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

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 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 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.