Improving The Properties Of CdZnTe Crystal By Decreasing The Free-space Volume In The Ampoule

Due to the high average atomic number and large band gap of CdZnTe material,CdZnTe detector has a large absorption coefficient, high count rate and can work atroom temperature, which induces broad prospects of CdZnTe detector in medicine,space science, airport, port security, nuclear waste monitoring and other nuclearapplications. As results of several constraints such as crystal performance, size, andcost, CdZnTe detectors are not widely used recently. Therefore, the key that is howto develop a new generation of CdZnTe nuclear detectors lies in the breakthrough onthe research of CdZnTe crystal growth.In this dissertation, optimization of CdZnTe crystal growth process and theinfluence of doping elements on the crystal performance are investigated. Bycontrolling the free-space volume in the quartz ampoule in the Bridgman growth ofCdZnTe, the Cd evaporation was suppressed and the stoichiometric compositionremained, leading to a better performance of the CZT crystals with less In-dopedcompensation. The main conclusions of this dissertation are summarized as follows:1. The temperature curve of the synthesizing CZT polycrystalline material wasoptimized by adding swing process. XRD analysis of CZT polycrystalline materialfound no characteristic diffraction peaks of Te and Zn, which showed that thepolycrystalline material was synthesized completely with no residual of Te or Zn. Byimproving the growth parameters of CZT, such as temperature, temperature gradientand the growth velocity the composition homogeneity was improved with themaximum deviation of Zn within2.4%axially and the fraction of single-crystallarger than35%.2. The free-space volume in the growing ampoule was compressed by a doubleenvelope crucible after synthesis of CZT polycrystalline ingot, in order to reduce theCd vacancies and Te-rich equal defects in CZT crystal due to the high vapor pressureof Cd component during the crystal growth. The relationship between the free-space volume in the growing ampoule and the defects in CZT, such as Te inclusion and Cdvacancy, was studied by IR transmission microscopy and positron annihilationtechnique (PAT). With the decreasing of the free-space volume in the ampoule, thedensity and size of Te inclusions reduced from6.67×104cm-2to2.36×103cm-2andthe average positron lifetime decreased from325.4ps to323.4ps, which indicated areduction of Cd vacancies. The IR transmittance and resistivity measurement ofCZT further demonstrated that decreasing the free-space volume in the ampoule candepress the defects in CZT crystal effectively.3. By decreasing the free-space volume in the Ampoule to5%, the relationshipbetween the In dopant and CZT crystal performance was studied. When the dopingconcentration of In was less than5×1016/cm3, the carrier concentration of CZTwafers decreased and the resistivity increased, with p-type conductivity. While theconcentration of In dopant was above5×1016/cm3, the conductivity type of CZTwafers changed from p-type to n-type. Furthermore, increasing the doping amountreduced the carrier mobility of CZT crystal. The positron annihilation lifetimespectra showed that Cd vacancies, vacancy clusters and micro-structural defects incrystals decreased after In doping. The size and density of Te inclusion in CZTwafers tended to decrease weakly, and IR transmittance significantly reduced. Theresults showed that CZT crystal with free-space volume in the Ampoule of5%andIn dopant concentration of5×1016/cm3had optimized performance, with theresistivity of1.8×1010cm and the carrier (electron) mobility of187cm2/Vs.4. The effects of decreasing free-space volume in the ampoule with In-dopedCZT crystal have been analyzed. In the ampoule without free-space compressionduring the In-doped CZT growth, the concentration of In dopant for semi-insulatingCZT is5×1017/cm3, while by decreasing the free-space volume in the Ampoule to5%, the concentration of In dopant required for conductivity type transformationfrom P to N type decreased to5×1016/cm3. With less In dopant, the carrier mobilityof CZT crystal increased. Therefore, decreasing free-space volume in the ampoule can reduce the extrinsic In dopant concentration effectively, which improves thecarrier mobility of CZT crystal.