Reactant-Dependent, 10⁸× Conductivity Modulation in Plasma-Enhanced Atomic Layer Deposition for Black TiO₂ Films

A rational choice of reactants during plasma-enhanced atomic layer deposition (PEALD) can lead to extreme changes in film properties. Herein, we demonstrate with this approach, a 108× change in electrical conductivity of an insulating TiO2 film to a highly conducting TiO2 film. The films are deposited using PEALD of titanium tetraisopropoxide (TTIP) with a 300 W plasma containing 5% H2 in argon as a reactant at 200 °C. The growth and film characteristics are compared to the ALD of TiO2 films grown using widely reported processes involving H2O and O2 plasma as reactants. In situ ellipsometry is used to extract the growth per cycle (GPC) of the TTIP + H2 plasma process, which is 0.013 nm/cycle. This can be compared to the GPC of 0.011 and 0.034 nm/cycle for the TTIP + H2O and TTIP + O2 plasma processes, respectively. X-ray photoelectron spectroscopy of the TTIP + H2 plasma films shows a reduced state, with a Ti3+/Ti4+ ratio of 72%, compared to 23.4 and 29.6% for the TTIP + H2O and TTIP + O2 plasma processes, respectively. The TTIP + H2 plasma film is amorphous as-deposited and has a root-mean-square surface roughness of 1.42 nm. The film demonstrates an apparent optical bandgap of 3.77 eV and strong free carrier absorption in the near-infrared. The room temperature resistivity of the film is measured to be 67 mΩ cm, with an activation energy for conduction of 0.142 eV and a free electron carrier concentration of 7.89 × 1021 cm–3. The TTIP + H2 plasma process makes the reduced TiO2 film appear "black" in color.