Lithium niobate crystal, combining its piezoelectric, nonlinear, electro-optical, and photorefractive properties, along with its stable physicochemical characteristics, has great potential for applications in integrated optics. However, designing thermal field for large-size lithium niobate crystal growth presents considerable challenges, considering the crucible shape being an important factor that significantly influences the crystal growth in which the diameter and height are compulsively restricted to the factors such as load capacity and crystal diameters. In this study, 4-inch congruent lithium niobate crystals were grown by using crucibles with two types of bottom shapes. The impacts of crucible bottom shape on the axial temperature gradient within the crystal and the melt near the crystal-melt interface, and the temperature distribution within the melt below the crystal-melt interface, were analyzed by numerical simulation. The impact of the crucible bottom shape on crystal growth was analyzed in contrast to crystal growth results. It is found that changes in the crucible bottom shape lead to variations in the temperature difference along the crucible sidewall and the temperature gradient within the melt, thereby altering the strength of natural convection in the melt. Compared to crucible with slipped bottom corner, the axial temperature gradient near the crystal-melt interface within the crystal and melt is large when using the crucible with curved bottom corner, and the axial temperature gradient within the melt below the crystal-melt interface is also large, and the natural convection is strong. Therefore, this study helps to solve the problems such as the unwanted crystal growth ridge spreading and the overgrowth of cellular interface.

Indium selenide (InSe) is a III-VI group semiconductor with interesting physical properties and has wide potential applications in the fields of photovoltaics, optics, thermoelectrics, and so on. However, the production of large-size InSe crystal is difficult due to the inconsistent melting of In and Se elements and peritectic reactions between InSe, In₆Se₇ and In₄Se₃ phases. In this work, a zone melting method, which has advantages of low cost and solid-liquid interface optimization, is employed for InSe crystal preparation. Because the initial mole ratio of In to Se is of great importance to InSe crystal growth, the non-stoichiometric In_0.52Se_0.48 solution was precisely used for growth based on the peritectic reaction of In-Se system, resulting in a InSe crystal productivity ratio at about 83%. An ingot with dimensions ϕ27 mm×130 mm is obtained with a typical slab-like InSe crystal in the size of ϕ27 mm×50 mm. The successfully peeled cleavage plane exhibits the good single-crystalline character as only (00l) peaks are detected in the X-ray diffraction pattern. This crystal has a hexagonal structure, and its elements are distributed uniformly in the matrix with transmittance of ~55.1% at 1800 nm wavelength, band gap energy of about 1.22 eV, a maximum electrical conductivity (σ) of about 1.55×10² S·m^-1 along the (001) direction, and a lowest thermal conductivity (κ) of about 0.48 W·m^-1·K^-1 perpendicular to the (001) direction at 800 K. These results imply that the zone melting method is indeed an effective approach for fabricating large-size InSe crystal, which could be applied for various fields. Above measured electrical and thermal behaviors are expected to provide a significant reference for InSe crystal application in the future.

The diamond film material holds great potential as a heat sink for GaN electronic devices. The diamond film layer with low stress, large dimensions, high quality, and an atomically smooth surface is crucial for enhancing the overall heat transfer capacity of GaN devices. This study presents a technique for growing and polishing polycrystalline diamond films on 3-inch(1 inch=2.54 cm) silicon substrates to facilitate the use of large-sized diamond film materials in radiator applications. Firstly, the study carries out multi-physical field self-consistent modelling of plasma in a microwave resonator. It then analyses the feasibility of depositing large diamond films using a microwave plasma chemical vapour deposition (MPCVD) device with a 2.45 GHz multi-mode ellipsoid resonator through simulation technology. The growth process parameters are optimized accordingly. After that, the diamond film is polished to meet the bonding requirements of GaN devices. The simulation results show that under the same microwave power input, the increase of chamber pressure leads to the increase of number density of plasma core electrons and H atoms, but the uniformity of radial distribution becomes worse. Diamond film is deposited under optimized conditions and mensurates that the thickness inhomogeneity of diamond film is 17%. In this process, methane at high concentration leads to pyramidal morphology of diamond grains dominated by (111) planes, accompanied by formation of twins. Full width at half maximum (FWHM) of the first-order characteristic peak of diamond in Raman spectrum is 7.4 cm⁻¹. After polishing, the surface roughness reaches 0.27 nm, the average bending degree of diamond film on silicon substrate is 13.84 μm, and the average internal stress is −40.7 MPa. Silicon substrate diamond wafers with large size, high crystal quality, low internal stress and atomically smooth surface are successfully prepared by the above method.

Since the beginning of the 21st century, the third generation wide band gap (E_g>2.3 eV) semiconductor materials represented by gallium nitride (GaN) and zinc oxide (ZnO) are becoming the core supporting materials for development of semiconductor industry. Due to difficult growth and high cost of GaN and ZnO single crystal, epitaxial technology is always used as the substrate materials to grow GaN and ZnO films. Therefore, it is crucial to find an ideal substrate material for the development of third generation semiconductor. Compared with traditional substrate materials, such as sapphire, 6H-SiC and GaAs, scandium magnesium aluminate (ScAlMgO₄) crystal, as a new self-peeling substrate material, has attracted much attention because of its small lattice mismatch rate (~1.4% and ~0.09%, respectively) and suitable thermal expansion coefficient with GaN and ZnO. In this paper, based on structure of ScAlMgO₄ crystal, the unique trigonal bipyramid coordination and natural superlattice structure, the basis for its thermal and electrical properties, are introduced in detail. In addition, the layered structure of ScAlMgO₄ crystal along the c-axis makes it self-peeling, which greatly reduces its preparation cost and has a good application prospect in the preparation of self-supported GaN films. However, the raw material of ScAlMgO₄ is difficult to synthesize, and the crystal growth method is single, mainly through the Czochralski method (Cz), and growing techniques now in China lag far behind that in Japan. Therefore, it is urgent to develop a new growth method of growing high quality and large size ScAlMgO₄ crystals to break the technical barriers.

Large-sized crystalline materials are the basic raw materials in semiconductors, lasers, and communications. Preparation of large-scale, high-quality crystalline materials has become a bottleneck restricting the development of related industries. Breaking through the preparation theory and technology of large-sized crystal materials is the key to obtaining high-quality large-sized crystals. Preparation process of crystal materials often undergoes nucleation and growth stages, including multiple processes at spatiotemporal scale: from atom/molecules, through clusters and nuclei, to bulk crystals. To further explore and accurately understand the crystal growth mechanism, we need intensively study the multiscale process,multi-scale in situ characterization techniques, and computational simulation methods. Among them, the latest in situ characterization methods for crystal growth includes optical microscopy, electron microscopy, vibration spectra, synchrotron radiation, neutron technology, and especially, machine learning method. Thus, the multi-scale computational simulation techniques for crystallization is introduced, for example, first principles calculation at atom/molecular scale, molecular dynamics simulation, Monte Carlo simulation, phase field simulation at mesoscopic scale, and finite element simulation at macroscopic scale. A single in situ characterization or simulation technique can only explore crystallization information over a specific time and space scale. To accurately and fully reflect the crystallization process, a combination of multi-scale methods is introduced. It can be speculated that the establishment of in situ characterization technology and computational simulation methods for the actual large-sized crystal growth environment will be the future development trend, which provides an important experimental and theoretical basis for developing crystallization theory and controlling crystal quality. Furthermore, it can be deduced that the combination of in situ characterization technology with machine learning and big data technology will be the trend for large-sized crystal growth.

Indium phosphide (InP) is a kind of important compound semiconductor material, now increasingly used in high frequency electronic devices and infrared optoelectronic devices. Currently, the price of InP devices is much higher than that of GaAs devices, mainly because of its low yield of single crystals and increase of epitaxy, and device process cost due to smaller wafer diameter. Increasing the diameter of InP single crystals is critical to reducing wafer and semiconductor process costs. The main difficulties in preparing large diameter InP single crystals are increasing crystal yield and reducing stress in the crystal. The vertical gradient freeze (VGF) and the liquid encapsulated Czochralski (LEC) methods are commonly used in the industry to prepare InP, while the VGF method has little success in preparing 6-inch InP crystals, and the crystals prepared by the LEC method tend to have higher stress and dislocation density. Here we reported a semi-sealed Czochralski (SSC) method to grow large diameter InP crystals. Numerical simulations were used to analyze the temperature distribution in melt, crystal, boron oxide, and atmosphere in LEC and SSC method, with emphasis on temperature field of the SSC method. As a simulation result, the temperature gradient in the crystal of SSC method is 17.4 K/cm, significantly lower thanthat of 28.7 K/cm in the LEC method. And temperature of atmosphere near the crystal shoulder in the diameter control stage of the SSC method is 504 K higher than that of the LEC method. Then the used thermal field of SSC method was optimized according to the simulation results, and 6-inch (1 inch=2.54 cm) S doped InP single crystals with low defect density and no cracks were prepared by this optimized method, which confirmed that the optimized SSC method is promising for growing large-size InP single crystals.

Currently, although Er³⁺ and Yb³⁺ co-doped YAG crystals are widely used in high power solid state lasers, there are still many challenges in growing large size, low defect doped YAG crystals using the Czochralski (Cz) method. In this paper, large-sized Er³⁺ and Yb³⁺ co-doped YAG bulk crystal with a diameter of 80 mm and a length of 230 mm was obtained by the fast Cz growth method. Their structure, doping concentration, optical absorption, luminescence performances, and etching defects were evaluated.According to the Raman detecting results, there is no significant variation in the peak positions and full width at half maxima (FWHM) of the Raman peaks at different locations on the wafer, indicating that the crystal structure and strain at central and edge section of thel wafer are uniformity. The etching results show that the corrosion pits are evenly distributed over the entire corrosion surfacewithout dislocation corrosion pit, which means that the crystals are highly near perfect. Strong luminescence peaks of Yb³⁺ and Er³⁺ at different wavelengths and glow discharge mass spectrometry results demonstrate the successful doping of rare earth ions in Er,Yb:YAG single crystals. This work successfully used the Cz method to grow large-sized, low-defect Er,Yb:YAG single crystals, confirming that the fast growth method is effective for doping double rare-earth ions in YAG crystals.

Terahertz (THz) technology has immersing potential applications in industrial non-destructive testing, scientific research and military engineering. However, as the most commonly used THz emission and detection electro-optical material, the ZnTe single crystal growth still faces great challenges. In order to achieve ZnTe single crystals with large size, good homogeneity and high performance, an accelerated crucible rotation technique (ACRT) was introduced in growing ZnTe crystals by temperature gradient solution growth method (TGSG). Intrinsic ZnTe single crystals with high crystalline quality were successfully prepared. Through the simulation of flow field and solute distribution at different rotation speeds, the influence of ACRT technology on the stability of solid-liquid interface and Te inclusions distribution in crystal growth were investigated. During the crystal growth, the ACRT technology can effectively promote the melt flow, improve the solute mass transfer ability and stabilize the solid-liquid interface, which not only avoids the appearance of mixed phase zone at the crystal tail, but also reduces the number and size of Te inclusions in the crystal. With the further optimizing parameters, a large size ZnTe single crystal with a diameter of 60 mm was prepared. Meanwhile, the high response area of terahertz exceeding 90% faces due to the great uniformity of ZnTe crystal, which meant the edge effect being significantly limited and the ZnTe crystal meeting the commercial imaging requirements. Therefore, introduction of ACRT technology can provide a new strategy for preparation of ZnTe based electro-optical crystals.

With the continuous development of CeF₃ crystals in laser and magneto-optical applications, the demand for CeF₃ single crystals with large size and high optical quality has become increasingly urgent, while the high viscosity and low thermal conductivity of CeF₃ melt always bring challenges to crystal growth process. In order to study the growth problem caused by low thermal conductivity of CeF₃ melt, the influence mechanism of the furnace structure and process parameters on temperature distribution and crystallographic interface during the growth process was explored. In this work, numerical simulations about the growth of large size CeF₃ crystal (ϕ80 mm) through the heat exchanger-Bridgman method were carried out to analyze the relationship between furnace structure and crystal/melt temperature distribution, the variation of interface shape in different growth stages, and the mechanism of thermal field structure on the growth interface. Results show that when the length of the heating element matches the length of the crucible, it is more conducive to construct a reasonable temperature gradient field. The unfavorable concave interface during the “shouldering” and “cylindering” growth stages can be effectively improved by adjusting temperature distribution on the ampoule wall through changing the baffle shape and adding a reflective screen. Therefore, the result not only deepens understanding of the crystallization habit of CeF₃ crystals, but also enlightens the furnace and growth interface optimization of other crystals’ Bridgman growth.

Zintl phase Mg₃X₂ (X=Sb, Bi) based thermoelectric materials have attracted much attention because of their non-toxic, low cost and high performance. Compared with polycrystalline materials, the Mg₃X₂ crystals are of great value in revealing material’s intrinsic and anisotropic thermoelectric properties, as well as providing effective strategies for enhancing electrical and thermal transport properties. Therefore, the recent progress of single crystal growth and thermoelectric properties for Mg₃X₂ crystals are systematically summarizes in this paper. Due to the volatility and causticity of Mg element, several different methods such as slow cooling method, directional solidification method, flux method, and flux Bridgman method are widely used for synthesizing Mg₃X₂ crystals, in which the flux Bridgman method is more competitive to prepare large size bulk crystals. Researchers found that both n-type and p-type Mg₃Sb₂ crystals show an anisotropy thermoelectric transport property. The crystal growth rate, the concentration of self-doped Mg element, the concentration of impurity doping or alloying elements have a great impact on both electrical and thermal transport properties for Mg₃Sb₂ crystals. So far, the p-type and n-type Mg₃Sb₂ crystals with ZT value of 0.68 and 0.82 are achieved, respectively. This paper reviews the recent progress of growth and thermoelectrics properties of Zintl phase Mg₃X₂-based crystals, revealing that the flux Bridgman method is the most effective method to produce large-sized Mg₃X₂-based crystals. Tuning chemical composition of Mg₃X₂-based crystal by doping and forming solid solution for optimal carrier concentration and band structure engineering is expected to further improve the thermoelectric performance of Mg₃X₂-based crystal. The above-mentioned growth method and research strategies provide a significant guidance for the in-depth understanding of the Mg₃X₂-based crystal in the future.

Yttrium iron garnet (Y₃Fe₅O₁₂, YIG) crystals have been widely used in microwave and magneto-optic devices due to their excellent magnetic and magneto-optical properties. Currently, the commercial material is YIG single crystal thin films, which is deposited on Gd₃Ga₅O₁₂ (GGG) substrate using liquid phase epitaxy technique. Herein, we report a new growth technology of YIG single crystal by top seeded solution growth (TSSG) technique from lead-free B₂O₃-BaF₂ flux. The maximum size and weight of the as-grown YIG crystal can be up to 43 mm× 46 mm×11 mm and 60 g, respectively. The crystals exhibit excellent comprehensive performances with narrow ferromagnetic resonance linewidth (0.679 Oe, 1 Oe=250/π A/m), high transparency (75%) and Faraday rotation angle (200 (°)·cm^-1@1310 nm and 160 (°)·cm^-1@1550 nm), indicating a good candidate in microwave and magneto-optic devices. More significantly, this growth technique is ideally suited to large size YIG or doped-YIG single crystals ascombined with the oriented seed crystal and lifting process, which can significantly decrease the manufacture cost.