Solubility of Impurities and Defect Impurity Interaction In II-VI Semiconductors
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SOLUBILITY OF IMPURITIES AND DEFECT IMPURITY INTERACTION IN II-VI SEMICONDUCTORS Y. MARFAING Laboratoire de Physique des Solides de Bellevue, C.N.R.S. - 92195 Meudon-Cedex, France ABSTRACT A description of incorporation and solubility limit of substitutional impurities is made using the alloy CdxHgloxTe as a model of analysis and a source of experimental data. Then non-equilibrium incorporation of impurities under light excitation is considered. A model of photo assisted doping is presented which accounts for the high doping efficiency reported for the donor indium in photoassisted grown CdTe. 1. INTRODUCTION Impurity doping of II-VI semiconductors poses particular problems : strong interaction of impurities with native defects often occurs leading to self-compensation phenomena ; moreover the solubility of electrically active impurities is in any case not very high, limited to a few 1018 cm- 3 . This result is in contrast with impurity doping of Si or III-V compounds where active concentrations above 1019 cm3 are commonly obtained. In the past, the attention to these problems has mainly been focused on wide band gap II-VI compounds (ZnSe, ZnTe, CdS) for the reason that very severe self-compensation phenomena prevent the conversion of conductivity type in these materials. Theoretical models have been worked out based on the chemical and electrical relations which link impurities and native defects under the conditions of thermodynamic equilibrium [1] [2] [3]. They provide an interpretation of compensation effects in terms of lattice defect properties : energy of formation, electronic energy levels. As these quantities are not a priori known, comparison with experiments does not lead to firm and definite conclusions. Another way of approach is to consider the low band gap materials - essentially the alloy CdxHgl-xTe - for which a larger amount of data is available. Incorporation of impurities has been carried out in a wide range of temperature and mercury vapour pressure, either during growth (Bridgman, LPE, MOCVD, MBE) or in post-growth anneals. In the first part of this paper we will show that a detailed analysis of these results provides a general understanding of the solubility limits in II-VI materials, in terms of impurity-defect interaction. Putting the problem of effective solubility in this way leads to the idea of non-equilibrium incorporation, where the feedback between impurities and defects might be relaxed. Photoassisted doping could reach such a result according to experiments recently reported in the literature. We will present a simple model of incorporation of impurities during epitaxial growth under light excitation which is able to explain some of the observed effects. This could open up the way to more systematic studies aiming to dope reproducibly the reluctant wide band gap materials.
Mat. Res. Soc. Symp. Proc. Vol. 161. ©1990 Materials Research Society
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DEFECT IMPURITY INTERACTION IN CdXHg 1_xTe ALLOYS
Due to the large number of data available CdxHglxTe is an exemplary medium for s
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