Optical and transport properties of a-Si:H,F/a-Si,Ge:H,F superlattices

Abstract

We present dark conductivities and photoconductivities of compositional superlattices (SL) with a-Si:H,F barrier and a-Si,Ge:H,F well layers measured parallel and perpendicular to the plane of the layers. The optical absorption spectra, photogenerated hole collection and electron drift data in these structures are also reported. The optical gaps E opt of the superlattices with high germanium concentration in the well are determined by the well layer's band gap. Quantum confinement is observed for narrow wells. The electronic transport properties of the superlattices perpendicular to the SL planes result from an interplay of two phenomena that are absent from bulk materials: the presence of barriers which the carriers can cross by either tunneling through or being thermally emitted over, and the quantum and/or structural effects of sandwiching a very thin layer (20 – 50 Å) of alloy between two a-Si barrier layers. The hole (μτ) p of the superlattices, measured by photocurrent collection, is approximately independent of E opt. The electron time-of-flight (μτ) n of the superlattices decreases with E opt and is determined primarily by thermal emission over the a-Si:H,F barriers.