Electrical Properties of Amorphous Selenium Based Photoconductive Devices for Application in X-ray Image Detectors
This PhD work has been devoted to researching the possibilities for dark current minimization in a-Se x-ray photoconductors devices through a systematic study of the charge transport (carrier mobility and carrier lifetimes) and dark currents in single and multilayered a-Se devices as a function of alloying, doping, deposition condition and other fabrication factors. The results of the studies are extensively discussed in the thesis. We have proposed a new technological method for dark current reduction in single and multilayered a-Se based photoconductor for x-ray detector applications. The new technology is based on original experimental findings which demonstrate that both hole transport and the dark currents in a-Se films are a very strong function of the substrate temperature (Tsubstrate) during the film deposition process. We have shown that the new technique reduces the dark currents to approximately the same levels as achievable with the previously existing methods for dark current reduction. However, the new method is simpler to implement, and offers some potential advantages, especially in cases when a very high image resolution (20 cycles/mm) and/or fast pixel readout (more than 30 times per second) are needed.
Using the new technology we have fabricated simple single and double (ni-like) photoconductor layers on prototype x-ray image detectors with CCD (Charge Coupled Device) readout circuits. Dark currents in the a-Se photoconductor layer were not a problem for detector operation at all tested electric fields. Compared to the currently available commercial systems for mammography, the prototype detectors have demonstrated an excellent imaging performance, in particular superior spatial resolution (20 cycles/mm). Thus, the newly proposed technology for dark current reduction has shown a potential for commercialization.
Advisor:Kasap, S.O.; Johanson, R.E.; Dodds, D.E.; Faried, S.O.; Sargent, Colin; Nguyen, Ha
School:University of Saskatchewan
School Location:Canada - Saskatchewan
Source Type:Master's Thesis
Keywords:amorphous state semiconductors photoconductivity a se photoconductors medical imaging radiation detectors high resolution digital mammography thick films experimental study electron hole pair lifetime localized states carrier trapping drift mobility charge alloying fabrication property relation arsenic additions doping chlorine deposition rate conditions boat temperature substrate annealing time of flight method dark current transients transient curves i t v curv x ray induced changes control reduction multilayered structures metal electrodes blocking layers double layer structure
Date of Publication:02/14/2007