Electrical Properties Of Grain Boundaries Research Articles

Improving dielectric properties of (Zn2+, Al3+) Co-doped CaCu3Ti4O12 perovskite ceramics by enhancing the grain boundary response

CaCu3Ti4O12 and CaCu2.95Zn0.05Ti4-xAlxO12 (x = 0.025, 0.05, and 0.10) ceramics were created using a conventional solid-state reaction method. A single phase of CaCu3Ti4O12 was present in all ceramics. Co-doping with Zn2+ and Al3+ resulted in a significantly increased grain size. According to the DFT results, both Zn and Al atoms preferentially occupy Cu sites, resulting in LPS processes caused by excess Cu concentrations at the grain boundary. CaCu2.95Zn0.05Ti4-xAlxO12 ceramics with dielectric permittivities of more than 7.00 × 104 and dielectric loss tangents less than 0.04 have been produced. It was also discovered that the temperature stability of dielectric permittivity could be boosted. Improved electrical properties of grain boundaries, particularly grain boundary resistance, in co-doped ceramics caused by the influence of metastable phases such as Cu-related phases and oxygen enrichment at the grain boundaries, might well be related to improved dielectric properties. The Cu-related phases and the structure without oxygen vacancy at the grain boundary can act as a stable insulative layer, preventing long-range charge migration. From our electron density calculations, the Zn and Al ions become more positive, whereas the O ions are more negative. Moreover, Cu+ and Ti3+ could be generated from the VO++ in the Zn05Al10 structure. Based on experimental and theoretical data, an internal barrier layer capacitor structure in these ceramics could account for the giant dielectric characteristics of these materials.

Direct prediction of electrical properties of grain boundaries from photoluminescence profiles using machine learning

We present a machine learning model to directly predict the carrier recombination velocity, vGB, at the grain boundary (GB) from the measured photoluminescence (PL) intensity profile by training it with numerical simulation results. As the training dataset, 1800 PL profiles were calculated with a combination of random values of four material properties—vGB, the GB inclination angle, and the carrier diffusion lengths in the grains on both sides of the GB. In addition, the measured noise was modeled artificially and applied to the simulated profiles. A neural network was constructed with the inputs of the PL profile and the outputs of the four properties. This served as the solver of the reverse problem of the computational simulation. The coefficient of determination and the root mean squared error of vlog, which is the common logarithm of vGB, for the test dataset were 0.97 and 0.245, respectively. This prediction error was sufficiently low for the practical estimation of vGB. Moreover, the calculation time was reduced by a factor of 198 000 compared to conventional numerical optimization of repeating the computational simulations. By utilizing this fast prediction method, continuous evaluation of vGB along a GB was demonstrated. The finding is expected to advance scientific investigation of the electrical properties of local defects.

Effects of Charge Compensation on Colossal Permittivity and Electrical Properties of Grain Boundary of CaCu3Ti4O12 Ceramics Substituted by Al3+ and Ta5+/Nb5.

The effects of charge compensation on dielectric and electrical properties of CaCu3Ti4-x(Al1/2Ta1/4Nb1/4)xO12 ceramics (x = 0−0.05) prepared by a solid-state reaction method were studied based on the configuration of defect dipoles. A single phase of CaCu3Ti4O12 was observed in all ceramics with a slight change in lattice parameters. The mean grain size of CaCu3Ti4-x(Al1/2Ta1/4Nb1/4)xO12 ceramics was slightly smaller than that of the undoped ceramic. The dielectric loss tangent can be reduced by a factor of 13 (tanδ ~0.017), while the dielectric permittivity was higher than 104 over a wide frequency range. Impedance spectroscopy showed that the significant decrease in tanδ was attributed to the highly increased resistance of the grain boundary by two orders of magnitude. The DFT calculation showed that the preferential sites of Al and Nb/Ta were closed together in the Ti sites, forming self-charge compensation, and resulting in the enhanced potential barrier height at the grain boundary. Therefore, the improved dielectric properties of CaCu3Ti4-x(Al1/2Ta1/4Nb1/4)xO12 ceramics associated with the enhanced electrical properties of grain boundaries. In addition, the non-Ohmic properties were also improved. Characterization of the grain boundaries under a DC bias showed the reduction of potential barrier height at the grain boundary. The overall results indicated that the origin of the colossal dielectric properties was caused by the internal barrier layer capacitor structure, in which the Schottky barriers at the grain boundaries were formed.

A Critical Examination of the Mott–Schottky Model of Grain-Boundary Space-Charge Layers in Oxide-Ion Conductors

The electrical properties of grain boundaries in ionic conductors are studied most frequently and most easily by Electrochemical Impedance Spectroscopy (EIS). The resistance data obtained in this manner are typically analyzed with the Mott–Schottky space-charge model to extract a space-charge potential. In this study, taking CeO2 containing acceptor-dopant cations and oxygen vacancies as our model system, we calculate impedance spectra by solving the drift–diffusion equation for oxygen vacancies for a bicrystal geometry with space-charge layers at the grain boundary. Three different cases are considered for the behavior of the acceptor-dopant cations: a uniform distribution (Mott–Schottky), an equilibrium distribution (Gouy–Chapman), and a distribution frozen-in from a much higher temperature (restricted equilibrium). Analyzing our impedance data for the restricted-equilibrium case with the Mott–Schottky model, we find that the obtained space-charge potentials are substantially underestimated. In view of such a discrepancy not normally being apparent (the true values being unknown), we propose a specific set of EIS experiments that allow the Mott–Schottky model to be discounted.

Insight into impurity scavenging effect of gadolinium-doped ceria electrolytes

In this study, Ce0.89+xGd0.1−2xSrxEu0.01O2−δ (x = 0–0.04) ceramic electrolytes sintered at different temperatures were prepared and the impurity scavenging effect of Sr2+ dopants on their grain boundary (GB) electrical properties were investigated. The phase compositions, microstructures, and ionic conductivities of the samples were investigated using X-ray diffraction, scanning electron microscopy, and AC impedance spectroscopy, respectively. The incorporation of Sr2+ improved the overall GB conduction of the electrolytes by decreasing their GB area and space-charge potential. It hindered conduction to some extent by weakening the impurity dilution effect. The scavenging effect could be observed only when the sintering temperature was higher than the eutectic temperature of Si GB impurities. At low sintering temperatures, the Sr2+ dopants could not scavenge the GB impurities and increased the GB impurity coverage proportion. The findings of this study provide insights into the optimisation of the GB electrical properties of oxygen-ion conductors by selecting appropriate dopants and sintering temperatures to induce the GB impurity scavenging effect.

Origin of significantly enhanced dielectric response and nonlinear electrical behavior in Ni2+-doped CaCu3Ti4O12: Influence of DC bias on electrical properties of grain boundary and associated giant dielectric properties

CaCu3-xNixTi4O12 (x = 0, 0.05, and 0.10) powders were synthesized using a solid state reaction method. Phase structure and microstructure analyses revealed that all sintered CaCu3-xNixTi4O12 ceramics were of a pure phase. The CaCu3Ti4O12 ceramics had a dense microstructure and grain sizes were enlarged by doping with Ni2+. Interestingly, the dielectric permittivity was significantly enhanced, whereas the loss tangent was greatly suppressed to ∼0.046–0.034 at 1 kHz. All sintered ceramics exhibited non-Ohmic characteristics. Clarification of the influences of DC bias showed that the dielectric permittivity and loss tangent values were increased by DC bias. The resistance of grain boundaries and the associated conduction activation energy of CaCu3-xNixTi4O12 ceramics were reduced as the DC bias voltage increased. Therefore, the observed non-Ohmic behavior and significantly enhanced dielectric properties should be closely related to variation in the Schottky barriers at the grain boundaries.

New insights into the 6H-type hexagonal perovskite solid solution BaTiO3 − δ: Influence of acceptor and donor doping on crystal structure and electrical properties

Polycrystalline 6H-type hexagonal Barium Titanate BaTi0.5(Fe0.33M0.17)O3−δ, where M=Mo or W was prepared by the solid state reaction method. The effects of the cationic substitution of Fe3+, Mo6+ and W6+ for titanium in the B site for BaTiO3 perovskite lattice on the symmetry and the electrical properties were investigated. It was found that the substitution of Ti4+ by (Fe3+Mo6+) gives a hexagonal perovskite structure, whereas the substitution by (Fe3+W6+) cations results in a mixture of double cubic and hexagonal pervoskite phases. The difference in the average radius of cations in the B site (rB=0.50×rTi+0.33×rFe+0.17×rM, (M=Mo or W)) created the difference in cell parameters. The conductivity obtained for BaTi0.5(Fe0.33Mo0.17)O3-δ sample at a temperature of 200°C was 2.3×10−3Scm−1, which is much greater than that established by BaTi0.5(Fe0.33W0.17)O3-δ sample at a temperature of 328°C (0.26×10−3Scm−1) This means that a mixed ionic-electronic conductor (MIEC) character may be present in the first compound. Impedance Spectroscopy data collected for our samples over a range of temperatures and frequencies showed the appearance of a phenomenon of blocking at the grain boundary level, which means that the compounds are highly influenced by the oxygen stoichiometry. The correlation between the electrical properties of grain boundaries and their chemical composition is consistent with the interpretation in terms of the space charge model with a positive excess charge in the grain boundary core.

(Invited) Electrical Atomic Force Microscopy for 2D Transition Metal Dichalcogenide Materials

As Si-based electronic devices are approaching their projected scaling limits, layered two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs) are extensively studied as potential new channel materials and fundamental building blocks of emerging sensors and devices. In this context, MoS2 and WSe2 to name a few, are now available for deposition using different top down approaches. However, their outstanding properties are often degraded during the fabrication processes required for the device integration. For example, 2D TMDs selective growth, patterning and tuning of the electronics properties still remain elusive. Here, electrical atomic force microscopy (AFM) and beam analysis techniques are used to develop a framework of analysis for 2D materials. The latter is applied to understand the local properties of MoS2 comparing pristine material and structures which are selectively grown and patterned. Different growth techniques are investigated. After modelling the tip-sample contact system, we assess the impact of the plasma-induced damages combining the electrical AFMs and Auger emission spectroscopy. We study the local electrical properties of grain boundaries and their transport in patterned MoS2 structures for field-effect-transistor devices by conductive atomic force microscopy (C-AFM).

(Invited) Electrical Atomic Force Microscopy for 2D Transition Metal Dichalcogenide Materials

As Si-based electronic devices are approaching their projected scaling limits, layered two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs) are extensively studied as potential new channel materials and fundamental building blocks of emerging sensors and devices.[1-2] In this context, MoS2, WS2 and WSe2 to name a few, are now available for deposition trough different top down approaches. However, their outstanding properties are often degraded during the fabrication processes required for the device integration. For example, the selective growth of 2D TMDs their patterning and the electronics properties fine tuning still remain elusive. Here we report on electrical atomic force microcopy (AFM) and beam analysis techniques which are used to develop a framework of analysis for 2D materials. The latter is applied to understand the local properties of MoS2 comparing pristine material and structures which are selectively grown and patterned.[3] Different growth techniques are investigated. After modelling the tip-sample contact system, we assess the impact of the plasma-induced damages combining the electrical AFMs and Auger emission spectroscopy. We study the local electrical properties of grain boundaries and their transport respectively in pristine and patterned structures for FET devices by conductive atomic force microscopy (C-AFM). [1] G. Fiori, F. Bonaccorso, G. Iannaccone, T. Palacios, D. Neumaier, A. Seabaugh, S. K. Banerjee, and L. Colombo, “Electronics based on two-dimensional materials,” Nat. Nanotechnol., vol. 9, no. 10, pp. 768–779, 2014.[2] Desai, S. B., Madhvapathy, S. R., Sachid, A. B., Llinas, J. P., Wang, Q., Ahn, G. H., Javey, A. (2016). MoS 2 transistors with 1-nanometer gate lengths, 354(6308), 2–6.[3] Chiappe, D., Asselberghs, I., Sutar, S., Iacovo, S., Afanas’Ev, V., Stesmans, A., … Thean, A. (2016). Controlled Sulfurization Process for the Synthesis of Large Area MoS2 Films and MoS2/WS2 Heterostructures. Advanced Materials Interfaces, 3(4), 1–10.

Anchoring Molecules on the Surface of ZnO and Effects on Intergranular Transport Properties

Improving electrical conductivity in thin film semiconductor devices such as solar cells, thermoelectric modules, or transparent conducting devices, remains a key point for better performances. As polycrystalline films have the favor of the industry for their lower fabrication cost, the limitation of the current is most of the time attributed to charge scattering at the grain boundaries due to high energy barriers at the interface between grains.(1) This work focuses on the understanding of intergranular electrical conductivity in the view of improving thin film semiconductor performances deposited via a soft chemistry deposition routes. In this goal, ZnO:Al was chosen as a model doped semiconductor for our investigations as it is a reference material in many semiconductor applications (sensors, solar cells, transparent conducting oxides)(2). Many studies have already demonstrated the role of adsorbed species onto ZnO thin films and nanostructures surfaces to improve sensor selectivity or charge transfer in dye sensitized solar cells for instance (3). With this in mind, we have performed liquid phase adsorption studies of organic molecules onto ZnO particles through solution conductimetry and IR. A theoretical modeling of the energy level of anchoring molecules was performed in order to fit the LUMO level to the conduction band. Conjugated and thiophene based molecules with different substituents and lengths were selected. Conductivity on pressed powders was studied as a function of the applied pressure to show the effect of the grain boundaries and of the powder compaction. A conclusion was drown on the participation of these molecules and their LUMO levels in the electrical conductivity of ZnO:Al. This study allows quantitative determination adsorbates on ZnO surface and puts into perspective the investigation of better matches between LUMO and conduction band. Literature: 1. Greuter F, Blatter G. Electrical properties of grain boundaries in polycrystalline compound semiconductors. Semicond Sci Technol. 1990;5(2):111. 2. Klaus Ellmer, Andreas Klein, Bernd Rech. Transparent Conductive Zinc Oxide. Springer Series in Material Science. 2008. 3. Galoppini E. Linkers for anchoring sensitizers to semiconductor nanoparticles. Coord Chem Rev. 2004 juillet;248(13–14):1283–97.

A progressive route for tailoring electrical transport in MoS2

Typically, molybdenum disulfide (MoS2) synthesized by chemical vapor deposition (CVD) is polycrystalline; as a result, the scattering of charge carriers at grain boundaries can lead to performances lower than those observed in exfoliated single-crystal MoS2. Until now, the electrical properties of grain boundaries have been indirectly studied without accurate knowledge of their location. Here, we present a technique to measure the electrical behavior of individual grain boundaries in CVD-grown MoS2, imaged with the help of aligned liquid crystals. Unexpectedly, the electrical conductance decreased by three orders of magnitude for the grain boundaries with the lowest on/off ratio. Our study provides a useful technique to fabricate devices on a single-crystal area, using optimized growth conditions and device geometry. The photoresponse, studied within a MoS2 single grain, showed that the device responsivity was comparable with that of the exfoliated MoS2-based photodetectors.

Dielectric constant versus voltage and non-Ohmic characteristics of Bi2/3Cu3Ti4O12 ceramics prepared by different methods

Bi2/3Cu3Ti4O12 (BCTO) ceramics were successfully prepared by traditional solid-state reaction method (BCTO-SS) and sol–gel method (BCTO-SG). Pure perovskite phase and dense structure were obtained in BCTO ceramics prepared by both methods. BCTO-SG ceramics showed a large dielectric constant of ~1.1×104 while BCTO-SS ceramics exhibited a low dielectric constant of ~3200. At 100kHz, the dielectric constant of BCTO-SS ceramics decreased with applied voltage increasing, while the dielectric constant of BCTO-SG ceramics increased with applied voltage increasing. Further study of the relationship between dielectric constant and voltage suggested BCTO-SG ceramics had larger defect concentration than BCTO-SS ceramics. The investigation of complex impedance indicated that the electrical properties of grain boundaries for all BCTO ceramics were evidently affected by applied voltages and the electrical properties of grains were independent of applied voltages. In addition, the non-Ohmic properties of BCTO ceramics were studied in detail. The non-linear coefficients of BCTO-SS and BCTO-SG ceramics were 1.65 and 1.01, respectively. The breakdown electric fields of BCTO-SS and BCTO-SG ceramics were found to be 1.21 and 0.48kV/cm, respectively. The potential barrier heights of BCTO-SS and BCTO-SG ceramics were calculated to be 0.549 and 0.485eV, indicating that the potential barriers at the grain boundaries for BCTO-SS and BCTO-SG ceramics are the Schottky-type barrier.

Direct observation of electrical properties of grain boundaries in sputter-deposited CdTe using scan-probe microwave reflectivity based capacitance measurements

Polycrystalline CdTe in 12% efficient solar cells has been studied using scanning microwave impedance microscopy (sMIM). The CdS/CdTe junctions were grown on transparent-conducting-oxide-coated soda lime glass using rf sputter deposition. sMIM based capacitance measurements were performed on the exposed surface of CdCl2 treated CdTe adjacent to thermal-evaporation-deposited Cu/Au back contacts. The sMIM instrument was operated at ∼3 GHz, and capacitance measurements were performed as a function of ac and dc voltage biases applied to the tip, with and without sample illumination. Although dc capacitance measurements are affected by sample topography, the differential capacitance measurement was shown to be topography independent. It was found that the grain boundaries exhibit a depleted carrier concentration as compared to the grain bulk. This depletion effect is enhanced under photo-generated carrier separation or under sufficiently large probe tip biases opposite to the majority carrier charge.

Aligned Growth of Hexagonal Boron Nitride Monolayer on Germanium.

A hexagonal boron nitride monolayer with aligned orientations is grown on reusable semiconducting germanium. The number of primary orientations of the h-BN domains depends on the symmetry of the underlying crystal face, and Ge (110) gives rise to only two opposite orientations. The structures and electrical properties of grain boundaries between h-BN domains with opposite orientations are also systematically analyzed.

Electrically Active and Inactive Single Grain Boundaries in Semiconducting BaTiO3

Bi‐crystal specimens were prepared from Nb and Mn‐doped BaTiO3 poly‐crystals with giant grains of millimeter order in size, and the resistance (R) versus temperature (T) characteristics of these individual grain boundaries was investigated. The electrically active grain boundaries that show normal positive temperature coefficient resistor (PTCR) behavior had no second phases or they were partially distributed along boundaries. On the other hand, electrically inactive grain boundaries that show flat R‐T characteristics were also observed, where continuous Ti‐rich second phases of Ba4Ti12O27 could be detected. Different interface state density and its resultant R–T characteristics were observed for each individual active boundary, which indicates the degree of oxidation and the formation of potential barrier can be different depending on the character of the grain‐boundary plane. The resistance of inactive boundaries was determined by that of insulating second phase showing negative temperature coefficient resistor (NTCR) behavior. These results demonstrate that continuous second phase surrounding a grain deactivates the electrical properties of grain boundary, and thus should be distinguished from insulating depletion layer near grain boundary.

Reversibility of (Ag,Cu)(In,Ga)Se2 electrical properties with the addition and removal of Na: Role of grain boundaries

The Na content of (Ag,Cu)(In,Ga)Se2 films was cyclically adjusted using a novel method involving cycles of water rinsing at 60 °C followed by heating in air at 200 °C to remove Na and evaporation of NaF to re-introduce Na back into the film. The low temperatures and short heating times ensure that Na is removed only from grain boundaries while leaving grain interiors unaffected. Cross-grain conductivity and Seebeck coefficient were measured during this removal procedure and both measurements decreased when Na was removed and both recovered upon the re-addition of Na, consistent with an increase in compensating donor defects in the absence of Na. These results demonstrate that Na reversibly affects the electrical properties of grain boundaries. We propose that Na reversibly passivates donor-like defects such as InCu double donors at grain boundaries.

EFFECT OF THE BASE COMPOSITION AND MICROSTRUCTURE ON THE LEVEL OF ABSORPTION OF ELECTROMAGNETIC RADIATION IN NICKEL−ZINC FERRITE

Promising absorbing materials include Ni—Zn−ferrites, as they quite intensively absorb electromagnetic waves in the 50 MHz to 1000 MHz frequency range. In this paper we have studied the electromagnetic properties of Ni—Zn ferrite absorbing materials obtained in different technological modes. We propose a model that allows one to evaluate the dielectric constant of the ferrite material depending on the parameters of the microstructure and electrical properties of grain boundaries. Influence of base composition and microstructure on the level of absorption of electromagnetic radiation by Ni—Zn ferrite absorbing materials has been found. An increase in Fe₂O₃ excess to 51 % has been found to shift the frequency interval of electromagnetic radiation absorption towards lower frequencies, and this effect can be explained by an increase in the dielectric and magnetic constants of ferrite. Introduction of excess Fe₂O₃ in step 2 of grinding proved to be more efficient. An increase in the sintering temperature to 1350 °C also provides for a shift of electromagnetic radiation absorption frequency interval towards lower frequencies, which can be explained by an increase of the dielectric and magnetic constants of ferrite and resonance frequency shift of domain walls due to the formation of a coarse−grained structure.

Electrical properties of grain boundaries in interfacially controlled functional ceramics

The mass and charge transport properties as well as structure and composition of grain boundaries in interfacially controlled electroceramic materials are reviewed. Recent advances in the field of donor doped (n-conducting) barium titanate are emphasized, comprising experimental methods such as the characterization of the structure and composition of grain boundaries, the experimental determination of the electrical properties as a function of temperature, oxygen partial pressure and dc bias, the investigation of diffusion along grain boundaries involving conductivity relaxation experiments and 18O - tracer exchange measurements. In addition, phenomenological modeling of transport properties of grain boundaries in donor doped BaTiO3 are outlined in detail, encompassing finite element simulations of diffusion processes as well as modified Schottky barrier models which are suitable for the calculation of the electrical properties as a function of temperature and dc bias.

Small Rb+ doping in CaCu3Ti4O12-A possible approach to reduce dielectric loss

Ca1−xRbxCu3Ti4O12 (x=0, 0.01, 0.02 and 0.03) ceramics were synthesized by the sol-gel method. Doping Rb+ reduces dielectric loss, which reaches minimum when x=0.02. By measuring properties of electrical conduction, larger leakage current density and height of grain-boundary Schottky potential barrier (ϕB) were found in the doped samples, and ϕB became maximum when x=0.02. These results are attributed to the increase in the amount of oxygen vacancies and the formation of Cu-rich/Ti-poor grain-boundary layers, and it can be concluded that the dielectric loss in CCTO ceramic can be reduced by manipulating the composition and electrical properties of grain boundary.

Impact of local structural and electrical properties of grain boundaries in polycrystalline HfO2 on reliability of SiOx interfacial layer

Using nanometer-resolution characterization techniques, we present a study of the local structural and electrical properties of grain boundaries (GBs) in polycrystalline high-κ (HK) dielectric and their role on the reliability of underlying interfacial layer (IL). A detailed understanding of this analysis requires characterization of HK/IL dielectrics with nanometer scale resolution. In this work, we present the impact of surface roughness, thickness and GBs containing high density of defects, in polycrystalline HfO2 dielectric on the performance of underlying SiOx (x⩽2) IL using atomic force microscopy and simulation (device and statistical) results. Our results show SiOx IL beneath the GBs and thinner HfO2 dielectric experiences enhanced electric field and is likely to trigger the breakdown of the SiOx IL.