Bulk Growth Research Articles

Investigation of the Growth Kinetics of SiC Crystals during Physical Vapor Transport Growth by the Application of In Situ 3D Computed Tomography Visualization

Computed tomography using X‐rays is applied during the bulk growth of silicon carbide (SiC) to investigate growth kinetics in situ during the physical vapor transport process. In addition to the standard SiC source material, in particular, a pure solid source SiC block is used. It is found that the growth rate is lowered as the sublimation of gaseous species is limited to the top part of the solid source. The morphological changes in the source area during growth differ significantly compared with the process when conventional powder is used. The formation of multiple growth centers on the surface of the seed is monitored in situ with a computed tomography system. In a series of experiments, the influence of the supersaturation on the growth is examined. The in situ computed tomography shows that the curvature of the growth interface is stronger influenced by the thermal field at higher pressures. A high supersaturation leads to the formation of rather smooth surface morphologies, whereas the formation of large steps on the surface is induced at lower supersaturations.

Catalytic Metal Nanoparticles Embedded in Conductive Metal-Organic Frameworks for Chemiresistors: Highly Active and Conductive Porous Materials.

Conductive porous materials having a high surface reactivity offer great promise for a broad range of applications. However, a general and scalable synthesis of such materials remains challenging. In this work, the facile synthesis of catalytic metal nanoparticles (NPs) embedded in 2D metal–organic frameworks (MOFs) is reported as highly active and conductive porous materials. After the assembly of 2D conductive MOFs (C‐MOFs), i.e., Cu3(hexahydroxytriphenylene)2 [Cu3(HHTP)2], Pd or Pt NPs are functionalized within the cavities of C‐MOFs by infiltration of metal ions and subsequent reduction. The unique structure of Cu3(HHTP)2 with a cavity size of 2 nm confines the bulk growth of metal NPs, resulting in ultra‐small (≈2 nm) and well‐dispersed metal NPs loaded in 2D C‐MOFs. The Pd or Pt NPs‐loaded Cu3(HHTP)2 exhibits remarkably improved NO2 sensing performance at room temperature due to the high reactivity of catalytic metal NPs and the high porosity of C‐MOFs. The catalytic effect of Pd and Pt NPs on NO2 sensing of Cu3(HHTP)2, in terms of reaction rate kinetics and activation energy, is demonstrated.

Relationship between dynamic instability of individual microtubules and flux of subunits into and out of polymer.

Behaviors of dynamic polymers such as microtubules and actin are frequently assessed at one or both of the following scales: (a) net assembly or disassembly of bulk polymer, (b) growth and shortening of individual filaments. Previous work has derived various forms of an equation to relate the rate of change in bulk polymer mass (i.e., flux of subunits into and out of polymer, often abbreviated as "J") to individual filament behaviors. However, these versions of the "J equation" differ in the variables used to quantify individual filament behavior, which correspond to different experimental approaches. For example, some variants of the J equation use dynamic instability parameters, obtained by following particular individual filaments for long periods of time. Another form of the equation uses measurements from many individuals followed over short time steps. We use a combination of derivations and computer simulations that mimic experiments to (a) relate the various forms of the J equation to each other, (b) determine conditions under which these J equation forms are and are not equivalent, and (c) identify aspects of the measurements that can affect the accuracy of each form of the J equation. Improved understanding of the J equation and its connections to experimentally measurable quantities will contribute to efforts to build a multiscale understanding of steady-state polymer behavior.

(Invited) Recent Progress in 4H-SiC CVD Growth for High-Voltage Power Devices

This paper reports recent progress in 4H-SiC CVD growth, aiming to obtain high-quality epilayers and bulk crystals with a high productivity for high-voltage power devices. The production of 150 mm-diameter 4H-SiC wafers has become the industry standard and the practical use of 4H-SiC unipolar devices such as Schottky barrier diodes and MOSFETS with a blocking voltage from 600 to 3,300 V started in various fields, including power conditioners for photo voltaic systems, air conditioning equipment, power supplies for IT and manufacturing industries, and inverters for electric cars and trains. Although bipolar devices such as PiN diodes and IGBTs may be more suitable for very high-voltage classes over ten kilo volts, the practical application of the bipolar devices is hindered because of the degradation phenomenon (“bipolar degradation”) in which forward on-state voltage of the devices increases after current stress applied due to the expansion of stacking faults in the crystal during current conduction. Based on the current understanding on the bipolar degradation phenomenon, basal plane dislocations (BPDs) are responsible for the nuclei of expanding single Shockley stacking faults (1SSFs) and the expansion of the 1SSFs is activated by minority carrier injection exceeding a certain level. Although epilayers with low-density BPDs (< 0.1 cm-2) were obtained by enhancing the conversion of BPDs in the substrate to threading edge dislocations (TEDs) at the beginning of epitaxial growth, the expansion of 1SSFs from the BPD segments below the conversion points was confirmed in the high current density operation [1]. This can be understood due to the injection of minority carriers into a region below the BPD-TED conversion points located near the epilayer and substrate interface or in the substrate where high-density BPDs (102 cm-2) are existing. Meanwhile, inserting a buffer layer with a short carrier lifetime between the substrate and drift epilayer has been proposed to prevent minority carrier injection into a region where BPDs remain. At this time, a thinner buffer layer is more preferable and a technique to obtain shorter carrier lifetimes compatible with the practical epitaxial growth and device fabrication processes will be promising. High nitrogen (N) doping yielding high-density free electrons has been shown as a promising method to enhance direct and Auger recombination and reduce carrier lifetimes [2, 3]. Short carrier lifetimes of 66-120 ns were obtained at RT-250 ºC for an epilayer doped with N at 5×1018 cm-3. We also attempted intentional vanadium (V) doping during epitaxial growth of a buffer layer to realize much shorter carrier lifetimes. The highly N+V-doped epilayer (N=5×1018, V=7×1014 cm-3) shows very short carrier lifetimes of ~20 ns or less at RT-250 ºC. Inserting a N- or N+V-doped buffer layer between a substrate and n- drift epilayer in the PiN diodes was confirmed to be effective to prevent the formation of 1SSFs from BPDs in the substrate. Besides continuous improvements in the quality of wafers, the strong demand to cut the cost of 4H-SiC devices from the application side may request to promote wafer productivity. Meanwhile, the development of 200 mm-diameter wafers has been demonstrated by sublimation growth in past conferences. Apart from enlarging wafer diameter, massively enhancing the growth rate or ingot length may pose the ultimate challenge in 4H-SiC bulk growth and trials featuring solution and high-temperature CVD growth methods are now underway. For high-temperature CVD growth, we attempted fast bulk crystal growth of 4H-SiC at a seed temperature of 2350-2550ºC using an H2-SiH4-C3H8-N2 gas system in a vertical CVD reactor [4]. By adjusting process parameters to enhance growth rates and improve crystal quality, fast 4H-SiC crystal growth at a growth rate exceeding 3 mm/h was achieved while maintaining the low dislocation density (threading screw dislocations: ~2.5×102 cm-2) of the seed crystal. A part of this work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Next-generation power electronics/Consistent R&D of next-generation SiC power electronics” (funding agency: NEDO). [1] A. Tanaka et al., J. Appl. Phys. 119, 095711 (2016), [2] T. Tawara et al., J. Appl. Phys. 120, 115101 (2016), [3] T. Tawara et al., J. Appl. Phys. 123, 025707 (2018), [4] N. Hoshino et al., J. Cryst. Growth 478, 9 (2017).

Impact of Varying Parameters on the Temperature Gradients in 100 mm Silicon Carbide Bulk Growth in a Computer Simulation Validated by Experimental Results

AbstractThe investigation of the interplay of experimental crystal growth runs and computer simulations of the physical vapor transport (PVT) growth process of silicon carbide (SiC) strongly supports the development of the growth technology toward larger crystalline diameters. To apply computer‐aided designs of the growth process, a material database is developed that enables quantitative calculations of the temperature distribution inside the growth setup. The model utilizing COMSOL Multiphysics is validated by experimental means using five 100 mm SiC crystal growth runs, two measurement runs in a 100 mm PVT setup, and one SiC crystal growth run in a 75 mm PVT setup, varying in power, pressure, coil position, and reactor geometry. Material data are varied and the impact on the thermal gradients is investigated to trace out critical and noncritical parameters for the accuracy of the simulation. The influence of ambient pressure on the isolation's performance is studied experimentally and this correlation is implemented into the model. Beyond the state of the art, this work presents a modeling approach and handling of material properties for a true scale up of the current 100 mm and 150 mm SiC sublimation growth technologies to 200 mm.

Influence of Morphological Changes in a Source Material on the Growth Interface of 4h-Sic Single Crystals.

In this study, the change of mass distribution in a source material is tracked using an in situ computer tomography (CT) setup during the bulk growth of 4H- silicon carbide (SiC) via physical vapor depostion (PVT). The changing properties of the source material due to recrystallization and densification are evaluated. Laser flash measurement showed that the thermal properties of different regions of the source material change significantly before and after the growth run. The Si-depleted area at the bottom of the crucible is thermally insulating, while the residual SiC source showed increased thermal conductivity compared to the initially charged powder. Ex situ CT measurements revealed a needle-like structure with elongated pores causing anisotropic behavior for the heat conductivity. Models to assess the thermal conductivity are applied in order to calculate the changes in the temperature field in the crucible and the changes in growth kinetics are discussed.

ZnCo2O4/ZnO induced lithium deposition in multi-scaled carbon/nickel frameworks for dendrite-free lithium metal anode

Abstract Lithium metal attracts growing attention as an ideal anode candidate for next generation lithium battery systems owing to its high capacity, low density, and low working potential. However, the volume expansion of the bulk and dendrite growth on the surface of lithium anode limits its practical application. Herein, we fabricate a composite lithium host featuring both multiple scaled structure and lithiophilic property to address obstacles at both aspects of bulk and surface simultaneously. In which, the multiple scaled structure provides void space to accommodate lithium volume change while zinc and cobalt oxides sites derived from Zeolitic Imidazolate Frameworks can react with lithium and form a stable solid electrolyte interphase, leading to a stable cycling of lithium symmetrical cell for more than 500 cycles with voltage hysteresis of only 88 mV at 2 mA cm−2 and 5 mAh cm−2. Moreover, full cells paired with LiFePO4 cathode can realize 500 cycles with 99.2% capacity retention, showing great potential for practical applications. The excellent electrochemical performance of the composite lithium anode proves the effectiveness of our anode design with multiple scaled structure and lithiophilic feature, which can be also expanded to other metal anodes for batteries.

Limitations during Vapor Phase Growth of Bulk (100) 3C-SiC Using 3C-SiC-on-SiC Seeding Stacks.

The growth of 3C-SiC shows technological challenges, such as high supersaturation, a silicon-rich gas phase and a high vertical temperature gradient. We have developed a transfer method creating high-quality 3C-SiC-on-SiC (100) seeding stacks, suitable for use in sublimation “sandwich” epitaxy (SE). This work presents simulation data on the change of supersaturation and the temperature gradient between source and seed for the bulk growth. A series of growth runs on increased source to seed distances was characterized by XRD and Raman spectroscopy. Results show a decrease in quality in terms of single-crystallinity with a decrease in supersaturation. Morphology analysis of as-grown material indicates an increasing protrusion dimension with increasing thickness. This effect limits the achievable maximal thickness. Additional polytype inclusions were observed, which began to occur with low supersaturation (S ≤ 0.06) and prolonged growth (increase of carbon gas-species).

Application of Defect Conversion Layer by Solution Growth for Reduction of TSDs in 4H-SiC Bulk Crystals by PVT Growth

We have developed the bulk growth technique to reduce threading screw dislocations (TSDs) by combining solution growth and PVT growth methods. More than 80 % of TSDs in original seed crystals were successfully converted to Frank defects on basal planes by the solution growth on 4° off C-face with Si-5at.% Ti solvent. After PVT growth on the as-grown surface of the conversion layer, TSDs in the original seed were successfully reduced. The presence of micrometer-size macrosteps in the initial stage of PVT growth is important to continue to propagate the converted Frank defects on basal planes during PVT bulk growth.

Photonics with hexagonal boron nitride

For more than seven decades, hexagonal boron nitride (hBN) has been employed as an inert, thermally stable engineering ceramic; since 2010, it has also been used as the optimal substrate for graphene in nanoelectronic and optoelectronic devices. Recent research has revealed that hBN exhibits a unique combination of optical properties that enable novel (nano)photonic functionalities. Specifically, hBN is a natural hyperbolic material in the mid-IR range, in which photonic material options are sparse. Furthermore, hBN hosts defects that can be engineered to obtain room-temperature, single-photon emission; exhibits strong second-order nonlinearities with broad implications for practical devices; and is a wide-bandgap semiconductor well suited for deep UV emitters and detectors. Inspired by these promising attributes, research on the properties of hBN and the development of large-area bulk and thin-film growth techniques has dramatically expanded. This Review offers a snapshot of current research exploring the properties underlying the use of hBN for future photonics functionalities and potential applications, and covers some of the remaining obstacles. Hexagonal boron nitride (hBN) is highly sought after for mid-IR nanophotonics, nonlinear and quantum optics, and as an efficient UV emitter. This Review surveys its fundamental physical properties, applications and synthesis.

Control of temperature distribution to suppress macro-defects in solution growth of 4H-SiC crystals

We investigated the surface roughening in 4H-SiC solution growth from the following two aspects: the roughening of the seed surface before seeding and the roughening caused by the adhesion of SiC particles during growth. First, we investigated the morphological changes of the seed surface before and after the melt-back process. The seed surface just before seeding was covered with macrosteps and 6H-SiC hillocks with a height of several micrometers. This surface roughening was caused by condensation of droplets of vaporized solvent on the seed surface. We found that the 6H-SiC hillocks were the origins of trench defects. The melt-back process completely removed the hillocks and produced a smooth surface adequate for successive bulk growth. Second, we investigated surface roughening caused by the formation of SiC particles. The adhesion of SiC particles on the growth surface introduced trench defects and polytype inclusions. The adhesion of SiC particles was suppressed by controlling the distribution of carbon supersaturation in the solvent.

Self-Limiting Growth of Two-Dimensional Palladium between Graphene Oxide Layers.

The ability of different materials to display self-limiting growth has recently attracted an enormous amount of attention because of the importance of nanoscale materials in applications for catalysis, energy conversion, (opto)electronics, and so forth. Here, we show that the electrochemical deposition of palladium (Pd) between graphene oxide (GO) sheets result in the self-limiting growth of 5-nm-thick Pd nanosheets. The self-limiting growth is found to be a consequence of the strong interaction of Pd with the confining GO sheets, which results in the bulk growth of Pd being energetically unfavorable for larger thicknesses. Furthermore, we have successfully carried out liquid exfoliation of the resulting Pd-GO laminates to isolate Pd nanosheets and have demonstrated their high efficiency in continuous flow catalysis and electrocatalysis.

Study on Carbon Particle Inclusions during 4H-SiC Growth by Using Physical Vapor Transport System

A study on carbon particle inclusions during 4H-SiC bulk growth is presented. Special attentions were paid to design of graphite growth compartment, size of SiC source materials, and process of seed crystal handling. It was found that common carbon inclusions with size of 30μm or less were attributed to carbon particles from graphitized SiC source. Less common carbon inclusions with size of over 100μm were also found and were attributed to poor seed crystal mounting process. In order to reduce carbon inclusions, several experiments were designed by using a NAURA Advanced Physical Vapor Transport (PVT) System APS130G. A graphite plate separator was inserted into the growth compartment to prevent the carbon particles from transporting to the growth surface. SiC powder materials with larger diameters were selected to reduce source graphitization. Additional clean process was performed to remove carbon particle residuals on graphite parts during seed mounting. The results showed significant improvement of carbon inclusion problems in SiC ingots and thus high-quality SiC wafers were made successfully.

Recent development of wide bandgap semiconductor SiC substrates and device

As a key representative material for the third-generation wide bandgap semiconductors, silicon carbide (SiC) is a promising wide band gap semiconductor material and can be used for the fabrication of high-power and high-frequency electronics, due to its superior physical properties，such as high thermal conductivity, wide band gap and high critical breakdown field. In recent years, bulk growth of SiC single crystals and the fabrication of devices have made significant progress. The paper introduces the growth techniques for SiC bulk and presents the relationship between the on-state resistance and voltage or laser energy. It also analyses the failure of devices.

Glioblastoma Unique Features Drive the Ways for Innovative Therapies in the Trunk-branch Era

Glioblastoma multiforme is a solid tumor with particular aspects due to its organ of origin and its development modalities. The brain is very sensitive to oxygen and glucose deprivation and it is the only organ that cannot be either transplanted or entirely removed. Furthermore, many clues and recent indirect experimental evidence indicate that the micro-infiltration of the whole brain parenchyma occurs in very early stages of tumor bulk growth or likely even before. As a consequence, the primary glioblastoma (IDH-wildtype, WHO 2016) is the only tumor where the malignant (i.e. distantly infiltrating the organ of origin) and deadly (i.e. leading cause to patient’s death) phases coincide and overlap in one single phase of its natural history. To date, the prognosis of optimally treated glioblastoma patients remains dismal despite recent fundamental progress in neurosurgical techniques which are enabling better maximal safe resection and survival outcome. Intratumor variegated heterogeneity of glioblastoma bulk due to trunk-branch evolution and very early micro-infiltration and settlement of neoplastic cells in the entire brain parenchyma are the reasons for resistance to current therapeutic treatments. With the aim of future innovative and effective therapies, this paper deals with the unique glioblastoma features, the appropriate research methods as well as the strategies to follow to overcome current causes of resistance.

Growth of β‐ΒaΒ2O4 Crystals from Solution in LiF–NaF Melt and Study of Phase Equilibria

AbstractPhase equilibria in the ΒaΒ2O4–LiF–NaF section of the quaternary reciprocal system Li, Na, Ba // BO2, F are studied by the solid‐phase synthesis, differential thermal analysis, and visual polythermal analysis techniques. It is shown that this system can be used for the growth of bulk β‐ΒaΒ2O4 crystals.

Glioblastoma Unique Features Drive the Ways for Innovative Therapies in the Trunk-branch Era.

Glioblastoma multiforme is a solid tumor with particular aspects due to its organ of origin and its development modalities. The brain is very sensitive to oxygen and glucose deprivation and it is the only organ that cannot be either transplanted or entirely removed. Furthermore, many clues and recent indirect experimental evidence indicate that the micro-infiltration of the whole brain parenchyma occurs in very early stages of tumor bulk growth or likely even before. As a consequence, the primary glioblastoma (IDH-wildtype, WHO 2016) is the only tumor where the malignant (i.e. distantly infiltrating the organ of origin) and deadly (i.e. leading cause to patient's death) phases coincide and overlap in one single phase of its natural history. To date, the prognosis of optimally treated glioblastoma patients remains dismal despite recent fundamental progress in neurosurgical techniques which are enabling better maximal safe resection and survival outcome. Intratumor variegated heterogeneity of glioblastoma bulk due to trunk-branch evolution and very early micro-infiltration and settlement of neoplastic cells in the entire brain parenchyma are the reasons for resistance to current therapeutic treatments. With the aim of future innovative and effective therapies, this paper deals with the unique glioblastoma features, the appropriate research methods as well as the strategies to follow to overcome current causes of resistance.

Constraining age and volume of lava flow invasions of the Alcantara valley, Etna volcano (Italy). New insights from paleomagnetic dating and 3D magnetic modeling

The Alcantara River marks the northern border of Etna volcano, and along its NE sector, it forms a narrow NW-SE elongated valley that is carved in the Mts. Peloritani sedimentary sequences. The valley has been invaded by several lava flows during the growth of the main bulk of the stratovolcano edifice in the past 60 ky (Ellittico and Mongibello volcanoes). In this paper, we have constrained the age of the main lava flows forming the valley floor and reconstructed the length and volume of the main lava flow that impacted this area through a multidisciplinary approach integrating stratigraphic and aeromagnetic data analysis together with new paleomagnetic and 14C dating. The new age determinations showed that the evolution of the present-day geological and hydrographic setting of the valley was mainly conditioned by a few flank eruptions that occurred in the lower N flank of Etna during the activity of the Mongibello volcano. In particular, between 13.9 and 9.7 ka BP the valley floor was filled by the so-called Alcantara lava flow for its entire length up to the river mouth, reaching a maximum length of about 24 km. Later, two other flank eruptions – producing the so-called Mt. Dolce and Solicchiata lava flows – occurred at 9.1–7.2 and 7.3–7.2 ka BP (respectively), covering the eruptive fissure and the proximal portion of the Alcantara lava field and generated the most recent northward shift of the riverbed. Finally, 3D magnetic modeling allowed us to calculate the total on- and off-shore volume of the Alcantara lava flow as 1 km3, a value comparable to those of the long-lasting historical eruptions of Etna. Conversely, the 24 km length reached by this lava flow is anomalously high when compared to lava flows of the past 15 ky, likely because its emplacement occurred within the valley floor, preventing any possibility of lateral expansion.

Bulk growth and an efficient mid-IR laser of high-quality Er:YSGG crystals

This work reports on the growth of the high quality large size Er:YSGG crystals and their highly efficient 2.8 nm laser.

Synthesis and mixing of complex halide perovskites by solvent-free solid-state methods

Halide perovskites are exciting photoactive semiconductors with exceptional photovoltaic and optoelectronic properties. Literature in this area focuses on solution phase aspects of these materials, for example in inks for solar cells, or the growth of bulk or nanosized crystals. Critically, varying solute-solvent interactions often cause heavily mixed perovskites to have compositions that strongly deviate from their synthetic loading. In contrast, complex halide perovskites prepared by solid-state methods in the absence of solvents display much more predictable compositions and significantly suppressed phase segregation. Further, because they generate less waste, solvent-free methods are often ‘greener’ and more industrially scalable. Herein, we review the solvent-free methods used to synthesize single composition ‘parent’ and heavily mixed perovskites in the solid-state. We discuss the known mechanisms for ion diffusion involved in these transformations, summarize and contrast their main benefits and features, and review their use in the preparation of mixed-cation and/or mixed-halide perovskites.