In situ mass spectroscopy and thermogravimetric studies of GaAs MOCVD gas phase and surface reactions

Abstract

Two special MOCVD reactors systems designed for in situ monitoring of gas phase and surface reactions are presented. The gas phase decomposition mechanisms of Ga(CH 3) 3, Ga(C 2H 5) 3 and As(CH 3) 3 in H 2, He and D 2 are investigated by molecular beam sampled mass spectroscopy. The data show that the initial thermal pyrolysis reaction of Ga(CH 3) 3 and As(CH 3) 3 is a unimolecular reaction leading to the loss of methyl groups. This is supported by the observations of similar decomposition temperatures in H 2 and He, an increased C 2H 6 formation in He, and the production of CH 3D in D 2 carrier gas. The decomposition of Ga(C 2H 5) 3 proceeds either via the loss of ethyl radicals or β-elimination of ethene, and the latter mechanism dominates above 650 K. This model is supported by variation in the product distribution of C 2H 4, C 2H 5·, C 2H 6 and C 4H 10 with pyrolysis temperature. A new thermogravimetric MOCVD system is used to measure surface reaction rates of Ga(CH 3) 3 and As(CH 3) 3 at low pressures, where mass transfer effects are absent. Data for varying total pressures and constant partial pressures of Ga(CH 3) 3 and As(CH 3) 3 indicate that the growth rate at reduced pressures is influenced by gas phase reactions. At low pressures surface reactions appear to dominate and at high temperatures the growth rate goes to zero and etching takes place.