Growth and characterization of CdTe single crystals for radiation detectors

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Abstract

To improve the productivity of CdTe radiation detectors, the crystal growth by traveling heater method (THM) as well as the quality of the fabricated detectors were investigated. In the THM growth, optimization of the solvent volume was found to be essential because it affects the shape of the growth interface. The use of the slightly tilted seed from 〈1 1 1〉B was also effective to limit the generation of twins having different directions. Single-crystal (1 1 1) wafers, larger than 30×30 mm2 were successfully obtained from a grown crystal of 50 mm diameter. Pt/CdTe/Pt detectors of dimensions 4×4×2 mm3, fabricated from the whole crystal ingot, showed an energy resolution (FWHM of 122 keV peak from a 57Co source) between 6% and 8%. Similarly, Pt/CdTe/In detectors of dimensions 2×2×0.5 mm3 showed a resolution better than 3%. These characteristics encourage the practical applications of various types of CdTe detectors.

Introduction

CdTe:Cl and CdZnTe crystals have been chosen as semiconductor materials for room-temperature-operating radiation detectors with both high quantum efficiency and high-energy resolution [1], [2]. CdTe:Cl is usually grown by the traveling heater method (THM), by which the crystal can be grown far below the melting point; the purity of the grown crystal is high enough to reduce the trap density [3]. In the THM grown crystal, Cl can be doped quite uniformly due to its very small segregation coefficient [4], and high-mobility-lifetime products are obtained throughout the crystal. These features of THM result in high yield of spectrometer grade detectors [3]. However, it has been thought that large single crystals suitable for practical applications cannot be grown by THM [5], probably because the diameter of crystals had been limited in early studies [6], [7], [8].

Recently, the demand for large-area crystal wafers has been increasing for applications in monolithic pixel or strip detectors. The previous research on CdZnTe monolithic detectors revealed that crystalline defects such as cracks, grain boundaries, twins and voids degrade the yield and performance of the detectors [9], [10], [11]. Consequently, it became clear that the wafers used in the detectors should be single crystals.

To improve the productivity of the various types of detectors, a THM growth technique for making CdTe:Cl single crystals of large diameter has been developed in our group. In this paper, we outline the growth technique and the performance of the fabricated detectors.

Section snippets

Design of THM furnace

Since the shape of the growth interface is considered to be the most important factor in growing a single crystal, the shape was predicted by calculating the temperature profile in the growing crystal using the finite element method. An example of the result is shown in Fig. 1. The results show that a smaller length of the heater element leads to improper shapes of the growth interface, such as a Gaussian-like shape (convex shape with inflection points), and that a longer length of the heater

The shape of the growth interface

The effects of the solvent volume (the length of the solvent zone) on the shape of the growth interface and the crystalline quality was investigated. Although the shapes of the growth interface in any experimental conditions were roughly convex towards the solvent, the detail of the shape was sensitive to the length of the solvent, as shown in Fig. 3. If the solvent length was longer than 50 mm or shorter than 30 mm, the convex shape included inflection points which was likely to introduce grain

Conclusions

CdTe single crystals of 50 mm diameter were successfully grown by the traveling heater method. The heater length and the solvent volume (zone length) were optimized to achieve the appropriate shape of the growth interface, and the seed orientation was chosen to limit the twins to one direction. The grown crystal was large enough to obtain (1 1 1)-oriented single-crystal wafers with an area as large as 30×30 mm2. Pt/CdTe/Pt detectors and Pt/CdTe/In detectors were fabricated to characterize the grown

Acknowledgements

The authors wish to thank Dr. Carl M. Stahle for his helpful discussion on this research.

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