X-RAY INDUCED EFFECT ON CHARGE CARRIER TRAPPING LIFETIME OF a-Se PHOTOCONDUCTORS AND THE RECOVERY PROCESS
Amorphous selenium (a-Se) alloy x-ray detectors are currently used in commercial mammographic detectors for breast cancer detection and diagnosis. They have been only recently commercialized and there are now at least five companies manufacturing these detectors. This work focuses on the study of the X-ray induce effects on the carrier trapping lifetime in a-Se, and the recovery process of the X-ray induced damage in the bulk of a-Se samples. The x-ray dose effect on the carrier trapping lifetime was studied alongside the temperature effect on the induced x-ray damage and recovery process. The carrier trapping lifetime reduces as the accumulated dose deposited in the a-Se samples increases. Upon the cessation of x-ray exposure, carrier lifetime recovered slowly (over many hours) back to its original state. The damage was not permanent. Several a-Se detectors samples have been exposed to high doses of x-ray and the recovery process has been observed under different temperature, 23.5 oC and 35.5 oC. The time of flight (TOF) measurement technique was employed to measure the carrier drift mobility and the interrupted filed time of flight (IFTOF) technique was used to measure the carrier trapping lifetime . All samples used in this project are pure a-Se for hole transport measurements, a-Se: 0.3%: 2.5ppm Cl and a-Se: 0.5%: 10ppm Cl for electron transport measurements. Sample thickness ranges from 50 μm to 200 μm with a variance of ±5 μm at different positions on the sample. The applied dose rate during the x-ray irradiation ranges from 1.9 Gy/s to 2.5 Gy/s. The difference in dose rate does not affect the change in the hole trapping lifetime but has a non-significant effect on the electron trapping lifetime. The rate of decrease in the hole normalized lifetime is more rapid at 35.5oC than at room temperature (23.5oC). The recovery processes were also observed to be more rapid at the higher temperature.
X-ray, Detector, photoconductor, lifetime, charge carriers
Master of Science (M.Sc.)
Electrical and Computer Engineering