The Structure of Amorphous Semiconductor: Metal Thin-Films

A transition from semiconducting to extend state conduction may be induced in certain amorphous semiconductor:metal alloys by increasing the metal concentration above a critical limit. Descriptions of the processes involved in such a transition have generally been based around investigations on electronic properties. However, without a knowledge of the atomic-scale structure of the alloys, it is difficult to ascribe a mechanism to an observed transition. In order to increase the understanding of such processes in semiconductor:metal systems, thin-film samples of three alloy systems (a-Si:Ni:H, a-Si:Sn:H and a-Ge:Au) have been prepared by rf co-sputtering over pertinent composition ranges, and micro-structural studies have been performed using extended X-ray absorption fine structure spectroscopy and other complementary techniques. For low metal concentrations (<20at.%), both a-Si:Ni:H and a-Ge:Au appear to consist of two separate phases: regions of an amorphous Ni:Si or a partially crystalline Au:Ge alloy being embedded in the remaining, modified amorphous matrix provided by a-Si:H and a-Ge respectively. In contrast, Sn atoms appear to substitute randomly into the a-Si:H network. The implications of these results for the interpretation of electrical conductivity data is discussed.