TiO2 films were grown by atomic layer deposition (ALD) with Ti(O-i-Pr)(4) and H2O. Below a critical film thickness the grown TiO2 films exhibited an amorphous structure. The growth rate of the amorphous films was constant at about 0.055 nm/cycle irrespective of the susceptor temperature. Films which exceeded the critical thickness showed a polycrystalline structure. The growth rate of the thicker crystallized films was higher by a factor of about 2 compared to the value of the thin amorphous films. In this study it is shown that the abrupt increase in the growth rate is caused by an increase in the density of hydroxyl groups on the reaction surface rather than by a certain surface roughening accompanying the crystallization process. (c) 2010 The Electrochemical Society. [DOI: 10.1149/1.3507258] All rights reserved.