Papers by Keyword: Superlattice

Abstract: This article briefly reviews recent developments of Landau-Ginzburg theory to ferroelectric phase transitions in superlattices. An overview of the contributions of Landau-type theory to study ferroelectric superlattices is given. Recent findings from first-principles calculations and experiments on intermixing, local polarization coupling and polar discontinuity at interfaces that are not address in these contributions are highlighted. This is followed by a review of recent developments of Landau-Ginzburg theory that addresses these emergent phenomena at interfaces, which is the focus of this review article. The Landau-Ginzburg approach to ferroelectric superlattices with spatial distribution of polarization is outlined. It describes the formation of intermixed layer with properties different from those of both layers. These intermixed layers are mutually coupled through the local polarization at interfaces. Polarization continuity or continuity at interfaces is determined by the nature of the intermixed layer formed at the interface region. Recent results obtained in investigating superlattices comprised primarily of ferroelectric and paraelectric materials are discussed. The results include modulated polarizations, phase transitions, dielectric susceptibilities and switching behaviors.

Abstract: The term "Multiferroic" is coined for a material possessing at least two ferroic orders in the same or composite phase (ferromagnetic, ferroelectric, ferroelastic); if the first two ferroic orders are linearly coupled together it is known as a magnetoelectric (ME) multiferroic. Two kinds of ME multiferroic memory devices are under extensive research based on the philosophy of "switching of polarization by magnetic fields and magnetization by electric fields." Successful switching of ferroic orders will provide an extra degree of freedom to create more logic states. The "switching of polarization by magnetic fields" is useful for magnetic field sensors and for memory elements if, for example, polarization switching is via a very small magnetic field from a coil underneath an integrated circuit. The electric control of magnetization is suitable for nondestructive low-power, high-density magnetically read and electrically written memory elements. If the system possesses additional features, such as propagating magnon (spin wave) excitations at room temperature, additional functional applications may be possible. Magnon-based logic (magnonic) systems have been initiated by various scientists, and prototype devices show potential for future complementary metal oxide semiconductor (CMOS) technology. Discovery of high polarization, magnetization, piezoelectric, spin waves (magnon), magneto-electric, photovoltaic, exchange bias coupling, etc. make bismuth ferrite, BiFeO₃, one of the widely investigated materials in this decade. Basic multiferroic features of well known room temperature single phase BiFeO₃ in bulk and thin films have been discussed. Functional magnetoelectric (ME) properties of some lead-based solid solution perovskite multiferroics are presented and these systems also have a bright future. The prospects and the limitations of the ME-based random access memory (MERAM) are explained in the context of recent discoveries and state of the art research.

Abstract: New MOCVD grown UV (ultra-violet) LEDs using low dislocation density GaN buffer layers on sapphire have been studied. Two different LED characteristics of GaN substrates, i.e. 5um-thick and 20um-thick buffer layers, on sapphire are compared with each other. The enhanced LED characteristics show ~29.5% reduction in current-voltage resistance, ~8.5% reduction in turn-on voltage and output power saturation at higher current. Better GaN buffer quality and heat dissipation due to the lower defect density are believed to the enhanced reason.

Abstract: We studied the relationship of the superlattice structure with the doping efficiency in detail by semi-classic calculation and follow experiment. The results show that period length of about 9nm and Al content of 30% is optimal. The Hall, AFM and PL measurement show that the best annealing temperature under atmosphere is about 540°C to 580°C. Finally we obtained the sample with resistivity of 0.31Ω·cm. It can be used for fabricating p type ohmic contact and active layer for blue LED.

Abstract: We studied the transport properties of short period AlGaN/GaN superlattice induced by terahertz field. Due to the strong polarize effect it is not precise enough to calculate the energy dispersion relation just using the nearest wells in tight binding method. The distortion of the conduction band profile by the polarization fields has been taken into account in our improved model. The calculations indicate that the potential wells of next to nearest neighbors cause anharmonic electron oscillations at the multiples of the fundamental Bloch frequency. The results show that the AlGaN/GaN superlattice is a promising candidate to convert radiation of frequency ω to radiation at frequency 3ω or even higher.

Abstract: Superlattice structure of SrTiO₃ and Nb-doped SrTiO₃ have been epitaxially grown on atomically flat surface of LaAlO₃ substrates by ion beam deposition method. Epitaxial superlattices were grown at 800 °C in the presence of partial oxygen pressure under optimizing growth conditions. The Nb-doped SrTiO₃ layers were varied from 2 to 15 unit cell thickness approximately, while SrTiO₃ layers are maintained at 15 unit cell thickness with 10 periods. The superlattices with various Nb-doped SrTiO₃ layer thicknesses were investigated using X-ray diffractometer (XRD) and atomic force microscope (AFM), in order to clearly understand structural properties and surface structure, which are significant for fabrication of the high quality superlattice structure.

Abstract: Two kinds of magnetron sputtering ZrAlN films containing 23%atAl were deposited. The first was multicomponent ZrAlN film, the second was structure gradient and mulilayer film, named superlattice ZrAlN film. The microstructure was studied by FESEM, TEM , XRD, its mechanical properties were evaluated by nano-indentation method. The fracture toughness of films were determined from the length of ‘radial cracks’ on the applied diamond identer load 1.96N. the results show that, multicomponent ZrAlN film has hardness value of 35GPa, fracutre toughness value of 1.97MPa·m^-0.5; while superlattice ZrAlN film has coresponding value of 40.1GPa and 3.17MPa·m^-0.5. TEM image illustrates the superlattice ZrAlN film period is 2.5nm, nanoidentation test shows superlattice film has higher elastic recovery parameter and lower plastic work.

Abstract: Based on the classical diffuse mismatch model (DMM), the dedicated Monte Carlo model for Si/Ge superlattice is proposed and the method to cope with the scattering at the interface is provided. In this model, phonons transport between two different materials can be taken as the movements between the different phonon dispersion relationships. If there is a corresponding position in the other material, the phonon with a given frequency will be able to pass through the interface, otherwise it will be reflected.

Abstract: We studied in detail the lattice transition and local lattice structure (including Jahn-Teller distortion) in LaMnO₃/SrMnO₃ surperlattices by classical atomistic simulation. For a certain doping density, it is found that the superlattices with short modulation period have small lattice energies and larger differences among lattice parameters a, b/√2 and c. The average La-Mn (Mn³⁺-O) distance is larger than the average Sr-Mn (Mn⁴⁺-O) distance for all doping densities and superlattice configurations at certain doping density. The standard deviation of Mn-O bond lengths and Jahn-Teller distortion of MnO₆ octahedra have been calculated. Both the standard deviation and Jahn-Teller distortion of Mn³⁺O₆ octahedra in the superlattices are much smaller than those of Mn³⁺O₆ octahedra in LaMnO₃, while Mn⁴⁺O₆ octahedra in the superlattices have the smallest lattice distortion, but larger than those in SrMnO₃.

Abstract: The magnetooptical and optical properties, and giant magnetoresistance (GMR) of MBE-grown Fe(tx, Å)/Cr10 Å (tx=0.3-30 Å) superlattices and nanostructured multilayers are studied. The data obtained are used for characterization of magnetic clusters in structures with ultrathin Fe layers (tFe<6.6 Å) and for estimation of interfacial electron scattering parameters in GMR superlattices.