Mustafa Ünal a b, Özden Başar Balbaşı b c, Salar H. Sedani a d, Mehmet Can Karaman a b, Gülçin Çelik b, Deniz Bender a, Ayşe Merve Genç b, Mehmet Parlak b c, Raşit Turan a b c d
a Graduate School of Micro and Nanotechnology, Middle East Technical University, 06800 Ankara, Turkey
b Crystal Growth Laboratory, Middle East Technical University, 06800 Ankara, Turkey
c Physics Department, Middle East Technical University, 06800 Ankara, Turkey
d Centre for Solar Energy Research and Applications (ODTÜ-GÜNAM), Middle East Technical University, 06800 Ankara, Turkey
## Abstract
Production of CdZnTe crystals for radiation detector applications through Bridgman and Vertical Gradient Freeze (VGF) techniques is challenging in many aspects. Due to the structural characteristics of CdZnTe, it is not possible to obtain a large single crystalline ingots from the melt growth technique. In addition, the segregation of dopants and Zn decrease the yield significantly. In order to achieve detector-grade CdZnTe crystals, zinc segregation behavior should be fully understood, and optimum dopant concentration (In) should be determined. In this work, indium-doped Cd0.9Zn0.1Te single crystals grown via the Vertical Gradient Freeze (VGF) technique were compared in terms of dopant segregation, defect concentration, electron mobility, and electrical resistivity. It is observed that Zn segregates towards the tip of the ingot while In segregates towards the heel. Zn segregation coefficient is calculated as 1.35 while In segregation coefficient is found to be 0.4 for the VGF method. The variation of Zn throughout the ingot and excessive In dopant concentration degraded the detector performance. CdZnTe crystals having In doping concentrations of more than 5 ppm were unresponsive to gamma radiation. (µτ)e of VGF-grown crystals in METU Crystal Growth Laboratory is calculated as 4–5 × 10−4 cm2/V.
## Introduction
Among the semiconductor materials,
Cadmium Zinc Telluride (CdZnTe) has attracted much interest owing to its high atomic number, large band gap, and high intrinsic µτ product. This novel alloy exhibits an attractive combination of good charge transport properties, high resistivity, and low leakage currents [1], [2]. These properties altogether make CdZnTe suitable for a wide range of detector applications. However, the choice of the growth method, kinetics, point defects, and tellurium inclusions have a profound effect on the uniformity, which in turn degrades X-ray and gamma-ray detector performance. To date, it is still a challenge to achieve defect-free, large-volume yields with high electrical resistivity [3]. There have been many efforts to upgrade the quality and the electrical properties of the crystal by modifying several factors such as; crucible rotation [4], annealing conditions [5], and doping concentrations [6], [7], [8]. Even though sub-grain boundaries [9] and Te inclusions [10] have effect on detector performance, the resistivity and transport properties are largely affected by point defects. In order to achieve detector-grade Cd0.9Zn0.1Te crystals, Zn segregation behavior throughout the ingot should be fully understood, and optimum dopant concentration should be determined. Introducing dopants into the crystalline structure has proven to be an efficient way to increase the resistivity of the crystal. Resistivity is known to be controlled by the pinning of the Fermi level near midgap by intrinsic and extrinsic dopants through a balance between shallow and deep-level impurities [11], [12], [13]. Dopants act as donors in the CdZnTe crystal and compensate for existing defects, reducing the trap centers, which in turn improves charge collection efficiency and increases the resistivity. Doping, particularly with Indium (In), has been the focus of research due to the formation of shallow defects [13], [14], [15], [16], [17]. However, the exact concentration of indium dopant to increase the resistivity remains to be unclear. It is reported that low In concentration doesn’t have a profound effect on the resistivity and shows a high leakage current. On the other hand, doping of excessive indium does not convert CdZnTe:In into n-type because Fermi level pinning results in high resistive crystal but transport properties are weakened. The optimum In doping concentration reported in the literature has a large span ranging [18]. As a result, this study aims to investigate the relationship between the dopant concentration and electrical properties of CdZnTe:In crystals while preserving Zn concentration. In this paper, indium-doped CdZnTe crystals grown via the Vertical Gradient Freeze (VGF) technique were compared in terms of zinc segregation, dopant segregation, compositional uniformity, electron mobility-lifetime product, and electrical resistivity.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/production-of-detector-grade-cdznte-crystal-with-vgf-furnace-by-analyzing-segregation-of-zn-and-in.html