Oxide materials start to play a very important role in electronic devices. Nowadays, it is possible to control the electronic properties of these materials with nanoscale precision. This class of oxide compounds with perovskite type crystal structure exhibits a broad range of functional properties, such as high dielectric permittivity, piezoelectricity and ferroelectricity, superconductivity, colossal magnetoresistance and ferromagnetism. Many of these phenomena occur in perovskite oxides that are lattice-matched within a few percent of one another. This enables fabrication of heteroepitaxial structures, in which the multiple degrees of freedom can be accessed, making a myriad of devices with novel functionalities possible.
Very small characteristic length scales of the order 0.1 - 1 nm determine the nature of the physical properties in such oxide electronics. Therefore, a growth control with a precision on the atomic level is essential for novel epitaxial heterostructures. For complex oxides, pulsed laser deposition (also called laser-MBE) has proven to be a growth technique in which the deposited material can be controlled at the atomic scale. Stoichiometric transfer, high deposition rate and tunable energy of the arriving particles are properties, which make 2-dimensional layer-by-layer growth feasible for various complex oxides with a surface roughness of only one unit cell. This makes fabrication of thin films, heterostructures and superlattices possible with high quality surfaces and interfaces, which can be applied in electronic devices. However, the combination of pulsed laser deposition with reflection high-energy electron diffraction (RHEED), to monitor in-situ the thin film growth, is essential to obtain this high quality. Monitoring of the RHEED intensity provides information about the surface morphology as well as the diffusion and nucleation processes. In this way atomically abrupt interfaces can be produced with a strong epitaxial relation between the two oxide layers.
The properties of oxide interfaces in such heterostructures can be exploited for applications in electronic devices. Interfaces and surfaces in such highly correlated systems are much more complex and offer far more application possibilities than interfaces involving only conventional metals and semiconductors. Interfaces alter the bulk electronic system sometimes with dramatic consequences; interfaces break the translational and the rotational symmetry, induce stress or strain, consequently altering the distances and bonds between the ions, giving rise to shift and distortion of the electronic states and energy levels and modifying the bands. By altering the electronic states, interfaces modify also electronic correlations. Because the correlations control the electronic behaviour of the material, their modification can induce dramatic changes of the collective electronic and magnetic properties, to the extent that phase transitions are induced.
Brief summary of research over last few years
In the past few years I have studied novel nanostructured oxide thin films with special structural and advanced functional properties at the incorporated interfaces, see Figure. Artificial perovskite structures were realized by controlled growth with atomic precision, to exploit the exceptional physical properties of 2-dimensional electron gases (2DEG) at the interfaces between LaAlO3 and SrTiO3 insulators. Following the first realization of electronic coupling [Nature Materials 2006] and the first observation of magnetism [Nature Materials 2007] in the years before, several detailed studies were performed on the origin of the interfacial 2DEG [PRL 2009a/2010a/2011; PRB 2010/2012a/2013].
The scope of my research was broadened during my Postdoc research at UC Berkeley into the field of multiferroic applications, in which magnetism is controlled by an electric field and vice versa. High-quality interfaces were realized in oxide heterostructures to study the magnetic and electronic interactions between various systems, such as (anti)ferromagnets and ferroelectrics [Nano Lett. 2008, PRB 2008/2010]. This led to the first observations of exchange bias interactions between the ferromagnet La0.7Sr0.3MnO3 and the multiferroic BiFeO3 [PRL 2010b], caused by local structural changes at the interfaces indicating an antiferrodistortive phase transition [PRL 2010c, Adv. Mater. 2011]. Detailed characterization of the ferroelectric domain structures led to the deterministic control of ferroelastic switching, enabling magneto-electric, domain-wall-based and strain-coupled devices [Nature Nanotech. 2009, PRL 2009b]. Furthermore, the controlled creation of conductive one-dimensional channels provided a pathway for the design of such integrated oxide electronic devices based on domain patterning [Nature Physics 2012]. We have determined the critical limit at 4-5 unit cells (i.e. 2 nm) in BiFeO3 thin films for controlled polarization switching [PRB 2012b], above which interfacial exchange bias coupling occurs to the ferromagnetic La0.7Sr0.3MnO3 layer [Adv. Mater. 2013]. Recently, we have studied in more detail the local strain and oxygen octahedral rotations at correlated perovskite interfaces [APL 2014a, APL Mat. 2014, Nature Comm. 2014].
As program director of the strategic research orientation “Nanomaterials for Energy” in the MESA+ Institute for Nanotechnology, I have initiated new research projects on thermoelectrics, photovoltaics and artificial photosynthesis, in which controlled fabrication of high-quality oxide multilayers was vital to achieve enhanced properties. Titanates and cobaltates are promising thermoelectric candidates because of their good thermal and chemical stability, making them ideal for high-temperature energy conversion applications. In my VENI project structural engineering of crystallinity and grain size has been applied to optimize their electronic as well as thermal properties [Chem. Mater. 2010, PRB 2011, APL 2011, RSC Adv. 2012], leading to a simultaneous doubling of the thermoelectric power factor and a strong suppression of the thermal conductivity [Adv. Energy Mater. 2014, APL 2014b/2015]. Recently, we have shown that incorporation of a strontium copper oxide nano-layer dramatically suppresses oxygen defects by reducing the kinetic barrier for oxygen surface exchange [Adv. Func. Mater. 2013], which is crucial in applications such as solid oxide fuel cells. Recently, I have received a personal VIDI grant of the Netherlands Organization for Scientific Research for a five year research project on ‘Self-assembled 3D Solid-State Batteries’.