Identification of Cl and Na Impurities in Inclusions of a Vapor-grown CdTe Doped with Zn and Cl
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M. Fiederle, T. Feltgen, and K. Benz Kristallographisches Institut, Universitat Freiburg, Hebelstrasse 25, D-79104, Freiburg, Germany
E. Diegueza) Departamento de Fisica de Materiales, Universidad Autonoma de Madrid, 28049 Madrid, Spain (Received 9 October 2001; accepted 13 February 2002)
Morphology and analysis of composition of inclusions were done by secondary electron microscopy and spatially resolved energy-dispersive analysis of x-ray on semiintrinsic CdTe:Cl and CdTe:Zn:Cl crystals grown from the vapor phase by the modified Markov technique and on undoped CdTe crystals grown from the melt by the Bridgman method. In CdTe:Cl and CdTe:Zn:Cl crystals nonstoichiometric inclusions of about 10–20 m were found, which contain high concentrations of Cl and Na impurities. The Cl is concentrated in small precipitates of 1–2 m inside these inclusions. After short-time low-temperature annealing (600 °C), the inclusions mostly disappeared.
I. INTRODUCTION
Potential industrial application of room-temperature x-ray detectors based on semiinsulating cadmium telluride requires the homogeneity of electrical properties and low concentration of native defects and impurities.1 In many cases Te-rich precipitates and inclusions enriched with impurities are the main sources of inhomogeneity of CdTe-based materials,2– 4 which creates the problem of resistivity control at the postgrowth heat treatment and during the operational period. As it was earlier shown,5,6 in CdTe crystals grown from the melt, tellurium precipitates are formed from native defects (VCd Tei) during the cooling process due to the strong retrograde character of Te solubility in solid CdTe, while the Te-rich inclusions are originated by melt droplet capture on the growth surface. This phenomenon leads to inhomogeneous distribution of impurities due to segregation. Moreover, taking into account that the solubility of many impurities in liquid Te is higher than in solid CdTe, the concentration of residual impurities in Te-rich defects can be considerably higher than in bulk material.7 Commercially available semiintrinsic CdTe has always a high concentration of the doping impurity, particularly chlorine,8–10 and it is well known that the
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 5, May 2002
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main Cl-related defects are a shallow donor (ClTe) and acceptor-like complex (ClTe–VCd).11,12 It is also known that the nonuniformity of the Cl distribution leads to deterioration of the resistivity uniformity,13 but so far a limited study of Cl precipitation has been done14 and to the best of our knowledge this problem is important both for the melt- and vapor-grown CdTe materials. This paper presents the results of the chemical analysis of inclusions in CdTe:Cl and CdTe:Zn:Cl grown from the vapor phase and CdTe grown from the melt for the comparison following our previous study of Te-rich inclusions and the deviation from the stoichiometry in the boundary
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