Role of zinc in CdZnTe radiation detectors

CdZnTe (CZT) crystals grown with Zn compositions of 0%, 10%, 15%, and 20% have been grown and detectors have been produced. Infrared transmissions measured on the wafers sliced from these crystals show that as the Zn composition increases, there is a reduction in the transmission toward longer wavelengths, indicating the existence of an increasing amount of larger Te-inclusions. For producing high resistivity materials, a higher concentration of indium is also required for CZT with higher Zn composition. The best detectors were produced in CZT with 10% Zn, while CdTe detectors are unable to resolve the ⁵⁷Co 122-keV peak. CZT detectors with 15% and 20% Zn in the melt display high counts 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 antisites (Te_Cd), to increase the density of V_Cd, and to enhance the diffusion rate of V_Cd. The higher amount of Te-inclusions in CZT with more Zn is caused by the rapid merge of V_Cd through fast diffusion. Because of the trapping by the Te-inclusions, detectors fabricated on CZT with 15% and 20% Zn are inferior to the 10% Zn CZT detectors. On the other hand, CdTe and CZT with Zn composition 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)², 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%.