The Structure Of a-Si(1-X)Sn(X)H Thin Films

The doping of a-Si:H with Sn is known to modify the electrical and optical properties of the material. The optical band gap decreases as the doping level is increased, however, there is no insulator-metal transition of the type observed, for example, when transition metals are used as dopants. In order to increase the understanding of the conductivity processes that occur in a-Si:metal:H alloys we have measured the atomic scale structure of a series of a-Si(1-x)Sn(x):H thin-films using EXAFS. Samples were prepared by RF reactive co-sputtering and both Si and Sn K-edge EXAFS examined. The results indicate that the Sn atoms are substituted randomly into the a-Si tetrahedral random network. Both Si and Sn atoms retain fourfold co-ordination over the composition range studied (0-less-than-or-equal-to-x-less-than-or-equal-to-0.18). In contrast to results obtained using transition metal dopants there is no local modification of the tetrahedral random network.