Catalytic growth of N-doped MgO on Mo(001)

A simple pathway to grow thin films of N-doped MgO (MgO:N), which has been found experimentally to be a ferromagnetic $d^0$ insulator, is presented. It relies on the catalytic properties of a Mo(001) substrate using the growth of Mg in a mixed atmosphere of O${}_2$ and N${}_2$. Scanning tunneling spectroscopy reveals that the films are insulating and exhibit an N-induced state slightly below the conduction band minimum.

Figure 1

(a) STM image of 7 ML MgO${}_{0.973}$N${}_{0.027}$ film on Mo(001) with thicker islands (bright areas) on top of a wetting layer of 1–2 ML; $(50\times 50)$ nm${}^2$, $U=3$ V, $I=0.5$ nA. (b) Line profile along the red line marked in panel (a) with the height of the island above the wetting layer marked. (c) Tapping mode AFM image of 40 ML MgO${}_{0.993}$N${}_{0.007}$ film on Mo(001) after transfer into an ambient environment; $(100\times 100)$ nm${}^2$, $f=281$ kHz, free amplitude $A=30$ nm, setpoint 60$\%$. (d) Line profile along the red line marked in panel (c).

Figure 2

(a) Differential AES spectra of 7 ML MgO${}_{0.979}$N${}_{0.031}$ with assignment of the peaks different elements recorded at a primary energy of $E=1$ keV. Above the O peak, there is no other peak related to another element. (b) Higher resolution spectra from panel (a) of N and O with the corresponding peak-to-peak height $Y_i$ marked. (c) Nitrogen concentration $x$ of 7-ML-thick MgO:N films as a function of the deposition temperature $T_{\mathrm{D}}$.

Figure 3

(a) Nitrogen concentration $x$ as a function of monolayers $d$. Different symbols mark different preparation methods, the red fit curve assumes a constant amount of N atoms homogeneously distributed within the MgO film, and the black curve assumes the same amount of N at the Mo/MgO interface only (see text). (b) Sketch of N${}_2$ incorporation. Left: N${}_2$ dissociation on Mo(001). Right: Incorporation of N into MgO leaving the amount of N independent of the MgO thickness.

Figure 4

(a) $dI/dU\left(U\right)$ spectra measured by STS on an 11-ML-high MgO${}_{0.96}$N${}_{0.04}$ island ($U_{\text{stab}}=3$ V, $I_{\text{stab}}=0.5$ nA, $U_{\text{mod}}=40$ mV) at several positions (straight lines) and on an 11-ML-thick pristine MgO island (dashed line). The average film thickness in both cases is 7 ML. (b) Calculated density of states (DOS) for substitutional and interstitial impurity as well as a N-N dimer at MgO surface. HOS: Highest occupied state. All states between $-$1.2 eV and 3 eV are N induced; CBM is marked by an arrow. The inset shows the calculated charge density of the unoccupied N-induced states of a N-N dimer at the surface. The large green and small white spheres show Mg and O, respectively.