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7669. Thermal vibrations at copper surfaces studied by low energy ion scattering
Classified abstracts 7667-7674
51 7667. An interrupted field time-of-flight 0WI'OF) technique in transient photocondnctivity measurements An interrupted field time-of-flight (IFTOF) technique that enables displacement-current-free measurement of the photocurrent in the conventional transient photoconductivity experiment is described and implemented. During the drift of the photoinjected charge carriers across the sample, the applied field is removed at time Tt and reapplied at time T2 = Tt + ti, where t~ is the interruption time. During the interruption period t~, the charge carriers interact with the deep traps so that the recovered photocurrent when the field is reapplied at T2 = T~ + t~indicates the concentration of carriers remaining in the transport band. Although the IFTOF technique has a number of distinct advantages for studying charge trapping and release processes in high-resistivity solids, it has generally not been adopted as a convenient experiment since the sample capacitance results in large displacement currents at switching. The present paper describes a Schering-type bridge network that is excited by a switchable floating high-voltage supply. Recently available high-voltage complementary TMOS transistors were used to switch voltages as high as 500 V. Trigger signals to initiate the various IFTOF events were simply and economically generated from TTL logic gates and IC timers while the required time delays were obtained via clocked digital countdown techniques. The IFTOF method was successfully applied to the examination of hole trapping processes in chlorinated a-Se:0.3%As xeroradiographic-type films for which the conventional TOF measurement indicated essentially trap-free photocurrent. Using the IFTOF technique, it is shown that over a time scale far exceeding the conventional TOF transit time, the photoinjected hole concentration under low field conditions decays almost exponentially with a well-defined trapping time z. Furthermore, by interrupting the electric field while the photoinjected charge packet is at different locations in the film, it is shown that the IFTOF technique may be used to examine the dependence of the trapping time on the distance into the film. IFTOF is therefore a valuable technique for studying trapping inhomogeneities in amorphous semiconductor films. It is expected that the simple displacement-current-free IFTOF technique described in this paper, with further improvements, may be applied to study charge carrier trapping and release kinetics in a wide variety of high-resistivity solids. S O Kasap et al, Rev Scient lnstrum, 61, 1990, 2080-2087. 51 7668. Central temperature of convectively cooled thin targets during ion beam analysis A simple expression for the central temperature of a convectively cooled target subjected to ion beam analysis has been obtained in the approximation that the convective-cooling mechanism is dominant. D W McColm and T A Cahill, Nucl Instrum Meth Phys Res, B51, 1990, 196-197. 52. ION A N D ELECTRON SCATTERING 52 7669. Thermal vibrations at copper surfaces studied by low energy ion scattering The thermal vibrations of surface atoms on Cu(110) have been studied as a function of temperature with impact collision ion scattering spectroscopy. Monte Carlo simulations of the scattering along chains of surface atoms show that for large vibration amplitudes more than two atoms have to be included to explain the experimental data. The mean square displacements obtained from the comparison between experiment and calculations agree well with the results of atom scattering and inverse photoemission experiments and the results from surface phonon calculations. The influence of correlations and anisotropy of the surface vibrations on impact collision ion scattering spectra is discussed. H Diirr et ai, Vacuum, 41, 1990, 376-378. 52 7670. Structure determination of the SrTiOa(100) surface The surface o f SrTiO3(100) was analysed by low energy diffraction. Intensity spectra were taken using a TV camera whose signal is evaluated under computer control. Model intensities were calculated fully dynamically for energies above 200 eV where details of the scattering potential are of minor influence. The best theory-experiment fit (Pendry R-factor = 0.53) shows the surface to be terminated by both Sr-O and O - T i - O layers. The top layers are buckled by oxygen ions displaced out of the surface by 0.08 and 0.16/~ in the Ti and Sr terminated domains, respectively.
The buckling, which creates a static dipole moment in the surface layer, is accompanied by a multilayer relaxation which for the first two layers is dtz(Ti)/d0 = + 2 % , d23(Ti)/d0 = - 2 % for the Ti termination and dlz(Sr)/d0 = - 10%, d23(Sr)/d0 = + 4 % for the Sr terminated domains. N Bickel et al, Vacuum, 41, 1990, 4 6 4 8 . 52 7671. Test of structural models for Cu(001)-(~/2 x 2~/2) R45°-O by LEED intensity analysis The Cu(001~(x/2 × 2~/2)R45°~O structure was investigated by LEED intensity analysis. Several structural models were considered in the calculations. The models proposed on the basis of HREELS results can be ruled out. It was confirmed that the missing row model proposed by Zeng et al gives the best agreement with the experimental data. The structural parameters for this model were refined considering a buckling of the first and second copper layers. A Atrei et al, Vacuum, 41, 1990, 333 336. 52 7672. Surface structure and oxidation of Si(001)-Bi The geometric structure, electronic state and reaction with 02 of the Si(001) surface with Bi overlayer have been studied by LEED, XPS and UPS. Bi(0001) was grown epitaxially with interface structure of Bi(0001~ Si(001) and Bi[2i]] Si[110] when Bi was deposited on Si(001)2 x 1 clean surface at room temperature. Subsequently annealing to 570-720 K caused the desorption of Bi, and successive phase transition of the 2 x n (10 > n > 5) surface structure was observed around 1 ML (i.e. atomic ratio of Bi to first layer Si is unity). The effect of an 02 exposure of I000 L at 570-720 K depended upon initial Bi coverage. At submonolayer Bi coverage, Oz exposure induced the oxidation of Si surface and the desorption of Bi. On the other hand, the 2 x n surfaces which corresponded to about 1 ML were stable against 02, indicating strong interaction between Bi and Si. At higher coverage, excess Bi atoms were oxidized. T Hanada and M Kawai, Vacuum, 41, 1990, 65(L651. 52 7673. Interesting aspects of reflection high-energy electron diffraction oscillations during growth of GaAs(100) The intensities of several reflection high-energy electron diffraction (RHEED) beams have been recorded during molecular-beam epitaxial growth of GaAs(100) using a novel video intensity measurement system that records multiple R H E E D beam intensities simultaneously. The R H E E D beam intensities were recorded at varying angles of incidence and crystal substrate azimuth angles. Strong oscillations in the intensities in specular and nonspecular beams with the same period but varying phases have been measured. As noted by other investigators, the phase relationship of the oscillations of the various beams has been found to vary with incident and azimuthal angles. The results are examined with regard to recent studies of the role of Kikuchi processes on the phase of the specular beam. In contrast to other reports, it is found that although the diffracted intensities in the vicinity of the elastically diffracted beams are influenced by inelastically scattered electrons from the Kikuchi lines, these effects cannot account for all the phase behavior observed. K D Jamison et al, J Vac Sci Technol, B8, 1990, 279 282. 52 7674. A review of the geometrical fundamentals of reflection high-energy electron diffraction with application to silicon surfaces Reflection high-energy electron diffraction (RHEED) is an experimentally simple technique, and yet a powerful one for examining the structure of a substrate surface and for monitoring the surface crystal structure and the crystallographic orientation of thin films during their growth. However, it can be difficult to learn to interpret the R H E E D patterns of new materials, because a practical and adequately detailed introduction to the technique is not generally available. To address this need, we develop the geometrical principles of R H E E D ; using the kinematic approximation, we show how a particular point of the sample surface's reciprocal net gives rise to a diffraction maximum at a particular location on the R H E E D viewing screen. We explain the origins of 'reciprocal lattice rods', R H E E D streaks, and Laue rings. We show how to calculate the streak spacing, and clarify the basic effect on the R H E E D pattern of using a nonzero angle of incidence for the incident beam. Crystalline nets, reciprocal nets, and their R H E E D patterns are derived for both (001) and (111) silicon surfaces and are compared to experimentally obtained patterns. J E Mahan et al, J Vac Sci Technol, A8, 1990, 3692 3700. 693
51 7667. An interrupted field time-of-flight 0WI'OF) technique in transient photocondnctivity measurements An interrupted field time-of-flight (IFTOF) technique that enables displacement-current-free measurement of the photocurrent in the conventional transient photoconductivity experiment is described and implemented. During the drift of the photoinjected charge carriers across the sample, the applied field is removed at time Tt and reapplied at time T2 = Tt + ti, where t~ is the interruption time. During the interruption period t~, the charge carriers interact with the deep traps so that the recovered photocurrent when the field is reapplied at T2 = T~ + t~indicates the concentration of carriers remaining in the transport band. Although the IFTOF technique has a number of distinct advantages for studying charge trapping and release processes in high-resistivity solids, it has generally not been adopted as a convenient experiment since the sample capacitance results in large displacement currents at switching. The present paper describes a Schering-type bridge network that is excited by a switchable floating high-voltage supply. Recently available high-voltage complementary TMOS transistors were used to switch voltages as high as 500 V. Trigger signals to initiate the various IFTOF events were simply and economically generated from TTL logic gates and IC timers while the required time delays were obtained via clocked digital countdown techniques. The IFTOF method was successfully applied to the examination of hole trapping processes in chlorinated a-Se:0.3%As xeroradiographic-type films for which the conventional TOF measurement indicated essentially trap-free photocurrent. Using the IFTOF technique, it is shown that over a time scale far exceeding the conventional TOF transit time, the photoinjected hole concentration under low field conditions decays almost exponentially with a well-defined trapping time z. Furthermore, by interrupting the electric field while the photoinjected charge packet is at different locations in the film, it is shown that the IFTOF technique may be used to examine the dependence of the trapping time on the distance into the film. IFTOF is therefore a valuable technique for studying trapping inhomogeneities in amorphous semiconductor films. It is expected that the simple displacement-current-free IFTOF technique described in this paper, with further improvements, may be applied to study charge carrier trapping and release kinetics in a wide variety of high-resistivity solids. S O Kasap et al, Rev Scient lnstrum, 61, 1990, 2080-2087. 51 7668. Central temperature of convectively cooled thin targets during ion beam analysis A simple expression for the central temperature of a convectively cooled target subjected to ion beam analysis has been obtained in the approximation that the convective-cooling mechanism is dominant. D W McColm and T A Cahill, Nucl Instrum Meth Phys Res, B51, 1990, 196-197. 52. ION A N D ELECTRON SCATTERING 52 7669. Thermal vibrations at copper surfaces studied by low energy ion scattering The thermal vibrations of surface atoms on Cu(110) have been studied as a function of temperature with impact collision ion scattering spectroscopy. Monte Carlo simulations of the scattering along chains of surface atoms show that for large vibration amplitudes more than two atoms have to be included to explain the experimental data. The mean square displacements obtained from the comparison between experiment and calculations agree well with the results of atom scattering and inverse photoemission experiments and the results from surface phonon calculations. The influence of correlations and anisotropy of the surface vibrations on impact collision ion scattering spectra is discussed. H Diirr et ai, Vacuum, 41, 1990, 376-378. 52 7670. Structure determination of the SrTiOa(100) surface The surface o f SrTiO3(100) was analysed by low energy diffraction. Intensity spectra were taken using a TV camera whose signal is evaluated under computer control. Model intensities were calculated fully dynamically for energies above 200 eV where details of the scattering potential are of minor influence. The best theory-experiment fit (Pendry R-factor = 0.53) shows the surface to be terminated by both Sr-O and O - T i - O layers. The top layers are buckled by oxygen ions displaced out of the surface by 0.08 and 0.16/~ in the Ti and Sr terminated domains, respectively.
The buckling, which creates a static dipole moment in the surface layer, is accompanied by a multilayer relaxation which for the first two layers is dtz(Ti)/d0 = + 2 % , d23(Ti)/d0 = - 2 % for the Ti termination and dlz(Sr)/d0 = - 10%, d23(Sr)/d0 = + 4 % for the Sr terminated domains. N Bickel et al, Vacuum, 41, 1990, 4 6 4 8 . 52 7671. Test of structural models for Cu(001)-(~/2 x 2~/2) R45°-O by LEED intensity analysis The Cu(001~(x/2 × 2~/2)R45°~O structure was investigated by LEED intensity analysis. Several structural models were considered in the calculations. The models proposed on the basis of HREELS results can be ruled out. It was confirmed that the missing row model proposed by Zeng et al gives the best agreement with the experimental data. The structural parameters for this model were refined considering a buckling of the first and second copper layers. A Atrei et al, Vacuum, 41, 1990, 333 336. 52 7672. Surface structure and oxidation of Si(001)-Bi The geometric structure, electronic state and reaction with 02 of the Si(001) surface with Bi overlayer have been studied by LEED, XPS and UPS. Bi(0001) was grown epitaxially with interface structure of Bi(0001~ Si(001) and Bi[2i]] Si[110] when Bi was deposited on Si(001)2 x 1 clean surface at room temperature. Subsequently annealing to 570-720 K caused the desorption of Bi, and successive phase transition of the 2 x n (10 > n > 5) surface structure was observed around 1 ML (i.e. atomic ratio of Bi to first layer Si is unity). The effect of an 02 exposure of I000 L at 570-720 K depended upon initial Bi coverage. At submonolayer Bi coverage, Oz exposure induced the oxidation of Si surface and the desorption of Bi. On the other hand, the 2 x n surfaces which corresponded to about 1 ML were stable against 02, indicating strong interaction between Bi and Si. At higher coverage, excess Bi atoms were oxidized. T Hanada and M Kawai, Vacuum, 41, 1990, 65(L651. 52 7673. Interesting aspects of reflection high-energy electron diffraction oscillations during growth of GaAs(100) The intensities of several reflection high-energy electron diffraction (RHEED) beams have been recorded during molecular-beam epitaxial growth of GaAs(100) using a novel video intensity measurement system that records multiple R H E E D beam intensities simultaneously. The R H E E D beam intensities were recorded at varying angles of incidence and crystal substrate azimuth angles. Strong oscillations in the intensities in specular and nonspecular beams with the same period but varying phases have been measured. As noted by other investigators, the phase relationship of the oscillations of the various beams has been found to vary with incident and azimuthal angles. The results are examined with regard to recent studies of the role of Kikuchi processes on the phase of the specular beam. In contrast to other reports, it is found that although the diffracted intensities in the vicinity of the elastically diffracted beams are influenced by inelastically scattered electrons from the Kikuchi lines, these effects cannot account for all the phase behavior observed. K D Jamison et al, J Vac Sci Technol, B8, 1990, 279 282. 52 7674. A review of the geometrical fundamentals of reflection high-energy electron diffraction with application to silicon surfaces Reflection high-energy electron diffraction (RHEED) is an experimentally simple technique, and yet a powerful one for examining the structure of a substrate surface and for monitoring the surface crystal structure and the crystallographic orientation of thin films during their growth. However, it can be difficult to learn to interpret the R H E E D patterns of new materials, because a practical and adequately detailed introduction to the technique is not generally available. To address this need, we develop the geometrical principles of R H E E D ; using the kinematic approximation, we show how a particular point of the sample surface's reciprocal net gives rise to a diffraction maximum at a particular location on the R H E E D viewing screen. We explain the origins of 'reciprocal lattice rods', R H E E D streaks, and Laue rings. We show how to calculate the streak spacing, and clarify the basic effect on the R H E E D pattern of using a nonzero angle of incidence for the incident beam. Crystalline nets, reciprocal nets, and their R H E E D patterns are derived for both (001) and (111) silicon surfaces and are compared to experimentally obtained patterns. J E Mahan et al, J Vac Sci Technol, A8, 1990, 3692 3700. 693