Polycrystalline Grains Research Articles

Capacitance Measurement of Cerium Oxide Thin Film Capacitors

IntroductionCerium oxide (CeOx) is an attractive material for large volume on-chip decoupling capacitors, owing to its high dielectric constant (k) of 28. Commonly, a definite frequency dispersion in the small-signal capacitance measurement is observed in the frequency sweep, and larger capacitance values can be obtained at low frequency in the kHz range. The dielectric relaxation is reported to originate from the interface polarization between the polycrystalline grains in the films. In this work, the thickness-dependent polarization and the capacitance measurement under a DC voltage application are characterized.Fabrication processCeOx films with thicknesses from 8 to 20 nm were deposited by e-beam evaporation on an n+Si substrate with a W layer atop. Another W layer was deposited on CeOx layers and patterned to form electrodes. After the Al back contact formation, the samples are annealed at 420oC.ResultsFig. 1 shows the frequency dispersion of the capacitors with different CeOx thicknesses measured at 0 V. The dielectric relaxation frequency shifts to higher frequency while thinning the thickness, which is useful for capacitor application. The transient current upon 1 V application is shown in fig. 2. After a high charge current, one can observe a decay in the transient current, indicating the delay in the polarization. The capacitance of the samples can be obtained by taking the integral of the charge current and is shown in fig. 3. Comparable capacitance values were obtained with the small-signal measurements. 2 to 3 times higher capacitance values can be achieved with respect to the ionic polarization.ConclusionHigh capacitance values measured in the small-signal setup were confirmed by direct current measurement by DC voltage application. An even higher capacitance can be obtained with the thinner film.

Post-processing ZnSe optical fibers with a micro-chemical vapor transport technique

Polycrystalline zinc selenide optical fibers and fiber lasers are expected to provide powerful capabilities for infrared waveguiding and laser technology. High pressure chemical vapor deposition, which is the only technique currently capable of producing zinc selenide optical fibers, leaves a geometric imperfection in the form of a central pore which is detrimental to mode quality. Chemical vapor transport with large temperature and pressure gradients not only fills this central pore but also encourages polycrystalline grain growth. Increased grain size and a reduction in defects such as twinning are demonstrated with transmission electron microscopy, Raman spectroscopy, and X-ray diffraction, supporting that high-quality material is produced from this method. Finally, the mode structure of the waveguide is improved allowing most of the guided optical intensity to be centrally positioned in the fiber core. Loss as low as 0.22 dB/cm at 1908nm is demonstrated as a result of the material improvement.

Hydrogen-doped In2O3 for silicon heterojunction solar cells: Identification of a critical threshold for water content and rf sputtering power

Hydrogen-doped indium oxide (IO:H) layers with very high carrier mobility have been developed by two-step fabrication procedure. In the first step IO:H films were deposited by radio frequency (RF) magnetron sputtering from In2O3 target in Ar/water vapour gas mixtures. Different sputtering powers and partial pressures of H2O (pH2O) have been explored and effects induced by changes of these process parameters on final film structure and on electro-optical properties have been investigated. In the second step a post-deposition annealing under vacuum were performed. During this treatment a complete and complex amorphous-crystalline transition occurred. Growth of large crystalline domains give rise to IO:H films with excellent carrier mobility values, up to 138 cm2/Vs. For each sputtering power used, the pH2O range has been identified which allows to obtain the highest carrier mobility value for IOH post-annealed films. Growth of large crystallites occurred in IO:H annealed films and, at this purpose, a selective chemical etching method has been developed to give better evidence for the polycrystalline grains. IO:H films appeared formed by fairly large crystalline domains with presence of sub-grains. Silicon heterojunction cells fabricated by using IO:H layers as front electrode, compared with those obtained from a conventional ITO showed improved short-circuit current density and the resulting conversion efficiency.

Direct diffraction measurement of critical resolved shear stresses and stress localisation in magnesium alloy

The main purpose of this work is to develop neutron diffraction methodology in order to determine stresses localised in polycrystalline grains during elastoplastic deformation, directly from experiment. As a result, for the first time, the von Mises stress for chosen grain orientations and Critical Resolved Shear Stresses (CRSS) for active slip systems were unambiguously measured without the help of crystallographic models, which introduce different theoretical assumptions. The stresses measured for groups of grains and the determined CRSS values are important characteristics of a material, which allow to study plastic deformation in textured material at different scales: slip system and grain, which play a key role in mechanical properties and formability of the material.The new method was successfully tested and applied to textured AZ31 alloy subjected to tensile deformation and the components of stress tensor were for the first time determined from measured lattice strains corresponding to chosen orientations of crystallite lattice. The obtained results positively verified hypotheses that, during plastic deformation, a large difference in the hardness as well as in the localised stresses occurs for grains having different lattice orientations. It was found directly from experiment that, the activation of basal glide, having small CRSS, does not lead to significant plastic deformation, and the activation of other non-basal systems (with higher CRSS) induces the development of plasticity at the macroscopic scale. The early plastic deformation occurring due to slip on basal system is small but it can destructively affect fatigue life, limiting applicability of the material for structural components. Finally, the comparison of experimental results with a modified version of Elastic-Plastic Self-Consistent (EPSC) model showed its capability to simulate the mechanical behaviour of such materials.

A numerical study into element type and mesh resolution for crystal plasticity finite element modeling of explicit grain structures

A large number of massive crystal-plasticity-finite-element (CPFE) simulations are performed and post-processed to reveal the effects of element type and mesh resolution on accuracy of predicted mechanical fields over explicit grain structures. A CPFE model coupled with Abaqus/Standard is used to simulate simple-tension and simple-shear deformations to facilitate such quantitative mesh sensitivity studies. A grid-based polycrystalline grain structure is created synthetically by a phase-field simulation and converted to interface-conformal hexahedral and tetrahedral meshes of variable resolution. Procedures for such interface-conformal mesh generation over complex shapes are developed. FE meshes consisting of either hexahedral or tetrahedral, fully integrated as linear or quadratic elements are used for the CPFE simulations. It is shown that quadratic tetrahedral and linear hexahedral elements are more accurate for CPFE modeling than linear tetrahedral and quadratic hexahedral elements. Furthermore, tetrahedral elements are more desirable due to fast mesh generation and flexibility to describe geometries of grain structures.

Switching Characteristics and Mechanism Using Al2O3 Interfacial Layer in Al/Cu/GdOx/Al2O3/TiN Memristor

Resistive switching characteristics by using the Al2O3 interfacial layer in an Al/Cu/GdOx/Al2O3/TiN memristor have been enhanced as compared to the Al/Cu/GdOx/TiN structure owing to the insertion of Al2O3 layer for the first time. Polycrystalline grain, chemical composition, and surface roughness of defective GdOx film have been investigated by transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and atomic force microscopy (AFM). For bipolar resistive switching (BRS) characteristics, the conduction mechanism of high resistance state (HRS) is a space-charge limited current for the Al/Cu/GdOx/TiN device while the Al/Cu/GdOx/Al2O3/TiN device shows Schottky emission. However, both devices show Ohmic at a low resistance state (LRS). After the device has been SET, the Cu filament evidences by both TEM and elemental mapping. Oxygen-rich at the Cu/GdOx interface and Al2O3 layer are confirmed by energy dispersive X-ray spectroscopy (EDS) line profile. The Al/Cu/GdOx/Al2O3/TiN memristor has lower RESET current, higher speed operation of 100 ns, long read pulse endurance of >109 cycles, good data retention, and the memristor with a large resistance ratio of >105 is operated at a low current of 1.5 µA. The complementary resistive switching (CRS) characteristics of the Al/Cu/GdOx/Al2O3/TiN memristor show also a low current operation as compared to the Al/Cu/GdOx/TiN device (300 µA vs. 3.1 mA). The transport mechanism is the Cu ion migration and it shows Ohmic at low field and hopping at high field regions. A larger hopping distance of 1.82 nm at the Cu/GdOx interface is obtained as compared to a hopping distance of 1.14 nm in the Al2O3 layer owing to a larger Cu filament length at the Cu/GdOx interface than the Al2O3 layer. Similarly, the CRS mechanism is explained by using the schematic model. The CRS characteristics show a stable state with long endurance of >1000 cycles at a pulse width of 1 µs owing to the insertion of Al2O3 interfacial layer in the Al/Cu/GdOx/Al2O3/TiN structure.

Optoelectronic properties of facile synthesized orthorhombic cesium lead bromide (CsPbBr3)

CsPbBr3 has been synthesized via a wet chemical technique instead of colloidal hot injection route. Crystalline structure and chemical composition of the material have been assured by Rietveld refinement and energy dispersive X-ray spectroscopy. Photoresponse capability of CsPbBr3 nanocrystal in cubic phase has been already explored. Here, intrinsic optoelectronic properties of orthorhombic CsPbBr3 have been investigated in detail. A prototype photodetector has been fabricated on a transparent conducting oxide-coated glass by using silver as the other electrode material. The photo-responsibility of CsPbBr3 arises due to the combined effect of electron–hole pair generation and trapping within material and recombination. The non-linearity of I–V characteristics clearly reveals the formation of well-established Schottky contact at perovskite–ITO junction. Responsivity of the detector increases from 1.4 to 1.73 mA W−1 after enhancing the irradiance from 25 to 100 mW cm−2 at 8 V bias voltages. Linear dynamic range has been accounted to study the linear photosensitivity of the device. In order to investigate the stability and reversibility, photocurrent has been measured under ON/OFF condition of frequency 20 s. The quantitative contribution of polycrystalline grain and grain boundary to charge mobilization has been evaluated by fitting the Cole–Cole plot under different solar intensities.

Characterization of sequential physical vapor deposited methylammonium lead tri-iodide perovskite thin films

Methylammonium lead tri-iodide perovskite (MAPbI3) thin film for solar cells, chemical formula CH3NH3PbI3, is synthesized in situ via sequential thermal vapor deposition of lead(II)iodide (PbI2) and methylammonium iodide (MAI) single layers. The structural, morphological, optical, and electrical attributes are highly dependent on annealing time and MAI thickness. X-ray diffractograms confirmed the tetragonal crystal structure of MAPbI3 with I4/mcm space group, good crystallinity which increases with the thickness of MAI and transformation of PbI2-deficient MAPbI3 to PbI2-rich MAPbI3 upon increasing annealing time. UV–vis optical spectra reveal a redshift in the onset of absorption from 750 to 780 nm as the MAI thickness increases and a slight blueshift as the annealing time increases. Field emission scanning electron microscopy micrographs show densely packed polycrystalline grains with negligible pinholes and full coverage. The current density-voltage (J-V) characteristics under illumination reveals that the photogenerated current decreases with an increase in annealing time. Space charge limited current analysis of dark current J-V curves shows that PbI2-deficient MAPbI3 has higher mobility than PbI2-rich MAPbI3 and trap density increases with annealing time.

The surface of halide perovskites from nano to bulk

The surface of a semiconductor often has a key role in determining its properties. For metal halide perovskites, understanding the surface features and their impact on the materials and devices is becoming increasingly important. At length scales down to the nanoscale regime, surface features become dominant in regulating the properties of perovskite materials, owing to the high surface-to-volume ratio. For perovskite bulk films in the micrometre range, defects and structural disorder readily form at the surface and affect device performance. Through concerted efforts to optimize processing techniques, high-quality perovskite thin films can now be fabricated with monolayer-like polycrystalline grains or even single crystals. Surface defects therefore remain the major obstacle to progress, pushing surface studies to the forefront of perovskite research. In this Review, we summarize and assess recent advances in the understanding of perovskite surfaces and surface strategies towards improving perovskite materials and the efficiency and stability of perovskite devices. The surface features of metal halide perovskites have a crucial role in determining their properties. This Review introduces the recent advances in the understanding of perovskite surfaces and surveys the surface strategies towards improving the properties of perovskite materials and the performance of perovskite optoelectronic devices.

Barite Concretions in Wadi Halfa Oolitic Ironstone Formation, North Sudan

During our examination of the outcrops of the sedimentary formations in northern Sudan, we found discoidal-shape grains of the heavy mineral, barite in a sandstone of the Wadi Halfa Oolitic Ironstone Formation, which was recorded by all the earlier workers as a reworked sandstone. Petrography-wise, the framework of the sandstone consists of very angular to angular quartz grains, in which monocrystalline grains dominate over polycrystalline grains. Barite is the main cementing material of this sandstone, which occurs as concretions. Barite concretions indicate that more of the original porosity has been destroyed by cementation rather than by compaction processes with the inter-granular porosity being reduced mainly due to cementation. The origin of these concretions, as a cementing material in the sandstone, is ascribed to the reaction of Ba with some soluble sulfate to form the extremely insoluble heavy barite that appears as rounded concretions. The sulfur of the sulfate may be from the hydrothermal fluids related to submarine volcanism and/or biogeochemical processes. The deposition of these concretions might have taken place not long after the formation of the sandstone. The source of the barium, however, remains an unsolved problem. Further work is needed to interpret the origin and occurrence of these concretions along the region of Wadi Halfa.

The Synergism of DMSO and Diethyl Ether for Highly Reproducible and Efficient MA0.5FA0.5PbI3 Perovskite Solar Cells

AbstractComposition and film quality of perovskite are crucial for the further improvement of perovskite solar cells (PSCs), including efficiency, reproducibility, and stability. Here, it is demonstrated that by simply mixing 50% of formamidinium (FA+) into methylammonium lead iodide (MAPbI3), a highly crystalline, stable phase, and compact, polycrystalline grain morphology perovskite is formed by using a solvent‐mediated phase transformation process via the synergism of dimethyl sulfoxide and diethyl ether, which shows long carrier lifetime, low trap state density, and a record certified 21.8% power conversion efficiency (PCE) in pure‐iodide, alkaline‐metal‐free MA0.5FA0.5PbI3 perovskite‐based PSCs. These PSCs show very high operational stability, with 85% PCE retention upon 1000 h 1 Sun intensity illumination. A 17.33% PCE module (6.5 × 7 cm2) is also demonstrated, attesting to the scalability of such devices.

Subkilohertz optical homogeneous linewidth and dephasing mechanisms inEr3+:Y2O3ceramics

We report an optical homogeneous linewidth of 580 $\pm$ 20 Hz of Er$^{3+}$:Y$_2$O$_3$ ceramics at millikelvin temperatures, narrowest so far in rare-earth doped ceramics. We show slow spectral diffusion of $\sim$2 kHz over a millisecond time scale. Temperature, field dependence of optical coherence and spectral diffusions reveal the remaining dephasing mechanism as elastic two-level systems in polycrystalline grain boundaries and superhyperfine interactions of Er$^{3+}$ with nuclear spins. In addition, we perform spectral holeburning and measure up to 5 s hole lifetimes. These spectroscopic results put Er$^{3+}$:Y$_2$O$_3$ ceramics as a promising candidate for telecommunication quantum memories and light-matter interfaces.

Ultrabroadband and sensitive cavity optomechanical magnetometry

Magnetostrictive optomechanical cavities provide a new optical readout approach to room-temperature magnetometry. Here we report ultrasensitive and ultrahigh bandwidth cavity optomechanical magnetometers constructed by embedding a grain of the magnetostrictive material Terfenol-D within a high quality ( Q ) optical microcavity on a silicon chip. By engineering their physical structure, we achieve a peak sensitivity of 26 pT / Hz comparable to the best cryogenic microscale magnetometers, along with a 3 dB bandwidth as high as 11.3 MHz. Two classes of magnetic response are observed, which we postulate arise from the crystallinity of the Terfenol-D. This allows single crystalline and polycrystalline grains to be distinguished at the level of a single particle. Our results may enable applications such as lab-on-chip nuclear magnetic spectroscopy and magnetic navigation.

Microstructure and mechanical properties of pure copper manufactured by selective laser melting

Selective laser melting (SLM) was used to fabricate copper samples under various processing parameters. The influence of laser linear energy density on the microstructures and mechanical properties of the SLM copper samples were investigated theoretically and experimentally. Based on the results, the optimal linear energy density that can result in the best relative density (99.10 ± 0.5%) and surface roughness (Ra = 12.72 ± 4.54 μm) was determined to be 0.50 J/mm which corresponds to the laser power and scanning speed of 200 W and 400 mm/s, 300 W and 600 mm/s, respectively. Under the optimal processing conditions, the microhardness and strength (i.e., yield strength and ultimate tensile strength) of the copper sample achieved the highest value. In addition, it is also found that the microstructure of the SLM pure copper samples was characterized by polycrystalline grains with columnar dendrites and equiaxed structures dispersed inside. The grain size showed a decreasing trend as linear energy density increased due to the improved intrinsic heat treatment effect. The paper proves that nearly full dense copper with desirable mechanical properties can be fabricated through SLM.

Observing Hot Spot Formation in Individual Explosive Crystals Under Shock Compression.

The formation of hot spots in dynamically compressed, plastic-bonded explosives is known to be the primary mechanism by which these materials ignite and initiate, but hot spots are small, fleeting, and hard to observe. Using a microscope equipped with laser-launched, miniflyer plates, we have studied hot spots in small grains of cyclotetramethylene-tetranitramine (HMX) embedded in a polyurethane binder, shocked to about 20 GPa. A nanosecond video with 4 μm spatial resolution is used to observe hot spot formation and growth, while nanosecond optical pyrometry measured temperature. Using individual ∼200 μm nominally single crystals of HMX (HMX-SC), we observed hot spots forming preferentially on corners or edges. These hot spots are about 4000 K. When there are multiple hot spots, the flame propagated along crystal edges, and the crystal is mostly combusted after about 300 ns. Using polycrystalline grains (HMX-PC), 6000 K hot spots are created near internal defects or crystal junctions. However, the thermal mass of the material at 6000 K is quite small, so after those hot spots cool down, the HMX combustion is similar to the single crystals. Comparing a HMX-based polymer-bonded explosive (PBX) to the individual polymer-bonded HMX-SC and HMX-PC grains shows that the myriad hot spots in the PBX are hotter than HMX-SC and colder than HMX-PC, but they persist for a longer time in PBX than in the individual grains.

Pure copper components fabricated by cold spray (CS) and selective laser melting (SLM) technology

To study effect of the different metal additive manufacturing techniques on microstructural evolution, phase constitution and the relationship between the microstructure and tensile behavior of pure Cu parts, components were manufactured by selective laser melting (SLM) technology and cold spraying (CS) technology, respectively. The microstructure of Cu parts was detected using an optical microscope (OM) and scanning electron microscopy (SEM). The XRD spectrum revealed that only Cu phase is formed in the SLM Cu and CS Cu samples. The microstructure of the SLM Cu samples is constituted by the polycrystalline grains with substructures including the columnar dendrites and the equiaxed structures. As for the CS Cu samples, only the equiaxed grains were detected. In terms of the main physical properties, the averaged electrical conductivity of SLM Cu sample is 41% IACS, while that of CS Cu sample is 73% IACS. For the microhardness, the mean microhardness value of the CS Cu and the SLM Cu samples is 144.2 ± 4.3 HV0.05 and 83.6 ± 5.2 HV0.05, respectively. With regard to the statistic mechanical properties, the yield strength (YS) and ultimate tensile strength (UTS) of the SLM Cu part is approximately 185.8 ± 6.1 MPa and 242.2 ± 8.2 MPa, respectively.

The Application of Electron Backscatter Diffraction on Halide Perovskite Materials

AbstractRecently, the application of electron backscatter diffraction (EBSD) in halide perovskites has enabled the correlation of the micro‐structural arrangement of polycrystalline grains with other properties (optical, electrical, mechanical, and chemical) in a “pixel‐by‐pixel” approach. Most studies so far have used an ultra‐sensitive electron beam detector that has sensitivity thousands of times higher than a traditional scintillator screen and charge coupled device camera, enabling much lower beam currents. An alternative approach has been the use of low vacuum measurement conditions to avoid charge buildup that leads to damage. This review focuses on introducing the classical EBSD technique to the halide perovskite community, where it has been highly underutilized due to beaminduced damage in these relatively unstable materials. Recent research is used to dispel some common misconceptions about grain boundaries in halide perovskites and highlight what has been learned by comparing and correlating EBSD with other techniques. Additionally, the remaining limitations, development challenges, and future of the EBSD technique for halide perovskites are discussed. Successful utilization of the EBSD technique as a common characterization tool in the halide perovskite community will enable scientists and engineers to develop maps of cross correlated properties, helping to unlock the full potential of this complex material system.

Mesoscale modeling of light transmission modulation in ceramics

We present a modeling approach for simulating and predicting spatially resolved properties of polycrystalline ceramics with coupled optical, elastic and dielectric degrees of freedom. This approach is implemented in the Ferret application, built upon the Multiphysics Object Oriented Simulation Environment (MOOSE) finite-element framework, that allows for the determination of the dependency of optical properties on polycrystalline grain orientation, size, and shape, as well as the action of externally applied electric fields and mechanical distortions. For the evaluation of optical transmission through a polycrystal we adopt a modification of the wave-retardation theory, originally developed by Raman and Viswanathan in 1955. The coupled model produces excellent agreement with experimental results obtained for polycrystalline Al2O3, Ce-doped Y3Al5O12, and ZnS which are used as test cases in this investigation, to evaluate both the visible and infrared parts of the electro-magnetic spectrum. Simulations of the optical properties modulation under different applied mechanical and electrical boundary conditions predict large changes to transmission, including switching from full transparency to opacity in some instances. The results of this investigation highlight a remarkable promise of functional nano- and micro-ceramics for a wide range of advanced engineering applications, including broad spectrum windows with high frequency transparency modulation, and multifunctional metamaterials by design.

Solid solubility of Mg and enhanced electrical conduction in the C-axis orientation of CuCr1-xMgxO2 polycrystals

ABSTRACT The c-axis orientation of CuCr1-xMgxO2 (0 ≤ x ≤ 0.08) ceramics was prepared through solid-state reactions. The effects of a Mg solid solution in CuCrO2 have been investigated in terms of electrical conductivity, thermopower, and microstructure. While x = 0–0.03, the polycrystalline grains in a single-phase delafossite grew along the ab-plane with increasing (00 l) Lotgering factors of up to 0.53. It exhibited semiconducting electrical transport. The room temperature conductivity increased by 3–4 orders of magnitude, even up to 12.03 S·cm−1, because of a reduction in Arrhenius activation energy. The Seebeck coefficient decreased from 828 to 257.8 μV·K−1 at 330 K. For the doped samples, due to the contribution of the phonon drag thermopower overmatching the variation of the carrier concentration to the thermopower, it increased with increasing temperature, which was the opposite to that observed in the undoped samples. While x = 0.04–0.08, the second-phase MgCr2O4 spinel appeared and the properties and microstructure hardly changed. The layered structure of the c-axis orientation led to dominant electrical transport along the ab-plane and reduced the grain boundary, leading to weakened defect scattering, which caused an enhancement in conductivity.

Influence of annealing temperature on the structural and magnetic properties of FeGaSiB thin films

Understanding the relationship between the structure and magnetic properties of soft magnetic thin films is important in the development of magnetic sensors and electronic devices. This work has systematically studied how the annealing temperature changes the morphology and hence the magnetic properties of 60 nm FeGaSiB thin films. The as-grown and 100 °C annealed films were amorphous/nanocrystalline with soft magnetic properties. As the annealing temperature increased, so did the coercive field of the films, which was found to be due to the formation of polycrystalline grains, with random texture. The grain size increased with increasing annealing temperature, while the Curie Temperature and magnetostriction constant decreased. For the saturation induction, there was a large decrease between the 100 °C annealed film and the 250 °C annealed film, due to the film morphology changing from amorphous to polycrystalline. Thus there is strong correlation between the crystallinity and the soft magnetic behaviour of FeGaSiB films.