Thermodynamical equilibrium gap-state distribution in undoped a-Si: H

Abstract

Based on weak-bond to dangling-bond conversion and including amorphous broadening of available defect energy levels, we have derived a quantitative thermodynamical equilibrium model for the gap-state distribution in a-Si:H. Results from the conventional defect pool model are compared with a generalized approach requiring both structural and electronic equilibrium of the formed defects. Independent of the microscopic defect reactions, both models predict for undoped a-Si: H splitting of the available defect band into three components: a band of D+ states above and bands of D − and D° states below the Fermi level, with a minimum of the gap-state density at the Fermi level. For typical values of the effective correlation energy (∼0·2eV) and r.m.s. defect bandwidth (0·1-0·15 eV) the models predict a 2-10 times higher density of charged compared with neutral defects in thermal equilibrium which strongly suggests a reinterpretation of experimental data with respect to the gap-state distribution in undoped a-Si: H.