Table of Content

15 April 2021, Volume 50 Issue 4

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Reviews

Melt Growth of Bulk Organic Crystals

JIANG Jinke, CUI Shuangyue, LIU Yang, TAO Xutang

2021, 50(4):  603-618. 

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Large-sized organic single crystals play an indispensable role in many fields such as terahertz generation, neutron detection and maser, which are essential to national security and welfare. However, the growth of large-sized organic crystals has always been a big challenge and moreover, due to the intrinsic characteristics of organic crystals such as low hardness, frangibility, and easy-to-cleavage, processing and application of organic crystals are also hard tasks. Considering the lack of available large-sized organic single crystals has seriously hampered the development of the related fields in our country, a full summary and analysis of organic crystals growth methods is necessary. Comparing with growth methods based on gas phase and liquid phase, growth of bulk and doped organic crystals from melt is superior in terms of size and doping control. However, regulating accurately the growth process of bulk-sized and hig-quality crystal from organic melt is difficult to realize due to the low crystallization capacity and thermal stability of organic melts. In order to provide theoretical basis and practical experience, the research progress of bulk organic crystals via melt growth is summarized in this paper. From the aspects of raw materials refining, seed crystals selection, ampoule design, solid-liquid interface control, growing and cooling rate, and thermal field optimization, both scientific problems and technical issues in melt growth of bulk organic single crystals are specifically analyzed.

Research Progress of Semi-Insulating Silicon Carbide Single Crystal Substrate

PENG Yan, CHEN Xiufang, XIE Xuejian, XU Xiangang, HU Xiaobo, YANG Xianglong, YU Guojian, WANG Yaohao

2021, 50(4):  619-628. 

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Silicon carbide (SiC) is considered as one of the most important wide band gap materials. It has many advantages, such as wide band gap, high breakdown electric field, high thermal conductivity, high electron saturation velocity and strong radiation resistance. The semiconductor devices based on SiC materials can not only operate at higher temperature, but also have higher reliability at high voltage and high frequency. In the past 20 years, with the development of material growth technology, manufacturing process and device physics, SiC materials and devices have been widely used in radar, 5G telecom technology, electric vehicles and other fields, which has an extremely important impact on the development of national defense industry, information security and economy. In the SiC based high-power semiconductor device industrial chain, high-quality SiC single crystal preparation and industrialization is the most important part. In this paper, the development of semi-insulating SiC single crystal substrate materials at home and abroad is summarized. The research history and current progresses of semi-insulating SiC in Shandong University are mainly introduced. The research and industrialization and existing challenges are discussed.

Applying Crystallization Theory to Reshape Future of Cocoon Silk Soft Materials

QIU Wu, LIU Xiangyang

2021, 50(4):  629-647. 

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Attributed to advanced structrual characterization techniques, mesoscopic hierarchical structures as well as crystal network structures are revealed in different varieties of soft materials including silk fibroin materials. Noting the close correlation between mesoscopic structures (especially the crystal network structures) and macroscopic performance of silk fibroin materials, comprehensive investigations on the formation of crystallites and crystal network are utmost important. Based on latest research studying on how silk fibroin molecules refold into higher levels of structures within silk fibroin materials, it is determined that the crystallization theory can well predict the formation process of silk fibroin materials and the corresponding kinetics. Nevertheless, the rational design and mesoscopic reconstruction of silk fibroin materials can be implemented by controlling the molecular crystallization, such as crystal density, orientation degree, doped molecular nucleation intiator and so on, which endows noval functions to silk fibroin materials and other soft materials.

Development of Disordered Laser Crystals and Their Ultrafast Lasers

YU Haohai, PAN Zhongben, ZHANG Huaijin, WANG Jiyang

2021, 50(4):  648-668. 

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Laser is the light amplification by stimulated emission of radiation, and its wavelength is dependent key electronic structures of active ions, especially those in the outmost levels. Up to now, the lasers have been developed for 60 years, and the laser gain crystals have become the most important materials in the laser regimes boosting the development of lasers and applications. Now, the ultrafast pulsed lasers with the advantages of high peak power and ultrafast pulse width have been successfully applied in many important areas, which pushes that the research on the laser crystals suitable for the ultrafast pulsed lasers has become more and more important. The key of researching on the ultrafast laser crystals is the discovery of the physical parameters and the growth of laser crystals with broadband gain spectra. In this review, the disordered laser crystals including more than 10 kinds of crystals grown by our group and others are reviewed, with the beginning of the discussion of physical parameters of spectra of gain media. The crystal growth, crystalline physics and laser devices are included and their pulsed laser results are also presented. We hope this paper can provide some references for the readers.

Classical and Non-Classical Crystallization Pathways in Pharmaceutical Crystallization

SONG Shuhong, YAO Changlin, WANG Lei, QU Yaqian, TAO Xutang

2021, 50(4):  669-684. 

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The crystallization pathways of crystals can be classical and non-classical. The classical crystallization pathways usually involve some simple chemical species, where the nucleation and growth are realized by adding monomers successively. After decades of research, the classical crystallization pathways have been fully understood and formed a relatively perfect theory. Recent studies have found that some materials, such as calcium carbonate (CaCO₃), crystallize in the non-classical pathway. Non-classical crystallization pathways have aroused extensive interest in academia. However, the non-classical crystallization pathways have not been fully and uniformly explained. They involve the multistep mechanisms and the formation of complex intermediate particles, which range from multi-ion complexes to aggregation of oriented and nearly oriented metastable nanocrystals. In the field of pharmaceutical crystallization, the introduction of the conformational degrees of freedom in organic molecular systems increases the complexity. Polymorphism of solid drugs exists due to the weak interactions between drug molecules. The physicochemical properties and bioavailability of drugs are closely related to their crystal forms. Meanwhile, the complex intermediate species appearing during crystallization affect the form of solid drugs. It’s thus in urgent need to strengthen the study about nucleation and growth pathways of crystalline drugs. Methods should be developed to provide absolute control over crystal nucleation and growth. This paper summarizes the classical and non-classical crystallization of drugs in solution and in melt, including the Ostwald’s law of stages, independent nucleation and cross nucleation. From the perspective of solution chemistry, molecules existing in concentrated solution may self-assemble, via hydrogen-bonds and aromatic stacking, or be solvated to form the structural synthons. Nucleation is closely related to the growth units and structural synthons in solution. In order to distinguish the two crystallization pathways, it is critical to find the information about molecular motion at the molecular level and the relationship between nuclei and structural synthons in each system. Non-classical crystallization implies both opportunities and challenges for pharmaceutical crystallization research.

Design and Synthesis of Efficient Photocatalyst Based on the Principal of Crystallography

WANG Zeyan, WANG Peng, LIU Yuanyuan, ZHENG Zhaoke, CHENG Hefeng, HUANG Baibiao

2021, 50(4):  685-707. 

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Photocatalysis is a new technique that can convert solar energy into chemical energy. Semiconductor photocatalysts can be used to split water to generate hydrogen, reduce CO₂ to organic species, or degrade organic pollutants into eco-friendly species. It has been regarded as one of the most promising strategy to solve energy and environmental problems in the future. However, limited by the poor visible light absorption and inefficient charge separation rate, the photocatalytic activity of the present photocatalysts is still far from the criteria for practical applications. To solve these problems, we have recently developed a series of new strategies based on the basic principle of crystallography to further extend the light absorption range of photocatalysts and stimulate the charge separation inside and at the surface of photocatalysts. And this work may provide some new threads on the design and synthesis of highly efficient photocatalysts.

Lithium Niobate Crystals in the Information Age: Progress and Prospect

LIU Hong, SANG Yuanhua, SUN Dehui, WANG Dongzhou, WANG Jiyang

2021, 50(4):  708-715. 

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Lithium niobate crystals have many physical properties, such as nonlinear effect, electro-optic effect, acousto-optic effect, photorefractive effect, piezoelectric effect, and pyroelectric effect, et al. They have been widely used in surface acoustic wave (SAW) device, electro-optic device, acousto-optic device, and so on. After over 60 years of development, lithium niobate single crystal is still active in the material science and application. With the continuous development of material properties, plenty of new functions, new devices, and new applications come out. Especially in recent years, the development of lithium niobate single crystal films provides a key material for the new generation of information and communication technology in the aspects of thin film filter and integrated electro-optic modulation. In this review paper, the growth of lithium niobate crystals, the preparation of crystal films, and the new applications of lithium niobate crystals in recent years are briefly reviewed, and the prospective application of lithium niobate crystals is discussed.

Single-Crystal Lithium Niobate Thin Films

LI Qingyun, ZHU Houbin, ZHANG Honghu, ZHANG Xiuquan, HU Hui

2021, 50(4):  716-723. 

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Lithium niobate crystals have excellent electro-optic, acousto-optic and nonlinear optical properties, and have broad transmission window. As an important optical material, it is widely used in the fields of communication and sensing. Single-crystal lithium niobate thin film (lithium niobate on insulator, LNOI) prepared by ion implantation and direct bonding retains the excellent physical properties of lithium niobate bulk material, and it has large refractive index contrast. Photonic devices based on this material have been greatly improved in terms of integration and device performance. The preparation process and applications of LNOI is introduced in this paper, and a 6-inch LNOI is demonstrated. A hybrid thin film can be formed by covering a single-crystal silicon thin film on the top of a LNOI. This hybrid thin film combines the excellent optical properties of LN and excellent electronic properties of silicon. 3-inch hybrid thin film is reported in this paper. X-ray diffraction shows that the silicon thin film is single-crystal. This hybrid material will have potential applications in the future integrated optoelectronic chips.

Research Progress of Oversized KDP/DKDP Crystals

ZHANG Liyuan, WANG Shenglai, LIU Hui, XU Longyun, LI Xianglin, SUN Xun, WANG Bo

2021, 50(4):  724-731. 

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KDP/DKDP crystal has the advantages of simple growth method, low cost and good optical properties, while the oversized KDP/DKDP crystal is the only single crystal material that can be used in high power laser engineering. However, there are many factors in the process of crystal growth, and the post-treatment of crystal will also affect the performance of crystal, which is directly related to the practical application of oversized KDP/DKDP crystal. Accordingly, this paper reviews the important research progress of oversized KDP/DKDP crystals in recent years, especially for the traditional growth and rapid growth problems and the corresponding countermeasures as well as the properties-related of research, and the transmittance, deteration level and laser-induced damage of oversized KDP/DKDP crystals are analyzed and discussed with key point.

Research Letter

Growth of High Melting Point Rare Earth Sesquioxide Scandium Oxide Crystal (Sc₂O₃)

ZHAO Hengyu, HOU Wentao, XUE Yanyan, WANG Qingguo, DONG Jianshu, LEI Zhenlin, XU Xiaodong, XU Jun

2021, 50(4):  732-734. 

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A new type of high temperature crystal growth furnace has been designed and fabricated with the structure of heat-exchange method (HEM) combined with temperature gradient technique (TGT). Such design could solve the problem of crucible pollution at high temperature and the insufficiency of crystal driving force that derived from distribution of heat. 2 inch Sc₂O₃ crystal is grown and presented in this work, which is recognized as the largest sesquioxide crystal in size. The Sc₂O₃ crystal is colorless and transparent with no crack on the surface and no bubbles or other macro-defects inside of the bulk. The size of bulk Sc₂O₃ crystal is measured to be φ55 mm×50 mm, 40 mm of which is in equal-diameter growth while the rest 15 mm belongs to shouldering part. (222) and (444) peaks are observed in the the XRD pattern of bulk Sc₂O₃ crystal, which indicates to be single phase. X-ray rocking curve (RC) shows good crystalline quality and FWHM of RC peak at 15.76° is determined to be 113″.

Research Articles

Growth and Property Characterization of Novel Piezoelectric Crystal YbBa₃(PO₄)₃

WU Guangda, FAN Mengdi, DU Yuehao, YAO Guiteng, MIAO Hongchen, CHENG Xiufeng, YU Fapeng, ZHAO Xian

2021, 50(4):  735-740. 

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Piezoelectric materials are widely used in vibration sensors, pressure sensors and ultrasonic piezoelectric transducers etc. It is of great significance to explore novel piezoelectric crystal materials for using in special piezoelectric sensor devices. Herein, a novel non-centrosymmetric barium ytterbium phosphate (YbBa₃(PO₄)₃,YbBP) piezoelectric crystal with high melting point (~1 800 ℃) was grown by traditional Czochralski method.The single crystal X-ray diffraction shows that this crystal belongs to the cubic symmetry with the space group of I43d and lattice parameters of a=b=c=1.043 5 nm. It is found that the YbBP single crystal is easier to grow along the direction perpendicular to the (013) facet. High resolution X-ray diffraction was carried out to evaluate the crystalline quality. The full width at half-maximum of rocking curves is 60.6″, indicating that the grown YbBP single crystal shows good quality. Moreover, the dielectric permittivity and piezoele ctric constant were evaluated by using the LCR bridge method, impedance and ultrasonic methods. The relative dielectric permittivity (ε₁₁) and piezoelectric strain constant (d₁₄) of YbBP crystal are 15.3 and 11.4 pC/N, respectively. The YbBP crystal not only possesses large piezoelectric performance but also has pure face-shear vibration mode, indicating the potentials of this crystal for piezoelectric sensing application.

Defects of Multi-Plate Sapphire Grown from EFG Technique

CHEN Weichao, LUO Ping, WANG Qingguo, TANG Huili, XUE Yanyan, DUAN Jinzhu, WANG Qinfeng, LEI Zhenlin, XU Jun

2021, 50(4):  741-746. 

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Multi-plate sapphire crystals were grown by EFG technique. Micro-laser Raman spectrum and ICP-AES were used to identify the chemical composition of the matters in bubbles, showing that these matters may contain sulphur. Growth striations at the surface of the crystal were concerned with fluctuation of growth rate and temperature as well as the working accuracy of the die. Chemical etching test shows that the crystal has a dislocation density of 4.2×10⁴ cm^-2 and there is no low angle grain boundary defect. X-ray rocking curve measurement shows a single peak with FWHM of 70.63″. Owing to graphite heat shade, the crystal contains F and F⁺ centers. The transmittance of the crystal is over 80% at light wavelength from 400 nm to 3 000 nm. Annealing the crystal in air can reduce optical absorption caused by color centers. The results of this paper are useful to theoretical study on formation of defects in sapphire and technical improvement on EFG sapphire industury.

Growth and Property Characterization of Er:Lu₂O₃ Single Crystals by EFG Method

WANG Guiji, YIN Yanru, JIA Zhitai, TAO Xutang

2021, 50(4):  747-751. 

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The high-purity Lu₂O₃ and Er₂O₃ were used as raw materials, using independently design and manufacture automatic equal diameter EFG furnace, 7.82% (atomic fraction) Er:Lu₂O₃ single crystal of φ25 mm×20 mm was grown by the EFG method, and the segregation coefficient is 0.92. Annealing conditions were explored. XRD result is a pure phase, and the XRF result proves that the impurity content is low. The absorption cross-sections around 972 nm and 1 535 nm are calculated using absorption spectra to be 3.24×10^-21 cm²and 8.43×10^-21 cm², and FWHM are 28.22 nm and 27.31 nm, respectively. The thermal property test results show that the thermal conductivity is 13.28 W·m^-1·K^-1 at 30 ℃. The crystal surface micromorphology was characterized by SEM.

Dislocation Distribution in SiC Wafers Studied by Lattice Distortion Detector

YIN Pengtao, YU Jinying, YANG Xianglong, CHEN Xiufang, XIE Xuejian, PENG Yan, XIAO Longfei, HU Xiaobo, XU Xiangang

2021, 50(4):  752-756. 

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The dislocation distribution of SiC wafers was studied by lattice distortion detector. The dislocation distribution of the whole SiC wafer or a local area was obtained by scanning the SiC wafers etched by molten KOH. Compared with the scanning corrosion mapping of the LEXT OLS4000 3D laser confocal microscope, the scanning corrosion mapping of the lattice distortion detector can fully display the dislocation etch pit information. According to the color and size of the etch pits, three types of threading dislocations were identified, in which black spot corrosion pits correspond to screw dislocations (TSD), small size white spot corrosion pits correspond to edge dislocations (TED), and large size white spot corrosion pits correspond to mixed type dislocations (TMD). The lattice distortion detector was used to study the dislocation density and distribution of 4-inch (101.6 nm) N-type 4H-SiC crystal at different growth stages. The results reveal that as the crystal grows, the dislocation density shows a gradual decrease trend. The total dislocation density of the wafer in the later stage of growth is reduced to nearly 1/3 of the total dislocation density of the wafer in the early stage of growth. TED occupies the largest proportion in the wafer and decreases the fastest during the growth process. TSD and BPD occupy a small proportion in the wafer, and the density gradually decreases. The results of 0004 rocking curves indicate high crystalline quality and virtually flat basal planes. It is helpful to feed back the information of the propagation and transformation characteristics of dislocation defects during the SiC crystal growth process, so as to guide the improvement of the SiC crystal growth process.

High Reliable Al-Free 808 nm Semiconductor Laser Diode Pump Source

LIU Peng, ZHU Zhen, CHEN Kang, WANG Rongkun, XIA Wei, XU Xiangang

2021, 50(4):  757-761. 

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For 808 nm high power laser used as pump source, Al-free active-region laser diode was designed and fabricated, consisting of InGaAsP/GaInP. In this work, a double asymmetric structure of cladding and waveguide layers to reduce the thermal resistance and optical loss of P-side layers were proposed. By optimizing the MOCVD growth of As and P hybrid material, InGaAsP single-crystal epitaxial film with steep interface was fabricated. The threshold current is 1.5 A at room temperature and the slope efficiency is 1.26 W/A. The output power is 10.5 W at 10 A and the power efficiency is 58%. Under continuous wave (CW) operation, the maximum output power is 23 W@24.5 A, while it can reach 54 W@50 A under quasi continuous wave (QCW) mode without catastrophic optical damage (COD). No power degradation or COD occurred for accelerated aging over 4 200 h at 15 A, showing high long-term reliability of Al-free active-region 808 nm laser diode.

Crystal Growth and Scintillation Luminescence Properties of Ti:Al₂O₃ Crystals Grown with Kyropoulos Method

WANG Qingguo, LIU Bo, LUO Ping, TANG Huili, WU Feng, KANG Sen, DUAN Jinzhu, WANG Qinfeng, XU Jun

2021, 50(4):  762-767. 

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Large-sized Ti:Al₂O₃ crystals with the weight of 115 kg were grown successfully by Kyropoulos method. The grown crystals have complete shape without any cracking. Large Ti:Al₂O₃ single crystal sample with diameter of 300 mm was prepared. The scintillation luminescence properties of the grown crystals were tested under the X-ray and α particle excitation. The results show that the scintillation luminescence of Ti:Al₂O₃ crystals include near infrared band and near ultraviolet band. The near infrared emission comes from the characteristic emission of Ti³⁺, which has high efficiency and slow decay time. The near ultraviolet emission comes from Ti-localized exciton emission and F⁺center, which has the fast decay time. The light output is related to the self-absorption caused by Ti-doping. Under the excitation of the α particles, the light yield reaches up to 1 130.5 pe/MeV, and the fast component is 29.6 pe/MeV.

Structure and Photoelectric Properties of ZnO Single Crystal Grown by Homoepitaxy

LIU Zhenhua, FAN Long, FU Yajun, WANG Jin, CAO Linhong, WU Weidong

2021, 50(4):  768-775. 

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The structure and photoelectric properties of ZnO single crystal grown by homoepitaxy before and after annealing was reported. The red-brown ZnO single crystals were grown by the chemical vapor transport (CVT) method. The ZnO single crystal annealed in a high-temperature oxygen atmosphere shows colorless and transparent. The structure of the ZnO single crystal before and after annealing was analyzed by XRD, XPS, EDS, and Raman. The effect of annealing on the defect type and structure of a single crystal was discussed. XRD results show that the growth direction of the ZnO single crystal is (002). The FWHM of ω-rocking curve of ZnO single crystal before and after annealing is 59″ and 31″, respectively. It indicates that the defects in the single crystal decrease significantly after annealing. The composition and valence of elements in the single crystal before and after annealing were analyzed by XPS and EDS, and the results show that the content ratio of Zn and O elements in the single crystal after high-temperature oxygen atmosphere treatment is closer to the theoretical value. Raman spectroscopy analyzed the high-temperature oxygen atmosphere. The different Raman vibration modes of ZnO single crystal before and after annealing were analyzed. Through UV spectrum data analysis, the optical band-gap of ZnO single crystal before and after annealing is obtained to be 3.05 eV and 3.2 eV, respectively. Finally, the ZnO single crystal’s electrical properties before and after annealing in a high-temperature oxygen atmosphere were analyzed by Hall test. The low-temperature electrical transport characteristics of ZnO single crystal before and after annealing were discussed in depth.

Growth and Luminescence Properties of InGaN/GaN Micro-Array

ZHANG Lifan, JIA Wei, DONG Hailiang, LI Tianbao, JIA Zhigang, XU Bingshe

2021, 50(4):  776-782. 

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As the most promising optoelectronic device in the 21st century, the research of epitaxial structure of light-emitting diode has been a hot topic for scientists.Three types of InGaN/GaN micro-array with different morphologies were grown on graphical sapphire substrate by selective regional epitaxy using metal organic chemical vapor deposition technology. The size of the array is 6 μm. The tablets array is dominated by (0001)c facet, the height is 0.6 μm; the platelets array includes one (0001)c facet and six equivalent (10-11) semi-polar facets, the height is 1.2 μm; the pyramid-like array is dominated by (10-11) semi-polar facet and the height is 5 μm. The luminescence properties of InGaN/GaN micro-array structure were characterized by micro-area photoluminescence spectrometer and the results show that InGaN/GaN micro-array with different morphologies could achieve multi-wavelength emission, but its luminescence properties are heavily dependent on the morphology. Cathodic fluorescence spectra results show that the InGaN/GaN platelets array’s wavelength covering red, green and blue was obviously different for different test positions, which is mainly attributed to surface migration effect and transverse gas phase diffusion resulting in nonuniform distribution of In content in each position. Moreover, the results of energy dispersive spectrometer analysis show that the In content on different facets are different, and the (0001)c facet content is more than that of (10-11) semi-polar facets. For the realization of multi-emission and new optoelectronic device design, the successful preparation InGaN/GaN micro-array is very significance.