A study of signal generation and charge collectionin a-Si:H diodes for radiation imaging

Its high radiation resistivity and large area capability are the expected advantages of this material together with its ability to provide a front-end readout electronics in the vicinity of the sensor element. Electrons and holes created by incoming charged particles, X rays, gamma rays, are drifted by the electric field inside a-Si:H diodes and this carrier movement induces signal charges on electrodes. Charge collection and signal generation process are analyzed in terms of carrier mobilities, lifetimes and electric field. Charge collection in thick a-Si:H diodes is often limited by deep-level trapping of carriers during transit and a finite charge integration time required for single particle counting in some applications and sometimes by volume recombination of carriers for detecting heavily ionizing particles such as alpha particles. The charge collection process is also strongly affected by the nonuniform electric field profiles in a-Si:H diodes caused by the fixed space charges inside the material under reverse bias. Signal generation due to a weak light pulse irradiating each end of a thick diode is measured as a function of a reverse bias and it gives a valuable information about the fixed space charges. Field profiles can be manipulated by either doping, electrode geometry, or combination of both to improve the charge collection process. One can apply a higher reverse bias on a diode with an equivalent thickness by providing buffer layers at each end of the diode and thus suppressing soft breakdown phenomena. X ray detection with a good sensitivity is demonstrated by an a-Si:H photodiode coupled to an evaporated CsI scintillator. The scintillation quality of evaporated CsI layers can be made almost identical to its single crystal counterpart. Fields of a-Si:H radiation detector application include high energy physics, medical imaging, materials science and life science.