ENVIRONMENTS OF ION-IMPLANTED AS AND GA IMPURITIES IN AMORPHOUS-SILICON

Glancing-angle x-ray-absorption fine-structure experiments are reporte d for arsenic and gallium implants in amorphous silicon. Total impurit y concentrations range from 5 x 10(19) to 10(21) atoms cm-3. The effec ts of hydrogen and chemical compensation have been explored. The hydro gen concentration in the films has been derived from the critical angl e for total external reflection. Evidence is found for extra hydrogen bound to impurities. The presence of phosphorus causes gallium to adop t fourfold-coordinated sites. From the shape of the fine-structure env elope, more than one impurity site has been established. In particular , both arsenic and gallium exhibit a minority well-ordered site that w e propose provides the source of electronically active centers. This a ssertion is supported by structural differences resulting from chemica l compensation and also thermal annealing of ion-implanted films. As t he impurity dilution increases, an approximate correspondence has been found between the densities of these well-ordered sites and those rep orted for dangling-bond silicon states in hydrogenated amorphous silic on.