Hole transport in selenium semiconductors using density functional theory and bulk Monte Carlo

Amorphous selenium is emerging as a viable large-area imaging detector with avalanche multiplication gain for low-light and low-dose radiation detection applications. A key feature of its avalanche process is that only holes become "hot" carriers and undergo impact ionization. Thus, understanding the transport of non-equilibrium hot holes in extended states is pivotal to all the device applications. One of the interesting aspects of elemental selenium is the similar general feature of the electronic structure for various phase modifications. This stems from the strikingly similar short-range order between the crystalline and amorphous phases of selenium. At high electric fields, hole mobility in amorphous selenium loses its activated behavior and saturates with transport shifted entirely from localized to extended states. Thus, we expect the general details of the extended-state hole-phonon interaction in the amorphous phase to be described by the band-transport lattice theory of its crystalline counterparts, namely, monoclinic and trigonal selenium. To that effect and due to the intrinsic meta-stability of the monoclinic phase and high trap density in prepared specimens, we study hole transport in crystalline trigonal selenium semiconductors using a bulk Monte Carlo technique to solve the semi-classical Boltzmann transport equation. We validated our transport model by showing the excellent match between experimentally calculated hole drift mobilities with that calculated using the bulk Monte Carlo technique. Furthermore, calculations of the field-dependent carrier energy showed that holes in selenium can break the thermal equilibrium and get hot at which point the rate of energy gain from the applied electric field exceeds that of energy loss from the lattice.