Imaging the buried MgO/Ag interface: Formation mechanism of the STM contrast

Scanning tunneling microscopy (STM) provides real-space electronic state information at the atomic scale that is most commonly used to study materials surfaces. An intriguing extension of the method is to attempt to study the electronic structure at an insulator/conductor interface by performing low-bias imaging above the surface of an ultrathin insulating layer on the conducting substrate. We use first-principles theory to examine the physical mechanisms giving rise to the formation of low-bias STM images in the MgO/Ag system. We show that the main features of the low-bias STM contrast are completely determined by the atoms on the surface of MgO which overcomes prior ambiguities in assigning observed STM features to atomic positions of the substrate or thin film in such an epitaxial thin film system. Hence, the low-bias contrast is formed by states at the Fermi level in the Ag that propagate evanescently through the lattice and atomic orbitals of the MgO on their way to the surface. We develop a number of analysis techniques based on an ab initio tight-binding representation that allows identification of the origin of the STM contrast in cases where previous approaches have proven ambiguous.

Figure 1

The unit cell describing two MLs of MgO on top of three MLs of Ag (001): “i” means “interface” and “s” stands for “surface.” The in-plane [110] and $\left[1\overline{1}0\right]$ directions are labeled as is the out-of-plane [001] direction.

Figure 2

Total (DOS) and projected density of states (PDOS) of (a) bulk MgO and (b) a two-ML free-standing MgO film in vacuum. The energy is referenced to the top of the valence band indicated by the vertical black line at zero energy. To help visualize the conduction-band states, DOS and PDOS are multiplied by a factor of three for energy above zero.

Figure 3

PDOS on MgO atomic orbitals of the two-ML MgO/Ag interfacial system. Energies are referenced to the Fermi level which is set to zero energy.

Figure 4

Constant-height near-zero-bias STM simulations of surfaces of (a) Ag (001), (b) two-ML MgO/Ag(001), and (c) three-ML MgO/Ag(001). The images are computed at 1 Å above the surfaces. The blue (red) circles denote the positions of Mg (O) atoms at the surface monolayer. The white circles denote positions of Ag atoms of the interfacial layer. The dashed squares outline the unit cell used in these calculations.

Figure 5

Dependence of STM images on the overlayer-substrate distance $d$ for two-ML MgO/Ag (001). (a) is based on our theoretically relaxed structure with $d=2.68$ Å and is identical to Fig. 4. (b) uses $d$ fixed to 2.47 Å. STM images are computed at 1 Å above the surfaces. The blue (red) circles denote the positions of Mg (O) atoms at the surface monolayer. The dashed squares outline the unit cell used in the calculations.

Figure 6

(a) Side view of the two-ML MgO/Ag(001) system with the MgO overlayer shifted horizontally in the $\left[110\right]$ direction by half of the unit cell from its relaxed position. (b) Corresponding constant-height near-zero-bias STM image simulated at 1 Å above the surface. The blue (red) circles denote the positions of Mg (O) atoms at the surface monolayer. The white circles denote positions of Ag atoms at the interfacial layer. The dashed squares outline the unit cell used in the simulation.

Figure 7

(a) Side view of the two-ML MgO/Ag(001) system with the MgO overlayer stretched horizontally. (b) Corresponding constant-height near-zero-bias STM image. The dashed rectangle outlines the unit cell used in the simulation.

Figure 8

(a) Side view of the two-ML MgO/Ag(001) system with only the interfacial MgO layer stretched horizontally. (b) Corresponding constant-height near-zero-bias STM image. The dashed rectangle outlines the unit cell used in the simulation.

Figure 9

Local density of states at the Fermi level for the structures shown in Figs. 7 and 8. The isosurface is shown at $\sim 10\%$ of the maximum value of the function.

Figure 10

Band structure versus in-plane wave vector $\mathbf{k}$ for two MLs of MgO on Ag (001). (Black) solid lines show the plane-wave-basis band structure and (red) dotted lines show the tight-binding band structure. The Fermi level is placed at 0 eV.

Figure 11

Constant-height LDOS simulations of the two-ML MgO/Ag(001) system based on the atomic-orbital tight-binding model and selective omission of orbitals in the computation of the LDOS at $E_{\mathrm{F}}$ in Eq. (2): (a) all orbitals included, (b) orbitals of all Ag atoms omitted, (c) orbitals of the surface Mg omitted, and (d) orbitals of the surface O omitted. All images are computed at 1 Å above the surface. The blue (red) circles denote the positions of Mg (O) atoms on the surface layer. The white circles denote positions of Ag atoms at the interfacial layer. The dashed squares outline the unit cells used in the calculations.

Figure 12

Constant-height LDOS simulations based on the plane-wave theory [panels (a) and (b)] and the tight-binding model [panels (c) and (d)] for the two-ML MgO/Ag(001) system. Panels (a) and (c) correspond to a relaxed geometry of the interface with MgO/Ag separation distance $d=2.68$ Å, while panels (b) and (d) correspond to fixed $d=2.47$ Å. The images are computed at 1 Å above the surface. The blue (red) circles denote the positions of Mg (O) atoms at the surface monolayer. The white circles denote positions of Ag atoms at the interfacial layer. The dashed squares outline the unit cells used in the calculations.

Figure 13

Isosurfaces of the square of the wave functions for the atomic orbitals of the Mg $3p_z$, O $2p_z$, Ag $5s$, and Ag $5p_z$ states. The isosurfaces are shown for $\sim 60\%$ of their respective maximum values.

Figure 14

Constant-height LDOS simulations of the two-ML MgO/Ag(001) system based on the atomic-orbital tight-binding model and selective zeroing of the interfacial couplings $H_{mi}^{\mathbf{k}}$: (a) unperturbed Hamiltonian, (b) zeroed ${\mathrm{Ag}}_{\mathrm{i}}-{\mathrm{Mg}}_{\mathrm{i}}$ coupling, (c) zeroed ${\mathrm{Ag}}_{\mathrm{i}}-{\mathrm{O}}_{\mathrm{i}}$ coupling. All images are computed at 1 Å above the surface. The blue (red) circles denote the positions of Mg (O) atoms on the surface layer. The white circles denote positions of Ag atoms at the interfacial layer. The dashed squares outline the unit cells used in the calculations.

Figure 15

Density of states for the two-ML MgO/three-ML Ag(001) system computed with various Brillouin zone integration techniques. The Fermi level in each case is indicated by a vertical line at zero energy.

Figure 16

(a) Constant-height and (b) constant-density LDOS at Fermi level for the two-ML MgO/three-ML Ag(001) system. The constant-height image is computed at 1 Å above the surface. The constant-density image is simulated for the density roughly corresponding to the density minimum (darkest region) in panel (a).

Figure 17

Constant-height near-zero-bias STM simulations of the two-ML MgO/Ag(001) system at (a) 1 Å (b) 2 Å, and (c) 3 Å above the surface.