Two-Step Annealing to Remove Te Secondary-Phase Defects in CdZnTe While Preserving the High Electrical Resistivity

Abstract

The presence of Te secondary-phase defects (i.e., Te inclusions and Te precipitates) is a major factor limiting the performance of CdZnTe (CZT) X-ray and gamma-ray radiation detectors. We find that Te secondary-phase defects in CZT crystals can be removed through postgrowth two-step annealing without creating new trapping centers (i.e., prismatic punching defects). Two-step annealing (with the first under a Cd pressure and the second one under a Te pressure) was demonstrated to be effective in removing the Te secondary-phase defects, while preserving the electrical resistivity of the CZT detector. The first step involves annealing of semi-insulating CZT under a Cd overpressure at 700 °C/600 °C (CZT/Cd) for 24 h, which completely eliminated the Te-rich secondary-phase defects (Te inclusions). However, it resulted in a lower resistivity of the samples (down to $2\times 10^{4-6}\,\,\Omega \cdot \text {cm}$ ). A subsequent annealing step involves processing CZT under a Te ambient condition at 540 °C/380 °C (CZT/Te) for 120 h, which restored the crystal’s resistivity to $6.4 \times 10^{10}\,\,\Omega \cdot \text {cm}$ without creating new Te secondary-phase defects. However, Te inclusions reappeared in the case of unnecessarily long Te ambient annealing. Pulse-height spectra taken with the two-step annealed CZT detectors showed an improved detector performance due to a reduced concentration and the size of Te secondary-phase defects.