Tuning of structural, optical band gap, and electrical properties of room-temperature-grown epitaxial thin films through the Fe2O3:NiO ratio

We have investigated the structural, optical band gap, and electrical properties of (Fe₂O₃)_0.5x:(NiO)_{1 - 0.5x} (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 Fe₂O₃ 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 Fe²⁺ ions to Fe³⁺ 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 Fe₂O₃:NiO affected the crystal structure, the electrical conductivity and the optical band gap of direct transition (3.35 to 2.99 eV).