Deep trapping and recombination in hydrogenated amorphous silicon

Abstract

In this paper, an attempt is made to resolve the issue why the electron mobility-lifetime product (μτ)_ss determined from the steady-state photoconductivity, is about two orders of magnitude greater than the same product (μτ)_cc determined from time-of-flight (TOF) charge collection measurements in hydrogenated amorphous silicon (a-Si: H). It is shown that this question can be answered quite satisfactorily with our present knowledge of a-Si: H. We adopt a phenomenological approach and derive an explicit relationship between (μτ)_ss and (μτ)_cc. It is shown that the ratio (μτ)_ss/(μτ)_cc is equal to the ratio of the free-electron recombination lifetime t_R in the steady-state photoconductivity to the free-electron deep-trapping time z_f encountered in the TOF charge collection experiment. We demonstrate that t_r/t_f, and thus (μτ)_ss/(μτ)_cc, can be as large as 100, depending on the values taken for the parameters involved. The reason why t_R is much greater than t_f is believed to lie in the asymmetry of the conduction-band and valence-band tails, which causes most of the dangling bonds to be doubly occupied in the steady state. Thermal emission of carriers from the dangling bonds is shown to contribute very little to the difference in magnitude between (μτ)_ss and (μτ)_cc.