Investigation of ghosting recovery techniques and mechanisms in multilayer selenium X-ray detector structures under low bias

Abstract

The ghosting recovery techniques and mechanisms in multilayer selenium X-ray detector structures for mammography are experimentally and theoretically investigated. The experiments have been carried out under low positive applied electric field (~1-2V/μm) since a very little ghost can be seen under normal operating applied electric field (10V/μm). A ghost removal technique is investigated by reversing the bias polarity during the natural recovery process. The theoretical model considers accumulated trapped charges and their effects (trap filling, recombination, electric field profile, and electric field dependent electron-hole pair creation), the carrier transport in the blocking layers, and the effects of charge injection from the metal contacts. We consider carrier trapping in both charged and neutral defect states. It has been assumed that the X-ray induced deep trap centers are neutral defects. The time dependent carrier detrapping and structural relaxation (recovery of meta-stable trap centers) are also considered. The sensitivity in a rested sample is recovered mainly by the carrier detrapping, the recombination of the injected carriers with the existing trapped carriers, and the relaxation of the X-ray induced deep trap centers. A faster sensitivity recovery is found by reversing the bias during the natural recovery process. During reverse bias huge number of holes are injected from the metal and recombine with the trapped electrons. This results in faster sensitivity recovery. The electric fields at the metal contacts increase with time at the beginning of the natural ghosting recovery process which leads to the initial increase of the dark current. Later the electric fields at the metal contacts decrease and hence the dark current decays over time during the natural recovery process. The theoretical model shows a very good agreement with the experimental results.