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3986. LEED analysis of a Cu(110) surface. (USA)
Classified abstracts 3982-3991
ions to neutrals. This ratio as a function of the energy of the sputtered particles has been obtained for a clean polycrystalline copper surface sputtered by 1000-3000 eV A.r +. The energy distributions of both neutral and ionized copper were measured with a retarding potential analyser (RPA) using potential modulation differentiation and signal averaging. The maximum for both distributions is identical and occurs near 2.5 eV. The energy distributions of neutrals is more sharply peaked than that of the ions, presumably as a consequence of more efficient neutralization of slow escaping ions by the mobile electrons of copper. The ion-neutral ratio, which is found to be proportional to E "/2 with 0.6 < n < 1.4, is compared with various ionization models. T R Lundquist, J Vac Sci Technol, 15 (2), 1978, 684--687. 36 3982. Combined low-energy ion scattering and X-ray photoelectron spectroscopy study of Ta205 bombarded by 500-3000 eV He ions. (USA) The low-energy ion backscatting spectra of Ta205 obtained with He as the probe gas exhibit a pronounced low-energy tail on the metal peak and a strong incident ion energy dependence of the oxygen to tantalum peak intensity ratio. To determine the effect differential scattering cross-sections, ion neutralization and sample reduction have on these spectral features, a combined ISS-XPS study has been carried out. The data indicate that the sample is not reduced by the He ion beam and that a resonant-type neutralization plays a dominant role both in tailing and in the variation of the oxygen to tantalum peak intensity ratio. G C Nelson, J Vac Sci Technol, 15 (2), 1978, 702-705. 36 3983. Determination of quantitative sputter rates of iron oxide by Auger electron spectroscopy (AES) and ellipsometry. (USA) AES was used to study the sputtering rates for thermally grown iron oxide films bombarded with 1-5 keV Ar +. Ellipsometric measurements of the films were used to determine the film thickness and to confirm composition. Calibration was made for sputtering rates as a function of ion flux and ion energy. Special attention was placed upon the effects of specimen-ion source and specimen-electron energy analyzer alignment in establishing this calibration. This work is relevant to experiments in which ion etching is used as a means of obtaining depth profiles of oxidized iron and ferrous alloys. R G Hart and G W Simmons, J Vac Sci TechnoL 15 (2), 1978, 714--717. 36 3984. Surface photovoltage experiments on SrTiOa electrodes. (USA) Electronic surface states play a significant role in charge transfer between the electrolyte and the n-type SrTiO2 semiconducting electrodes used in the photoelectrolysis of water. To clarify the role of these states, we have measured the wavelength response of the photovoltage developed between the SrTiO3 surface and a vibrating Au reference electrode, both in vacuum and in a water vapour ambient. When the SrTiOa is exposed to subband-gap radiation, the amplitude and sign of the photovoltage vary with photon energy. Significant changes in the spectrum result from the introduction of water vapour. The energy-dependent structure is interpreted in terms of photon-induced electronic transitions between the states near the surface and conduction or valence band states. J G Mavroides and D F Kolesar, J Vac Sci Technol, 15 (2), 1978, 538-541. 36 3985. Temperature effects in polarized low-energy electron scattering from solids and liquids. (USA) The effects of temperature on the polarization of low-energy electrons scattered from surfaces of liquids, polycrystals and single crystals are discussed. The polarization of the single scattering component of the intensity is shown to be independent of temperature, whereas that of the multiple scattering intensity is temperature dependent. Differences between polarizations observed for electron scattering from free atoms and from polycrystals and liquids are qualitatively explained and a quantitative theory is outlined. The implications for polarized low-energy electron diffraction from single-crystal surfaces are also discussed. W N Unertl, J Vac Sci Technol, 15 (2), 1978, 591-594. 36 3986. LEED analysis of a Cu(ll0) surface. (USA) Experimental and calculated 1-V profiles for the (0]') diffraction beam from a Cu(ll0) surface are presented and compared. Preliminary 200
results of this comparison suggest that the outermost atomic layer is contracted by ~ 1 0 ~ relative to the bulk interlayer spacing and there is some surface that the second-third layer spacing may be slightly contracted. In order to demonstrate the reproducibility of the data obtained in this investigation, degeneracies of beams related by symmetry are also illustrated. J R Noonan et al, J Vac Sci Technol, 15 (2), 1978, 619-621. 36 3987. LEED-Auger study of the (li0) surface of a 1 ~ Si-Fe single crystal. (USA) An optimal procedure involving ion bombardment at elevated temperatures for surface cleaning is described. The observed LEED pattern basically consists of the ideal (II0) surface of a bcc structure together with extra spots indicating additional periodicity in the (001) direction. A possible explanation is proposed. Intensity profiles for different beams and their temperature dependence are discussed. An effective surface Debye temperature of 300 K and a predicted bulk value of about 420 K are deduced. Annealing at temperatures above 780°C leads to diffusion of sulfur atoms from the bulk and to ordering on the surface. The resulting LEED pattern is described in terms of a rationally related lattice with a unit net [(16/3)1/z × (8/3) I/2] with respect to the basic (lI0)-surface p lattice of a bee crystal. (South Africa). P E Viljoen et al, J Vac Sci Technol, 15 (2), 1978, 626-630. 36 3988. Surface structure and orbital symmetries of (II0) surface states of GaAs. (USA) The orbital symmetries of (I I0) surface electronic states of GaAs for three different surface-relaxation models are analysed using the tightbinding method. Although the atomic displacements involved in all three relaxations are basically very similar, they give rise to different orderings of some surface states when these states are characterized by their orbital symmetries. Angle-resolved photoemission measurements using polarized radiation, therefore, provide a useful tool for distinguishing between competing surface structures determined from low-energy electron-diffraction studies. A comparison of the recent, preliminary measurements by Smith, Williams and Lapeyre with our theoretical results show that only the 'bond-relaxation' model for the surface is in agreement with the experimental results. Local densities of surface states and the orbital symmetries of surface states are discussed. D J Chadi, J Vac Sci Technol, 15 (2), 1978, 631-636. 36 3989. Some effects of anisotropie vibrations in LEED. (USA) A formalism is presented which enables effects of certain types of vibrational anistropy to be included in LEED calculations. Results from a demonstration calculation are presented and compared with a similar calculation where no anisotropy was present. M I.Jlehla and H L Davis, J Vac Sci Technol, 15 (2), 1978, 642-646. 36 3990. N V V Auger spectra from W(100). (USA) The N V V Auger spectrum from a clean W(100) surface has been measured in the second derivative, d2N(E)/dE 2, mode to enhance fine structure. This measurement is compared with spectra generated from both the self-convolution of the tungsten valence-band bulk density of states (obtained from a relativistic APW energy band calculation) and a 'restricted convolution' in which only transitions involving electrons from the same valence energy are allowed. The restricted convolution for a model of the Auger process in which both N6 VV and N.~VV transitions contribute offers the best match of theory and experiment. No distinct evidence of Auger emission involving the surface resonance present on W(100) is observed. Effects of H2 and 02 adsorption on the Auger spectrum of the W(100) surface are reported. S P Withrow et al, J Vac Sci Technol, 15 (2), 1978, 511-517. 36 3991. X-ray photoelectron spectroscopy with X-ray photons of higher energy. (USA) There have been few studies using ionizing photons higher in energy than AI Ko (1487 eV). However, characteristic X-rays such as Au Mo (2123 eV) and Ag L, (2984 eV) are nearly as narrow in energy, and can provide deeper core photoelectron lines a n d their companion core-type Auger lines in the spectra of 26 elements that otherwise cannot be recorded. Such data are of special value in identifying chemical states by the two-dimensional approach. This involves
ions to neutrals. This ratio as a function of the energy of the sputtered particles has been obtained for a clean polycrystalline copper surface sputtered by 1000-3000 eV A.r +. The energy distributions of both neutral and ionized copper were measured with a retarding potential analyser (RPA) using potential modulation differentiation and signal averaging. The maximum for both distributions is identical and occurs near 2.5 eV. The energy distributions of neutrals is more sharply peaked than that of the ions, presumably as a consequence of more efficient neutralization of slow escaping ions by the mobile electrons of copper. The ion-neutral ratio, which is found to be proportional to E "/2 with 0.6 < n < 1.4, is compared with various ionization models. T R Lundquist, J Vac Sci Technol, 15 (2), 1978, 684--687. 36 3982. Combined low-energy ion scattering and X-ray photoelectron spectroscopy study of Ta205 bombarded by 500-3000 eV He ions. (USA) The low-energy ion backscatting spectra of Ta205 obtained with He as the probe gas exhibit a pronounced low-energy tail on the metal peak and a strong incident ion energy dependence of the oxygen to tantalum peak intensity ratio. To determine the effect differential scattering cross-sections, ion neutralization and sample reduction have on these spectral features, a combined ISS-XPS study has been carried out. The data indicate that the sample is not reduced by the He ion beam and that a resonant-type neutralization plays a dominant role both in tailing and in the variation of the oxygen to tantalum peak intensity ratio. G C Nelson, J Vac Sci Technol, 15 (2), 1978, 702-705. 36 3983. Determination of quantitative sputter rates of iron oxide by Auger electron spectroscopy (AES) and ellipsometry. (USA) AES was used to study the sputtering rates for thermally grown iron oxide films bombarded with 1-5 keV Ar +. Ellipsometric measurements of the films were used to determine the film thickness and to confirm composition. Calibration was made for sputtering rates as a function of ion flux and ion energy. Special attention was placed upon the effects of specimen-ion source and specimen-electron energy analyzer alignment in establishing this calibration. This work is relevant to experiments in which ion etching is used as a means of obtaining depth profiles of oxidized iron and ferrous alloys. R G Hart and G W Simmons, J Vac Sci TechnoL 15 (2), 1978, 714--717. 36 3984. Surface photovoltage experiments on SrTiOa electrodes. (USA) Electronic surface states play a significant role in charge transfer between the electrolyte and the n-type SrTiO2 semiconducting electrodes used in the photoelectrolysis of water. To clarify the role of these states, we have measured the wavelength response of the photovoltage developed between the SrTiO3 surface and a vibrating Au reference electrode, both in vacuum and in a water vapour ambient. When the SrTiOa is exposed to subband-gap radiation, the amplitude and sign of the photovoltage vary with photon energy. Significant changes in the spectrum result from the introduction of water vapour. The energy-dependent structure is interpreted in terms of photon-induced electronic transitions between the states near the surface and conduction or valence band states. J G Mavroides and D F Kolesar, J Vac Sci Technol, 15 (2), 1978, 538-541. 36 3985. Temperature effects in polarized low-energy electron scattering from solids and liquids. (USA) The effects of temperature on the polarization of low-energy electrons scattered from surfaces of liquids, polycrystals and single crystals are discussed. The polarization of the single scattering component of the intensity is shown to be independent of temperature, whereas that of the multiple scattering intensity is temperature dependent. Differences between polarizations observed for electron scattering from free atoms and from polycrystals and liquids are qualitatively explained and a quantitative theory is outlined. The implications for polarized low-energy electron diffraction from single-crystal surfaces are also discussed. W N Unertl, J Vac Sci Technol, 15 (2), 1978, 591-594. 36 3986. LEED analysis of a Cu(ll0) surface. (USA) Experimental and calculated 1-V profiles for the (0]') diffraction beam from a Cu(ll0) surface are presented and compared. Preliminary 200
results of this comparison suggest that the outermost atomic layer is contracted by ~ 1 0 ~ relative to the bulk interlayer spacing and there is some surface that the second-third layer spacing may be slightly contracted. In order to demonstrate the reproducibility of the data obtained in this investigation, degeneracies of beams related by symmetry are also illustrated. J R Noonan et al, J Vac Sci Technol, 15 (2), 1978, 619-621. 36 3987. LEED-Auger study of the (li0) surface of a 1 ~ Si-Fe single crystal. (USA) An optimal procedure involving ion bombardment at elevated temperatures for surface cleaning is described. The observed LEED pattern basically consists of the ideal (II0) surface of a bcc structure together with extra spots indicating additional periodicity in the (001) direction. A possible explanation is proposed. Intensity profiles for different beams and their temperature dependence are discussed. An effective surface Debye temperature of 300 K and a predicted bulk value of about 420 K are deduced. Annealing at temperatures above 780°C leads to diffusion of sulfur atoms from the bulk and to ordering on the surface. The resulting LEED pattern is described in terms of a rationally related lattice with a unit net [(16/3)1/z × (8/3) I/2] with respect to the basic (lI0)-surface p lattice of a bee crystal. (South Africa). P E Viljoen et al, J Vac Sci Technol, 15 (2), 1978, 626-630. 36 3988. Surface structure and orbital symmetries of (II0) surface states of GaAs. (USA) The orbital symmetries of (I I0) surface electronic states of GaAs for three different surface-relaxation models are analysed using the tightbinding method. Although the atomic displacements involved in all three relaxations are basically very similar, they give rise to different orderings of some surface states when these states are characterized by their orbital symmetries. Angle-resolved photoemission measurements using polarized radiation, therefore, provide a useful tool for distinguishing between competing surface structures determined from low-energy electron-diffraction studies. A comparison of the recent, preliminary measurements by Smith, Williams and Lapeyre with our theoretical results show that only the 'bond-relaxation' model for the surface is in agreement with the experimental results. Local densities of surface states and the orbital symmetries of surface states are discussed. D J Chadi, J Vac Sci Technol, 15 (2), 1978, 631-636. 36 3989. Some effects of anisotropie vibrations in LEED. (USA) A formalism is presented which enables effects of certain types of vibrational anistropy to be included in LEED calculations. Results from a demonstration calculation are presented and compared with a similar calculation where no anisotropy was present. M I.Jlehla and H L Davis, J Vac Sci Technol, 15 (2), 1978, 642-646. 36 3990. N V V Auger spectra from W(100). (USA) The N V V Auger spectrum from a clean W(100) surface has been measured in the second derivative, d2N(E)/dE 2, mode to enhance fine structure. This measurement is compared with spectra generated from both the self-convolution of the tungsten valence-band bulk density of states (obtained from a relativistic APW energy band calculation) and a 'restricted convolution' in which only transitions involving electrons from the same valence energy are allowed. The restricted convolution for a model of the Auger process in which both N6 VV and N.~VV transitions contribute offers the best match of theory and experiment. No distinct evidence of Auger emission involving the surface resonance present on W(100) is observed. Effects of H2 and 02 adsorption on the Auger spectrum of the W(100) surface are reported. S P Withrow et al, J Vac Sci Technol, 15 (2), 1978, 511-517. 36 3991. X-ray photoelectron spectroscopy with X-ray photons of higher energy. (USA) There have been few studies using ionizing photons higher in energy than AI Ko (1487 eV). However, characteristic X-rays such as Au Mo (2123 eV) and Ag L, (2984 eV) are nearly as narrow in energy, and can provide deeper core photoelectron lines a n d their companion core-type Auger lines in the spectra of 26 elements that otherwise cannot be recorded. Such data are of special value in identifying chemical states by the two-dimensional approach. This involves