Özden B. Balbaşı a b, Mustafa Ünal b c d, Ayşe Merve Genç b c, Gülçin Çelik b c, Mehmet Parlak a b c, Raşit Turan a b c d
a Department of Physics, Middle East Technical University (METU), 06800 Ankara, Turkey
b Crystal Growth Laboratory, Middle East Technical University (METU), 06800 Ankara, Turkey
c Center for Solar Energy Research and Applications (GÜNAM), Middle East Technical University, 06800 Ankara, Turkey
d Micro and Nanotechnology Program of Graduate School of Natural and Applied Sciences, Middle East Technical University (METU), 06800 Ankara, Turkey
##Abstract
Due to the requirement for superheating above the melting point in the homogenization process of CdZnTe material, complete seed melting remains as a significant problem for the seeded vertical gradient freeze (VGF) growth of CdZnTe. In addition, due to the typical low single crystal yields in VGF growth of CdZnTe, obtaining single crystal seed material with desired geometries could be considered as a challenge. In this study, single crystal CdZnTe seeds of cylindrical geometry with 10 mm diameter and up to 30 mm length were successfully obtained using a Computer Numerical Control (CNC) cutting system. Moreover, to avoid complete seed melting, a novel design of the growth crucible and support system along with radiation shielding around seed region is employed. Temperature profiling and growth experiments were conducted to observe seed melting and seeded crystal growth of CdZnTe. Initial results of a seeded VGF growth using a single crystal CdZnTe seed is reported.
## Introduction
Cadmium Zinc Telluride (CdZnTe) has been gaining popularity in recent years as a solid-state x-ray and gamma-ray detector material owing to its exceptional properties. High resistivity, low ionization energy, high atomic number, high energy resolution and large bandgap energy enables CdZnTe to be considered as an excellent candidate for room temperature detector applications [1]. In addition, CdZnTe can also be utilized as a substrate material for the epitaxial growth of Mercury Cadmium Telluride (HgCdTe) which is a key technology for far-infrared detectors with exceptional performance [2]. However, due to undesirable growth parameters such as high melt viscosity, and low thermal conductivity, stacking fault energy and critical resolved sheer stress, it remains a challenge to obtain CdZnTe ingots with high quality and good yields [3]. CdZnTe is most widely grown by directional solidification methods in which source materials are initially melted in a crucible and then solidified from the tip to the end. In order to achieve more control over the thermal environment during the growth, more complicated multi-zone furnaces are being developed. While some of these furnaces operate by moving the charge like Bridgman, others keep the charge fixed and move the thermal gradient using electronic control systems. Vertical Bridgman [4] including Vertical Gradient Freeze (VGF) [5], The Travelling Heater Method (THM) [6] and Vapor Phase Transport [7] are the most conventional and commonly used growth methods. Traditionally, seeded crystal growth techniques result in higher single crystal yield for bulk growth techniques since previously loaded single crystal seeds could act as a starting point for large area single crystal solidification process in the earlier stages of the growth. However, since superheating above the melting point is required for homogenization before growth, complete seed melting remains a problem detrimental to the seeded VGF method [8].
In this manuscript, preliminary studies on seeded VGF growth of CdZnTe are presented. Single crystalline cylindrical seeds were obtained from previously grown ingots using a CNC cutting system. The internal design of the VGF furnace was explored in order to prevent complete seed melting during the superheated homogenization stage.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/investigation-of-seeded-vertical-gradient-freeze-vgf-growth-of-cdznte-bulk-crystals.html