Evaporation induced diameter control in fiber crystal growth by micro‐pulling‐down technique: Bi4Ge3O12

Diameter self‐control was established in Bi₄Ge₃O₁₂ fiber crystal growth by micro‐pulling‐down technique. In accordance with Bi₂O₃‐GeO₂ phase diagram, the diameter was controlled due to compensation of solidification with evaporation of volatile Bi₂O₃ self‐flux charged into the crucible with excess. The crucibles had capillary channels of 310 or 650 μm in outer diameter. The crystals up to 400 mm long and 50‐300 μm in diameter were grown at pulling‐down rates of 0.04‐1.00 mm/min. The melt composition and the pulling rate were generally only two parameters determining solidification rate. As a result, crystals with uniform (± 10%) diameter and aspect ratio up to 10⁴ were produced without automation of the process. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of Vanadyl Pyrophosphate Single Crystals

The present work communicates about the first successful attempt at growing of sizeable single crystals of vanadyl pyrophosphate (VO)₂P₂O₇ (VOPO) — a low dimensional antiferromagnet. The growth of VOPO is complicated by two features of this compound. The first is a strong sensitivity of the oxydation state of vanadium (and stability of VOPO) to the oxygen content in the growth atmosphere. The second is a tendency of the VOPO-melt to glass formation during the cooling due to its high viscosity. Therefore the growth has to be carried out with a very low growth rate and in an atmosphere with exactly controllable oxygen content. The best results were achieved with a combination of Czochralski and Kyropoulos techniques, i.e. pulling of crystals with simultaneous cooling of the melt. Crystals of VOPO with sizes up to 10 × 5 × 3 mm³ have been grown. The growth from the melt is accompanied by growth from gaseous phase also (sublimation). Since the VOPO phase has a homogeneity range in oxygen content, attention was paid to the composition of the grown crystals. Growth experiments combined with TGA and XRD measurements show, that the oxygen content and thus the oxidation state of vanadium in the crystals can be adjusted accurately.     

Crystal pulling with a floating nonwetted shaper

In crystal growth by pulling from the melt, the crystal cross-section can be controlled with the help of a shaper, which in the case of semiconductor and metal single crystals is usually lighter than and not wetted by the melt. In order to better control the crystal cross-section, we controlled the temperature near the aperture of the shaper during crystal growth. Graphite was used as a floating shaper and in order to allow close observation of crystal growth, a low melting-point material, i.e., Sn, was chosen as the melt. Single crystals were grown with excellent control over the cross-sectional shape and size. Single crystals were also grown under two other conditions, i.e., no temperature control and crucible temperature control. The control over the crystal cross-section in these two cases, however, was inferior to that in the case of shaper (aperture) temperature control. This difference was explained with the help of knowledge of the shaper (aperture) temperature, which was measured in each of the three cases.     

The Czochralski Method ‐ where we are 90 years after Jan Czochralski’s invention

The Czochralski method, i.e. pulling a crystal from the melt, became the most important technology for the production of large semiconductor (Si, GaAs, InP, GaP …) and optical crystals (oxides, CaF₂ …). The present status is achieved by a profound analysis of the mechanisms of heat and species transport which are relevant for the stability of the Czochralski growth process and the performance of the growing crystal. It was clearly demonstrated in the last few years that modeling by numerical simulation is an indispensable tool to analyze the Czochralski process and to understand the governing mechanisms. The contribution presents examples of this kind of modeling the Czochralski technique in correlation with experimental investigations in order to illustrate the present status of understanding relevant processing phenomena. Furthermore, it is shown what problems need still to be solved in the future in order to further improve the yield and quality of Czochralski‐grown crystals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Edge-Defined Film-Fed (EFG) Growth of Rare-Earth Orthovanadates REVO4 (RE=Y,Gd): Approaches to Attain High-Quality Shaped Growth

In spite of their superior laser and polarizer properties rare-earth orthovanadates (REVO₄) single crystals have not been adopted yet into extensive industrial applications because of crystal growth difficulties. The main problems of CZ technique are compositional change and diameter instability. This work presents the first attempt to apply the edge-defined film-fed growth (EFG) technique by which well-shaped REVO₄crystals have been grown directly. The capillary properties of YVO₄ and GdVO₄ melt have been measured. The applicability of shaped growth for rare-earth orthovanadate family was approved by successful EFG growth of transparent rod-like macro-defect-free single crystals of YVO₄ and GdVO₄. We address two main approaches to enhance the quality of EFG crystals: (i) meniscus and crystal shape stability dependence on die top shape and (ii) the strategy of effective operating control. Concave die top was found to be the best choice for high-quality EFG growth of REVO₄ along [001] direction. The spectral analysis of weight signal from growing crystal was shown to be a useful feedforward clue to prevent crystallinity degradation at a very early stage. A reasonable stability of the EFG process was achieved using [211], [101], [001] and [100] pulling directions.     

Growth of LaBGeO5 crystal fibers by the micro‐pulling down technique

The LaBGeO₅ compound (LBGO) was prepared by solid state reaction at 1050°C and characterized by XRD and DTA analysis. The direct growth of LBGO fibers from the melt using micro‐pulling down technique was unsuccessful because of its high viscosity. The study of the LBGO‐LiF phase diagram showed that LiF could be considered as a convenient flux to reduce viscosity of the melt during the growth process. Several crystal fibers were then grown and characterized by Raman spectroscopy. To decrease the high volatility of LiF, B₂O₃ was added to the melt. A white cloudy fiber was obtained from LiF‐B₂O₃ flux and checked by Raman spectroscopy and scanning electron microscopy.     

Automated pulling of large-diameter alkali halide scintillation single crystals from the melt

Principles of a new high-productivity automated method for pulling large-diameter alkali halide crystals are described. On the stage of radial growth, the melt geometry is varied continuously by its level elevation in the conical crucible due to feeding by the melted raw material controlled by a lifting electrocontact probe. The melt level is stable when the crystal is grown in height. An automated system to control the crystal diameter has been developed using time intervals between feeding operations as the controlling parameter. This system allows control over the crystal diameter to an accuracy better than 1% over the range from 400 to 450 mm at the pulling rate from 6 to 6.5 mm/h. The method is used to produce scintillation alkali halide single crystals on an industrial scale.     

Near‐stoichiometric LiNbO3:ZrO2 single crystal growth by micro pulling down method

Crack‐free, rod‐shaped single crystals of undoped and 0.5, 0.7 and 1.0 mol% ZrO₂‐doped LiNbO₃ with a near‐stoichiometric composition were grown by the micro‐pulling down (μ‐PD) method. The structural properties of the grown crystals were examined by powder X‐ray diffraction (XRD). Electron probe micro analysis (EPMA) of the near‐stoichiometric LiNbO₃ single crystals revealed the homogeneous incorporation of Zr ions. The change in the refractive index and IR transmission spectra of the grown crystals were examined as a function of the Zr concentration. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical Properties of Er Doped Congruent and Stoichiometric LiNbO3 Single Crystals

Erbium doped LiNbO₃ (Er:LiNbO₃) single crystal fibers were grown free of cracks along c‐axis by the micro‐pulling down (μ‐PD) method. We have investigated the up‐conversion property with the change of doped Er₂O₃ concentration and the starting melt composition. An enhancement of green upconversion according host matrix is also observed the stoichiometric LiNbO₃. And, the dependence of the green emission according to Er³⁺ concentration is analyzed. The possible application of the Er³⁺ doped stoichiometric LiNbO₃ single crystal fiber for up‐conversion based optical devices is discussed.     

Thermal stress relaxation phenomenon through forming the interstitial region in CZ silicon pulled with rapid and slow cooling heat shields

This review article aims to clarify a mechanism of point defects formation in a CZ Si crystal through an experimental arrangement using the two kinds of heat shields with different slow-pulling periods. Point defects in a melt grown silicon crystal have been studied for a long time. The author and his co-researchers have reported about “Mechanism for generating interstitial atoms by thermal stress during silicon crystal growth” [in Progress in Crystal Growth and Characterization of Materials, 66 (2019) 36-46]. The experimental arrangement includes constant growing, changing pulling rate and finally detaching crystals from the melt. The two types of heat shields were used to change the cooling history of the grown crystals, for changing a temperature gradient at a bulk part in the grown crystal, G_b. In order to prove that the formation of an interstitial region or a boundary of vacancies (Vs)/interstitials (Is) in a silicon crystal is a phenomenon of relaxing thermal stress, the author explains that a G_b in a crystal forms thermal stress and causes some silicon atoms at lattice positions to move to the closest interstitial sites to relax the stress. The author defines a new term of metastable interstitial atom, I’, or I's as the plural of I’. The I’ coexists with the metastable vacancy V’ from where the I’ is displaced. The plural of V’ is defined to be V's. The author defines the above state to be a complex (I’+ V’), or (I ’+ V’)s as the plural of (I’+ V’), and explains that the (I’+ V’) s convert to Is and form the Is region. The (I’+ V’) is considered as the Frenkel pair-like complex.The crystals were firstly pulled with a high pulling rate, and the pulling rate was consequently decreased to a slow one. Then the crystals were pulled with the slow constant pulling rate for different periods making different cooling processes. Finally, the grown crystals were detached from the melt and cooled rapidly. Characterization of defects, such as Vs, Is, and defect-free (D-F) regions were identified in X-ray topographs (XAOP(s)). Wafer lifetime mapping (WLTM(s)) allows confirming dislocation loop (DL) regions. The results show that the Is are generated depending on the pulling period of the slow pulling and the shapes of the heat shields. The Is and DL regions are formed in a region at temperatures near the melting point. The Is form an Is region through a defect-free (D-F) region, forming the Vs/Is boundary. When the thermal stress weakens, the DL region changes to the Is region; the Is region changes to the D-F region; and the D-F region changes to the Vs region. Temperature gradient distribution is induced toward various directions at different parts of the growing crystal depending on the different slow-pulling periods. The temperature gradient, G_b, includes a temperature gradient from the cooled region shaded by the heat shield to the growth interface and a temperature gradient from the upper surface cooled during the long-time growth to the growth interface. The G_b exceeding a certain threshold at near the melting point forms thermal stress, generating Is to relax the stress.     

Improved growth of PbI2 single crystals

The phase equilibrium and the crystallization process of lead iodide (PbI₂) melt have been primarily investigated according to the lead–iodine phase diagram. It is found that the iodine evaporation and the segregated lead deposition are the two important factors that affect the PbI₂ crystal quality. The new method of Pulling U-type quartz growth ampoule has been made to impede the decomposition of PbI₂ and the vaporization and condensation of iodine. An orange and translucent PbI₂ single crystal of large size was obtained by the improved growth method, i.e. U-type ampoule pulling. Resistivity of the as-grown crystal is up to 4×10¹¹ Ω cm, and IR transmission is up to 45% in the region from 7800 to 450 cm⁻¹. Therefore, the improved growth method is a promising convenient new method for the growth of high quality PbI₂ crystals.     

On the Iso‐Diameter Growth of β ‐BaB2O4 (BBO) Crystals by Flux Pulling Method

The iso‐diameter growth of β ‐BaB₂O₄ (BBO) crystals by the flux pulling method have been studied based on the phase equilibrium diagram in the BaB₂O₄‐Na₂O pseudo‐binary system and from the interface stability. The mathematical expressions for the cooling rate in the growth of the crystals with constant diameter under stable growth conditions are derived, the experimental phenomena such as diameter contraction and difficulty to grow a lengthy crystal by the flux pulling method are explained, the prerequisite for iso‐diameter BBO crystal growth from the flux is suggested; a new continuous charging flux pulling method is introduced to grow large‐sized high quality crystals with a relative high growth rate.     

Characterization of dislocations in a LiNbO3 single crystal grown by micro pulling down method

LiNbO₃ single crystals grown by the micro pulling down (μ-PD) method have been revealed to be as free of dislocations and subgrain boundaries up to 500 μm in diameter. On the other hand, μ-PD LiNbO₃ single crystals grown along the x-axis in diameter of 800 μm were observed to be dislocated due to the size effect of crystal. The Burgers vectors of dislocations were determined to be [22 01], [101 1], and [01 11] by X-ray topography.     

On the void distribution and size in shaped sapphire crystals

Ribbon and rod sapphire pulling has been performed in three different crystal growth equipments in order to study the effect of the installation, of the atmosphere, of the die shape, of the feed material and of the pulling rate on the distribution, number and diameter of the characteristic voids (micro‐bubbles) in the crystals. The location of the bubbles in the crystals depends on the die geometry; however, in most cases they are essentially located close to the crystal periphery and then can be efficiently removed by lapping. After statistical analysis of the results, it is demonstrated that the number of gas moles incorporated in the crystals, inside the voids, is totally independent of any growth parameter. It is also shown that the bubble diameter depends only on the pulling rate. Consequently, for a given pulling rate, the number of bubbles auto‐adjusts in order to satisfy the constant molar gas incorporation. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phase Matched Wavelength Dependent on Atomic Displacement in Potassium Lithium Niobate Crystals Grown by Micro Pulling Down Method

Phase-matched wavelength in potassium lithium niobate (KLN) crystals grown by the micro pulling down (μ-PD) method dependent on the atomic displacement changed by melt composition was investigated. It has been found that the polar components of the atomic displacement of the metal atoms Nb(1) and Nb(2) increase with increasing Nb content of melt. Also, phase matched wavelength for second harmonic generation (SHG) properties was determined from ultra-violet to green region by increasing Nb content of melt.     

Micro‐Pulling Down Growth of Co‐doped Lithium Niobate Single Crystal Fibers According Er and Mg Contents and Photolumenescence Properties

The Er³⁺doped Mg:LiNbO₃single crystal fibers employed in our experiment were grown in air by a micro‐pulling down (μ‐PD) method from host materials of a congruent Li/Nb (0.945) ratio which were melt‐doped with a nominal molar concentration of 1, 3, 5% MgO and 0.6% Er₂O₃. The X‐ray diffraction analysis results indicated that the co‐doped crystals main tained the same structural characteristics as the undoped LiNbO₃, however the lattice parameters with Mg differed; c (Å) value decreased, and a (Å) increased than of pure LiNbO₃. The influence of dopants on the photoluminescence (PL) properties of the Er:Mg:LiNbO₃ single crystal fibers excited by laser lines of 514 nm was reported. Also, the PL properties according to temperature and the excitation power of Er:Mg:LiNbO₃ crystal fibers were analyzed.     

ø300 mm直拉硅单晶生长过程中的变晶现象及其影响因素

直拉法生长直径300 mm硅单晶过程中,直径均匀是获得高品质硅单晶的关键。在生产实践中发现,当硅晶体进入等径生长阶段,过高的提拉速度会引起晶体发生扭晶现象,导致晶线断裂随即变晶,对等径生长不利。本文采用数值模拟和理论相结合的方法分析了ø300 mm硅单晶生长过程中扭晶现象的成因,建立了不同提拉速度下晶体直径与熔体温度分布的关系,分析了晶体发生扭晶的影响因素。结果表明,随着提拉速度的增加,熔体自由表面产生过冷区且该过冷区随提拉速度的增加不断扩大,过冷区的产生是导致晶体发生扭晶的主要原因。提出了一种基于有限元热场数值模拟的最大稳定提拉速度的判别方法,并给出了通过改变晶体旋转速度来改善熔体自由表面温度分布的工艺措施建议,从而避免晶体扭晶现象的发生。研究结果对设计大尺寸硅单晶生长热场具有一定的指导作用。     

Comparison of Ba(B1-xAlx)2O4 Single Crystals on their Diameters

Small diameter crystals of Ba(B_1-xAl_x)₂O₄ were grown by Floating zone pulling down method and Micro-pulling down method to be compared with large diameter crystals grown by Czochralski method. While the Czochralski grown crystal was opaque, the crystal transparency was actually improved with the decrease of crystal diameter, i.e., the decrease of constitutional supercooling. The reason was the increase of temperature gradient caused by down-sizing of the crystal diameter deduced by down-sizing of the growth furnace. The adequate diameter for transparent crystal was determined depending on the Al content.     

Mechanism for generating interstitial atoms by thermal stress during silicon crystal growth

It has been known that, in growing silicon from melts, vacancies (Vs) predominantly exist in crystals obtained by high-rate growth, while interstitial atoms (Is) predominantly exist in crystals obtained by low-rate growth. To reveal the cause, the temperature distributions in growing crystal surfaces were measured. From this result, it was presumed that the high-rate growth causes a small temperature gradient between the growth interface and the interior of the crystal; in contrast, the low-rate growth causes a large temperature gradient between the growth interface and the interior of the crystal. However, this presumption is opposite to the commonly-accepted notion in melt growth. In order to experimentally demonstrate that the low-rate growth increases the temperature gradient and consequently generates Is, crystals were filled with vacancies by the high-rate growth, and then the pulling was stopped as the extreme condition of the low-rate growth. Nevertheless, the crystals continued to grow spontaneously after the pulling was stopped. Hence, simultaneously with the pulling-stop, the temperature of the melts was increased to melt the spontaneously grown portions, so that the diameters were restored to sizes at the moment of pulling-stop. Then, the crystals were cooled as the cooling time elapsed, and the temperature gradient in the crystals was increased. By using X-ray topographs before and after oxygen precipitation in combination with a minority carrier lifetime distribution, a time-dependent change in the defect type distribution was successfully observed in a three-dimensional manner from the growth interface to the low-temperature portion where the cooling progressed. This result revealed that Vs are uniformly introduced in a grown crystal regardless of the pulling rate as long as the growth continues, and the Vs agglomerate as a void and remain in the crystal, unless recombined with Is. On the other hand, Is are generated only in a region where the temperature gradient is large by low-rate growth. In particular, the generation starts near the peripheral portion in the vicinity of the solid–liquid interface. First, the generated Is are recombined with Vs introduced into the growth interface, so that a recombination region is always formed which is regarded as substantially defect free. Excessively generated Is after the recombination agglomerate and form a dislocation loop region. Unlike conventional Voronkov's diffusion model, Is hardly diffuse over a long distance. Is are generated by re-heating after growth.[In a steady state, the crystal growth rate is synonymous with the pulling rate. Meanwhile, when an atypical operation is performed, the pulling rate is specifically used.]This review on point defects formation intends to contribute further silicon crystals development, because electronic devices are aimed to have finer structures, and there is a demand for more perfect crystals with controlled point defects.