An examination of organometallic thermal stability and its relevance to low-temperature MOCVD growth of HgCdTe
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I. INTRODUCTION The MOCVD (metalorganic chemical vapor deposition) technique has been utilized with considerable success for the growth of a wide range of materials, particularly III-V and II-VI compounds. In the MOCVD process, the vapors from the precursor compounds are pyrolyzed on or in close proximity to a heated substrate. An important pyrolytic mechanism is free radical homolysis of the relatively weak carbon-metal bonds. Since a neighboring organic group can have a different effect on the stability of the parent compound and resultant free radicals, the stability of carbon-metal bonds can be altered. Consequently, the choice of the precursors can directly affect the pyrolytic growth process. The MOCVD growth of HgCdTe is an emerging technology for producing infrared detectors.1 An area of active research is the development of a low-temperature HgCdTe growth technique. Using the more conventional precursors of dimethylcadmium (DMCd), diethyltelluride (DETe), and elemental mercury, a growth temperature of approximately 400-420 °C is required. However, lower growth temperatures are desirable for several reasons. Diffusional processes are relatively rapid in HgCdTe at 400 °C resulting in broadened heterojunctions and homojunctions. In the extreme case of quantum-well structures, a growth temperature of approximately 200 °C is necessary to minimize interdiffusion. The growth of HgCdTe on foreign substrates can be adversely affected at 400 °C. Gallium outdiffusion from GaAs substrates is a concern at 400 °C as well as the thermal stability of InSb substrate surfaces. Finally, the density of mercury vacancies in HgCdTe at 400 °C is quite high, which can affect interdiffusion and substitutional doping. The potential for reducing the MOCVD growth J. Mater. Res. 3 (2), Mar/Apr 1988
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temperature is considerable. Using molecular beam epitaxy,2 MBE, high-quality epitaxial films have been grown at 180-200 °C, which demonstrates that there is sufficient surface kinetic energy for epitaxial HgCdTe growth at very low temperatures. With the proper precursors, similar growth temperatures should be possible by MOCVD. The 400 °C MOCVD growth temperature limit is due to the relatively high stability of DETe. As discussed below, a systematic reduction in growth temperature has been achieved using diisopropyltelluride3 (DIPTe), ditertiarybutyltelluride45 (DTBTe), and diallyltelluride6 (D ATe).
II. STABILITY CONSIDERATIONS Low-temperature HgCdTe growth requires the use of a tellurium compound less stable than DETe. The organic chemistry of tellurium is quite extensive7 with a wide range of possible compounds. Therefore, guidelines are needed for identifying promising low-temperature compounds. Hydrocarbon organometallic compounds possessing carbon-metal single bonds are considered here. Molecules containing other heteroatoms or metal-metal bonds (such as the ditellurides) are not considered. The rate determining step for the pyrolysis of organometallic compounds is the breaking of the carbonmetal bond
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