CHARACTERIZATION OF SECONDARY PHASES IN Cd0.9Zn0.1Te DETECTOR GRADE SEMICONDUCTOR BY IR TRANSMISSION MICROSCOPE AND IMPLEMENTATION OF AMPOULE ROTATION TECHNIQUE IN MODIFIED VERTICAL BRIDGMAN GROWTH TO MINIMIZE THE SECONDARY PHASES
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Presence of secondary phases (precipitates/inclusions) in cadmium zinc telluride (CZT) crystals degrades the performance of the materials as radiation detectors. The infrared (IR) transmission microscope was used to quantify the inclusions in CZT, grown by modified vertical Bridgman technique (MVB). Precipitates were unable to resolve due to their small size (< 1 μm) and limitation of the microscope. Density and the distribution of the inclusions in axial (growth) and radial directions of the ingot were measured to find the correlation between the inclusions and mobility-lifetime product (μτ). Study showed that the μτ product was greatly affected by the inclusions > 4 μm in diameters. Different solid-liquid growth interface shapes were analyzed to study their effects on the size and concentration of the inclusions. Application of various cooling schemes during crystal growths had significant effects on the size and concentration of the inclusions. Quenching and fast cooling resulted in reduced diameters of the inclusions and relatively better crystal properties. Analyses the effects of impurities on the detector properties were performed and attempts were made to establish the correlation between the impurities and the μτ. Surface dislocations were revealed in CZT single crystals using Everson etchant which were analyzed by IR and scanning electron microscope (SEM). It was observed that cooling schemes and excess amount of Cd/Te have strong effects on etch pits density (EPD). In order to reduce the size and density of the inclusions, ampoule rotation technique was applied to MVB during growth. Effects of various rotation profiles on the inclusions properties were analyzed and found that diameters of the inclusions were reduced to < 10 μm and the densities were increased significantly. No significant differences have been observed in mobility-lifetime values for ampoule rotation and without ampoule rotation grown crystals.