The optical absorption edge in layer structures

Abstract

The layer structures GaSe and GaS are composed of isomorphic, 4-fold atomic layers, and differ only in the manner in which these layers are stacked. The Se and S atoms occupy equivalent sites and it is possible to obtain crystals with a continuous range of composition between pure GaSe and pure GaS. The optical absorption of a series of mixed crystals of the system GaSe x S 1 − x has been measured at 12° K near the absorption edge. The line structure associated with the edge, generally held to be excitonic in nature, is present for all compositions but shows a rapid fall in intensity with increasing sulphur content. Absorption and reflectivity measurements on the pure crystals indicate that the edge arises from a direct transition in GaSe and from an indirect transition in GaS. The small thickness of the individual layers significantly affects the behaviour of excitons in such structures and the strong carrier-phonon interaction gives rise to polarization of the lattice by optically excited carriers. It is suggested that the observed lines in GaSe, GaS and the absence of hydrogen like behaviour may then result. Absorption and photoconductivity show a marked dependence on the orientation of the polarization of the incident illumination with respect to the c-axis of the crystals. A model is proposed to account for this dependence.