Search

A phenomenological Landau-Devonshire theory is applied to construct the thermodynamic potential of (110) oriented Ba(Zr0.08Ti0.92)O3 thin film and investigate the phase transition, ferroelectric, dielectric and piezoelectric properties. Three ferroelectric phases, such as tetragonal a1 phase, orthorhombic a2c and triclinic γ phase are stable in “misfit strain-temperature” phase diagram. Two triple points are found in the phase diagram and 90° polarization switching can occur directly between tetragonal a1 phase and orthorhombic a2c phase. Complicated phase transitions are induced by the electric field in triclinic γ phase due to the nonlinear coupling terms appeared in the thermodynamic potential. The longitudinal piezoelectric coefficient of (110) oriented Ba(Zr0.08Ti0.92)O3 thin film is smaller than that of (001) oriented film. Moreover, negative longitudinal piezoelectric coefficient can be obtained in triclinic γ phase.

The phenomenological Landau-Devonshire theory is applied to investigate the electrocaloric effects of bulk and thin film Ba(Zr0.08Ti0.92)O3 single crystals. The electrocaloric coefficient of 1.36 × 10−3 C/m2K and adiabatic temperature change of 1.0 °C are obtained in tetragonal T phase for bulk Ba(Zr0.08Ti0.92)O3 single crystal with the electric field of 50 kV/cm and field change of 200 kV/cm, respectively. Ba(Zr0.08Ti0.92)O3 thin film with tensile misfit strain has the larger electrocaloric effect in monoclinic M2 phase than that of tetragonal T phase which exists in compressive misfit strain. The compressive misfit strain does not change the maximum of electrocaloric coefficient, but enlarges the maximum of adiabatic temperature change. However, the tensile misfit strain improves the maxima of electrocaloric coefficient and adiabatic temperature change. In addition, the effects of electric field and temperature on electrocaloric effect are studied in tetragonal T phase and monoclinic M2 phase with different compressive and tensile misfit strains respectively. The large electrocaloric coefficient and adiabatic temperature change at the misfit strain of 0.4% are 7.4 × 10−3 C/m2K and 20 °C for Ba(Zr0.08Ti0.92)O3 thin film with the electric field of 50 kV/cm and the field change of 200 kV/cm, respectively. The giant electrocaloric effect in Ba(Zr0.08Ti0.92)O3 thin film may provide the potential for practical cooling device applications.

A phenomenological Landau-Devonshire theory is used to investigate the “misfit strain-temperature” phase diagram and physical properties of (001) oriented KNbO3 film. Three ferroelectric phases, such as c phase, aa phase, and γ phase, occur in the phase diagram. In-plane compressive misfit strain is beneficial to form c phase and tensile strain makes aa phase stable. Physical properties of KNbO3 film largely depend on misfit strain and electric field. At room temperature, γ phase of KNbO3 film with the tensile misfit strain has larger piezoelectric and pyroelectric coefficients than that of c phase with the compressive strain. Large piezoelectric and pyroelectric coefficients are 180 pm/V and 8.5 × 10−4 C/m2K respectively at the electric field of 100 KV/cm and misfit strain of 6.7 × 10−3, which are comparable with the experimental results of Pb(ZrxTi1−x)O3 film. Therefore, choosing the appropriate substrates can be used to improve the physical properties of KNbO3 film. In addition, the applied electric field decreases the dielectric, piezoelectric and pyroelectric properties of (001) oriented KNbO3 film.

Based on the phenomenological Landau-Devonshire theory, the phase diagram and physical properties of (110) oriented KNbO3 film are investigated. The “misfit strain-misfit strain” phase diagram at room temperature is constructed in the frame of eighth-order thermodynamic potential. Three tetragonal phases and two orthorhombic phases are stable in phase diagram due to the strain anisotropy. The large in-plane compressive or tensile misfit strains favor the tetragonal phases. Moreover, both the tensile misfit strain u1 (or u2) and compressive strain u2 (or u1) induce the orthorhombic phases. Large piezoelectric coefficient is obtained in orthorhombic phases which have the small in-plane tensile misfit strains. It makes (110) oriented KNbO3 film suitable for applications in electromechanical devices.

Orientation dependence of phase diagrams and physical properties of Ba0.6Sr0.4TiO3 films are investigated by using a phenomenological Landau-Devonshire theory. New ferroelectric phases, such as the tetragonal a 1 phase and the orthorhombic a 2 c phase in (110) oriented film and the monoclinic M A phase in (111) oriented film, appear in the “misfit strain-temperature” phase diagrams as compared with (001) oriented film. Moreover, the phase diagrams of (110) and (111) oriented films are more complex than that of (001) oriented film due to the nonlinear coupling terms appeared in the thermodynamic potential. The dielectric and piezoelectric properties largely depend on the misfit strain and orientation. (111) oriented film has the better piezoelectric property than (110) oriented film. Furthermore, the compressive misfit strain is prone to induce the larger piezoelectric property than tensile misfit strain.

The adsorption of K on the n-GaAs(I 0 0) surface was investigated by X-ray photoelectron spectroscopy (XPS) and synchrotron radiation photoemission spectroscopy (SR-PES). The Ga3d and As3d core level was measured for clean and K adsorbed GaAs(I 0 0) surface. The adsorption of K induced chemical reaction between K and As, and the K-As reactant formed when the K coverage theta > I ML. The chemical reaction between K and Ga did not occur, but Ga atoms were exchanged by K atoms. From the data of band bending, the Schottky barrier is 0.70 eV. The Fermi-level pinning was not caused by defect levels. The probable reason is that the dangling bonds of surface Ga atoms were filled by the outer-shell electrons of K atoms, forming a half-filled surface state. The Fermi-level pinning was caused by this half-filled surface state. (c) 2004 Elsevier B.V. All rights reserved.

A new sulfur passivation method for GaAs, CH 3 CSNH 2 treatment, has been developed. By Synchrotron Radiation Photoemission Spectroscopy(SRPES), the chemical states and electronic aspects of the passivated surfaces are investigated. It is found that the oxide layer of the GaAs is effectively removed, and Sulfur atoms bond both to Ga and As atom at room temperature. In addition, the behavior of Mg deposited on the passivated surfaces with and without annealing has been investigated. It is found that Ga atoms can be exchanged from GaS bond, and diffuse into Mg overlayer, but the sulfur atoms remain at the interfaces. At higher Mg coverage, a MgGa alloy may be formed due to excess Mg atoms reacting with Ga atoms at the overlayer.

Synchrotron Radiation Photoemission Spectroscopy (SRPES) has been used to investigate the chemical states and electronic states of a [NH 4 ] 2 S x treated GaSb(100) surface. We have found that the oxides of Ga and Sb are removed and the sulfides of Ga and Sb are formed on the surface. After sulfurized GaSb(100) was annealed, the SbS bond is broken to form elemental Sb, while the GaS bonds terminate the surface; these results imply that ammonia sulfide has a passivating role for GaSb. At room temperature (RT), deposited Mg on passivated surface has been also investigated. It is found that Ga atoms can be exchanged by Mg atoms and diffuse into Mg overlayer. Moreover, the Schottky barrier height of the Mg overlayer on the sulfurized GaSb surface was determined to be about 0.3eV.

The adsorption of K on the n-type GaAs(100) and p-type InP(100) surfaces were investigated with core-level and valence-band photoemission using synchrotron radiation. The characteristics of both interfaces were similar to each other in many ways. It was found that the chemical reaction between the adsorbate K and the anions of the substrates were occurred with the K adsorption, and the interface reactants were K As and K P compounds, respectively. However, there was no chemical reaction between K and the cations of the substrate, but the cations could be exchanged by K atoms and segregated to the surfaces. From the data of band bending on K/GaAs(100) and K/InP(100), the Schottky barrier heights were about 0.72 eV and 0.75 eV, respectively. We believe that the Fermi-level pinning may be caused by half-filled sates rather than by defect levels. These states are formed by the outer-shell electrons of K atoms that fill the empty states of the dangling bonds of the surface cations exchanged by the K atoms.

The effect of molecular nitrogen exposure on the surfaces of InP(100) modified by potassium overlayers is investigated by core-level and valence-band photoemission spectroscopy using Synchrotron radiation. In comparison with InP(110) surface, we found the promotion is much stronger for InP(100) surface due to the central role of surface defects in the promotion; furthermore, in contrast with K-promoted oxidation of InP(100) where the bonding is observed between indium and oxygen, indium atoms did not react directly with nitrogen atoms during the K-promoted nitridation of InP(100).

The electrical resistivity and the temperature coefficient of resistance (TCR) of periodic Pd/Mo bimetal films are reported for the bilayer wavelength in the range 1.5-13 nm by varying the ratio of Pd and Mo thicknesses in the temperature range from 77 to 300 K. The resistivity is found to be nearly proportional to the inverse of the bilayer wavelength. The most striking feature is that the TCR is decreased with bilayer wavelength and the change in sign of the TCR as Pd/Mo thickness ratio is varied. Taking into account the effects of electron weak localization in Mo layer, the experimental results can be explained.

The effects of exposure of molecular nitrogen and oxygen on p-type InP(100) surfaces modified by potassium overlayers were studied by core level and valence band (VB) photoemission using synchrotron radiation. On the K pre-covered surface, the potassium coverage enhanced the nitrogen and oxygen sticking coefficients dramatically. As far as the process of nitrogen adsorption is concerned, nitrogen atoms react mainly with P atoms rather than not react directly with indium atoms. Two kinds of nitride complexes, InPX x and InPN x + y were formed at the K-precovered InP(100) surface. In case of oxygen adsorption, O may bond with K and produce the peroxides O 2 2− and superoxides O 2 − . Following this a few kinds of phosphate phases, In(PO 4 ) x , were formed on the surface. In comparison with InP(110) surface, we found that the oxidation and nitridation promotion for the InP(100) surface was much stronger. The reasons may be the number of surface defects as well as the polarity of the InP(100) surface.