Surface reactions in Al2O3 growth from trimethylaluminium and water by atomic layer epitaxy

The layer by layer growth of Al2O3 from trimethylaluminium (TMA) and water vapour by atomic layer epitaxy (ALE) has been studied on porous large surface area silica. The use of this support enables the study of each reaction sequence brought to surface saturation by using elemental determinations, Fourier transform infrared (FTIR) and solid state nuclear magnetic resonance (NMR) spectroscopy. The chemisorption of TMA as a function of the preheat temperature of silica shows that bonding takes place to isolated OH groups and siloxane bridges. Increasing the preheat temperature of silica creates an increased amount of siloxane bridges. Increasing the chemisorption temperature of TMA leads to the reaction of double and triple siloxane bridges as measured by 29Si NMR. This type of reaction has not been presented earlier. The first five reaction cycles of TMA and water on silica show that, after the first TMA reaction; 2.8 Al/nm2 are present whereas the following reaction cycles increase Al by 0.5 and 0.3 Al/nm2 alternatively in the cycles 2–5. Water removes AlCH3 groups already at 120°C, but SiCH3 groups are not affected so much and are responsible for the slow progress of growth for the next layers.