Boundary Barrier Research Articles

Individual core-shell structured ZnSnO3 nanowires as photoconductors

Photoconducting properties of individual ZnSnO3 nanowires are investigated by performing transport measurement with UV and green laser illumination on/off circles. ZnSnO3 nanowires are of core-shell structures with the presence of a grain boundary between the inner and outer layer. Upon exposure to UV illumination, the current of one single ZnSnO3 nanowire greatly increases by about three orders of magnitude, from 0.3 to 162 nA. And the conductance increases by a factor of 22 under green laser illumination. Such behaviors are ascribed to that the illuminations adjust the grain boundary barrier. These results demonstrate a promising approach to fabricate nanosized photoconductors and optoelectronic switches.

Enhanced magnetoresistance in double perovskite Sr 2FeMoO 6 through SrMoO 4 tunneling barriers

We synthesized the single phase Sr 2FeMoO 6 (sample B) and Sr 2FeMoO 6 with 10% SrMoO 4 impurity phase (sample C) by sintering the material in different environments. Field emission scanning electron microscope (FE-SEM) and energy-dispersive X-ray analysis (EDAX) results clearly indicate that in sample C, the insulating SrMoO 4 phase is segregated at the grain boundaries of Sr 2FeMoO 6 grains and produces the tunneling barriers in the conduction of charge carriers between Sr 2FeMoO 6 grains. X-ray diffraction (XRD) results show that generation of SrMoO 4 phase increases the antisite defect in sample C. Both of the samples show semiconducting behaviour in the entire measured temperature range (80–300 K). The resistance of sample C is more as compared to sample B while the value of magnetization is less. The 38% enhancement in magnetoresistance (MR) at 80 K is observed for the sample C (having 10% insulating SrMoO 4 phase) as compared to single phase sample B at 3 kOe. It is suggested that this enhancement in MR is due to intergrain tunneling through insulating nonmagnetic SrMoO 4 grain boundary barriers in the Sr 2FeMoO 6 matrix and subsequent antisite defects, leading to enhancement in low field magnetoresistance (LFMR). This appears to be a simple way to produce tunneling barriers as compared to artificial fabrication of multilayer structures to enhance the low field magnetoresistance.

High permittivity in Zr doped NiO ceramics

We report on measurements of the dielectric permittivity of NiO-based ceramics doped with Zr (ZNO). Samples were prepared by the traditional solid-state reaction method. The concentration of Zr has an effect on the dielectric properties of ZNO ceramics. High permittivity values (∼104) were observed which remain almost constant from 200 K to 350 K at low frequencies. The high-dielectric-constant response of the ZNO ceramics is attributed mainly to a grain boundary (internal) barrier layer capacitance.

Extremely high oxygen sensing of individual ZnSnO3 nanowires arising from grain boundary barrier modulation

Extremely high oxygen sensing is realized from individual ZnSnO3 nanowires with abundant grain boundaries. The current across one single ZnSnO3 nanowire increases by about six orders of magnitude, from 1.20×10−7to3.78×10−1μA, as the oxygen pressure decreases from 3.7×104to1.0×10−4Pa. Such a drastic sensing is ascribed to grain boundary barrier modulation. This interpretation is confirmed by the sensing experiments under UV illumination. The results demonstrate a promising approach to realize miniaturized and highly sensitive oxygen sensors.

Effect of Ta 2O 5 in (Ca, Si, Ta)-doped TiO 2 ceramic varistors

TiO 2 varistors doped with 0.2 mol% Ca, 0.4 mol% Si and different concentrations of Ta were obtained by ceramic sintering processing at 1350 °C. The effect of Ta on the microstructures, nonlinear electrical behavior and dielectric properties of the (Ca, Si, Ta)-doped TiO 2 ceramics were investigated. The ceramics have nonlinear coefficients of α = 3.0–5.0 and ultrahigh relative dielectric constants which is up to 10 4. Experimental evidence shows that small quantities of Ta 2O 5 improve the nonlinear properties of the samples significantly. It was found that an optimal doping composition of 0.8 mol% Ta 2O 5 leads to a low breakdown voltage of 14.7 V/mm, a high nonlinear constant of 4.8 and an ultrahigh electrical permittivity of 5.0 × 10 4 and tg δ = 0.66 (measured at 1 kHz), which is consistent with the highest and narrowest grain boundary barriers of the ceramics. In view of these electrical characteristics, the TiO 2–0.8 mol% Ta 2O 5 ceramic is a viable candidate for capacitor–varistor functional devices. The characteristics of the ceramics can be explained by the effect and the maximum of the substitution of Ta 5+ for Ti 4+.

Conducting grain boundaries in the high-dielectric-constant ceramic CaCu3Ti4O12

To clarify the electrical property of grain boundaries, the fine-grained ceramics CaCu3Ti4O12 have been treated with the hydrofluoric acid to remove the parts of grain boundaries. The dielectric response difference between the etched samples and the pristine ones indicates that the ceramic CaCu3Ti4O12 consists of insulating or semiconducting grains with conducting grain boundaries. Therefore, the giant dielectric phenomenon is supposed not to derive from the grain boundary barrier layer capacitance effect. The possible mechanism is discussed.

Origins of shallow level and hole mobility in codoped p-type ZnO thin films

A combination study of structural, optical, and electrical properties has been carried out on N–In codoped p-type ZnO thin films for the origins of shallow level and hole mobility. The observed small activation energy of ∼20meV for the hole concentration corresponds well to the results from photoluminescence and conductivity data, revealing the grain boundary trapping nature of the shallow level. The achieved hole mobility is mainly due to the lack of grain boundary barrier effect, and the codoping yielded weak ionized impurity scattering. The authors have also revealed the scattering and conduction mechanisms in these p-ZnO films.

Sintering effects on structure and dielectric properties of dielectrics CaCu3Ti4O12

Due to the continuous demand for miniaturization of electronics, there is an interest in developing dielectric materials with higher permittivity. Recently, the CaCu3Ti4O12 (CCTO) has been reported [1–3], has a very high permittivity (~10) at room temperature and very small temperature dependence in a broad temperature range. However, it appears that the dielectric properties of CCTO are very sensitive to processing. [4–7] Permittivities from 478 to 300,000 have been obtained for CCTO prepared by different routes. The original work, done by Subramanian et al. [1] and Ramirez et al. [2] using conventional mixed-oxide processing via mortar and pestle, showed that room temperature permittivities ranged from 10,000 to 20,000. Other researchers using the same technique have reported permittivities of 478, 2400 and 18,700 [8–10]. The highest permittivity reported as derived from impedance spectroscopy measurements was 300,000 for CCTO made from powder that was mixed via ball milling and sintered for 24 h at 1100 C [11]. Though different hypotheses have been put forth to explain the high permittivity of CCTO, its origin is still not fully justified. At present, the grain boundary barrier layer capacitance model of extrinsic mechanism is the most favorable one [12–19]. It is believed that insulating surfaces form on semi-conducting grains during the sintering of CCTO ceramics, which results in an electronically heterogeneous material very similar to the nature of internal barrier layers capacitors (IBLC). The dielectric properties of these IBLCS are very sensitive to different processing parameters. In the present work, the CCTO ceramics are prepared by the solid-state reaction method. The sintering effects on the phase evolution, structure and dielectric properties of CCTO ceramics have been investigated.

Study for streamline of arbitrary shaped homogeneous reservoirs with impermeable barriers

The steady-state flow mathematical model of arbitrary shaped homogeneous reservoirs with impermeable barrier is constructed in this paper. By using Boundary Element Method (BEM), the mathematical model is solved. And a streamline generating technique is presented. The figures of streamlines are plotted and analyzed considering the effect of complex boundary and impermeable barriers. Through analyzing, it indicates that the size, shape and orientation of impermeable barriers have various degree of influence on the streamlines. So, if there are impermeable barriers in reservoir according to the geological materials, the influence of impermeable barriers must be considered when adjusting flood pattern and injection strategy.

Direct current bias effects on grain boundary Schottky barriers in CaCu3Ti4O12

CaCu 3 Ti 4 O 12 exhibits an unusually high dielectric constant on the order of 105 and highly nonlinear I-V characteristics. Impedance spectroscopy measurements carried out in this work point to the crucial role played by grain boundary barriers in controlling the electrical properties of this material. Under dc bias, the grain boundary resistance decreases, followed by a precipitous breakdown at higher applied voltages. The barrier height is estimated to be ∼0.82eV. The grain conductivity shows a transition from a negative temperature coefficient of resistance with activation energy of ∼0.08eV to a positive temperature coefficient of resistance at 280 °C suggesting a transition from impurity ionization to scattering controlled mobility in the carrier saturation region.

Stray-field effects in submicronYBa2Cu3O7−xbicrystal grain boundary junctions

We have investigated the magnetic field dependence of the critical current in submicron ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ [001] tilt bicrystal grain boundary junctions. The good homogeneity of submicron grain boundary barrier interfaces, together with the large magnetic field needed to modulate the Josephson current, allow to observe several features not observed in micron size junctions, like a nonconstant modulation period, strongly dependent on the maximum applied magnetic field. Experimental results are well accounted for by a theoretical model taking into account the magnetization of the nearby electrode, and the strong interaction of its stray field with the junction interface. The analysis of the nature of this interaction and the influence of the device geometry are also relevant for potential applications of high critical temperature LSI devices.

Electrical properties of ZnO varistors prepared by direct mixing of constituent phases

Varistor samples containing different amounts of constituent phases were prepared by direct mixing of constituent phases. Detailed electrical characterization was performed to explain the influence of minor phases (spinel and intergranular phases) on overall properties. Characterization included investigation of the non-linear coefficients (?), breakdown electric field (EB), leakage currents (JL), grain boundary barrier hight (?B) and constant ? from current-voltage characteristics, as well as calculation of activation energies for conduction (EA) from ac impedance spectroscopy in the temperature interval 30-410?C. Varistors sintered at 1100 ?C for 1 h showed pronounced differences in electrical properties depending on relative molar ratios of the phases. Results were discussed in the sense of possible reduction of the content of minor phases in ZnO varistors.

Tunnelling magnetoresistance of double perovskiteSr2FeMoO6 enhanced by grain boundary adjustment

Polycrystalline double perovskiteSr2FeMoO6 samples were synthesized by a wet chemistry method under controlled reductionconditions. Single-phase ‘bulrush-like’ samples of grain size about 110 nm in width and>2 µm in length were readily obtained at temperatures as low as950 °C, and they showed a large room-temperature tunnelling magnetoresistance(Δρ/ρ0)of −21% at a low magnetic field of 10 kOe. Utilizing the reactivity ofSr2FeMoO6 to water and a sonochemical technique, we manipulated the properties of grain boundary (GB)barriers, and managed to enhance the room-temperature tunnelling magnetoresistance to−35%. The magnitude of the low-field magnetoresistance appears to depend strongly on GBproperties, and can be regulated by controlling the reaction time with water.

Varistors based on Ta-doped TiO 2

The nonlinear current ( I)–voltage ( V) characteristics of titanium dioxide are examined when doped with small quantities (0.05–0.5 at.%) of tantalum pentaoxide. For optimum compositions, the nonlinear coefficients are found to be in the range of 25–30 and the breakdown field strength ( E B) is ∼4000 V/cm. The obtained α- and E B-values are higher than the previously reported values for TiO 2 ceramics. The acceptor like surface states at the grain boundary adsorb oxygen during sintering and cooling, leading to formation of grain boundary barrier. The grain boundary barrier height ( Φ B) is calculated using Schottky equation.

Multiple Bromodomain Genes Are Involved in Restricting the Spread of Heterochromatic Silencing at theSaccharomyces cerevisiae HMR-tRNA Boundary

The transfer RNA gene downstream from the HMR locus in S. cerevisiae functions as part of a boundary (barrier) element that restricts the spread of heterochromatic gene silencing into the downstream region of chromosome III. A genetic screen for identifying additional genes that, when mutated, allow inappropriate spreading of silencing from HMR through the tRNA gene was performed. YTA7, a gene containing bromodomain and ATPase homologies, was identified multiple times. Previously, others had shown that the bromodomain protein Bdf1p functions to restrict silencing at yeast euchromatin-heterochromatin boundaries; therefore we deleted nonessential bromodomain-containing genes to test their effects on heterochromatin spreading. Deletion of RSC2, coding for a component of the RSC chromatin-remodeling complex, resulted in a significant spread of silencing at HMR. Since the bromodomain of YTA7 lacks a key tyrosine residue shown to be important for acetyllysine binding in other bromodomains, we confirmed that a GST-Yta7p bromodomain fusion was capable of binding to histones in vitro. Epistasis analysis suggests that YTA7 and the HMR-tRNA function independently to restrict the spread of silencing, while RSC2 may function through the tRNA element. Our results suggest that multiple bromodomain proteins are involved in restricting the propagation of heterochromatin at HMR.

Effect of Dy 2O 3 and La 2O 3 on the microstructure and electrical properties of WO 3 ceramics

Dy 2O 3 and La 2O 3-doped WO 3 ceramics were fabricated by conventional ceramic technique and the microstructure, phase structure and electrical properties were studied. There are some porous slice-shaped materials in Dy-doped samples and club-shaped materials in La-doped samples. It is believed that slice-shaped materials (Dy-rich phase) are amorphism with weak conductivity to form the Schottky barrier. However, club-shaped materials (La-rich phase) are crystalline state with good conductivity so that the barrier is broken down, leading the nearly linear current–voltage characteristics. The grain boundary barrier layer capacitor (GBBLC) model cannot account for the high dielectric constant of La-doped sample because of the good conductivity of grain boundary materials. The high dielectric constant is assumed to arise from the high dielectric polarization of La-rich phase in the grain boundary.

Electrical transport parameters of heavily-doped zinc oxide and zinc magnesium oxide single and multilayer films heteroepitaxially grown on oxide single crystals

Heavily doped epitaxial ZnO:Al and Zn 1− x Mg x O:Al films were grown by radio frequency magnetron sputtering onto single crystalline substrates (sapphire, MgO, silicon) and characterized by structural and electrical measurements. It is the aim of this investigation to better understand the carrier transport and the doping mechanisms in heavily doped transparent conducting oxide (TCO) films. It was found that the crystallographic film quality determines only partly the mobilities and the carrier concentrations: ZnO:Al films on a-plane (110) sapphire and on MgO (100) exhibit the highest mobilities. The oxygen partial pressure during the deposition from ceramic targets is more important influencing especially the carrier concentration N of the films. Though the films grew epitaxially grain boundaries are still existent, which reduce the mobility due to electrical grain boundary barriers for N < 3 · 10 20 cm − 3 . From annealing experiments the role of point defects and dislocations for the carrier transport could be estimated. For carrier concentrations above 3 · 10 20 cm − 3 ionized impurity scattering limits the mobility, which is in agreement with our earlier review [K. Ellmer, J. Phys. D: Appl. Phys. 34 (2001) 3097].

Analytical on-state current model of polycrystalline silicon thin-film transistors including the kink effect

A simple analytical expression for the on-state current of polycrystalline silicon thin-film transistors is presented, valid in both linear and saturation regimes including the impact ionization effect. The maximum channel electric field and the avalanche multiplication factor are described in terms of an average trapped charge density at the grain boundary, varying with gate voltage due to the continuous energy distribution of the grain boundary trap states. Based on the parameters of the charge inversion voltage, effective carrier mobility and grain boundary barrier height which are extracted from the transfer characteristic at low drain voltage, the model reproduces the experimental output characteristics in devices with different gate lengths using few fitting parameters.

Effect of Co2O3 on the microstructure and electrical properties of Ta-doped SnO2 varistors

The effect of Co2O3 on the microstructure and electrical properties of Ta-doped SnO2 varistors was investigated. It was found that a sample doped with 0.1 mol% Co2O3 had the highest nonlinear coefficient α = 33, the highest breakdown electrical field EB = 872 V mm−1 and the lowest relative dielectrical constant εr = 598 (measured at 1 kHz). However, 0.1 mol% Co2O3 is not sufficient for densification of SnO2 ceramics, and the relative density of the sample doped with 0.1 mol% Co2O3 (85.8%) is much lower than that of the samples doped with 0.3, 0.5, 0.8 and 1.2 mol% Co2O3 (about 98%). The highest breakdown electrical field and lowest relative dielectric constant of the sample doped with 0.1 mol% Co2O3 are mainly the result of the loose microstructure and the smallest average grain size. The measurements of grain boundary barrier height, ΦB, and grain boundary resistance, RGB, indicate that CoSn× should be located at the depletion layer and is important to the formation of the grain boundary barrier.

Unusual Dielectric Relaxation in Lightly Doped n-Type Rhombohedral BaTi0.85Zr0.15O3:Ta Ferroelectric Ceramics

The dielectric properties of tantalum-doped BaTi0.85Zr0.15O3 were investigated. The conventional solid-state reaction method was used to make the ceramic samples. The composition was close to the pinch point of the three structure phase transitions in BaTiO3. Powder X-ray diffraction results indicated that the samples were in a slightly distorted rhombohedral structure at room temperature, and the dielectric measurement showed that only a cubic-to-rhombohedral phase transition occurred. Instead of a sharp peak characteristic of a ferroelectric (FE) transition, the large dielectric response was similar to those in nonferroelectric perovskites CaCu3Ti4O12, AFe1/2B1/2O3 (A = Ba, Sr, Ca; B = Nb, Ta, Sb), and IBLC (internal barrier layer capacitance) BaTiO3-based FE ceramics. However, unlike its FE analogue, the conventional BaTiO3-based IBLC ceramics, our samples showed weaker temperature dependence of relative permittivity which spans over a wide temperature range. A Maxwell−Wagner-type relaxation in the FE state was observed. Moreover, another relaxation mode appears from the vicinity of the FE phase transition temperature TC and extends over a narrow temperature range, with unusual slowing down of relaxation rate as temperature rises, which is contrary to what is commonly observed. We associate these unusual phenomena with the slightly distorted rhombohedral structure of our samples and the presence of the grain boundary barrier layer where oxygen vacancies distribute nonhomogeneously.