Do gas phase adducts form during metalorganic vapour phase epitaxial growth of gallium arsenide?

The new primary arsine hex-5-enylarsine (hexAsH₂) has been synthesized and decomposed in the liquid and gas phases in the presence and absence of trimethylgallium. From the gas phase decomposition by itself in vacuo the main product is hex-1-ene, although smaller amounts of cyclic C₆ products and of conjugated hexadienes are observed. Under growth conditions (1 atm of H₂ > 350°C) the hex-1-ene formed fragments completely because of hexAsH₂ first gives the adduct [Me₃Ga·AshexH₂] but this sequentially loses two moles of methane before giving GaAs, methylenecyclopentane and methane, showing that methylenecyclopentane will necessarily be a product if adduct formation occurs. In the gas phase in vacuo the major C₆ product from Me₃Ga and hexAsH₂ is hex-1-ene although there is a small increase in the proportion of methylenecyclopentane and cyclohexene, suggesting that two pathways, one involving adducts (minor) and the other not, operate. In hydrogen, the major C₆ products are methylenecyclopentane and cyclohexane confirming that adducts can be important, but, since hex-1-ene fragments under these conditions it is not possible to quantify the extent of adduct formation. These results are analysed in terms of the calculated equilibrium constant for adduct formation together with the residence time in the cold zone of the reactor and the probability of bimolecular reactions occurring. It is concluded that comparative studies under a variety of different conditions are essential for a full understanding of the processes occurring during growth.