Relationship between resistive switching characteristics and band diagrams of $\text{Ti}/{\text{Pr}}_{1-x}{\text{Ca}}_x{\text{MnO}}_3$ junctions

We investigated the Ca-composition dependence of the resistive switching (RS) characteristics and band diagrams in $\text{Ti}/{\text{Pr}}_{1-x}{\text{Ca}}_x{\text{MnO}}_3$ $\left[\text{PCMO}\left(x\right)\right]$ junctions and the impact of oxygen vacancies on the band diagrams. Hysteretic current-voltage characteristics, i.e., the RS effect, were observed for $\text{Ti}/\text{PCMO}\left(x\right)$ junctions with $x<0.8$, whereas junctions consisting of $n$-type semiconducting $\text{PCMO}\left(x\right)$ with $x>0.8$ showed almost no RS effect. The RS ratio $R_{\text{H}}/R_{\text{L}}$, where $R_{\text{H}}$ and $R_{\text{L}}$ are resistances of high- and low-resistance states, respectively, showed a clear $x$ dependence: $R_{\text{H}}/R_{\text{L}}$ increased with increasing $x$ and was maximized at $x\sim 0.4$. Cross-sectional transmission electron microscope images of the switched $\text{Ti}/\text{PCMO}\left(x\right)$ junctions confirmed the formation of amorphous ${\text{TiO}}_y$ layers at the interfaces. Electron energy-loss measurements of the $\text{Mn-}L$ edge indicated that oxygen-deficient $\text{PCMO}\left(x\right)$ layers were formed at the interface due to the electrochemical migration of oxygen ions. Optical-absorption measurements of oxygen-deficient $\text{PCMO}\left(x\right)$ films revealed that the formation of oxygen vacancies increased the band gap of $\text{PCMO}\left(x\right)$. On the basis of the results, we propose a possible model involving the change in barrier height and/or width for the hole-carrier conduction at the $\text{PCMO}\left(x\right)$ interface induced by the electrochemical migration of oxygen vacancies as the mechanism of the RS effect.

Figure 1

(Color online) (a) Out-of-plane lattice constant of Ca-composition-spread epitaxial ${\text{Pr}}_{1-x}{\text{Ca}}_x{\text{MnO}}_3$ [$\text{PCMO}\left(x\right)$] films on ${\text{SrTiO}}_3$ (100) substrate measured by local x-ray diffraction. Out-of-plane lattice constants of ${\text{PrMnO}}_3$ (PMO), ${\text{Pr}}_{0.5}{\text{Ca}}_{0.5}{\text{MnO}}_3$ [PCMO(0.5)], and ${\text{CaMnO}}_3$ (CMO) films separately deposited on STO substrates are shown on the right vertical axis. Estimated spatial change of $x$ in the composition-spread film is shown on the upper horizontal axis (see text). (b) Schematic of the sample used for $I\text{−}V$ measurements of $\text{Ti}/\text{PCMO}\left(x\right)$ junctions. The real sample was 10 mm long and $\sim 5\text{mm}$ wide, containing 41 junctions with various Ca compositions.

Figure 2

(Color online) $I\text{−}V$ characteristics of $\text{Ti}/\text{PCMO}\left(x\right)$ junctions with $x=0$, 0.4, 0.6, and 1 measured at room temperature. Except for the junction with $x=1$, hysteretic behavior was observed for all values. $R_{\text{L}}$ and $R_{\text{H}}$ were determined from the current measured at $V=+1\text{V}$.

Figure 3

Ca-composition, $x$, dependence of resistive switching ratio $R_{\text{H}}/R_{\text{L}}$ measured at room temperature. $R_{\text{H}}/R_{\text{L}}$ apparently depended on the Ca composition $x$, and had a maximum value $\left(>30\right)$ at around $x=0.4$. $\text{Ti}/\text{PCMO}\left(x\right)$ junctions with $x>0.8$ showed little change in resistance.

Figure 4

Cross-sectional TEM images of Ti/PCMO(0.5) junctions observed (a) before and (b) after switching operations at room temperature. In as-prepared junctions, a thin amorphous ${\text{TiO}}_y$ layer $\left(<1\text{nm}\right)$ was confirmed between the Ti and PCMO(0.5) layers. Switching operations (application of voltage stress) increased the thickness of the amorphous ${\text{TiO}}_y$ layer to $\sim 10\text{nm}$. After the switching operations, about 10-nm thick amorphous ${\text{TiO}}_y$ layers were also observed in (c) Ti/PCMO(0) (${\text{PrMnO}}_3$; PMO) and (d) Ti/PCMO(1) (${\text{CaMnO}}_3$; CMO) junctions.

Figure 5

(Color online) (a) Electron energy-loss spectra of $\text{Mn-}L$ edge at several positions in Ti/PCMO(0.5) junction and (b) cross-sectional TEM image of Ti/PCMO(0.5) junction. Positions where each spectrum was measured are indicated in (b). Spectra were normalized by the intensity of the $\text{Mn-}{L}_2$ peak.

Figure 6

(Color online) (a) Optical-absorption spectra of reduced (solid line) and oxygenated (dotted line) $\text{PCMO}\left(x\right)$ films on STO substrates with $x=0$, 0.5, and 1 measured at room temperature. The broken straight lines are fits to the rising part of the absorption spectra, and the band gap was estimated from the intersection of the broken line at the optical-absorption coefficient $\alpha =0$. (b) $1/C^2\text{−}V$ characteristics of reduced and oxygenated $\text{PCMO}\left(x\right)/\text{Nb}:\text{STO}$ junctions with $x=0$, 0.5, and 1 measured at room temperature. The $1/C^2\text{−}V$ characteristics showed a linear relationship, indicating the formation of depletion layers at the interface. Extrapolation of the straight line to $1/C^2=0$ gives the built-in potential $V_{\text{bi}}$. The insets of the upper panels show optical-absorption spectrum and $1/C^2\text{−}V$ curve of as-grown ${\text{PrMnO}}_3$ (PMO) samples which were prepared separately from the samples used in the reduction and oxygenation experiments. The band gap and built-in potential of the as-grown samples are almost identical to those of the oxygenated ones.

Figure 7

Hall resistivity of $\text{PCMO}\left(x\right)$ layers with $x=0.86$ and 1 versus magnetic field at room temperature.

Figure 8

Energetic alignments of conduction and valence bands of $\text{PCMO}\left(x\right)$ layers with $x=0$, 0.5, and 1 to Fermi level of Ti. CB,VB, and ${\text{E}}_{\text{F}}$ are the conduction band, the valence band, and the Fermi level, respectively.

Figure 9

(Color online) (a) Typical $I\text{−}V$ characteristics of Ti/PCMO(0.5) junction. The insets in (a) illustrate schematic models of migration of oxygen vacancies depending on the polarity of an applied voltage. Possible band diagrams of Ti/PCMO(0.5) interface (b) as-prepared, (c) in a high-resistance state (HRS), and (d) in a low-resistance state (LRS). In the HRS, the oxygen-deficient PCMO(0.5) layer at the interface, having a larger band gap, acts as a barrier to hole-carrier conduction.

Figure 10

(a) Typical $I\text{−}V$ characteristics of Ti/CMO junction. The insets illustrate a model of migration of oxygen vacancies. Possible band diagrams of Ti/CMO interface (b) as-prepared, (c) after applying a negative voltage bias, and (d) after applying a positive voltage bias.