Persistent photoconductivity studies in a-Si:H/nc-Si:H thin film superlattices

The electronic properties of undoped a-Si:H/nc-Si:H superlattice structures have been investigated by photoconductivity measurements. Multilayer structures having alternate layers of a-Si:H and nc-Si:H were deposited on corning 1737 glass substrate by Hot wire chemical vapor deposition technique, keeping the total thickness of films constant at 700 nm. Dark and photo conductivity along with persistent photoconductivity (PPC) are measured in coplanar geometry using Ag paste as electrodes. Quite interestingly room temperature PPC has been observed in these undoped a-Si:H/nc-Si:H superlattice structures. PPC decay time constant, its dependence on exposure time, electric field, number of periods and annealing temperature have been studied in detail. The origin of PPC is understood in terms of competition between carriers transport in the lateral direction due to external field and along the depth due to band bending at a-Si:H/nc-Si:H interfaces. Carriers trapped in the interfaces states or the separation of carriers due to band bending are likely to be responsible for observed PPC.