Analysis of CZT crystals and detectors grown in Russia and the Ukraine by high-pressure Bridgman methods

Sandia National Laboratories (SNL) is leading an effort to evaluate vertica l high pressure Bridgman (VHPB) Cd1-xZnxTe (CZT) crystals grown in the form er Soviet Union (FSU) (Ukraine and Russia), in order to study the parameter s limiting the crystal quality and the radiation detector performance. The stoichiometry of the CZT crystals, with 0.04<x<0.25, has been determined by methods such as proton-induced x-ray emission (PIXE), x-ray diffraction (X RD), microprobe analysis and laser ablation ICP mass spectroscopy (LA-ICP/M S). Other methods such as triaxial double crystal x-ray diffraction (TADXRD ), infrared transmission spectroscopy (IR), atomic force microscopy (AFM), thermoelectric emission spectroscopy (TEES) and laser induced transient cha rge technique (TCT) were also used to evaluate the material properties. We have measured the zinc distribution in a CZT ingot along the axial directio n and also its homogeneity. The (Cd+Zn)/Te average ratio measured on the Uk raine crystals was 1.2, compared to the ratio of 0.9-1.06 on the Russian in gots. The IR transmission showed highly decorated grain boundaries with pre cipitates and hollow bubbles. Microprobe elemental analysis and LA-ICP/MS s howed carbon precipitates in the CZT bulk and carbon deposits along grain b oundaries. The higher concentration of impurities and the imperfect crystal linity lead to shorter electron and hole lifetimes in the range of 0.5-2 an d 0.1 mu s, respectively, compared to 3-20 and 1-7 mu s measured on U.S spe ctrometer grade CZT detectors. These results are consistent with the lower resistivity and worse crystalline perfection of these crystals, compared to U.S.-grown CZT. However, recently grown CZT from FSU exhibited better dete ctor performance and good response to alpha particles.