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Atomic layer deposition (ALD) of TiO2 thin film from TiCl4 and H2O has been intensively studied since the invention of ALD method to grow thin films via chemical adsorptions of two precursors. However the role of HCl which is a gaseous byproduct in ALD chemistry for TiO2 growth is still intriguing in terms of the growth mechanism. In order to investigate the role of HCl in TiO2 ALD, HCl pulse and its purging steps are inserted in a typical sequence of TiCl4 pulse-purge-H2O pulse-purge. When they are inserted after the first-half reaction (chemisorption of TiCl4), the grown thickness of TiO2 becomes thinner or thicker at lower or higher growth temperatures than 300 o C, respectively. However the insertion after the second-half reaction (chemisorption of H2O) results in severely reduced thicknesses in all growth temperatures. By using the result, we explain the growth mechanism and the role of HCl in TiO2 ALD.

When atomic layer deposition (ALD) is performed on a porous material by using an organometallic precursor, minimum exposure time of the precursor for complete coverage becomes much longer since the ALD is limited by Knudsen diffusion in the pores. In the previous report by Min et al. (Ref. 23), shrinking core model (SCM) was proposed to predict the minimum exposure time of diethylzinc for ZnO ALD on a porous cylindrical alumina monolith. According to the SCM, the minimum exposure time of the precursor is influenced by volumetric density of adsorption sites, effective diffusion coefficient, precursor concentration in gas phase and size of the porous monolith. Here we modify the SCM in order to consider undesirable adsorption of byproduct molecules. TiO2 ALD was performed on the cylindrical alumina monolith by using titanium tetrachloride (TiCl4) and water. We observed that the byproduct (i.e., HCl) of TiO2 ALD can chemically adsorb on adsorption sites, unlike the behavior of the byproduct (i.e., ethane) of ZnO ALD. Consequently, the minimum exposure time of TiCl4 (~16 min) was significantly much shorter than that (~ 71 min) of DEZ. The predicted minimum exposure time by the modified SCM well agrees with the observed time. In addition, the modified SCM gives an effective diffusion coefficient of TiCl4 of ~1.78 × 10 �2 cm 2 /s in the porous alumina monolith.