Research on Electrodes of CdZnTe Nuclear Radiation Detectors

Research on Electrodes of CdZnTe Nuclear Radiation Detectors
 
Major: Materials Physics and Chemistry
 
Postgraduate Student: Zhang Dongmin
 
Supervisor: Professor Zhu Shifu
 
CdZnTe (Cadmium Zinc Telluride) crystal is an excellent material for manufacturing room-temperature γ-ray and X-ray detectors. The crystal has a high average atomic number and strong stopping power for rays; it features a large band gap, high resistivity, and small leakage current; the product of carrier mobility and lifetime is relatively large, and the charge collection efficiency is high; it has good chemical stability and favorable mechanical processing performance. At room temperature, the CdZnTe detector has high detection efficiency and good energy resolution for nuclear radiation. Therefore, it has attracted much attention in recent years and has become one of the current前沿 research hotspots. However, in the preparation process of CdZnTe detectors, the surface passivation of wafers and the preparation of surface electrodes have a great impact on their performance.
 
For this reason, this thesis focuses on researching the key processes in the preparation of CdZnTe nuclear radiation detectors—the surface passivation of wafers and the preparation process of surface electrodes. The main contents are as follows:
 
It is found that the surface of CdZnTe wafers after mechanical processing has imperfections. Passivation treatment can form an oxide layer with insulating properties on the surface, which can reduce the surface leakage current of the wafer and is a very good method for manufacturing MIS (Metal - Insulator - Semiconductor) contact electrodes. The passivation time of the wafer determines the passivation effect, and the optimized passivation process can effectively improve the surface structure, composition, and performance of the wafer.
 
A new optimal surface treatment process for improving the performance of CdZnTe detector wafers is developed: Use BM etching solution (5% Br + methanol) and LB etching solution (2% Br + 20% lactic acid + ethylene glycol) to etch and polish the wafer for 1 minute each, and then use NH₄F/H₂O₂ solution to passivate the wafer for 30 minutes. This can minimize the surface leakage current of the wafer and achieve the best electrical performance. During the passivation process, the high-resistance oxide generated on the surface of the CdZnTe wafer is beneficial for increasing the resistivity and reducing the surface leakage current. The passivated CdZnTe wafer can be used for the preparation of room-temperature nuclear radiation detectors.
 
High-purity C, Pt, and Au materials are used to prepare MS (Metal - Semiconductor) and MIS contacts of C, Pt, and Au electrodes respectively by ion sputtering, and the I-V characteristics of the detectors are tested and compared. The results show that: The Pt electrode material forms a Schottky barrier contact on the wafer surface, which is beneficial for reducing the leakage current during the detector's operation and improving the energy resolution. The Pt electrode has relatively good comprehensive performance in both MS and MIS structures and is a better material for manufacturing the electrodes of CdZnTe detectors.
 
When the CdZnTe detector is fabricated into an MSM (Metal - Semiconductor - Metal) structured electrode, the dark current is relatively large, thus limiting the wide application of this electrode-structured detector. However, the leakage current of the CdZnTe detector with MIS-structured electrodes after etching and passivation treatment is significantly smaller than that of the detector with MSM-structured electrodes. Therefore, the MIS contact has better leakage current characteristics than the MS contact. Under the same applied voltage, its leakage current is smaller, and it is more suitable as a contact electrode structure for detectors. In the MIS structure, for asymmetric electrodes, changing the polarity of the electrodes will have a certain impact on the surface leakage current of the CdZnTe detector.
 
Keywords: CdZnTe crystal; detector; passivation; electrode; leakage current; MS; MIS