Zhe Huang, Siyuan Wu, Baishan Chen, Siwei Tang, Yunzhu Ma, Wensheng Liu
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, China
## Abstract
Defects deteriorate the optical and electronic properties of the
Cadmium Zinc Telluride (CZT) single crystal. However, the growth of defects, such as vacancies and precipitation, could not be avoided in the industrial-grade preparation of CZT single crystal. Therefore, it is crucial to explore ways to eliminate defects after growth. This paper employs one-step heat treatment with the CdTe compound as an annealing atmosphere source. After annealing, some large-size Te inclusions are eliminated in CZT crystal, but nano Te precipitates still exist. At the same time, the crystal surface becomes rougher, caused by the thermal migration of Te droplets, dislocation proliferation, and aggregation. Increasing annealing time, dislocations migrate, decompose, and interact with each other to form dislocation walls, stacking faults or terminating at the surface, reducing dislocation etch pit density. In addition, a large number of ordered phases are still found in the crystal. Single annealing with CdTe significantly improved the crystal's optical and electrical properties. After 180 h annealing, the infrared (IR) transmittance of the as-grown wafer increased from 40% to 65%, and resistivity increased from 2.2 × 109 Ω cm to 1.7 × 1011 Ω cm. Compared to reducing the Te precipitation phase, these increasements were mainly dominated by replacing point defects, like Cd vacancies.
## Introduction
With a wide band gap, high energy resolution, and adjustable lattice constant, cadmium zinc telluride (CZT) has been used as x-ray and gamma-ray detectors commonly in cutting-edge fields such as anti-terrorism equipment, medical imaging devices, and remote sensing satellites. Large-size and defect-free CZT single crystals are essential to the high performance of these devices [1,2]. However, because of its thermophysical characteristics, such as low heat conductivity and high equilibrium segregation coefficient, the growth conditions in the melt growth of industrial-grade CZT single crystal are complex and challenging to control. Since the inadequate and non-uniformly heat transfer around the solid-liquid interface, defects such as Cd vacancies (VCd), dislocations, and precipitations would be generated easily during crystal growth. These defects interrupt the periodicity of the lattice and degrade the optical and electrical performance of the CZT wafer [[3], [4], [5], [6]]. Many investigations have demonstrated that Te inclusions and precipitates are a major impediment to achieving good energy resolution in long-drift detectors. Moreover, extended defects such as dislocations have also been identified as an important performance-limiting factor, as they act as trapping centers and degrade carrier transport properties. Therefore, there is an urgent need to eliminate unwanted defects for the commercialization of large-volume, high-performance CZT detector devices [7].
Although various growth methods have been attempted, such as Modified Verticle Bridgeman method (MVB), Traveling Heater Method (THM), they have been unable to produce crystals with both low defect densities and high resistivity. Post-growth thermal annealing is considered a crucial step in achieving such high-performance devices. In recent researches, kinds of annealing post-treatments were proposed to eliminate the defects. Usually, Cd, Te, or CdZn powder was used as an annealing atmosphere source in heat treatment process. However, one-step annealing in Te-vapor or Cd-vapor caused a decrease in resistivity due to the change of free carriers and Fermi energy level, despite the reduction of defects, such as Te precipitates [[8], [9], [10]]. The two-step annealing method (first annealing in Cd-vapors, then in Te-vapors) can effectively eliminate the precipitated phase and maintain high resistivity. However, a different defect equilibrium and additional defects, dislocation cluster and prismatic defects, appeared after two-step annealing, with strong consequences on trapping carriers which lead to a decrease in device performance [[9], [10], [11]]. Therefore, a simple annealing process is needed, not only to maintain the chemical stoichiometry but also effectively reduce defects and improve performance.
In this paper, CZT single crystal was annealed one step with CdTe vapor as the annealing atmosphere source, which is more effective and straightforward than traditional methods. The mechanism of atomic migration during annealing is analyzed by the evolution of defects before and after annealing. Moreover, optical and electrical properties were tested to verify the improvement of the wafer quality.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/tailoring-the-defects-and-resistivity-in-cdznte-single-crystal-via-one-step-annealing-with-cdte-compound.html