For at least a decade now, the thin border between two different oxides (heterointerface), has been a playground for scientists working in the field of complex oxides, which are crystalline materials formed by combining different transition-metal oxides. Complex oxides were brought into the spotlight in 2006 when a two-dimensional electron gas (2DEG) was discovered at the interface between two electrical insulators, nonpolar strontium titanate (SrTiO₃) and polar lanthanum aluminate (LaAlO₃). Heterointerfaces in complex oxides exhibit a wide range of intriguing physical properties, ranging from ionic conduction to ferroelectricity, magnetism, and superconductivity, which rarely exist in conventional materials systems.

In a study published in a recent issue of Nature Nanotechnology, researchers from the United States and China have measured conductivity at the interface of a ferroelectric/insulator complex metal oxide consisting of a ferroelectric BiFeO₃ (BFO) film and an insulating TbScO₃ (TSO) film. The observed conductivity was shown to be strongly dependent on the polarization of the ferroelectric domains of BFO and the direction of conductivity measurement.

A closer look at a cross-section of the complex oxide using a transmission electron microscope revealed two areas: the BFO part, divided in stripe-domains, atop of a single-crystalline TSO substrate. Within each stripe-domain, the crystal cells have, on average, the same value of electric dipole moments; in other words, the polarization in each domain points in the same direction, either toward or away from the interface of the oxides. As a result, electrons or holes, respectively, transfer to the interface to create a 2DEG or a 2D hole gas (2DHG), forming alternating n- and p-doped regions, and a “polar discontinuity” appears across the ferroelectric/insulator interface.

“This ‘polar discontinuity’ has the immediate advantage that it allows modulation of the interfacial electronic conductivity via polarization switching, enabling unprecedented control of multiple degrees of freedom and facilitating the design of novel functionalities, because of the spontaneous, electrically-switchable polarization that ferroelectric materials possess,” says Xiaoqing Pan, professor at the University of California, Irvine, who led the team of researchers from the University of California, Irvine, University of Nebraska, The Pennsylvania State University, Cornell University the Kavli Institute at Cornell, and Nanjing University, China.

As a consequence, the conducting path at the domain walls is blocked because of the formation of p-n junctions: the interface is conducting along the BFO domain stripes, whereas in the direction perpendicular to these stripe domains, the interface is insulating. “This finding provides a clear physical picture for polarization-induced interfacial conduction at the ferroelectric/insulator interfaces and offers a high flexibility to engineer the interfacial properties by using polarization and domain walls,” Pan says.  

The researchers used molecular beam epitaxy to grow 400-nm-thick BFO films on the top of the TSO substrates. They measured the conductivity at the interface by conductive atomic force microscopy (C-AFM). C-AFM is a mode of atomic force microscopy that, apart from generating a topographic image of the material, also measures the electric current flow between the AFM probe (tip) and the surface of the substrate, thus producing a “current image.”

“Direct measurement of the interface conductivity was one of the main challenges of this research,” Pan says. They achieved this by probing the local conductance and ferroelectric polarization over an ultrathin cross-section of the heterostructure using C-AFM. In previous studies, C-AFM was usually applied to test the top surface of ferroelectric films. “It is this innovative approach that led to the discovery of the anisotropic polarization-controlled conductance at a ferroelectric-insulator interface,” Pan adds.

Recent research shows that the domain walls exhibit their own distinct chemistry and magnetic behavior. According to Pan, these emergent characteristics have fostered the realization that the domain walls can be used as functional elements in novel nanoelectronic devices. “We found that the domain wall conductivity strongly depends on their crystallographic orientation, i.e., it can vary by an order of magnitude along different directions,” he says. The team is now focusing their efforts on this finding, which they believe will help clarify a long-standing controversy between groups that report conflicting results regarding domain wall conductivity—the direction in which the conductivity is measured has to be taken into account.

“The paper reports an impressive experimental accomplishment, with the interpretation backed by solid theoretical calculations. The authors employ atomic-resolution STEM [scanning transmission electron microscope] images that give a detailed polarization map and atomic-resolution EELS [electron energy loss spectroscope] maps that establish the sharpness of the interfaces. Such data are essential to construct reliable interface structural models for calculations. Sadly, many papers in this field lack such data, which makes the link between calculations and observed interface-induced phenomena rather speculative,” says Sokrates Pantelides, distinguished professor of physics and engineering at Vanderbilt University.

Pantelides says that the “well-documented results of this paper open new venues for the fabrication of complex interfaces with controlled n-type or p-type conducting channels in specific orientations, setting the stage for applications in nanoelectronics.”

Read the abstract in Nature Nanotechnology.