Influence on the transport properties of the deposition temperature of a-Si:H films deposited from mixtures of silane in helium at high deposition rates

We study the electronic properties of hydrogenated amorphous silicon (a-Si:H) deposited at high rates (up to 11 Å/s) in a silane-helium mixture containing 60% of helium and compare them with those of "standard" a-Si:H deposited at 250°C in pure silane. For the films obtained under helium dilution in silane, increasing the deposition temperature from 250°C to 350°C leads to improved electronic properties (an increase of the trap-limited mobility along with a decrease in the density of states (DOS) of the conduction band tail), while the characteristic features deduced from photothermal deflection spectroscopy absorption spectra (Urbach tail, deep defect density), which are mainly related to the DOS below midgap, remain unchanged. The changes in the electronic properties are correlated to changes in the hydrogen-related structure of the material.