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 10 8 × change in electrical conductivity of an insulating TiO 2 film to a highly conducting TiO 2 film. The films are deposited using PEALD of titanium tetraisopropoxide (TTIP) with a 300 W plasma containing 5% H 2 in argon as a reactant at 200 °C. The growth and film characteristics are compared to the ALD of TiO 2 films grown using widely reported processes involving H 2 O and O 2 plasma as reactants. In situ ellipsometry is used to extract the growth per cycle (GPC) of the TTIP + H 2 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 + H 2 O and TTIP + O 2 plasma processes, respectively. X-ray photoelectron spectroscopy of the TTIP + H 2 plasma films shows a reduced state, with a Ti 3+ /Ti 4+ ratio of 72%, compared to 23.4 and 29.6% for the TTIP + H 2 O and TTIP + O 2 plasma processes, respectively. The TTIP + H 2 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 × 10 21 cm –3 . The TTIP + H 2 plasma process makes the reduced TiO 2 film appear “black” in color.