Ferroelectric Polarization-Modulated Interfacial Fine Structures Involving Two-Dimensional Electron Gases in...

Interfacial fine structures of bare LaAlO₃/SrTiO₃ (LAO/STO) heterostructures are compared with those of LAO/STO heterostructures capped with upward-polarized Pb(Zr_0.1,Ti_0.9)O₃ (PZT_up) or downward-polarized Pb(Zr_0.5,Ti_0.5)O₃ (PZT_down) overlayers by aberration-corrected scanning transmission electron microscopy experiments. By combining the acquired electron energy-loss spectroscopy mapping, we are able to directly observe electron transfer from Ti⁴⁺ to Ti³⁺ and ionic displacements at the interface of bare LAO/STO and PZT_down/LAO/STO heterostructure unit cell by unit cell. No evidence of Ti³⁺ is observed at the interface of the PZT_up/LAO/STO samples. Furthermore, the confinement of the two-dimensional electron gas (2DEG) at the interface is determined by atomic-column spatial resolution. Compared with the bare LAO/STO interface, the 2DEG density at the LAO/STO interface is enhanced or depressed by the PZT_down or PZT_up overlayer, respectively. Our microscopy studies shed light on the mechanism of ferroelectric modulation of interfacial transport at polar/nonpolar oxide heterointerfaces, which may facilitate applications of these materials as nonvolatile memory.