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.

Reactive RF magnetron sputtering epitaxy of NiO thin films on (0001) sapphire and (100) MgO substrates

“…Meanwhile, Kokubun et al reported an increase in the lattice parameters at high heat treatment temperatures caused by Mg diffusion from the MgO substrate to NiO epilayer during the heat treatment in the solgel process. 37) Therefore, it is assumed that there could be Mg diffusion from the MgO substrate near the interface in our experiment. One of the reasons for this irregular interface is the roughness of the MgO substrate before deposition of NiO at 500 °C.…”

“…Epitaxial NiO thin films are grown using oxidation of Ni, 28,29) pulsed laser deposition, 5,6,17,18,21,30,31) electron beam evaporation, 32) molecular beam epitaxy, 33) atomic layer deposition, 34) mist chemical vapor deposition, 35) and sol-gel method. 36,37) Among these, sputtering is the most suitable approach for depositing large-area films with well-controlled compositions economically. However, the epitaxial growth mechanisms of NiO thin films grown by sputtering have not yet been clarified because of the relatively few reported studies.…”