Charge transport through O-deficient Au-MgO-Au junctions

Metal-oxide heterostructures have been attracting considerable attention in recent years due to various technological applications. We present results of electronic structure and transport calculations for the Au-MgO-Au (metal-insulator-metal) heterostructure based on density-functional theory and the nonequilibrium Green’s functions method. The dependence of the conductance of the heterostructure on the thickness of the MgO interlayer and the interface spacing is studied. In addition, we address the effects of O vacancies. We observe deviations from an exponentially suppressed conductance with growing interlayer thickness caused by Au-O chemical bonds. Electronic states tracing back to O vacancies can increase the conductance. Furthermore, this effect can be enhanced by enlarging the interface spacing as the vacancy induced Mg states are shifted toward the Fermi energy.

Figure 1

(Color online) Heterostructures under investigation (from left to right): $n=1,2,3$ unit cells of MgO sandwiched between Au leads. Interface conf. I is shown, see the text for details.

Figure 2

(Color online) Projected DOS of atoms at the interface for $n=1$ (left-hand side) and of O atoms in the center of the MgO interlayer for $n=1,2,3$ (right-hand side).

Figure 3

(Color online) Transmission coefficient (left-hand side) and I-V characteristic (right-hand side) for $n=1,2,3$.

Figure 4

(Color online) Projected DOS of atoms at the interface for $n=1$ (left-hand side) and of O atoms in the center of the MgO interlayer for $n=1,2,3$ (right-hand side).

Figure 5

(Color online) Transmission coefficient for $n=1,2,3$. Logarithmic plot of the data in a reduced energy range is shown on the right-hand side.

Figure 6

(Color online) Projected DOS of atoms at the interface at $d=3.06\text{Å}$ for $n=1$ (left-hand side) and of O atoms in the center of the MgO interlayer at different values of the interface spacing for $n=1$ (right-hand side).

Figure 7

(Color online) Transmission coefficient (left-hand side) and I-V characteristic (right-hand side) at interface spacings of $2.05\text{Å}$ and $2.50\text{Å}$ for $n=1$.

Figure 8

(Color online) Projected DOS of atoms at the interface (left-hand side) and in the center of the MgO interlayer (right-hand side) for $n=1$ and a symmetric vacancy, where $d=2.05\text{Å}$.

Figure 9

(Color online) Comparison of the transmission coefficient for $n=1$ and $n=2$.

Figure 10

(Color online) Transmission coefficient (left-hand side) and I-V characteristic (right-hand side) for $n=2$ and various vacancy configurations.

Figure 11

(Color online) Projected DOS of atoms at the interface (left-hand side) and in the center of the MgO interlayer (right-hand side) for $n=1$ and a symmetric vacancy, where $d=3.06\text{Å}$.