Mechanical and electrical properties of hydrogen or helium diluted a-Si:H prepared at low temperatures

Hydrogen or helium dilution of silane has been used to improve charge transport characteristics or deposition rate for thick amorphous silicon layers. In both cases, mechanical instability, such as peeling-off, due to high strain energy stored in the a-Si:H film is one of the major concerns. Growing the a-Si:H film at temperatures below 150°C brought the residual stress in the film to a level low enough to prevent delamination. The defect density increased at the same time, but it could be recovered by annealing at 160°C for 100 hours without affecting the stress state. In the hydrogen-diluted material, the deposition condition for optimum mobility and defect density values corresponded to the onset of the microcrystalline formation, and this relationship was confirmed by Transmission Electron Microscopy (TEM). Existence of a small amount of an ordered phase in the helium-diluted material, which was hardly observed by the TEM, was revealed by IR absorption spectra and supported by the change in ratio of photo-to-dark conductivity compared to that of undiluted material.