A Qualitative Model for Predicting Alkyl Stability and its Relevance to Organometallic Growth of HgCdTe and GaAlAs
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A QUALITATIVE MODEL FOR PREDICTING ALKYL STABILITY AND ITS RELEVANCE TO ORGANOMETALLIC GROWTH OF HgCdTe AND GaAIAs WILLIAM E. HOKE Raytheon Co., 131 Spring St., Lexington, MA
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ABSTRACT A qualitative stability model established for hydrocarbon molecules and extended to organometallic compounds is applied to alkyl organometallic compounds used in thin film growth. For hydrocarbon molecules the strength of a carbon-hydrogen bond is reduced by neighboring alkyl groups, The mechanism for this effect is delocalization of the unpaired electronic charge in the resultant free radical by neighboring alkyl groups. The delocalization effect is present in organometallic compounds and is illustrated here for several alkyl organometallic systems. Two applications of the delocalization effect in thin film growth are discussed. First the thermal stability of organotellurium compounds is substantially reduced in branched compounds which permits significantly lower HgCdTe metalorganic growth temperatures. Also carbon incorporation in GaAs and GaAIAs is reduced with ethyl instead of methyl organometallic compounds. An important factor contributing to this result is the weaker carbon-metal bonds in the ethyl compounds.
INTRODUCTION The MOCVD (metalorganic chemical vapor deposition} technique has been used to grow epitaxial thin films of a wide range of compounds. Considerable interest in recent years has been on the MOCVD growth of III-V compounds such as GaAs and GaA1As as well as II-VI compounds such as HgCdTe. In the MOCVD process the starting organometallic sources are pyrolyzed in the substrate region, Typically the weakest bonds in these compounds are the carbon-metal bonds. Consequently the strengths of these bonds can directly affect the pyrolysis temperature and film growth process. Since the strength of a carbon-metal bond is affected by neighboring organic groups, the choice of organometallic sources is an important consideration. As discussed below the stability properties of the organometallic compounds determine the low temperature limit on MOCVD HgCdTe growth temperature as well as affect carbon incorporation in GaAs and GaA1As films,
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Epitaxial films of HgCdTe can be degraded by relatively high growth temperatures, which are typically 400-440 0 C for MOCVD. At these temperatures diffusional processes broaden heterojunction interfaces and make the growth of novel quantum well structures infeasible. The growth of HgCdTe films on foreign substrates can be contaminated from substrate autodoping. Furthermore the equilibrium density of mercury vacancies increases sharply with temperature. Consequently the use of alternative organometallic sources which permits lower HgCdTe growth temperatures is attractive [1]. Carbon is an acceptor impurity in epitaxial films of GaAs and GaAlAs. Incorporation of carbon from the organometallic sources has been observed in MOCVD growth of these materials [2). Organometallic sources are also used in an ultra-high vacuum thin film growth technique termed gas-source molecular beam e
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