Medium and high-energy x-ray diffraction has been used to study the atomic structure of pure amorphous Si prepared by MeV Si implantation into crystalline silicon. Both as-implanted and annealed samples were studied. The inelastically scattered x rays were removed by fitting the energy spectrum for the scattered x rays. The atomic scattering factor of silicon, previously known reliably up to 20 Å Ϫ1 , has been extended to 55 Å Ϫ1. The radial distribution function of amorphous Si, before and after annealing, has been determined through an unbiased Fourier transformation of the normalized scattering data. Gaussian fits to the first neighbor peak in these functions shows that scattering data out to at least 40 Å Ϫ1 is required to reliably determine the radial distribution function. The first-shell coordination number increases from 3.79 to 3.88 upon thermal annealing at 600°C, whereas that of crystalline Si determined from similar measurements on a Si powder analyzed using the same technique is 4.0. Amorphous Si is therefore under coordinated relative to crystalline Si. Noise in the distribution function, caused by statistical variations in the scattering data at high-momentum transfer, has been reduced without affecting the experimental resolution through filtering of the interference function after subtracting the contribution of the first-neighbor peak. The difference induced by thermal annealing in the remainder of the radial distribution functions, thus revealed, is much smaller than previously believed. ͓S0163-1829͑99͒00943-1͔ I. INTRODUCTION
The structure factor S͑Q͒ of high purity amorphous Si membranes prepared by ion implantation was measured over an extended Q range (0.03 55 Å 21). Calculation of the first neighbor shell coordination (C 1) as a function of maximum Q indicates that measurement of S͑Q͒ out to at least 40 Å 21 is required to reliably determine the radial distribution function (RDF). A 2% change in C 1 and subtle changes in the rest of the RDF were observed upon annealing, consistent with point defect removal. After annealing at 600 ± C, C 1 3.88, which would explain why amorphous Si is less dense than crystalline Si.
New features of the nanoscale structure of amorphous (a)-Si produced by ion-implantation-induced amorphization of crystalline (c)-Si have been determined by the technique of small-angle x-ray scattering (SAXS). Si ion energies up to 17 MeV were used to generate a thick amorphous layer (8 μm) on a c-Si wafer to enable the SAXS measurements. As-implanted and thermally annealed (up to 540 °C) a-Si were studied. No nanovoids were detected within a sensitivity of 0.1 vol %, but the atomic-scale structure produced a measurable diffuse scattering signal that decreased with increasing anneal temperatures. These measurements show that the known density deficit of 1.8% in a-Si relative to c-Si cannot be due to voids and that a-Si is homogeneous on nm length scale.
The compositional dependence of the fundamental bandgap of pseudomorphic GaAs 1Àx Bi x layers on GaAs substrates is studied at room temperature by optical transmission and photoluminescence spectroscopies. All GaAs 1Àx Bi x films (0 x 17.8%) show direct optical bandgaps, which decrease with increasing Bi content, closely following density functional theory predictions. The smallest measured bandgap is 0.52 eV ($2.4 lm) at 17.8% Bi. Extrapolating a fit to the data, the GaAs 1Àx Bi x bandgap is predicted to reach 0 eV at 35% Bi. Below the GaAs 1Àx Bi x bandgap, exponential absorption band tails are observed with Urbach energies 3-6 times larger than that of bulk GaAs. The Urbach parameter increases with Bi content up to 5.5% Bi, and remains constant at higher concentrations. The lattice constant and Bi content of GaAs 1Àx Bi x layers (0 < x 19.4%) are studied using high resolution x-ray diffraction and Rutherford backscattering spectroscopy. The relaxed lattice constant of hypothetical zincblende GaBi is estimated to be 6.33 6 0.05 Å , from extrapolation of the Rutherford backscattering spectrometry and x-ray diffraction data.
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