Radial distribution studies of amorphous GexTe1−x alloys

Radial distribution studies of amorphous Ge χ Te 1−χ alloys have been calculated from X-ray diffraction data for x = 0.11 and 0.54. These show peaks at approximately 2.7 A and 4.2 A. The absence of a peak at the crystalline GeTe first neighbor separation of 3 A is shown to imply that the local coordinations in the amorphous materials are different from those in crystalline GeTe. The areas under the first neighbor peaks indicated that it is very unlikely that the coordination numbers of the Ge and Te atoms are the same. These areas are consistent with models in which the average Ge and Te coordinations are four and two, respectively. This implies that the similarity in the optical absorption edges of crystalline and amorphous GeTe cannot be explained by similarities in short range structures. The similarities in bonding over a wide range of compositions in these amorphous alloys appear to lend support to Mott's picture for the failure of many impurities in many amorphous semiconductors to add significantly to the conductivity. The form of bonding postulated appears to be inconsistent with a microcrystalline picture for these alloys.