Electron-hole pair creation energy in amorphous selenium: geminate versus columnar recombination

Amorphous selenium (a-Se) is one of the most successful photoconductors for direct-conversion X-ray detectors. However, the initial carrier recombination is believed to be responsible for high electron-hole pair (EHP) creation energy in a-Se. The simultaneously generated electron and its hole twin can recombine (geminate recombination) or the non-geminate electrons and holes in the columnar track of the primary photoelectron can also recombine (columnar recombination). The question of which mechanism (geminate or columnar) dominates in X-ray irradiation has not been resolved. In this paper, we examine these two recombination mechanisms and analyze them by fitting with published experimental data. The analysis and results are consistent with the columnar recombination mechanism at X-ray irradiation. We also propose an empirical expression for the electric field and photon energy-dependent EHP creation energy in a-Se at room temperature.