IR and NMR study of the chemisorption of ammonia on trimethylaluminum-modified silica

The saturating reaction of ammonia was studied on trimethylaluminum (TMA)-modified porous silica. This reaction step completes a reaction cycle of TMA and ammonia in aluminum nitride growth by atomic layer chemical vapor deposition (ALCVD), a technique based on well-separated saturating gas-solid reactions. The reaction was studied from 423 to 823 K. In addition, the separate reactions of TMA at 423 K and ammonia at 823 K were studied on silica dehydroxylated at 1023 K. The reaction products on the surface were identified by IR and Si-29, C-13, and H-1 NMR spectroscopy, and they were quantified by element determinations and H-1 NMR. In the reaction of TMA on silica, methyl groups were attached to the surface indirectly through aluminum and through direct bonding to silicon; In the subsequent ammonia reaction, ligand exchange of ammonia with the methyl groups occurred at all reaction temperatures, resulting in primary amino groups and the release of methane. Also, secondary amino groups were found on the surface, and quantitative determinations indicated the presence of tertiary amino groups, especially at high reaction temperatures. In addition, especially at low reaction temperatures, ammonia chemisorbed associatively on the TMA-modified silica. All of the methyl groups bonded to aluminum were removed with ammonia at 573-623 K, and about 80% of the methyl groups bonded to silicon were removed at 823 K; amino groups bonded to both aluminum and silicon were left behind. The higher the reaction temperature, the smaller was the average number of hydrogen atoms (x) in the amino groups (NHx).