Bulk Crystal Growth Research Articles

Colossal grain growth yields single-crystal metal foils by contact-free annealing.

Single-crystal metals have distinctive properties owing to the absence of grain boundaries and strong anisotropy. Commercial single-crystal metals are usually synthesized by bulk crystal growth or by deposition of thin films onto substrates, and they are expensive and small. We prepared extremely large single-crystal metal foils by "contact-free annealing" from commercial polycrystalline foils. The colossal grain growth (up to 32 square centimeters) is achieved by minimizing contact stresses, resulting in a preferred in-plane and out-of-plane crystal orientation, and is driven by surface energy minimization during the rotation of the crystal lattice followed by "consumption" of neighboring grains. Industrial-scale production of single-crystal metal foils is possible as a result of this discovery.

Revealing the role of calcium codoping on optical and scintillation homogeneity in Lu2SiO5:Ce single crystals

The presence of impurities and dopants (or codopants) can influence solute transport at the solid-liquid interface and its stability during bulk crystal growth, which ultimately affect the crystal uniformity. Codoping with Ca2+ can significantly improve the performance of Lu2SiO5:Ce single crystals, well-known scintillators for nuclear medical imaging, albeit at the expense of growth stability due to surface tension reduction. However, an understanding of the correlation between crystal growth properties and radial performance uniformity in Czochralski-grown LSO:Ce, Ca single crystals is still lacking. In this work, we reveal this essential correlation by studying the roles of Ca2+ concentration on the radial distribution of stable Ce3+ ions and oxygen vacancy (VO) related defects as well as solute transport at the solid-liquid interface. Through mapping of optical and scintillation properties across cross-sections of 0.1 at% and 0.4 at% Ca codoped boules and the defect formation energies derived from density functional theory (DFT) calculations, a direct link between Ca2+ ions, stable Ce3+ or Ce4+ ions, and {CaLu + VO} complex defects is established. The light yield enhancement in Ca2+ codoped LSO:Ce single crystals is attributed to the dissociation of spatially correlated Ce ions and oxygen vacancies. Based on the quantitative comparison of photoluminescence (PL) and radioluminescence (RL) emissions from Ce1 (seven-coordinated) and Ce2 (six-coordinated) sites, the preferential occupation of stable Ce4+ is believed to depend on the concentration of stable Ce4+. The underlying causes of symmetry and variation in the radial distribution of optical and scintillation properties, as consequences of Ca2+ distribution, are discussed from the perspective of crystal growth.

Observation of Hybrid MAPbBr3 Perovskite Bulk Crystals Grown by Repeated Crystallizations

In this study, the inverse temperature crystallization method was used to produce bulk crystal CH3NH3PbBr3 (MAPbBr3) perovskite, and repeated crystallization processes were carried out to obtain a larger crystal size and develop a bulk material for application in semiconductor devices. The material and optoelectronic properties of the MAPbBr3 perovskite crystals after the repeated processes were characterized. The X-ray diffraction (XRD) patterns of all samples demonstrated a pure perovskite phase. One strong diffraction peak located at 29.4°, which corresponds to the (200) perovskite plane, was observed after the first growth cycle. The mobilities for the samples after the first, second, and third growth cycles were calculated and resulted to be 0.9, 5.6, and 54.7 cm2/Vs, respectively, according to Mott–Gurney law. A higher mobility after the multiple crystallization processes indicated that the surface states caused by voids in the crystals favored electron transition in the perovskite material.

Dislocation Behavior in Bulk Crystals Grown by TSSG Method

The dislocation behavior during bulk crystal growth on the 4H-SiC (000-1) C-face using the solution method was investigated. A 2 inch wafer with a 4° off angle was fabricated from a bulk crystal grown by the TSSG method, and the dislocations in the crystal were evaluated using synchrotron X-ray topography and TEM observation. From the topograph images, it was found that the TSD density remarkably decreased as the growth progressed. Furthermore, the TEM observation suggested that TSD decreases as the threading dislocations convert to in-plane defects toward the center of the crystal. Conventionally, it was considered that conversion of threading dislocations hardly occurs in solution growth on the C-face. However, it is thought that this phenomenon was not observable because the conversion efficiency is remarkably low. We speculate that dislocations may be transformed by suddenly making macrosteps during bulk growth.

The synergy of modeling and novel experiments for melt crystal growth research

Computational modeling and novel experiments, when performed together, can enable the identification of new, fundamental mechanisms important for the growth of bulk crystals from the melt. In this paper, we present a compelling example of this synergy via the discovery of previously unascertained physical mechanisms that govern the engulfment of silicon carbide particles during the growth of crystalline silicon.

Enhanced spin-ordering temperature in ultrathin FeTe films grown on a topological insulator

We studied the temperature dependence of the diagonal double-stripe spin order in one and two unit cell thick layers of FeTe grown on the topological insulator Bi_2Te_3 via spin-polarized scanning tunneling microscopy. The spin order persists up to temperatures which are higher than the transition temperature reported for bulk Fe_1+yTe with lowest possible excess Fe content y. The enhanced spin order stability is assigned to a strongly decreased y with respect to the lowest values achievable in bulk crystal growth, and effects due to the interface between the FeTe and the topological insulator. The result is relevant for understanding the recent observation of a coexistence of superconducting correlations and spin order in this system.

Nucleation- and Emergence-Limited Growth of Ice from Pores.

Nucleation of ice from vapor on atmospheric aerosols has been attributed to the condensation and freezing of supercooled water in small pores. Here we use wedge pores on mica to directly observe the growth of ice in confinement prior to the growth of bulk crystals. We report a transition in behavior with a decreasing temperature: At low temperatures, the limiting step is not nucleation but a free energy barrier associated with the growth of ice through a narrow pore mouth to become a bulk phase.

Bulk crystal growth and their effective third order nonlinear optical properties of 2-(4-fluorobenzylidene) malononitrile (FBM) single crystal

2-(4-fluorobenzylidene) malononitrile (FBM), an organic third order nonlinear (TONLO) single crystal with the dimensions of 32 × 7 × 11 mm3, has been successfully grown in acetone solution by slow evaporation technique at 35 °C. The crystal system (triclinic), space group (P-1) and crystalline purity of the titular crystal were measured by single crystal and powder X-ray diffraction, respectively. The molecular weight and the multiple functional groups of the FBM material were confirmed through the mass and FT-IR spectral analysis. UV–Vis–NIR spectral study enroles that the FBM crystal exhibits excellent transparency (83%) in the entire visible and near infra-red region with a wide bandgap 2.90 eV. The low dielectric constant (εr) value of FBM crystal is appreciable for microelectronics industry applications. Thermal stability and melting point (130.09 °C) were ascertained by TGA-DSC analysis. The laser-induced surface damage threshold (LDT) value of FBM specimen is found to be 2.14 GW/cm2, it is fairly good compared to other reported NLO crystals. The third - order nonlinear optical character of the FBM crystal was confirmed through the typical single beam Z-scan technique. All these finding authorized that the organic crystal of FBM is favorably suitable for NLO applications.

NeoGrowth silicon: A new high purity, low‐oxygen crystal growth technique for photovoltaic substrates

AbstractSolarWorld has developed a new entrant in the field of crystal growth for silicon photovoltaic substrates. The NeoGrowth technique is a contactless bulk crystal growth method for producing single crystal ingots. NeoGrowth material can be produced at a throughput on par with G5 multicrystalline silicon, but with p‐type as‐grown minority carrier lifetimes exceeding 600 microseconds for a 1.5‐ohm cm resistivity. The silicon has low oxygen, and light‐induced degradation is measured at 0.5% to 0.7% in passivated emitter rear contact–based modules. In the first report of results from this technique, p‐type resistivity can be managed within a range of 1.5 to 2.0 ohm cm over the entire ingot. Dislocation density is shown to be typically in the 104 to 105/cm2 range but can be managed down to even lower levels. After cell processing steps, minority carrier lifetime can exceed 1.5 milliseconds for p‐type material and cell efficiencies on industrial cells range up to 20.9%.

A review of Ga2O3 materials, processing, and devices

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (ε) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

Experimental and quantum chemical studies on N[sbnd]H[sbnd][sbnd][sbnd]O hydrogen bonded helical chain type Morpholinium 2-chloro-4-nitrobenzoate: A phasematchable organic nonlinear optical material

We report on the studies performed on Morpholinium 2-chloro-4-nitrobenzoate (M2C4N), a second order nonlinear optical (NLO) material, which has been proved to crystallize with chiral structure as compared to its other two variants. The synthesized powder was studied for its composition, crystalline phase and NLO efficiency and phasematchability. The solubility and the metastable zone width (MSZW) of the title compound were measured for the growth of bulk crystals of M2C4N. A smoky pattern observed in the middle region of the crystals that could prevent it from optical applications was greatly reduced, through suitable modifications in the growth process. The optical properties such as luminescence and laser damage threshold resistance were studied for the bulk crystals whereas the molecular vibrations of M2C4N were studied through Fourier transform infrared (FTIR) and FT-Raman spectral analysis using the polycrystalline powders derived from the single crystals. In addition, quantum chemical studies on M2C4N molecule were performed by using density functional theory (DFT) at the B3LYP/6-311++G (d, p) basis set. These studies showed that the M2C4N is a phasematchable NLO crystal and could be used for device fabrication.

Bulk crystal growth, spectral, optical, thermal, electrical and third-order NLO properties of benzylidene malononitrile derivative single crystal: a promising material for nonlinear optical device applications

Bulk organic intramolecular charge transfer nonlinear optical single crystal of 2-(2,4-dimethoxybenzylidene) malononitrile (DMM) with the sizes of 24 × 18 × 13 mm3 have been successfully grown by slow evaporation solution growth technique at 35 °C using acetone as the solvent. The lattice parameters of the grown DMM crystal was evaluated by single crystal X-ray diffraction analysis. The optical transmittance (T) data was taken from the well-polished crystal of DMM, and cut-off wavelength (λ = 483 nm) was identified by UV–Vis spectral studies. Thermal stability and melting point (145.78 °C) were studied with TGA–DSC analysis. The low value of the dielectric constant (er) of DMM suggests that the crystal can be used in the microelectronics industry. The laser-induced damage threshold experiment shows that the grown DMM bulk crystal possess an excellent resistance to laser radiation with a high threshold up to 1.75 GW/cm2, much larger than those of several known organic and inorganic NLO materials. The DMM crystal exhibits positive optical non-linearity and reverse saturation absorption. It also exhibited a nonlinear refractive index (n2) in the order of 10−11 m2/W, a nonlinear absorption coefficient (β) in the order of 10−5 m/W, third order non-linear susceptibility (χ(3)) in the order of 10−5 esu and a second-order molecular hyperpolarizability (γ) in the order of 10−33 esu. All the above results indicate that DMM has a potential application as a useful NLO candidate.

Bulk Crystal Growth of Cr-doped MgAl2O4 spinel by Czochralski method and properties characterization

Cr3+-doped MgAl2O4 single crystal with size of Ø30 mm × 70 mm, was firstly successfully grown by the Czochralski method and the effective segregation coefficient of Cr3+ was determined. The X-ray powder pattern was studied, and the X-ray rocking curve indicates a high crystalline quality of the grown crystal. The thermal conductivity was measured from 270 K to 370 K, and the optical absorption measurements in the 250–1000 nm spectral regions was performed at room temperature. Furthermore, emission spectra, excitation spectra and fluorescence decay curve of Cr:MgAl2O4 crystal were also recorded.

(Invited) P-Type 3C-SiC Photocathode for Solar to Hydrogen Energy Conversion

Solar to hydrogen conversion technology attracts attention as renewable energy generation. In general, to perform solar to hydrogen conversion, photocatalytic materials are employed. However, most of photocatalytic materials cannot absorb visible light resulting in low conversion efficiency and degrade during operation. Thus, we need to find new photocatalytic materials which have both high conversion efficiencies and resistance to corrosion. Silicon carbide (SiC) is a chemically stable and photocatalytic semiconductor material. SiC can be grown by different atomic configurations resulting in a large number of polytypes. Among SiC polytypes, 3C-SiC has the smallest band gap (2.3 eV), and thus 3C-SiC is capable to absorb significant part of solar light. In addition, conductive type of SiC can be controlled by doping. Photocatalytic materials with p-type conduction operate as cathode and reduce hydrogen in water. Cathodic operation prevents oxidation of photocatalytic materials. Therefore, p-type 3C-SiC will be an efficient and durable photocatalyst. However, bulk crystal growth of 3C-SiC is very difficult because this polytype is stable only in growth at much lower temperature than sublimation of SiC, which is a typical condition for bulk SiC growth. This difficulty makes 3C-SiC growth only by chemical vapor deposition (CVD) on Si or 4H- or 6H-SiC substrates. Such the heteroepitaxial growth induces large number of structural defects or polytype mixing owing to lattice mismatch or inheritance of the substrate polytype. For our recent 3C-SiC photocathode fabrication, we employed p+-type 4H-SiC substrates with the (0001) Si-face inclined 0.7° to <1120> direction. Growth on the 4H-SiC substrates with quasi on-axis Si-face prevents polytype mixing and p+-type conductivity of the substrate facilitates ohmic contact formation on the substrate side. We performed 3C-SiC growth on the substrates by CVD with silane and propane as source gases and in an Al acceptor doping condition. A photograph of the grown crystal is shown in Fig. 1(a). After the growth, we fabricated ohmic contact on the substrate side of the crystals. The 3C-SiC surface is active surface as the photocathode. In addition, we formed Pt or Pd cocatalysts on the 3C-SiC surface to enhance hydrogen generation by the photocathodes. The schematic of the photocathode structure is shown in Fig. 1(b). We measured photocurrent-potential characteristics for the photocathodes in 1 M H2SO4 aqueous solution using chopped solar simulator light with 100 mW/cm2. The results are shown in Fig. 1(c). For bare surface, cathodic photocurrent was observed below 0.6 V vs SCE, and it reached -1.4 A/cm2 at -1 V vs SCE. On the other hand, with cocatalysts, photocurrents were observed at higher potential, and in the case of with Pt cocatalysts, it was observed from 0.9 V vs SCE. In addition, at -1 V vs SCE, the photocurrent was -2.3 A/cm2. In solar to hydrogen conversion, the Faraday efficiencies of the photocathodes were near to 100% and the photocathodes were not degraded during experiments. Solar to hydrogen conversion efficiency for the photocathode with Pt cocatalysts was 0.52 %. Considering such a high efficiency and possibility of further improvement, SiC is a very promising material for the solar to hydrogen conversion technology. Figure 1

Fast growth of n-type 4H-SiC bulk crystal by gas-source method

Fast growth of n-type 4H-SiC crystals was attempted using a high-temperature gas-source method. High growth rates exceeding 9mm/h were archived at a seed temperature of 2550°C, although the formation of macro-step bunching caused doping fluctuation and voids in the grown crystal. We investigated a trade-off between growth-rate enhancement and macro-step formation and how to improve the trade-off. By controlling the growth conditions, the growth of highly nitrogen-doped 4H-SiC crystals without the doping fluctuation and void formation were accomplished under a high growth rate exceeding 3mm/h, maintaining the density of threading screw dislocations in the same level with the seed crystal. The influence of growth parameters on nitrogen incorporations into grown crystals was also surveyed.

Finding the Next Deep-Ultraviolet Nonlinear Optical Material: NH4B4O6F.

Nonlinear optical materials are essential for the development of solid-state lasers. KBe2BO3F2 (KBBF) is a unique nonlinear optical material for generation of deep-ultraviolet coherent light; however, its industrial application is limited. Here, we report a new material NH4B4O6F, which exhibits a wide deep-ultraviolet transparent range and suitable birefringence that enables frequency doubling below 200 nm. NH4B4O6F possesses large nonlinear coefficients about 2.5 times that of KBBF. In addition, it is easy to grow bulk crystals and does not contain toxic elements.

Strong yellow emission of high-conductivity bulk ZnO single crystals irradiated with high-power gyrotron beam

We report the strong yellow emission of bulk ZnO single crystals irradiated with the high-power gyrotron beam. Hydrothermally grown bulk crystals with high conductivity are irradiated at room temperature with up to 60-W output of a sub-terahertz gyrotron wave source. During gyrotron irradiation, the high-conductivity crystals exhibit intense emissions with a peak of around 2 eV (600 nm) and a longer-wavelength tail. The sample temperatures were also elevated from room temperature to above 1000 K by irradiation. However, when heated up to 1250 K using a heater without irradiation, the ZnO crystals do not exhibit similar visible emissions. We then use the generalized Planck's radiation in non-equilibrium states as an explanation of our experimental observations. The emission peak intensity can be enhanced by the gyrotron-induced non-equilibrium states, and the emission peak position can be related to the Urbach energy. With high intensities in the visible wavelengths, the emissions of the irradiated crystals can be readily observed with our bare eyes or with inexpensive digital cameras. As the spatial distribution of the yellow emission reflects the gyrotron beam pattern, the bulk ZnO single crystals can then be utilized for the quick diagnosis of gyrotron beam patterns and positions.

Monolayer Tungsten Disulfide (WS2 ) via Chlorine-Driven Chemical Vapor Transport.

Large-scale production of high-quality tungsten disulfide (WS2 ) monolayers is a prerequisite for potential device applications using this promising transition metal dichalcogenide semiconductor. The most researched technique is chemical vapor deposition, typically involving the reaction of sulfur vapors with tungsten oxide. Other techniques such as physical vapor deposition have been explored with some success, but low vapor pressures make growth difficult. This study demonstrates a growth process that relies on halide-driven vapor transport commonly utilized in bulk crystal growth. Using a small amount of sodium chloride salt as a source of chlorine, nonvolatile WS2 can react to form gaseous tungsten chloride and sulfur. With an open tube system, a controlled reaction generates mono and few-layer WS2 crystals. Optical and physical characterization of the monolayer material shows good uniformity and triangular domains over 50 µm in length. Photoluminescence transient measurements show similar nonlinear exciton dynamics as exfoliated flakes, attributed to multiparticle physics. Requiring only the powder of the desired crystal and appropriate halide salt as precursors, the technique has the potential to realize other layered materials that are challenging to grow with current processes.

Active Crystal Growth Techniques for Quantum Materials

We review recent advances in crystal growth techniques, focusing on the development of novel quantum materials. Recent progress in instrumentation design, opening new avenues in bulk crystal growth of oxide and intermetallic compounds, is highlighted. Specifically, we illustrate leading techniques that allow for the active control of crystal nucleation/growth and provide platforms for the realization of single crystals with ultrahigh purity and minimized defects. Advances in the postgrowth manipulation of crystals, as well as the impact of purification techniques on the stabilization of delicate quantum phases, are also discussed. Throughout, we highlight new scientific avenues opened by access to high-purity single-crystal samples.

Development and melt growth of novel scintillating halide crystals

Melt growth of scintillating halide crystals is reviewed. The vertical Bridgman growth technique is still considered as very popular method that enables production of relatively large and commercially attractive crystals. On the other hand, the micro-pulling-down method is preferable when fabrication of small samples, sufficient for preliminary characterization of their optical and/or scintillation performance, is required. Moreover, bulk crystal growth is also available using the micro-pulling-down furnace. The examples of growths of various halide crystals by industrially friendly melt growth techniques including Czochralski and edge-defined film-fed growth methods are also discussed. Finally, traveling molten zone growth that in some degree corresponds to horizontal zone melting is briefly overviewed.