Zhe Huang, Siyuan Wu, Baishan Chen, Yunzhu Ma, Siwei Tang, Wensheng Liu
National Key Laboratory of Science and Technology on High-strength Structural Materials, Central South University, Changsha, Hunan 410083, China
## Abstract
In this study, heat treatments were employed using a CdCl2 solution as the annealing medium to investigate the evolution of internal defects and photoelectric properties of CdZnTe crystals. It was observed that CdCl2 crystals formed on the surface of the annealed crystals, and their size increased exponentially with prolonged annealing time. It could be a medium for Cl transport. The liquid-phase annealing process was found to be gentle, resulting in minimal damage and the absence of large defects migrating or accumulating dislocations and less nano-defects on the crystal surface than sample annealed in vapor condition. After a 30 hour annealing, the infrared (IR) transmittance reached 55%, and the resistivity increased to 1.36×1010 Ω·cm. The presence of Cd2+ facilitated the filling of Cd vacancies (VCd) defects through diffusion reduction, while the diffusion of Cl- introduced complex defect with extra VCd, causing the Fermi level to approach the center of the band gap. Consequently, a high-resistance CdZnTe material was achieved.
## Introduction
Cadmium Zinc Telluride (CdZnTe or CZT) is highly regarded for its exceptional properties, including a wide band gap, high energy resolution, and adjustable lattice constant. Given the increasing demand for nuclear security, CZT has become a preferred material for room-temperature detection of X-rays and gamma-rays in advanced applications such as nuclear radiation detectors, anti-terrorism equipment, and medical imaging devices. However, to optimize the performance of CZT devices, it is crucial to achieve large, defect-free single crystals[1]. The growth of CZT single crystals for industrial purposes presents complex challenges due to specific thermophysical properties, such as low heat conductivity and a high equilibrium segregation coefficient[2], [3], [4], [5]. These properties can lead to inadequate and non-uniform heat transfer around the solid-liquid interface, resulting in the formation of defects, including vacancies (VCd), dislocations, and precipitates/inclusions, which disrupt the regularity of the crystal lattice and degrade the optical and electrical performance of CZT wafers. To overcome these challenges, post-treatments, particularly annealing, are necessary. Traditional annealing atmospheres generated from either cadmium (Cd) or tellurium (Te) powder or compacts have been used. However, annealing in a single Te-rich or Cd-rich atmosphere can decrease resistivity due to changes in free carrier generation and the Fermi energy level[6], [7], [8]. Currently, the two-step annealing method, involving an initial annealing step in a Te-rich atmosphere followed by a subsequent annealing step in a Cd-rich atmosphere, effectively eliminates the precipitated phase and maintains high resistivity in CZT crystals[9], [10]. Nevertheless, this method can introduce new defects, such as prismatic defects and increased dislocation density[11], [12], [13]. Furthermore, in traditional vapor phase annealing processes, the migration of Te atoms or droplets to the crystal surface through diffusion or temperature gradients can cause damage to the crystal surface and generate thermal stress, leading to the proliferation of dislocations. To ensure the purity of the reaction conditions, vapor phase annealing requires a large vacuum chamber capable of accommodating increasingly larger single crystals, thereby increasing equipment costs and operational complexity. If a molten metal is used as the annealing source, which requires high temperatures, the reaction becomes excessively strong and difficult to regulate. In comparison, the liquid annealing environment of the liquid phase offers greater control over the defect reaction process and requires simpler equipment. Liquids possess a fluid nature and lack a definite shape, allowing liquid-phase processing to be independent of sample shape and size, effectively flowing and covering the crystal surfaces. Therefore, liquid phase annealing represents a promising approach for achieving non-destructive enhancement of defects in single crystal samples.
A novel, damage-free annealing method applicable to single crystals of any shape is proposed in this study. Specifically, CZT single crystal wafers were annealed and modified in a CdCl2 solution. The surface morphology of the crystals after annealing in the liquid phase environment was examined. The evolution of defects within the crystal was investigated, and the optical and electrical properties were compared to those obtained through vapor traditional annealing. The observed improvement in performance is primarily attributed to the reduction of Vcd compensated by Cl and Cdi and the alteration of the Fermi energy level. The relatively mild and damage-free nature of liquid-phase annealing on the crystal surface opens up new avenues for regulating defects in large single crystals.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/enhanced-performance-cdznte-single-crystal-with-few-surface-damages-via-solution-based-annealing.html