Abstract

There has been a rejuvenated interest in amorphous selenium (a-Se) and its alloys as a photoconductive material in the arena of digital direct conversion flat panel X-ray image detectors (FPXI) for diagnostic medical imaging. However the a-Se photoconductive layer in FPXIs have to go through high applied field (up to 10V/µm) and makes one of the significant difficulties related to FPXIs by creating current in the absence of radiation known as dark current. This thesis deals with the effect of dark current under physics based theoretical modeling. The density of the defect states of deposited n layer of multilayer a-Se detectors are determined by analyzing the transient dark current behaviors. This analysis is important for the origin of time and bias dependent dark current being steady state. The improved model has been applied to alkaline doped n-layer and cold- deposited n-i based detectors and validated by the experimental results. This validation strongly supports the physical mechanisms responsible for the transient behaviour of dark current in the X-ray imaging detectors. The theoretical investigation of the density of states (DOS) of both n-layers (alkaline doped and cold deposited) link up the contribution between the trap concentrations and transient behaviour of the dark current.
The observation of impact ionization, which leads to avalanche multiplication on the amorphous selenium, has great impact on the low dose medical application such as general radiographic and fluoroscopic applications. However, the dark current can be high and very critical due to extremely high field and the avalanche nature of dark current. In this thesis, a physics-based analytical model for describing the transient and steady-state dark current is developed by considering bulk thermal generation, transient carrier depletion and avalanche multiplication. The model is validated by the published experimental results.