Photoconductivity of amorphous GaAs

Abstract

Abstract The spectral response of photoconductivity has been measured in samples of amorphous gallium arsenide deposited by rf sputtering of monocrystalline GaAs targets with and without hydrogen. The maximum of the normalized spectral photoconductivity (hν ≅ 2.0 eV) as a function of hydrogen pressure, pH2, increases when pH2 ≤ 0.1 Pa and then tends to a constant value. The values of the Tauc optical gap, Eg, and those of the photoconductive threshold, Et,ph are similar. Their difference for the same sample tends to decrease with increasing pH2. The same trend is shown by the difference between the Urbach energy computed either by photothermal deflection spectroscopy, E0, or by photoconductivity absorption coefficient data, E0,ph. These results and the dependence of other quantities, such as the activation energy of the conductivity, Eσ, on pH2, are interpreted in terms of the possible effects that hydrogenation can induce in a compound semiconductor. They are: a less disordered network reducing the band tail depth; decrease of the bond-angle fluctuation and ineffectiveness of hydrogen on the density of wrong bonds. The first two contributions are strictly linked to the deposition conditions of the films which determine, for example, the deposition rate of the film.