Modulating the Electrical Transport in the Two-Dimensional Electron Gas at ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ Heterostructures by Interfacial Flexoelectricity

Thin film flexoelectricity is attracting more attention because of its enhanced effect and potential application in electronic devices. Here we find that a mechanical bending induced flexoelectricity significantly modulates the electrical transport properties of the interfacial two-dimensional electron gas (2DEG) at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ ($\mathrm{LAO}/\mathrm{STO}$) heterostructure. Under variant bending states, both the carrier density and mobility of the 2DEG are changed according to the flexoelectric polarization direction, showing an electric field effect modulation. By measuring the flexoelectric response of LAO, it is found that the effective flexoelectricity in the LAO thin film is enhanced by 3 orders compared to its bulk. These results broaden the horizon of study on the flexoelectricity effect in the hetero-oxide interface and more research on the oxide interfacial flexoelectricity may be stimulated.

Figure 1

Bending of the $\mathrm{LAO}/\mathrm{STO}$ heterostructure. (a) The bending fixture setup for the three-point bending on $\mathrm{LAO}/\mathrm{STO}$ samples. The sample illustrated is the $n$-shape bending status. Our definitions of the $n$-shape and $u$-shape bending are illustrated in (b) and (c), respectively. The yellow arrows represent the opposite flexoelectric polarizations in the LAO film under the two different bending status.

Figure 2

The interface resistance change percentage under $u$-shape (negative strain gradient) and $n$-shape (positive strain gradient) bending. To illustrate the resistance change more obviously, the $y$-axis scale is adjusted according to the maximum resistance change values under the corresponding bending status.

Figure 3

The flexoelectric field effect across the $\mathrm{LAO}/\mathrm{STO}$ heterostructure. The output voltage between the $\mathrm{LAO}/\mathrm{STO}$ interface and the Au top electrode on LAO film surface under a periodic dynamic (a) $n$-shape bending and (b) $u$-shape bending. The blue curves are the voltage signal probed from the $\mathrm{LAO}/\mathrm{STO}$ interface. The red curves are the bending displacement at the center of the sample. (c) The experimental interfacial electrical transport property change under different bending states: the circles are carrier densities and the squares are mobilities. The negative and positive strain gradients represent $u$-shape and $n$-shape bending, respectively. The corresponding sheet resistance change is shown in Fig. S4 of the Supplemental Material [28], where the strain gradient values are estimated from the sample’s maximum bending limit.

Figure 4

Flexoelectric polarization versus the applied strain gradient measured at room temperature. The slopes are calculated as the flexoelectric coefficients of (a) LAO film on ${\mathrm{TiO}}_2$-terminated STO, (b) LAO bulk, (c) STO bulk, (d) LAO film on Nb:STO, and (e) STO film on Nb:STO. All the materials are in the (001) direction.