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The growth of high quality II-VI devices by MOVPE is dependent upon the progress achieved in certain key research activities. Here, the precursors are the crucial point for a successful epitaxy. The metallorganics Et 2 Te, i Pr 2 Te, MeTe-allyl, Et 2 Zn, Me 2 Zn and the Me 2 Zn-Et 3 N adduct are used for the growth of ZnTe epilayers. The relevant properties of these alkyls such as purity, vapour pressures, RT-stability and bond disruption energies are discussed. Special emphasis is put on the thermal decomposition. Reaction mechanisms and kinetics were studied in an isothermal ersatz reactor and by mass spectroscopy. Literature results are quoted to complete the interpretation. The evaluation of the measurements is often complicated due to the condensation and autocatalysis. Homogeneous and heterogeneous pyrolysis compete with each other ( i Pr 2 Te). Homolysis followed by radical reactions is the dominant reaction mechanism for Me 2 Zn, Et 2 Zn and MeTe-allyl. β-hydrogen elimination competes with homolysis in certain cases (Et 2 Te, i Pr 2 Te). For most precursor combinations, strong interactions in pyrolysis are observed. For the relatively stable Te-alkyls Me 2 Te and Et 2 Te, the decomposition is enhanced by co-pyrolysis with Et 2 Zn. For the less stable Te-alkyls i Pr 2 Te and MeTe-allyl, the decomposition kinetics remain essentially unchanged with Et 2 Zn. The activation energies and prefactors for the elemental reaction steps are discussed. Basic considerations lead to an expression which allows the evaluation of E a for the surface reaction in a flow tube. Under similar experimental conditions, activation energies for the surface pyrolysis were found to be comparable to the growth activation energies. |
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