Jijun Zhang, Yongwu Qi, Wanping Liu, Jiongjiong Wei, Hao Liu, Xiaoyan Liang, Linjun Wang
School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
## Abstract
Nowadays, Traveling Heater Method (THM) is demonstrated to be one of the most promising ways to grow high-quality CdZnTe (CZT) crystals. The THM is essentially a melt-solution growth method, in which the CZT solute diffuses from the dissolution interface to the growth interface in the Te solution and deposits on the CZT seed. In this work, the effects of Te solution volumes on the structure of CZT solute, growth interface, defects and electrical properties of CZT crystals grown by THM were investigated. CZT crystals (φ30 × 80 mm3) were grown by THM with different volumes of Te solution (H/D = 1.5, 1, and 0.5, where H is the height and D is the diameter of Te solution). The distribution and morphology of CZT solutes in different Te solutions were investigated, which shown that the large-sized CZT solute in Te solution is more conducive to single-crystal growth and the growth temperature is calculated to be 806–809 °C. The growth interfaces correlated with Te inclusions in the as-grown CZT crystals were revealed, which shown that the CZT crystal grown with Te solution of H/D = 1 has a planar growth interface and the lowest Te concentration. The size of Te inclusion is mainly in the range of 3–10 μm in all CZT crystals grown by THM. The IR transmittance of different CZT wafers was conducted in the range of 4000–500 cm−1. The resistivity and energy spectra of CZT detectors under 241Am irradiation were measured at room temperature. The experiment results demonstrated that the Te solution volume with H/D = 1.0 is optimal for the stable growth of high-quality CZT crystals by THM.
## Introduction
Cd1-xZnxTe (CdZnTe, or CZT) is an II - VI ternary compound semiconductor in which the Cd element in CdTe matrix is substituted by a small amount of Zn element. The addition of Zn changes the lattice constant and bandgap of CZT [1], making it suitable for different applications. When x = 0.04, CZT is used as one of the most suitable substrate for epitaxial growth of HgCdTe which is an excellent material for high-performance mid-infrared (8–12 μm) detectors [2], [3], [4]. When x = 0.1, CZT is the main candidate material for room-temperature X-ray and gamma ray detectors, which were widely used in various applications such as nuclear medical imaging [5], [6], environmental monitoring [7], and space exploration [8].
At present, CZT crystals are mainly grown by Traveling Heater Method (THM), Vertical Bridgman (VB), and Vertical Gradient Freezing (VGF) methods. Compared to VB and VGF methods grown from melt, THM is essentially a melt-solution method, which has advantages of low growth temperature (700–900 °C), good components uniformity, high purity, and low defect density, making it a reliable way for growing detector-grade CZT crystals [9]. During the THM growth, the polycrystalline CZT source ingot dissolves into the Te solution region at the dissolution interface as solute, then transports to the growth interface and crystallizes on the seed under supercooling conditions. Stelian and Duffar [10] found that an eddy current flow existed in the Te solution region with the height H of 2.5 cm (H/D = 0.75, D is the ingot diameter) and accelerated the transport of Te solvent towards the growth interface, making an unstable growth interface. Peterson et al. [11] found that the flow rate in the Te solution region reduces significantly with the decrease of the Te solution region, and the growth interface transforms from concave to slightly convex. Jouglar et al. found that the growth temperature determines the maximum growth rate during THM growth under a certain temperature gradient [12]. Peterson et al. demonstrated that a large temperature gradient strengthens the back wind wave vortices near the growth interface and decreases the stability of the interface [11]. The above simulation results indicate that the volume of Te solution region has dominant effect on the THM growth of CZT, including the growth rate, flow rate, solute transport, and growth interface morphology. However, seldom work has been experimentally performed on the THM growth of CZT crystals with different Te solution volume.
In this work, CZT crystal ingots (φ 30 × 80 mm3) were grown by THM with different volumes of Te solution, and the morphology and composition distribution of Te solution zones were analyzed. The growth interface, Te inclusions, and electrical properties of the CZT crystals grown under different volumes of Te solution were characterized and analyzed. The influence of Te solution zones on the quality of CZT crystals grown by THM was investigated.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/the-effect-of-te-solution-volume-on-the-growth-of-cdznte-crystals-by-traveling-heater-method.html