Conduction at a Ferroelectric Interface

Typical logic elements utilizing the field effect rely on the change in carrier concentration due to the field in the channel region of the device. Ferroelectric-field-effect devices provide a nonvolatile version of this effect due to the stable polarization order parameter in the ferroelectric. In this work, we describe an oxide/oxide ferroelectric heterostructure device based on (001)-oriented ${\mathrm{PbZr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3\text{−}{\mathrm{LaNiO}}_3$ where the dominant change in conductivity is a result of a significant mobility change in the interfacial channel region. The effect is confined to a few atomic layers at the interface and is reversible by switching the ferroelectric polarization. More interestingly, in one polarization state, the field effect induces a 1.7-eV shift of the interfacial bands to create a new conducting channel in the interfacial PbO layer of the ferroelectric.

Figure 1

Resistivity and switching behavior of ${\mathrm{PbZr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3/{\mathrm{LaNiO}}_3$ devices. (a) Schematic of the $\mathrm{PZT}/{\mathrm{LaNiO}}_3$ devices, shown with gold electrodes. (b) Cross-sectional transmission electron micrograph of a PZT/4-u.c. ${\mathrm{LaNiO}}_3$ device. (c) Resistivity versus temperature for $\mathrm{PZT}/{\mathrm{LaNiO}}_3$ devices with ${\mathrm{LaNiO}}_3$ thicknesses of 3, 4, and 8 u.c. (solid and dotted lines correspond to accumulation and depletion of holes, respectively). (d) The polarization-electric field ($P\text{−}E$) loop obtained from a structure of 150 nm PZT/3 u.c. ${\mathrm{LaNiO}}_3$. The remanent polarization is approximately equal to $30\mu \mathrm{C}/{\mathrm{cm}}^2$. In (e), the room-temperature resistivity of a 3-u.c. ${\mathrm{LaNiO}}_3$ device is shown as a function of time as the system is switched from the low-resistance state (accumulation of holes) to the high-resistance state (depletion of holes). (f) Polarization-dependent carrier mobility determined from Hall measurements for a PZT/4-u.c. ${\mathrm{LaNiO}}_3$ device.

Figure 2

Electronic structure of PTO/4-u.c. ${\mathrm{LaNiO}}_3$ strained to the theoretical in-plane lattice constant of ${\mathrm{LaAlO}}_3$. A 2D projection of the local density of electronic states derived from first-principles calculations integrated within $\pm k_{B}T\mathrm{eV}$ of the Fermi level ($T=300\mathrm{K}$) at the interface of ${\mathrm{PbTiO}}_3$ and ${\mathrm{LaNiO}}_3$ for (a) the accumulation state and (b) the depletion state. The red contours indicate a higher density of states, and the white areas indicate a lower density of states. (c) Temperature-dependent resistivity measurements for devices with 3-u.c.-thick ${\mathrm{LaNiO}}_3$ in the accumulation (solid blue line) and depletion states (dashed blue line). The resistivity, normalized to 1 u.c. of interfacial PZT, is also shown (red line). Expanded versions of (a) and (b) are shown in Fig. S1.

Figure 3

Band structure of $\mathrm{PTO}/{\mathrm{LaNiO}}_3$ strained to the theoretical in-plane lattice constant of ${\mathrm{LaAlO}}_3$. Band structure for (a) accumulation and (b) depletion, where the zero of energy is the Fermi level in each case and a ($1\times 1$) interfacial unit cell is employed. Red-colored bands correspond to dominant ${\mathrm{LaNiO}}_3$ character, while blue-colored bands indicate strong contributions from the interfacial PbO layer. The tops of the PbO-dominated bands shift from about $-1.2\mathrm{eV}$ in depletion to approximately 0.5 eV in accumulation. (c) The layer-resolved LDOS at the interface for accumulation and (d) depletion. The ${\mathrm{PbTiO}}_3$ is insulating in depletion and is insulating in accumulation away from the interface, which has LDOS at the Fermi level.