High‐temperature annealing behavior of μτ products of electrons and holes in a‐Si:H

The mobility‐lifetime products of electrons and holes [(μτ)_e and (μτ)_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 3×10¹⁵–2×10¹⁸ cm⁻³ by annealing at high temperatures. N_d and the Urbach tail slope E_ov 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 σ_pc have been measured. The results show that there is a correlation between N_d and E_ov which is consistent with equilibrium theory. (μτ)_e and (μτ)_h change in quite different ways as N_d increases, namely, (μτ)_e decreases as a linear function of the inverse of N_d. However, (μτ)_h remains almost constant when N_d≤5×10¹⁶ 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.