Room-temperature selective epitaxial growth of CoO (1 1 1) and Co3O4 (1 1 1) thin films with atomic steps by pulsed laser deposition

Abstract

Cobalt oxide epitaxial thin films of both rock-salt CoO and spinel Co3O4 were selectively synthesized on atomically stepped α-Al2O3 (0001) 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°C). The CoO (111) film was obtained in ultra-high vacuum of 1×10−8Torr, while the Co3O4 (111) film was grown in 1×10−2Torr of O2. X-ray reciprocal space mapping results indicated that the in-plane mismatches of the {1=10} planes of CoO (111) and Co3O4 (111) 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 [111] axis and shrinkage in the (111) 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 0.1nm indicating suppressed grain growth on the substrates at room temperature. The optical bandgap of the epitaxial CoO (111) film was estimated to be 2.72eV accompanied with a broad absorption attributable to non-stoichiometry or d-d transition. The bandgap of the Co3O4 (111) film was evaluated as 1.42eV, and also absorption at 1.86eV 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.