Tai-Bor Wu

NON-OHMIC CHARACTERISTICS OF ZNO-V2O5 CERAMICS

The densification behavior, microstructure, and electrical properties of ZnO‐V2O5 ceramics were studied with V2O5 as the only additive ranging from 0.01 to 1.0 mol %. The addition of V2O5 to zinc oxide shows a tendency to enhance the densification rate and promote grain growth. However, a microstructure that consisted of anomalously grown grains was found for the specimens containing V2O5≥0.05 mol % when sintered at 1100 °C for 2 h. The x‐ray diffraction and SEM‐EDS microanalysis revealed that the sintered specimens had a two‐phase microstructure, i.e., a vanadium‐rich intergranular phase formed between ZnO grains. The formation of the grain boundary barrier layer was confirmed by the non‐ohmic I‐V behavior and the quick drop of apparent dielectric constant with increasing frequency of the ceramics. A nonlinearity coefficient of 2.4–2.8 was obtained at a current density of 10 mA/cm2 for a series ZnO‐V2O5 ceramics, and a Schottky barrier height of 0.44–0.47 eV (at 25 °C) was determined from the I‐V and C‐V...

Structural characteristics of epitaxial BaTiO3/LaNiO3 superlattice

Artificial superlattices consisting of ferroelectric BaTiO3 (BTO) and conductive LaNiO3 (LNO) sublayers were epitaxially grown on Nb-doped SrTiO3(001) single crystal substrates by a dual-gun rf magnetron sputtering system. A symmetric sublayer structure with the designed thickness varying in the range from 3 nm to 70 nm was adopted. The formation of superlattice structure was confirmed from the (00L) Bragg reflection of x ray and the depth profile of secondary ion mass spectrometry. The in-plane diffraction shows that the BTO and LNO sublayers have the same in-plane lattice spacing for the superlattices with stacking periodicity below 16 nm. The lattice parameter obtained from in-plane diffraction also exhibits a partial but nearly constant relaxation of in-plane strain in the superlattices, even though the sublayer thickness is below the critical value for generation of misfit dislocations. X-ray reflectivity measurement reveals that all the above superlattices have about the same interface roughness of ...

X‐ray absorption spectroscopic studies of sputter‐deposited LaNiO3 thin films on Si substrate

X‐ray absorption spectroscopy (XAS) was applied to investigate the growth behavior of LaNiO3 thin films on Si substrate deposited via radio frequency magnetron sputtering at high temperature. The thickness of deposited film was always proportional to the sputtering time. However, the Ni‐to‐La ratio in the film was found to decrease with increasing substrate temperature. It is due to a loss of Ni on high‐temperature deposition which lowers the film growth rate and leads to a gradual structural change. Nevertheless, the oxidation states of both Ni atom and La atom in the thin films were not influenced by the substrate temperature or sputtering time. All the XAS evidence was consistent with the results from x‐ray reflectivity, x‐ray diffraction, and chemical analysis.

Characteristics of constrained ferroelectricity in PbZrO3/BaZrO3 superlattice films

An Artificially layered perovskite composed of antiferroelectric PbZrO3 and paraelectric BaZrO3 (BZO) was fabricated on LaNiO3∕Pt∕Ti∕SiO2∕Si substrates at 475 °C by radio-frequency magnetron sputtering. It had an (001)-oriented superlattice structure with an average composition of (Pb0.75Ba0.25)ZrO3 (PBZ). X-ray diffraction, cross-sectional transmission electron microscopy, and a depth profile of a secondary-ion mass spectrometer confirmed the formation of superlattice structure with designed composition modulation. Ferroelectricity was induced in the superlattice films, and the ferroelectric as well as the dielectric properties were enhanced with reducing the stacking periodicity. The remanent polarization Pr and coercive field Ec were found linearly dependent on the applied voltage but independent of the measurement temperature up to 100 °C. The retention loss of superlattice films was small and significantly less than that of (Pb1−xBax)ZrO3 (PBZ) solid-solution films either at room temperature or 100 °...

Charging characteristics of Au nanocrystals embedded in metal-oxide-semiconductor structures

The charging characteristics of metal-oxide-semiconductor (p-type) structures containing Au nanocrystals in SiO2 gate oxide were studied. The Au nanocrystals of 2–3nm in diameter are self-assembled from the agglomeration of an ultrathin Au layer embedded in SiO2 matrix by annealing at 600°C. A large hysteresis loop is found in the capacitance-voltage (C-V) relation even at a low operating voltage (2V), indicating its significant charge storage effect. Different charging rates for two kinds of trapped carriers (electron and hole) were found from C-V measurement under various scan rates. The relatively stable retention characteristic for holes trapped in the Au nanocrystals at room temperature was also demonstrated.

Evaluation of Strain-Dependent Dielectric Properties in BaTiO3 ∕ LaNiO3 and ( Ba , Sr ) TiO3 ∕ LaNiO3 Artificial Superlattice Films on LaNiO3-Coated SrTiO3 Substrates

We formed strained artificial superlattices of BaTiO 3 /LaNiO 3 (BTO/LNO) and (Ba 0 . 4 8 Sr 0 . 5 2 )TiO 3 /LaNiO 3 (BST/LNO) on LaNiO 3 -coated SrTiO 3 (001) substrates by dual-gun radio-frequency magnetron sputtering. The (00L) crystal truncation rod intensity profiles confirm the formation of a superlattice structure. The BTO (or BST) sublayer in the artificial superlattice is under biaxial compressive stress whereas the LNO sublayer is under biaxial tensile stress. Both BTO/LNO and BST/LNO superlattices of total thickness -60 nm exhibit some degree of strain relaxation, even though the modulation length is less than the critical value to generate misfit dislocations. These BTO/LNO and BST/LNO artificial superlattices show a dielectric constant significantly enhanced relative to BTO and BST single layers of the same effective thickness. From a macroscopic point of view, the strain in the superlattice structure contributes significantly to the dielectric enhancement.

A texture study of anisotropic sintered Nd‐Fe‐B magnets

An x‐ray pole figure technique was used to study the development of magnetocrystalline texture in the sintering of Nd‐Fe‐B magnets. The effect of excess Nd on the texture and remanence is emphasized. It is found that a medium content of excess Nd enhances the (006) texture and remanence. However, a high content of excess Nd may deteriorate both texture and remanence, and in addition, the tilt of texture away from the prealigned direction also becomes more significant. It is believed that the floating and rotation of grains on the thick layer of liquid phase during sintering cause the deterioration.

On the suppression of hydrogen degradation in PbZr0.4Ti0.6O3 ferroelectric capacitors with PtOx top electrode

The effect of oxygen content in the sputtered Pt oxide (PtOx, x=0.4∼1), top electrode on suppressing the hydrogen degradation of ferroelectric capacitors of PbZr0.4Ti0.6O3 (PZT) was studied. It was found that the ferroelectric property becomes significantly degraded along with a positive voltage offset in the polarization-field hysteresis loop after forming gas (N2+H2) annealing at 200°C of the PZT capacitors made with Pt top electrode. The leakage current in the degraded capacitors also greatly increases due to the reduction of Schottky barrier. However, the degradation of electrical property can be suppressed by using PtOx as top electrode, and the ferroelectric characteristics are well retained ever for capacitors with size shrunk down to submicron scale. The secondary-ion-mass spectroscopy and elastic recoil detection analysis reveal that the catalytic reaction of Pt in dissociation of hydrogen molecules from forming gas can be effectively interrupted by increasing the oxygen content in the PtOx elect...

Effects of postannealing on the electrical properties of Ta2O5 thin films deposited on TiN/T

The effects of postannealing on the structural and leakage current characteristics of Ta2O5 thin films deposited on a TiN/Ti bottom electrode by chemical-vapor deposition at 350 °C were investigated. The Ta2O5 film of 50 nm thickness shows an amorphous phase and smooth surface morphology after annealing in N2O plasma at 350 °C, but the film treated with rapid thermal annealing in nitrogen (RTN) at 750 °C prior to N2O plasma annealing at 350 °C has an orthorhombic structure and rugged surface morphology. In addition, the TiSi phase also appears in the specimen of RTN-treated Ta2O5 film. Moreover, the residual carbon can be effectively eliminated and the oxygen deficiency can be well compensated by the N2O plasma annealing for both amorphous and crystalline Ta2O5 films. The I–V relation of the amorphous Ta2O5 films is asymmetric with respect to the biasing polarity, which indicates that the leakage current is primarily dominated by the Schottky emission. On the other hand, the crystalline Ta2O5 film has a symmetrical I–V relation, suggesting that the non-Ohmic leakage in the crystalline Ta2O5 film is related to the Poole–Frenkel effect. The barrier heights φB for the non-Ohmic leakage in the amorphous and crystalline Ta2O5 films, as evaluated from the temperature dependence of the I–V relation, are 0.52 and 0.82 eV, respectively.The effects of postannealing on the structural and leakage current characteristics of Ta2O5 thin films deposited on a TiN/Ti bottom electrode by chemical-vapor deposition at 350 °C were investigated. The Ta2O5 film of 50 nm thickness shows an amorphous phase and smooth surface morphology after annealing in N2O plasma at 350 °C, but the film treated with rapid thermal annealing in nitrogen (RTN) at 750 °C prior to N2O plasma annealing at 350 °C has an orthorhombic structure and rugged surface morphology. In addition, the TiSi phase also appears in the specimen of RTN-treated Ta2O5 film. Moreover, the residual carbon can be effectively eliminated and the oxygen deficiency can be well compensated by the N2O plasma annealing for both amorphous and crystalline Ta2O5 films. The I–V relation of the amorphous Ta2O5 films is asymmetric with respect to the biasing polarity, which indicates that the leakage current is primarily dominated by the Schottky emission. On the other hand, the crystalline Ta2O5 film has a s...

Convective solution transport - An improved technique for the growth of big crystals of the superconducting α-FeSe using KCl as solvent

An improved technique of convective solution transport using KCl as solvent at 840–790 °C (where mass transport takes place across a vertical temperature gradient) is described for the growth of crystals of the recently discovered superconductor α-FeSex (x = 1−0.8). The crystals were annealed in situ at 400–350 °C for 20–30 h to improve the superconducting properties. Hexagonal plate like crystals measuring 5–6 mm across and 0.25–0.5 mm thick were obtained. High resolution transmission electron microscopy (HRTEM) measurements show good crystallinity and the energy dispersive x-ray analysis (EDX) gives a composition very close to the starting powders. The zero resistance temperature of the crystals was found to increase from 6.5 to 9 K as the composition is decreased from x = 0.95 to 0.9 and decrease thereafter. Similar behavior was also observed in the powder x-ray diffraction (XRD) patterns and Raman spectra with the main peak shifting to higher value until 0.9 and decrease thereafter. In addition the XR...