Tuning of structural, optical band gap, and electrical properties of room-temperature-grown epitaxial thin films through the FeO:NiO ratio.

We have investigated the structural, optical band gap, and electrical properties of (FeO):(NiO) (x = 0.3, 0.4, 0.5, 0.6 and 0.7) epitaxial thin films grown on an atomically smooth substrate at room temperature. With increasing FeO content, the rock-salt structure of the thin films transformed to a spinel structure above x = 0.6. In terms of the local structure, the increased ratio of Fe ions to Fe ions indicates that the octahedral sites of FeO were continuously transformed into distorted octahedral and tetrahedral sites. On the other hand, the NiO matrix was not affected by the local structure change. Chemical composition of FeO:NiO affected the crystal structure, the electrical conductivity and the optical band gap of direct transition (3.35 to 2.99 eV).