The Optical Properties of Amorphous ZnTe and CdTe

Abstract

Amorphous films of ZnTe and CdTe have been prepared by vacuum deposition of the compounds onto −150 °C sapphire substrates. Structural studies reported earlier by Brown and Brodie indicate that the crystallite size in the amorphous phase is less than 25 Å, and that the basic crystalline tetrahedral units of the zinc-blende structure are retained in the amorphous phase.Both the reflectivity and absorption spectra display only one broad maximum associated with transitions involving energy states near the center of the Brillouin zone and in the [Formula: see text] directions, the bond directions between neighboring atoms in the tetrahedra. These bonds are not disturbed by the lack of long range order. The strong E2 transition observed with polycrystalline samples is absent in the amorphous films. This transition is associated with energy states near the X point in the zone and the nature of these states is changed. The X point is in the [Formula: see text] direction at the zone boundary and the bonds associated with this direction connect neighboring tetrahedra. The fundamental band edge is less affected by disorder. It is as steep as the polycrystalline band edge but shifted to lower energy. The edge temperature dependence is the same as that of the crystal but transitions just above the band edge are indirect in contrast to the crystalline case where they are k conserving.