Influence of atomic hydrogen annealing on the optical properties of vacuum-deposited a-Si:H films

The influence of atomic hydrogen annealing on the optical parameters of a-Si:H films was studied using spectrophotometric measurements of the film transmittance and reflectance in the wavelength range 200–3000 nm. In this annealing, the deposition of a thin layer and treatment with atomic hydrogen were repeated alternately, where the thickness of the thin cyclic layer, dcyc, and the treatment time of each cycle, tca, were kept fixed for each sample. A series of different samples with average thickness of 0.5 μm and different dcyc and tca were prepared. It was found that the refractive index, n, and the optical energy gap, Eg, increase as the treatment time, tca, increases from 0 to 60 s, while at tca=120 s both n and Eg decrease. Also, both the refractive index and the optical energy gap decrease with increasing the relative diffusion length of hydrogen, √tca/dcyc from 0.39 to 0.77. The widening of Eg is due to the structural relaxation resulting from impingement of atomic hydrogen on the growing surface. Thus, a good-quality a-Si:H with Urbach parameter 65 mev and optical energy gap of 1.78 eV was successfully prepared.