Experimental study of the interfacial cobalt oxide in ${\text{Co}}_3{\text{O}}_4/\alpha -{\text{Al}}_2{\text{O}}_3\left(0001\right)$ epitaxial films

A detailed spectroscopic and structural characterization of ultrathin cobalt oxide films grown by O-assisted molecular-beam epitaxy on $\alpha -{\text{Al}}_2{\text{O}}_3\left(0001\right)$ single crystals is reported. The experimental results show that the cobalt oxide films become progressively more disordered with increasing thickness, starting from the early stages of deposition. Low-energy electron-diffraction patterns suggest that the unit cell remains similar to that of $\alpha {\text{-Al}}_2{\text{O}}_3\left(0001\right)$ up to a thickness of $17\text{Å}$, while at larger thicknesses a pattern identified with that of ${\text{Co}}_3{\text{O}}_4\left(111\right)$ becomes visible. X-ray photoelectron spectroscopy reveals sudden changes in the shape of the $\text{Co}2p$ lines from 3.4 to $17\text{Å}$ cobalt oxide thickness, indicating the transition from an interfacial cobalt oxide layer toward [111]-oriented ${\text{Co}}_3{\text{O}}_4$. In particular, the absence of characteristic satellite peaks in the $\text{Co}2p$ lines indicates the formation of a trivalent, octahedrally coordinated, interfacial cobalt oxide layer during the early stages of growth, identified as the ${\text{Co}}_2{\text{O}}_3$ corundum phase.

Figure 1

RHEED patterns of the cobalt oxide film along two different azimuths of the $\alpha {\text{-Al}}_2{\text{O}}_3\left(0001\right)$ surface at several stages of the film growth, as labeled. The incident electron-beam energy is 15 keV.

Figure 2

(a) LEED patterns of the cobalt oxide film at several stages of film growth, as labeled. The incident electron-beam energy is 100.0 eV for the patterns shown, except for the $34\text{Å}$ (116.8 eV) and $120\text{Å}$ films (138.2 eV). (b) LEED line profiles across the diffraction spots labeled A and B in (a) for the different cobalt oxide thickness; the profile for ${\text{Co}}_3{\text{O}}_4\left(111\right)$ corresponds to a LEED pattern of a 38 nm ${\text{Co}}_3{\text{O}}_4\left(111\right)/\alpha {\text{-Al}}_2{\text{O}}_3\left(0001\right)$ film annealed in air at $600°\text{C}$ for 14 h (104.3 eV), shown in (c). The dashed rhombus in (a) and (c) represent the LEED unit cells of $\alpha {\text{-Al}}_2{\text{O}}_3$ and ${\text{Co}}_3{\text{O}}_4$, respectively.

Figure 3

XPS spectra as a function of cobalt oxide film thickness at the $\text{Co}2p$ and $\text{O}1s$ edges. Arrows indicate the energy position of the O KVV loss peaks, dashed lines indicate main peak positions, and dotted lines indicate the energy position of the ${\text{Co}}_3{\text{O}}_4$ satellite peaks. Data have been shifted vertically for convenient display.

Figure 4

Variation in the $\text{O}1s$ XPS peak full width at half maximum as a function of cobalt oxide thickness.

Figure 5

(Color online) (a) Atomic structure of the stoichiometric corundum (0001) surface. (b) Model for the epitaxial growth of the ${\text{Co}}_3{\text{O}}_4\left(111\right)$ structure on the corundum (0001) surface, showing the formation of the octahedral ${\text{Co}}^{3+}$ layer. (c) Alternative growth model, showing the formation of the mixed-valence tetrahedral ${\text{Co}}^{2+}$-octahedral ${\text{Co}}^{3+}$-tetrahedral ${\text{Co}}^{2+}$ layer of ${\text{Co}}_3{\text{O}}_4\left(111\right)$ on the corundum (0001) surface. For each surface, the dashed area indicates the primitive surface unit cell.