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Transition metal spinel oxides comprised of earth-abundant Mn and Co have long been explored for their use in catalytic reactions and energy storage. However, understanding functional properties can be challenging due to differences in sample preparation and the ultimate structural properties of the materials. Epitaxial thin film synthesis provides a novel means of producing precisely controlled materials to explore the variations reported in the literature. In this work, MnxCo3−xO4 samples from x = 0 to x = 1.28 were synthesized through molecular beam epitaxy and characterized to develop a material properties map as a function of stoichiometry. Films were characterized via in situ x-ray photoelectron spectroscopy, x-ray diffraction, scanning transmission electron microscopy, and polarized K-edge x-ray absorption spectroscopy. Mn cations within this range were found to be octahedrally coordinated, in line with an inverse spinel structure. Samples largely show mixed Mn3+ and Mn4+ character with evidence of phase segregation tendencies with the increasing Mn content and increasing Mn3+ formal charge. Phase segregation may occur due to structural incompatibility between cubic and tetragonal crystal structures associated with Mn4+ and Jahn–Teller active Mn3+ octahedra, respectively. Our results help in explaining the reported differences across samples in these promising materials for renewable energy technologies. 

This work demonstrates the advantage of carrying out silicon ion (Si+) implantation at high temperatures for forming controlled heavily doped regions in gallium oxide. Room temperature (RT, 25 °C) and high temperature (HT, 600 °C) Si implants were carried out into MBE grown (010) β-Ga2O3 films to form ∼350 nm deep Si-doped layers with average concentrations up to ∼1.2 × 1020 cm−3. For such high concentrations, the RT sample was too resistive for measurement, but the HT samples had 82.1% Si dopant activation efficiency with a high sheet electron concentration of 3.3 × 1015 cm−2 and an excellent mobility of 92.8 cm2/V·s at room temperature. X-ray diffraction measurements indicate that HT implantation prevents the formation of other Ga2O3 phases and results in reduced structural defects and lattice damage. These results are highly encouraging for achieving ultra-low resistance heavily doped Ga2O3 layers using ion implantation.