X-ray sensitivity and x-ray induced charge transport changes in stabilized amorphous selenium films

• Main Author: Nesdoly, Mark Timothy Alexander
• Other Authors: Kasap, Safa O.
• Format: Others
• Language: en
• Published: University of Saskatchewan 2000
• Subjects: selenium films; electrical engineering; charge coupled devices; photoconductivity; physics; X-rays
• Online Access: http://library.usask.ca/theses/available/etd-10212004-002307

This study investigated the mobility and trapping of charges and the recombination of x-ray induced charges in a-Se. X-ray induced changes in these parameters were also examined. Prior to exposure to x-rays, the mobilities and deep trapping lifetimes of both holes and electrons were constant. Exposure to x-rays caused no change in the mobility of these charges. Immediately following exposure, the hole deep trapping lifetime would fall ~30% while the electron deep trapping lifetime would only change ~10%. The deep trapping lifetimes continued to change unpredictably several hours after the initial exposure. Following an extended rest period, the charge lifetimes within the a-Se film would return to a stable state, but not necessarily equal to the initial lifetime prior to the x-ray exposure. These changes were proposed to occur because of a relaxation or reordering of the atoms in a-Se, similar to accepted changes thought to occur resulting from exposure to visible light. Analysis of the experimental evidence suggests that intimate valence alternation pair (IVAP) charged defects are created by x-ray irradiation. These defects are relatively unstable, disappearing within two hours after irradiation. Since the hole and electron lifetimes continued to change for at least 12 hours, it was concluded that the traditional view of deep charge trapping into IVAP defects cannot be dominant. A new charge trapping theory, consistent with published optically induced effects, is proposed in this work to explain these observations. The energy required to create a free electron-hole pair in a-Se by exposure to x-rays was measured. This energy was found to exhibit a strong field and temperature dependence, with little dependence on the mean x-ray beam energy. These findings are consistent with the geminate recombination theory, generally agreed to be the dominant charge loss mechanism with optical photons in a-Se. The persistent x-ray photocurrent was found to be thermally activated below ~250 K with an activation energy of 0.16 eV. This does not correspond to the energy level of any known traps in a-Se, and lends further support to the charge trapping theory developed earlier.