Aluminum oxide (Al2O3) grown by atomic layer deposition (ALD) is currently under investigation for use as a high-kappa gate dielectric alternative to SiO2. Cluster calculations employing hybrid density functional theory have been carried out to examine the chemical reaction pathways between the ALD precursors, trimethylaluminum (TMA) and H2O, with the H/Si(100)- 2x1 surface. Results obtained using Si9H14 and Si15H20, dimer and double dimer clusters to represent the surface active site are in good agreement, providing a consistent view of reaction energetics on the H/Si(100)- 2x1 surface. The adsorption energies for TMA and H2O on the surface are calculated to be 0.02 and 0.15 eV, respectively. For the reaction between H2O and the H/Si(100)- 2x1 surface, hydroxylation of the surface accompanied by loss of H-2 was found to be the preferred pathway having an activation energy and overall reaction enthalpy of 1.60 eV and -0.75 eV, both of which are greater than or equal to0.70 eV lower than the corresponding values for the possible H/D exchange reaction. TMA exposure of the H/Si(100)- 2x1 surface favors the deposition of - Al(CH3)(2) with loss of CH4, having a barrier height of 1.30 eV and reaction enthalpy of -0.31 eV, which are 0.10 and 0.40 eV lower than the surface methylation pathway (H/CH3 exchange) and 2.64 and 0.45 eV lower in energy than the H-2 loss reaction, that results in the deposition of - CH2 -Al(CH3)(2) to the surface. Therefore, the dominant reactions identified in this work are those with direct implication in the Al2O3 ALD growth mechanism, leading to the formation of Si - O and Si - Al species on the H/ Si(100)- 2x1 surface. (C) 2003 American Institute of Physics.