Shangshu Li, Chao Xu, Changhe Zhou
National Key Laboratory of Infrared Detection Technologies, Shanghai Institute of Technical Physics (SITP), Chinese Academy of Sciences (CAS), Shanghai 200083, China
## Abstract
A convex growth interface shape is desired but is usually hard to form during the Bridgman process of CdZnTe crystal due to its low thermal conductivity. In this work, the convex interface shape was first analyzed and the mathematical criterion was determined. Theoretical analysis indicates that a convex interface can form when the derivative of axial temperature gradient () near the interface is negative. The temperature profiles were then proposed according to the theoretical results, and correlative temperature gradient characteristics were presented. The 90 mm diameter CdZnTe crystal has been grown by VGF technique using the proposed temperature profiles. The experimental result shows the interface shape is always convex throughout the entire growth run, which could make a better CdZnTe single-crystalline yield possible.
## Introduction
CdZnTe has drawn much attention for its application in radiation detection, photovoltaic devices, and HgCdTe infrared focal plane arrays (IRFPAs) [1], [2], [3], [4], [5], [6]. Travelling Heat Method (THM) and Bridgman process (including Vertical Bridgman (VB) and Vertical Gradient Freeze (VGF)) are the main CdZnTe bulk crystal growth techniques [7]. Controlling growth interface is crucial to bulk crystal growth. During CdZnTe THM process, a convex interface shape significantly gives rise to single-crystalline growth [8], [9]. On the other hand, maintaining a convex interface shape during CdZnTe Bridgman growth is relatively difficult because of the low thermal conductivity.
The influences of operating parameters and thermal environment on interface shape have been investigated in many simulation works [10], [11], [12], [13], [14]. However, there are few reports (especially experimental results) on the interface shape evolution during CdZnTe VGF growth process. In our previous work [15], both simulation and experiment results show that interface shape evolves from convex into concave, and the convex interface growth length can reach 40 mm by using a quartz&graphite composite support. It seems difficult to achieve convex interface growth during entire VGF process by configuration design alone. For VB process, the localized heating method with a booster heater can be used to control growth interface shape [16], [17]. In detail, the localized heating beside the growth front could lead to a convex interface shape because it maintains the inward radial heat fluxes during VB growth. Inspired by the idea of radial heat flux control, we considered that a similar effect may be realized in the VGF growth due to the more flexible thermal control of multi-zone VGF furnace. Thus, in this work we proposed the novel temperature profile based on the radial heat flux control, that can result in a convex interface during the entire CdZnTe VGF growth. The mathematical criterion for convex interface shape was derived from basic heat conduction theory. The temperature profiles of different growth stages were given according to the theoretical criterion, and the preliminary CdZnTe growth results were presented.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/controllable-convex-interface-shape-during-vertical-gradient-freeze-vgf-growth-of-cdznte-crystal.html