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

The saturating reaction of ammonia was studied on trimethylaluminum (TMA)-m odified porous silica. This reaction step completes a reaction cycle of TMA and ammonia in aluminum nitride growth by atomic layer chemical vapor depo sition (ALCVD), a technique based on well-separated saturating gas-solid re actions. The reaction was studied from 423 to 823 K. In addition, the separ ate reactions of TMA at 423 K and ammonia at 823 K were studied on silica d ehydroxylated at 1023 K. The reaction products on the surface were identifi ed by IR and Si-29, C-13, and H-1 NMR spectroscopy, and they were quantifie d 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, liga nd exchange of ammonia with the methyl groups occurred at all reaction temp eratures, resulting in primary amino groups and the release of methane. Als o, secondary amino groups were found on the surface, and quantitative deter minations indicated the presence of tertiary amino groups, especially at hi gh reaction temperatures. In addition, especially at low reaction temperatu res, ammonia chemisorbed associatively on the TMA-modified silica. All of t he 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 hi gher the reaction temperature, the smaller was the average number of hydrog en atoms (x) in the amino groups (NHx).