Simulation of thickness dependence in ferroelectric thin films

The thickness dependence of coercive field (E_C) and remanent polarization (P_r) in ferroelectric thin films has been numerically simulated using a two-dimensional four-state Potts model. In this model, each of the dipoles in the film is assigned to one of the four states corresponding to the four different mutually perpendicular orientations. Neighboring dipoles with the same orientation are then grouped together to form a domain. Four different kinds of domains exist. In the presence of the surface layer near the electrode/film interface, the thickness dependence of both coercive field and remanent polarization are simulated.     

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.     

Thickness dependent magnetic and ferroelectric properties of LaNiO3 buffered BiFeO3 thin films

BiFeO₃ (BFO) thin films with thickness increasing from 40 to 480 nm were successfully grown on LaNiO₃ (LNO) buffered Pt/Ti/SiO₂/Si(100) substrate and the effects of thickness evolution on magnetic and ferroelectric properties are investigated. The LNO buffer layer promotes the growth and crystallization of BFO thin films. Highly (100) orientation is induced for all BFO films regardless of the film thickness together with the dense microstructure. All BFO films exhibited weak ferromagnetic response at room temperature and saturation magnetization is found to decrease with increase in film thickness. Well saturated ferroelectric hysteresis loops were obtained for thicker films; however, the leakage current dominated the ferroelectric properties in thinner films. The leakage current density decreased by three orders of magnitude for 335 nm film compared to 40 nm film, giving rise to enhanced ferroelectric properties for thicker films. The mechanisms for the evolution of ferromagnetic and ferroelectric characteristics are discussed.     

A polarizability model for the ferroelectric mode in semiconducting SrTiO3

The temperature dependence of the ferroelectric mode frequency in SrTiO₃ with oxygen vacancies and free electrons, is quantitatively explained on the basis of a microscopic model for ferroelectric soft modes.     

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.

     

Microstructural investigations of Y2O3 thin films deposited by laser ablation on MgO

The epitaxial relationships and the microstructure of thin Y₂O₃ film, in situ grown by laser ablation at 700 °C under 0.5 mbar oxygen pressure on MgO substrate, are studied by X-ray and HRTEM investigations. X-ray φ-scanexperiments show that Y₂O₃ exhibits two different crystallographic growing directions <111> and <100> on (100) single-crystalline MgO substrate. The <111> growing direction appears with four different epitaxial relationships where the <110> Y₂O₃ in-plane direction is parallel to the <110> in-plane direction of MgO. The <100> growing direction appears with one epitaxial relationship, cube Y₂O₃ on cube MgO. The HRTEM experiments show the columnar aspect of the Y₂O₃ thin film. Both (111) and (100) Y₂O₃-oriented grains are observed in good agreement with the X-ray experiments. Received: 16 May 2000 / Accepted: 22 May 2000 / Published online: 13 September 2000     

Thickness dependence of optical parameters for ZnTe thin films deposited by electron beam gun evaporation technique

Zinc telluride thin films with different thicknesses have been deposited by electron beam gun evaporation system onto glass substrates at room temperature. X-ray and electron diffraction techniques have been employed to determine the crystal structure and the particle size of the deposited films. The stoichiometry of the deposited films was confirmed by means of energy-dispersive X-ray spectrometry. The optical transmission and reflection spectrum of the deposited films have been recorded in the wavelength optical range 450-2500 nm. The variation of the optical parameters, i.e. refractive index, n, extinction coefficient, k, with thickness of the deposited films has been investigated. The refractive index dispersion in the transmission and low absorption region is adequately described by the single-oscillator model, whereby the values of the oscillator strength, oscillator position, dispersion parameter as well as the high-frequency dielectric constant were calculated for different film thickness. Graphical representations of the surface and volume energy loss function were also presented.     

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.     

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)     

Orientation dependence of the electrocaloric effect of ferroelectric bilayer thin films

An analytical thermodynamic theory is applied to investigate the electrocaloric effect of ferroelectric BaTiO₃/SrTiO₃ bilayer thin films with different orientations at room temperature. Theoretical analysis indicates that the strong electrostatic coupling between the layers results in the suppression of ferroelectricity at a critical relative thickness which occurs approximately at 50%, 23%, and 12% of SrTiO₃ fraction in the (001), (110), and (111) bilayer thin films, respectively. The ferroelectric bilayer thin films are respected to have the largest electrocaloric effect at this critical relative thickness. Moreover, the electrocaloric effect strongly depends on the orientation and the (110) oriented bilayer thin films have the largest electrocaloric effect. Consequently, control of the orientation and the relative thickness of SrTiO₃ layer can be used to adjust the electrocaloric effect of ferroelectric bilayer thin films, which may provide the potential for practical application in refrigeration devices.     

Structural and magnetic characterization of LaSrMnO3 thin films deposited by laser ablation on MgO substrates

La_2/3Sr_1/3MnO_3?δ thin films were deposited by laser ablation on MgO substrates under low oxygen pressure cool down. Their structural and magnetic properties are presented. The magnetic and electrical resistivity measurements indicate a reduction of the Curie and the metal–insulator transition temperatures due to the formation of magnetic inhomogeneneous films, where clusters of a metallic phase are mixed in a magnetically disordered insulating matrix. By a low-angle X-ray reflectivity study we show that the thin films are chemically inhomogeneous with an oxygen deficiency in bulk of the film when compared with the film/air interfacial region.     

Thickness dependence and annealing effects on the reflectance of thin Cd,Ga and In films freshly deposited at 77 K

The reflectance of thin Cd, Ga and In films with various thicknesses is measured during isochronal annealing above 77 K, immediately after deposition. The irreversibility of temperature dependence of reflectance factor is related to annihilation of lattice defects, growth of crystallites and granulation.     

Soft modes in semiconducting SrTiO3: II. The ferroelectric mode

The frequency, temperature and wavevector dependence of the ferroelectric soft mode in semiconducting SrTiO₃ was investigated by temperature derivative Raman spectroscopy and by inelastic neutron scattering. A tremendous shift in mode frequency with free electron and oxygen vacancy concentration, respectively, was observed.     

Thickness dependence of superconductivity for In/Mo thin films

We report on the superconducting characteristics of the Indium thin films on molybdenum under-layer as a function of the In film thickness. Our molybdenum under-layer with thickness of 50 Å does not cause the occurrence of superconductivity until 1.5 K and the sheet resistance has logarithmic temperature dependence observed in the present investigation. As thickness of In increased, the oscillation phenomenon of T_C was observed at early stage of deposition and the value of T_C is higher than the that for bulk of In. Furthermore, it is found that with increase of the In thickness, there are large differences of the strengths of the upper critical magnetic field H_C2(T), resistivity and T_C between films with thickness below and above 100 Å. On the other hand, the T_C decreases monotonously as sheet resistance increases, when the T_C is plotted against sheet resistance. To clarify the relation of superconducting characteristics and the surface structure of the films with different thickness, we have performed surface observation by atomic force microscope. As a result, we have found that the surface changes from homogeneous structure to inhomogeneous (or percolative) structure, when the thickness of in films pass through about 100 Å. Superconductivity of In/Mo films with relatively thick-inhomogeneous films cannot be explained in terms of the simple percolation theory. Therefore, we analysis the experimental data of H_C2(T) near T_C, using a extended Landau–Ginzburg model. It is found out that our In/Mo films must consider some factors; such as, grain size, the distance of grain space, and the strength of couplings between grains.     

Thickness dependence of the magnetic properties and magnetoimpedance effect in CoFeAlO thin films

The change of the magnetic properties and magnetoimpedance effect of Co–Fe–Al–O thin films with film thicknesses 50–1200 nm has been investigated. The coercivity and the anisotropy field changed strongly with increase of film thickness, while the saturation induction almost remained unchanged. The maximum value of GMI effect obtained about 33% for a film thickness of 1200 nm.     

Dielectric and ferroelectric properties of pulsed-laser deposited BaTiO3 films

3 films were produced by KrF excimer-laser ablation. Films deposited on fused silica substrates were polycrystalline without preferential orientation and had cubic rather than tetragonal structure. BaTiO₃/Au/Ti/fused silica films showed a large dielectric constant, which increased with the thickness of the film, but poor ferroelectric properties. This behavior seems to be related to the small size of grains. On (100)MgO substrates oriented films were obtained. BaTiO₃/YBa₂Cu₃O_7-δ/(100)MgO films showed a large dielectric constant also and improved ferroelectric properties. Although this indicates a larger degree of tetragonality, the tetragonal structure of single crystal BaTiO₃ has not yet developed. Localized reduction and metallization of BaTiO₃/(100)MgO films by means of Ar⁺-laser radiation was demonstrated. This technique allows to produce conducting patterns in a single-s tep process. Received: 6 January 1997/Accepted: 21 April 1997     

Thickness dependence of photoluminescence-decay profiles of exciton-exciton scattering in ZnO thin films

We have investigated the photoluminescence (PL) dynamics of ZnO thin films under intense excitation conditions using an optical-Kerr-gating method. The PL bands originating from exciton-exciton scattering (P emission) and biexciton (M emission) have been observed at 10 K. The ultrashort gating time of 0.6 ps has enabled us to obtain precise information of the temporal profiles of the peak energies and the intensities of the P- and M-PL bands. We have found that the decay time of the P emission becomes longer with increasing film thickness, while that of the M emission is independent of the film thickness. Although the decay time of the P emission is an increasing function of the film thickness, the relation is not in proportion, which is contrary to the predicted proportionality based on a simple model of photon-like polariton propagation.