Reactant-Dependent,\n10<sup>8</sup>× Conductivity\nModulation in Plasma-Enhanced Atomic Layer Deposition for Black TiO<sub>2</sub> Films

A rational choice of reactants during plasma-enhanced\natomic layer\ndeposition (PEALD) can lead to extreme changes in film properties.\nHerein, we demonstrate with this approach, a 10<sup>8</sup>×\nchange in electrical conductivity of an insulating TiO<sub>2</sub> film to a highly conducting TiO<sub>2</sub> film. The films are\ndeposited using PEALD of titanium tetraisopropoxide (TTIP) with a\n300 W plasma containing 5% H<sub>2</sub> in argon as a reactant at\n200 °C. The growth and film characteristics are compared to the\nALD of TiO<sub>2</sub> films grown using widely reported processes\ninvolving H<sub>2</sub>O and O<sub>2</sub> plasma as reactants. <i>In situ</i> ellipsometry is used to extract the growth per cycle\n(GPC) of the TTIP + H<sub>2</sub> plasma process, which is 0.013 nm/cycle.\nThis can be compared to the GPC of 0.011 and 0.034 nm/cycle for the\nTTIP + H<sub>2</sub>O and TTIP + O<sub>2</sub> plasma processes, respectively.\nX-ray photoelectron spectroscopy of the TTIP + H<sub>2</sub> plasma\nfilms shows a reduced state, with a Ti<sup>3+</sup>/Ti<sup>4+</sup> ratio of 72%, compared to 23.4 and 29.6% for the TTIP + H<sub>2</sub>O and TTIP + O<sub>2</sub> plasma processes, respectively. The TTIP\n+ H<sub>2</sub> plasma film is amorphous as-deposited and has a root-mean-square\nsurface roughness of 1.42 nm. The film demonstrates an apparent optical\nbandgap of 3.77 eV and strong free carrier absorption in the near-infrared.\nThe room temperature resistivity of the film is measured to be 67\nmΩ cm, with an activation energy for conduction of 0.142 eV\nand a free electron carrier concentration of 7.89 × 10<sup>21</sup> cm<sup>–3</sup>. The TTIP + H<sub>2</sub> plasma process\nmakes the reduced TiO<sub>2</sub> film appear “black”\nin color.