Sequential Infiltration Synthesis process: SIS processes were performed at 70°C in a commercial cross-flow ALD reactor (Savannah S200, Ultratech Cambridge NanoTech) utilizing TMA and H2O as organometallic and oxygen precursors, respectively. Before starting the process, the samples were kept at 70 °C for 30 minutes under N2 flow (100 sccm, 0.66 Torr) to guarantee proper thermalization of the growth chamber. The SIS cycle comprised the following steps: TMA pulse, TMA exposure, TMA purge, H2O pulse, H2O exposure and H2O purge. The duration (tTMA) of the TMA pulses was varied from 25 to 40 ms, while the duration of the H2O pulse was 15 ms. The chamber was kept in static vacuum during the 90 s long TMA and 60 s long H2O exposure steps by closing inlet and outlet valves. TMA purge time was 60 s. H2O purge time was 180 s to guarantee complete removal of reaction by-products. A scheme of the SIS cycle and of the pressure evolution into the ALD reactor is reported in Figure . A change of the tTMA value determines a variation of the chamber pressure, consistently with the idea of a modification of TMA partial pressure in the growth chamber. After the SIS process, the samples were exposed to O2 plasma (40 W, 10 min) to remove the polymer, resulting in the formation of Al2O3 films on the Si substrate. Spectroscopic Ellipsometry: SE data were collected using a M-2000F (J. A. Woollam Co.