We use density functional theory (DFT) to investigate non-growth ligand exchange reactions (NGLE) during the precursor half-cycle of atomic layer deposition (ALD) of HfO2 using HfCl4 and H2O as part of a detailed atomistic mechanism. The non-growth ligand exchange (NGLE) reactions of HfCl4 on as-grown OH terminated HfO2 involve the formation of hydroxychlorides and dihydroxychlorides and have lower activation barriers compared to ligand exchange reactions, suggesting that these side-reactions are significant and competitive with ligand exchange. Consequently, NGLE reactions are predicted to be a significant source of chlorine contamination of HfO2 ALD films deposited using HfCl4 and can reduce the efficiency of metal deposition of the ALD process. However, on OH terminated SiO2 these NGLE reactions are predicted to have appreciably larger barriers than ligand exchange reactions and consequently play a less significant role for the initial deposition of HfO2 on SiO2 substrates. (c) 2006 Elsevier B.V. All rights reserved.