Single Grain Boundary Research Articles

Atomic-scale manipulation of potential barriers at SrTiO3 grain boundaries

In oxide grain boundaries (GBs), oxygen ions and their vacancies serve as a common denominator in controlling properties such as GB barrier height and capacitance. Therefore, it is critical to analyze, control and manipulate oxygen and vacancies at oxide interfaces as most of the practical devices are almost always influenced by the presence of electrostatic potential barriers at interfaces. Here, we report adjustment of a single GB potential barrier via manipulation of oxygen vacancy concentration using simple oxidation and reduction treatments. We validate our analysis with aberration-corrected HAADF imaging and column-by-column EELS coupled with macroscopic transport measurements of isolated GBs to gain important insight into the physical attributes of GB potential barriers.

Diffusion Impurity Drag of Twin Grain Boundaries and Triple Junctions Motion in Zinc

The experimental results on motion of single grain boundaries (GBs) of natural mechanical twin and single fabricated twin GBs as well as on fabricated twin GBs in system with triple junction (TJ) are obtained. The mobility of natural mechanical twin GBs, fabricated single GBs and fabricated GBs with TJ are compared. For the first time the effect of detachment of moving TJ and single natural twin GB from adsorbed atoms is reported. The results on single GB migration are considered in context of triple junction migration as the step to grain growth, i.e. “polycrystal” experiments.

Enhanced superconducting properties of bicrystallineYBa2Cu3Ox and alkali metals under pressure

Nearly hydrostatic pressures are found to enhance the critical current densityJc through singlegrain boundaries in YBa2Cu3Ox rings and to induce superconductivity in Li. Whereas Li becomes superconducting above20 GPa at temperatures as high as 15 K, no superconductivity was observed above 4 K in Nato 65 GPa nor in K above 4 K to 43.5 GPa or above 1.5 K to 35 GPa.

ZnO 바리스터의 단입계면 분석을 위한 마이크로 전극 제작과 전기적 특성 해석

To investigate the electrical properties at the single grainboundary of ZnO varistors, micro-electrodes were fabricated on the surface which was polished and thermally etched. Our micro-electrode had 2000 <TEX>$\AA$</TEX> silicon nitride layer between micro-electrode and ZnO surface. This layer was deposited by PECVD and etched by RIE after photoresistor pattering process using by mask 1. The metal patterning of micro-electrodes used lift-off method. We found that the breakdown voltage of single grainboundary is about 3.5∼4.2 V at 0.1 mA on I-V curves. Also, capacitance-voltage measurement at single grainboundary gave several parameters( <TEX>$N_{d}$</TEX>, <TEX>$N_{t}$</TEX>, <TEX>$\Phi$</TEX><TEX>$_{b}$</TEX>, t) which were related with grainboundary.ary.

Influence of the grain boundary network on the critical current density of deformation-texturedYBa2Cu3O7−xcoated conductors made by metal-organic deposition

Coated conductors (CC) are quasisingle crystals consisting of a series-parallel network of predominantly low angle grain boundaries, whose network misorientation distribution determines the current-carrying characteristics. To deepen our understanding of the influence of the grain boundary network in CC, we present a study of critical current density $({J}_{c})$ and electric field--current density $(E\text{\ensuremath{-}}J)$ characteristics over a broad range of magnetic fields and temperatures in a series of three 1-\ensuremath{\mu}m-thick ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\text{\ensuremath{-}}x}$ samples with variable textures, including a single-crystal film and two deformation-textured coated conductors made by a metal-organic deposition (MOD) process. We also investigated the influence of the number of grains within the bridge width by successively narrowing the bridge on the better-textured CC. We found clear evidence of crossover from grain boundary (GB) network control to grain control of ${J}_{c}$ by study of the magnitude of ${J}_{c}$ and the shape of the $E\text{\ensuremath{-}}J$ characteristic. In the narrowest tracks, we were able to isolate the effect of single GBs, finding dissipation only when the misorientation exceeded $\ensuremath{\sim}7\ifmmode^\circ\else\textdegree\fi{}$, more than twice that seen in classic bicrystal experiments. We found that the influence of global texture on ${J}_{c}$ (77 K, 0 T) is highly nonlinear, ${J}_{c}$ rising by more than 2.5 times as the full width at half maximum decreases only by $\ensuremath{\sim}2\ifmmode^\circ\else\textdegree\fi{}$. Very interestingly, the MOD-CC exhibited no grain boundary dissipation signature in their $E\text{\ensuremath{-}}J$ characteristics in wide bridges, showing that global influence of grain boundary network is very much attenuated even for the grain boundary network containing misorientations that would generate significant dissipation in thin film bicrystals.

Modeling of grain boundary barrier modulation in ZnO invisible thin film transistors

We model grain boundaries (GBs) for polycrystalline ZnO thin film transistors (TFTs). Experimental result shows a non-linear increase of drain current and gradual enhancement of field effect mobility with increasing gate bias. Our initial single GB model was unable to explain the experimentally obtained results, where we considered the peak defect distribution at the mid gap. Realizing from the experimentally obtained results, we remodeled the grain boundary considering the peak distribution close to the conduction band, which then better replicates the experimental observation. We describe here the transfer characteristic of experimental ZnO TFT in linear region with calculated potential profiles. Appropriate grain boundary modeling signifies that the slower decrease in potential barrier in grain boundary with applied gate voltage is responsible for such non-linear changes in drain current and gradual enhancement of mobility.

Low temperature poly-Si TFTs for display application

Abstract We have fabricated new polycrystalline silicon (poly-Si) thin film transistors (TFTs) with a single grain-boundary by a simple excimer laser annealing (ELA) method which employs a selectively floating amorphous silicon (a-Si) active layer and Al patterns. A thermally insulating air-gap between the floating a-Si and substrate successfully induces the lateral grain growth. The proposed poly-Si TFT exhibits high mobility of 331 cm 2 /V s due to the high-quality grains. The other proposed method employs a lateral grain growth phenomenon obtained by excimer laser irradiation on an a-Si layer with pre-patterned aluminum film. The aluminum patterns act as a masking layer of incident laser beam for the selective melting of a-Si layer. The n-channel poly-Si TFT fabricated by the proposed method shows considerably improved I – V characteristics, such as high field effect mobility exceeding 240 cm 2 /V s.

Characterization of an individual grain boundary in metal-induced laterally crystallized polycrystalline silicon thin-film devices

A distinct grain boundary (GB) is formed when two crystallization fronts collide in metal-induced laterally crystallized (MILC) polycrystalline silicon (poly-Si) thin films. This has been used to study carrier transport across a single dominant GB. The average number of traps per unit area is found to be about 1.6/spl times/ 10/sup 12//cm/sup 2/ in this GB, significantly higher than that associated with the regular GBs in the bulk of MILC poly-Si. Though this single GB occupies a negligible fraction of the total device volume, it has been found to significantly affect both the resistance of MILC resistors and the leakage current of MILC thin-film transistors.

Formation of Potential Barrier Related to Grain‐Boundary Character in Semiconducting Barium Titanate

Resistance–temperature (R–T) characteristics were measured directly at single‐grain boundaries in 0.1‐mol%‐niobium‐doped barium titanate bicrystals that had been fabricated from polycrystalline sinters, to determine a geometrical grain‐boundary character dependence of the positive temperature coefficient of resistivity (PTCR) effect. Both random boundaries and low‐Σ boundaries exhibit a similar grain‐boundary character dependence of the PTCR effect through a simple geometrical analysis, using the coincidence of reciprocal lattice points. Differences of the R–T characteristics in individual boundaries have been explained in terms of the formation of a potential barrier that is associated with the oxidation of grain boundaries during cooling, after sintering or annealing. The grain‐boundary character is likely to affect the diffusivity of O2− ions and, hence, is crucial to the formation of the potential barrier.

Critical currents and microstructures of LPE grown YBCO bicrystal films with large single facet grain boundaries

The microstructures and electric transport properties across grain boundaries (GBs) are studied for YBaCuO bicrystal films grown by liquid phase epitaxy (LPE) method. Large single facet GBs with typical size of 10–100 μm are obtained in the films on MgO bicrystal substrates. Microstructures and atomic arrangements of GBs with different misorientation angles ϑ are investigated by TEM. The critical current densities across [001] tilt GBs as a function of ϑ show that J c values are almost constant up to ∼12° and then decrease exponentially, as usually observed for bicrystal films grown by physical vapor deposition methods. The presence of a plateau region up to 12° demonstrates a feature of the clean and meandering free GB structure in the LPE bicrystal films. The temperature and field dependencies of I c for 24° GB are presented and discussed based on the microstructures.

Microstructural studies of high-T/sub c/ superconducting Josephson junctions to understand junction properties

Transmission electron microscopy (TEM) is applied to the study of the relationship between the microstructures and the electrical properties of Josephson junctions. Typical microstructures for several artificial barrier junctions and grain boundary (GB) junctions are reviewed. In the case of artificial barrier junctions, it was found that the barrier layer coverage can be enhanced by using a-axis oriented bottom electrodes and homoepitaxy growth. TEM observations of multi-layer junctions with PrBa/sub 2/Cu/sub 3/O/sub y/ (PrBCO) or YBa/sub 2/Fe/sub 3/O/sub y/ (YBFeO) barrier layers grown by quasi-homoepitaxy showed prefect coverage and good crystallinity. Liquid phase-epitaxy was successfully used to obtain large single facet GBs over 50 pin by growing YBa/sub 2/Cu/sub 3/O/sub y/ (YBCO) films on bicrystal substrates. Microstructures and atomic arrangements of these straight bicrystal GBs are presented.

Superconducting properties of natural and artificial grain boundaries in bulk melt-textured YBCO

A range of experimental techniques have been used to characterize melt-processed YBa 2Cu 3O 7− δ samples containing single-grain boundaries. Both natural high-angle boundaries, which sometimes appear during the grain growth process, and artificial low-angle boundaries, obtained by joining two single domains, have been investigated. Electrical resistivity, current–voltage characteristics, magnetic moment measurements and Hall probe mapping techniques have been employed to investigate the boundaries. Results are compared with the properties of single domain material (i.e., containing no grain boundary) for which T c≈89 K and J c‖ ab (77 K, 1 T)>10 4 A/cm 2. Resistance measurements across all the grain boundaries show a stronger dependence on current and magnetic field than that measured within the grains and exhibit a pronounced resistive `tail'. The I– V curves obtained for the high-angle natural grain boundary are sharp and differ from the rounded I– V curves which are characteristic of single-grains. Field mapping measurements used to evaluate the critical current anisotropy are in agreement with magnetisation measurements. The limitations of this technique for investigating boundaries are discussed. It was found that current anisotropy can conceal a weak link between two grains, leading to a false indication of single-grain behaviour. Artificially engineered boundaries are shown to have significant potential for applications in high fields at 77 K.

Quasi-particle and Josephson tunnelling characteristics of the single-grain boundary in BaPb1-xBixO3 bicrystals

Bulk bicrystals of the oxide superconductor BaPb1-xBixO3, with parameter x=0.27 and critical temperature 10.9 K, were grown by crystallization in a PbO-Bi2O3-BaO melt. Current-voltage (I-V) characteristics of the single-grain boundary, constituting a plane between superconducting blocks, were measured. The I-V characteristics had a shape typical of the Josephson junction with a critical current density of lower than 0.1 A cm2 at 4.2 K. The temperature dependences of the critical current density of the single-grain boundary were measured. Clear peaks on the dynamic conductance against bias voltage dependences of the bicrystals were observed and interpreted as energy-gap peculiarities. The temperature dependence of the energy gap of BaPb1-xBixO3 and the ratio 2 Delta (0)/kTc=3.6 were determined.

Electrical properties of ZnO/Bi/sub 2/O/sub 3/ two-layer composites

A ZnO/Bi/sub 2/O/sub 3/ two-layer composite formed by RF sputtering is studied as a model of a single-grain boundary in a ceramic ZnO varistor to clarify the mechanisms of degradation and nonlinear conduction and to realize a thin-film varistor. The current-voltage characteristics and the thermally stimulated currents (TSC) are measured. The composite under a reverse bias shows highly nonlinear conduction comparable to that in an actual ceramic varistor. The breakdown voltage of ZnO/Bi/sub 2/O/sub 3/ two-layer specimens increases with the thickness of the Bi/sub 2/O/sub 3/ layer and thus can be controlled by changing this layer. This is explained in terms of enhancement of the electric field at the ZnO/Bi/sub 2/O/sub 3/ interface by space-charge accumulation in the Bi/sub 2/O/sub 3/ layer and at the interface. TSC for reverse-bias poling voltage flows in the direction of polarization at temperatures >120 degrees C. This is explained by the space-charge accumulation in the Bi/sub 2/O/sub 3/ bulk and at the interface.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Critical current measurements in single crystals and single-grain boundaries in YBa2Cu3O7 films

The temperature, magnetic field, and orientation dependence of the critical current density of superconducting YBa <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> Cu <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</inf> films have been determined from transport measurements. The results support a model of flux creep within single grains and weak-link coupling across grain boundaries.