CHEMISORPTION OF TRIMETHYLALUMINUM AND AMMONIA ON SILICA - MECHANISMS FOR THE FORMATION OF AL-N BONDS AND THE ELIMINATION OF METHYL-GROUPS BONDED TO ALUMINUM

Infrared spectroscopy and temperature-programmed desorption (TPD) have been used to study the formation and decomposition of a potential AlN precursor on the surface of silica. At 300 K, ammonia reacts with the monomethylaluminum surface complex produced by trimethylaluminum chemisorption to form a structure analogous to the TMA:NH3 adduct known in the condensed phase. The surface adduct is stable in high vacuum and demonstrates that strong Al-N bonds can be produced on the surface at low temperatures. Additional Al-N bonds are formed when these species react to yield -NH-2- (amino) groups that bridge between aluminum centers. Thermal decomposition of the methylaluminum:ND3 surface adduct leads to the desorption of CH-3D below 600 K, suggesting that the formation of bridging amino groups is the coproduct of a reaction in which ammonia directly hydrogenates methyl groups bonded to aluminum. Collectively, these results imply that adjacent adducts decompose via an interadsorbate reaction mechanism that synergistically decreases the source of carbon contamination and promotes the formation of contiguous Al-N bonds on the surface at low temperatures. TPD results and the coverage dependence of bridging amino formation suggest that the extent of Al-N bonding is limited by the desorption of ammonia and by the proximity and orientation of the surface adduct.