ELECTRON LIFETIME AND ITS DEPENDENCE ON TEMPERATURE AND DOSE IN a-Se PHOTOCONDUCTORS

Electron transport in vacuum deposited a-Se films has been investigated by Interrupted-Field Time-of-Flight (IFTOF) transient photoconductivity experiments to examine the effect of sample temperature (T) and applied electric field (F) on X-ray induced changes in the electron lifetime. Upon exposure to x-rays, the electron lifetime decreases. The decrease in normalized lifetime is almost linearly proportional to the absorbed dose, and is more significant at higher temperatures. Upon the cessation of x-ray irradiation, the lifetime recovers towards its equilibrium value through a structural relaxation process, and is characterized by a structural relaxation time. The structural relaxation time decreases with temperature in an Arrhenius fashion, and exhibits an activation energy that is roughly 1.4 eV. The structural relaxation time at room temperature (21 C) is 2 – 4 hrs whereas at 35 C, 6 – 10 mins. These measurements are important in characterizing the charge collection efficiency of a-Se based x-ray detectors, and its dependence on x-ray exposure and temperature. The results indicate that the rate of change of electron lifetime per unit exposure is less than 2%/Gy