to disorder induced extrinsic surface states which are known to lie in the bulk band gap and have been observed many times via Fermi level pinning. The minima at 4 and 6.5 eV suggest a quenching of surface states observed on the clean surface. It should be mentioned that argon ion bombardment has been used previously as a tool to discriminate between surface and bulk features in an EELS study of silicon [ 191and GaAs 1201.The insensitivity of the 4 and 7.5 eV surface states to oxygen leads to the conclusion that these electronic states
sputtered clean
9 t; ii
-
.
i-s
0
2 L (E-EV,,)/eV
6
8
Fig. 2. Several structural features of spectra in fig. 1 (left panel) are quenched by oxygen adsorption or argon ion damage. Difference spectra showing the energetic position of these surface emissions are displayed in the right hand panel.
D. Straub et al. / Intrinsic unoccupied surface states at GaP(lI0)
14
do not tail into the vacuum. Their existence could therefore be due to the relaxation which is known to affect at least the three topmost atomic layers. The geometry of these layers is of course heavily distorted by argon ion treatment, consistent with the observed quenching. Alternatively we propose that these states are of p,, p,, orbital character which could also account for their reduced sensitivity to oxygen exposure.
Acknowledgements
This work was financially supported by the Deutsche Forschungsgemeinschaft. We want to thank Dr. M. Scheffler for providing us with his theoretical GaP data prior to pdblication. We have benefited from numerous discussions with Dr. H. Scheidt. We want to thank Professor H. Ltith for providing the samples used in this work.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [1 1]
[ 121 [13] [14] [15] [16]
[17] [18] [19] 1201
V. Dose, H.-J. Gossmann and D. Straub, Phys. Rev. Letters 47 (1981) 608. V. Dose, H.-J. Gossmann and D. Straub, Surface Sci. ,117 (1982) 387. R.E. Allen, H.P. Hjalmarson and J.D. Dow, Surface Sci. 110 (1981) L625. F. Manghi, C.M. Bertoni, C. Calandra and E. Molinari, Phys. Rev. B24 (1981) 6029. C.M. Bertoni. 0. Bisi, F. Manghi and C. Calandra, J. Vacuum Sci. Technol. 15 (1978) 1256. W. Gudat and D.E. Eastman, J. Vacuum Sci. Technol. 13 (1976) 831. D. Norman, I.T. McGovern and C. Norris, Phys. Letters 63A (1977) 384. D.E. Eastman, T.C. Chiang, P. Heimann and F.J. Himpsel, Phys. Rev. Letters 45 (1980) 656. J.L. van Laar, A. Huijser and T.L. van Rooy, J. Vacuum Sci. Technol. 14 (1977) 894. A. Huijser, J.L. van Laar and T.L. van Rooy, Surface Sci. 62 (1977) 472. G.M. Guichar, C.A. Sebenne and C.D. Thuault, J. Vacuum Sci. Technol. 16 (1979) 1212. V. Dose, Appl. Phys. 14 (1977) 117. J.B. Pendry, Phys. Rev. Letters 45 (1980) 1356. G. Denninger, V. Dose and H. Scheidt, Appl. Phys. 18 (1979) 375; V. Dose Progr. Surface Sci. 13(1983) 225. M. Conrad, V. Dose, Th. Fauster and H. Scheidt, Appl. Phys. 20 (1979) 37. W.E. Spicer, P.W. Chye, P.R. Skeath, C.Y. Su and I. Lindau, J. Vacuum Sci. Technol. 16 (1979) 1422; W.E. Spicer, I. Lindau, P.R. Skeath and C.Y. Su, J. Vacuum Sci. Technol. 17 (1980) 1019. M. Scheffler, J. Bemholc, N.O. Lipari and S.T. Pantelides, Phys. Rev. B, submitted. H. Gant and W. Miinch, private communication. J.E. Rowe and H. Ibach, Phys. Rev. Letters 31 (1973) 102. R. Ludeke and L. Esaki, Phys. Rev. Letters 33 (1974) 653.
9
INVESTIGATION OF INTRINSIC UNOCCUPIED AT GaP(110) BY INVERSE PHOTOEMISSION
SURFACE
STATES
D. STRAUB, V. DOSE and W. ALTMANN Physikalisches
Instiiut der Universitsit, Am Hubland, D-8700
Received 5 May 1983; accepted for publication
Wiirzburg, Fed. Rep. of Germany
1 July 1983
GaP is the only III-V semiconductor with intrinsic unoccupied electronic states in the bulk band gap. We have employed inverse photoemission or isochromat spectroscopy to measure the density of these unoccupied states. Spectra were taken at quantum energies of 73 and 9.7 eV. The experimental data were analyzed by comparing the measured spectra at both quantum energies with a calculated bulk density of states. The analysis lead to three empty surface states at 2.2, 4.0 and about 7.5 eV above valence band maximum (VBM). The sensitivity of emission features at these energies to oxygen exposure and argon ion damage provides experimental evidence for their surface nature.
1. Introduction
The electronic structure of III-V compound semiconductors continues to receive considerable attention. A variety of experimental methods has been employed to attack this problem. For occupied electronic states these include core level and valence band photoelectron spectroscopy in the ultraviolet and the X-ray region. Much less research has been devoted to empty electronic states mainly employing electron energy loss and partial yield techniques. Concerning empty electronic states research has focused on the existence and energetic position of surface states relative to the bulk band gap. This is of interest not only in technological connections but is also intimately related to surface reconstruction and relaxation models. For GaAs( 110) for example, empty electronic surface states have been predicted for an ideally terminated surface while surface relaxation moves these states into the conduction band. This has been verified experimentally only recently by isochromat measurements [ 1,2]. Partial yield and energy loss techniques, both of which involve core hole production, suffer from final state excitor& shifts and have lead to erroneous positioning of surface states in the gap of GaAs( 110). For GaP(ll0) the situation is slightly different. Regardless of the type of surface reconstruction theory predicts empty electronic surface states in the 2.26 eV wide bulk band gap [3-51. Partial yield data by Gudat and Eastman 0039-6028/83/0000-0000/$03.00
0 1983 North-Holland
[6], and Norman et al. [7] identify an empty surface state at GaP(ll0) near midgap, Even if we consider the latest estimate for the excitonic binding energy at GaP(110) of 0.8 eV [8] this state remains in the bulk band gap. Work function in combination with photoemission measurements provide another access to the energetic position of surface states. From measurements on both p- and n-type GaP(ll0) samples it was concluded that a band bending of 0.5 eV occurs on vacuum cleaved n-type GaPfllO) samples while bands are practically flat on p-type GaP(I 10). This leads to an estimate for the lower limit of the energetic position of the empty surface state of 1.7 eV above VBM (valence band ma~mum) f9- 1l]. Fermi Ievel pinning may occur via intrinsic or defect induced extrinsic surface states Depending on the doping a density of states as low as 10’2/cm2 is sufficient to pin the Fermi level, It is therefore impossible to draw conclusions on the position of intrinsic surface states which are expected to have a density of the order of 1015/cm2 from work function and ionization energy measurements only. As pointed out earlier [ 1,2], inverse photoemission spectroscopy [ 12,131 is a suitable technique to reveal the energetic position of such intrinsic surface states since its sensitivity is limited to a density of about 0.1 to 0.01 states per eV per atom and since it does not suffer from excitor& energy shifts.
2. Experimentat
The experiment was carried out in an ion pumped UHV system at a pressure of 2 x lo-” mbar during data acquisition. The system is equipped with isochromat spectrometers for quantum energies of 9.7 and 73 eV, argon ion gun, quadrupole residual gas analyzer, Kelvin vibrating capacitor, LEED, and retarding field Auger facilities, The 9.7 eV isochromat spectrometer uses the energy selective properties of a Geiger-Mtiller counter with CaF, entrance window and He-I, filhng gas. The mean detection energy is 9.74 eV with a rms deviation of f0.35 eV 1141. Spectra with this spectrometer were taken at 100 meV energy increments with a total charge of 1 mC per measured point, The 73 eV spectrometer is of a conceptionally different type [ 151.It employs the quasi discontinuity of X-ray absorption at the Al L,,, edge which, when combined with potential modulation and lock-in techniques, makes up for a pseudomonochromator with a pass energy of 73 eV. Spectra with this spectrometer were taken at 200 meV energy increments with a modulation of 0.3 V,,, and 40 mC accumulated charge per data point. The reason for employing two spectrometers is the largely different electron penetration depths associated with the two operating energies. A lightly Te-doped n-type GaP(I 10) sample (n = 1.5 X 10’7/cm3) cleaved under atmospheric pressure was used for this experiment. Before each measurement the sample was prepared by ion ~mbardment and annealing until
D. Straub et al. / Intrinsic unoccupied surface states at GaP(ll0)
11
no contaminants could be detected by Auger analysis and a sharp LEED pattern with low background intensity was obtained. Isochromat spectroscopy refers energies to the Fermi level of the sample. The electronic states on the other hand are conveniently referred to the VBM. It is therefore of great importance to determine the position of the Fermi level with respect to the band edges. Work function measurements were carried out to this end on GaP( 110) and the well known GaAs( 110) [ 161 as a reference. Using the ionization energies determined by Huijser et al. [lo] we arrive at a Fermi level pinning at 1.11 eV above VBM. Note that this value differs from the previous quotation for an UHV cleaved sample. It is, however, in splendid agreement with results of Huijser et al. [lo], who report a Fermi level pinning at midgap due to surface defects. Such defects are inevitably created on samples prepared by ion etching and annealing. Consistently, neither surface damage of a freshly prepared sample by argon ion bombardment until the LEED pattern was hardly visible nor oxygen exposure up to 4.5 x lo3 L appeared to move the Fermi level. A similar insensitivity of the Fermi level position to oxygen exposure was observed by Normal et al. [7].
3. Results and discussion The solid circles in fig. 1 display isochromat spectra from clean GaP( 110) at quantum energies of 9.7 and 73 eV. The structural features in both spectra are by and large identical. Note that considerable emission tails into the region below CBM (conduction band minimum), i.e. into the band gap, in both spectra. As expected, this band gap emission is more intense for the 73 eV data due to enhanced surface sensitivity.
0
2
L (E-E
6 VBM)JeV
8
0
2
L
6
6
(E-E,,,lleV
Fig. 1. Solid circles are isochromat spectra from a clean well ordered GaP( 110) sample recorded at quantum energies of 9.7 and 73 eV respectively. The long-dashed curve is the bulk density of
states. The short dashes represent the surface contribution to the total emission (full line).
12
D. Straub et al. / Intrinsic unoccupied surface states at GaP(I 10)
In order to interpret the spectra in terms of surface and volume contributions to the overall emission, we compare the experimental data to a theoretical bulk density of states calculation shown as the long dashed curves. Theoretical results are from a very recent selfconsistent pseudopotential calculation [17]. They have been convoluted with the instrument response functions of the two spectrometers in order to account for the finite experimental resolution. Major discrepancies in structure between theory and experiment occur mainly in three regions, namely at CBM and at 4 and 7.5 eV above VBM where experimental emission maxima coincide with relative minima of the bulk density of states. This leads to the following procedure. A hypothetical surface density of states was constructed from three Gaussians of roughly twice the instrumental width with initial positions at 2, 4 and 7 eV, and arbitrary amplitudes. The total density of states consisting of the rigorously calculated bulk and the hypothetical surface contributions was then used to determine the amplitudes and positions of the Gaussians via a least squares fit to the experimental data. As a constraint, the positions had to be the same in the 9.7 and-73 eV data. The result of this procedure is shown as the short-dashed curve in fig. 1 for the surface density of states and the solid curve for the overall fit. The energetic positions of the surface states are 2.2,4.0 and 7.5 eV respectively. As expected, we find an enhanced surface contribution to the total emission in the 73 eV data with particularly strong enhancement of the 2.2 eV surface peak. The surface state at CBM is believed to be related to the exciton observed in partial yield spectroscopy. Since the energy of the VBM was determined independently in our experiment, it is possible to derive a new estimate for the excitonic binding energy at GaP(llO). From partial yield measurements the surface state called B; is placed at 1.3 eV above VBM. Our analysis rather indicates 2.2 eV above VBM which leads to an excitonic shift of 0.9 eV, in splendid agreement with a recent estimate by Eastman et al. of 0.8 eV [S]. Partial yield spectroscopy reveals only the lowest empty surface state and gives no indication of the states at 4 and 7.5 eV observed in this work. Several sets of data are available on electron energy loss at GaP( 110) surfaces. Van Laar et al. report two empty surface states from an analysis of EELS data at 2.2 and 8.2 eV above VBM [9]. Their spectra contain a 10.8 eV loss also which could be attributed to a transition from the B, occupied surface state at 6.5 eV below VBM to our 4 eV surface state. Though Van Laar et al. consider this possibility, they prefer to interpret the 10.8 eV loss to be due to a surface plasmon. Very recent energy loss measurements by Gant and Month [ 181have revealed a loss at 21.8 eV. Assuming an excitonic shift of 0.8- 1.0 eV, this loss could be due to a transition from the Ga(3d) core level to the 4 eV surface state observed in this work. Theoretical work on empty surface states at GaP(110) has so far been focused onto the lowest state at CBM. Its energy has been predicted to be
D. Straub et al. / Intrinsic unoccupied surface states at GaP(I10)
13
1.9-2.15 eV regardless of the rotation angle describing the surface relaxation [3]. Calculations for higher energies would be interesting since higher lying states are expected to depend sensitively, in position and amplitude, on the nature of the relaxation. Our analysis of bremsstrahlung isochromats from GaP( 110) in terms of bulk and surface emission relied heavily on theoretical data on the bulk density of states. Two other means of identification of surface features have been used too. The GaP sample was exposed to 4.5 x IO3 L of oxygen in order to saturate dangling bond states. From AES the oxygen coverage could be estimated to be less than 0.1 ML. These data were collected for 9.7 eV quantum energy. The difference spectrum of oxygen exposed minus clean is displayed in fig. 2. It shows a single peak at CBM indicating that oxygen adsorption quenches only this state. In accord with previous partial yield interpretations, this state is therefore assigned to derive from the gallium dangling bond. Oxygen adsorption has no influence on the states at 4 and 7.5 eV. As a second diagnostic tool, we exposed the well prepared GaP( 110) surface to gentle argon ion bombardment such that the LEED spots of the ordered surface were still visible. The difference of spectra from the well prepared and the damaged surface is also displayed in fig. 2. Three prominent features show up. While argon ion bombardment leads to enhanced emission at CBM we get attenuations of the 4 and 6.5 eV isochromat features. The enhancement at CBM is attributed to disorder induced extrinsic surface states which are known to lie in the bulk band gap and have been observed many times via Fermi level pinning. The minima at 4 and 6.5 eV suggest a quenching of surface states observed on the clean surface. It should be mentioned that argon ion bombardment has been used previously as a tool to discriminate between surface and bulk features in an EELS study of silicon [ 191and GaAs 1201.The insensitivity of the 4 and 7.5 eV surface states to oxygen leads to the conclusion that these electronic states
sputtered clean
9 t; ii
-
.
i-s
0
2 L (E-EV,,)/eV
6
8
Fig. 2. Several structural features of spectra in fig. 1 (left panel) are quenched by oxygen adsorption or argon ion damage. Difference spectra showing the energetic position of these surface emissions are displayed in the right hand panel.
D. Straub et al. / Intrinsic unoccupied surface states at GaP(lI0)
14
do not tail into the vacuum. Their existence could therefore be due to the relaxation which is known to affect at least the three topmost atomic layers. The geometry of these layers is of course heavily distorted by argon ion treatment, consistent with the observed quenching. Alternatively we propose that these states are of p,, p,, orbital character which could also account for their reduced sensitivity to oxygen exposure.
Acknowledgements
This work was financially supported by the Deutsche Forschungsgemeinschaft. We want to thank Dr. M. Scheffler for providing us with his theoretical GaP data prior to pdblication. We have benefited from numerous discussions with Dr. H. Scheidt. We want to thank Professor H. Ltith for providing the samples used in this work.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [1 1]
[ 121 [13] [14] [15] [16]
[17] [18] [19] 1201
V. Dose, H.-J. Gossmann and D. Straub, Phys. Rev. Letters 47 (1981) 608. V. Dose, H.-J. Gossmann and D. Straub, Surface Sci. ,117 (1982) 387. R.E. Allen, H.P. Hjalmarson and J.D. Dow, Surface Sci. 110 (1981) L625. F. Manghi, C.M. Bertoni, C. Calandra and E. Molinari, Phys. Rev. B24 (1981) 6029. C.M. Bertoni. 0. Bisi, F. Manghi and C. Calandra, J. Vacuum Sci. Technol. 15 (1978) 1256. W. Gudat and D.E. Eastman, J. Vacuum Sci. Technol. 13 (1976) 831. D. Norman, I.T. McGovern and C. Norris, Phys. Letters 63A (1977) 384. D.E. Eastman, T.C. Chiang, P. Heimann and F.J. Himpsel, Phys. Rev. Letters 45 (1980) 656. J.L. van Laar, A. Huijser and T.L. van Rooy, J. Vacuum Sci. Technol. 14 (1977) 894. A. Huijser, J.L. van Laar and T.L. van Rooy, Surface Sci. 62 (1977) 472. G.M. Guichar, C.A. Sebenne and C.D. Thuault, J. Vacuum Sci. Technol. 16 (1979) 1212. V. Dose, Appl. Phys. 14 (1977) 117. J.B. Pendry, Phys. Rev. Letters 45 (1980) 1356. G. Denninger, V. Dose and H. Scheidt, Appl. Phys. 18 (1979) 375; V. Dose Progr. Surface Sci. 13(1983) 225. M. Conrad, V. Dose, Th. Fauster and H. Scheidt, Appl. Phys. 20 (1979) 37. W.E. Spicer, P.W. Chye, P.R. Skeath, C.Y. Su and I. Lindau, J. Vacuum Sci. Technol. 16 (1979) 1422; W.E. Spicer, I. Lindau, P.R. Skeath and C.Y. Su, J. Vacuum Sci. Technol. 17 (1980) 1019. M. Scheffler, J. Bemholc, N.O. Lipari and S.T. Pantelides, Phys. Rev. B, submitted. H. Gant and W. Miinch, private communication. J.E. Rowe and H. Ibach, Phys. Rev. Letters 31 (1973) 102. R. Ludeke and L. Esaki, Phys. Rev. Letters 33 (1974) 653.