Characterization of semi-insulating CdTe crystals grown by horizontal seeded physical vapor transport

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Abstract

CdTe crystals were grown by horizontal seeded physical vapor transport technique in uncoated and boron nitride coated fused silica ampoules with the source materials near the congruent sublimation condition. The grown crystals were characterized by current–voltage measurements, low-temperature photoluminescence spectroscopy, near IR transmission optical microscopy, spark source mass spectroscopy and chemical etching. The measured resistivities of the crystals were in the high-108 Ω cm range. The photoluminescence spectra of the crystal grown in the boron nitride coated ampoule showed similar features previously observed in the CdTe crystals doped with group III elements. Although the crystal was contaminated with boron, the boron nitride coating of the growth ampoule has yielded a single crystal with no inclusions or precipitates.

Introduction

High-resistivity CdTe crystals with high mobility-lifetime product are needed for room temperature nuclear radiation detectors 1, 2, 3, 4, 5, 6, 7. The desired high resistivity in CdTe is usually accomplished by the introduction of dopants, such as halogens 8, 9, 10. In Ref. [6], Ti 11, 12or V 11, 12, to compensate for the native defect centers, which are generally thought to be related to Cd vacancies although a Te anti-site defect was proposed recently as the predominant native defect 13, 14. The requirement of a high mobility-lifetime product means that the crystal needs to show good crystalline quality and have a low concentration of defects, including native or impurity point defects as well as the two- and three-dimensional structural defects such as grain boundaries and Te inclusions or precipitates.

In the past, the main method used to grow detector grade CdTe material was the traveling heater method (THM) from Te-rich solvent 2, 4, 5, 15, 16, 17. This low-temperature technique has some advantages over the other high-temperature growth methods, such as the Bridgman technique, in that it results in less thermal strain in the grown crystals and it extracted impurities 2, 15, 16as well as reduced possible contamination from or through the fused silica ampoule 2, 18. The disadvantage of this non-stoichiometric (Te-excess) growth condition is the introduction of Te inclusions and precipitates as well as the native defects such as Cd vacancies [6]. Consequently, in previous investigation doping was necessary to compensate for the native-point defects. The resistivity of the undoped or doped sample grown by the THM is generally in the 105–109 Ω cm range 17, 19.

Vapor growth techniques offer similar advantages as the THM in that the growth temperature is considerably lower than the melting point. Physical vapor transport (PVT) and its various modifications were used to grow undoped and doped CdTe 9, 10, 20, 21, 22. The resistivity of the undoped CdTe crystals varied from 1×103 to 3×109 Ω cm. This variation was mainly determined by the deviations from stoichiometry of the samples. The Cl- and V-doped CdTe samples grown by PVT showed a resistivity as high as 1.5×1010 Ω cm, however, the problem of polarization causing time instability or degradation during the performance of the nuclear detectors has been reported on Cl-doped CdTe [23]. In theory, the highest resistivity that a CdTe crystal can possibly achieve can be estimated from intrinsic carrier concentration as determined from the energy band gap and carrier mobilities. An intrinsic CdTe sample has no electrically active defects, including native-point defects such as vacancies, interstitials, anti-sites, etc., as well as foreign impurities. In principle, with accurate control of the stoichiometry during the PVT growth process one should obtain CdTe crystal with high resistivity. However, it is difficult to reproducibly grow CdTe crystals with the desired electrical properties and free from Te-precipitates when the partial pressures of the predominant vapor species, Cd and Te2, vary over orders of magnitude at the growth temperature due to deviations from stoichiometry of the source materials [24].

Additionally, oxygen and other impurities from fused silica crucibles can cause significant contamination to the grown crystals during the growth process even with the employment of carbon coated fused silica ampoules. Therefore, about a decade ago, the high-pressure Bridgman (HPB) method was adopted to grow high-resistivity crystals of undoped CdTe and CdZnTe 25, 26. With the high inert pressure applied over the growth chamber, graphite could be used as the growth crucible. Using the HPB technique, the undoped CdTe crystals showed a resistivity as high as 3×109 Ω cm, although the problem of Te-inclusions and precipitates, due to the retrograde Te solubility of CdTe at the elevated growth temperatures, has not been solved.

Recently, a heat-treatment technique on the CdTe starting material for the PVT process aiming at adjusting the vapor-phase stoichiometry toward that of the congruent sublimation condition has been demonstrated [27]. In this paper, we report the results of the growth and characterization of the CdTe crystals grown by a horizontal seeded PVT process using this heat treatment technique. The prevention of contamination from the fused silica ampoules was also investigated by growing the CdTe crystal in a boron nitride (BN) coated (inside) fused silica ampoule. The quality of the grown crystals were assessed by current–voltage measurements, low-temperature photoluminescence (PL) spectroscopy, near IR transmission optical microscopy, spark source mass spectroscopy (SSMS) and chemical etching.

Section snippets

Experimental procedure

The starting materials were homogenized from pure elements. The homogenization ampoules were made from 24 mm OD, 20 mm ID (24×20) fused silica tubing supplied by Heraeus Amersil, Inc. The starting elements were quadruple-zone refined (QZR) or double zone refined, six–nine grade, Cd rods and QZR, six–nine grade Te bars from Johnson Matthey, Inc. The elements were weighed to the accuracy of 0.1 mg. A total amount of 100–220 g of the pure elements were loaded into each ampoule with the nominal Te

Results and discussion

Table 1 gives the SSMS results on the impurity levels in the homogenized CdTe starting material and in the grown CdTe crystal. The major impurities in both samples are S and In and a low level of Li and S contamination was found from the growth process.

Table 2 gives the resistivities and current density of both CdTe-10 and CdTe-22 crystals. Fig. 2 shows the IV curves of these samples. The intrinsic carrier concentration of CdTe at 300 K is about 1×106 cm−3 [29]and the highest mobilities measured

Conclusions

In this article, we have presented the results on the growth and characterization of CdTe crystals grown by horizontal seeded PVT with the source material near the congruent sublimation condition. The prevention of contamination from the fused silica ampoules was also investigated by growing the CdTe crystal in a boron nitride (BN) coated fused silica ampoule. The resistivities of the grown crystals were in the high-108 Ω cm range. The PL spectra of the crystal grown in the BN coated ampoule

Acknowledgements

This work was supported by National Aeronautics and Space Administration through the Fisk University center for Photonic Materials and Devices, Grant No. NAGW-2925 and by Microgravity Research Division, National Aeronautics and Space Administration.

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