Grain Boundary Potential Research Articles

Point-Defect Segregation and Space-Charge Potentials at the Σ5(310)[001] Grain Boundary in Ceria

The atomistic structure and point-defect thermodynamics of the model Σ5(310)[001] grain boundary in CeO2 were explored with atomistic simulations. An interface with a double-diamond-shaped structural repeat unit was found to have the lowest energy. Segregation energies were calculated for oxygen vacancies, electron polarons, gadolinium and scandium acceptor cations, and tantalum donor cations. These energies deviate strongly from their bulk values over the same length scale, thus indicating a structural grain-boundary width of approximately 1.5 nm. However, an analysis revealed no unambiguous correlation between segregation energies and local structural descriptors, such as interatomic distance or coordination number. From the segregation energies, the grain-boundary space-charge potential in Gouy–Chapman and restricted-equilibrium regimes was calculated as a function of temperature for dilute solutions of (i) oxygen vacancies and acceptor cations and (ii) electron polarons and donor cations. For the latter, the space-charge potential is predicted to change from negative to positive in the restricted-equilibrium regime. For the former, the calculation of the space-charge potential from atomistic segregation energies is shown to require the inclusion of the segregation energies for acceptor cations. Nevertheless, the space-charge potential in the restricted-equilibrium regime can be described well with an empirical model employing a single effective oxygen-vacancy segregation energy.

Improved Charge Carrier Transport Across Grain Boundaries in N‐type PbSe by Dopant Segregation

Doping is an important and routine method to tune the properties of semiconductors. Dopants accumulated at grain boundaries (GBs) can exert a profound influence on microstructures and transport properties of heat and charge. To unravel the effect of dopant accumulation at GBs on the scattering of electrons, individual high‐angle GBs in three PbSe samples doped with different amounts of Cu using a home‐designed correlative characterization platform combining electron backscatter diffraction, microcircuit transport property measurements, and atom probe tomography are studied. The findings reveal that the segregation of Cu dopants to GBs reduces the GB potential barrier height. Once the GB phase reaches an equilibrium with saturated Cu, the extra Cu dopants distribute homogeneously inside the grains, compensating for vacancies and improving the electrical conductivity of the PbSe grains. The results correlate the Cu distribution at GBs and grains with local electrical properties, enlightening strategies for manipulating advanced functional materials by GB segregation engineering.

Effects of Poly-Si Grain Boundary on Retention Characteristics under Cross-Temperature Conditions in 3-D NAND Flash Memory.

Electrical characteristics with various program temperatures (TPGM) in three-dimensional (3-D) NAND flash memory are investigated. The cross-temperature conditions of the TPGM up to 120 °C and the read temperature (TREAD) at 30 °C are used to analyze the influence of grain boundaries (GB) on the bit line current (IBL) and threshold voltage (VT). The VT shift in the E-P-E pattern is successfully decomposed into the charge loss (ΔVT,CL) component and the poly-Si GB (ΔVT,GB) component. The extracted ΔVT,GB increases at higher TPGM due to the reduced GB potential barrier. Additionally, the ΔVT,GB is evaluated using the Technology Computer Aided Design (TCAD) simulation, depending on the GB position (XGB) and the bit line voltage (VBL).

Pressure-Engineered Ti3C2Tx MXene with Enhanced Conductivity and Accelerated Reaction Kinetics of Lithium Storage.

We studied the structure-function relationship of compressed Ti3C2Tx MXene using high-pressure in situ synchrotron radiation, impedance spectroscopy, Hall effect measurements, and first-principles calculations. With increasing pressure, the conductivity of Ti3C2Tx MXene increases along with its continued lattice shrinkage. A pressure range of 0.4-2.2 GPa exhibits a sharp decrease in resistance, which decreases by more than one order of magnitude from 3.3 × 104 to 1.4 × 103 Ω. A pressure range of 2.2-6.6 GPa exhibits a steady resistance with a slight decrease of 0.2%. As the pressure drops to atmospheric conditions, the resistance increases slightly to 4.2 × 103 Ω. This is accompanied by a transformation of the semiconductor into metal. An irreversible increase in conductivity is observed owing to an increase in the electron concentration and a decrease in the grain-boundary potential barrier. Furthermore, abundant Ti3C2Tx undergoing prepressure treatments (0.4, 2.0, and 4.0 GPa) was first prepared using a double-anvil hydraulic press. The recycled samples retain an accordion-like layered structure with slight lattice shrinkage while the voids between the sheets contract considerably, increasing the density. Correspondingly, electrochemical results show a pressure threshold of 2.0 GPa based on the rapid quenching from the hydraulic press. This weakens the electric polarization in redox reactions and increases the ionic transport rate for the formation of a Ti3C2Tx anode owing to pressure improving the conductivity and interlaminar densification. Our study shows a new, simple, and universal way to regulate various MXenes and also promotes the application of MXene-based materials in energy storage and related fields.

Alloying Pr-Tb-Cu diffusion source with Ni for enhancing both coercivity and corrosion resistance of Nd-Fe-B magnets

Grain boundary diffusion of heavy rare earth-based alloys can effectively enhance the coercivity of Nd-Fe-B magnets. Although the light rare earth elements can further improve the diffusion efficiency of Dy and Tb, they generally lead to reduced anti-corrosion properties of the magnets. Here, we successfully enhanced the coercivity and corrosion resistance of Nd-Fe-B magnets by GBD of Ni alloyed Pr-Tb-Cu diffusion source. The ternary Pr 35 Tb 35 Cu 30 alloy diffusion increased the coercivity of Nd-Fe-B magnet from 1330 to 1923 kA/m, but the coercivity was enhanced to 1990 kA/m by Pr 35 Tb 35 Ni 15 Cu 15 alloy diffusion. The formation of high-anisotropy (Nd,Tb) 2 Fe 14 B phase is the main reason for the coercivity improvement. Partially substitution of Cu by Ni can reduce the melting point of Pr 35 Tb 35 Cu 30 alloy, which is effective in promoting the infiltration of Tb. Cu is also a necessary element for improving the diffusion process, and the diffusion of Pr 35 Tb 35 Ni 30 alloy only increased the coercivity to 1747 kA/m. More interestingly, the diffusion of Pr 35 Tb 35 Ni 15 Cu 15 also enhanced the chemical stability of the magnet in NaCl solution. Scanning Kelvin probe force microscopy characterized that the modified GB phase in Pr 35 Tb 35 Ni 15 Cu 15 treated magnet exhibits a higher Volta potential than the 2:14:1 main phase, which is the main reason for the improved corrosion resistance. The present results suggest that the non-rare earth elements in the alloy diffusion sources play important roles in enhancing both coercivity and corrosion resistance of Nd-Fe-B magnets. • Both coercivity and corrosion resistance of Nd-Fe-B magnet are distinctly enhanced. • Ni alloying lowers melting point of Pr-Tb-Cu and deepens the infiltration of Tb. • Ni alloying results in higher potential of grain boundary than that of 2:14:1 phase. • Scanning Kelvin probe force microscopy was used to measure local potential.

Improving critical current density of Nb3Sn by optimizing pinning potential of grain boundary and grain size

Nb3Sn superconductor is of significant interest for applications in constructing high-field magnets beyond the limit of NbTi. However, its critical current density decreases rapidly at high magnetic fields (12 T) and the state-of-the-art level of Nb3Sn superconductors still cannot meet the requirements of the planned future accelerator magnets. The primary flux pinning centers for Nb3Sn wire mainly arise from the grain boundaries (GBs). In the present paper, we theoretically investigate, through time-dependent Ginzburg-Landau theory and with graphics-processing unit parallel technique, the vortex pinning and the critical current density in large-scale polycrystalline Nb3Sn superconductor by varying the pinning potential of GB and grain size at various magnetic fields. Unlike the conventional dot-like pinning systems, it is found that the critical current is not a monotonous function by suppressing the superconductivity of the GBs. The optimal pinning potential of GB for maximum critical current density strongly depends on magnetic fields. Furthermore, we find that the critical current density can be significantly enhanced by reducing grain size at low magnetic fields, while increase of critical current density cannot always be observed at high magnetic fields. Actually, critical current density even decreases by reducing grain size, which depends on the superconductivity of GBs. The findings in the paper provide theoretical foundations to achieve further improvement of Nb3Sn with optimizing the flux pinning.

The Hydrogenation Impact on Electronic Properties of p-Diamond/n-Si Heterojunctions.

The undoped polycrystalline diamond films (PDFs) have been deposited on n-type silicon (Si) by Hot Filament Chemical Vapor Deposition (HF CVD) technique. The reaction gases are a mixture of methane and hydrogen. The obtained PDFs were characterized by scanning electron microscopy (SEM) and Raman spectroscopy which, in addition to the diamond phase, also confirms the presence of sp hybridized carbon bonds. As-grown CVD diamond layers are hydrogen terminated and show p-type conductivity. The effect of the level of hydrogenation on the electrical properties of p-diamond/n-Si heterojunctions has been investigated by temperature dependent current–voltage (J-V/T) characteristics. The obtained results suggest that the energy distribution of interface states at the grain boundary (GB) subjected to hydrogenation becomes shallower, and the hole capture cross-section can be reduced. Hydrogenation can lead to a significant reduction of the GB potential barrier. These results can be interesting from the point of view of hydrogen passivation of GBs in microelectronics.

Direct Microscopy Imaging of Nonuniform Carrier Transport in Polycrystalline Cadmium Telluride

Summary Inhomogeneous microscopic carrier transport is difficult to study, but important in many condensed-matter applications. For example, the role of grain boundaries (GBs) in polycrystalline semiconductors has been controversial for 20 years. In cadmium telluride (CdTe) solar cells, electron-beam-induced current (EBIC) measurements consistently demonstrate enhanced current collection along GBs, which is argued as evidence for interpenetrating CdTe p-n current-collection networks critical to high efficiency. Conversely, cathodoluminescence (CL) measurements consistently indicate that GBs are deleterious low-lifetime regions. Here, we apply transport imaging (TI) in conjunction with spatially correlated EBIC, CL, and scanning Kelvin probe force microscopy measurements to understand carrier drift, diffusion, and recombination in polycrystalline CdTe. We simultaneously observe GB potential wells, reduced carrier lifetime at GBs, and seemingly contradictory enhanced GB current collection, and then describe their coexistence with microscopic TI and physical arguments. The results provide visualization of inhomogeneous transport that is critical to understanding and engineering polycrystalline solar technology.

Enhanced dielectric properties and grain boundary potentials in sulfur-doped CaCu3Ti4O12 thin films

CaCu3Ti4O12 (CCTO) has been reported to possess a colossal dielectric constant owing to the intrinsic interfacial polarization via charge accumulations across the grain boundary. Herein, we explore the effects of unusual anion-doping on the dielectric properties of sputter-deposited CCTO thin films using an example of sulfur-doping. A post-annealing process of the films was utilized in a flowing H2S atmosphere for the sulfur-doping. The incorporation of sulfur into the perovskite structure was evidenced with the changes in chemical states, such as the reduced cations of Cu+ and Ti3+, the increased concentration of oxygen vacancies, and the formation of S-O bonds. The sulfurized CCTO thin films demonstrated an enhanced relative permittivity of ∼620 at 100 Hz, which is substantially better than that of the unsulfurized film. Direct measurement of the grain-boundary potential using Kelvin probe force microscopy suggests that the enhanced relative permittivity is associated with an increased Schottky barrier height.

Nanoscale control of grain boundary potential barrier, dopant density and filled trap state density for higher efficiency perovskite solar cells

AbstractIn this work, grain boundary (GB) potential barrier ( Δφ GB), dopant density (Pnet), and filled trap state density (PGB,trap) were manipulated at the nanoscale by exposing the fabricated perovskite films to various relative humidity (RH) environments. Spatial mapping of surface potential in the perovskite film revealed higher positive potential at GBs than inside the grains. The average Δφ GB, Pnet, and PGB,trap in the perovskite films decreased from 0% RH to 25% RH exposure, but increased when the RH increased to 35% RH and 45% RH. This clearly indicated that perovskite solar cells fabricated at 25% RH led to the lowest average GB potential, smallest dopant density, and least filled trap states density. This is consistent with the highest photovoltaic efficiency of 18.16% at 25% RH among the different relative humidities from 0% to 45% RH.image

Grain-boundary-limited carrier mobility in polycrystalline silicon with negative temperature dependence: modeling carrier conduction through grain-boundary traps based on trap-assisted tunneling

Seto’s model based on thermionic emission (TE) theory explains that the grain-boundary (GB)-limited carrier mobility in polycrystalline silicon (poly-Si) has a positive temperature dependence. Although this model is widely accepted as a standard model for GB-limited carrier mobility in poly-Si, many experimental measurements under steady-state conditions support extremely weak or negative temperature dependence. In this report, we formulate carrier conduction through GB traps by utilizing the trap-assisted tunneling (TAT) model based on non-radiative multi-phonon transition theory. Self-consistent calculation of poly-Si under steady-state conditions reveals that TAT current exceeds TE current in regions with high GB potential barriers. In contrast to Seto’s model, our novel model reproduces the negative dependence of GB-limited carrier mobility on temperature. From the viewpoint of consistency with experimental measurements, our model seems to be suitable for describing carrier conduction in poly-Si under steady-state conditions.

Modifying Grain Boundary Ionic/Electronic Transport in Nano-Sr- and Mg- Doped LaGaO3-δ by Sintering Variations

Perovskite La0.9Sr0.1Ga0.9Mg0.1O3-δ (LSGM) is one of the fastest known oxide ion conductors, with reported enhanced p-type electronic transference numbers at grain boundaries, attributed to space charge effects. As this material is applied as a solid oxide fuel/electrolysis cell electrolyte, it is of interest to learn how its mixed conductivity may be tailored. Field assisted sintering technique/spark plasma sintering (FAST/SPS) and conventional sintering without field or pressure were employed to prepare pellets with various grain sizes, in order to systematically assess the influence of processing route on the mixed conductivity. AC-impedance spectroscopy and the brick layer model were applied to determine local conductivities as a function of temperature, oxygen partial pressure, and dc bias. With increasing sintering temperature and grain size, the following trends were observed: larger electrical grain boundary (GB) widths, higher GB potentials, lower specific GB conductivity, greater dc-bias dependence of GB conductivity, higher pO2-dependence of GB conductivity indicating higher electronic transference numbers, and lower pre-exponential factor for specific GB conductivity. These results suggest an increasing GB space charge effect with increasing sintering temperature/grain size, which coincided with increasing compositional uniformity by TEM and EDS. The results confirm that sintering route is an important variable for tailoring mixed conduction.

Temperature Dependence According to Grain Boundary Potential Barrier Variation in Vertical NAND Flash Cell with Polycrystalline-Silicon Channel.

We investigate the electrical characteristics according to changing temperature on trap distribution in the energy gap of grain boundary (GB) and interface trap density (D(it)) between polycrystalline-silicon (poly-Si) channel and tunnel oxide in Vertical NAND (VNAND) flash cell with poly-Si channel. We confirmed that there are two factors changing GB potential barrier height such as trap distribution in GB and D(it) using technology computer-aided design (TCAD) simulation. Also, we found that the electrical characteristics according to changing temperature are significantly dependent on height and position of GB potential barrier in VNAND flash cell with poly-Si channel. We expect that it is required to develop more accurate extraction method for trap distribution in each GB and D(it) for better understanding temperature dependence of electrical characteristics in VNAND Flash cell.

Probing Grain-Boundary Chemistry and Electronic Structure in Proton-Conducting Oxides by Atom Probe Tomography.

A laser-assisted atom-probe-tomographic (LAAPT) method has been developed and applied to measure and characterize the three-dimensional atomic and electronic nanostructure at an yttrium-doped barium zirconate (BaZr0.9Y0.1O3-δ, BZY10) grain boundary. Proton-conducting perovskites, such as BZY10, are attracting intense interest for a variety of energy conversion applications. However, their implementation has been hindered, in part, because of high grain-boundary (GB) resistance that is attributed to a positive GB space-charge layer (SCL). In this study, LAAPT is used to analyze BZY10 GB chemistry in three dimensions with subnanometer resolution. From this analysis, maps of the charge density and electrostatic potential arising at the GBs are derived, revealing for the first time direct chemical evidence that a positive SCL indeed exists at these GBs. These maps reveal new insights on the inhomogeneity of the SCL region and produce an average GB potential barrier of approximately 580 mV, agreeing with previous indirect electrochemical measurements.

Thermo-kinetic mechanisms for grain boundary structure multiplicity, thermal instability and defect interactions

Grain boundaries (GBs) provide a source and/or a sink for crystal defects and store elastic energy due to the non-uniform atomic bonding structure of the GB core. GB structures are thermodynamically driven to transition to the lowest energy configuration possible; however to date there has been little evidence to explain why specific GB structures have a low energy state. Furthermore, there is little quantitative demonstration of the significance of physical and GB structure characteristics on the GB energy, thermal stability, and the effect of temporary local GB structure transformations on defect interactions. This paper evaluates the defect interactions and structure stability of multiple Σ5(310) GB structures in bi-crystals of pure aluminium, and systematically investigates the features at 0 K to characterise multiple metastable structures. Structure stability is evaluated by utilising unstable vacancy defects to initiate GB transformations, and using nudged elastic band simulations to quantify this with the activation energy. The emission of stable vacancy defects from the ‘stable’ and metastable grain boundaries is also evaluated in the same manner. A detailed analysis of dislocation nucleation at the atomistic scale demonstrates that local transformations of GB structure between stable and metastable intermediates can provide a mechanism to accommodate the generation of crystal defects. Kinetic (time-dependent) effects that compete with energetic driving forces for structural transformations of GBs are shown to cause a significant effect on the activation properties that may exceed the influence of GB potential energy. The results demonstrate that GB structural multiplicity can be associated with the generation and absorption of dislocations and vacancies. This paper demonstrates the suitability of atomistic simulations coupled with nudged elastic band simulations to evaluate fundamental thermodynamic properties of pure FCC metals. Overall, this paper demonstrates an inherent link between the atomic structure and the thermal, energy and structure stability properties of grain boundaries; and reveals the mechanisms for GB-defect interactions involving localised GB structure transformation.

Molecular Dynamics Simulation of Helium Atom Diffusion in Iron–Chromium Bicrystal Grain

Helium has very low solubility in metals leading to formation of helium clusters and complexes with vacancies/interstitials. Clusters and complexes then diffuse and coalesce to form helium bubbles of nanometric size or higher. These bubbles are one of the reasons for the brittle failure of materials. In order to study the diffusion process, molecular dynamics simulations of helium diffusion in FeCr grain boundaries are carried out for different orientations of the bicrystal. A total of eight different configurations of the bicrystal, have been studied in the temperature range of 700–1,000 K. Potential energy analysis of host and grain boundary (GB) atoms predicts average GB potential energy in the range of $$-$$ 3.7 to $$-$$ 3.6 eV, which is 0.2 eV higher than that in the host matrix. The migration energy of helium in grain boundary is found to be 0.3–0.7 eV, which is an order of magnitude larger than that in the bulk crystal. The grain boundary width calculated for all the bi-crystals lie within 11–14 A and the cage distance of helium atom is of the order of the bond length (2.87 A) of the host atoms. For $$\sum 9 \langle 110 \rangle \{1 \ 1 \ 4\}$$ orientation more than 100 helium trajectories have been analysed to measure the statistical variation of migration energy.

Enhanced Thermopower in Nano-SrTiO3 Via Rare Earth Doping

Doped perovskite titanates are known to exhibit non-linear electrical conductivity when they are in polycrystalline form. This non-linear behavior is attributed to the barrier potential at the grain boundary (GB). In this paper, we focus on the influence of the barrier potential on the thermopower (S) of rare earth (RE)-doped SrTiO3 with a grain size of 20 nm. |S| was found to increase linearly for smaller RE dopants despite the use of identical carrier concentrations. Capacitance–voltage measurements showed that the barrier potential increased linearly for the smaller RE dopants. Energy-selective scattering by the GB potential was evaluated theoretically, and allowed us to qualitatively explain the enhanced S in this system. The results indicate that the GB nature, which is a crucial factor for nanostructured thermoelectric materials, can be tuned by RE doping in this system.

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.

An Explicit Analytical Solution to the Grain Boundary Barrier Height in Undoped Polycrystalline Semiconductor Thin-Film Transistors

Based on discrete grain analysis and U-shaped distribution of density of states (DOS) for grain boundary (GB) traps, a physical-based explicit analytical solution to the GB potential barrier height (ψ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> ) is developed for undoped polycrystalline semiconductor thin-film transistors (TFTs). The explicit solution is derived using the Lambert W function, without additional approximations introduced. The validity and accuracy of the solution is demonstrated comparing the model with both numerical calculations and experimental ψ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> data of polycrystalline Si TFTs. Furthermore, it is found that a previous widely used Seto's model could be consistent to the proposed model in the above-threshold region, in this case deep states DOS dominates ψ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> , where the monoenergetic trap density of Seto's model roughly corresponds to the deep states DOS multiplying by 3-4 units of the thermal energy kT. Finally, the analytical model is applied in ZnO TFTs.

Effects of grain-boundary potential barrier height and its fluctuation on conductivity of polycrystalline semiconductors in the ionized-impurity-scattering dominated case

We propose a new conduction model based on an energy filtering model for thermionic emission over non-uniform grain-boundary barriers in polycrystalline semiconductors of arbitrary degeneracy in which the relaxation time of carriers is dominated by ionized-impurity scattering. It is shown that the non-linear curves of Arrhenius plots of electrical conductivity in several polycrystalline samples of different semiconductors (FeS2, WO3, SnO2, ZnO) in literature are well fitted using the model with the mean value and the standard deviation of the grain-boundary potential barrier height and the concentration of ionized impurities as the three fitting parameters.