Wafer-scale development, characterization, and high temperature stabilization of epitaxial Cr₂O₃ films grown on Ru(0001)

This work outlines conditions suitable for the heteroepitaxial growth of Cr2O3(0001) films (1.5-20 nm thick) on a Ru(0001)-terminated substrate. Optimized growth is achieved by sputter deposition of Cr within a 4 mTorr Ar/O-2 20% ambient at Ru temperatures ranging from 450 to 600 degrees C. The Cr2O3 film adopts a 30 degrees rotated honeycomb configuration with respect to the underlying Ru(0001) substrate and exhibits a hexagonal lattice parameter consistent with that for bulk Cr2O3(0001). Heating to 700 degrees C within the same environment during film preparation leads to Ru oxidation. Exposure to temperatures at or above 400 degrees C in a vacuum, Ar, or Ar/H-2 3% leads to chromia film degradation characterized by increased Ru 3d XPS intensity coupled with concomitant Cr 2p and O 1s peak attenuations when compared to data collected from unannealed films. An ill-defined but hexagonally well-ordered RuxCryOz surface structure is noted after heating the film in this manner. Heating within a wet Ar/H-2 3% environment preserves the Cr2O3(0001)/Ru(0001) heterolayer structure to temperatures of at least 950 degrees C. Heating an Ru-Cr2O3-Ru heterostacked film to 950 degrees C within this environment is shown by cross-sectional scanning/transmission electron microscopy (S/TEM) to provide clear evidence of retained epitaxial bicrystalline oxide interlayer structure, interlayer immiscibility, and epitaxial registry between the top and bottom Ru layers. Subtle effects marked by O enrichment and O 1s and Cr 2p shifts to increased binding energies are noted by XPS in the near-Ru regions of Cr2O3(0001)/Ru(0001) and Ru(0001)/Cr2O3(0001)/Ru(0001) films after annealing to different temperatures in different sets of environmental conditions.