Non-linear current–voltage characteristics related to native defects in SrTiO3 and ZnO bicrystals

SrTiO₃ and ZnO bicrystals with various types of boundaries were fabricated in order to examine their current—voltage characteristics across single grain boundaries. Their grain boundary structures were also investigated by high-resolution transmission electron microscopy. In Nb-doped SrTiO₃, electron transport behaviors depend on the type of boundaries. Random type boundaries exhibit highly non-linear current—voltage characteristics, while low angle boundaries show a slight non-linearity. On the contrary, undoped ZnO does not exhibit non-linear current—voltage characteristics in any type of boundaries including random ones. It is suggested that the differences observed in current—voltage properties between the two systems are mainly due to the difference in the accumulation behavior of acceptor-like native defects at grain boundaries. A clear non-linearity is obtained by means of Co-doping even for the highly coherent ∑ 1 boundary in a ZnO bicrystal. This is considered to result from the production of acceptor-like native defects by Co-doping.     

Electron transport behaviors across single grain boundaries in <Emphasis Type="Italic">n</Emphasis>-type BaTiO<Subscript>3</Subscript>, SrTiO<Subscript>3</Subscript> and ZnO

In some electroceramic materials, their unique electrical properties are due to potential barriers, i.e., double Schottky barriers (DSBs), formed at grain boundaries. So far, some researchers have revealed that the electrical properties of DSB are closely related to grain boundary characters, especially grain boundary coherency. For example, highly coherent boundary does not give PTCR or varistic property, while random types exhibit clear resistivity jump or abrupt current increment. Therefore, a concept of grain boundary design will be required for future device manufacturing, even in bulk materials. But it has not been clarified yet why the electron transport behaviors depend on them. In order to address this question, it is necessary to carry out a systematic experiment focusing on single grain boundaries using well-defined bicrystals.In the present study, we have summarized our studies with a special interest in electron transport behavior across single grain boundaries for n-type BaTiO₃, SrTiO₃ and ZnO.     

Effect of Na-diffusion on the electrical properties of SrTiO3

Grain boundary characteristics of Na-diffused SrTiO₃ were investigated. It was observed that Na ions are segregated in grain boundaries by secondary ion mass spectroscopy analysis. Using I-V characteristics and C-V measurement, the non-linearity coefficient and the potential barrier height were found to be 15.6, 0.82 eV, respectively. The I-V characteristics, complex plane analysis, and dc voltage step technique were performed to study the electrical properties of a single grain boundary. From the results of complex plane analysis, the deep electronic level was varied from 0.10 to 0.31 eV, which can be explained by the substitution of Na ions for Sr ions.     

Microstructural study of YBa2Cu3O7/SrTiO3/YBa2Cu3O7 heteroepitaxial trilayer films grown on (100) SrTiO3 substrates

Detailed transmission electron microscopic study has been carried out on heteroepitaxial YBa₂Cu₃O₇/SrTiO₃/YBa₂Cu₃O₇ trilayer thin films grown on (100)SrTiO₃ substrates prepared by DC and RF magnetron sputtering. The microstructural results showed the existence of somea-axis-oriented YBCO grains 20–90 nm wide in thec-axis-oriented YBCO matrix. Some of thea-axis grains in the lower YBCO thin film layer have protruded into the above SrTiO₃ layer, which may cause short circuit between the two YBCO superconducting layers. This is unsuitable for the application of trilayer thin films for microelectronic devices. The defects on the surface of the substrates would also influence the growth quality of the YBCO thin films.     

Study on electrical properties of Ni-doped SrTiO3 ceramics using impedance spectroscopy

The ceramics, SrTiO₃ (ST) and 0.4, 0.8 atom % Ni doped SrTiO₃, were prepared by solid state reaction route. The average grain size of undoped and doped samples was measured and found to be 1.2, 1.9 and 3.7 μm, respectively. The impedance measurements were conducted at 400–600°C to separate grain and grain boundary contributions. The grain and grain boundaries relaxation frequencies were shifted to higher frequency with temperature. Bulk resistance of doped and undoped ST ceramics was more or less the same. Single grain boundary resistance of doped sample was higher than that of undoped one, indicating that GB resistance increases with acceptor doping. Activation energies were calculated to confirm the same.     

Structural and thermal characterization of La5Ca9Cu24O41 thin films grown by pulsed laser deposition on (1 1 0) SrTiO3 substrates

In the present study stoichiometric, b-axis oriented La₅Ca₉Cu₂₄O₄₁ thin films were grown by pulsed laser deposition on (1 1 0) SrTiO₃ substrates in the temperature range 600-750 °C. High resolution transmission electron microscopy was employed to investigate the growth mechanism and the epitaxial relationship between the SrTiO₃ substrates and the La₅Ca₉Cu₂₄O₄₁ films grown at 700 °C. The 3-ω method was used to measure the cross-plane thermal conductivity of La₅Ca₉Cu₂₄O₄₁ films in the temperature range 50-350 K. The observed glass-like behavior is attributed to atomic-scale defects, grain boundaries and an interfacial layer formed between film and substrate.     

DC SQUIDs based on YBCO step edge junctions

We describe the fabrication and testing of dc SQUIDs (Superconducting QUantum Interference Devices) obtained by photolithographic patterning of YBa₂Cu₃O_7–x thin films deposited both on SrTiO₃ and MgO substrates. The Josephson junctions in the superconducting loop are of the step-edge type, where the weak link is obtained through the growth of grains with different orientations across suitably prepared steps previously etched on the substrate surface. TheI–V characteristics of the devices tested show multiple branches and instabilities having a weak dependence on the external magnetic field, probably due to formation of junction clusters on the substrate step. The SQUID devices showed quantum interference behavior and an easily detectable voltage modulation with the applied magnetic field at a temperature of 77 K.     

Electronic Structure of Mn Acceptor Impurity Incorporated SrTiO3 Using Embedded Cluster Method

We have performed atomic-level first principle electronic structure calculations on doped grain boundaries (GB) in SrTiO₃. This was motivated by the electron holography experiments, which were able to quantify the electrostatic potential and the associated space charge distribution across the Mn-doped GB in this material. The embedded cluster Discrete Variational (DV)-X method was used to determine the charge and the densities of states for several idealized models of a single crystal and symmetrical tilt grain boundaries in SrTiO₃. Special attention was given to the role of Mn⁺² and Mn⁺³ acceptors substituting for Ti⁺⁴ resulting in charge segregation across the grain boundaries, which was shown in the electron holography experiments. We have found that Mn replacing Ti prefers to have valence charge around +2 and this picture agrees with the experimental observation of negative grain boundary charges in the GB core.     

Current–Voltage Characteristics of TlInX2–TlSmX2 (X = S, Se, Te) Alloys

The steady-state current–voltage characteristics of TlInX₂–TlSmX₂ alloys were measured as a function of sample composition and thickness. The results demonstrate that the alloys studied exhibit threshold switching. The threshold voltage decreases with increasing mean atomic weight and can be stabilized by heat treatment. The most stable current–voltage characteristics are obtained for thin-film samples.     

HRTEM and EELS Studies on Structural Defects in the Superconductor Sr2CuO2+δCl2−y Newly Synthesized Under High Pressure

Structural defects in the newly high-pressure synthesized superconductor Sr₂CuO_2+δCl_2−y have been studied by means of high-resolution transmission electron microscopy and parallel electron energy-loss spectroscopy. Two types of grain boundaries were found in the material. One is the amorphous boundaries usually lying in the ab-planes. The other is the structurally intact grain boundaries, and most of them are large-angle boundaries. One of the large angles frequently observed is the 90°〈110〉 asymmetrical tilt boundary. In some domains, periodicity lengths were changed due to the lattice distortion usually associated with atom deficiency. These domains can be regarded as “second-phase particles” embedded in the perfect grains. O-K electron energy-loss spectroscopy results suggest that those defect domains are in the hole-poor states. Quantitative composition analysis from energy dispersive of X-ray suggests the hole-poorness is caused by oxygen deficiency. Dislocations, such as the a[110] and c[001] edge dislocations, were also observed in the material. In most cases, those dislocations are very complex and exhibit stacking faults. PACS: 68.37.Lp, 67.80.Mg, 74.72.Jt     

A high-resolution transmission electron microscope study of a zinc oxide varistor

Investigations were made of varistor microstructure, the morphology of Bi₂O₃ at multiple ZnO grain junctions, Bi₂O₃/ZnO grain boundaries and ZnO/ZnO grain boundaries (especially whether Bi₂O₃ is present or not at the ZnO/ZnO grain boundary) by means of high-resolution transmission electron microscopy and X-ray microanalysis in the scanning transmission electron microscope. Bi₂O₃ at multiple ZnO grain junctions consists of small particles of 0.1m in diameter, and they are vitrified to some extent. It is suggested that bismuth ions dissolve into ZnO grains over a 30 nm range from a Bi₂O₃/ZnO grain boundary; however, there is no bismuth at ZnO/ZnO grain boundaries.     

Fabrication and Characterization of YBa2Cu3O7 ? x Thin Film Step Junctions on MgO Substrates

We have investigated the tunneling properties of proximity-effect superconductor-normal-superconductor junctions based on high-temperature superconductor YBCO thin films and normal Ag layer. The systematic analysis of the properties of the films and the fabrication process which leads to a good-quality junctions is discussed in detail. The properties of the tunneling barrier are investigated by dc and rf measurements: the dc properties reveal that our junctions fully behave as expected from the de Gennes model. Regular Shapiro steps in the current–voltage characteristics are obtained as a result of microwave irradiation, indicating reliable rf properties of the junctions.     

Synthesis and Processing of Doped Hg1Ba2Ca2Cu3Oy Superconductors

A series of quenching experiments were conducted to understand the sequence of reactions that occur during the synthesis of doped Hg1223, (Hg, A)Ba₂Ca₂Cu₃O _y , A = Re, Bi, and Pb (HgA1223). The formation and decomposition of the intermediate phases during the high-temperature reaction were followed as a function of temperature. HgA1223 phase forms over a wide range of temperatures, 750–950°C, 750–880°C, and 840–880°C for A = Re, Pb, and Bi, respectively. At T<750°C, HgA1212 phase forms for A = Re and Pb. Based on the results of quenching experiments, heat treatment conditions were optimized for the synthesis of pure HgA1223 phase using commercial BaCaCuO precursor powders. A reduced-temperature annealing stage after the high-temperature reaction helps in grain growth and improves the microstructural characteristics of HgA1223 samples. Control of Hg pressure during the reaction is crucial for achieving phase purity, grain growth, and texture in the final products. A novel approach for the control of Hg pressure during the synthesis of HgA1223, which consists of using CaHgO₂ as an external Hg source, is reported. HgA1223 samples synthesized using the new synthesis protocol exhibit improved microstructural and superconducting properties.     

Determination of the atomic structure of a Σ13 SrTiO<Subscript>3</Subscript> grain boundary

New elements of a symmetric [001] 67.4^∘ SrTiO₃ near Σ 13 tilt grain boundary are identified by a quantitative analysis of lattice images, reconstructed electron exit waves, and HAADF images. The analysis reveals local, geometrical variations of structural grain boundary units that relate to the presence of defects introduced by a tilt deviation of 0.65 + 0.02 degrees from the perfect Σ13 geometry. Sr and TiO columns are discriminated in HAADF images while the reconstructed electron exit wave reveals all oxygen columns in addition. Both methods depict the crystal and boundary structure directly while lattice imaging with a high voltage instrument requires image simulations to link the image intensity to structure. For the first time we observe a Sr column splitting by 90 pm that supports theoretical predictions. An inhomogeneous, preferential etching at the grain boundary is attributed to local charge variations and hampers a quantitative investigation and local stoichiometry. The near Σ13 boundary forms a dense and compact structure with chemically identical columns in close proximity. Therefore, it is different from the relaxed, bulk like configurations described in previous reports.     

Sintering and properties of low-firing non-ohmic SrTiO3 ceramics

The sintering and properties of non-ohmic SrTiO₃ ceramics in the presence of Nb⁵⁺ as a donor dopant and Li⁺ as an acceptor dopant and sintering agent have been studied. Different sintering parameters, such as sintering temperature, sintering time, cooling rate, and the ambient atmosphere at the cooling stage, were investigated. The changes induced by each parameter on the electrical properties have been discussed. The segregation of lithium at the grain boundaries is mainly fulfilled at the cooling stage. The correct slow cooling rate and oxygen atmosphere are both favourable to the segregation of lithium at the grain boundaries, and thus result in an improvement of non-linear I–V characteristics. The lithium content and different lithium salts (Li₂CO₃, LiNO₃, LiF) were also investigated.     

On the limiting factors of the critical current density in high-Tc superconducting ceramics

The temperature dependence of the critical current density at high temperatures and in weak applied magnetic field for YBa₂Cu₃O_7–y ceramic samples with a pronounced granular character is analyzed. The experimental results can be explained in terms of thermally activated motion of the intergrain Josephson vortices at grain boundaries, which may be an indication that the actual limiting factor for the critical current density in ceramic samples results from a weak pinning force density for the intergrain vortices rather than from the weak-link quenching.     

Microstructure of Yb and Li co-doped ZnO by electron microscopy

The microstructure and microanalysis of Yb³⁺ and Li⁺ co-doped ZnO has been studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Co-doping ZnO with Li⁺ and Yb³⁺ by solid state reaction method produces a new LiYbO₂ phase. Formation of the LiYbO₂ is intermediated by ZnO. Nucleation and growth of LiYbO₂ occur at ZnO surfaces and at grain boundaries. It is also found that the LiYbO₂ particle is strongly bonded to the ZnO subtract, and thus a reaction (or diffusion) zone is likely to exist between LiYbO₂ and ZnO. The diffusion region may then accommodate a large amount of Yb³⁺ and Li⁺ ion in the ZnO.     

Proximity Effect in Bulk LaBa2Cu3O7?y Samples with Ag Additions

Bulk LaBa ₂ Cu ₃ O _(7–y) samples with different Ag additions were investigated. It was shown that Ag does not enter the crystallographic structure of the superconductor and segregates on the grain boundary region. Current path in these samples occurs through the proximity-connected grains, and this was confirmed from the temperature dependence of the critical current density and mutual inductance. By using the theory developed for the thin-film structures, we conclude that growing of the Ag content increased the effective cross-section and the normal metal thickness. The effective cross-section prevails at low concentration, increasing the current. The normal metal thickness dominates at higher concentration, leading to saturation or even lowering of the critical current.     

Study of (Bi4−x La x ) Ti3O12 Powders and Ceramics Prepared by Sol-Gel Method

La-modified (Bi_4–x La _x )Ti₃O₁₂ (abbreviated as BLT) powders were prepared by sol-gel processing methods. The powders were characterized by differential thermal analysis (DTA) and laser-diffraction particle-size analysis. The (Bi_4–x La _x )Ti₃O₁₂ ceramics were prepared from the powders and characterized by X-ray diffraction (XRD). The results indicate that the sol-gel method can be used to prepare nanometer powder for the new types of BLT ferroelectric ceramics. The solid reaction of BLT powders occurs at 730°C approximately resulting in the growth of BLT grain. The average grain size may be varied in the range 60–500 nm, depending on the calcination temperature of the powders. The (Bi_4–x La _x )Ti₃O₁₂ ceramics prepared from the powders were polycrystalline materials with completely monoclinic (Bi_4–x La _x )Ti₃O₁₂ phase.