Tailoring Nickel\nOxide Thin Films: Comparative Study\nof Oxidizing Agents in Thermal and Plasma-Enhanced Atomic Layer Deposition

Thermal atomic layer deposition (TALD) and plasma atomic\nlayer\ndeposition (PALD) were used for producing thin NiO<sub><i>x</i></sub> films from nickel(II) acetylacetonate Ni(acac)<sub>2</sub>, employing different oxidizing agents (deionized water H<sub>2</sub>O, ozone O<sub>3</sub>, and molecular oxygen O<sub>2</sub>). The\nfilms were deposited at 300 °C (TALD) and 220 °C (PALD)\nover glass substrates; their physical and chemical properties were\nconsiderably influenced by the choice of oxidizing agents. In particular,\nALD(H<sub>2</sub>O) samples had a low growth per cycle (GPC) and a\nhigh concentration of defects. The best NiO<sub><i>x</i></sub> parameters were achieved with PALD(O<sub>2</sub>), featuring\nhigh GPC (0.07 nm/cycle), high optical transparency in the visible\nregion, electrical resistivity (1.18 × 10<sup>4</sup> Ω·cm),\ngood carrier concentration (8.82 × 10<sup>13</sup> cm<sup>–3</sup>), and common mobility (5.98 cm<sup>2</sup>/V·s). The resulting\nNiO<sub><i>x</i></sub> films are polycrystalline and homogeneous\nin thickness and composition. According to ultraviolet photoelectron\nspectroscopy (UPS), work function φ and the valence band maximum <i>E</i><sub>V</sub> can be tuned by the choice of the coreactant\nemployed, with variations of up to ∼1 eV between TALD and PALD\nsynthesis. Our results suggest that PALD permits one to achieve a\nbetter energy band alignment of NiO<sub><i>x</i></sub> and\nCsFAMAPbBrI perovskite, which is promising for solar cell applications.