Sweep-out measurements of band-tail carriers in a-Si: H

Abstract

The sweep-out experiment, used to measure the density of band-tail carriers in hydrogenated amorphous silicon (a-Si:H) is described. The experimental configuration, sample structure and analysis are discussed in some detail. The band-tail carrier density n_BT is obtained with good accuracy from the charge collection, but their energy distribution is not easily deduced from the time dependence of the current. The sweep-out technique is applied to doped material and the results are related to the transport, the density-of-states distribution and the thermal equilibration of the localized states. In the equilibrium regime, n_BT is activated with a doping-dependent energy, which is explained by the thermodynamics of the defect and dopant states. The measurements of n_BT are combined with conductivity data and the density of states, to give the drift mobility of doped material. Sweep-out measurements also prove that the doping efficiency of a-Si: H depends on the position of the Fermi energy, as predicted by the thermodynamic models. Lastly, it is shown that the technique can be used to obtain the photoconductivity and response time of undoped a-Si: H.