New Organotellurium Precursors for the Pyrolytic and Photolytic Deposition of Hg 1-x Cd x Te
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NEW ORGANOTELLURIUM PRECURSORS FOR THE PYROLYTIC AND PHOTOLYTIC DEPOSITION OF Hg1 _xCdxTe ROBERT W. GEDRIDGE, JR.,* KELVIN T. HIGA, AND ROBIN A.NISSAN Chemistry Division, Research Department, Naval Weapons Center, China Lake, CA 93555 ABSTRACT Organometallic precursors with low decomposition temperatures are essential in the fabrication of high performance mercury cadmium telluride (Hgl-xCdxTe) infrared detectors by pyrolytic and photolytic metal-organic chemical vapor deposition (MOCVD). Film growth temperature is governed by the relative stability and/or reactivity of the organotellurium precursor, which is determined by the strength of the Te-C bonds. Since the rate-determining step in the pyrolysis of organometallic compounds involves bond breaking and free radical formation, we have concentrated on the synthesis of a variety of organotellurium precursors with substituents that possess low activation energies for the formation of hydrocarbon free radicals. The synthesis, characterization, and properties of methylallyltelluride, ethylallyltelluride, isopropylallyltelluride, tertiarybutylallyl-telluride, methylbenzyltelluride, and methylpentadienyltelluride are reported. These unsymmetrical tellurides were characterized by 1H, 13 C, and 125 Te NMR spectroscopy. The potential applicability of these organotellurium precursors to lower film-growth temperatures in MOCVD is discussed. INTRODUCTION Metal-organic chemical vapor deposition (MOCVD) is an increasingly valuable technique used to prepare high quality mercury cadmium telluride (Hgl.xCdxTe) semiconductor films. The development of pyrolytic and photolytic MOCVD growth of mercury cadmium telluride superlattices and focal plane arrays is essential in the fabrication of high performance infrared detectors [1]. Low substrate temperature is the critical requirement for growth of complex structures that contain sharp heterojunction interfaces. However, conventional MOCVD of mercury cadmium telluride with elemental mercury, dimethylcadmium, and diethyltelluride between 390-450 0C [2] results in film degradation due to mercury evaporation and diffusion. Lower growth temperatures can be achieved by a variety of methods which include the use of alternative organotellurium source compounds [3-7] and photo-assisted MOCVD [1,8]. Lowering the growth temperature of mercury cadmium telluride is governed by the relative stability and/or reactivity of the organotellurium source compound, which is determined by the strength of the Te-C bonds. The rate-determining step in the pyrolysis of organometallic compounds is homolytic cleavage of the metalcarbon bond, which is generally the weakest bond in the molecule. The relative thermal stability of organotellurium precursors parallels the activation energy for breaking the C-H bond in the parent hydrocarbon and formation of a hydrocarbon free radical [9]. Lower activation energies for breaking the C-H bond result when the hydrocarbon free radical is stabilized by resonance. Thus allyl (C-H = 87 kcal/mol), benzyl (88 kcal/mol), and pen
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