The quasineutrality condition in amorphous semiconductors: Reformulation of the ‘lifetime/relaxation’ criterion

The concepts of lifetime and relaxation semiconductors introduced by van Roosbroeck and Casey are reconsidered for <i>amorphous</i> semiconductors and the effect of localized states on the lifetime/relaxation criterion specified: The quantity to be considered is τ_ρ/<i>T</i>_d, where <i>T</i>_d is (as before) the dielectric relaxation time, but τ_ρ is now the lifetime of the <i>total</i> charge <i>including</i> charge stored in localized states. τ_ρ is equal to the free carrier lifetime times a correction factor that is much larger than unity for amorphous semiconductors. Bandtail states and dangling bonds act differently on the criterion. The correction factor is frequency-dependent and decreases at higher frequencies. Two illustrative experimental examples (low substrate temperature a-Si:H, a-SiC:H alloys) are given, showing borderline cases.