Amorphous and Nanocrystalline Semiconductors for Optoelectronic Applications: Photoconductors, Detectors and Sensors Based on a-Se Alloys and Optoelectronic Glasses

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The DG program on optoelectronic materials for radiation detection and imaging includes a major study on doped amorphous Se-As alloys (a-Se:As) and a complimentary study on rare-earth doped glasses and glass ceramics (containing nanocrystals). a-Se alloys have one clear distinct advantage over crystalline semiconductors that they can be conveniently fabricated over large areas, which has led to their extensive commercial use in flat panel x-ray image detectors. The long term objective is to build on strength by extending our scientific and technological knowledge on doped a-Se alloy based semiconductors and devices by discoveries at the material and device levels; and advancing knowledge at the fundamental level. The design of next generation a-Se detectors, in particular for use in general radiology, CT and tomosynthesis, also needs extensive knowledge of the electrical properties of the photoconductor used in the three layers, e.g. drift mobilities, carrier trapping and recombination lifetimes, sensitivity, dark current, excess noise, thermal generation rate, x-ray induced effects (such as defect creation or damage) and relaxation effects - these alloys are glasses that age and exhibit relaxation effects. Given the availability of the Canadian Light Source (CLS), we intend to continue to use of the synchrotron facility in our research and continue to collaborate with the CLS staff; and also continue with our industrial partner in relating results to practical detectors. The research program will encompass all aspects of photoconductor research from present to new a-Se alloys (doped) in a concerted effort to relate detector performance to fundamental semiconductor properties through physical models. It will also include Sm3+ doped glasses and glass ceramics for Microbeam Radiation Therapy in which we need to accurately measure the dose profile in the microbeam at the CLS BMIT line (peak dose 2 - 3 kGy with a width 20 um). There will be a number of projects within this DG program. Typical examples: (a) Study of optoelectronic properties of new doped a-Se alloys as a function of temperature and electric field toward enhanced p-i-n structures (thicker i-layer) with lower dark currents and higher applied fields. (b) Study of the x-ray energy dependence of photoconductivity of a-Se alloys, MTF, DQE as a function of operating field, temperature, total dose and dose rate. (d) Study of the effects of x-ray irradiation on the photoconductor properties and hence on the sensitivity and DQE of the actual detector. (e) Discovering new Sm-doped optoelectronic glasses (OEGs) that are tissue equivalent for better MRT dose distribution monitoring and addressing current problems (energy dependence). The impact of the work is the generation of new knowledge on doped a-Se alloys with new compositions for the design and development of next generation flat panel x-ray detectors and new OEGs for high-dose dosimetry.