Comparison of experiment and theory of the photoconductivity of a-Si:H up to a generation rate of 10²⁸cm^-3s^-1

We have studied the dependence of the photoconductivity σ_p on photocarrier generation rate G in intrinsic a-Si:H at 300K between G=10¹²cm^-3s^-1 and 10²⁸cm^-3s^-1. Below a certain value G_o, we find σ_o=AG^γ with γ=0.9±0.05 and the values of A vary considerably with defect concentration N_d which signifies monomolecular recombination through defects. Above G_o the recombination is bimolecular, γ=0.5±0.02 and A=(6±3) ×10^-15Ω^-1cm ^1/2 s ^1/2 is independent of N_d. The transition value G_o is about 3×10²⁰cm^-3s^-1 for high quality annealed a-Si:H and increases with N_d. A simulation of σ_p(G) assuming conduction in and recombination from extended states fits our experiments within a capture coefficient C_t=(6±2)×10^-9cm³s^-1 of carriers to their opposite tail states. Our C_t is close to the value (5±2)×10^-9cm³s^-1 obtained from optical measurements but higher than (0.5±0.1)×10^-9cm³s^-1 determined from photoelectric studies. Below T=150K our model calculations overestimate σ_p because the tunneling transitions, becoming important for recombination and conduction, are neglected.