The far infrared absorbing properties of pure amorphous silicon and various amorphous silicon-aluminium and amorphous silicon-arsenic alloys

Abstract

Room-temperature far infrared absorption spectra (20cm-1 – 400cm-1) have been obtained for a number of pure amorphous-silicon samples and a number of variously constituted amorphous-silicon-aluminium and amorphous-silicon-arsenic alloys. To minimise thin film interference effects the measurements were performed on weakly absorbing thin films using a sensitive multi-pass Fourier transform spectroscopic technique. Based on the assumption that the vibrational density of states of amorphous-silicon is not significantly altered by the introduction of small concentrations of impurity, the alloy absorption data has been interpreted in terms of an impurity induced modification of the amorphous-silicon photon-vibration matrix-elements. Two models were proposed to account for the far infrared activity of the vibrational excitations in amorphous-silicon. The higher wavenumber photon-vibration coupling model depended upon the degree of intrinsic disorder and involved the formation of a disorder induced dipole moment by the transfer of charge from extended to compressed adjacent covalent bonds, this model is clearly not appropriate when describing the photon-vibration coupling of the lower wavenumber 'bond bending' vibrational excitations. The far infrared activity of the low wavenumber (below approximately 120cm -1) vibrational modes was seen to be due to the in-phase displacement of the charge distributions that are associated with the structural defects in amorphous-silicon, a photon-vibration coupling model that is supported by the apparent frequency independence of the low wavenumber (below approximately 120cm -1) photon-vibration matrix-element and its observed sensitivity to extrinsic disorder.