Transmission electron microscopy study of epitaxial La0.8MnO3 thin films on SrTiO3

The structure and microstructure of La_0.8MnO₃ thin films on SrTiO₃ substrates, fabricated by pulsed laser deposition at substrate temperatures of 873?K and 1073?K, have been studied by transmission electron microscopy. In both films, columnar growth morphology has been observed. The columnar grain size is found to increase with increasing substrate temperature. In the film deposited at a substrate temperature of 1073?K, there is only one rhombohedral phase. However, two phases, a rhombohedral one and an orthorhombic one, have been observed in the film deposited at 873?K.     

Local ferroelectricity in SrTiO3 thin films

The temperature-dependent polarization of SrTiO3 thin films is investigated using confocal scanning optical microscopy. A homogeneous out-of-plane and an inhomogeneous in-plane ferroelectric phase are identified from images of the linear electro-optic response. Both hysteretic and nonhysteretic behavior are observed under a dc bias field. Unlike classical transitions in bulk ferroelectrics, local ferroelectricity is observed at temperatures far above the dielectric permittivity maximum. The results demonstrate the utility of local probe experiments in understanding inhomogeneous ferroelectrics.     

Enhanced strain relaxation induced by epitaxial layer growth mode of MgO thin films

Growth of MgO films on silicon substrate was conducted by KrF excimer pulsed-laser ablation system. Two kinds of growth mode were revealed in situ by reflection high energy electron diffraction. It was found that the layer growth mode of MgO thin films could remarkably reduce the misfit strain originated from the different lattice constant and thermal expansion coefficiency between MgO films and Si. An enhanced strain relaxation was discovered for MgO films, which were grown with the layer growth mode, in the film thickness range of 40-100 nm. The value of critical thickness for the formation of misfit dislocation agrees well with the calculated one. This exceptional phenomenon should be ascribed to the layer growth mode of epitaxial MgO films.     

In-plane and out-of-plane ferroelectric instabilities in epitaxial SrTiO3 films

The in-plane and out-of-plane ferroelectric instabilities in compressed (100)-epitaxial SrTiO3 films were examined by infrared reflection spectroscopy. The strongly stiffened in-plane soft mode frequency softened very slowly on cooling. On the other hand, the silent mode appeared at around 150 K, indicating an out-of-plane ferroelectric transition. This behavior points to a split of in-plane and out-of-plane ferroelectric instability temperatures due to the lowered symmetry of the SrTiO3 lattice caused by mechanical misfit strain. Infrared spectroscopy provides a possibility to detect such an effect in the strained epitaxial ferroelectric films.     

Phenomenological description of thin SrTiO3 films

The constants of the phenomenological potential of SrTiO₃ are calculated allowing for recent experimental data. The α₁₁, α₁₂, and t ₄₄ constants are refined by more than one order of magnitude as compared to the earlier values. The T-P phase diagrams for the case of the uniaxial [001] and [110] loading are calculated. The temperature vs. misfit-strain phase diagram for a thin SrTiO₃ film is constructed, in which pure ferroelectric phases exist at low temperatures.

     

High-quality epitaxial LaNiO3 thin films on SrTiO3(100) and LaAlO3(100)

LaNiO₃ thin films have been deposited by pulsed laser deposition on SrTiO₃(100) and LaAlO₃(100) substrates. The processing conditions have been investigated in order to optimize electrical resistivity, crystal quality, and surface morphology. Excellent properties are achieved at moderate substrate temperature and relatively low oxygen pressure, without the need for annealing. Thickness exerts an important influence on electrical transport, as the electrical resistivity increases quickly in films thicker than a few tens of nanometer. The surface of the films on LaAlO₃ is very flat in all the studied thickness range, but the films on SrTiO₃ develop a pattern of boundaries and even cracks as the thickness is higher. Below the critical thickness, high-quality epitaxial films with very smooth surface and low electrical resistivity are obtained under the optimum conditions of substrate temperature and oxygen pressure. The optimum processing conditions are different depending on the substrate, and control is especially critical in films deposited on SrTiO₃.     

Polar dependent in-plane electric transport of epitaxial ZnO thin films on SrTiO(3) substrates

Polar (001) and nonpolar (110) ZnO epitaxial thin films were grown on SrTiO(3) substrates by the pulsed laser deposition method and the in-plane electric transport was investigated. Both films display semiconducting behavior. The polar thin films have linear I-V relations with mobility increasing almost linearly with temperature. In contrast, for nonpolar ZnO thin films, the I-V curves are symmetric and nonlinear with room temperature resistivity 30 times larger than that of polar thin films. We conclude that in nonpolar ZnO thin films the bound polarization charge induced barrier limits the carrier transport. Instead, for polar thin films, the polar effect on the in-plane transport is negligible, and the charged dislocation scattering is dominant. Our observations suggest the polar effect should be considered in the design of ZnO related devices.     

Nanoscale resistive switching in SrTiO3 thin films

The local conductivity of SrTiO₃ thin films epitaxially grown on SrRuO₃‐buffered SrTiO₃ single crystals has been investigated in detail with an atomic force microscope equipped with a conducting tip (LC‐AFM). These experiments demonstrate that the conductivity of SrTiO₃ thin films originates from nanoscale well‐conducting filaments connecting the surface to the SrRuO₃ bottom electrode. The electrical conduction of the filaments is shown to be reversibly modulated over several orders of magnitude by application of an appropriate electrical field. We analyze the resistive switching by addressing individual filaments with the AFM tip as well as by scanning areas up to the µm scale. Temperature dependent measurements reveal that resistive switching on a macroscopic scale can be traced down to the insulator‐to‐metal transition of the independently switchable filaments. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Metal-insulator transition in SrTiO(3-x) thin films induced by frozen-out carriers

We report optical, electrical and magnetotransport properties of oxygen deficient SrTiO(3) (SrTiO(3-x)) thin films fabricated by pulsed laser deposition technique. The oxygen vacancies (O(vac)) in the thin film are expected to be uniform. By comparing its electrical properties to those of bulk SrTiO(3-x), it was found that O(vac) in bulk SrTiO(3-x) is far from uniform over the whole material. The metal-insulator transition (MIT) observed in the SrTiO(3-x) film was found to be induced by the carrier freeze-out effect. The low temperature frozen state can be reexcited by Joule heating, electric and intriguingly magnetic field.     

Stabilizing graphitic thin films of wurtzite materials by epitaxial strain

Recent theory [Phys. Rev. Lett. 96, 066102 (2006)] and experiment [Phys. Rev. Lett. 99, 026102 (2007)] show that (0001) ultrathin films of wurtzite (WZ) materials surprisingly transform into a stable graphitelike structure, but the stability is limited to thicknesses of only a few atomic layers. Using first-principles calculations of both freestanding and substrate-supported thin films, we predict that the thickness range of stable graphitic films depends sensitively on strain and can be substantially extended to much thicker films by epitaxial tensile strain. Moreover, the band gap of the stable strained graphitic films can be tuned over a wide range either above or below that of the bulk WZ phase.     

Morphology dependence of the dielectric properties of epitaxial BaTiO3 films and epitaxial BaTiO3/SrTiO3 multilayers

Epitaxial BaTiO₃ films and epitaxial BaTiO₃/SrTiO₃ multilayers were grown by pulsed laser deposition on vicinal surfaces of (001)-oriented Nb-doped SrTiO₃ (SrTiO₃:Nb) single-crystal substrates. Atomic force microscopy was used to investigate the surface topography of the deposited films. The morphology of the films, of the BaTiO₃/SrTiO₃ interfaces, and of the column boundaries was investigated by cross-sectional high-resolution transmission electron microscopy. Measurements of the dielectric properties were performed by comparing BaTiO₃ films and BaTiO₃/SrTiO₃ multilayers of different numbers of individual layers, but equal overall thickness. The dielectric loss saturates for a thickness above 300 nm and linearly decreases with decreasing film thickness below a thickness of 75 nm. At the same thickness of 75 nm, the thickness dependence of the dielectric constant also exhibits a change in the linear slope both for BaTiO₃ films and BaTiO₃/SrTiO₃ multilayers. This behaviour is explained by the change observed in the grain morphology at a thickness of 75 nm. For the thickness dependence of the dielectric constant, two phenomenological models are considered, viz. a series-capacitor model and a dead-layer model. PACS 77.22.-d; 77.22.Ch; 77.55.+f; 77.22.Gm; 77.84.Dy; 81.15.-z ; 81.16.Mk; 81.65.-b; 68.55.-a; 68.37.-d     

Optical properties of BiFeO3 epitaxial thin films

The properties of neodymium-doped BiFeO₃ nanosized films on magnesium oxide single-crystal substrates are studied. The films are obtained using high-frequency sputtering with the aid of layered growth. The structural perfection of the films is analyzed using the X-ray diffraction. The transmission of the films with different thicknesses is studied in the wavelength interval 200–1100 nm. The spectra are processed with the aid of a dispersion formula for permittivity of a sum of oscillators with allowance for damping, so that direct and indirect transitions can be revealed. The absorption edges are estimated to be 2.81 and 2.78 eV for the direct transitions of the films with thicknesses of 14 and 60 nm, respectively.     

Optical properties of epitaxial BiFeO3 thin films

Optical properties of epitaxial BiFeO₃ thin films grown via pulsed-laser deposition on (110) DyScO₃ substrates have been investigated. Their near-normal spectroscopic reflectivity was measured in the spectral range 2 to 14 eV at room temperature, while spectroscopic ellipsometry in the spectral range 1–6 eV was measured in the temperature range from 300 to 775 K. The optical response functions have been calculated and a direct optical gap was determined varying from 2.75 to 2.70 eV in this temperature range.     

Effect of misfit strain on the electrocaloric effect in epitaxial SrTiO3 thin films

Based on a phenomenological Landau-Devonshire theory, the effect of misfit strain on the electrocaloric effect in epitaxial SrTiO₃ thin films is investigated. Theoretical analysis indicates that both the electrocaloric coefficient and the adiabatic temperature change strongly depend on the misfit strain. Moreover, the maximum of electrocaloric coefficient almost does not change with increasing the compressive or tensile misfit strain. However, the enhancement of misfit strain enlarges both the adiabatic temperature change and the temperature at which it is maximized. Most importantly, the largest room-temperature electrocaloric effect can be attained at a critical misfit strain. Therefore, it may open more opportunities for practical application in refrigeration devices.     

Flexoelectricity induced increase of critical thickness in epitaxial ferroelectric thin films

Flexoelectricity describes the coupling between polarization and strain/stress gradients in insulating crystals. In this paper, using the Landau–Ginsburg–Devonshire phenomenological approach, we found that flexoelectricity could increase the theoretical critical thickness in epitaxial BaTiO₃ thin films, below which the switchable spontaneous polarization vanishes. This increase is remarkable in tensile films while trivial in compressive films due to the electrostriction caused decrease of potential barrier, which can be easily destroyed by the flexoelectricity, between the ferroelectric state and the paraelectric state in tensile films. In addition, the films are still in a uni-polar state even below the critical thickness due to the flexoelectric effect.     

Strain phase diagram and domain orientation in SrTiO3 thin films

SrTiO3 thin films were used as a model system to study the effects of strain and epitaxial constraint on structural phase transitions of perovskite films. The basic phenomena revealed will apply to a variety of important structural transitions including the ferroelectric transition. Highly strained SrTiO3 films were grown on different substrates, providing both compressive and tensile strain. The measured strain-temperature phase diagram is qualitatively consistent with theory; however, the increase in the phase transition temperature is much larger than predicted. Because of the epitaxial strain and substrate clamping, the SrTiO3 lattice is tetragonal at all temperatures. The phase transitions involve only changes in internal symmetry. The low temperature phase under tensile strain has a unique structure with orthorhombic Cmcm space group but a tetragonal lattice, an interesting consequence of epitaxial constraint.     

Effect of misfit strain on the electrocaloric effect of polydomain epitaxial ferroelectric thin films

The effect of misfit strain on the electrocaloric effect in polydomain epitaxial BaTiO 3 thin films at room temperature is investigated using the Ginzburg-Landau-Devonshire thermodynamic theory. Numerical calculations indicate that the misfit strain has a large impact on the ferroelectric polarization states and the electrocaloric effect. Most importantly, the electrocaloric effect in the polydomain ca 1 /ca 2 /ca 1 /ca 2 phase is much larger than that in the monodomain c phase and the other polydomain phases. Consequently, a large electrocaloric effect can be obtained by carefully controlling the misfit strain, which may provide potential applications in refrigeration devices.     

EPR spectra of a GdMnO3 thin film on a SrTiO3 substrate

Electron paramagnetic resonance (EPR) spectra of a GdMnO₃/SrTiO₃ thin film in the X band have been measured in the temperature interval from 200 to 450 K. Signals from two types of paramagnetic centers have been observed in the spectra. The first paramagnetic center is a subsystem of Gd³⁺ ions, in the EPR spectrum of which the fine structure lines are resolved below 350 K. The second paramagnetic center is a system of manganese and gadolinium ions, in the EPR spectrum of which an exchange-narrowed line is observed with the width ΔH several times less than the width ΔH of an exchange-narrowed line observed in the GdMnO₃ single crystal. Unusual magnetic properties are due to the mismatch of the lattice parameters of the GdMnO₃ thin film and the SrTiO₃ substrate.     

溶胶-凝胶法制备BaTiO3/SrTiO3多层膜的介电增强效应

采用溶胶凝胶法,在PtTiSiO2Si衬底上逐层制备了BaTiO3SrTiO3多层膜.从多层膜的XRD图可看出明显的双峰,分别对应为BaTiO3和SrTiO3的特征峰,表明样品已形成了多层膜结构.与同厚度的Ba05Sr05TiO3单层膜比较,BaTiO3SrTiO3多层膜的介电系数得到了明显的增强,在频率为10kHz时,周期为66nm的BaTiO3SrTiO3多层膜相对于同厚度的Ba05Sr05TiO3薄膜的介电系数从245增强到595,而损耗依然保持较低,分别为0029和0033.研究同时表明,BaTi 关键词： 溶胶-凝胶法 多层膜 介电增强