Use of Pulse-Height Spectroscopy to Characterize the Hole Conduction Mechanism of a Polyimide Blocking Layer Used in Amorphous-Selenium Radiation Detectors

We demonstrate the use of pulse-height spectroscopy (PHS) to extract the internal electric field of an amorphous-selenium (a-Se) detector having a polyimide (PI) blocking layer. PHS enables more accurate measurement of the internal electric field of the detector because single-photon interactions in radiation detectors do not distort the internal electric field significantly. We fabricated a set of a-Se detectors, each having a PI layer with different thicknesses, and measured instantaneous electric field within the a-Se and PI layers using PHS, as well as the dark current of each detector. We also investigated the detector response under X-ray pulse illumination to determine the optimal thickness of PI necessary to achieve the best photo-to-dark-current ratio for low radiation dose imaging applications. Finally, we represent an analytical model of the steady-state dark-current behavior and a hole conduction mechanism in PI, which incorporates the Poole-Frankel emission model, and compared our model with the experimental results. The PHS approach reported in this article can enable the selection and design of an optimal blocking layer material when integrated with the existing [e.g., a-Se and cadmium zinc telluride (CZT)] and emerging direct-conversion radiation semiconductors (e.g., PbO and TIBr).