Available Technology

BSA 15-02: Novel electrode contacts to CdZnTe for X-ray and gamma-ray detector applications

CdZnTe (CZT) has made a significant impact as a material for room-temperature nuclear-radiation detectors due to its potential impact in applications related to nonproliferation, homeland security, medical imaging, and gamma-ray telescopes. However, the poor adhesion of common metals (used as electrodes) to CdZnTe (CZT)/CdTe surfaces has been a long-standing challenge for radiation detectors. Because of the large mismatch in the thermal-expansion coefficient between the metal contacts and CZT, the contacts can undergo stress and mechanical degradation, which is the main cause for device instability over the long term. To resolve this long-standing issue, a novel nonmetallic contact viz., aluminum-doped ZnO (AZO) to CZT is invented for radiation-detector devices.
Patent Abstract: 
To have a stable contact on CZT, the thermal expansion coefficient of the contact material should be close to the value for CZT, and it should have good adhesion. In this invention a nonmetallic contact was adopted viz., aluminum-doped ZnO (AZO) to CZT. AZO offers several advantages over the common metals presently used for fabricating radiation detectors. The hardness of ZnO is 4.7 GPa which is 8–20 times higher than the commonly used metal contacts (e.g., gold, platinum and indium) for CZT detectors. In addition, the coefficient of thermal expansion of ZnO is close to that of CZT and because it is an oxide it ensures better chemical bonding and chemical stability of the interface. The current–voltage characteristics (I-V) for AZO contact is similar to the gold contact but with lower dark current, which is advantageous because the AZO/CZT detector can be operated at higher voltages and with less noise compared to gold contacted detectors.
The new contact approach offers a promising alternative contact-layer to the common metals for durable CdTe-based X-ray and gamma-ray detectors operating at room temperature. The higher hardness of AZO compared to the common metals ensures a higher throughput for the fabrication process, and the damages due to scratches during handling will be reduced drastically.
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