STRATEGIC APPROACHES TOWARDS SOLVING CRITICAL CHALLENGES IN CRYSTAL GROWTH OF DETECTOR GRADE Cd0.9Zn0.1Te INCLUDING MELT MIXING TECHNIQUES
Over the last few decades Cadmium Zinc Telluride (CZT) has emerged as a novel material for radiation detection in homeland security as well as medical applications and as substrates for epitaxial growth of infrared detector, Mercury Cadmium Telluride. There are, however, several critical issues regarding the bulk growth of CZT which renders it unsuitable or cost ineffective for these applications. These challenges can be summarized as non-uniformity (due to Zinc and intentional dopant segregation), longer growth times (because of lower thermal conductivity of the melt), Tellurium (Te) secondary phases (SP), unstable growth interface (due to lower thermal conductivity and higher heat of fusion) and non-stoichiometric growth conditions. Systematic approaches towards solving these problems will be discussed along with the consequences and future research directions. By application of a proper mixing technique to the CZT melt, most inhomogeneity problems that are common in growth techniques using non rotating crucibles can be overcome. A unique custom made ampoule rotation system was developed for mixing the CZT melt by applying several rotation parameters. Experiments were performed with different growth temperature profiles, rotation parameters and charge/dopant concentrations. Improvements in Zinc distribution profiles and the shape of the liquid/solid interface during the growth process were determined by room temperature photoluminescence (RT-PL) mapping of radial and longitudinal-cut CZT wafers. SP diameters were limited to a maximum of 10 µm after application of ampoule rotation, with distribution maxima at 6 µm. Growth interface studies revealed several unique features which will be discussed in the dissertation. The spectral, optical and electrical properties of the detectors fabricated from all these growths will be presented. The statistical variations of different properties along the grown ingots will be discussed.