Ion-Implantation Getiering of Impurities in CdTe
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ION-IMPLANTATION GETIERING OF IMPURITIES IN CdTe M. H. Jin, K. M. James, C. E. Jones 1 and J. L. Merz
Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106 ABSTRACT
This is the first reported use of ion-implantation damage gettering of impurities in CdTe to provide high-quality substrates for the epitaxial growth of appropriate binary or ternary compounds, or for related applications. We describe the results of photoluminescence (PL) measurements performed on samples of Bridgeman-grown CdTe to study both the annealing behavior and gettering effects in this material. From the PL results, it was found that impurity gettering occurs at temperatures at which liquid phase epitaxy take place (-500°C) so that these two fabrication procedures are compatible. It was also found that the optimum anneal time at this temperature is four hours. INTRODUCTION CdTe is an extremely useful material for optical devices which require a direct bandgap in the near infrared. Bulk CdTe is of interest as a substrate for the epitaxial growth of HgCdTe, and for the formation of superlattices in combination with other II-VI compounds. However, in order to properly control the liquid phase epitaxial (LPE) growth of HgCdTe on CdTe, a knowledge of the annealing behavior of the CdTe substrate is important. It is also necessary to identify and eliminate unwanted impurities in the CdTe that would diffuse into epitaxial layers during growth. One technique for the removal of impurities involves gettering. Considerable work has been done with both ion-implantation damage gettering and surface gettering on Si and on GaAs [1,2,3]. However, this technique has not yet been applied to CdTe. It is possible that ion-implantation damage gettering could be useful in providing better substrates for epitaxial growth or for other applications. It is especially important to study the annealing behavior and gettering effects at the growth temperatures used for LPE. EXPERIMENTAL TECHNIQUES Eight samples studied in this investigation were taken from the same slice of a Bridgeman-grown boule of CdTe. The crystals were not intentionally doped and they are high purity p-type (-lEl4cm-3 ). All surfaces are of (111) orientation. Low temperature (1.4K) PL experiments were performed on all of these samples in order to study the uniformity of the original samples. The samples were illuminated with the 5145A line of an Ar+ laser with an incident power of 1.2mW focused on the samples. The diameter of the focused laser spot was approximately 200 gtm. The PL data on as-received samples showed that there was no significant variation either along the surface of the samples or as a function of depth below the surface, which was determined by step-etching the surface. Three of these samples were ion-implanted with 200keV Kr++ ions on one side of the wafer at room temperature, to a dose of lEl6cm-2 ; the equivalent energy for Kr+ ions was therefore 400keV. These samples were then annealed at 5000 C, which is in the temperature range use
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