Xin Wan a, Yang Kang a b, Heming Wei a, Ran Jiang a, Yu Liu a, Renying Cheng a, Xue Tian a, Tingting Tan a, Gangqiang Zha a b, Yingrui Li b, Kun Cao a
a School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, Shanxi 710072, China
b Research and Development Institute of Northwestern Polytechnical University in Shenzhen, Shenzhen, Guangdong, 518057, China
## Abstract
The radiation detection performance of CZT epitaxial single crystal grown by the Close Space Sublimation (CSS) method is hindered by inherent defects. However, significant improvement in this work is achieved through annealing in a Te2 atmosphere. The saturation channel value of the total energy peak of the detector for 241Am α-particles has shown a substantial increase from 1900 to 6000, resulting in a notable enhancement of the carrier collection efficiency. After annealing, the detector's energy resolution to 241Am gamma-rays can reach 8.3%, representing the highest energy resolution achieved by CSS growth of CZT materials. This remarkable achievement holds great promise for its potential application in the field of low-energy detection. Analysis of the laser-beam-induced current (LBIC) curve results reveals that the surface recombination of the detector is more pronounced prior to annealing, adversely affecting the transport process of photogenerated carriers. The low-temperature photoluminescence (PL) results and I–V curve demonstrate a significant compensation effect on the shallow acceptor level VCd after annealing, leading to a reduced concentration of free carriers and subsequently leading to lower leakage current and improved signal-to-noise ratio for the detector. The concurrent reduction in leakage current and surface recombination contributes collectively to improving the radiation detection performance of the CZT detector. These findings underscore the importance of annealing in a Te2 atmosphere as a viable method for mitigating defects and optimizing the performance of CZT epitaxial single crystals, showcasing the potential for broader applications in the field of low-energy detection.
## Introduction
Cadmium Zinc Telluride (CZT) stands out as a significant ternary compound semiconductor, offering a multitude of applications in high-energy X-ray and γ-ray radiation detection, nuclear spectroscopy, and medical imaging (Shen et al., 2019; Shkir et al., 2019; Sordo et al., 2009; Takahashi and Watanabe, 2001). Its direct band gap structure, coupled with high resistivity, a high atomic number, and excellent carrier mobility, renders it highly desirable for such purposes. With the development of large panel array detectors, there is an increasing demand for large-scale CZT single crystals (Szeles and Derby, 2021).
However, the challenges associated with growing sizable CZT crystals and their high production costs have prompted exploration into CZT epitaxial single crystals as viable alternatives. A variety of deposition methods for CZT epitaxial single crystal have been reported, including Molecular Beam Epitaxy (MBE) (Pan et al., 2023), Hot Wall Epitaxy (HWE) (Vuichyk et al., 2021), Metal-organic Chemical Vapor Deposition (MOCVD) (Kartopu et al., 2021) and Close Space Sublimation (CSS) (Li et al., 2023). Among these methods, the CSS method is a more suitable method for large area (2/4/6 inches) epitaxial single crystal growth due to its high deposition rate (10 μm/min) and simple growth system. Epitaxial single crystals grown using CSS can be scaled up to create large-area array detectors, potentially replacing amorphous selenium (a-Se) in digital radiography (DR) surface imaging technology.
The advancement of CZT epitaxial single crystal grown using the CSS method has indeed shown promising results. However, several challenges remain before it can fully replace CZT crystals, particularly in large-scale applications. One significant issue is the presence of crystal heterogeneity during the growth process, especially in larger CZT crystals. Previous studies have highlighted this challenge, indicating that crystal heterogeneity can impact the overall performance of CZT epitaxial single crystals (Wan et al., 2023a). Furthermore, the performance of CZT epitaxial single crystal is determined by various types of point defects, with shallow level ones affecting the free carrier concentration and thus impacting the detector's electrical performance, while deep level defects act as recombination or capture centers that affect the carrier collection efficiency (Guo et al., 2019).
To address these challenges and improve the performance of CZT epitaxial single crystals, an annealing process is often employed. Annealing helps regulate point defects within the crystal structure, leading to enhanced performance. However, the effectiveness of annealing depends on several factors, including the annealing atmosphere, time, and temperature (Sharma et al., 2022). Specifically, the type of point defect is affected by the annealing atmosphere while the concentration is determined by the annealing time (Yu et al., 2011; Yu and Jie, 2013). Additionally, the annealing temperature plays a crucial role in determining which types of defects can be formed, based on their respective formation energies (Wanwan et al., 2006; Kim et al., 2016). The design of a suitable annealing process is crucial for widely utilizing CZT epitaxial single crystal instead of CZT crystal, thereby achieving excellent radiation detection performance.
In our previous work, the CZT epitaxial film was annealed in Te2 atmosphere with a thickness of approximately 200 μm, resulting in an improvement in the resistivity of the film (Wan et al., 2023b). However, it exhibited no response to 241Am gamma-ray and thus cannot be used as energy spectrum detector. The present study proposes a novel annealing process for 1 mm-thick CZT epitaxial single crystal. The resulting annealed CZT detector offers exceptional energy resolution to gamma rays, making it highly suitable for gamma-ray spectrum detection.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/improvement-of-radiation-detection-performance-of-detector-prepared-by-cdznte-epitaxial-single-crystal.html