SIGN REVERSAL IN TRANSIENT PHOTOCONDUCTIVITY IN THE PRESENCE OF OPTICAL BIAS IN UNDOPED HOMOGENEOUS A-SI-H

An undershoot or sign reversal in the transient photoconductive respon se to pulse illumination in the presence of optical bias has been obse rved in homogeneous films of undoped amorphous silicon. This report is the first of experimental observation, to our knowledge. The undersho ot is seen in the regime of linear response to the photoexcitation pul se, when the background steady state generation rate, G(ss) > 10(18) c m(-3) s(-1). The time, t(us), at which sign reversal occurs varies inv ersely and sublinearly with generation rate. When the quasi-Fermi leve l is maintained constant at ( E-Fn - E-F) = 0.24 eV, t(us) is thermall y activated with energy 0.36 eV. Direct application of theory to the F ourier transform of the time resolved transient photoconductivity data yields a figure of 1.5 x 10(-8) cm(3) s(-1) for the recombination coe fficient. Numerical simulation not including dangling bonds can only b e induced to exhibit an undershoot by adopting capture coefficients su ch that recombination occurs by the path of electron capture by trappe d holes. (C) 1998 Elsevier Science B.V. All rights reserved.