Ferroelectric Polarization-Modulated Interfacial Fine Structures Involving Two-Dimensional Electron Gases in Pb(Zr,Ti)O-3/LaAlO3/SrTiO3 Heterostructures

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)O-3 (PZT(up)) or downward-polarized Pb(Zr-0.5,Ti-0.5)O-3 (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.