Spectral response of amorphous–nano-crystalline silicon thin films

A representative set of amorphous–nano-crystalline Si thin films was deposited by radio-frequency plasma enhanced chemical vapor deposition using silane highly diluted by hydrogen. By Raman spectroscopy it was found that the variation of silane to hydrogen ratio resulted in films with crystal fraction between 0 and 55 vol.% and individual crystal sizes between 2 and 20 nm with bi-modal, broad size distribution. High resolution transmission microscopy, done on certain number of samples, confirmed the nano-meter size of crystallites and bi-modal size distribution. The optical properties measured by Fourier transform photocurrent spectroscopy and photo thermal deflection spectroscopy correspond to the material with structure between amorphous and crystalline. The spectral distribution of relative quantum efficiency of photovoltaic solar cell made from this material shows ‘blue shift’ with increase of crystal to amorphous fraction. This result is discussed as a possible consequence of quantum effects accompanied with actual size and size distribution of crystals.