HOMOLYSIS VERSUS BETA-HYDRIDE ELIMINATION IN THE DECOMPOSITION OF TRIALKYLGALLIUM ON GAAS(100)

The decomposition of various surface alkyl groups on GaAs(100), which were generated by dissociative adsorption of the corresponding trialky lgallium precursors R3Ga (R = Me, Et, Et-d(5), Pr-n, Pr-i, Bu-n and Bu -t), was studied by temperature programmed desorption. Two elimination pathways were observed, homolysis and beta-hydride elimination. The f ormer reaction results in the formation of alkyl radicals and the latt er gives alkene and H-2 as products. On GaAs(100), both reactions are observed in the decomposition of all surface alkyl groups, except for methyl that reacts by homolysis. For each surface alkyl group. homolys is always occurs at slightly lower temperature than beta-hydride elimi nation. Experiments with perdeuterated triethylgallium reveal that sur face Et groups do not undergo coupling with coadsorbed deuterium on th e surface to form ethane, and that ethane forms in subsequent wall rea ctions that involve E-a radicals. The activation energy E-a for homoly sis followed the trend Me > Et > Pr-n > Bu-n > Pr-i > Bu-t, which refl ects the strength of alkyl-surface bonds as well as the increased stab ility of the alkyl radical. The E-a for beta-hydride elimination follo ws closely the E-a for homolysis and exhibits similar behavior in term s of magnitude and trend, i.e. Et > Pr-n > Bu-n approximate to Pr-i > Bu-t, suggesting that breaking the alkyl-surface bond contributes to t he activation energy for both homolysis and beta-hydride elimination r eactions. The alkyl-sulface bond energy (Delta N-h) and the heat of re action for beta-hydride elimination (Delta H-beta) for all surface alk yls are calculated from the desorption temperatures of their products. (C) 1998 Elsevier Science B.V. All rights reserved.