Successive reactions of gaseous trimethylaluminium and ammonia on porous alumina

Abstract

Successive reactions of gaseous trimethylaluminium (TMA) and ammonia on porous alumina were studied with the goal of finding suitable process conditions for preparing aluminium nitride (AlN) by atomic layer deposition (ALD), a technique based on separate saturating gas–solid reactions. The reaction of TMA was studied at 353–623 K on alumina dehydroxylated at 473–1073 K, and the following reaction of ammonia at 423–823 K. Reference samples were prepared by reacting ammonia at 623 and 823 K with alumina dehydroxylated at 833 K. The samples were characterised by elemental analysis of carbon and nitrogen and by IR and ¹H NMR. TMA reacted with alumina in a saturating manner at 353–573 K. Reaction took place through ligand exchange with surface OH groups, with release of methane, and through dissociation of TMA on pairs of coordinatively unsaturated Al and O ions. Aluminium-bonded methyl groups remained on the surface. Decomposition of TMA occurred at 600 K and above. Ammonia had reacted with most of the methyl groups by 573 K, but 723 K was required to remove them all. Primary, secondary and tertiary amino groups were formed in the reaction, and ammonia molecules were adsorbed on the surface. The average H/N ratio in the amino groups decreased with increasing reaction temperature. Good temperatures for AlN deposition by ALD seem to be any temperature up to 573 K for the TMA reaction and 723 K or above for the ammonia reaction.