Morphology of Monolayer MgO Films on Ag(100): Switching from Corrugated Islands to Extended Flat Terraces

The ability to engineer nearly perfect ultrathin oxide layers, up to the limit of monolayer thickness, is a key issue for nanotechnological applications. Here we face the difficult and important case of ultrathin MgO films on Ag(100), for which no extended and well-ordered layers could thus far be produced in the monolayer limit. We demonstrate that their final morphology depends not only on the usual growth parameters (crystal temperature, metal flux, and oxygen partial pressure), but also on aftergrowth treatments controlling so far neglected thermodynamics constraints. We thus succeed in tuning the shape of the oxide films from irregular, nanometer-sized, monolayer-thick islands to slightly larger, perfectly squared, bilayer islands, to extended monolayers limited apparently only by substrate steps.

Figure 1

(a)–(c), top and bottom: STM Images of MgO films of 0.7 ML nominal thickness grown under different conditions. (a) $T_g=450\mathrm{K}$, FC; (b) $T_g=773\mathrm{K}$, FC; (c) $T_g=773\mathrm{K}$, SC. For all panels, image size $21\times 21{\mathrm{nm}}^2$, $I=0.2\mathrm{nA}$. (d) Atomically resolved image of clean Ag(100), used for calibration. High symmetry directions are marked by arrows. Image size $2.4\times 2.4{\mathrm{nm}}^2$, $V=0.1$, $I=0.2\mathrm{nA}$. (e) Height profiles of the different MgO structures cut along the lines marked in the bottom panels of (a)–(c) ($⟨001⟩$ direction, topographic conditions).

Figure 2

$dI/dV$ spectrum of the MgO film of preparation (c), recorded at the point marked in the left-hand inset (image size $17\times 17{\mathrm{nm}}^2$, $V=1.0\mathrm{V}$, $I=0.2\mathrm{nA}$) and showing the gap around the Fermi level. Right-hand inset: Enlargement of the spectrum highlighting the presence of an Ag state around 1.8 V.

Figure 3

$\mathrm{O}1s$ and $\mathrm{Mg}1s$ photoemission spectra corresponding to 0.7 ML MgO films grown following different protocols. Spectra (a), (b), and (c) correspond to preparations of Fig. 1 with the same labels. Traces were scaled on the secondary electron background of the bottom spectrum and then rigidly up-shifted for a better visualization.

Figure 4

HREEL spectra of the 0.7 ML MgO films grown following the different protocols described in the text. Bottom spectra, $T_g=450\mathrm{K}$; top spectra, $T_g=773\mathrm{K}$ (up-shifted for sake of clarity).