Transient-photocurrent study of localized states at the conduction-band edge of a-Si: H

Abstract

Measurements of the transient secondary photocurrent in undoped and phosphorus-doped a-Si: H have been made in order to study the nature of localized states at the conduction band edge as well as the relaxation process of excess carriers. The decay rates of the transient photocurrent before recombination or deep-trapping processes begin show weak temperature dependence in both samples. Recombination or deep-trapping times differ by three orders of magnitude between undoped and phosphorus-doped samples, although their activation energies are identical (∼ 0·2 eV). All the results indicate that the transient photocurrent in the recombination-free or deep-trapping-free-time range cannot be interpreted in terms of the time evolution of excess carriers in the exponentially distributed gap states. It is likely that well-defined states are located at about 0·2 eV below the conductionband mobility edge in a-Si: H.