Magnesium Diffusion from MgO Substrates in Sol–Gel‐Derived NiO Epitaxial Films: Effects of Heat Treatment Temperature and Li Doping

Abstract

The effects of heat treatment temperature and Li doping on the structural properties of sol–gel‐derived NiO epitaxial films on MgO (100) substrates are investigated. X‐ray diffraction measurements indicate that the out‐of‐plane lattice parameters of undoped NiO films increase as the heat treatment temperature increases. This increase is particularly significant in films treated above 900 °C, in which out‐of‐plane and in‐plane lattice parameters are larger than the expected values for bulk NiO. This result cannot be explained by the strain originating in the lattice mismatch between NiO and MgO. In addition, the bandgap drastically increases in undoped films treated above 900 °C. Secondary ion mass spectrometry depth profiling demonstrates that undoped films treated above 900 °C are Ni1−xMgxO alloys generated by the diffusion of Mg from MgO substrates into NiO films, resulting in the anomalous characteristics of the prepared films. Furthermore, it indicates that the diffusion of Mg is reduced in Li‐doped films, which can be explained on the basis of defect chemistry, in which Ni vacancies decrease with the increase in Li concentration. Consequently, the increases in the lattice parameter and bandgap at high heat treatment temperatures are reduced in Li‐doped films.