Columnar recombination for X-ray generated electron-holes in amorphous selenium and its significance in a-Se x-ray detectors

Although amorphous selenium (a-Se) has a long and successful history of application in optical and X-ray imaging, some of its fundamental properties are still puzzling. In particularly, the mechanism of carrier recombination following x-ray excitation and electric field and temperature dependences of the electron-hole pair creation energy (W-ehp) remain unclear. Using the combination of X-ray photocurrent and pulse height spectroscopy measurements, we measure W-ehp in a wide range of temperatures (218-320K) and electric fields (10-100 V/mm) and show that the conventional columnar recombination model which assumes Langevin recombination within a column (a primary electron track) fails to explain experimental results in a wide range of electric fields and temperatures. The reason for the failure of the conventional model is revealed in this work, and the theory of the columnar recombination is modified to include the saturation of the recombination rate at high electric field in order to account for the experimental results in the entire range of fields and temperatures. (C) 2016 AIP Publishing LLC.