Grant: Discovery Grants Program - Individual

The current DG program is on amorphous semiconductors for x-ray detector applications with the present extension being solely on multilayer doped amorphous selenium (a-Se) alloys. 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 imaging detectors. Over 70% of mammographic x-ray detectors today use a-Se based technologies. Furthermore, a-Se is the only large area semiconductor so far that exhibits avalanche multiplication under sufficiently high fields. This enables avalanche a-Se detectors to be used in indirect conversion with a suitable scintillator, which opens up the chess radiology modality to a-Se detectors. The long-term objective is to extend 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 third generation a-Se detectors, in particular for use in general radiology, CT and tomosynthesis needs extensive knowledge of the electrical properties of the photoconductor, i.e. drift mobilities, carrier lifetimes, sensitivity, dark current, excess noise, avalanche multiplication noise, thermal generation rate, x-ray induced effects and relaxation effects - a-Se alloys exhibit relaxation effects over time. Although much progress has been achieved in the last DG cycle, there is still much left to be discovered. The DG program will encompass all aspects of photoconductor research from present to new a-Se alloys (doped and alloyed) in a concerted effort to relate detector performance to fundamental semiconductor properties through physical models: (A) The study of optoelectronic properties of new doped a-Se alloys as a function of temperature and electric field toward enhanced n-i-p structures, avalanche a-Se photodetectors, and novel vertical a-Se detectors that can be used at high energies and also potentially in dual energy imaging (B) The study of excess or flicker noise (or 1/f noise) in various a-Se structures as a function of applied field, temperature, device configuration, and pre-irradiation. (C) The development of new noise models for a-Se detectors towards improved detective quantum efficiency (DQE) models. Unfortunately, very little is known on the excess noise due to the dark current in a-Se alloy-based detectors (the shot noise assumption in the literature is wrong), which will be investigated in this work. The DG program will investigate halogen-doped a-Se1-xTex alloys whose bandgap can be matched to the emission wavelength of the scintillator and hence can enable high DQE large area indirect conversion detectors over all modalities. The results will be directly useful to the Canadian x-ray detector industry and to the advancement of x-ray diagnosis in health care; and enhanced industrial and security imaging as well.