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Surface and interface states on GaAs (110): effects of atomic and electronic rearrangements
Classified
abstracts
3366-3458
Classified abstracts on this page 3366-3472
Editor’s note The label immediately following the title of each item and that at the end of each abstract indicates country I. General engineering 11. GASES AND
vacuum
science
and
SOLIDS
11 3366. CO2 laser radiation absorption in propane. The absorption coefficient of propane has been measured using a low-power COz laser at a variety of wavenumbers over the range 940-1081 cm-‘. At 944 cm-’ the absorption coefficient of propane was found to vary linearly with pressure, its value being 8.5 x 1O-s cm-’ torr-I. This coefficient was essentially unaltered by adding nitrogen to propane to produce an atmospheric mixture. J A McMordie and G W Sentance, JPhys D: ApplPhys, 10 (10). 1977, L127-L130. 11 3367. Electron-beam effects daring oxygen adsorption. (GB) The use of Auger electron spectroscopy to monitor the oxidation of GaP (110) is discussed. Although the electron beam is turned off during exposure and for some time afterwards, a strong dependence is found of the oxygen Auger peak height on the elapsed time since the electron beam was switched on again. A mechanism for this is proposed, involving adsorption of oxygen on to the filament of the electron gun and the ceramic supports of the filament. The gradual diffusion of oxygen into the electron beam followed by ionization and projection towards the sample is then possible. It is suggested that, in such experiments, the filament should be heated, with the beam deflected away from the sample, for several hours before reliable results can be obtained. D Norman and D K Skinner,JPhys D: ApplPhys, 10 (1 l), 1977, L151L154. 3368. Some thoughts on the existence of empty surface states and the effect of surface order on sorption. Experimental and computational information on the existence of empty surface bands in the upper portion of the band gap on nonpolar compound semiconductor surfaces is reviewed and it is shown that agreement is poor. However, both sources of information have reached the point where meaningful agreement can be achieved, thus presenting one of the first cases of quantitative experimental and theoretical self-consistency of the electronic properties of semiconductor surfaces. The dominant influence of surface atomic order on sorption at free nonpolar surfaces of compound semiconductors is also reviewed and it is suggested that both experimenters and theorists must incorporate this order-dependence into their work before a fundamental understanding of sorption can be achieved. P Mark et al, J Vat Sci Technol, 14 (4), 1977,865-869. 11 3369. Surface and interface states on GaAs (110) : effects of atomic and electronic rearrangements. (USA) Research during the last year has led to a better understanding of the electronic and atomic structure of the (110) surface-s of III-V semiconductors. In this paper we will briefly review thesenew developments as well as point out areas where agreement has been found between various experimental results presented in the literature. It is now generally agreed that there are no intrinsic surface states in the band gap on GaAs and the smaller band-gap materials (e.g. GaSb, InAs and GaSb) and that Schottky barrier pinning must be due to states produced when the metal adlayer is applied. Particular attention is focused in this paper on the large surface rearrangement which takes place on the (110) GaAs surface and effects of the strain which may be produced in joining this rearranged surface layer to the rest of GaAs crystal. It is pointed out that this may lead to variations in the surface rearrangement which can produce variations in the valence electronic structure at the surface. Such variations are shown in experimental energy distribution curves obtained by the photoemission technique which samples principally the last two molecular layers. It is further shown that surprisingly small amounts of chemisorbed oxygen can
denotes the country or origin of publication, of origin of work (where known).
produce first-order effects in the valence-band electronic structure. On all GaAs (110) surfaces studied, a phaselike transformation was observed with a few hundredths of a monolayer coverage of chemisorbed oxygen. Near this coverage. the Ga 3 d exciton structure disappears and the oxygen uptake increases significantly. On certain samples, first-order changes in the valence-band electronic structure were observed at a coverage of a hundredth of a monolayer or lower. These transformations are discussed in terms of the electronic and atomic configurations at the surface. Experimental data showing As and Ga 3 dchemical shifts for oxidation as well as chemisorption are also presented and used to point out difficulties to be expected in passivating, practical surfaces. In particular, the effect of mixed As and Ga oxides the desirability of bonding passivating layers to the GaAs through As bonds, and the effect of strain-induced interface states are discussed. W E Spicer et al, J Vuc Sci Technol, 14 (4). 1977,885-895. 11 3370. Electronic surface properties of Ga and In containing III-V compounds. (USA) Based upon contact potential-difference measurements, photoemission experiments, and low-energy-loss spectra of GaSb, GaAs, Gap, InAs and InP, we propose an empirical model for the surface states at the (110) cleavage face of all Ga and In containing III-V compounds with the zincblende structure. Three tiled and two empty surface-state bands were identified. The position of two of the filled states for the GaAs (110) face are consistent with available data from angle-resolved photoemission experiments. However, for any other states no experimental data are available. J van Laar et ai, J Vuc Sci Technol, 14 (4), 1977,894-898. 11 3371. Ionicity effects on compound semiconductor (110) surfaces. (USA) Properties of clean and controllably oxidized surfaces of in situ cleaved GaAs, CdTe, ZnTe, ZnSe and ZnS were probed by synchrotron radiation-induced photoelectron spectroscopy. We concentrate on variations in submonolayer oxygen adsorption due to changing semiconductor ionicity. A roughly exponential dependence of O2 sticking coefficient on electronegativity difference correlates well with estimates based on other techniques when the molecular state of the adsorbate is considered. Monitoring the O2 interaction by the semiconductor substrate core-level chemical shift, changes in surface bonding with ionicity are shown by the cation behaviour. We also point out that the predominant angular momentum of the intrinsic empty surface states is an important characteristic. Using the dipole selection rules governing photoemission partial-yield transitions, significant anion s-like empty surface state density exists on all (110) surfaces studied. Increasing ionicity appears mainly to change the atomic character of cation-derived empty surface states from p- to s-like. Core-level transitions to these surface states are strongly influenced by final-state effects. Our self-consistent measurements of p-core exciton-binding energies show a large increase in surface finalstate effects with increasing ionicity. while bulk conduction-band-edge excitons become weaker. The varying bonding requirements and possibly the associated surface-atom positions provide a unifying concept for understanding these ionicity effects. R S Bauer,J Vuc Sci Technol, 14 (4), 1977,899-903. 11 3372. Oxygen sorption and excitonic effects on GaAs surfaces. (USA) It is now clear that excitations from the Ga-3 dcore level into the empty surface states are highly excitonic in nature. These transitions can be studied by partial yield or low-energy electron-loss spectroscopy, and in the past their disappearance with oxygen exposure has been attributed to oxygen bonding on surface Ga atoms and destroying the empty surface states. By combining partial yield, ultraviolet photoemission and soft X-ray photoemission data, it is shown that this disappearance is not due to oxygen bonding on Ga atoms but due to exciton extinction from oxygen-induced changes in the surface electronic structure at very low coverages. P W Chye et al,J Vuc Sci Technof, 14 (4), 1977,917-919. 97
abstracts
3366-3458
Classified abstracts on this page 3366-3472
Editor’s note The label immediately following the title of each item and that at the end of each abstract indicates country I. General engineering 11. GASES AND
vacuum
science
and
SOLIDS
11 3366. CO2 laser radiation absorption in propane. The absorption coefficient of propane has been measured using a low-power COz laser at a variety of wavenumbers over the range 940-1081 cm-‘. At 944 cm-’ the absorption coefficient of propane was found to vary linearly with pressure, its value being 8.5 x 1O-s cm-’ torr-I. This coefficient was essentially unaltered by adding nitrogen to propane to produce an atmospheric mixture. J A McMordie and G W Sentance, JPhys D: ApplPhys, 10 (10). 1977, L127-L130. 11 3367. Electron-beam effects daring oxygen adsorption. (GB) The use of Auger electron spectroscopy to monitor the oxidation of GaP (110) is discussed. Although the electron beam is turned off during exposure and for some time afterwards, a strong dependence is found of the oxygen Auger peak height on the elapsed time since the electron beam was switched on again. A mechanism for this is proposed, involving adsorption of oxygen on to the filament of the electron gun and the ceramic supports of the filament. The gradual diffusion of oxygen into the electron beam followed by ionization and projection towards the sample is then possible. It is suggested that, in such experiments, the filament should be heated, with the beam deflected away from the sample, for several hours before reliable results can be obtained. D Norman and D K Skinner,JPhys D: ApplPhys, 10 (1 l), 1977, L151L154. 3368. Some thoughts on the existence of empty surface states and the effect of surface order on sorption. Experimental and computational information on the existence of empty surface bands in the upper portion of the band gap on nonpolar compound semiconductor surfaces is reviewed and it is shown that agreement is poor. However, both sources of information have reached the point where meaningful agreement can be achieved, thus presenting one of the first cases of quantitative experimental and theoretical self-consistency of the electronic properties of semiconductor surfaces. The dominant influence of surface atomic order on sorption at free nonpolar surfaces of compound semiconductors is also reviewed and it is suggested that both experimenters and theorists must incorporate this order-dependence into their work before a fundamental understanding of sorption can be achieved. P Mark et al, J Vat Sci Technol, 14 (4), 1977,865-869. 11 3369. Surface and interface states on GaAs (110) : effects of atomic and electronic rearrangements. (USA) Research during the last year has led to a better understanding of the electronic and atomic structure of the (110) surface-s of III-V semiconductors. In this paper we will briefly review thesenew developments as well as point out areas where agreement has been found between various experimental results presented in the literature. It is now generally agreed that there are no intrinsic surface states in the band gap on GaAs and the smaller band-gap materials (e.g. GaSb, InAs and GaSb) and that Schottky barrier pinning must be due to states produced when the metal adlayer is applied. Particular attention is focused in this paper on the large surface rearrangement which takes place on the (110) GaAs surface and effects of the strain which may be produced in joining this rearranged surface layer to the rest of GaAs crystal. It is pointed out that this may lead to variations in the surface rearrangement which can produce variations in the valence electronic structure at the surface. Such variations are shown in experimental energy distribution curves obtained by the photoemission technique which samples principally the last two molecular layers. It is further shown that surprisingly small amounts of chemisorbed oxygen can
denotes the country or origin of publication, of origin of work (where known).
produce first-order effects in the valence-band electronic structure. On all GaAs (110) surfaces studied, a phaselike transformation was observed with a few hundredths of a monolayer coverage of chemisorbed oxygen. Near this coverage. the Ga 3 d exciton structure disappears and the oxygen uptake increases significantly. On certain samples, first-order changes in the valence-band electronic structure were observed at a coverage of a hundredth of a monolayer or lower. These transformations are discussed in terms of the electronic and atomic configurations at the surface. Experimental data showing As and Ga 3 dchemical shifts for oxidation as well as chemisorption are also presented and used to point out difficulties to be expected in passivating, practical surfaces. In particular, the effect of mixed As and Ga oxides the desirability of bonding passivating layers to the GaAs through As bonds, and the effect of strain-induced interface states are discussed. W E Spicer et al, J Vuc Sci Technol, 14 (4). 1977,885-895. 11 3370. Electronic surface properties of Ga and In containing III-V compounds. (USA) Based upon contact potential-difference measurements, photoemission experiments, and low-energy-loss spectra of GaSb, GaAs, Gap, InAs and InP, we propose an empirical model for the surface states at the (110) cleavage face of all Ga and In containing III-V compounds with the zincblende structure. Three tiled and two empty surface-state bands were identified. The position of two of the filled states for the GaAs (110) face are consistent with available data from angle-resolved photoemission experiments. However, for any other states no experimental data are available. J van Laar et ai, J Vuc Sci Technol, 14 (4), 1977,894-898. 11 3371. Ionicity effects on compound semiconductor (110) surfaces. (USA) Properties of clean and controllably oxidized surfaces of in situ cleaved GaAs, CdTe, ZnTe, ZnSe and ZnS were probed by synchrotron radiation-induced photoelectron spectroscopy. We concentrate on variations in submonolayer oxygen adsorption due to changing semiconductor ionicity. A roughly exponential dependence of O2 sticking coefficient on electronegativity difference correlates well with estimates based on other techniques when the molecular state of the adsorbate is considered. Monitoring the O2 interaction by the semiconductor substrate core-level chemical shift, changes in surface bonding with ionicity are shown by the cation behaviour. We also point out that the predominant angular momentum of the intrinsic empty surface states is an important characteristic. Using the dipole selection rules governing photoemission partial-yield transitions, significant anion s-like empty surface state density exists on all (110) surfaces studied. Increasing ionicity appears mainly to change the atomic character of cation-derived empty surface states from p- to s-like. Core-level transitions to these surface states are strongly influenced by final-state effects. Our self-consistent measurements of p-core exciton-binding energies show a large increase in surface finalstate effects with increasing ionicity. while bulk conduction-band-edge excitons become weaker. The varying bonding requirements and possibly the associated surface-atom positions provide a unifying concept for understanding these ionicity effects. R S Bauer,J Vuc Sci Technol, 14 (4), 1977,899-903. 11 3372. Oxygen sorption and excitonic effects on GaAs surfaces. (USA) It is now clear that excitations from the Ga-3 dcore level into the empty surface states are highly excitonic in nature. These transitions can be studied by partial yield or low-energy electron-loss spectroscopy, and in the past their disappearance with oxygen exposure has been attributed to oxygen bonding on surface Ga atoms and destroying the empty surface states. By combining partial yield, ultraviolet photoemission and soft X-ray photoemission data, it is shown that this disappearance is not due to oxygen bonding on Ga atoms but due to exciton extinction from oxygen-induced changes in the surface electronic structure at very low coverages. P W Chye et al,J Vuc Sci Technof, 14 (4), 1977,917-919. 97