Differences between transient photoconductivity in a-Se sandwich(bulk) and co-planar(interface) structures

Abstract

During the development of a novel photoconductive digital medical X-ray imaging device, we performed transient photoconductive measurements of both electrons and holes in a-Se structures that were either of a sandwich or co-planar structure. In sandwich type measurements, carriers move through the bulk away from interfaces. In co-planar structures, carriers move near an interface. The photoconductive properties of a-Se have been extensively characterised, using photoconductive time-of-flight (TOF) methods. These measurements have been performed using sandwich structures consisting of a-Se deposited on a bottom conducting substrate and a top thin semi-transparent biasing electrode. A weak, brief (compared to transit times) light pulse is applied to the semi-transparent electrode and the resulting photoconductive transient current pulse is used to determine carrier properties. In a-Se medical X-ray applications, the a-Se layer is quite thick (150–500 $\upmu$m) causing long carrier transit times. We were interested in reducing these long photoconductive transit (readout) times by instead moving the collected image charges comparatively short distances (10–20 $\upmu$m) laterally between co-planar image pixel electrodes and neighbouring readout electrodes. In the exploration of this concept, we studied experimentally the transient photoconductivity of co-planar a-Se structures. We found, unexpectedly, the transient photoconductivity measurements of the co-planar structures to be quite different from those of the sandwich (bulk) type. We concluded that the co-planar a-Se photoconductivity was totally dominated by a high density of interface trapping states. It is recommended that the measurement of bulk carrier properties via TOF using co-planar structures carefully take into account interface states.