Environments of ion-implanted As and Ga impurities in amorphous silicon.

Glancing-angle x-ray-absorption fine-structure experiments are reported for arsenic and gallium implants in amorphous silicon. Total impurity concentrations range from 5\ifmmode\times\else\texttimes\fi{}${10}^{19}$ to ${10}^{21}$ atoms ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. The effects of hydrogen and chemical compensation have been explored. The hydrogen concentration in the films has been derived from the critical angle for total external reflection. Evidence is found for extra hydrogen bound to impurities. The presence of phosphorus causes gallium to adopt fourfold-coordinated sites. From the shape of the fine-structure envelope, more than one impurity site has been established. In particular, both arsenic and gallium exhibit a minority well-ordered site that we propose provides the source of electronically active centers. This assertion is supported by structural differences resulting from chemical compensation and also thermal annealing of ion-implanted films. As the impurity dilution increases, an approximate correspondence has been found between the densities of these well-ordered sites and those reported for dangling-bond silicon states in hydrogenated amorphous silicon.