PHOTORESPONSE TIME AND DENSITY OF LOCALIZED STATES IN THE MOBILITY GAP OF a-Si:H ALLOYS PREPARED BY R.F. GLOW DISCHARGE

Abstract
Taking advantage of the well known Stevenson-Keys and Haynes-Shockley techniques we describe a way to achieve the photoresponse time and the density of localized states in the mobility gap of a-Si : H alloys produced by bias r.f. glow discharge. The effect of incident light upon samples under the action of d.c. bias plus an a.c. controlled pulse has been investigated. In the dark, the gate voltage is related with the surface charges through Poisson's equation. Under these conditions the differential capacitance (Sze) is given by : C_D = (εε0e²g_α)^1/2 · F(φ_s) Where : εε0 is the dielectric constant ; e is the electric charge ; g_α the density of localized states ; F(φ_s) is a function of the surface potential φ_s. Which is determined after conductance and decay time measurements. The main effect of incident light is the increasing of the differential capacitance due to the decrease in the response time of minority carriers and/or the decrease of surface potencial φ_s due to space charge layer reduction. This is explained by the shift of the quasi Fermi levels upwards (n-type material). The values of the density of localized states, minority carriers life time and the signal of charges have been calculated.