Measurement of photoconductivity of thin amorphous selenium films induced by x-ray radiation



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A new x-ray image tube that has two major advantages over existing devices, namely the reduction of x-ray exposure and high resolving power, would be available only if a thin film material of high sensitivity to x-rays and low conductivity under normal conditions can be found. Amorphous selenium was chosen in the first phase to be tested as an x-ray sensitive material. Aluminum-amorphous selenium-aluminum sandwich samples of various thicknesses were evaporated in three consecutive steps. The space-charge -limited current density J was then measured as a function of the voltage V. According to Rose and Lampert, the trapping density and its distribution in energy in an insulator can be deduced from the J-V curves. An exponential trap distribution was obtained experimentally which confirmed the predicted theoretical results. A system of trapping and recombination equations of exponential trap distribution was solved. The rise, decay and steady state changes in conductivity as a function of time under different radiation conditions were calculated and compared with the experimental results. It was found that amorphous selenium is not an ideal target material for our purpose.