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CZT crystals with Zn contents of 0%, 10%, 15%, and 20% have been grown and detectors have been produced. Infrared transmission measured on the wafers sliced from these crystals shows that as the Zn content increases, there is a reduction in the transmission toward longer wavelengths, indicating the existence of an increasing amount of larger Te-precipitates. For producing high resistivity materials, a higher concentration of indium is also required for CZT with higher Zn content. The best detectors were produced in CZT with 10% Zn, while CdTe detectors are unable to resolve the 57Co 122 keV peak and CZT detectors with 15% and 20% Zn display high noise levels at energies below this peak. The above results are explained by a model that the role of Zn in CZT is to reduce the density of TeCd to increase the density of VCd, and to enhance the diffusion rate of VCd. The higher amount of Te-precipitates in CZT with more Zn is caused by the rapid merge of VCd through fast diffusion of VCd. Because of the trapping by the Te-precipitates, detectors fabricated on CZT with 10% and 20% Zn are inferior to the 10% Zn CZT detectors. On the other hand, CdTe and CZT with Zn content less than 7% Zn have a high concentration of TeCd, VCd, and complexes such as TeCd·VCd and TeCd·(VCd), which are also trapping centers. As a result, the detectors fabricated on these crystals are also inferior to the 10% Zn detectors. The optimal Zn content for CZT grown using our technique is therefore near 10%.