Multiscale Modeling of High Field Hole Transport and Excess Noise in Avalanche Amorphous Selenium Layers

Amorphous selenium as a wide-bandgap disordered photoconductive material has achieved deterministic single-carrier hole avalanche gains ~ 1000, while exhibiting a very low excess noise factor. In the range of impact ionization at high electric fields, `hot' hole mobility in amorphous selenium loses its activated behavior and saturates with transport shifted entirely from localized to extended states. This necessitates the need to gain insight into the relaxation dynamics and particle trajectories of the non-equilibrium `hot' holes in extended states. A continuum of electrical and semiconducting properties across allotropic forms of a disordered structure, has been long attributed to a similarity in short-range order. We probed this retainment of short range order between amorphous selenium and trigonal selenium using molecular dynamics simulations, which, allowed us to model the general details of the extended-state hole-phonon interaction in the amorphous phase by modeling the band-transport lattice theory of its crystalline counterpart, trigonal selenium. The energy and phonon band structure along with the density of states and acoustic/optical deformation potentials for trigonal selenium was calculated using density functional theory. The density functional theory calculated hole-phonon coupling parameters were fed into an in-house bulk Monte Carlo algorithm to study high field hole transport in amorphous selenium layers while accounting for disorder in the amorphous phase. This study makes a strong case for the need of a microscopic dynamical model of hole transport in selenium semiconductors.