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Aluminum nitride (AlN) usually exhibits a microstructure consisting of equiaxed grains. However, these grains do not allow for the positive control of the microstructure and mechanical properties of AlN. In this work, we attempted to control the microstructure of AlN through the addition of whiskers and investigated the relationship between the microstructure and mechanical properties. The sintered AlN showed a microstructure consisting of large, rod-like grains, which originated from the whiskers. Furthermore, the microstructure varied with the size and amount of whiskers added, and so did the flexural strength and fracture toughness. The flexural strength and fracture toughness of the whisker-added AlN samples ranged from 197 to 267 MPa and 5.4 to 6.4 MPa m1/2, respectively. In contrast, those of AlN without whiskers were 336 MPa and 3.1 MPa m1/2, respectively.

We previously reported that giant crystal enantiomeric excess (CEE) can be obtained when sodium chlorate (NaClO3) chiral crystallization from a solution is induced by the excitation of localized su...

To design a time-dependent control recipe which can ensure consistently suitable growth conditions in an unsteady growth system with dynamic environmental changes, an adaptive control method based on high-speed machine learning prediction models was proposed and applied to the solution growth of SiC crystals. This approach comprised three parts, namely, a quasi-unsteady computational fluid dynamics (CFD) model for thermal and flow field simulation, machine learning models for approximating the simulation results and giving instant prediction, and an optimization algorithm for searching the optimal growth conditions. First, the evolution of the flow, temperature and carbon concentration fields over 50 h unsteady growth following an original recipe with fixed control parameters was analyzed by CFD simulation. Then, adaptive control was applied to design a time-dependent growth process with a 100-timestep sequence. The hybrid of machine learning models and CFD simulation accelerated the entire design and optimization process by 300 times, compared with CFD simulations alone. The adaptive control facilitated superior performance compared with the fixed recipe, where the single SiC crystal thickness increased by ∼30% and the growth interface was more uniform. Further, crucible dissolution and polycrystal precipitation were suppressed by ∼50%, enabling longer growth time and more stable growth. It is the first time that the importance of adaptive control during long-term SiC solution growth is discussed, and the method proposed in this study demonstrated the potential for real-time optimization in the future.

In the design of a crystal growth system, the ability to efficiently regulate intertwined geometrical parameters is crucial for its successful development and commercialization. However, the traditional experimental and computational methods consume tremendous amounts of time and resources. To address this problem, a machine learning approach was developed in this study to accelerate the geometry optimization process. It was found that the combination of machine learning with a genetic algorithm could generate various possible solutions through a global search at a relatively high speed, which lie outside the solution range of the experimental optimization methods that are currently used. By applying this technique, an optimal geometrical design was obtained for a 150 mm top-seed solution growth system, indicating that the proposed method represents an innovative and attractive strategy for the development of crystal growth systems with superior characteristics.

The control of crystallization not only impacts the production of functional crystalline materials or pharmaceuticals but also provides profit to investigate the fundamental mechanism of crystalliz...

Li metal anodes are plagued by low coulombic efficiency due to their interfacial instability. Many approaches were proposed to cope with this problem; however, little attention has been given to the current collector of Li anodes. In this study, we investigate the crystal orientation dependence of the cycling stability of Li anodes on single-crystal Cu(111), (101), and (001) and polycrystalline Cu current collectors. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) show that (111) and (001) achieved high current efficiency and low interfacial resistance, while (101) and polycrystalline Cu exhibited low cyclabilities. X-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES) analysis revealed that the thickness of the solid electrolyte interphase (SEI) varies with the Cu crystal orientation, and the SEI is the thinnest on the single-crystal Cu(111). This tendency can be explained by the orientation dependence of the surface energy of Cu, which corresponds to the chemical activity of the surfaces. Our result advocates the importance of considering Cu orientation for interfacial engineering of Li metal anodes.

Thermal conduction was investigated in titanium-chromium oxide natural superlattices with an ordered arrangement of crystallographic shear planes, which were nearly pristine interfaces for thermal phonons. Thermal conductivity exhibited a clear maximum value at around 50 K, which indicates interface roughness as small as 20 pm based on the calculation of thermal conductivity by a modified Debye-Callaway model. With an ordered arrangement of nearly pristine interfaces at nanoscale, the wave contribution to phonon transport was estimated to be predominant at lower temperatures (less than 100 K) and as much as 30 % even at room temperature. We found that the interface roughness greatly influences the thermal conduction in natural superlattice systems, which may lead to variations in the reported value of thermal conductivity. We have demonstrated that the wave contribution of thermal conductivity become apparent in bulk crystal having nano scale periodic structure with pristine interfaces.

A three-dimensional numerical simulation study has been carried for the Top-Seeded Solution Growth (TSSG) process of single crystal SiC. During growth, Marangoni and forced convections (flows) develop in the growth solution due to free surface tension gradient and seed rotation, respectively. Relative contributions of such flows were numerically examined. Since supersaturation in the growth solution near the crystal surface is a key factor affecting the stability of crystal morphology, our study has focused on the influence of these flows on the distributions of spatial and temporal flow velocity and solution supersaturation. Simulation results show that the contribution of Marangoni convection gives rise to the development of spoke-like flow patterns and a downward flow below the seed crystal that leads to a non-uniform supersaturation distribution at the center and at the edge near the seed crystal surface. Additionally, a characteristic frequency associated with the supersaturation oscillation appears to be due to the Marangoni instability. Results also show that, although the forced convection induced by seed rotation may weaken such supersaturation non-uniformity in the center and suppress the development of such spoke-like patterns, its effect near the edge is not significant.

Previous numerical simulations carried out for the TSSG process of SiC crystal have shown that the downward flow developing in the melt under the seed crystal induces radial growth rate non-uniformity. In order to minimize the adverse effect of this flow, an adjoint-based sensitivity analysis was performed and an optimal crucible temperature condition was determined. The analysis at the initial state showed that the middle region of the crucible sidewall is the most sensitive region for the optimization of the objective functional. At the optimized state, growth rate uniformity is significantly improved without any reduction in growth rate. Furthermore, this optimized state is stable within a range of crucible temperature perturbations.

A 4H-SiC 4° off-wafer fabricated from a bulk crystal grown using the solution method has high quality with extremely low-density threading screw (TSD) and basal plane (BPD) dislocations. For application to electronic devices, we formed an epitaxial layer on the solution-method-prepared wafer via chemical vapor deposition and evaluated the BPD in the epitaxial layer using synchrotron X-ray topography and molten KOH etching. The BPD density of the epitaxial layer formed on the solution-grown crystals was extremely low. Bulk crystals fabricated as wafers by the solution method are expected to be applied to high-voltage bipolar devices that do not suffer from degradation of forward characteristics.

Optical trapping-induced crystallization (OTIC) has provided significant impacts on the control of crystallization. Although consecutive research studies have implied that the optical potential for...

We demonstrate that plasmonic trapping can control crystallization of acetaminophen from its aqueous solution. Irradiation of a focused continuous-wave near-infrared laser to a plasmonic Au nanolattice supporting a thin film of a saturated solution allowed acetaminophen molecules to crystallize in annular distribution with the center of the focal spot. The annularly distributed crystals can be spatially manipulated by changing the position of the laser focal spot. The annular pattern is rationalized by competition between electrical field gradient force as an attractive force to the focal spot and thermophoretic force as a repulsive force. It is also found that, upon stopping the laser irradiation, the crystals first transformed to highly concentrated droplets rather than directly dissolving to the solution. Relaxation of the droplets by self-diffusion to the solution followed to the crystal/droplet transformation. These two-step dissociation dynamics indicate that not only plasmonic trapping of the molec...

The conversion of threading screw dislocations (TSDs) to defects on basal planes during SiC solution growth caused by advancing macrosteps is a key factor for high-quality crystal growth. To study this effect for large growth areas, a 2 inch 4H-SiC crystal was grown on the C face of a 1° off-axis seed crystal using the top-seeded solution growth method. Step-flow growth towards the [110] direction occurred over the entire growth surface. Inhomogeneities in the surface morphology were observed. The step height increased monotonically along the step-flow direction. The non-uniform step height resulted in a spatial distribution of TSD conversion. Little conversion took place in the upstream area of step flow with small steps. In contrast, conversion happened frequently in the centre of the crystal and in the downstream area, with a conversion ratio as high as 80% in these areas. According to a numerical simulation, the solution flow direction was outward near the growth surface. The solution flow affected the steps adversely in the upstream and downstream areas, resulting in a spatial distribution of step height and TSD conversion ratio. This study revealed that the solution flow direction is a key factor for the growth of large high-quality crystals.

We investigate the intensity interference between pairs of electrons using a spin-polarized electron beam having a high polarization and a narrow energy width. We observe spin-dependent antibunching on the basis of coincident counts of electron pairs performed with a spin-polarized transmission electron microscope, which could control the spin-polarization without any changes in the electron optics. The experimental results show that the time correlation was only affected by the spin polarization, demonstrating that the antibunching is associated with fermionic statistics. The coherent spin-polarized electron beam facilitates the extraction of intrinsic quantum interference.

Recently, quality of free-standing gallium nitride (GaN) substrate has been improved. For example, single crystal GaN substrate with threading dislocation density (TDD) of 104 cm-2 is available. Meanwhile, there is a progress in observation method of dislocation in GaN. In addition to conventional etch pits method, X-ray topography, and CL observation, recently, multiphoton PL microscope which can observe dislocation three-dimensionally is actively being carried out. However, these methods are not simple observation because these are destructive observation or require a lot of labor and time. In this study, we attempted crystal evaluation of GaN with a newly developed birefringence microscope, which can be used with the same convenience as the conventional optical microscope. GaN is a material having birefringence, so that strain around dislocation can be seen with birefringence microscope as retardation[1]. Therefore, we tried observing commercially available GaN substrate with TDD of 106, 105, 104 cm-2. As results, an useless random contrast image was obtained with the substrate with TDD of 106 cm-2. With the substrate with TDD of 105 cm-2, distinctive contrasts that seem to coincide with the dislocation cores were obtained, but it was an unclear image. On the other hand, with the substrate with TDD of 104 cm-2, we could obtain contrasts that seems to be independent dislocation cores and surrounding strain field. It seems that the strain field of each dislocation overlapped because the distance between dislocations was too close in substrate with TDD of 106 and 105 cm-2. For the substrate with TDD of 104 cm-2, comparison with the X-ray topography image taken at BL8S2 of Aichi SR was carried out, and it was confirmed that the dislocation was exist at the position of the contrast in the image taken by birefringence microscope. However, we also found that there is no contrast at some dislocation positions. This is considered to be caused by that no strain occurs in the planar direction when the threading dislocation is pure screw dislocation. In some of the dislocations, butterfly type contrast which is theoretically expected to appear around edge dislocation was observed. We consider that this novel birefringent method is very useful method to discriminate edge component and its direction of Burgers vector, and also to discriminate screw dislocation in combination with other methods.

The contrast features of synchrotron X-ray topographic images of screw-type basal plane dislocations (BPDs) in on-axis 4H-SiC wafers have been studied. Screw BPD images are categorized into two types: one exhibiting a white line bordered by black lines and the other a pure black line contrast. Similar images for off-axis specimens and the corresponding ray-tracing simulations demonstrate that these contrasts can be attributed to the depth of the screw BPDs below the crystal surface. The correlation of the contrast features between simulations and the screw BPD topography images can be used to estimate the depth.

We have developed a reinforcement learning (RL) model to control the melt flow in the radio frequency (RF) top-seeded solution growth (TSSG) process for growing more uniform SiC crystals with a higher growth rate. In the study, the electromagnetic field (EM) strength is controlled by the RL model to weaken the influence of Marangoni convection. The RL model is trained through a two-dimensional (2D) numerical simulation of the TSSG process. As a result, the growth rate under the control of the RL model is improved significantly. The optimized RF-coil parameters based on the control strategy for the 2D melt flow are used in a three-dimensional (3D) numerical simulation for model validation, which predicts a higher and more uniform growth rate. It is shown that the present RL model can significantly reduce the development cost and offers a useful means of finding the optimal RF-coil parameters.

We observed the behavior of double Shockley stacking faults (DSFs) in 4H-SiC crystals with nitrogen concentrations of 1.0×1019-2.6×1019 cm−3 over an extensive temperature range (1380-1910 K) by in-situ synchrotron X-ray topography. For a nitrogen concentration of 2.6×1019 cm−3, the expansion velocity of the DSFs exponentially increased with temperature for temperatures in the range from 1370 to 1650 K. In contrast, at a nitrogen concentration of 1.0×1019 cm−3, this velocity decreased above 1610 K and the DSFs shrank above 1730 K. The DSF energy, considered to be the driving force for DSF expansion, was quantitatively estimated from the radius of the curvature of bowed-out partial dislocations (PDs) pinned by threading screw dislocations (TSDs), showing a positive temperature dependence and lying in the range from −0.6 to 0.8 mJ/m2 at a nitrogen concentration of 1.0×1019 cm−3. The DSF expansion and shrinkage behavior can be understood by the simple temperature and nitrogen concentration dependence of the DSF energy.

Evaporation of the volatile solute from the liquid phase is a common problem for the crystal grown from solution, especially for the growth under high temperature. In the present study, a numerical model was constructed to quantitatively investigate the evaporation process in crystal growth. This model was applied in top-seeded solution growth (TSSG) of SiC crystal to simulate the transport of aluminum (Al), which is important for crystal surface morphology but easy to vaporize. The transport path of Al in the growth system was determined by analyzing the possible reactions on different boundaries. Accordingly, an improved structure was proposed to suppress the evaporation loss of Al during long-term growth, and was compared with the original case both numerically and experimentally. The simulation results showed that the improved structure could effectively decrease the Al loss by over 70%, and meanwhile had almost no influence on the thermal and flow environment in the solution. For the experimental results, the improved case presented much lower spontaneous nucleation possibilities and higher step height on the crystal surface, which matched well with the features of high Al addition in literatures. Therefore, the improved structure proposed in the present study was proven to be effective to enhance the composition stability of solution during long-term SiC solution growth, and this numerical method could be applied in the growth of other crystals facing the similar problem.

We have investigated thermal conduction in bulk titanium oxide natural superlattices with crystallographic shear (CS) structures, in which dense planar faults are introduced with different periodicities, prepared by reductive annealing of rutile TiO2 and crystal growth by the floating zone method. High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) revealed that (132)rutile and (121)rutile CS planes with interspacings of 2.7 and 1.0 nm were introduced in the mother rutile structure. Time-domain thermoreflectance (TDTR) revealed that the thermal conductivity decreased by the introduction of CS planes, but that the decrease is not monotonic with increasing density of CS planes. Calculation of the thermal conductivity and the mean free path for phonons revealed that a crossover from incoherent to coherent thermal conduction took place, and coherent interfaces with nanoscale periodicity were formed as thermodynamically stable phases in bulk titanium oxide natural superlattices.

For solution growth of silicon carbide, it is significant to understanding the evolutionary mechanism of step bunching. This study infers that solute’s incorporation into steps and transport together determines step bunching progress. The occurrence of step bunching is due to the depletion of solute in the region with high step density, caused by a high step kinetic coefficient. On the other hand, by promoting the transport of the solute in the solution, the step speed becomes uniform, thereby the step bunching can be prevented. Furthermore, we proposed a non-dimensional Damkohler number for crystal growth in step-flow mode. It correlates incorporation rates with bulk diffusion rates and can build a phase map of growth rates and step bunching stability. Several solvents are located in the phase map, demonstrating the possible usage of the phase map as a pointer for solvent designing.

The improvement mechanism of sputtered AlN films by high temperature annealing in nitrogen ambient has been investigated. Sputtered AlN films were annealed at 1100–1700 °C and their microstructures were observed by scanning transmission electron microscopy. A two-layer structure consisting of columnar domains with different sizes was observed in the as-grown sputtered AlN films. The first layer with 10 nm thickness located at the AlN/sapphire interface, comprised columnar domains with diameters of nm order and was compressively strained owing to the lattice mismatch between AlN and sapphire. The diameter of columnar domains in the other layer was approximately 25 nm. The columnar domains split into irregularly shaped and coalesced at an elevated annealing temperature, resulting in improved crystal quality. When the annealing temperature was 1700 °C, the domain boundaries in AlN films were almost annihilated and the full width at half maximum of the (0 0 0 2)- and (1 0 −1 2)-plane X-ray rocking curves were improved to as low as 49 and 310 arcsec, respectively. The polarity switched from N-polar to Al-polar after about 4–10 AlN layers in the growth direction. The oxygen element content slightly increased at the polarity inversion boundary, which may have caused the polarity to switch.

A set of numerical simulations were conducted to investigate the effect of applied static magnetic field on the convective flows and carbon concentration in the Top-Seeded Solution Growth (TSSG) process of SiC. The applied magnetic field is considered as vertical and cusp in an axisymmetric configuration. Seed rotation was also included in the simulations. The numerical results show that the applications of VMF (vertical magnetic field) and weak CMF (cusp magnetic field) are effective in suppressing Marangoni convection. The application of VMF slows down growth rate but reduces the variation of growth rate towards a more constant rate. Seed rotation appears to be not beneficial in the presence of CMF alone. However, the combined effect of seed rotation with VMF enhances growth and makes the growth rate variation smaller.

The explosive increase in the normal growth rates for dislocation free tetragonal hen egg white (HEW) lysozyme crystals was observed from the point of (C – Ceq), corresponding to a high supersatura...

We have found that large chiral symmetry breaking in chiral crystallization can be achieved by irradiating a several milliwatts focused laser to a plasmonic nanolattice immersed in a stagnant NaClO3 saturated aqueous solution. Several hundreds of chiral crystals with the same handedness showed up in the solution after the laser irradiation in contrast to spontaneous crystallization. In situ microscopic observation for the early stage of the crystallization in the vicinity of the focal spot revealed that microbubble generation followed by large supersaturation increase, in which supersaturation reaches 360%, promotes several numbers of crystal nucleation in the vicinity of the bubble as “mother” crystal. The generation of the microbubble induced Marangoni convection, the velocity of which reaches several hundreds of micrometers per second, crushing the first appearing chiral crystal into pieces by microfluidic shear. Namely, secondary nucleation caused by microfluidic shear amplified the number of “daughte...