Chowdhury, Mazharul Huq
Electrical Properties of Grain Boundaries in Low Doped Polycrystalline Materials with Applications to Detectors.
Masters thesis, Concordia University.
- Accepted Version
Polycrystalline materials are widely used in large area electronic devices such as flat panel x-ray image detectors, and solar cells due to their suitability to deposit over large area at low cost. The performance of polycrystalline-based flat panel detectors are showing encouraging results (good sensitivity, good resolution and acceptable dark current) and give possibility to replace existing x-ray film/screen cassette. Therefore large area polycrystalline based flat panel detectors have opened new clinical possibilities and the polycrystalline solar cells give the opportunity of manufacturing low cost photovoltaic cells. Consequently, active research has been carried out to find out suitable polycrystalline materials (e.g. HgI2, CdTe, CdZnTe, PbI2, PbO etc) for various large area applications. However a polycrystalline material is composed of micro crystallites joined together by grain boundaries (complex structure, consisting of a few atomic layers of disordered atoms) which posses trap centers for charge carriers. Therefore, grain boundaries can trap a large amount of charges during detector operation. A potential barrier for drifting carriers may exist at the grain boundary, which controls the carrier mobility. Moreover, the performance of these polycrystalline detectors are affected due to the polarization phenomena (any change in the performance of the detector after the detector biasing) under applied bias. Therefore, in this research work, an analytical model is developed to study the electrical properties (electric field and potential distributions, potential barrier height, and polarization phenomenon) of polycrystalline materials at different doping levels for detector and solar cell applications by considering an arbitrary amount of grain boundary charge and a finite width of grain boundary region. The general grain boundary model is also applicable to highly doped polycrystalline materials. The electric field and potential distributions are obtained by solving the Poisson’s equation in both depleted grains and grain boundary regions. The electric field and potential distributions across the detector are analyzed under various doping, trapping and applied biases. The electric field collapses, i.e., a nearly zero average electric field region exists in some part of the biased detector at high trapped charge densities at the grain boundaries. The model explains the conditions of existence of a zero average field region, i.e., it explains the polarization mechanisms in polycrystalline materials. The potential barrier at the grain boundary exists if the electric field changes its sign at the opposite side of the grain boundary. The potential barrier does not exist in all grain boundaries in the low doped polycrystalline detector and it never exists in intrinsic polycrystalline detectors under applied bias condition provided there is no charge trapping in the grain.
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