HIGH-TEMPERATURE ANNEALING BEHAVIOR OF MU-TAU PRODUCTS OF ELECTRONS AND HOLES IN A-SI-H

The mobility-lifetime products of electrons and holes [(mu-tau)e and (mu-tau)h] in undoped hydrogenated amorphous silicon samples have been studied by photoconductivity and ambipolar diffusion length measurements. The density of dangling bonds N(d) in the samples is changed over a range of 3 x 10(15)-2 x 10(18) cm-3 by annealing at high temperatures. N(d) and the Urbach tail slope E(0-upsilon) have been determined by the constant photocurrent method. In addition, the optical gap, the activation energy of dark conductivity, and the exponent governing the intensity dependence of sigma(pc) have been measured. The results show that there is a correlation between N(d) and E(0-upsilon) which is consistent with equilibrium theory. (mu-tau)e and (mu-tau)h change in quite different ways as N(d) increases, namely, (mu-tau)e decreases as a linear function of the inverse of N(d). However, (mu-tau)h remains almost constant when N(d) less-than-or-equal-to 5 x 10(16) cm-3, then decreases fast for higher N(d). The asymmetric dependence of transport properties of electrons and holes on N(d) suggests that for electrons recombination through dangling bond states is dominant; but, for holes, recombination mainly proceeds through deep band tail states, especially when N(d) is relatively low.