A PICOSUN R-200 atomic layer deposition (ALD) system was used to deposit 100 nm of a-TiO2 on the Si substrates using water and tetrakis(dimethylamino)titanium (TDMAT) as the O and Ti precursors, respectively. The sample stage was held at 120 °C during the deposition. A 1.6 s pulse of TDMAT and 7.0 s N2 purge was followed by a 0.1 s water pulse followed by a 6.0 s N2 purge to deposit a single layer of a-TiO2. These steps were repeated until the desired thickness was achieved. Thickness of the a-TiO2 layer was confirmed using a J.A. Woollam Co. alpha-SE ellipsometer. For devices with a c-TiO2 protection layer, the samples were subsequently heated to 500 °C for 1 h on a hotplate in air.