Optical properties of amorphous arsenic

Abstract

The optical constants of sputtered and bulk amorphous As have been obtained from 0-03 to 30 eV by Kramers-Krönig analysis of the reflectivity spectra. The optical gap for oxygen free films occurs at 1-1 eV. The slope of the exponential absorption edge (12 eV⁻¹) is contrasted with slopes reported for other elemental amorphous semiconductors. The steepness is observed to decrease monotonically as the coordination increases. Similarities between the reflection spectra of crystalline and amorphous forms of As indicate similarities in coordination and bonding. The Penn model has been used to correlate detailed differences in the optical properties with differences in bonding anticipated from the structures. The average energy gap is estimated to be greater for amorphous As (3-5 eV) than for rhombohodral As(2-9 eV). This difference is attributed to the loss of the crystalline interlayer interaction which strengthens the covalent bond in the amorphous form. Examination of the sum rules and the energy loss spectra of crystalline and amorphous As suggest that at the plasma frequency all five valence electrons are taking part in the optical absorption. Application of the plasma sum rule to rhombohedral and amorphous As yields an estimate of 0-2 for the ratio of crystalline inter/intralayer electron charge density. Finally, the optical density of states of sputtered a-As is employed to deduce a profile for the valence band density of states which is in good agreement with the X-ray photoemission spectrum. The two spectra are then used to elicit an overall shape for the conduction band density of states consistent with the form of the matrix elements appropriate to the Penn model. In contrast to amorphous Ge, the conduction band is not step-like but appears to increase rather gradually from the band edge. The possibility of some structure within the conduction band is considered in relation to the profile of the 3d absorption spectrum.