Epitaxial growth of undoped and Li-doped NiO thin films on α-Al2O3 substrates by mist chemical vapor deposition

Abstract

Undoped and Li-doped NiO thin films were grown on α-Al₂O₃ (0 0 0 1) substrates by mist chemical vapor deposition. Both undoped and Li-doped NiO thin films grew bi-epitaxially on the substrates with crystallographic orientation relationships of NiO(1 1 1)[ 1 0] || α-Al₂O₃(0 0 0 1)[0 1 0] and NiO(1 1 1)[1 0] || α-Al₂O₃(0 0 0 1)[0 1 0]. In the Li-doped NiO thin film, a periodic structure was observed, in accordance with a mirror-symmetrical oxygen layer on the terraces of the substrate. Both undoped and Li-doped NiO thin films exhibited high transmittance (>80%) in the visible-light region and optical bandgaps of 3.7–3.8 eV. The undoped NiO thin film showed insulating properties and a resistivity of 10⁶ Ω cm or higher. In contrast, the Li-doped NiO thin films had resistivities of 10¹–10⁵ Ω cm, depending on the Li precursor concentration. Furthermore, they exhibited positive Seebeck coefficients, indicating their p-type conductivity. These results indicate that Li dopants effectively act as acceptors in NiO thin films.