Biepitaxial growth of high-quality semiconducting NiO thin films on (0001) Al ₂O ₃ substrates: Microstructural characterization and electrical properties

This paper reports the effects of substrate temperature on the microstructural characteristics and electrical properties of p-type semiconducting NiO thin films grown on (0001) Al ₂O ₃ substrates. NiO thin films were biepitaxially grown on (0001) Al ₂O ₃ substrates by radiofrequency magnetron sputtering, and they showed specific crystallographic orientation relationships: [1̄1̄0] _NiO∥[011̄0] _Al2O3, [1̄12̄] _NiO∥[21̄1̄0] _Al2O3 (in-plane), and [1̄11] _NiO∥[0001̄] _Al2O3 (out-of-plane). Thus, a low lattice mismatch of 7.52% was obtained between the NiO thin films and the (0001) Al ₂O ₃ substrates. The film grown at 600 °C consisted of cubic and rhombohedral NiO grains, while the NiO thin films grown at substrate temperature below 400 °C only consisted of cubic NiO grains. Atoms at the grain boundaries between the cubic and the rhombohedral NiO grains perfectly coincided with each other because of the same atomic stacking sequences along [11̄1̄] _c-NiO and [0003] _r-NiO and with equal interatomic distances. Further, the paper discussed the observations of the perfectly coinciding nickel and oxygen atoms at the grain boundaries between the cubic and the rhombohedral NiO grains using high-resolution transmission electron microscopy (HRTEM) along with atomic modeling on the atomic scale. In addition, the dependence of the electrical properties of the NiO thin films on the substrate temperature and crystallinity is presented in this paper.