The authors have studied the initial stages of the atomic layer deposition (ALD) of HfO2 onto Si by means of x-ray photoelectron spectroscopy using synchrotron radiation. The ALD was obtained using HfCl4 and H2O as precursors. The investigation was carried out in situ giving the possibility to determine the properties of the grown film after each ALD cycle. The Si 2p, O 1s, and Hf 4d +Cl 2p spectra show the growth of HfO2 in a smooth way until the complete formation of two oxide layers. The averaged growth rate is found to be 0.33 (one layer after three cycles) in accordance with previous works but, within the formation of one oxide layer, each ALD cycle behaves in a distinct way: the oxidation step in the various cycles shows a different efficiency leading to the inclusion of Cl impurities into the Hf oxide. In relation to the experimental results we discuss the origin of the Cl contamination proposing a mechanism based on the adsorption geometry of HfCl4 onto the-OH terminated substrate. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3021023]