Gundlach oscillations and Coulomb blockade of Co nanoislands on MgO/Mo(100) investigated by scanning tunneling spectroscopy at 300 K

Ultrathin MgO films on Mo(100) with a thickness up to 12 ML are studied by scanning tunneling microscopy and spectroscopy at room temperature. The spatial variation in the work function within the MgO film is mapped by field emission resonance states (Gundlach oscillations) using $dz/dU$ spectroscopy. We found circular spots with significantly reduced work function $\left(\Delta \Phi =0.6\text{eV}\right)$, which are assigned to charged defects within the MgO film. On top of the MgO films, small Co clusters are deposited with an average contact area of $A_{\text{Co}}\simeq 4{\text{nm}}^2$. These islands exhibit Coulomb oscillations in $dI/dU$-spectra at room temperature. Good agreement with orthodox theory is achieved showing variations in the background charge $Q_0$ for islands at different positions, which are in accordance with the work function differences determined by the Gundlach oscillations.

Figure 1

(Color online) (a) $\left(200\times 200\right){\text{nm}}^2$ STM image of 7–8 ML MgO film on Mo(100) $\left(U=3\text{V},I=0.5\text{nA}\right)$; the typical Moiré pattern caused by the lattice mismatch of Mo and MgO is visible; the marked crystallographic directions of Mo are determined by LEED. The lines emphasize the deviation of the Moiré pattern from the Mo[001] direction within different MgO facets. (b) Zoom into the MgO film as marked by a dashed line in image (a); the Moiré pattern and a screw dislocation indicated by an arrow is visible. (c) MgO sample consisting of 11–12 ML thick islands and valleys in between consisting of 1–2 ML MgO. The inset shows the Moiré pattern still appearing on MgO islands of 11 ML. (d) Spectra of $dI/dU\left(U\right)$ measured on MgO areas of 2, 7, and 11 ML thickness as indicated $\left(U_{\text{stab}}=3\text{V},I_{\text{stab}}=0.45\text{nA},U_{\text{mod}}=40\text{mV}\right)$; the deduced band gap is marked.

Figure 2

(Color online) (a) $\left(40\times 40\right){\text{nm}}^2$ STM image of 7–8 ML thick MgO film $\left(U=3.5\text{V},I=0.5\text{nA}\right)$. (b) $dz/dU$ map recorded at $U=3.5\text{V}$ of the same area as shown in (a) $\left(I=0.5\text{nA},U_{\text{mod}}=60\text{mV}\right)$; only the grooves in between the islands exhibit spectroscopic contrast. (c) $dz/dU$ map recorded at $U=4.0\text{V}$ of the same sample area as (a) and (b) $\left(I=0.5\text{nA},U_{\text{mod}}=60\text{mV}\right)$; additional bright spots appear on parts of the MgO film. (d) Sketch of the potential scheme within the tunneling barrier explaining the field resonance states; $E_F$, $E_V$, and $\Phi$ marked at tip and sample are the Fermi levels, vacuum levels and work functions, respectively; the black line in the vacuum region is the vacuum level influenced by applied electric field and the image potential of tip and sample (white line without image potential); FER states are marked by $n=1$, $n=2$, $n=3$, and $n=4$. (e) $dz/dU$ spectra taken at the positions marked in c) $\left(U_{\text{stab}}=3.5\text{V},I_{\text{stab}}=0.5\text{nA},U_{\text{mod}}=100\text{mV}\right)$; the first two FER states $n=1$ and $n=2$ are labeled for curve 1; the $n=1$ state shifts by about $\Delta E=620\text{meV}$ between curve 1 and curve 2. Inset: three FER peaks measured on a dark region of c). (f) $dI/dU\left(U\right)$ spectra recorded on two of the positions marked in c) $\left(U_{\text{stab}}=4.0\text{V},I_{\text{stab}}=0.5\text{nA},U_{\text{mod}}=26\text{mV}\right)$; the apparent band gap exhibits a peak at $U=-2.7\text{V}$ only in curve 1, which indicates the defect state.

Figure 3

(Color online) (a) Schematic drawing of the tunnel junction consisting of tip/vacuum/Co-island and Co-island/MgO/Mo(100) together with equivalent circuit diagram. (b) STM image $\left(25\times 20\right){\text{nm}}^2$ of Co nanoislands on MgO $\left(U=3.5\text{V},I=0.5\text{nA}\right)$. (c)–(f) $dI/dU\left(U\right)$ spectra recorded on Co islands with different sizes $A_{\text{Co}}$ as indicated $\left(U_{\text{stab}}=3.5\text{V},I_{\text{stab}}=0.7\text{nA},U_{\text{mod}}=40\text{mV}\right)$; the Coulomb gap $\Delta U_0$ and the charging voltage $\Delta U$ are marked in (c); spectrum (f) is measured on a Co island on 10 ML MgO, spectra (c)–(e) are recorded on an island of 11 ML MgO.