Tailoring Nickel Oxide Thin Films: Comparative Study of Oxidizing Agents in Thermal and Plasma-Enhanced Atomic Layer Deposition

Thermal atomic layer deposition (TALD) and plasma atomic layer deposition (PALD) were used for producing thin NiO _<i>x</i> films from nickel(II) acetylacetonate Ni(acac)₂, employing different oxidizing agents (deionized water H₂O, ozone O₃, and molecular oxygen O₂). The films were deposited at 300 °C (TALD) and 220 °C (PALD) over glass substrates; their physical and chemical properties were considerably influenced by the choice of oxidizing agents. In particular, ALD(H₂O) samples had a low growth per cycle (GPC) and a high concentration of defects. The best NiO _<i>x</i> parameters were achieved with PALD(O₂), featuring high GPC (0.07 nm/cycle), high optical transparency in the visible region, electrical resistivity (1.18 × 10⁴ Ω·cm), good carrier concentration (8.82 × 10¹³ cm^-3), and common mobility (5.98 cm²/V·s). The resulting NiO _<i>x</i> films are polycrystalline and homogeneous in thickness and composition. According to ultraviolet photoelectron spectroscopy (UPS), work function φ and the valence band maximum <i>E</i> _V can be tuned by the choice of the coreactant employed, with variations of up to ∼1 eV between TALD and PALD synthesis. Our results suggest that PALD permits one to achieve a better energy band alignment of NiO _<i>x</i> and CsFAMAPbBrI perovskite, which is promising for solar cell applications.