Improvement of CdMnTe Detector Performance by MnTe Purification
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Improvement of CdMnTe Detector Performance by MnTe Purification K. H. Kim, A. E. Bolotnikov, G. S. Camarda, R. Tappero, A. Hossain, Y. Cui, G. Yang, R. Gul, and R. B. James Brookhaven National Laboratory, Upton, NY 11973, USA ABSTRACT Residual impurities in manganese (Mn) are a big obstacle to obtaining high- performance CdMnTe (CMT) X-ray and gamma-ray detectors. Generally, the zone-refining method is an effective way to improve the material’s purity. In this work, we purified the MnTe compounds combining the zone-refining method with molten Te that has a very high solubility. We confirmed the improved purity of the material by glow-discharge mass spectrometry (GDMS). We also found that CMT crystals from a multiple refined MnTe source, grown by the vertical Bridgman method, yielded better performing detectors. INTRODUCTION Although the physical properties of CdMnTe (CMT) and CdZnTe (CZT) are very similar, CMT crystals show poorer performance as X-ray detectors; furthermore, controlling their electrical properties was very difficult due to the high concentrations of residual impurities arising from the starting material [1,2]. Despite these drawbacks, one important difference favoring the usage of CMT rather than CZT is the fact that segregation coefficient of Mn in the CdTe host is close to 1, thereby ensuring a uniform alloy composition within large-volume ingots. Reportedly, cadmium vacancies in CdTe-based semiconductors are successfully compensated by doping with indium, chlorine, or aluminum [1,2]. The concentrations of native points defects (NPDs) in CdTe crystals at room temperatures (RT) is about 1013 – 1015 ɫm-3 [3,4]. Maintaining a concentration of residual impurities below this level ensures their minimal effect on the crystals’ electrical properties. Typically, the most effective way to minimize uncontrollable impurities overall by combinations of repeated vacuum-distillation and zone-refining [4,5]. Such methods resulted in impurity contents as low as 1015-1016 ɫm-3, depending upon the particular impurities. In our experiment, we improved the purity of MnTe via the molten Te zone-refining method, and verified it in the better performance of the resulting CMT detector. EXPERIMENT The CMT:In crystals were grown by the vertical Bridgman method using a mixture of CdTe (6N), MnTe, and excess Te. At high temperatures, Mn readily reacts with the quartz tubing even if its inner wall is completely carbon coated. The MnTe was synthesized at 1026 C at Terich conditions, i.e., with 44% of Mn (4N) and 56% of Te (6N). The source manganese first was cleaned with diluted HNO3 in methanol. Then, vitreous graphite tubing containing Mn and Te was loaded in quartz tube, and sealed under a vacuum of 10-6 torr. Then, the temperature was raised at a rate of 0.5 C/min to 960 C to avoid uncontrolled exothermic reactions, and maintained there for one week until the reaction was completed. Finally, we raised the temperature to 1026 C and held it there for 24 hours. We charged the synthesized MnTe in the carbon-coated quartz
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