Room-temperature selective epitaxial growth of CoO (111) and Co3O4 (111) thin films with atomic steps by pulsed laser deposition

Cobalt oxide epitaxial thin films of both rock-salt CoO and spinel Co3O4 were selectively synthesized on atomically stepped alpha-Al2O3 (0 0 0 1) substrates at room temperature under well-controlled oxygen pressures by pulsed laser deposition. X-ray diffraction and reflection high-energy electron diffraction analyses demonstrated that the CoO and Co3O4 films were grown with phase control and good epitaxial quality at room temperature (20 degrees C). The Co0 (1 1 1) film was obtained in ultra-high vacuum of 1 x 10(-8) Torr, while the Co3O4 (1 1 1) film was grown in 1 x 10(-2) Torr of O-2. X-ray reciprocal space mapping results indicated that the in-plane mismatches of the {1 = 10} planes of CoO (1 1 1) and Co3O4 (1 1 1) films with the substrate were 4.5% and 2.5%, respectively. The films were almost entirely relaxed with ratios of expansion less than +2%; the films underwent slight elongation along the [1 1 1] axis and shrinkage in the (1 1 1) plane. The surfaces of the as-grown CoO and Co3O4 thin films revealed atomic steps reflective of those on the substrates. Their root-mean-square roughness values were about 0.1 nm indicating suppressed grain growth on the substrates at room temperature. The optical bandgap of the epitaxial CoO (1 1 1) film was estimated to be 2.72 eV accompanied with a broad absorption attributable to non-stoichiometry or d-d transition. The bandgap of the Co3O4 (1 1 1) film was evaluated as 1.42 eV, and also absorption at 1.86 eV was observed. The obtained room-temperature epitaxial growth of CoO and Co3O4 thin films contributes to enhance their catalytic ability and quality of layer-stacking devices in terms of improving surface/interface flatness and specific surface area. (C) 2015 Elsevier B.V. All rights reserved.