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The effects of surface damage on surface plasmon excitations in doped InSb(100)
Applied Surface Science 45 (1990) 85-90 North-Holland
85
The effects of surface damage on surface plasmon excitations in doped InSb(100) T.S. Jones, M.Q. D i n g *, N.V. R i c h a r d s o n Surface Science Research Centre, The University of Liverpool, P.O. Box 147, Liverpool, L69 3BX, UK and
C.F. M c C o n v i l l e * * Royal Signals and Radar Establishment, St. Andrews Road, Malvern, Worcestershire, WR14 3PS, UK
Received 5 January 1990; accepted for publication 8 May 1990
High-resolution electron energy loss spectroscopy(HREELS) has been used to study the (100) surface of n-type InSb (Te doped, 1 × 1018 c m - 3 ) . Clean, ordered surfaces were prepared by cycles of low-energy argon ion bombardment and annealing. Throughout the annealing process, a series of surface reconstructions were observed as a function of annealing temperature, by low-energy electron diffraction (LEED). Specular HREELS studies, carried out at 300 K, showed that the plasmon loss energy changed dramatically with post-bombardment annealing temperature. These changes in plasmon energy are discussed in relation to the particular surface reconstruction and the degree of surface damage induced by the ion sputtering process. n =
1. Introduction Semiconductor device processing and fabrication often involves the use of ion beam etching for surface cleaning, dopant implantation and the formarion of buried junctions. Since it is well known that even low-energy ion bombardment ( < 1000 eV) can cause surface damage, the study of this and the effect of post-bombardment annealing on the structural and electronic properties of semiconductor surfaces, is of considerable technological interest. High-resolution electron energy loss spectroscopy (HREELS) is proving to be a valuable probe of the vibrational and electronic excitations
* Permanent address: Beijing Vacuum Electronics Research Institute, P.O. Box 749, Beijing, People's Rep. of China. ** Present address: Department of Physics, University of Warwick, Coventry, CV4 7AL, UK.
of I I I - V semiconductor surfaces [1-8]. The majority of these studies have concentrated on the (110) surfaces of materials, such as GaAs [1-3] and InSb [7], which are formed by cleavage in vacuum. The study of the (100) surfaces is complicated by the difficulties associated with the production of clean, ordered structures. Two methods of preparation have been employed, the first of which involves a combination of ion sputtering and annealing cycles. The second method employs a protective capping layer deposited on material grown by molecular beam epitaxy (MBE), which can subsequently be thermally desorbed in vacuum. Both methods have their own problems. Cycles of ion bombardment are known to produce damage which may not be totally removed by annealing and can lead to a depletion of carriers extending well below the surface of the material [9]. The capping of I I I - V semiconductor surfaces, usually with the Group V element, requires desorption of the capping material at high temperatures. With a
0169-4332/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
86
T.S. Jones et al. / Surface plasmon excitations in doped lnSb(lO0)
low melting point material, such as InSb, this requires heating above the non-congruent temperature with the possibility of preferential Sb loss and difficulties in determining the exact stoichiometric ratio of the final surface [4-6]. In this paper, we show that the energy of the plasmon excitation for InSb(100), as measured by HREELS, is extremely sensitive to the details of the surface preparation. The (100) surface of InSb has a number of clean surface reconstructions. The majority of these have been formed in-situ by MBE and have been characterised using reflection high-energy electron diffraction ( R H E E D ) [10]. Recent low-energy electron diffraction (LEED) studies have also shown that a combination of ion b o m b a r d m e n t and annealing cycles results in one of two indium-rich surface reconstructions, a (4 × 1) (or streaky (4 x 2) pattern) and a c(8 x 2) structure [11-15]. The work of Jones et al. [12,15] has also shown that adsorption of iodine (or chlorine) on the clean (4 x 1) and c(8 × 2) surfaces, followed by desorption at 260 o C, reduces the surface indium concentration to given an antimony-rich c(4 × 4) surface.
mixture of lactic, nitric and hydrofluoric acids to remove sub-surface damage and produce a stable oxide surface. In-situ cleaning involved cycles of low-energy argon ion b o m b a r d m e n t (500 eV primary energy, 1 - 2 /~A ion current, - 6 0 min) followed by annealing to a m a x i m u m temperature of 700 K for a period of approximately 10 min. The temperature was monitored using a c h r o m e l alumel thermocouple fixed close to the sample. Surface cleanliness was monitored using AES and following each annealing sequence, L E E D observations and H R E E L S measurements were carried out at 300 K.
3. Results and discussion Fig. 1 shows specular H R E E L spectra recorded on the InSb(100) sample prior to argon ion
&5eV
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2. Experimental ~,. The experiments were carried out in a U H V chamber (base pressure - 5 x 10 -11 mbar) equipped with HREELS, LEED, Auger electron spectroscopy (AES) and a mass spectrometer. The H R E E L S spectrometer (VSW Scientific Instruments Ltd.) consists of a fixed monochromator and rotatable analyser, both of the 180 ° hemispherical deflector type, with four element lenses. All H R E E L spectra were recorded in specular scattering geometry (i.e. 0 i = 0~ = 40 °, all angles refer to the surface normal). The typical resolution of the elastic peak in specular scattering geometry was 7 meV full-width at half-maximum (FWHM). The InSb sample consisted of a 8-10 Inn-I2 slice of a 500 /~m thick material, with n-type doping (Te doped, n ~ 1 x 1018 cm-3), grown in the (100) orientation (MCP Electronic Materials Ltd). The sample was mechanical polished and solvent cleaned by the manufacturer. Prior to insertion in vacuum, the sample was chemically etched with a
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Fig. 1. Specular (Oi = 8 s = 40 ° ) HREEL spectra recorded on the oxide-coated n-type InSb(100) (n ~ 1 x 1018cm -3) sample before argon-ion sputtering. The sample was annealed to 550 K and the spectra were recorded at 300 K. Two incident electron energies were employed, i.e. E i = 5 and 35 eV.
T.S. Jones et aL / Surface plasmon excitations in doped InSb(lO0) bombardment, i.e. before any structural damage induced by the sputtering process. The sample was annealed to - 5 5 0 K before the measurements were carried out. Two loss features can clearly be seen at 47 and 98 meV respectively, with a broad feature also present at - 150 meV in the lower energy spectrum (i.e. E i = 5 e V ) . The 47 meV loss is more intense at the higher incident beam energy whilst the higher energy features, at 98 and 150 meV, are more intense at lower incident electron energies. We assign the loss at 47 meV to the conduction band plasmon excitation, whilst the higher energy features are vibrational modes associated with S b - O vibrations of an oxide layer. These higher energy vibrations have also been reported by Evans et al. [6] for Sb capped InSb(100) (n = 1 x 1018 cm -3) before desorption of the capping layer. In their work, H R E E L spectra were recorded only at 5 eV and the plasmon excitation, which dominates at the higher incident electron energies (fig. 1), was not observed. The plasmon energy is extremely close to that expected for a bulk doping level of 1 x 10 ]8 cm -3 (bulk plasmon frequency, ~0b = 48 meV at 300 K, assuming an effective mass of 0•0372 [16]). After argon ion bombardment, a number of different L E E D structures were observed during the annealing process. At 500 K, a relatively sharp (4 x 1) pattern was observed, with a series of additional faint spots/streaks between the (4 × 1) features. These additional streaks imply that a (4 × 2) structure may be present consistent with previous L E E D studies on i o n - b o m b a r d e d InSb(100) [12]. The structure at 500 K can be described as a ( 4 x l ) , or a ( 4 x 2 ) with onedimensional disorder. Annealing the sample to 600 K results in a c(8 X 2) L E E D pattern. Such a pattern has also been reported in a recent photoemission study by John et al. [13], however, only after annealing to 700 K. Fig. 2 shows specular H R E E L spectra recorded after argon ion bombardment of the highly doped (n --- 1 x 1018 cm -3) InSb(100) sample, followed by annealing to a series of different temperatures upto a maximum value of 700 K. After argon ion bombardment only and no annealing, a broad elastic peak can be seen with an unresolved shoulder on the loss energy side. Increasing the
87 700K
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Fig. 2. Specular (8i =0s=40 ° ) HREEL spectra recorded on n-type InSb(100) (n --1 × 10TM cm-3) after argon-ionbombardment and a series of different annealing temperatures upto 700 K. All spectra were recorded with an incident electron energy of 26 eV and with the sample temperature fixed at 300 K. annealing temperature to 500 K results in the elastic peak becoming sharper while the loss feature moves out to an increased energy (65 meV at 500 K). A second, lower energy peak can also be seen at 23 meV. The LEED observations show that this annealing temperature corresponds to the formation of the (4 x 1) (or 4 × 2)) structure. Further increases in the annealing temperature to 700 K show that the lower loss feature at 23 meV does not shift in energy, but that the higher loss feature gradually moves to a lower energy• After annealing to 700 K, the two peaks are clearly resolved in the H R E E L spectrum, with loss energies of 23 and 46 meV respectively. This correlates with the formation of the c(8 x 2) L E E D pattern. It can also be noted from the spectra shown in fig. 2 that as the annealing temperature is increased above 500 K, the F W H M value of both the elastic peak and the loss features is significantly reduced.
T.S. Jones et al. / Surface plasmon excitations in doped InSb(lO0)
88
The loss at 23 meV, which is independent of the post-bombardment annealing temperature, is assigned as the dipole active, surface phonon mode (the Fuchs-Kliewer surface phonon). The energy of this mode is in excellent agreement with other HREELS studies of both capped InSb(100) [6] and cleaved InSb(ll0) [7]. The second loss feature, which shifts dramatically in energy with postbombardment annealing temperature (fig. 3), is assigned as the conduction band plasmon excitation, which we interpret as a bulk plasmon in the near-surface region. These assignments are confirmed by measurements of the intensity of the two loss features as a function of incident electron beam energy. At low incident electron energies (i.e. E i < 10 eV), the phonon is considerably stronger in intensity than the plasmon loss. However, at higher electron energies ( E i > 20 eV), the relative intensity of the two modes is reversed [17]. Such an observation is in good agreement with other electron energy dependent HREELS studies of doped I I I - V semiconductor surfaces [1-7]. It is well documented that ion bombardment of semiconductor materials results in lattice damage and crystal imperfections. In the case of I I I - V semiconductors, preferential sputtering of the Group V element occurs, leaving the surface enriched in the Group III element [9,18,19]. For GaAs, there is a damage threshold for the argon ion impact energy (approximately 100 eV) below InSb (100) n - lx1018cm-3 >o
E
70
o r
50 o co o
~_
40 3o 200 300 400 500 600 700 800 Annealing Temperature (K)
Fig. 3. Plasmon loss energy (meV) plotted as a function of annealing temperature (K) for specular HREELS measurements on n-type InSb(100) (n ---1× 1018cm-3) after argon-ion sputtering.
which full recovery is possible upon annealing [9]. For higher argon ion energies (typically 200-1000 eV), full recovery is not attainable because of cascade collisions resulting in a disordered surface layer. In our experiments, the primary ion energy employed was fixed at 500 eV. Lattice imperfections induced by the sputtering process involves the creation of vacancies, interstitials and the implantation of argon ions in the material. This results in the formation of defect states which pin the Fermi level in the band gap. At this doping level, the Fermi level in the bulk lies 150 meV above the bottom of the conduction band [6]. The upward band bending which results then produces a depletion layer in the near-surface region. The overall effect of this lattice damage is to trap conduction band electrons in the near-surface region and is a likely explanation for the very low plasmon frequency observed in the H R E E L S experiment, compared with that expected for a bulk doping level of 1 X 1018 cm -3 (0~b = 48 meV). The annealing process serves to repair some of the structural damage, as confirmed by the observed sequence of LEED patterns. The gradual increase in the plasmon energy as the annealing temperature is increased from 300 K (fig. 2) implies that the free-carrier concentration in the near-surface region recovers substantially tending towards the bulk value. After annealing to 500 K, the observed plasmon energy is 65 meV, which is higher than that expected from the bulk doping level. This suggests that there is an accumulation of free carriers in the near-surface region. Further increases in the annealing temperature ( > 500 K) result in a decrease in the plasmon loss energy, such that at 700 K, the plasmon energy is 46 meV. This is close to the plasmon energy expected for a bulk doping concentration of 1 x 10 x8 cm -3. This indicates that at higher annealing temperatures (i.e. between 600 and 700 K) there is a decrease in the concentration of free carriers in the nearsurface region. The lowering in loss energy at higher annealing temperatures is accompanied by the transformation of the L E E D pattern from a ( 4 × 1) (or disordered (4 x 2)) into a c(8 x 2) structure. For InSb, the non-congruent temperature of the
7".S. Jones et al. / Surface plasmon excitations m doped lnSb(lO0)
material (i.e. the temperature at which one of the two elements of the compound semiconductor is lost from the surface) is - 6 0 0 K. Above this temperature, antimony is preferentially lost from the material leaving the surface indium-rich. Although our AES measurements did not show a noticeable reduction in Sb composition, the transition to a c(8 × 2) structure at annealing temperatures of - 6 0 0 K may result in some loss of surface Sb during the atomic rearrangement at the surface. Such a small loss of Sb would be extremely difficult to detect with AES since the Auger signal is integrated over a depth of several atomic layers. A loss of surface Sb may result in the incipient formation of a I n / I n S b interface which would be consistent with the decrease in plasmon energy observed in the HREELS experiment. It can also be noted from fig. 2 that at high annealing temperatures, there is a considerable decrease in the half-width of the plasmon feature. This suggests that the carrier lifetime is increased at higher annealing temperatures and is consistent with the removal of surface damage. So far we have related the changes in plasmon energy which occur with annealing temperature to be due to repairing of the surface damage induced by the ion sputtering process. However, diffusion of the dopant material may also be important at the higher annealing temperatures. In a recent study by F/Srster et al. [20], shifts in the plasmon energy of n-type Si(100) were attributed to diffusion of the dopant (in this case phosphorus) into the bulk at high annealing temperatures. For our work on InSb, this would result in a decrease in free carriers in the near-surface region and therefore a reduced plasmon energy at high annealing temperatures. This is indeed observed for annealing temperatures above 500 K. Work is in progress aimed at studying InSb(100) with a wide range of bulk doping concentrations in order to determine whether dopant diffusion is important in determining the plasmon energy observed in HREELS.
4. Conclusion HREELS has been used to study the (100) surface of n-type InSb (n = 1 x 1018 cm -3) pre-
89
pared by a combination of low-energy argon ion bombardment and annealing cycles. In particular, the loss energy of the plasmon excitation is found to be extremely sensitive to the details of the surface preparation. These changes in plasmon loss energy are accompanied by a series of LEED patterns. After argon-ion sputtering only, the plasmon energy (37 meV) is considerable lower than expected for such a bulk doping level used (calculated bulk plasmon frequency 48 meV). This is explained by defect states which pin the Fermi level in the band gap resulting in upward band bending and producing a depletion layer. Annealing results in increases in the plasmon energy which are attributed to a repair of the structural damage. At 500 K, the plasmon energy is 65 meV, suggesting the formation of an accumulation layer. Further increases in annealing temperature results in a gradual decrease in plasmon energy (46 meV at 700 K) to a value close to that expected for the bulk doping concentration used. This is interpreted as arising from either band bending associated with a surface markedly enriched with indium, or by dopant diffusion into the bulk.
Acknowledgements Useful discussions with R.G. Egdell, S. Holloway, P.T. Andrews and P. Weightman are gratefully acknowledged. R.G. Jones and R.G. Egdell are thanked for sending us copies of manuscripts prior to publication.
References [1] R. Matz and H. Li~th, Phys. Rev. Lett. 46 (1981) 500. [2] Y. Chen, S. Nannarone, J. Schaefer, J.C. Hermanson and G.J. Lapeyre, Phys. Rev. B 39 (1989) 7653. [3] M.G. Betti, U. del Pennino and C. Mariani, Phys. Rev. B 39 (1989) 5887. [4] Z.J. Gray-Grychowski, R.A. Stradling, R.G. Egdell, P.J. Dobson, B.A. Joyce and K. Woodbridge, Solid State Commun. 59 (1986) 703. [5] Z.J. Gray-Grychowski, R.G. Egdell, B.A. Joyce, R.A. Stradfing and K. Woodbridge, Surf. Sci. 186 (1987) 482. [6] S.D. Evans, L.L. Can, R.G. Egdell, R. Droopad, S.D. Parker and R.A. Stradling, Surf. Sci. 226 (1990) 169. [7] A. Ritz and H. Liith, Phys. Rev. Lett. 52 (1984) 1242.
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T.S. Jones et al. / Surface plasmon excitations in doped InSb(lO0)
[8] Y. Chen, J.C. Hermanson and G.J. Lapeyre, Phys. Rev. B 39 (1989) 12682. [9] L.H. Dubois and G.P. Schwartz, J. Vac. Sci. Technol. B 2 (1984) 101. [10] K. Oe, S. Ando and K. Sugiyama, J. Appl. Phys. 19 (1980) L417. [11] D.P. Woodruff and K. Horn, Vacuum 33 (1983) 633. [12] R.G. Jones, N.K. Singh and C.F. McConville, Surf. Sci. 208 (1989) L34. [13] P. John, T. Miller and T.C. Chiang, Phys. Rev. B 39 (1989) 1730. [14] T.S. Jones, M.R. Ashton, N.V. Richardson and C.F. McConville, J. Phys. (Condens. Matter) 1 (1989) 207.
[15] A.P. Mowbray and R.G. Jones, Vacuum, submitted. [16] T. Inaoka, D.M. Newns and R.G. Egdell, Surf. Sci. 186 (1987) 290. [17] T.S. Jones, M.Q. Ding, N.V. Richardson and C.F. McConville, to be published. [18] I.L. Singer, J.S. Murday and L.R. Cooper, Surf. Sci. 108 (1981) 7. [19] D.M. Hill, F. Xu, Z. Lin and J.H. Weaver, Phys. Rev. B 38 (1988) 1893. [20] A. FSrster, J.M. Layet and H. Liath, Appl. Phys. A 47 (1988) 95.
85
The effects of surface damage on surface plasmon excitations in doped InSb(100) T.S. Jones, M.Q. D i n g *, N.V. R i c h a r d s o n Surface Science Research Centre, The University of Liverpool, P.O. Box 147, Liverpool, L69 3BX, UK and
C.F. M c C o n v i l l e * * Royal Signals and Radar Establishment, St. Andrews Road, Malvern, Worcestershire, WR14 3PS, UK
Received 5 January 1990; accepted for publication 8 May 1990
High-resolution electron energy loss spectroscopy(HREELS) has been used to study the (100) surface of n-type InSb (Te doped, 1 × 1018 c m - 3 ) . Clean, ordered surfaces were prepared by cycles of low-energy argon ion bombardment and annealing. Throughout the annealing process, a series of surface reconstructions were observed as a function of annealing temperature, by low-energy electron diffraction (LEED). Specular HREELS studies, carried out at 300 K, showed that the plasmon loss energy changed dramatically with post-bombardment annealing temperature. These changes in plasmon energy are discussed in relation to the particular surface reconstruction and the degree of surface damage induced by the ion sputtering process. n =
1. Introduction Semiconductor device processing and fabrication often involves the use of ion beam etching for surface cleaning, dopant implantation and the formarion of buried junctions. Since it is well known that even low-energy ion bombardment ( < 1000 eV) can cause surface damage, the study of this and the effect of post-bombardment annealing on the structural and electronic properties of semiconductor surfaces, is of considerable technological interest. High-resolution electron energy loss spectroscopy (HREELS) is proving to be a valuable probe of the vibrational and electronic excitations
* Permanent address: Beijing Vacuum Electronics Research Institute, P.O. Box 749, Beijing, People's Rep. of China. ** Present address: Department of Physics, University of Warwick, Coventry, CV4 7AL, UK.
of I I I - V semiconductor surfaces [1-8]. The majority of these studies have concentrated on the (110) surfaces of materials, such as GaAs [1-3] and InSb [7], which are formed by cleavage in vacuum. The study of the (100) surfaces is complicated by the difficulties associated with the production of clean, ordered structures. Two methods of preparation have been employed, the first of which involves a combination of ion sputtering and annealing cycles. The second method employs a protective capping layer deposited on material grown by molecular beam epitaxy (MBE), which can subsequently be thermally desorbed in vacuum. Both methods have their own problems. Cycles of ion bombardment are known to produce damage which may not be totally removed by annealing and can lead to a depletion of carriers extending well below the surface of the material [9]. The capping of I I I - V semiconductor surfaces, usually with the Group V element, requires desorption of the capping material at high temperatures. With a
0169-4332/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
86
T.S. Jones et al. / Surface plasmon excitations in doped lnSb(lO0)
low melting point material, such as InSb, this requires heating above the non-congruent temperature with the possibility of preferential Sb loss and difficulties in determining the exact stoichiometric ratio of the final surface [4-6]. In this paper, we show that the energy of the plasmon excitation for InSb(100), as measured by HREELS, is extremely sensitive to the details of the surface preparation. The (100) surface of InSb has a number of clean surface reconstructions. The majority of these have been formed in-situ by MBE and have been characterised using reflection high-energy electron diffraction ( R H E E D ) [10]. Recent low-energy electron diffraction (LEED) studies have also shown that a combination of ion b o m b a r d m e n t and annealing cycles results in one of two indium-rich surface reconstructions, a (4 × 1) (or streaky (4 x 2) pattern) and a c(8 x 2) structure [11-15]. The work of Jones et al. [12,15] has also shown that adsorption of iodine (or chlorine) on the clean (4 x 1) and c(8 × 2) surfaces, followed by desorption at 260 o C, reduces the surface indium concentration to given an antimony-rich c(4 × 4) surface.
mixture of lactic, nitric and hydrofluoric acids to remove sub-surface damage and produce a stable oxide surface. In-situ cleaning involved cycles of low-energy argon ion b o m b a r d m e n t (500 eV primary energy, 1 - 2 /~A ion current, - 6 0 min) followed by annealing to a m a x i m u m temperature of 700 K for a period of approximately 10 min. The temperature was monitored using a c h r o m e l alumel thermocouple fixed close to the sample. Surface cleanliness was monitored using AES and following each annealing sequence, L E E D observations and H R E E L S measurements were carried out at 300 K.
3. Results and discussion Fig. 1 shows specular H R E E L spectra recorded on the InSb(100) sample prior to argon ion
&5eV
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2. Experimental ~,. The experiments were carried out in a U H V chamber (base pressure - 5 x 10 -11 mbar) equipped with HREELS, LEED, Auger electron spectroscopy (AES) and a mass spectrometer. The H R E E L S spectrometer (VSW Scientific Instruments Ltd.) consists of a fixed monochromator and rotatable analyser, both of the 180 ° hemispherical deflector type, with four element lenses. All H R E E L spectra were recorded in specular scattering geometry (i.e. 0 i = 0~ = 40 °, all angles refer to the surface normal). The typical resolution of the elastic peak in specular scattering geometry was 7 meV full-width at half-maximum (FWHM). The InSb sample consisted of a 8-10 Inn-I2 slice of a 500 /~m thick material, with n-type doping (Te doped, n ~ 1 x 1018 cm-3), grown in the (100) orientation (MCP Electronic Materials Ltd). The sample was mechanical polished and solvent cleaned by the manufacturer. Prior to insertion in vacuum, the sample was chemically etched with a
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Fig. 1. Specular (Oi = 8 s = 40 ° ) HREEL spectra recorded on the oxide-coated n-type InSb(100) (n ~ 1 x 1018cm -3) sample before argon-ion sputtering. The sample was annealed to 550 K and the spectra were recorded at 300 K. Two incident electron energies were employed, i.e. E i = 5 and 35 eV.
T.S. Jones et aL / Surface plasmon excitations in doped InSb(lO0) bombardment, i.e. before any structural damage induced by the sputtering process. The sample was annealed to - 5 5 0 K before the measurements were carried out. Two loss features can clearly be seen at 47 and 98 meV respectively, with a broad feature also present at - 150 meV in the lower energy spectrum (i.e. E i = 5 e V ) . The 47 meV loss is more intense at the higher incident beam energy whilst the higher energy features, at 98 and 150 meV, are more intense at lower incident electron energies. We assign the loss at 47 meV to the conduction band plasmon excitation, whilst the higher energy features are vibrational modes associated with S b - O vibrations of an oxide layer. These higher energy vibrations have also been reported by Evans et al. [6] for Sb capped InSb(100) (n = 1 x 1018 cm -3) before desorption of the capping layer. In their work, H R E E L spectra were recorded only at 5 eV and the plasmon excitation, which dominates at the higher incident electron energies (fig. 1), was not observed. The plasmon energy is extremely close to that expected for a bulk doping level of 1 x 10 ]8 cm -3 (bulk plasmon frequency, ~0b = 48 meV at 300 K, assuming an effective mass of 0•0372 [16]). After argon ion bombardment, a number of different L E E D structures were observed during the annealing process. At 500 K, a relatively sharp (4 x 1) pattern was observed, with a series of additional faint spots/streaks between the (4 × 1) features. These additional streaks imply that a (4 × 2) structure may be present consistent with previous L E E D studies on i o n - b o m b a r d e d InSb(100) [12]. The structure at 500 K can be described as a ( 4 x l ) , or a ( 4 x 2 ) with onedimensional disorder. Annealing the sample to 600 K results in a c(8 X 2) L E E D pattern. Such a pattern has also been reported in a recent photoemission study by John et al. [13], however, only after annealing to 700 K. Fig. 2 shows specular H R E E L spectra recorded after argon ion bombardment of the highly doped (n --- 1 x 1018 cm -3) InSb(100) sample, followed by annealing to a series of different temperatures upto a maximum value of 700 K. After argon ion bombardment only and no annealing, a broad elastic peak can be seen with an unresolved shoulder on the loss energy side. Increasing the
87 700K
....-..
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Fig. 2. Specular (8i =0s=40 ° ) HREEL spectra recorded on n-type InSb(100) (n --1 × 10TM cm-3) after argon-ionbombardment and a series of different annealing temperatures upto 700 K. All spectra were recorded with an incident electron energy of 26 eV and with the sample temperature fixed at 300 K. annealing temperature to 500 K results in the elastic peak becoming sharper while the loss feature moves out to an increased energy (65 meV at 500 K). A second, lower energy peak can also be seen at 23 meV. The LEED observations show that this annealing temperature corresponds to the formation of the (4 x 1) (or 4 × 2)) structure. Further increases in the annealing temperature to 700 K show that the lower loss feature at 23 meV does not shift in energy, but that the higher loss feature gradually moves to a lower energy• After annealing to 700 K, the two peaks are clearly resolved in the H R E E L spectrum, with loss energies of 23 and 46 meV respectively. This correlates with the formation of the c(8 x 2) L E E D pattern. It can also be noted from the spectra shown in fig. 2 that as the annealing temperature is increased above 500 K, the F W H M value of both the elastic peak and the loss features is significantly reduced.
T.S. Jones et al. / Surface plasmon excitations in doped InSb(lO0)
88
The loss at 23 meV, which is independent of the post-bombardment annealing temperature, is assigned as the dipole active, surface phonon mode (the Fuchs-Kliewer surface phonon). The energy of this mode is in excellent agreement with other HREELS studies of both capped InSb(100) [6] and cleaved InSb(ll0) [7]. The second loss feature, which shifts dramatically in energy with postbombardment annealing temperature (fig. 3), is assigned as the conduction band plasmon excitation, which we interpret as a bulk plasmon in the near-surface region. These assignments are confirmed by measurements of the intensity of the two loss features as a function of incident electron beam energy. At low incident electron energies (i.e. E i < 10 eV), the phonon is considerably stronger in intensity than the plasmon loss. However, at higher electron energies ( E i > 20 eV), the relative intensity of the two modes is reversed [17]. Such an observation is in good agreement with other electron energy dependent HREELS studies of doped I I I - V semiconductor surfaces [1-7]. It is well documented that ion bombardment of semiconductor materials results in lattice damage and crystal imperfections. In the case of I I I - V semiconductors, preferential sputtering of the Group V element occurs, leaving the surface enriched in the Group III element [9,18,19]. For GaAs, there is a damage threshold for the argon ion impact energy (approximately 100 eV) below InSb (100) n - lx1018cm-3 >o
E
70
o r
50 o co o
~_
40 3o 200 300 400 500 600 700 800 Annealing Temperature (K)
Fig. 3. Plasmon loss energy (meV) plotted as a function of annealing temperature (K) for specular HREELS measurements on n-type InSb(100) (n ---1× 1018cm-3) after argon-ion sputtering.
which full recovery is possible upon annealing [9]. For higher argon ion energies (typically 200-1000 eV), full recovery is not attainable because of cascade collisions resulting in a disordered surface layer. In our experiments, the primary ion energy employed was fixed at 500 eV. Lattice imperfections induced by the sputtering process involves the creation of vacancies, interstitials and the implantation of argon ions in the material. This results in the formation of defect states which pin the Fermi level in the band gap. At this doping level, the Fermi level in the bulk lies 150 meV above the bottom of the conduction band [6]. The upward band bending which results then produces a depletion layer in the near-surface region. The overall effect of this lattice damage is to trap conduction band electrons in the near-surface region and is a likely explanation for the very low plasmon frequency observed in the H R E E L S experiment, compared with that expected for a bulk doping level of 1 X 1018 cm -3 (0~b = 48 meV). The annealing process serves to repair some of the structural damage, as confirmed by the observed sequence of LEED patterns. The gradual increase in the plasmon energy as the annealing temperature is increased from 300 K (fig. 2) implies that the free-carrier concentration in the near-surface region recovers substantially tending towards the bulk value. After annealing to 500 K, the observed plasmon energy is 65 meV, which is higher than that expected from the bulk doping level. This suggests that there is an accumulation of free carriers in the near-surface region. Further increases in the annealing temperature ( > 500 K) result in a decrease in the plasmon loss energy, such that at 700 K, the plasmon energy is 46 meV. This is close to the plasmon energy expected for a bulk doping concentration of 1 x 10 x8 cm -3. This indicates that at higher annealing temperatures (i.e. between 600 and 700 K) there is a decrease in the concentration of free carriers in the nearsurface region. The lowering in loss energy at higher annealing temperatures is accompanied by the transformation of the L E E D pattern from a ( 4 × 1) (or disordered (4 x 2)) into a c(8 x 2) structure. For InSb, the non-congruent temperature of the
7".S. Jones et al. / Surface plasmon excitations m doped lnSb(lO0)
material (i.e. the temperature at which one of the two elements of the compound semiconductor is lost from the surface) is - 6 0 0 K. Above this temperature, antimony is preferentially lost from the material leaving the surface indium-rich. Although our AES measurements did not show a noticeable reduction in Sb composition, the transition to a c(8 × 2) structure at annealing temperatures of - 6 0 0 K may result in some loss of surface Sb during the atomic rearrangement at the surface. Such a small loss of Sb would be extremely difficult to detect with AES since the Auger signal is integrated over a depth of several atomic layers. A loss of surface Sb may result in the incipient formation of a I n / I n S b interface which would be consistent with the decrease in plasmon energy observed in the HREELS experiment. It can also be noted from fig. 2 that at high annealing temperatures, there is a considerable decrease in the half-width of the plasmon feature. This suggests that the carrier lifetime is increased at higher annealing temperatures and is consistent with the removal of surface damage. So far we have related the changes in plasmon energy which occur with annealing temperature to be due to repairing of the surface damage induced by the ion sputtering process. However, diffusion of the dopant material may also be important at the higher annealing temperatures. In a recent study by F/Srster et al. [20], shifts in the plasmon energy of n-type Si(100) were attributed to diffusion of the dopant (in this case phosphorus) into the bulk at high annealing temperatures. For our work on InSb, this would result in a decrease in free carriers in the near-surface region and therefore a reduced plasmon energy at high annealing temperatures. This is indeed observed for annealing temperatures above 500 K. Work is in progress aimed at studying InSb(100) with a wide range of bulk doping concentrations in order to determine whether dopant diffusion is important in determining the plasmon energy observed in HREELS.
4. Conclusion HREELS has been used to study the (100) surface of n-type InSb (n = 1 x 1018 cm -3) pre-
89
pared by a combination of low-energy argon ion bombardment and annealing cycles. In particular, the loss energy of the plasmon excitation is found to be extremely sensitive to the details of the surface preparation. These changes in plasmon loss energy are accompanied by a series of LEED patterns. After argon-ion sputtering only, the plasmon energy (37 meV) is considerable lower than expected for such a bulk doping level used (calculated bulk plasmon frequency 48 meV). This is explained by defect states which pin the Fermi level in the band gap resulting in upward band bending and producing a depletion layer. Annealing results in increases in the plasmon energy which are attributed to a repair of the structural damage. At 500 K, the plasmon energy is 65 meV, suggesting the formation of an accumulation layer. Further increases in annealing temperature results in a gradual decrease in plasmon energy (46 meV at 700 K) to a value close to that expected for the bulk doping concentration used. This is interpreted as arising from either band bending associated with a surface markedly enriched with indium, or by dopant diffusion into the bulk.
Acknowledgements Useful discussions with R.G. Egdell, S. Holloway, P.T. Andrews and P. Weightman are gratefully acknowledged. R.G. Jones and R.G. Egdell are thanked for sending us copies of manuscripts prior to publication.
References [1] R. Matz and H. Li~th, Phys. Rev. Lett. 46 (1981) 500. [2] Y. Chen, S. Nannarone, J. Schaefer, J.C. Hermanson and G.J. Lapeyre, Phys. Rev. B 39 (1989) 7653. [3] M.G. Betti, U. del Pennino and C. Mariani, Phys. Rev. B 39 (1989) 5887. [4] Z.J. Gray-Grychowski, R.A. Stradling, R.G. Egdell, P.J. Dobson, B.A. Joyce and K. Woodbridge, Solid State Commun. 59 (1986) 703. [5] Z.J. Gray-Grychowski, R.G. Egdell, B.A. Joyce, R.A. Stradfing and K. Woodbridge, Surf. Sci. 186 (1987) 482. [6] S.D. Evans, L.L. Can, R.G. Egdell, R. Droopad, S.D. Parker and R.A. Stradling, Surf. Sci. 226 (1990) 169. [7] A. Ritz and H. Liith, Phys. Rev. Lett. 52 (1984) 1242.
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[8] Y. Chen, J.C. Hermanson and G.J. Lapeyre, Phys. Rev. B 39 (1989) 12682. [9] L.H. Dubois and G.P. Schwartz, J. Vac. Sci. Technol. B 2 (1984) 101. [10] K. Oe, S. Ando and K. Sugiyama, J. Appl. Phys. 19 (1980) L417. [11] D.P. Woodruff and K. Horn, Vacuum 33 (1983) 633. [12] R.G. Jones, N.K. Singh and C.F. McConville, Surf. Sci. 208 (1989) L34. [13] P. John, T. Miller and T.C. Chiang, Phys. Rev. B 39 (1989) 1730. [14] T.S. Jones, M.R. Ashton, N.V. Richardson and C.F. McConville, J. Phys. (Condens. Matter) 1 (1989) 207.
[15] A.P. Mowbray and R.G. Jones, Vacuum, submitted. [16] T. Inaoka, D.M. Newns and R.G. Egdell, Surf. Sci. 186 (1987) 290. [17] T.S. Jones, M.Q. Ding, N.V. Richardson and C.F. McConville, to be published. [18] I.L. Singer, J.S. Murday and L.R. Cooper, Surf. Sci. 108 (1981) 7. [19] D.M. Hill, F. Xu, Z. Lin and J.H. Weaver, Phys. Rev. B 38 (1988) 1893. [20] A. FSrster, J.M. Layet and H. Liath, Appl. Phys. A 47 (1988) 95.