Role of zinc in CdZnTe radiation detectors: why zinc? How much?

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 Te_Cd to increase the density of V_Cd, and to enhance the diffusion rate of V_Cd. The higher amount of Te-precipitates in CZT with more Zn is caused by the rapid merge of V_Cd through fast diffusion of V_Cd. 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 Te_Cd, V_Cd, and complexes such as Te_Cd·V_Cd and Te_Cd·(V_Cd), 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%.