Trapping in hydrogenated amorphous silicon studied by dual-beam modulated photoconductivity

Abstract

The steady-state photoconductivity of hydrogenated amorphous silicon (a-Si: H) has been studied by means of the dual beam modulated photoconductivity (DBMP) method. The frequency-dependent spectrum of DBMP reveals the existence of two independent densities of trapping centres in the gap, which are the doubly occupied dangling-bond distribution and the localized states of the conduction-band tail. Steady-state photoconductivity is due to thermal emission from these two distributions of trapped electrons. The relative contribution of each is strongly dependent on temperature. This interpretation is supported by the details of the temperature dependence studies. In particular, the variations in the γ exponent of photoconductivity (σ_p ∞ f _p ^γ, f _p being the photogeneration rate) as a function of temperature are found to be correlated to the variations in the partial photoconductivities.