High-quality, large (10 cm long and 2.5 cm diameter), nuclear spectrometer grade Cd^sub 0.9^Zn^sub 0.1^Te (CZT) single crystals have been grown by a controlled vertical Bridgman technique using in-house zone refined precursor materials (Cd, Zn, and Te). A state-of-the-art computer model, multizone adaptive scheme for transport and phase-change processes (MASTRAP), is used to model heat and mass transfer in the Bridgman growth system and to predict the stress distribution in the as-grown CZT crystal and optimize the thermal profile. The model accounts for heat transfer in the multiphase system, convection in the melt, and interface dynamics. The grown semi-insulating (SI) CZT crystals have demonstrated promising results for high-resolution room-temperature radiation detectors due to their high dark resistivity (ρ [asymptotically =] 2.8 × 10^sup 11^ ohm cm), good charge-transport properties [electron and hole mobility-lifetime product, µτ^sub e^ [asymptotically =] (2-5) × 10^sup -3^ and µτ^sub h^ [asymptotically =] (3-5) × 10^sup -5^ respectively, and low cost of production. Spectroscopic ellipsometry and optical transmission measurements were carried out on the grown CZT crystals using two-modulator generalized ellipsometry (2-MGE). The refractive index n and extinction coefficient k were determined by mathematically eliminating the ~3-nm surface roughness layer. Nuclear detection measurements on the single-element CZT detectors with ^sup 241^Am and ^sup 137^Cs clearly detected 59.6 and 662 keV energies with energy resolution (FWHM) of 2.4 keV (4.0%) and 9.2 keV (1.4%), respectively.

Cadmium zinc telluride (CZT) has emerged as one of the most attractive and promising materials for room-temperature γ- and x-ray spectroscopy. CZT material has the advantages of high average atomic number (Z [asymptotically =] 50), high density (5.8 g/cm^sup 3^), and wide bandgap 01.50 eV at 300 K), yielding CZT detectors that are highly efficient at room temperature and above.1 Currently used Si and Ge detectors can only work efficiently at liquid-nitrogen temperature, which is expensive and inconvenient. The energy required for generating one electron-hole pair in CZT (~5 eV) is much less than that required for scintillation crystals coupled to photomultiplier tubes (-50 eV), resulting in better energy resolution. CZT materials also have shown improved spectral performance using novel, single-carrier detector designs, such as a Frisch ring,2,3 small pixel effect,4 and coplanar grid.5 Due to these advantages, CZT has been the material of choice for x- and γ-ray detectors for medical imaging, infrared focal plane array, national security, environmental monitoring, and space astronomy.6-9

Although tremendous efforts have been made to grow large, high-quality CZT crystals, the production of CZT, dominated by the high-pressure Bridgman method, suffers from low yields and small device sizes. Current CZT growth technology continues to endure problems, including easy defect formation, such as grains and twinning, precipitation and inclusion of Cd and Te, cracking due to thermal stresses, and nonuniform crystal composition caused by zinc segregation. During growth, the quality of the as-grown crystal is significantly influenced by complex transport phenomena taking place in the furnace. The melt flow driven by the buoyancy force has been realized to significantly affect the solid/melt interface shape and dopant impurity distribution in the as-grown crystal, giving rise to radial and axial segregation that adversely affects device quality.10 In addition, the inhomogeneous temperature distribution as well as wall contact can cause mechanical stresses in the crystal and result in a high dislocation density. Achieving the dopant uniformity in the grown CZT crystal requires precise control of the melt flow and heat and mass transfer in the growth system.

An alternative modified vertical Bridgman growth technique for CZT crystals has been adopted at EIC to produce large-volume detector-grade single crystals in high yield. The growth process has been studied numerically using an integrated model that combines formulation of global heat transfer and thermal elastic stresses. Using the elastic stress submodel, thermal stresses in the growing crystal caused by the nonuniform temperature distribution can be predicted. Special attention is directed to the interaction between the crystal and the ampoule. The global temperature distribution in the furnace, the flow patterns in the melt, and the interface shapes are presented herein.

The CZT crystal grown from zone-refined (ZR) precursor materials using the growth furnace at EIC showed very good charge-transport properties, i.e., high mobility-lifetime product for electrons while maintaining high bulk resistivity. Spectroscopic ellipsometry and optical transmission measurements of grown CZT crystal using two-modular generalized ellipsometry (2-MGE) are presented. Detection performances of CZT detectors with ^sup 241^Am and ^sup 137^Cs are also reported.