Growth and Characterization of ZnSeTe Epilayers and Superlattices
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GROWTH AND CHARACTERIZATION OF ZNSETE EPILAYERS AND SUPERLATTICES M. C. PHILLIPS, D. H. CHOW, D. T. J. Watson, Sr., *Hughes Research
Y. RAJAKARUNANAYAKE, J. 0. MCCALDIN, R. H. MILES*, A. COLLINS, T. C. MCGILL Laboratory of Applied Physics, Caltech, Pasadena, CA 91125 Laboratories, Malibu, CA 90265
ABSTRACT Little has been published about Te-rich ZnSeETel_= grown at low temperatures, in spite of some successes in the fabrication of wide band gap light emitting devices from ZnSeTe alloys grown at higher temperatures. We present x-ray diffraction and photoluminescence (PL) spectra for ZnSeTe epilayers and ZnSeTe/ZnTe superlattices grown by molecular beaCm epitaxy (MBE). These we compare with measurements on ZnTe, ZnSe and CdZnTe epilayers and on CdZnTe/ZnTe superlattices grown under similar conditions and also with data published for ZnSeTe alloys grown at high temperatures. Equilibrium phase diagrams for the ZnSeTe alloy system suggest a large miscibility gap at MBE growth temperatures; this may account for some unusual features in the (PL) spectra and for large line widths in the x-ray data. In spite of these possible miscibility problems, we find that ZnSeTe alloys luminesce brightly.
INTRODUCTION Extensive efforts during the 1960s to fabricate light emitters from wide band gap II-VI materials produced devices which were not efficient enough at room temperature to be commercially successful [1]. The high temperature growth techniques available at that time yielded II-VI materials of poor structural quality, and conventional doping did not provide p-n junctions in any of the binary compounds with band gaps larger than that of CdTe. No adequately lattice matched heterojunctions between p- and ntype binaries were found to have band alignments suitable for minority carrier injection. Aven et al. [2] made p-n homojunctions in the pseudobinary ZnSe.Tel_.. for roughly 0.1 < x < 0.6. Light emitting diodes made from these alloys were quite efficient at 70K, but very inefficient at room temperature due to the lack of a radiative center capable of competing with nonradiative recombination at impurities or structural defects [1-31. Sanguine expectations that the high purity and low temperatures of MBE growth would cure both the doping and the structural problems [4] have been only partially satisfied, with reliable doping of both conductivity types in ZnSe or ZnTe still an elusive goal [5]. The material quality of ZnSe and ZnTe is, however, greatly improved by MBE growth, so that it seems reasonable to revisit the ZnSeTe system, where radiative efficiency rather than doping was a problem. Curiously, we find only one report of the growth of ZnSe.Tel-. alloys by MBE for x not near 1 [6], and this growth study included no PL and little discussion of x-ray diffraction. Further work has concentrated exclusively on Te isoelectronic centers in ZnSe [71 and on highly strained ZnSe/ZnTe superlattices [8]. are results of PL and Given below x-ray characterization of Te-rich ZnSeTe epilayers and alloy-layer superlattices. We find quali
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