A new method for growing detector-grade cadmium zinc telluride crystals

Results will be reported on a new technique to grow cadmium zinc telluride crystals for radiation detector applications without the use of an expensive, high-pressure chamber. This technique, based on the Modified Vertical Bridgman (MVB) method, has partly resolved problems resulting from the lack of control of the thermal environment during growth, leading to better melt stabilization, interface control, and crystallinity. For example, single crystal volumes exceeding 120 cm³ have been produced, which is 5-10 times larger that the typical single-crystal volumes produced using the high-pressure Bridgman method. The crystals exhibit simultaneously high electrical resistivity, good uniformity, and excellent electron mobility-lifetime products. The yield of spectrometer-grade crystals is >50%. Electrical resistivities in the range of 2-9×10¹⁰ Ohm-cm are obtained, and electron mobility-lifetime products up to 1×10^-2 cm²/V are measured using collimated low-energy gamma rays. These material advances have enabled production of large-volume planar detectors, co-planar grid detectors, and large-area pixellated imaging arrays. The detectors exhibit good counting efficiency, excellent peak-to-valley ratios, and low leakage currents. Individual pixels of multi-element arrays have spectral resolutions of up to 3% for uncollimated 122-keV photons. Co-planar grid detectors with dimensions of (10 mm)³ show resolutions of up to 2.2%, and energy resolutions of 1.2% are measured with 5.4 MeV alpha particles. Prototype large-volume planar detectors were fabricated, and the photopeak at 122 keV was approximately Gaussian in shape with practically all of the events in the peak, instead of counts at lower channels. New records for detector thickness (25 mm) and volume efficiency (3600 mm³) have been achieved. Results on the material and detector properties will be presented.