MOCVD Growth and Doping of ZnSe and Related II-VI Materials
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MOCVD GROWTH AND DOPING OF ZnSe AND RELATED Il-VI MATERIALS HIROSHI KUKIMOTO Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 227, Japan ABSTRACT Recent progress in metalorganic chemical vapor deposition (MOCVD) of wide bandgap fI-VI materials, especially of ZnSe, ZnS and their alloys, is discussed with emphasis on the general principles for obtaining uniform and high quality epitaxial layers and the current major issue of impurity doping for achieving conductivity control. The surface morphology and crystalline quality can be improved by a suitable choice of source materials and by latticematching the epitaxial layer to the substrate. By using appropriate sources, high conductivity ntype epitaxial layers of ZnSe and ZnS doped with impurities from group HI and VII of the periodic table have been successfully grown by low temperature MOCVD. We have also grown p-type ZnSe layers with carrier concentration ranging from low 1016 to high 1017 cm-3 using Li 3N as the dopant. Extensive studies are now focussed on the better p-type control. High purity source materials, appropriate p-type dopants and low temperature growth are important keys. INTRODUCTION Wide bandgap II-VI materials of ZnSe, ZnS and their alloys potentially offer optical devices operating in the short wavelength spectral region which cannot be covered by III-V materials other than nitrides. The fl-VI devices expected along this line includes blue lightemitting diodes and short-wavelength (green to blue) lasers. The studies of these wide bandgap fl-VI materials were extensively performed in the past, but encountered two major problems. One was the difficulty in growing high quality epitaxial layers on commercially available largearea subsrtates such as GaAs, GaP, InP and Si by the conventional techniques established for Ir-V epitaxial growth. Neither liquid phase epitaxy by using a variety of solvent metals nor conventional vapor phase epitaxy by using hydrogen carrier transport of constituent elements or by sublimation of compounds met a significant success, primarily owing to high vapor pressure of constituent elements (Zn, Se and S) at growth temperature. The other was the hardship of conductivity control by doping with donor and acceptor impurities due to so-called selfcompensation which is typical of these wide bandgap materials. Upon doping with a donor impurity in ZnSe or ZnS for achieving n-type conductivity control, for example, a Zn-vacancy acting as an acceptor tends to be generated, resulting in charge compensation. Similarly, p-type conductivity control is hampered by the formation of S- or Se-vacancies which act as the donors compensating doped acceptor impurities. Metalorganic chemical vapor deposition (MOCVD) has proven in recent years to be a very effective method for growing a variety of epitaxial layers of rn-V materials and their devices. The versatility of MOCVD has already been suggested in the early pioneering work of Manasevit [1]. The application of MOCVD for the growth
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