The effects of two intrinsic deep levels on electrical compensation in semi-insulating CdTe and Cd-Zn-Te crystals are reported here. These levels were found in samples grown by conventional Bridgman and highpressure Bridgman techniques. The levels were observed with thermoelectric effect spectroscopy at distinct temperatures corresponding to thermal ionization energies of E d1 ϭE v ϩ0.735Ϯ0.005 eV and E d2 ϭE v ϩ0.743Ϯ0.005 eV. The first level is associated with the doubly ionized Cd vacancy acceptor and the second level was tentatively identified as the Te antisite (Te Cd ), which is thought to be complexed with a vacancy. The second level was found to electrically compensate CdTe and Cd-Zn-Te to produce high resistivity crystals, provided that the Cd vacancy concentration is sufficiently reduced during crystal growth or by post-growth thermal processing.The relatively wide band gap, large atomic numbers, and good carrier mobility render CdTe and Cd-Zn-Te ͑Refs. 1 and 2͒ as promising materials for room-temperature radiation sensor applications. High resistivity is necessary but not sufficient for production of CdTe and Cd-Zn-Te radiation detectors. To realize the maximum resistivity of CdTe and CdZn-Te crystals allowed by the band gap a net carrier concentration of 10 8 cm Ϫ3 or lower has to be achieved. Early attempts to produce high resistivity CdTe and Cd-Zn-Te crystals by common growth techniques resulted in materials with native defect or impurity concentrations on the order of 10 15 cm Ϫ3 and carrier concentrations orders of magnitude in excess of the requirements for intrinsic resistivity. Recently, high resistivity CdTe and Cd-Zn-Te crystals have been grown with both high-pressure Bridgman 3 ͑HPB͒ 4 and vertical Bridgman with overpressure control ͑VBOC͒ techniques. In this paper we show that intrinsic defects compensate CdTe and Cd-Zn-Te resulting in semi-insulating materials in spite of a total impurity concentration of ϳ10 15 cm Ϫ3 , which is achievable with current technologies.Samples of CdTe and Cd-Zn-Te were characterized with current-voltage measurements (I-V), glow discharge mass spectroscopy ͑GDMS͒, and thermoelectric effect spectroscopy ͑TEES͒. 5 The electrical transport properties of the samples including bulk electrical resistivity were studied by I-V measurements. The thermal activation energies of the various defect levels were extracted from TEES data. Results on four different representative CdTe and Cd-Zn-Te crystals are presented in this paper. The samples were grown by eV PRODUCTS using the HPB technique and by Johnson Matthey Electronics ͑JME, now Honeywell Electronic Materials͒ using the VBOC technique. The eV samples included Cd 0.9 Zn 0.1 Te and CdTe samples, as well as an Al-doped CdTe sample. 6 The JME samples were all Cd 0.85 Zn 0.15 Te. The total residual impurity concentration was on the order of 10 15 cm Ϫ3 for all samples studied. Glow-discharge mass spectroscopy ͑GDMS͒ data for selected elements, which are discussed in the literature as candidates for midgap trap, are shown...
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