MBE Growth of Mercury Cadmium Telluride: Issues and Practical Solutions
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MBE GROWTH OF MERCURY CADMIUM TELLURIDE: ISSUES AND PRACTICAL SOLUTIONS J. W. COOK, JR., K. A. HARRIS, and J. F. SCHETZINA North Carolina State University, Department of Physics, Raleigh, NC 27695-8202 ABSTRACT The growth of thin films of mercury-based materials by molecular beam epitaxy (MBE) presents significant experimental problems which must be overcome in order to successfully grow infrared detector materials such as mercury cadmium telluride (MCT). Many of the problems associated with the use of Hg in MBE arise from its high room temperature vapor pressure (2 mTorr) and its low sticking coefficient. The MBE system must be designed for Hg usage by considering such things as the ultra high vacuum pumping system, the Hg source, Hg containment, and Hg removal. In addition, Hg is a toxic heavy metal and must be handled appropriately. Other problems involved with the growth of MCT are associated with the design of the MBE furnaces which are used to evaporate cadmium telluride and tellurium. A system designed specifically for the growth of Hg-based materials has been designed and constructed at North Carolina State University. The features of this system which were included specifically to deal with the problems stated above will be discussed. In addition, information on the growth and characterization of MCT films deposited in the system will be presented. INTRODUCTION The first MBE growth of MCT by Faurie and Millionfl] in 1981 demonstrated the feasibility of using this elegant thin film growth technique for depositing epitaxial layers of this important infrared detector material. However, there has been very little discussion in the literature of the significant practical problems which must be overcome in order to successfully deposit MCT films by MBE. In this paper the problems are identified and solutions are presented. The solutions to the problems are discussed from the experience obtained at North Carolina State University in designing and constructing a MBE system specifically for the growth of MCT and other Hg-based materials. Many of the problems in the growth of MCT by MBE arise from the high vapor pressure of Hg at room temperature. Since MBE growth usually takes place at pressures in the 10-10 Torr range, the use of Hg necessitates the use of extensive liquid nitrogen shrouding for Hg containment and special sources which allow the Hg be confined when film growth is not in progress. In addition, care must be taken in choosing a suitable ultra high vacuum pumping system for the growth chamber. It should be noted that the ion pumping systems commonly used in the MBE growth of III-V materials can be severely damaged if Hg inadvertently enters the pumps. Other problems arise because of the low sticking coefficient of Hg. Mat. Res. Soc. Symp. Proc. Vol. 90.
1987 Materials Research Society
420
While Faurie and Million first overcame the high vapor pressure problem of Hg by using HgTe and a source of Hg, they found that it was necessary to use elemental Hg in order to obtain a large enough flux to compe
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