The use of hex-5-enylarsine as a chemically designed precursor to probe the mechanisms of the metalorganic vapour phase epitaxy growth of gallium arsenide; consequences for reactor design

Ex-situ Fourier transform infrared spectroscopy has been employed to study the mechanisms of the metalorganic vapour phase epitaxy growth of GaAs, using hex-5-enylarsine and trimethylgallium as precursors. Hex-5-enylarsine was synthesized for the purpose of distinguishing between reductive elimination, free radical and β-hydrogen elimination reactions since all three pathways are theoretically available for its decomposition. However, the reaction under MOVPE conditions is not as simple as envisaged in that fragmentation of the hex-5-enyl alkene chain competes effectively with C-As bond cleavage as a decomposition pathway. Thus for hex-5-enylarsine in dihydrogen decomposition is observed to commence at temperatures of ca. 500°C yielding as yet unassigned alkylarsines, methane and some ethene as products. At higher temperatures ethane, propane are formed while at very high temperatures (> 700°C) there is evidence for the formation of ethyne and 1,3-butadiene. In the presence of trimethylgallium (TMGa), an involatile solid forms at room temperature via the elimination of methane, strongly supporting the proposal that gas-phase adduct formation occurs as the primary process. At higher temperatures, ca. 350°C, this solid begins to decompose yielding the alkylarsines noted above. In addition, methylarsine and dimethylarsine are formed, together with methylene cyclopentane and cyclohexane. The appearance of these latter products appears to correlate with the onset of decomposition of TMGa and is attributed to a mechanism involving the formation of a radical intermediate which may then undergo 1,5 or 1,6 cyclization.