Dimethylaluminum chloride (DMACl) as an aluminum source has shown promising potential to replace more expensive and commonly used trimethylaluminum in the semiconductor industry for atomic layer deposited (ALD) thin films. Here, the Al2O3 DMACl-process is modified by replacing the common ALD oxidant, water, by ozone that offers several benefits including shorter purge time, layer-by-layer growth, and improved film adhesion. It is shown that the introduction of the ozone instead of water increases carbon and chlorine content in the Al2O3, while long ozone pulses increase the amount of interfacial hydrogen at silicon surface. These are found to be beneficial effects regarding the surface passivation and thus final device operation. Heat treatments (at 400 and 800 °C) are found to be essential for high quality surface passivation similar to ALD Al2O3 deposited from conventional precursors, which is correlated with the changes at the interface and related impurity distributions. The optimal deposition temperature is found to be 250 °C, which provides the best chemical passivation after thermal treatments.