Nucleation and growth of AlN : self-limiting reactions and the regeneration of active sites using sequential exposures of trimethylaluminum and ammonia on silica at 600 K

The sequential reactions of trimethylaluminum (TMA) and ammonia have been studied in the 1-Torr pressure regime at 600 K with FTIR (Fourier transform infrared spectroscopy) and XPS (X-ray photoelectron spectroscopy). Transmission FTIR spectra acquired through a silica substrate reveal that extended ammonia exposure can overcome the desporption of ammonia from the methylaluminum:ammonia surface adduct identified previously. This increases the number of reactions between methyl groups and ammonia on adjacent adducts and thereby maximizes the population of bridging amino groups on the surface (Al-NH[sub 2]-Al). The resulting NH[sub 2] species can then react with TMA from an additional exposure to produce a new layer of methylaluminum species. Repetitions of these sequencial reactions demonstrate that the respective TMA and ammonia reactions are self-limiting, regenerating active sites suitable for promoting chemisorption of the next precursor for the layer-by-layer growth of aluminum nitride. These studies suggest that in addition to providing transport for the source elements, functional groups that remain chemisorbed on the surface may also be used to enhance the uptake of the next precursor and influence bond directionality during film growth at low temperatures. Although the crystallinity of the aluminim nitride film is expected to be limited by the amorphous nature of the silica substrate, the results from a simple conformational analysis suggest a general rule that can be applied to any substrate; if a concerted reaction between dissimilar functional groups is the only mechanism available for growth, then this mechanism will ultimately limit long-range order in the resulting film. 35 refs., 8 figs.