J.-S. Chung

Giant Flexoelectric Effect in Ferroelectric Epitaxial Thin Films

We report on nanoscale strain gradients in ferroelectric HoMnO(3) epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.

Ferroelectric properties of SrRuO3∕BaTiO3∕SrRuO3 ultrathin film capacitors free from passive layers

Structural studies on ultrathin SrRuO3∕BaTiO3∕SrRuO3 capacitors, with BaTiO3 thicknesses of between 5nm and 30nm, show well-defined interfaces between ferroelectric BaTiO3 and electrode SrRuO3 layers. In these capacitors, we cannot observe any extrinsic electrical effects due to either the formation of an insulating interfacial passive layer or passive-layer-induced charge injection. Such high-quality interfaces result in very good fatigue endurance, even for the 5nm thick BaTiO3 capacitor.Structural studies on ultrathin SrRuO3∕BaTiO3∕SrRuO3 capacitors, with BaTiO3 thicknesses of between 5nm and 30nm, show well-defined interfaces between ferroelectric BaTiO3 and electrode SrRuO3 layers. In these capacitors, we cannot observe any extrinsic electrical effects due to either the formation of an insulating interfacial passive layer or passive-layer-induced charge injection. Such high-quality interfaces result in very good fatigue endurance, even for the 5nm thick BaTiO3 capacitor.

Localized electronic states induced by defects and possible origin of ferroelectricity in strontium titanate thin films

Several defect configurations including oxygen vacancies have been investigated as possible origins of the reported room-temperature ferroelectricity of strontium titanate (STO) thin films [Y. S. Kim et al., Appl. Phys. Lett. 91, 042908 (2007)]. First-principles calculations revealed that the Sr–O–O vacancy complexes create deep localized states in the band gap of SrTiO3 without affecting its insulating property. These results are consistent with electronic structural changes determined from optical transmission and x-ray absorption measurements. Our work suggests importance of oxygen vacancies and their complexes in understanding of electronic properties of perovskite oxide thin films, including STO.

Origin of metal–insulator transition temperature enhancement in underdoped lanthanum manganite films

The possibility of controlling transport properties of colossal magnetoresistance manganite films using substrate-induced strain has attracted great interest. We have investigated transport properties of La0.9Ca0.1MnO3, La0.92Ba0.08MnO3, La0.8Ba0.2MnO3, and LaMnO3 films. When the films were post-annealed at proper conditions, all of them showed metal–insulator transitions. Their transition temperatures TMI were much higher than the corresponding bulk values, irrespective of the type of substrate-induced biaxial strain. This surprising fact demonstrated that strain could not be the main origin of the TMI enhancement observed in the underdoped (dopant concentration x<0.3) manganite films. We suggested that TMI enhancements should be attributed mostly to the cationic vacancies in the post-annealed films.

Surface versus bulk characterizations of electronic inhomogeneity in a VO2 thin film

thin film, theMIT should occur during the percolation process. We also used surface-sensitive scanning tunneling spectros-copy STS to investigate the microscopic evolution of the MIT near the surface. Similar to the XRD results,STS maps revealed a systematic decrease in the metallic phase as temperature decreased. However, this rate ofchange was much slower than the rate observed with XRD, indicating that the electronic inhomogeneity nearthe surface differs greatly from that inside the film. We investigated several possible origins of this discrepancyand postulated that the variety in the strain states near the surface plays an important role in the broad MITobserved using STS. We also explored the possible involvement of such strain effects in other correlatedelectron oxide systems with strong electron-lattice interactions.DOI: 10.1103/PhysRevB.76.075118 PACS number s : 71.30. h, 68.47.Gh, 71.27. a

Epitaxial stabilization of artificial hexagonal GdMnO3 thin films and their magnetic properties

The authors investigated the role of oxygen partial pressure on the epitaxial growth of an artificial hexagonal GdMnO3 phase, which should exist in an orthorhombic structure in bulk. The hexagonal GdMnO3 film showed diverse, but obvious, magnetic phase transitions with highly enhanced ferromagnetic properties. Its remnant magnetization at 4.2K is higher than those of other hexagonal RMnO3 (R=Ho, Er, and Yb) compounds, and the Curie temperature increases by around 25K. The results demonstrate that the epitaxial stabilization technique is a promising method for fabricating an artificial material with enhanced magnetic properties.The authors investigated the role of oxygen partial pressure on the epitaxial growth of an artificial hexagonal GdMnO3 phase, which should exist in an orthorhombic structure in bulk. The hexagonal GdMnO3 film showed diverse, but obvious, magnetic phase transitions with highly enhanced ferromagnetic properties. Its remnant magnetization at 4.2K is higher than those of other hexagonal RMnO3 (R=Ho, Er, and Yb) compounds, and the Curie temperature increases by around 25K. The results demonstrate that the epitaxial stabilization technique is a promising method for fabricating an artificial material with enhanced magnetic properties.

Electric-field-controlled directional motion of ferroelectric domain walls in multiferroic BiFeO3 films

We describe the directional ferroelectric domain wall motion in a multiferroic BiFeO3 thin film, which was grown epitaxially on a vicinal (001) SrTiO3 substrate. A structural analysis of the film shows that a strain gradient is developed in our film, which creates a symmetry breaking in a ferroelectric double-well potential. The asymmetric double-well potential can cause ferroelectric domain walls to move sideways with preferred directionality under a vertical electric field. Our results suggest the possibility of controlling the direction of domain growth with an electric field by imposing constraints on ferroelectric films, such as a strain gradient.

Polarity-dependent kinetics of ferroelectric switching in epitaxial BiFeO3(111) capacitors

We report on the intriguing polarity-dependent kinetics of polarization switching in epitaxial BiFeO3(111) capacitors. Two seemingly incompatible switching kinetics were observed depending on the polarity of the applied switching bias. Under a negative switching bias, the polarization switching process occurs mainly through sideways domain wall motion, but under a positive switching bias, domain nucleation governs the polarization reversal. The modified piezoresponse force microscopy reveals these polarity-dependent ferroelectric domain evolutions. This polarity dependence of ferroelectric switching kinetics is attributed to defect-related local fields that have different distributions near film/electrode interfaces, probably due to structural relaxation in the BiFeO3(111) film.

Defect-related room-temperature ferroelectricity in tensile-strained SrTiO3 thin films on GdScO3 (110) substrates

We investigate room-temperature (RT) ferroelectricity in tensile-strained SrTiO3 (STO) thin films grown on GdScO3 (110) substrates. To separate the strain and the defect dipole effect, we apply an electric field to measure the polarization in the direction perpendicular to the elongation axis, and the RT ferroelectric polarization is found to be perpendicular to that axis. These results clearly demonstrate the importance of the contribution of defect dipoles to the RT ferroelectricity observed in STO thin films.

Photoluminescence induced by thermal annealing in SrTiO3 thin film

We have grown SrTiO3 thin films by rf-sputtering and studied its photoluminescence (PL) property after postannealing treatments. While the as-grown film does not show any PL signal, visible frequency PL emissions are induced by high temperature (T>550 °C) annealing. When subsequent low-T (50 °C) and long term (>8 months) annealing was made, the PL-spectra evolved into another pattern in which four distinct luminescence peaks appear simultaneously at λ=1.8, 2.2, 2.7, and 3.1 eV. We propose that these remarkable room temperature PL effects are due to both metastable and energetically stabilized defect states formed inside the band gap.We have grown SrTiO3 thin films by rf-sputtering and studied its photoluminescence (PL) property after postannealing treatments. While the as-grown film does not show any PL signal, visible frequency PL emissions are induced by high temperature (T>550 °C) annealing. When subsequent low-T (50 °C) and long term (>8 months) annealing was made, the PL-spectra evolved into another pattern in which four distinct luminescence peaks appear simultaneously at λ=1.8, 2.2, 2.7, and 3.1 eV. We propose that these remarkable room temperature PL effects are due to both metastable and energetically stabilized defect states formed inside the band gap.