Microscopic modeling of high-field charge transport in amorphous selenium

Abstract

Avalanche multiplication of charge carriers as a result of successive impact ionization has led to the development of solid state avalanche photo-detectors. Crystalline based avalanche photodiodes have found a variety of applications including laser range finders and fiber optic telecommunications. Recently, there is a growing interest to employ amorphous semiconductors due to their economically favourbale costs and capability to be readily prepared in the desired size and structure with high efficiency. Selenium is the only material that has been reported to clearly feature the avalanche phenomenon in the amorphous phase in a practical electric field. Selenium based avalanche photo-diodes motivated commercialization of TV camera tubes which are capable of capturing images at extremely low light intensities. In addition, amorphous Selenium exhibits a high potential for development of x−ray and γ−ray detectors for medical imaging devices. Hence, studying the electronic properties of Selenium is worthwhile for advancement of functional amorphous materials that feature impact ionization. The energy loss mechanism that prevents the carriers from gaining sufficient kinetic energy to initiate impact ionization is inelastic scattering of electrons and holes with optical phonons. The latter interaction in Selenium is analyzed in this work. To overcome the computational difficulties, a crystalline structure of Selenium was studied, however it is of interest to extend the outcomes to amorphous phase. Here, we assume that the calculated results based on trigonal Selenium structure can be also translated into the amorphous structure. This assumption is supported by further studies of density of states and phonon density of states in both amorphous and crystalline phases of Selenium. In addition, validity of our assumption is further confirmed by simulating an amorphous Selenium structure. Volume deformation potential was studied for both trigonal and the simulated amorphous selenium.