Investigation of Ghosting Recovery Mechanisms in Selenium X-ray Detector Structures for Mammography

The ghosting recovery 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
$(\sim 1\ {\rm V}/\mu{\rm m})$
. 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, detrapping, structural relaxation and electric field dependent electron-hole pair creation), and effects of charge injection from the metal contacts. Carrier trapping in both charged and neutral defect states has been considered in the model. It has been found that the X-ray induced deep trap centers are charged defects. A faster sensitivity recovery is found by reversing the bias during the natural recovery process. During the reverse bias, a huge number of carriers are injected from the metal contacts, and fill the existing trap centers. This results in an abrupt recovery of the relative sensitivity. However, the relative sensitivity slightly decreases with time after this abrupt recovery due to the release of the trapped electrons as well as the long recovery time of the induced trap centers. The theoretical model shows a very good agreement with the experimental results.