Electronic Properties Of Grain Boundaries Research Articles

Strengthened proximity effect at grain boundaries to enhance inter-grain supercurrent in Ba1-xKxFe2As2 superconductors

Iron-based superconductors have great potential for high-power applications due to their prominent high-field properties. One of the central issues in enhancing the critical current density of iron-based superconducting wires is to reveal the roles and limitations of grain boundaries in supercurrent transport. Here, we finely tuned the electronic properties of grain boundaries by doping Ba1-xKxFe2As2 superconductors in a wide range (0.25 ≤ x ≤ 0.598). It is found that the intra-grain Jcintra peaks near x∼0.287, while the inter-grain Jcinter has a maximum at about x∼0.458. Remarkably, the grain boundary transparency parameter defined as ε = Jcinter/Jcintra rises monotonically with doping. Through detailed microscopic analysis, we suggest that the FeAs segregation phase commonly existing at grain boundaries and the adjacent grains constitute superconductor-normal metal-superconductor (SNS) Josephson junctions which play a key role in transporting supercurrent. A sandwich model based on the proximity effect and the SNS junction is proposed to well interpret our data. It is found that overdoping in superconducting grains largely strengthens the proximity effect and consequently enhances the intergrain supercurrent. Our results will shed new insights and inspirations for improving the application parameters of iron-based superconductors by grain boundary engineering.

Unveiling the Electronic Structure of Grain Boundaries in Anatase with Electron Microscopy and First-Principles Modeling.

Polycrystalline anatase titanium dioxide has drawn great interest, because of its potential applications in high-efficiency photovoltaics and photocatalysts. There has been speculation on the electronic properties of grain boundaries but little direct evidence, because grain boundaries in anatase are challenging to probe experimentally and to model. We present a combined experimental and theoretical study of anatase grain boundaries that have been fabricated by epitaxial growth on a bicrystalline substrate, allowing accurate atomic-scale models to be determined. The electronic structure in the vicinity of stoichiometric grain boundaries is relatively benign to device performance but segregation of oxygen vacancies introduces barriers to electron transport, because of the development of a space charge region. An intrinsically oxygen-deficient boundary exhibits charge trapping consistent with electron energy loss spectroscopy measurements. We discuss strategies for the synthesis of polycrystalline anatase in order to minimize the formation of such deleterious grain boundaries.

The impact of Kelvin probe force microscopy operation modes and environment on grain boundary band bending in perovskite and Cu(In,Ga)Se2 solar cells

An in-depth understanding of the electronic properties of grain boundaries (GBs) in polycrystalline semiconductor absorbers is of high importance since their charge carrier recombination rates may be very high and hence limit the solar cell device performance. Kelvin Probe Force Microscopy (KPFM) is the method of choice to investigate GB band bending on the nanometer scale and thereby helps to develop passivation strategies. Here, it is shown that the workfunction, measured with amplitude modulation (AM)-KPFM, which is by far the most common KPFM measurement mode, is prone to exhibit measurement artifacts at grain boundaries on typical solar cell absorbers such as Cu(In,Ga)Se2 and CH3NH3PbI3. This is a direct consequence of a change in the cantilever–sample distance that varies on rough samples. Furthermore, we critically discuss the impact of different environments (air versus vacuum) and show that air exposure alters the GB and facet contrast, which leads to erroneous interpretations of the GB physics. Frequency modulation (FM)-KPFM measurements on non-air-exposed CIGSe and perovskite absorbers show that the amount of band bending measured at the GB is negligible and that the electronic landscape of the semiconductor surface is dominated by facet-related contrast due to the polycrystalline nature of the absorbers.

Impact of Electronic Properties of Grain Boundaries on the Solid Electrolyte Interphases (SEIs) in Li-ion Batteries

Electron leakage through the solid-state electrolyte interphase (SEI) in Li-ion batteries causes the reduction of the electrolyte and the consumption of Li-ions, decreasing the battery capacity and...

Nanoscopic study of the compositions, structures, and electronic properties of grain boundaries in Cu(InGa)Se2 photovoltaic thin films

Polycrystalline Cu(InGa)Se2 (CIGS) thin film is a well-known light absorber material for photovoltaic solar cells. However, the mechanism of the benignity of its grain boundary (GB) remains in dispute. By using a combination of experimental techniques, we have systematically investigated the compositions, structures, and properties of the grain interior (GI) and GB of the polycrystalline CIGS films at nanometer scale. A definitive Cu deficient layer with thickness depending on the overall Cu content of the films was observed near the non Σ3 GBs with distinctly different characteristics from GIs. The band alignment between GB and GI was discovered to be of type II with downward offset for both conduction and valence bands at GB, well correlating to the local copper deficiency and structure. Our findings expressively support the type inversion and large hole barrier in this GB layer, and establish a comprehensive mechanism for the benignity of the CIGS grain boundaries towards carrier recombination.

Direct Observation of CdCl2 Treatment Induced Grain Boundary Carrier Depletion in CdTe Solar Cells Using Scanning Probe Microwave Reflectivity Based Capacitance Measurements

Device grade polycrystalline CdTe films, grown by rf-sputtering (yielding up to 13% efficient devices) and close-spaced sublimation (CSS) (yielding up to 16% efficient devices), have been analyzed using scanning microwave impedance microscopy (sMIM). In order to study the effect of Cl on the electronic properties of grain boundaries in CdTe, sMIM based capacitance measurements were performed on the surfaces of as-deposited and CdCl2 annealed CdTe films. The measurements were performed with an sMIM instrument operating at ∼3 GHz probe signal frequency in conjunction with a combination of ac and dc voltage biases applied to the tip. It was found that CdCl2 annealed CdTe samples deposited using both CSS and sputtering exhibit carrier depletion along the grain boundaries as compared to the grain bulk. The grain boundary carrier depletion was not observed in samples that have not been CdCl2 annealed and hence have no incorporated Cl.

Identifying the Electronic Properties of Grain Boundaries in CdTe Thin-film Solar Cells Using Electron Backscatter Diffraction and Electron Beam Induced Current Techniques

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

(Invited) Structure and Electronic Properties of Polycrystalline Dielectrics

We present an overview of the theoretical approaches that can be employed to model polycrystalline oxides along with a discussion of their limitations and associated challenges. We then present results for two metal oxide materials, MgO and HfO2, where theory and experiment have come together to provide insight into the structure and electronic properties of grain boundaries. Finally, we conclude with a discussion and outlook.

Effect of alkali metals on the electronic properties of grain boundaries on a polycrystalline silicon surface

The effect of alkali metals on carrier drift from grain boundaries to a polycrystalline silicon surface is experimentally studied. The results obtained show that an increase in the dopant concentration during the diffusion, desorption, and adsorption of alkali metals along grain boundaries leads to an increase in the potential barrier.

Electronic properties of grain boundaries in Cu(In,Ga)Se2 thin films with various Ga-contents

We present a study on the electronic properties of grain boundaries (GBs) in polycrystalline Cu(In,Ga)Se2 (CIGSe) thin films by means of Kelvin probe force microscopy. As grown as well as KCN-treated films were investigated comparatively. No influence of the chemical treatment on the electronic properties of GBs was found. GBs generally exhibited large variations in their electronic properties. By means of a novel method of data analysis both potential barriers for holes and electrons were found at GBs, in a range from −118mV to +114mV, as well as GBs without potential barrier. No dependence of the electronic GB-properties on the Ga-content was detected. Consequently, we conclude that there is no correlation between the electronic properties of GBs and the obtained maximum efficiencies of CIGSe thin film solar cells as a function of the Ga-content.

Grain boundary mediated leakage current in polycrystalline HfO 2 films

In this work, we combine conductive atomic force microscopy (CAFM) and first principles calculations to investigate leakage current in thin polycrystalline HfO 2 films. A clear correlation between the presence of grain boundaries and increased leakage current through the film is demonstrated. The effect is a result of a number of related factors, including local reduction in the oxide film thickness near grain boundaries, the intrinsic electronic properties of grain boundaries which enhance direct tunnelling relative to the bulk, and segregation of oxygen vacancy defects which increase trap assisted tunnelling currents. These results highlight the important role of grain boundaries in determining the electrical properties of polycrystalline HfO 2 films with relevance to applications in advanced logic and memory devices.

Adsorption of alkali metals and their effect on electronic properties of grain boundaries in bulk of polycrystalline silicon

The adsorption of alkali metals and their effect on the electronic properties of grain boundaries in bulk of polycrystalline silicon has been studied experimentally. The results obtained show that the potential barrier grows during diffusion and adsorption of alkali metal atoms along grain boundaries.

Structural and electronic properties of grain boundaries in graphite: Planes of periodically distributed point defects

We report on scanning tunneling microscopy and spectroscopy of grain boundaries in highly oriented pyrolytic graphite. Grain boundaries showed a periodic structure and an enhanced charge density compared to the bare graphite surface. Two possible periodic structures have been observed along grain boundaries. A geometrical model producing periodically distributed point defects on the basal plane of graphite has been proposed to explain the structure of grain boundaries. Scanning tunneling spectroscopy on grain boundaries revealed two strong localized states at -0.3 V and 0.4 V.

Grain boundaries in Cu(In, Ga)(Se, S)2 thin-film solar cells

The paper reviews the current status of the research on grain boundaries in polycrystalline Cu(In, Ga)(S, Se)2 alloys used as absorber materials for thin-film solar cells. We discuss the different concepts that are available to explain the relatively low electronic activity of grain boundaries in these materials. Numerical simulations that have been undergone so far to model the polycrystalline solar cells are briefly summarized. In addition, we give an overview on the experiments that have been conducted so far to elucidate the structural, defect-chemical, and electronic properties of grain boundaries in Cu(In, Ga)(S, Se)2 thin-films.

Effect of Copassivation of Cl and Cu on CdTe Grain Boundaries

Using a first-principles method, we investigate the structural and electronic properties of grain boundaries (GBs) in polycrystalline CdTe and the effects of copassivation of elements with far distinct electronegativities. Of the two types of GBs studied in this Letter, we find that the Cd core is less harmful to the carrier transport, but is difficult to passivate with impurities such as Cl and Cu, whereas the Te core creates a high defect density below the conduction band minimum, but all these levels can be removed by copassivation of Cl and Cu. Our analysis indicates that for most polycrystalline systems copassivation or multipassivation is required to passivate the GBs.

Dependence of Solar Cell Performance on Electronic Properties at Grain Boundaries in Polycrystalline Silicon Thin Films Deposited by Atmospheric Pressure Chemical Vapor Deposition

The relationship between the electronic properties of grain boundaries and polycrystalline silicon (poly-Si) thin-film solar cell performance was investigated. The poly-Si thin films were directly deposited on foreign substrates by atmospheric pressure chemical vapor deposition. We evaluated the interface state density at the grain boundaries of poly-Si thin films deposited under different deposition conditions by measuring the temperature dependence of Hall mobility. We found a strong correlation between the interface state density and poly-Si thin film solar cell performance. The decrease in interface state density resulted in the improvement of the solar cell performance, which was caused by decreasing the minority carrier recombination velocity at grain boundaries.

Atomic and electronic structure of high-energy grain boundaries in silicon and carbon

We have studied the structure and electronic properties of a few high-energy twist grain boundaries, the (0 0 1) φ=53.1°, or Σ5, and the (0 0 1) φ=43.6°, or Σ29, in the covalently bonded systems Si and C by means of tight-binding molecular dynamics. We describe the parallelization of the code and the main results obtained for the structural and electronic properties of the grain boundaries. The role of structural disorder in silicon is compared to the chemical (bonding) disorder in carbon.

Atomic Scale Characterization of Oxygen Vacancy Segregation at SrTiO3 Grain Boundaries

AbstractWe have examined the structure, composition and bonding at an un-doped 58° [001] tilt grain-boundary in SrTiO3 in order to investigate the control that the grain boundary exerts over the bulk properties. Room temperature and in-situ heating experiments show that there is a segregation of oxygen vacancies to the grain boundary that is increased at elevated temperatures and is independent of the cation arrangement. These measurements indicate that the widely observed electronic properties of grain boundaries may be due to an excess of mobile oxygen vacancies that cause a highly doped n-type region in the close proximity ( ≍ 1 unit cell) to the boundary. These results are shown to be consistent with both theoretical models and lower resolution chemical analysis.

Intrinsic weak link originating from tilt in contacts between d x 2 -y 2 wave superconductors

wave superconductors, which form a contact at a tilt angle. The contact is a weak link or a Josephson junction, with properties determined by the misalignment angle. Complementing existing theories that discuss the influence of dx2-y2 symmetry on the charge transfer across junctions, the ideas presented provide insights into the origin of the junction formed in such configurations. The mechanism discussed is of relevance for the understanding of the electronic properties of grain boundaries in superconductors, including the angular dependence of the critical current density.

Electronic properties of grain boundaries in SrTiO3 and BaTiO3 ceramics

The grain boundary space charge depletion layer in acceptor-doped SrTiO3 and BaTiO3 ceramics is investigated by an impedance analysis in the time domain and is described in terms of a simplified back-to-back double Schottky barrier model. A numerical simulation is employed to analyze the defect chemistry in the depletion layer and to elaborate a refined Schottky model. The local distribution of the donor-type grain boundary states causing the depletion layer and the resulting band bending are discussed.