Schottky-type Barrier Research Articles

Electrical properties of rutile-type FeTiMO6 (M = Ta,Nb)

Various electrical properties of polycrystalline rutile-type compounds FeTiTaO6 and FeTiNbO6 were studied between room temperature and ∼750 K. The main purpose of this investigation was to analyse to what degree the giant relaxor-type dielectric constant ϵ(ω), with broad and dispersive maxima in ϵ(ω), suggested to be dictated by the presence of polar nanodomains due to the variety of heterovalent cations, is affected by extrinsic properties such as grain boundary and sample-electrode interfacial effects. For FeTiNbO6, the voltage dependence of the magnitude of capacitance Cp(ω) in the whole frequency range, pointing to Schottky-type barriers, is strong indication of a considerable influence by electrode contact effects. For FeTiTaO6, the dependence of Cp(ω) on the preparation conditions, including the cooling rate, points to the effect of microstructural properties, likely in part by oxygen deficiency leading to inhomogeneities at grain boundaries and resulting in high-capacitive layers. Values of DC conductivity σDC could be determined for the bulk, grain boundaries and electrode contacts in certain temperature ranges. The bulk contribution of FeTiTaO6 to σDC is characterized by activation energy EA = 0.35 eV and σDC(295 K) ∼ 3 × 10 − 6 Ω − 1cm − 1 and for FeTiNbO6 by EA = 0.31 eV and σDC(295 K) ∼ 7 × 10 − 5 Ω − 1cm − 1. Grain boundaries and electrode contacts are related to much lower σDC and higher EA values, i.e. they exhibit higher resistivities by interlayer effects. The characteristic features of the frequency dependence of AC conductivity of both oxides are marked by grain boundary and electrode effects. Relaxation processes were established from loss data, being likely due to these effects. The thermopower is negative showing weak variation with temperature and pointing to a charge transfer polaron-hopping mechanism in the bulk of both compounds. The Mossbauer spectrum of FeTiNbO6 shows besides the Fe3 + component a trace of Fe2 + .

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.

Reduction of the PtGe/Ge Electron Schottky-Barrier Height by Rapid Thermal Diffusion of Phosphorous Dopants

The electron Schottky-barrier height (SBH) of platinum germanide diodes on germanium (PtGe/Ge) is tuned by means of incorporation of phosphorous dopants from a spin-on-dopant resist. Thereby a highly doped surface layer on Ge substrates is formed in a rapid thermal diffusion process before the formation of PtGe. Applying a diffusion process of the P atoms at temperatures above , an ohmic contact behavior is found for the originally Schottky-type barrier diodes and evidence is given for a lowered electron SBH. The contacts exhibit barrier heights to electrons as low as . In contrast, increased barrier heights for holes of are found. The results of the electrical characterization are further supported by time of flight secondary-ion mass spectrometry measurements where a pileup of P dopants at the PtGe to Ge interface region is observed. In summary, a damage-free process scheme for the reduction in the electron SBHs in PtGe/Ge diodes is given. The presented approach can pave the way to ohmic-type n-contacts to germanium and is applicable to planar as well as three-dimensional device structures.

Dielectric properties of thin ferroelectric Sn2P2S6 films deposited by thermal evaporation

The structure, microstructure, and temperature and field dependences of the dielectric properties of thin (0.5–8.0µm) Sn2P2S6 ferroelectric films deposited onto glass and aluminum foil substrates by thermal vacuum evaporation in a quasi-closed volume are studied. The film-thickness and frequency dependences of the dielectric properties and the unipolarity of the C–V characteristics are explained by the presence of near-surface Schottky-type barrier layers.

Distribution of electric fields in ZnS:Mn single crystals during electroluminescence

Brightness-voltage characteristics of the ZnS:Mn single crystals and their dependence on the frequency and strength of the electric field are studied. A model of the mechanism of excitation of electroluminescence in these crystals on the basis of excitation of the electroluminescence centers by electrons accelerated in the electric fields of the Schottky-type barriers is suggested. Such barriers are formed by dislocations at the sites of interrupting the stacking faults by them. The experimental and calculated data are in good agreement.

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.

Optical and nonlinear electrical properties of SnO 2–polyaniline nanocomposites

Nanostructured tin dioxide (SnO 2) is synthesized in the form of colloidal sol. Aniline monomer is polymerized in colloidal sol of SnO 2 to prepare inorganic–organic hybrid nanocomposites. Optical band gap increases from 3.74 eV to 4.23 eV with increase of polyaniline concentration. The observed nonlinear current–voltage characteristics are satisfactorily explained using the Schottky type barriers. The temperature dependence of conductivity reveals three dimensional Mott's hopping process.

Nonlinear Electrical Properties of (Sc, Ta) Doped TiO&lt;sub&gt;2&lt;/sub&gt; Varistor Ceramics

The nonlinear electrical properties of TiO2-based varistor doped with 0.25mol% Ta2O5 and different contents of Sc2O3 were investigated. It was found that the TiO2 varistor ceramic doped with 0.10mol% Sc2O3 exhibited an optimal nonlinear coefficient of 7.8, a breakdown electrical field of 16.0V/mm, and relative dielectric constant of 1.27 × 105 (measured at 1 kHz). In order to analyze the effect of Sc2O3 on TiO2 varistors, studies were made on the capacitance versus voltage characteristics. A Schottky-type barrier, which is assumed as the origin of varistor behavior, was inferred from the C-V measurement. The barrier height and donor concentration were obtained as 0.41eV and 1.21 × 1026cm-3, respectively, for sample doped with 0.10mol% Sc2O3. Analogized to the ZnO varistors, the formation mechanism of Schottky-type barrier was discussed in this paper by the theory of defect in crystal lattice.

Optical and diode like current–voltage characteristics of SnO2–polypyrrole nanocomposites

Nanocomposites based on conducting polypyrrole (PPY) and inorganic SnO2 have been synthesized by the soft chemical route. Absorption spectra of PPY–SnO2 nanocomposites show peaks at wavelengths of 310–340 nm corresponding to nanosized SnO2 and two broad bands present at around 450–475 nm and in the range 600–900 nm are due to polypyrrole. The absorption band at lower wavelength indicates a blue shift of about 30 nm with respect to bulk SnO2. Direct current conductivity of nanocomposites as a function of temperature for different concentrations of polypyrrole have been interpreted by three-dimensional Mott's variable range hopping process. The observed diode like current–voltage characteristics are satisfactorily explained using the Schottky type barriers.

Non-Ohmic and dielectric properties of a Ca2Cu2Ti4O12 polycrystalline system

An investigation was made into the non-Ohmic and dielectric properties of a Ca2Cu2Ti4O12 perovskite-type system. Compared to the traditional CaCu3Ti4O12-based composition, the imbalance between the Ca and Cu atoms caused the formation of a polycrystalline system presenting ∼33.3mol% of CaCu3Ti4O12 (traditional composition) and ∼66.7mol% of CaTiO3. As for non-Ohmic properties, the effect of this Ca and Cu atom imbalance was that a nonlinear electric behavior of ∼1500 was obtained. This high nonlinear electrical behavior emerged in detriment to the ultrahigh dielectric property frequently reported. The high non-Ohmic property was explained by the existence of Schottky-type barriers, whose formation mechanism may be similar to that proposed for traditional metal oxide non-Ohmic devices, according to similarities discussed herein.

Dielectric spectroscopy analysis of CaCu3Ti4O12 polycrystalline systems

Dielectric spectroscopy was used in this study to examine CaCu3Ti4O12 polycrystalline samples. The analysis involved systems presenting low non-Ohmic properties, and the grain’s internal domain was evaluated separately from the contribution of barrier-layer capacitances associated with Schottky-type barriers in this type of material. The effect of oxygen-rich atmosphere and high cooling rate was evaluated, revealing a strong increase in the dielectric properties of the CaCu3Ti4O12 system under these conditions. This effect was attributed to a chemical change in the grain’s internal domain, which may be considered an internal barrier-layer capacitance of the polycrystalline material.

Supported oxides as combustion catalysts and as humidity sensors. Tuning the surface behavior by inter-phase charge transfer

The changes of the electrical properties of semiconductor oxide catalysts, when measured in situ provide useful information about surface processes as adsorption/desorption/reaction occur with charge transfer. The electrical properties of polycrystalline powders, mainly controlled by inter-grain Schottky-type barriers, are sensible to the “quality” of the surface/inter-grain domains, to sample morphology and to changes in charge and coverage of the surface due to operating conditions. This can be exploited to study oxide catalysts dispersed on supports at nanoscale levels for answering questions like: (i) adsorption ability of the surface layer and (ii) the influence of the active phase and of the support. It is demonstrated that for low loading supported SnO 2 or V 2O 5 catalysts (also important as sensor materials), the conductivity plots show the “fingertip” of the support, which controls the behavior of the supported phase. As an example, the lower propylene combustion activity of SnO 2 when dispersed on γ-Al 2O 3 is explained by hydrophile/proton conductor properties of the support; γ-Al 2O 3 diminishes the oxygen adsorption ability of SnO 2 by providing a higher concentration of molecularly adsorbed water and surface OH groups towards its surface. It is proposed that by selecting appropriate n-type semiconductor oxide supports (as TiO 2), and by increasing the surface conductivity one can increase the oxygen adsorption ability of the supported phase, with increased activity for combustion/chemical sensing. This suggests the possibility of tuning the surface properties of the supported oxide by inter-phase charge (electron or ion) transfer.

The Nature of Contact between Pd Leads and Semiconducting Carbon Nanotubes

The contact between semiconducting single-wall carbon nanotubes (SWCNTs) and metallic leads is of central importance to potential electronic device applications. We investigate the nature of the contact of SWCNTs with Pd leads in a fully covered geometry that closely resembles experimental setups. We employ first-principles calculations within density functional theory to obtain the equilibrium structure for representative semiconducting SWCNTs embedded in Pd and analyze their electronic structure features, charge-transfer effects, electrostatic potentials, and Fermi level alignment at the interfaces with the metal contact. We find that there is no electrostatic or Schottky-type barrier to electron transfer between the metal and the nanotube.

Control mechanisms for transport and nonlinear optical response in organic materials: a tale of twists and barriers

Simple electronic structure models are used to address two significant challenges in organic materials chemistry, the design of chromophores for strong electro-optic response (and low-energy optical absorption), and the prediction of relative mobilities and charge injection barriers for conductive oligomers. For electro-optic response, we examine two chromophore classes where twisting around an inter-ring bond can tune the electronic structure from aromatic (zwitterionic) to quinoid (neutral). The calculated nonlinear response develops a very strong maximum (βμ∼1500×10−30 esu) at twist angles near 80°. For the transport behavior, structure/function correlations are presented for three series of oligomers, based on calculations of bandwidths (as functions of geometry) and of reorganization energies. Transport type appears to be fixed less by these mobility factors than by the injection barriers. The simplest estimates for these Schottky-type barriers, using frontier orbital energies from density functional calculations, predict carrier n-type or p-type behavior remarkably well.