Atomic-Scale Characterization of Charge Distributions at Oxide Interfaces and Néel-Type Domain Walls Using Scanning Transmission Electron Microscopy
Ming-Wen Chu1*
1Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
* Presenter:Ming-Wen Chu,
The mature semiconductor technology is closely entangled with the capability of designing and engineering the carrier distribution across the heterointerfaces. The discovery of two-dimensional electron gas (2DEG) at the heterointerface between insulating LaAlO3 (LAO) and SrTiO3 (STO) in 2004 boasts the otherwise opportunity of oxide electronics and rejuvenates the interest in charged domain walls (DWs, i.e., 2D homointerfaces). With more than a decade of researches on LAO/STO, the origin for the metallic heterointerface, however, remains unsettled and the current status of the physics of charged DWs shares a similar situation. To resolve the puzzles, a quantitative addressing of the structural and intervening chemical and electronic characteristics of all interfacial unit cells is indispensable and can be achieved by atomic-resolution scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy (EELS). Using STEM-EELS, we unveiled that the metallicity onset at the LAO/STO interface is accompanied with head-to-head (HH) ferroelectric(FE)-like polarizations across the interface due to strain-rejuvenated FE-like instabilities in the materials. The divergent depolarization fields of the HH polarizations cast the interface into an electron reservoir, leading to the accumulation of screening 2DEG [Nat. Commun. 7, 12773 (2016)]. We then tackled the coexistence of HH and tail-to-tail (TT) DWs in hybrid improper FEs (Ca,Sr)3Ti2O7 (CSTO), with the respective DWs demanding for screening charges with an opposite sign while only electrons available in n-type titanates. Using STEM-EELS and also group-theoretical analyses, we established the existence of a hidden antipolar order parameter in the CSTO, turning the order-parameter spaces to be multicomponent with the joint formation of Néel-type twin-like antipolar domain walls between the HH and TT polar domains. This distinct domain topology lifts the polar divergences between the domains, casting an emergent exemplification of charged DW screening by an antipolar ingredient [M. H. Lee et al., Phys. Rev. Lett. 119, 157601 (2017); cover of the issue].

Keywords: scanning transmission electron microscopy, electron energy-loss spectroscopy, oxide interface, domain wall, ferroelectric