Sign reversal in transient photoconductivity in the presence of optical bias in undoped homogeneous a-Si:H

Abstract

An undershoot or sign reversal in the transient photoconductive response to pulse illumination in the presence of optical bias has been observed in homogeneous films of undoped amorphous silicon. This report is the first of experimental observation, to our knowledge. The undershoot is seen in the regime of linear response to the photoexcitation pulse, when the background steady state generation rate, Gss>1018 cm−3 s−1. The time, tus, at which sign reversal occurs varies inversely and sublinearly with generation rate. When the quasi-Fermi level is maintained constant at (EFn−EF)=0.24 eV, tus is thermally activated with energy 0.36 eV. Direct application of theory to the Fourier transform of the time resolved transient photoconductivity data yields a figure of 1.5×10−8 cm3 s−1 for the recombination coefficient. Numerical simulation not including dangling bonds can only be induced to exhibit an undershoot by adopting capture coefficients such that recombination occurs by the path of electron capture by trapped holes.