Probing the mechanisms of growth of gallium arsenide by metalorganic vapor phase epitaxy using experimental and theoretical studies of designed precursors

Abstract

Decomposition of o-CH₃C₆H₄AsD₂ in the gas phase at 600–1000°C produces toluenes with 0-3 D atoms in the methyl group. It is shown that this cannot be accounted for by conventional mechanisms involving initial As-C bond cleavage or reductive elimination, but rather that the first step is As-D bond cleavage and this is followed by reductive elimination of o-CH₃C₆H₄D or H atom transfer to give o-HDAsC₆H₄CH₂· which abstracts D from an intact o-tolylarsine to give o-CH₂DC₆H₄AsHD. Repetition of these steps can lead to multiple D incorporation. The free energies of activation for reductive elimination or multiple D incorporation are found to be very similar. Theoretical studies on the decomposition of^tBuAsH₂ show that the first step for decomposition can be As-H bond cleavage to give^tBuAsH· or loss of H₂ to^tBuAs.^tBuAsH· decomposes to^tBu· which abstracts H· from^tBuAsH₂ to give 2-methylpropane or by β-H abstraction to give 2-methylpropene.^tBuAs, on the other hand only gives 2-methylpropene,via a β-H abstraction mechanism. Measured effects of total reactor pressure on product distribution are modeled qualitatively. Hex-5-enylarsine also decomposesvia initial As-H bond cleavage followed by reductive elimination of 1-hexene. However, it reacts in the liquid or solution phase with Me₃Ga to give the adduct. [Me₃Ga.AsH₂hex]. On heating, this loses methane to give first [Me₂Ga.AsHhex]₃ then [MeGa.Ashex]_n. Finally, GaAs is produced with the formation of methane and methylenecyclopentane. The last product indicates a free radical mechanism involving cleavage of the As-hex bond for the last step. In the gas phase at 600°C, GaAs is again formed but the major C₆ product is 1-hexene. This is interpreted as meaning that the adduct, [Me₃Ga.AsH₂hex] is not formed in the gas phase under growth conditions.