Cluster calculations employing hybrid density functional theory have been carried out to examine the initial surface reactions in atomic layer deposition (ALD) of TiN thin films on the SiO2 surface using TiCl4 and NH3 as precursors. The potential energy surface ( PES) of both half-reactions at different temperatures is presented. The first half-reaction between TiCl4 with the SiO2 surface is activated with an activation barrier of 0.78 eV and an exothermicity of 0.38 eV, suggesting that it is thermodynamically favourable. Also, the NH3 half-reaction begins with the formation of amido complexes by the replacement of Cl atoms by NH2, which is endothermic by 0.58 eV with a physisorbed HCl state (HCl-PS1). Formation of the amido complexes can be followed by an elimination reaction to form imido complexes, which has a relatively high activation barrier of 2.51 eV. In addition, the effect of the reaction temperature on the Cl impurity concentrations and film growth rate in the ALD process is also discussed.