NUCLEATION AND GROWTH OF AIN - SELF-LIMITING REACTIONS AND THE REGENERATION OF ACTIVE-SITES USING SEQUENTIAL EXPOSURES OF TRIMETHYLALUMINUMAND AMMONIA ON SILICA AT 600 K

The sequential reactions of trimethylaluminum (TMA) and ammonia have b een studied in the 1-Torr pressure regime at 600 K with FTIR (Fourier transform infrared spectroscopy) and XPS (X-ray photoelectron spectros copy). Transmission FTIR spectra acquired through a silica substrate r eveal that extended ammonia exposure can overcome the desorption of am monia from the methylaluminum:ammonia surface adduct identified previo usly. This increases the number of reactions between methyl groups and ammonia on adjacent adducts and thereby maximizes the population of b ridging amino groups on the surface (Al-NH2-Al). The resulting NH2 spe cies can then react with TMA from an additional exposure to produce a new layer of methylaluminum species. Repetitions of these sequential r eactions demonstrate that the respective TMA and ammonia reactions are self-limiting, regenerating active sites suitable for promoting chemi sorption of the next precursor for the layer-by-layer growth of alumin um nitride. These studies suggest that in addition to providing transp ort for the source elements, functional groups that remain chemisorbed on the surface may also be used to enhance the uptake of the next pre cursor and influence bond directionality during film growth at low tem peratures. Although the crystallinity of the aluminum 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 bet ween dissimilar functional groups is the only mechanism available for growth, then this mechanism will ultimately limit long-range order in the resulting film.