Schottky-type Potential Barrier Research Articles

Enhancement of non-linear properties in SnO2 varistors by ZnO doping

Abstract The effect of Zn doping content in SnO 2 -based varistors was investigated. The general formula was ((Sn 0.985 Co 0.01 Nb 0.005 O β ) 1−x (ZnO) x ) ; x = 0, 0.005, 0.01, 0.02 and 0.04. The powders synthesized by chemical precipitation were 4.4 – 6.1 nm in crystallite size along both [110] and [101]. Only cassiterite (SnO 2 ) phase was observed within the limit of detection of X-ray diffraction (XRD). Zn doping yielded great enhancement in sinterability. High densities were achieved in the x = 0.04 sample at 1150 °C whereas it took 100 °C higher to achieve the similar density for the undoped sample. Substitution of Zn cations into the Sn lattice was believed to generate oxygen vacancies, facilitating diffusion and mass transport. As a result, the average grain sizes initially increased at x = 0.005 and gradually decreased at higher Zn content possibly due to existence of secondary phases. The optimal composition with the greatest breakdown voltage (6384 V cm −1 ) and non-linear coefficient (17.2) occurred at x = 0.02. This improvement in varistor properties has been attributed by formation of more effective Schottky-type potential barrier. The correlations between varistor properties and other microstructural aspects were also discussed.

Sensitivity of metal oxide gas sensors to non-parabolic intergranular barriers

We analyze the electrical conductivity of polycrystalline semiconductor solids due to the presence of Schottky-type potential barriers formed at intergrains. The density of charged dopants along the grains is usually considered constant leading to parabolic intergranular potential barriers. If temperature is high enough to allow sufficiently mobility to the dopants, their resulting equilibrium distribution is far from constant leading to potential barriers that show a strong non-parabolic character. Implications for the electrical conductivity and then on the sensitivity to barrier height and sample doping in metal oxide gas sensors are discussed.

Evidence of the pressure-induced conductivity switching of yttrium-doped SrTiO3

The electrical transport properties of undoped and yttrium-doped strontium titanate (Sr(Ti1 – xYx)O3 – δ, x = 0, 0.02) under high pressure were investigated with in situ impedance spectroscopy measurements. A pressure-induced conductivity switching for undoped and 2 mole% Y-doped strontium titanate is observed at around ~10.0 and 7.0 GPa respectively, which are caused by a cubic to tetragonal I4/mcm phase transition. The decrease of the phase transition point of 2 mole% Y-doped strontium titanate can be attributed to larger Y3+ atoms occupying the B-site and the creation of more oxygen vacancies, which lead to octahedra tilting and symmetry breaking. The results of the voltage-bias dependence of grain-boundary impedance of undoped and 2 mole% Y-doped strontium titanate at different pressures revealed that Schottky-type potential barriers formed at grain boundaries are the key factor for the accumulation of oxygen vacancy at the interface under pressure.

Grain size effect on the giant dielectric and nonlinear electrical behaviors of Bi1/2Na1/2Cu3Ti4O12 ceramics

Single phase Bi1/2Na1/2Cu3Ti4O12 (BNCTO) ceramics with different grain sizes (1.4–4.3 μm) are prepared by a modified Pechini method to investigate their giant dielectric and nonlinear electrical behaviors. The results show that the giant dielectric and nonlinear electrical behaviors are strongly dependent on grain size. With the increment of grain size, the dielectric constant increases monotonically from 14110 (for 1.4 μm sample) to 36183 (for 4.3 μm sample) at 1 kHz, in accompaniment with the breakdown voltage reducing from 112.5 to 43.2 V/mm and the nonlinear coefficient reducing from 4.9 to 3.4. On the basis of the internal barrier layer capacitor (IBLC) model and the IBLC model of Schottky-type potential barrier, an interpretation of the grain size effect on the giant dielectric and nonlinear electrical behaviors is presented.

Current–voltage characteristics of grain boundaries in polycrystalline Sr-doped LaGaO3

The oxide-ion current across grain boundaries in a polycrystalline LaGaO(3) ceramic doped with 1 mol% Sr(2+) increases non-linearly with the applied dc-bias to present a transition from ohmic to superohmic where the bias exceeds the thermal voltage, verifying that a Schottky-type potential barrier exists at the grain boundary in acceptor-doped LaGaO(3) to limit the internal current.

Direct evidence of potential barriers at grain boundaries in Y-doped BaZrO3 from dc-bias dependence measurements

Y-doped BaZrO3 (BZY) is an outstanding proton conductor under wet atmosphere at relatively low temperatures (<600 °C). Dense polycrystalline ceramics of this material used in intermediate-temperature solid oxide fuel cells as solid electrolytes, however, suffer from uniquely high resistance across grain boundaries in the materials. It has long been speculated that such high resistance at the grain boundaries is originated from potential barriers which may be formed at the grain boundaries. Here we provide direct experimental evidence that Schottky-type potential barriers indeed exist at grain boundaries in BZY. The results of our study on dc-bias dependence of grain-boundary impedance in 5, 3 and 2 mol% Y-doped BaZrO3 (BZY5, BZY3, and BZY2, respectively) clearly show that the relation between the current across a grain boundary and the applied dc-bias is ohmic where the dc-bias is lower than the thermal voltage, while that becomes superohmic at sufficiently high biases. In addition, it is found that the capacitance of the grain boundary decreases with the bias. These observations are consistent with what is expected from the thermionic emission mechanism for a current across a potential barrier. Hence we unambiguously verified that Schottky-type potential barriers are formed at grain boundaries in BZY to serve as extra internal resistors, and are responsible for high grain-boundary resistance observed in polycrystalline BZY ceramics. The barrier heights estimated for BZY5, BZY3, and BZY2 are 0.51, 0.85, and 0.95 V, respectively.

Visualization of bias-dependent potential barriers using scanning gate microscopy in copper-phthalocyanine field-effect transistor

Potential barriers and their contribution have been visualized in an organic field-effect transistor composed of copper-phthalocyanine (CuPc) thin film via scanning gate microscopy (SGM). The SGM response shows a peak when a biased tip situates on both edges of the Au electrode at the lower source-drain voltages. It indicates that the electric field from the tip modulates the Schottky-type potential barrier at the CuPc∕Au interface and the barriers strongly restrict the carrier (hole) injection and/or emission at the CuPc channel. On the other hand, a significant peak appears only at the source (hole-injection) side at higher bias voltages. The difference indicates that the contribution of the barrier to the transport changes with the bias condition. The electrostatic force microscopy response, which is simultaneously obtained with the SGM image, supports these considerations. Moreover, it is confirmed that the peak height is related to the distribution of current injection into the channel.

Dopant-concentration dependence of grain-boundary conductivity in ceria: A space-charge analysis

Here we elucidate the origin of the characteristic development of grain-boundary conductivity and of its activation energy measured in Gd-doped ceria (GDC) ceramics upon change in the dopant concentration in the context of the space-charge effect. The grain-boundary conductivity in GDC was measured as a function of the dopant concentration (1–20 cat.%). The total conductivity of the sample with 1 cat.% dopant is lower than the bulk conductivity by several orders of magnitude largely due to exceedingly high grain-boundary resistance in the sample, consistent with the results previously reported in literature. The specific grain-boundary conductivity of GDC however rapidly increases with increasing dopant content to approach a plateau at concentrations above 15 cat.%, leading to the total conductivity being close to its bulk conductivity at the relatively high dopant level. On the other hand, the grain-boundary width estimated experimentally in GDC is found to decrease substantially as the dopant concentration increases, implying that the observed increase in the grain-boundary conductivity is correlated with such reduction in the grain-boundary width. In addition, the values of the grain-boundary width determined at each dopant concentration are in quantitative agreement with those calculated based on a space-charge model, in which the grain-boundary resistance is attributed to the Schottky-type potential barrier present at the grain boundaries. Furthermore, the values of activation energy of the grain-boundary conductivity measured at different dopant concentrations can also be reproduced quantitatively using the space-charge model. It is therefore verifed unambiguously that the characteristic increase in the grain-boundary conductivity measured in GDC upon increasing dopant concentration results from concurrent reduction in the height of the potential barrier formed inherently at the grain boundaries.

Two mechanisms of conduction in polycrystalline SnO 2

Impedance spectroscopy was carried out on SnO 2 thick films exposed to different atmospheres. Oxygen desorption and adsorption during heating and cooling processes, respectively, onto the grain surface affects the resistance and electrical capacitance of the sensors. Double parabolic Schottky-type potential barriers at grain boundaries were assumed to analyze the tunneling and thermionic contributions to conductivity. In addition, the influence of the temperature on the oxygen diffusion into the grains annihilating oxygen vacancies was studied. It was deduced that in-diffusion substantially affects the conductivity.

Bias-driven local density of states alterations and transport in ballistic molecular devices

We study dynamic nonequilibrium electron charging phenomena in ballistic molecular devices at room temperature that compromise their response to bias and whose nature is evidently distinguishable from static Schottky-type potential barriers. Using various metallic/semiconducting carbon nanotubes and alkane dithiol molecules as active parts of a molecular bridge, we perform self-consistent quantum transport calculations under the nonequilibrium Green's function formalism coupled to a three-dimensional Poisson solver for a mutual description of chemistry and electrostatics. Our results sketch a particular tracking relationship between the device's local density of states and the contact electrochemical potentials that can effectively condition the conduction process by altering the electronic structure of the molecular system. Such change is unassociated to electronic/phononic scattering effects while its extent is highly correlated to the conducting character of the system, giving rise to an increase of the intrinsic resistance of molecules with a semiconducting character and a symmetric mass-center disposition.

Separation of dielectric and space charge polarizations in CaCu3Ti4O12∕CaTiO3 composite polycrystalline systems

The complex analysis of dielectric/capacitance is a very useful approach to separate different polarization contributions existing in polycrystalline ceramics. In this letter, the authors use this type of spectroscopic analysis to separate the bulk’s dielectric dipolar relaxation contributions from the polarization contribution due to space charge in the grain boundaries of a CaCu3Ti4O12∕CaTiO3 polycrystalline composite system. The bulk dielectric dipolar relaxation was attributed to the self-intertwined domain structures from the CaCu3Ti4O12 phase coupled to the dipole relaxation from the CaTiO3 phase, while the space charge relaxation was attributed to the Schottky-type potential barrier responsible for the highly non-Ohmic properties observed in this composite polycrystalline system.

Magnetic properties of neodymium-iron-boron sintered from granulated powders: Y.Kishimoto, O.Yamashita. (Sumitomo Special Metals Co Ltd, Osaka, Japan.) J. Jpn Soc. Powder Powder Metall., vol 44, no 10, 1997, 951–957. (In Japanese.)

The effect of Zn doping content in SnO2-based varistors was investigated. The general formula was ((Sn0.985Co0.01Nb0.005Oβ)1−x(ZnO)x) ; x = 0, 0.005, 0.01, 0.02 and 0.04. The powders synthesized by chemical precipitation were 4.4 – 6.1 nm in crystallite size along both [110] and [101]. Only cassiterite (SnO2) phase was observed within the limit of detection of X-ray diffraction (XRD). Zn doping yielded great enhancement in sinterability. High densities were achieved in the x = 0.04 sample at 1150 °C whereas it took 100 °C higher to achieve the similar density for the undoped sample. Substitution of Zn cations into the Sn lattice was believed to generate oxygen vacancies, facilitating diffusion and mass transport. As a result, the average grain sizes initially increased at x = 0.005 and gradually decreased at higher Zn content possibly due to existence of secondary phases. The optimal composition with the greatest breakdown voltage (6384 V cm−1) and non-linear coefficient (17.2) occurred at x = 0.02. This improvement in varistor properties has been attributed by formation of more effective Schottky-type potential barrier. The correlations between varistor properties and other microstructural aspects were also discussed.

High electric field conduction mechanisms in electrode poling of electro-optic polymers

We investigated electrical conduction phenomena, which occurred during high electric field electrode poling of side chain χ(2) polymers. Electric current and second harmonic intensity were measured simultaneously in poling experiments performed on samples with and without an additional inorganic poly–methyl–siloxane layer. Current densities appeared to be limited by a Schottky-type potential barrier at the electrode/insulator interface. The field dependence of the current density was found to be Schottky charge emission for medium field strengths (EPOL≤100 V/μm) whereas it was dominated by Fowler–Nordheim tunneling at higher poling fields. In the presence of the siloxane layer, a significant suppression of tunneling current was observed. This leads to a reduced probability of singular dielectric breakdown events and shifts the limit of avalanche breakdown to higher internal effective poling field strengths.