Modifying the crystal structures of Fe2O3-doped NiO epitaxial thin films grown at room temperature by controlling the oxygen partial pressure

Abstract

Herein, we investigated the influence of oxygen partial pressure (pO2) on average long- and short-range transformations in the crystal structures of ultra-smooth Fe2O3-doped NiO thin films that were epitaxially grown at room temperature. The films were deposited via pulsed laser deposition, and the oxygen content in the films was controlled by adjusting the pO2 to 1, 10-3, and 10-5 Pa. Result showed that d-spacing and lattice strain decreased with increasing pO2. The thin film grown at 10-5 Pa had a rock salt structure, whereas films grown at pO2s of 10-3 and 1 Pa had spinel structures. The local structures and valence states indicate that local Fe coordination transitioned from a distorted octahedral geometry to a tetrahedral/distorted octahedral geometry at higher pO2s. Additionally, the valence of Fe increased from Fe2+ to Fe3+, indicating structural phase transformation, which is supported by the X-ray diffraction analysis results. In contrast, the valence and local environment of Ni varied little. The structural transformation of the films was accompanied by the narrowing of the optical bandgap for direct transition and an increase in electrical resistivity.