High-energy x-ray diffraction study of pure amorphous silicon

Abstract

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 ${\mathrm{\AA{}}}^{\mathrm{\ensuremath{-}}1}$, has been extended to 55 ${\mathrm{\AA{}}}^{\mathrm{\ensuremath{-}}1}$. 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 ${\mathrm{\AA{}}}^{\mathrm{\ensuremath{-}}1}$ 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 \ifmmode^\circ\else\textdegree\fi{}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.