P. Vijayakumar a, Edward Prabu Amaladass a b, K. Ganesan a b, R.M. Sarguna a, Varsha Roy a, S. Ganesamoorthy a b
a Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, 603 102, India
b Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai, 400094, India
## Abstract
We report on the indigenous design and development of laboratory scale travelling heater method (THM) system to grow detector grade Cd0.9Zn0.1Te (CdZnTe) single crystals. THM system mainly consists of two-zone furnace with a tuneable temperature gradient (30–80 °C/cm), high precision translation (1–25 mm/day) and rotation (1–50 rpm) assemblies to meet the stringent conditions that are essential to grow detector grade CdZnTe single crystals. Further, a load cell in the THM system enables continuous monitoring of the growth. Systematic growth experiments were performed to optimize the various growth parameters in order to achieve large grain single crystals. Herein, the effect of temperature gradient and growth rate on the increase in grain size is discussed in detail. Each successful growth experiment yields a minimum of four detector grade elements of dimensions 10 × 10 × 5 mm3 from a starting charge of 100 g of CdZnTe. The crystalline nature and quality of the detector elements were evaluated using Laue, NIR transmission spectroscopy and I–V characteristics. Crystals with resistivity greater than ∼109 - 1010 Ω-cm were identified for testing gamma ray detection. The photo peak of 137Cs was resolved with an energy resolution of 4.2% at 662 keV and its measured electron mobility lifetime product is found to be ∼3.3 × 10−3 cm2/V. The demonstration of the gamma ray detection with a relatively high μτ product is the testimony to the successful growth of detector grade CdZnTe single crystals by an indigenously developed THM system.
## Introduction
Cadmium Zinc Telluride (CdZnTe) is one of the most promising room-temperature semiconductor gamma radiation detector material due to its wide energy band gap and high atomic number [[1], [2], [3], [4]]. The research activities carried out on detector technology over the past few decades have showcased that CdZnTe is an excellent substitute for scintillation detectors in terms of high sensitivity, better energy resolution and high spatial resolution at room temperature [[4], [5], [6], [7]]. In the recent past, CdZnTe detector with an energy resolution of less than ≈0.5% at 662 keV of 137Cs had already been demonstrated [7] and commercial detectors are also available in the market. Even though its resolution is inferior to the resolution exhibited by HPGe detectors (0.2% at 662 keV), the ease of operation with no cooling requirements renders the superiority of CdZnTe detectors for gamma-ray spectrometric applications. Moreover, the pixelated CdZnTe detectors are also becoming indispensable for room temperature detection of hard X-rays in medical and industrial imaging applications [2,7,8]. Despite having several advantages, its high production cost and the complications involved in the crystal growth impose huge challenges to grow large volume detector grade CdZnTe crystals at reasonable cost. Presently, only a few crystal growth industries are involved in the production and commercialization of large size CdZnTe single crystals for gamma radiation detector and spectroscopic imaging applications. But, the availability of detector grade CdZnTe crystals/wafers with homogeneous and low defect density is still limited. This leads to an exploration of crystal growth methods at laboratory scale in pursuit to obtain detector grade CdZnTe crystals. However, the successful demonstration of growth of detector grade CdZnTe single crystals by academia is still limited in the literature.
The growth of CdZnTe crystals was initially started with different varieties of Bridgman (BM) techniques, such as vertical BM, horizontal BM and high pressure BM methods [[9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]]. Further, the vertical gradient freeze (VGF), travelling heater method (THM) and physical vapour transport were also used for the growth of CdZnTe single crystals. In Bridgman and VGF techniques, the crystal growth is conducted at the melting point of CdZnTe (1115 °C) which results in continuous loss of Cd, Zn segregation, and compositional inhomogeneity along the growth axis. Also, the melt becomes rich in Te during growth. Further, the Te inclusions with sizes ranging from 5 to 60 μm occur depending upon the growth conditions. Note that the Te precipitates/inclusions, especially those larger than 10 μm, causes electron trapping leading to deterioration in the signal quality and severe degradation of detector performance [20].
The THM is the most successful crystal growth method that has been shown to produce detector-grade CdTe and CdZnTe crystals consistently by different industries including ACRORAD, Co., Ltd, Japan [3,4,13,[16], [17], [18], [19],21]. The main advantage of THM is the possibility of growing crystals at lower growth temperature due to the addition of excess Te which is used as a solvent. Since the growth temperature is low and only a partial solute is in the molten state, the defect density and Te precipitates can be minimized significantly and also uniform Zn concentration along the radial and axial growth directions can be obtained. However, the significant advantages of THM are hampered by slow growth rate of ∼1–5 mm/day and the formation of multi-grains which limits the possibility of obtaining useable volume of detector grade single crystal from the entire volume of the grown ingot. Hence, one needs to have highly sophisticated crystal growth facility with fine tunability on various process parameters to control the crystal growth kinetics which enables high structural quality and low defect density CdZnTe single crystals. Further, the optimization of THM process parameters leading to single large grain is an extremely challenging task.
In literature, several reports are available on the growth of CdZnTe single crystals by THM. However, it is understood that the growth of high-quality CdZnTe single crystals involves several technical challenges due to the inherent materials properties such as very low thermal conductivity (0.01 W/cm.K), high segregation co-efficient of Zn (Keff ∼1.35) and propensity to defects due to high vapour pressure of Cd and growth instability at the solid-liquid interface. The THM system can overcome some of these challenges with its unique design. The THM system was initially developed for the growth of HgCdTe for infrared imaging applications and later, THM is extensively used for growth of CdTe based II-VI compounds [[15], [16], [17], [18], [19],[22], [23], [24]].
In this report, we highlight the indigenous design and development of laboratory scale THM system for the growth of small volume CdZnTe crystal in a small diameter quartz ampoule but capable of yielding a large useable volume of detector elements from a single growth run. To achieve this goal, a large number of growth experiments were performed systematically. Herewith, we discuss the effect of temperature gradient and growth rate on the growth of large grain single crystals with a diameter of 20 mm. After the successful growth, the CdZnTe crystals were characterized and the suitable CdZnTe detector elements were tested for the detection of gamma rays from 137Cs source and the results are discussed here.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/development-of-travelling-heater-method-for-growth-of-detector-grade-cdznte-single-crystals.html