Figure 1

(Color online) STM topograph (a) and simultaneously acquired DOS map (b) for a $15\mathrm{\AA }$ sputtered, annealed MgO film on Fe, acquired at a tip bias of $2\mathrm{V}$, $100\mathrm{pA}$. There is little overall correlation between electronic structure and topography. DOS measurements $(dI∕dV)$ across the surface [(c), dotted lines], show uniform band-gap values $(\sim 1.5\mathrm{V})$; structure in the DOS map is due to a variation in strength of band tails. Curves are offset for clarity. The solid curve is from an identical, but unannealed sample. The band gap is $(\sim 0.5\mathrm{V})$ smaller.Reuse & Permissions

Figure 2

(Color online) DOS measurements for several $20\mathrm{\AA }$ film preparation methods (a). Sputtering produces a band gap of $\sim 1.5\mathrm{eV}$, which annealing further increases to $\sim 2.5\mathrm{eV}$. Annealing sputtered films broadens the band gap and reduces vacancy-defect sites. E-beam evaporation results in band gaps that are not improved upon annealing. Measurements for $30\mathrm{\AA }$ (b) films show that as the surface moves away from the strained, disordered $\mathrm{Fe}∕\mathrm{Mg}\mathrm{O}$ interface the vacancy-defect density drops and the band gap increases.Reuse & Permissions

Figure 3

(Color online) DOS measurements (a) for a $20\mathrm{\AA }$ annealed, sputter deposited MgO film, taken at the lowest points in the DOS map (inset, circled) show peaks characteristic of oxygen vacancy sites 1.75 and $0.5\mathrm{V}$ above the Fermi level. Image parameters were $-2.5\mathrm{V}$ tip bias and $100\mathrm{pA}$. (b) shows a peak from a similar DOS spot in an unannealed $20\mathrm{\AA }$ film. The discrete levels become smeared out into a band tail in the more disordered unannealed sample. (c) DOS measurements on a defective site from a $30\mathrm{\AA }$ sputtered, annealed MgO film. Curves are offset for clarity. The initial curve exhibits very close to the full $7.8\mathrm{eV}$ MgO bulk band gap, while a defect state (see arrows) due to a Mg vacancy appears in subsequent measurements as the STM tip drifts. This state is $\sim 1\mathrm{V}$ above the valence-band maxima (VBM).Reuse & Permissions