Studies on charge collection and transport properties on semi-insulating materials in the presence of a non-uniform electric field

Abstract

II–VI semiconductors obtain a real interest in the scientific community for their wide applications in several fields: from optoelectronic and solar cell technology to applications as radiation detectors. The charge collection efficiency (C.C.E.) – i.e. the ratio between the photo-generated charge and the charge collected by the electrodes – is closely related both to the material transport properties and to the internal electric field. It could be exploited to study the transport parameters of these materials and the electric field profile inside the devices under irradiation. Under opportune conditions, C.C.E., as a function of applied bias, should follow Hecht or Many's equation, depending on the penetration length of incident radiation. A central requirement in both these models is a uniform internal electric field but in real devices this condition is seldom satisfied due to the presence of spatial charge, as demonstrated by Pockels measurements and bad photocurrent curve fitting. The authors, starting from the Ramo–Shockley theorem and assuming a linear shape of the electric field, present a new model to describe the C.C.E. as a function of applied bias, with the mobility life-time product μτ and the field slope as parameters. This model, with respect to the previous ones, gives information also about the internal electric field profile and, despite its simplicity, it seems to be a good approximation in several cases, as shown by the experimental analyses reported here.