Oxygen-Deficient Line Defects in an Ultrathin Aluminum Oxide Film

A model for the straight antiphase domain boundary of the ultrathin aluminum oxide film on the NiAl(110) substrate is derived from scanning tunneling microscopy measurements and density-functional theory calculations. Although the local bonding environment of the perfect film is maintained, the structure is oxygen deficient and possesses a favorable adsorption site. The domain boundary exhibits a downwards band bending and three characteristic unoccupied electronic states, in excellent agreement with scanning tunneling spectroscopy measurements.

Figure 1

(a)–(d) Experimental and theoretical STM images of the domain boundary: (a) 80 K, $-570\mathrm{mV}$, 0.34 nA; (b) RT, $-0.4\mathrm{mV}$, 0.5 nA; (c) 5 K, $-250\mathrm{mV}$, 0.5 nA; (d) simulation based on the Tersoff-Hamann approach. Models of (e) the perfect film and (f) the domain boundary. In (g) only the oxygen (${\mathrm{O}}_i$) and aluminum (${\mathrm{Al}}_i$) atoms close to the NiAl substrate are shown for the domain boundary model. Calculated atom positions are also shown as circles in the STM images. The unit cell used in the simulation is denoted by the white rectangle, and the dashed lines show where extra atoms are inserted in the boundary between two oxide domains. Pentagons in the interface and some characteristic units in the surface are highlighted by white and yellow lines, respectively.

Figure 2

PDOS of (a) the perfect oxide and (b) the cell containing a domain boundary, calculated using the hybrid functional. Three peaks are highlighted by the vertical arrows, and the conduction band (CB) and valence band (VB) edges are marked. The energies are aligned at the Fermi level.

Figure 3

(a) Isosurface (light blue) of the density distribution corresponding to the states in the vicinity of the first peak in the conduction band. The Al atoms (dark blue), on which the lowest defect state is localized, are marked as Al1 and Al2. For guidance, the domain boundary is indicated by the dashed white lines and the same features as in Fig. 1f are highlighted by yellow lines. (b) Optimized structure with an additional O atom forming bonds with the atoms Al1, Al2, and Al3.