Defect Chemistry Model Research Articles

Influence of temperature and interface charge on the grain-boundary conductivity in acceptor-doped SrTiO3 ceramics

The temperature dependence of both the grain-boundary potential barrier height and the conductivity across the grain-boundary space-charge depletion layer in acceptor-doped SrTiO3 ceramics has been investigated by a numerical simulation technique. The underlying model is that of a back-to-back double Schottky barrier at the grain boundary. The influence of the amount of positively charged donorlike grain-boundary interface states on the charge transport behavior of the grain-boundary region is also discussed. An interpretation in terms of a defect chemistry model of the bulk and the space-charge depletion layer, which on both sides surrounds the grain-boundary core is presented. The temperature behavior of the potential barrier at the grain boundary can be divided into three different regimes: a linear regime, and subsequently, saturation and decreasing regimes. The theoretical explanation for this behavior is given. Two different spatial conductivity profiles at the grain boundaries have to be considered, which clearly can be identified by two different characteristic thermal activation energies of the effective grain-boundary conductivity. The barrier height itself is not equal to the the thermal activation energy of the effective grain-boundary conductivity. The electrical characteristics of the grain boundaries can be influenced deliberately by decorating the grain boundaries with suitable dopants.

Simulation of Electrical Properties of Grain Boundaries in Titanate Ceramics

AbstractUsing a numerical simulation method, the electrostatic properties of grain boundaries in titanate ceramics (SrTiO3 and BaTiO3) are calculated by the combination of a defect chemistry model and the general potential equation. Based on these results and a simple three‐dimensional “brick‐wall‐model” of the ceramic microstructure, an equivalent network is developed for the ceramic body which consists of a great number of R C‐branches. Using this network it is possible to calculate the frequency dependence of the complex impedance of the ceramic. The results of the impedance simulation can be compared to experimental results. This comparison yields information about the charge transport across and along the grain boundaries and about the influence of the temperature, the bulk dopant concentrations, and the donor‐ or acceptor‐like grain boundary interface states on the electrical properties of the grain boundaries. By using the simulation technique one gets additional information to interpret and understand the experimental results and to model the physical and electrical behaviour of the grain boundaries which mainly determine the electrical characteristics of the ceramic.

Grain‐Boundary Defect Chemistry of Acceptor‐Doped Titanates: Inversion Layer and Low‐Field Conduction

The electronic and ionic conduction across grain‐boundary (GB) space‐charge depletion layers in acceptor‐doped SrTiO3 ceramics have been investigated by admittance analysis in the time domain. The dependence on the acceptor concentration, the oxygen partial pressure during equilibration, and the temperature dependence of the GB conductivity are discussed and interpreted in terms of a defect‐chemistry model of the GB region using a numerical simulation technique.

Protonic conduction in La 2Zr 2O 7-based pyrochlore materials

Electrochemical pumping of oxygen can be used to adjust the oxygen partial pressure without changing the total content of hydrogen containing species (H 2+H 2O), and this is suitable for studying materials with proton conductivity selectively dependent on water vapour partial pressure. This method was used to demonstrate that lanthanum zirconate based materials possess a significant protonic conductivity contribution at temperatures lower than about 1000°C, and to revise the defect chemistry model. Y for Zr substitution enhances the electron hole, oxygen ion and protonic conductivity contributions, as expected for addition of an acceptor species. The effective mobility of oxygen vacancies, and the equilibrium constant for formation of anti-Frenkel defects were evaluated by taking into account the oxygen ion conductivities in Y-containing and undoped lanthanum zirconate.

Defects and transport of the brownmillerite oxides with high oxygen ion conductivity — Ba 2In 2O 5

A defect chemistry model is proposed for the brownmillerite oxides with high oxygen ion conductivity. Ba 2In 2O 5 was chosen as the model material and its electrical conductivity and transport properties have been studied in detail. The oxygen ion conduction above the order-disorder temperature, T d ≈ 925 °C, and the mixed ionic-electronic conduction below T d , was studied by conductivity and EMF measurements as a function of temperature and oxygen activity. The main defects are intrinsic anion Frenkel defects below T d , and above T d the oxide can be treated as acceptor-doped perovskite with extrinsic oxygen vacancies. Charge compensation involves only ionic defects over the whole P O 2 range used in this study. The formation and mobility enthalpies of the Frenkel defects, the redox enthalpies, and the band gap have been obtained for this oxide. The proposed model is in good agreement with the experimental results. Decomposition of Ba 2In 2O 5 with reduction of In 3+ is indicated by the conductivity measurements. Degradation in CO 2 atmospheres with formation of BaCO 3 and volatilization of In 2O 3 was also observed by XRD and EDS techniques. It is clear that In 3+ will have to be replaced by an element with a more stable 3 + oxidation state (e.g. Y, Er, Ga), if a brownmillerite compound is to be a useful solid electrolyte over a wide range of oxygen activities.

Electrical characteristics of MOCVD deposited Ba1−xSrxTiO3/ YBa2Cu3O7-xON LaAlO3substrate

Abstract High dielectric constant material Ba0·6Sr0·4TiO3 (BST) was heteroepitaxially grown over YBa2Cu3O7-x (YBCO) on Lanthanum aluminate (LaAlO3) substrates. The electrical characteristics of the BST film was studied by depositing electron beam evaporated platinum (Pt). The capacitancevoltage (C-V) measurements at various temperatures up to 100°C show negative temperature coefficient for capacitance, confirming the paraelectric phase. The capacitance vs voltage and current vs voltage characteristics show asymmetry with the polarity of the applied voltage. The asymmetry in the capacitance and current-voltage characteristics can be explained by defect chemistry model of electrode/oxide interface.

Defect chemistry model of the ferroelectricelectrode interface

Abstract The interactions between the electrodes and titanate ceramic materials are examined using numerical defect chemistry models. A time independent model solves for the steady-state concentrations of electrons and holes, ionized acceptors, and oxygen vacancy concentrations throughout the ferroelectric film as a function of electrical potential, temperature, and oxygen partial pressure. These calculations reveal a basic mechanism by which electrically active space charge regions may form in the ferroelectric material. The combination of atmospheric oxygen partial pressure with ionization energies of naturally occurring acceptor impurities of around one electron-volt prevents full ionization of the acceptors. However, the acceptors respond strongly to an electric field, and the percentage of ionized acceptors in the interface region may range from 0% to 100% with the application of a few tenths of a volt. The response of the partially ionized impurities to the electric field is the proposed mechanism for the space charge activity in thin-film devices. These models also show that the electrochemical potential induced by the contacts can produce a substantial increase of the oxygen vacancy concentration at the metdceramic interface. A negative contact potential reduces the ceramic, lowering the oxygen concentration in the interface region. The reduction is increased by an externally applied negative bias, which lowers the oxygen concentration at the interface even more. The calculations confirm previous models that treat the space charge layer as a depleted n-type semiconducting region. These results give hrther support to the Schottky barrier models that have been proposed to explain the capacitance vs. voltage and current vs. voltage characteristics of ferroelectric capacitors.

Defect Chemistry of CuInSe2

The defect chemistry model is developed as a function of the nonmolecularity (ΔX) and nonstoichiometry (ΔY), and the formation energies of defects are also considered. In this model, the p-type conductivity can occur in the region with ΔX<0 and ΔY<0 and n-type conductivity can occur in the region with ΔX>0 and ΔY>0. For above given regions, it could be possible to define a critical relation as ΔXc=8ΔY/(1-2ΔY). The p-type conductivity occurs in the ΔX (more negative value)<ΔXc region at a constant ΔY value. As the decreasing of ΔX negative value, the type conversion appears near the ΔX equals to ΔXc region, and then it was dominated by n-type conductivity.

Conductive and transparent Al — doped ZnO thin films prepared by rf magnetron sputtering

Highly conductive ZnO thin films with Al 2O 3 contents of 0–10 atomic% have been prepared by rf magnetron sputtering. The resistivity of the as — deposited films varies from 3.2 × 10 4 to 1.1 × 10 −2 Ω-cm dependent on different experimental conditions. The minimum transmittance in the visible region (450–800 nm) is above 80% for all films, while a 92% transmission is observed in the sample with a thickness of 400 nm. Transmittance of the sample with a resistivity of 1.1× 10 −2 Ω-cm is above 86 % in the visible region. All films have a strongly preferred orientation of the c axis perpendicular to the target surface. A defect chemistry model is proposed to investigate the effect of Al 2O 3 dopants in the ZnO film. Doping of Al 2O 3 up to several at% forms a substitutional solid solution with excess electrons dominating the electrical condition. The introduction of more Al 2O 3 results in the formation of the oxygen interstitial, which leads to an expansion in the lattice and peak shift in the X — ray diffraction profile.

Relation between electrical properties and composition in CuInSe2 single crystals

Abstract The electrical properties and the elemental composition of as-grown CuInSe2 single crystals are measured and compared with the predictions of an intrinsic defect chemistry model. It is found that the validity range of the intrinsic defect model is limited to small deviations from the ideal stoichiometry of the compound. The critical values for the deviations from molecularity and valence stoichiometry are given. Preliminary results are reported on changes of composition and electrical parameters during annealing.