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Spatial Mapping of the Mobility-Lifetime (microtau) Production in Cadmium Zinc Telluride Nuclear Radiation Detectors Using Transport Imaging

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Master's thesis

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Cadmium zinc telluride Cd1-xZnxTe is an important material for room temperature nuclear radiation detectors due to its high stopping power for gamma rays combined with its good electron transport. However, CdZnTe crystals are susceptible to growth defects such as grain boundaries, twin boundaries, and tellurium Te inclusions which can compromise desirable energy resolution and electronhole charge collection properties. The presence of these defects ultimately degrades the effectiveness of the nuclear radiation detector material. The ability to map electron and hole transport properties at high spatial resolution can provide new insight into the roles of individual defects. Experimentally, this study employs high-resolution 5 m transport imaging to explore the effect of localized crystal defects on the spatial variation of carrier transport properties. The ambipolar diffusion length Ld and associated free carrier mobility-lifetime product are determined by imaging the recombination luminescence from carriers generated by an electron beam. Localized defects often are marked by regions of low intensity luminescence. At the same time, we observe increasing ambipolar diffusion length in the region immediately surrounding the defects. One explanation is that the gettering of point defects, such as interstitials and vacancies, associated with the formation of microscopic precipitates results in localized increases in the product. Initial results indicate that these variations occur over a region extending 10 m from the edge of the inclusion. Mathematically, this study employs the minority carrier diffusion equation to model the 3D diffusion of free charge carriers away from a point source. A non-linear least squares program using exact methods and asymptotic expansion methods is then used to fit this model to transport data imagery.

Subject Categories:

  • Inorganic Chemistry
  • Miscellaneous Detection and Detectors
  • Nuclear Physics and Elementary Particle Physics

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