ON THERMAL QUENCHING OF THE PHOTOCONDUCTIVITY IN HYDROGENATED AMORPHOUS-SILICON

Thermal quenching (TQ) of the photoconductivity sigma(p) is the decrea se in sigma(p) with increasing temperature. We present an explanation for TQ of sigma(p) usually observed above 100 K in undoped and weakly doped hydrogenated amorphous silicon (a-Si:H). With computer simulatio ns employing the theory of Simmons and Taylor, we show that TQ is caus ed by the natural density of gap states of a-Si:H. The onset of therma l quenching occurs at the temperature T-TQ where the trapped hole dens ity in the valence band tail has decreased to twice the density N-D of dangling bonds. We elucidate the experimental observation that T-TQ s hifts to lower temperatures as the Fermi level shifts toward the valen ce band or as N-D is increased and explain the reported superlinear de pendence of the inverse photoconductivity sigma(p)(-1) on N-D. We test and discuss the validity of the Simmons-Taylor theory by comparing th e simulated and experimental temperature dependences of the Rose expon ent gamma, which relates the photoconductivity and the generation rate .