Lu Liang a b, Lingyan Xu a b, Chi Qin a b, Yingming Wang a b, Zhentao Qin a b, Chongqi Liu a b, Lixiang Lian a b, Ce Zheng c, Yadong Xu a b, Wanqi Jie a b
a State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
b Key Laboratory of Radiation Detection Materials and Devices, Ministry of Industry and Information Technology, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an 710072, China
c Science and Technology on Thermo Structural Composite Materials Laboratory, Northwestern Polytechnical University, Xi'an 710072, China
## Abstract
Consideration of irradiation effects is important for in-orbit operation and radiation protection in aerospace applications. The configuration of irradiation defects and their impact on deteriorating the carrier transport properties of CZT crystals need to be clarified. In this study, the microdefect evolution mechanism of CdZnTe (CZT) crystals under 10 MeV Chlorine (Cl) ion irradiation at flux of 1 × 1010∼1 × 1012 n·cm−2 are investigated. The stacking faults first appear in the CZT crystal at 4.5 × 10−4 displacement per atom (dpa), which are categorized into slip-type, gap-type and vacancy-type. In order to hinder the further expansion of stacking fault, S-shaped stacking fault dipole appears. When the dpa reaches 4.5 × 10−2, the local phase transition occurs in the damage zone to form CuPt-A ordered phase with ZnTe/CdTe periodic arrangement because the fluctuation in local composition and strain caused by cascade collisions. The deterioration of electric field distribution and charge collection caused by irradiation damage layer leads to the decrease of γ-ray detection ability. The energy resolution (ER) of the CZT detector for γ-ray can be maintained at 30.6% when the ion flux accumulates to 1 × 1012 n·cm−2, revealing high irradiation hardness. Consequently, these findings provide a theoretical basis for the radiation damage recovery of CZT detectors, further demonstrating that CZT crystals are the most competitive new generation of room-temperature nuclear radiation detection materials, and can provide radiation protection strategies for similar compound semiconductor materials.
## 1. Introduction
The importance of the effects of radiation on semiconductor detectors can be traced back to 1963, when Explorer 1 crossed the Van Allen radiation belt, causing the internal transistors of the Geiger counter to fail due to radiation exposure. This event drew attention to the vulnerability of semiconductor detectors to radiation-induced damage. In 1965, the discovery of damage defects in single crystal silicon by W et al. [1] marked the beginning of a systematic study of radiation damage in semiconductor materials. Since then, understanding and mitigating the effects of radiation on semiconductor detectors have become imperative to improve their reliability and performance in a variety of critical applications. In contemporary times, the rapid growth of industrial imaging, deep space exploration and high energy physics research has escalated the need for enhanced serviceability of radiation detection materials. Therefore, the understanding of radiation damage and the implementation of protective measures remain critical.
As an emerging ternary compound semiconductor material,
Cadmium Zinc Telluride (CdZnTe, CZT) is characterized by several features such as high resistivity, suitable band gap, high atomic number and high electron mobility [2,3]. The CZT has demonstrated its effectiveness as a radiation detector on board the Practitioner 10 satellite and successfully acquired space radiation data. In addition, the CZT is the core component of the radiation dosimeters on board the manned spacecrafts Shenzhou VII to Shenzhou X, playing a key role in ensuring the safety of astronauts during space missions. The space radiation environment is characterized by the presence of heavy ions of different energies, including Galactic Cosmic Rays (GCRs), Solar Energetic Particle Events (SEPs), and the Earth's radiation belts [4,5]. A high concentration of defective damage layers due to inhomogeneous deposition of heavy ions in CZT materials can degrade detector performance and even lead to loss of functionality. Various types of defects in CZT crystals, including point defects [6,7], extend defects [8,9], precipitation phases, and inclusions [10,11], significantly degrade their optoelectronic properties [[12], [13], [14]]. Therefore, understanding the radiation resistance and heavy ion irradiation damage mechanisms of CZT crystals is essential to ensure the reliability of the devices in a space radiation environment.
Numerous studies have elucidated that the damage characteristics of semiconductor materials under heavy ion irradiation exhibit a complex relationship with factors such as ion type, energy, flux rate, and total flux. Kucheyev et al. [15,16] observed that light ion irradiation of single crystals can induce amorphization owing to intense chemical reactions within GaN crystal lattice. Wendler et al. [17] investigated the irradiation of SiC by Si ions and found that the damage evolution was positively correlated with the presence of displaced atoms. Point defect clusters and extended defects occur when the relative volume of total defects exceeds ∼6 × 10−3. Romano et al. [18] discovered that amorphization of Ge crystal surfaces occurs when exposed to irradiation with Ge ions at 30 keV and a flux of 1 × 1014 n·cm−2. In addition, they observed that the depth of the amorphous layer increased with irradiation flux, showing a porous structure. Wallace et al. [19] compared the results of Si crystals irradiated with 0.5 MeV and 1.35 MeV Xe ions, and observed a clear correlation between the defect relaxation time constants and the ion energies. Baubekova et al. [20] investigated the annealing kinetics of MgO single crystals irradiated with 0.23 GeV Xe ions, and they noted a significant decrease in the migration energy of the interstitials due to the irradiation-induced special distribution of f-type centers and oxygen interstitials.
Currently, studies on the damage caused by heavy ion irradiation to CZTs are mainly concerned with their effects on the Au-CZT interface as well as on the photoelectric and detection properties [[21], [22], [23]]. In view of the close relationship between the crystal quality and the photoelectric properties, a number of studies have been conducted to explore the changes in the lattice structure of CZTs under specific conditions. Zhang et al. [[24], [25], [26], [27], [28]] discovered that CZT crystals more readily form nanocrystals or nano-twins, demonstrating their superior mechanical properties under pressure and deformation. Liu et al. [29] investigated the phase transition behavior of CZT crystal under a pressure of 1.75 GPa using in-situ TEM nanoindentation. Nanostructures consisting of single crystals, slip bands, stacking faults, superlattices and new tetragonal phases were observed. Li et al. [30] observed CuPt-A ordered phase with Te and Zn periodically arranged along (111) plane in CZT single crystals. The formation of this ordered phase may be attributed to the high localized Zn concentration. Reduction of stacking defects in CdTe thin films significantly improves the solar conversion efficiency [31,32], which further emphasizes the critical role of microstructural defects in photovoltaic functional materials. As a complex ternary compound semiconductor, CZT is prone to form microdefects such as dislocations and stacking faults during the growth process because of its low thermal conductivity and strong ionic bonds [33]. Collision cascades between heavy ions and CZT atoms as well as energy loss of phonons lead to a sharp increase in lattice temperature, which generates a large number of microdefects.
In order to elucidate the internal factors of irradiation leading to the deterioration of the photovoltaic properties of CZTs, this study investigates the evolution of microdefects introduced by heavy ions in the crystals. The distribution and structural characteristics of microdefects under different ion irradiation fluxes are deeply analyzed. Meanwhile, the heavy ion irradiation hardness of CZT crystals was evaluated, which provides a theoretical basis for irradiation damage recovery and irradiation protection strategies for similar semiconductor materials. In addition, these results provide a reference for the irradiation resistance of CZT in high-throughput irradiation environments.
## 2. Experimental methods
Three series of indium-doped CZT wafers with dimension of 10 × 10 × 1 mm3 prepared by Imdetek Ltd were utilized. The crystals were cut from the same position of an ingot, and the resistivity of the CZT crystals is ∼1010 Ω cm. Irradiation experiments were carried out at the State Key Laboratory of Nuclear Physics and Technology, Peking University. The irradiation process at room temperature involved 10 MeV Cl ions with the fluxes of 1 × 1010 n·cm−2, 1 × 1011 n·cm−2 and 1 × 1012 n·cm−2, respectively.
The energy loss and damage depth distribution of ions in CZT crystals were calculated using SRIM Monte Carlo simulation. Under the excitation wavelength of 532 nm and laser power of 2 mW, the lattice stresses of CZT crystals before and after irradiation were characterized by micro confocal Raman spectroscopy. The irradiation defects and crystal quality of CZT crystal were characterized using photoluminescence (PL) spectra under 488 nm Ar+ laser at 10 K wavelength.
Specimens for transmission electron microscopy (TEM) characterization were prepared from cross sections of irradiated surfaces by focused ion beam (FIB) milling. The obtained flakes were 6 × 6 μm in size and ∼50 nm in thickness. Subsequently, the flakes were subjected to low-energy gallium ion milling to eliminate the amorphous thin layers formed on both surfaces during the FIB process. High-resolution transmission electron microscopy (HRTEM, Talos 200 × microscope, Thermofisher) and dual Cs corrector transmission electron microscope were employed to examine the microstructure of irradiated CZT crystal. Bright field (BF), HRTEM, selected area electron diffraction (SAED), and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) techniques were employed.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/microstructure-evolution-of-cdznte-crystals-irradiated-by-heavy-ions.html