Semi-insulating cadmium telluride at low impurity concentrations
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Semi-insulating cadmium telluride at low impurity concentrations M. Fiederle,a) V. Babentsov,b) A. Fauler, W. Witte, and K.W. Benz Kristallographisches Institut, Albert Ludwigs Universität, Hebelstrasse 25, Freiburger Materialforschungszentrum, Stefan-Meier-Strasse 21, Freiburg D-79104, Germany
R.B. James Energy, Environment and National Security Directorate, Brookhaven National Laboratory, Building 460, Upton, New York 11973 (Received 3 June 2003; accepted 3 October 2003)
We report a substantial reduction in the impurity concentration of semi-insulating CdTe:Ge grown by the vertical Bridgman method by using sublimation of the feed material. Specific resistivity (dark) values of up to 3 × 109 ⍀ cm were obtained for samples with a relatively high photosensitivity (PS) value and optimal compensation. Concentrations of impurities in the feed and as-grown crystals were determined by the glow discharge mass spectroscopy (GDMS) method. The energy levels in the band-gap were studied by photoluminescence (PL), and the data were correlated with the GDMS measurements. The highest values of dark and PS were observed in the regions where the PL bands via the deep levels of Ge and Te antisite were present. There is an increasing need for high-quality CdTe and (Cd,Zn)Te alloys for producing room temperature x- and ␥-ray detectors. Meeting the need requires that several problems with the growth and electrical compensation are overcome.1 Semi-insulating CdTe is obtained by the proper compensation of residual acceptors by the deep donor, such as Ge. Previously, it has been shown that detector-grade CdTe:Ge can be grown by the Bridgman method at a doping level of N ⳱ 1016 cm–3. It was also reported that reducing the impurity concentration to 1014–1015 cm–3 improves the spectroscopic ability of this material2,3 due to decreasing the trapping by deep levels. Thus, the optimal compensation and carrier lifetimes are reached at the lowest concentrations of residual impurities and dopants. To obtain optimal compensation and material properties, the vapor growth technique at lower temperature (850 °C) has been applied.4–7 In recent years, considerable effort to reduce the impurity concentration in CdTe crystals has also focused on purification of the feed material by the normal freezing, or sublimation– crystallization methods.8–11 The resulting undoped CdTe crystals were of high purity, but with resistivity lower than 107–108 ⍀ cm. Reference 11 mentions undoped a)
Address all correspondence to this author. e-mail: [email protected] b) Permanent address: Institute for Semiconductor Physics, Pr. Nauki, 45, Kiev 03028, Ukraine. J. Mater. Res., Vol. 19, No. 2, Feb 2004
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CdTe with a resistivity of 4 × 109 ⍀ cm, but the compensation mechanism was not studied. In this report, we present results on the growth and characterization of semi-insulating CdTe:Ge crystals with low concentrations of residual impurities (Na, Li, K, Cu, Ag). The low concentrations were achieved
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