Local structures of Ga atoms in amorphous silicon and hydrogenated amorphous silicon before and after synchrotron x-ray irradiation

The influence of intense x-ray irradiation on the local bonding structure of Ga dopants in both hydrogen-free (a-Si) and hydrogenated (a-Si:H) amorphous Si thin films has been studied. Prior to x-ray exposure, extended x-ray absorption fine structure measurements revealed that H reduced the static disorder around the Ga atoms in amorphous Si. Thereafter, x-ray irradiation modified the local structure in the a-Si and a-Si:H samples. The Ga coordination number increased from <3.5 to ~3.80 atoms for both types of amorphous material as consistent with a greater fraction of electrically active, tetrahedrally coordinated dopant atoms. Also, greater structural disorder was observed around Ga atoms after x-ray irradiation suggesting that tetrahedrally coordinated Ga atoms were less well ordered than in threefold-coordinated sites.