Negative differential resistance induced by Mn substitution at ${\text{SrRuO}}_3/\text{Nb}:{\text{SrTiO}}_3$ Schottky interfaces

We observed a strong modulation in the current-voltage characteristics of $\text{Sr}{\text{RuO}}_3/\text{Nb}:{\text{SrTiO}}_3$ Schottky junctions by Mn substitution in ${\text{SrRuO}}_3$, which induces a metal-insulator transition in the bulk. The temperature dependence of the junction ideality factor indicates an increased spatial inhomogeneity of the interface potential with substitution. Furthermore, negative differential resistance was observed at low temperatures, indicating the formation of a resonant state by Mn substitution. By spatially varying the position of the Mn dopants across the interface with single unit cell control, we can isolate the origin of this resonant state to the interface ${\text{SrRuO}}_3$ layer. These results demonstrate a conceptually different approach to controlling the interface states by utilizing the highly sensitive response of conducting perovskites to impurities.

Figure 1

(Color online) Temperature-dependent resistivity $\left(\rho \right)$ of the ${\text{SrRuO}}_3$ (bold) and ${\text{SrRu}}_{0.95}{\text{Mn}}_{0.05}{\text{O}}_3$ (solid) thin films that were grown on ${\text{SrTiO}}_3\left(100\right)$ substrates. The increased resistivity and the reduced $T_C$ are evident for the ${\text{SrRu}}_{0.95}{\text{Mn}}_{0.05}{\text{O}}_3$ thin film.

Figure 2

(Color online) Temperature dependence of the $I\text{−}V$ characteristics in the (a) ${\text{SrRuO}}_3/\text{Nb}:{\text{SrTiO}}_3$ and (b) ${\text{SrRu}}_{0.95}{\text{Mn}}_{0.05}{\text{O}}_3/\text{Nb}:{\text{SrTiO}}_3$ junctions. Measurements were taken between 300 and 20 K in 40 K steps.

Figure 3

(Color online) (a) Richardson plot of ${\text{SrRuO}}_3$ $\left({\text{SrRu}}_{0.95}{\text{Mn}}_{0.05}{\text{O}}_3\right)/\text{Nb}:{\text{SrTiO}}_3$ junctions showing a crossover from the thermoionic field emission to the field emission. (b) Evaluation of the Schottky barrier height spatial inhomogeneity from the ideality factors. The circles (squares) denote ${\text{SrRuO}}_3$ $\left({\text{SrRu}}_{0.95}{\text{Mn}}_{0.05}{\text{O}}_3\right)$ and the lines are linear fits to the experimental data in the high temperature regime.

Figure 4

(Color online) Temperature dependence of the (a) $I\text{−}V$ characteristics and (b) the normalized conductance in a ${\text{SrRu}}_{0.95}{\text{Mn}}_{0.05}{\text{O}}_3/{\text{SrTiO}}_3$ junction. Solid (dotted) lines in (b) denote the voltage sweeps in the positive (negative) direction. The NDR emerges as the temperature is decreased.

Figure 5

(Color online) (a) Schematic diagram of the artificial structures grown to systematically vary the distribution of the Mn ions with respect to the interface. (1) ${\text{SrRuO}}_3/{\left[{\text{SrMnO}}_3\right]}_{1uc}/{\text{Nb:SrTiO}}_3$, (2) ${\text{SrRu}}_{0.95}{\text{Mn}}_{0.05}{\text{O}}_3/\text{Nb}:{\text{SrTiO}}_3$, (3) ${\text{SrRuO}}_3/{\left[{\text{SrRu}}_{0.95}{\text{Mn}}_{0.05}{\text{O}}_3\right]}_{1uc}/\text{Nb}:{\text{SrTiO}}_3$, (4) ${\text{SrRuO}}_3/\text{Nb}:{\text{SrTiO}}_3$, and (5) ${\text{SrRuO}}_3/{\left[{\text{SrTi}}_{0.95}{\text{Mn}}_{0.05}{\text{O}}_3\right]}_{1uc}/\text{Nb}:{\text{SrTiO}}_3$. (b) The $I\text{−}V$ characteristics and (c) the normalized conductance of the structures illustrated in (a). The NDR is observed only in structures (2) and (3).

Figure 6

Annular dark field STEM images of a $\left({\text{SrRu}}_{1-x}{\text{Mn}}_x{\text{O}}_3/{\text{SrTiO}}_3\right)$ multilayer. (a) Overview of the structure. (b) High magnification image of the abrupt ${\text{SrRu}}_{0.95}{\text{Mn}}_{0.05}{\text{O}}_3/{\text{SrTiO}}_3$ interface.

Figure 7

(Color online) ${\text{Mn-L}}_{2,3}$ electron energy loss spectra from the three Mn-doped ${\text{SrRuO}}_3$ layers imaged in Fig. 6a. For comparison, the ${\text{Mn}}^{3+}$ and ${\text{Mn}}^{4+}$ reference spectra are shown.

Figure 8

(a) A schematic of the density of states for ${\text{SrRuO}}_3$ and ${\text{SrMnO}}_3$. (b) An interface band diagram illustrating the localized state on the metal side of the interface. $V_F$ is the applied forward bias voltage, $E_F$ is the Fermi level, ${\Phi }_{SB}$ the Schottky barrier height, and $E_{\text{loc}}$ is the localized state at the interface.