Charge Carrier Transport and Photogeneration at Very High Electric Fields in Amorphous Selenium

The flat-panel digital X-ray detectors (e.g. amorphous selenium, a-Se, based detectors) are replacing the film-based technology in various diagnostic medical imaging modalities such as mammography and chest radiography. Whereas, there is a huge demand for lowering the irradiation dose in various medical imaging modalities, the present flat-panel digital X-ray imaging technology is severely challenged under low dose conditions. To date, amorphous selenium (a-Se) is one of the most highly developed photoconductors used in digital X-ray imaging, which exhibits impact ionization and usuable carrier multiplication. The viability of avalanche multiplication can increase the signal strength and improve the signal to noise ratio for application in low dose medical X-ray imaging detectors. In spite of the interesting outlook of a-Se, some of its fundamental properties are still not fully understood. Specifically, an understanding of carrier transport at extremely high field in a-Se is in a very premature state. Therefore, an extensive research work is vital to clearly understand the fundamental underlying physics of carrier generation, multiplication, and transport mechanisms in a-Se. In this dissertation, a physics-based model is developed to investigate the mechanisms of the electric field and temperature dependent effective drift mobility of holes and electrons and also the impact ionization in a-Se. The models consider the density of states distribution near the band edges, field enhancement release rate from the shallow traps, and carrier heating. The lucky-drift model for a-Se is developed based on the observed field dependent microscopic mobility. The validation of the developed models via comparison with the experimental data verifies the mechanisms behind the electric field and temperature dependent behaviours of impact ionization coefficient in a-Se. The density of state function near the band edges, consisting of an exponential tail and a Gaussian peak, successfully described the electric field and temperature-dependent effective drift mobility characteristics in a-Se. The photogeneration efficiency in a-Se under optical excitation strongly depends on photon wavelength and electric field. A physics-based model is proposed to investigate the physical mechanism of charge carrier photogeneration in a-Se under high electric fields. The exact extension of Onsager theory can explain the photogeneration efficiency in a-Se at extremely high electric field. The mechanism of carrier recombination following X-ray excitation and hence the evaluation of electric field and X-ray photon energy dependent electron-hole pair (EHP) creation energy (amount of energy needed to produce a detectable free EHP upon the absorption of an X-ray photon) in a-Se have been topics of a very vital debate over the last two decades. These issues are addressed in this thesis. Towards this end, a physics-based analytical model is developed via incorporating a few valid assumptions to study the initial recombination mechanisms of X-ray generated EHPs in a-Se. The analytical model is later verified by a full phase numerical model, considering three-dimensional coupled continuity equations of electrons and holes under carrier drift, diffusion and bimolecular recombination. The corresponsding calculations of EHP creation energy with wide variations of X-ray energy, electric field and temperature are verified with respect to the available published experimental data. According to this, it is found that the columnar recombination model is capable of describing the electric field, temperature and photon energy dependent EHP creation energy in a-Se for high-energy photons. The theoretical work of this thesis unveil the physics of the charge carrier transport and photogeneration mechanisms in a-Se at very high electric fields, which is vital to optimum design of avalanche a-Se detectors. This work will also provide a guideline for further improvement of the radiation imaging detectors.