Kyropoulos Research Articles

Numerical study on the effect of additional resistive heating and crystal rotation on sapphire single crystals (Al2O3) grown by the Kyropoulos method

Reduction of melt–crystal interface convexity during Kyropoulos growth of sapphire single crystals through rotation and crucible bottom heating.

Crystal growth, electronic structure and optical properties of Sr2Mg(BO3)2

Single crystals of Sr2Mg(BO3)2 (SMBO) were grown by Kyropoulos method. X-ray powder diffraction (XRD) analysis, transmission spectrum, thermal properties, band structure, density of states and charge distribution as well as Raman spectra of SMBO were described. The as-grown SMBO crystals show wide transparency range with UV cut-off below 180nm. A direct band gap of 4.66eV was obtained from the calculated electronic structure results. The calculated band structure and density of states results indicated the top valence band is determined by O 2p states whereas the low conduction band mainly consists of Sr 5s states. Twelve Raman peaks were observed in the experimental spectrum, fewer than the number predicted by the site group analysis. Raman peaks of SMBO were assigned combining first-principle calculation and site group analysis results. The strongest peak at 917cm−1 in the experimental spectrum is assigned to symmetric stretching mode A1′(ν1) of free BO3 units. SMBO is a potential Raman crystal which can be used in deep UV laser frequency conversion.

Chirality versus dichroism: Competition and role in conical diffraction displayed from the Nd:Bi2ZnOB2O6 acentric biaxial laser crystal

We have grown acentric chiral Nd-doped BZBO single crystals by the Kyropoulos method, cut perpendicular to the optical axis. With a focused beam at wavelengths in the spectral domain of transparency with various polarizations, the conoscopy patterns revealed the specific role of chirality: disappearance of the optical axis replaced by two C-directions, linear polarization rotation and spiral patterns. The specific rotatory power was measured. The output near field exhibits the influence of chirality on the conical diffraction. Going to wavelengths in the spectral domain of absorption corresponding to the 4I9/2 → 4F5/2 Nd3+ transition, we first studied the chirality/dichroism competition from propagation along the C-directions, characterizing the circular and elliptic eigen-modes. We exhibited the wavelengths at which the ellipse degenerates linearly, signature of the chirality/dichroism balancing. The second aspect is the existence of the optical axis which can be restored if the dichroism is high enough. In this case a singular Voigt wave can propagate. From the experimental determination of the optical parameters, a theoretical description has been proposed, leading to quite well agreement in the far field (conoscopy) and in the near field (conical diffraction).

Refractive index of infrared-transparent polycrystalline alumina

The refractive index of fully dense, infrared-transparent polycrystalline alumina (PCA) with a mean grain size of ∼0.6 μm is reported for the wavelength range 0.85 to 5.0 μm over the temperature range T=296 to 498 K. The temperature-dependent Sellmeier equation is n2−1=(A+B[T2−To2])λ2/[λ2−(λ1+C[T2−To2])2]+Dλ2/(λ2−λ22), where λ is expressed in μm, To=295.15 K, A=2.07156, B=6.273×10−8, λ1=0.091293, C=−1.9516×10−8, D=5.62675, λ2=18.5533, and the root-mean square deviation from measurements is 0.0002. This paper describes how to predict the refractive index of fully dense isotropic PCA with randomly oriented grains using the ordinary and extraordinary refractive indices (no and ne) of sapphire spatially averaged over the surface of a hemisphere. The refractive index of alumina at 296 and 470 K agrees within ±0.0002 with the predicted values. Similarly, the ordinary and extraordinary optical constants ko and ke are used to predict the absorption coefficient of alumina. The refractive indices no and ne of sapphire grown at Rubicon Technologies by the Kyropoulos method were measured at 295 K and agree with published Sellmeier equations for sapphire grown by other methods within ±0.0002.

Bifunctional Bi2ZnOB2O6:Nd3+ Single Crystal for Near Infrared Lasers: Luminescence and μ-Raman Investigations

Bi2ZnOB2O6 single crystal doped with Nd3+ ions is characterized by high values of nonlinear optical coefficients as well as the effective luminescence of excited Nd3+ ions, which make this system a unique candidate for near infrared to visible laser converters. The investigated Bi2ZnOB2O6:Nd3+ single crystal was grown by means of the Kyropoulos method. The vibrational properties of Bi2ZnOB2O6:Nd3+ were studied using μ-Raman spectroscopy. In particular, the Raman-active modes detected in parallel and cross-polarizations were assigned to the vibrations of the characteristic molecular groups BO3, BO4, ZnO4, and BiO6. In the absorption spectra of Bi2ZnOB2O6:Nd3+ the bands related to the optical transitions from the 4I9/2 ground state to the excited states of Nd3+ ions were detected at 432, 515, 528, 533, 577, 586, 689, 750, 810, 874, and 1605 nm. Moreover, the strong emission of Bi2ZnOB2O6:Nd3+ with a maximum at about 1062 nm (4F3/2 → 4I11/2 transition) was detected under the excitation at 514 nm. The decay k...

Interface effect on titanium distribution during Ti-doped sapphire crystals grown by the Kyropoulos method

Large ingot Ti-doped sapphire crystals were successfully grown by Kyropoulos method. Optical characterization of 10 cm diameter crystals shows non-uniform radial distribution of titanium. The measurements of Ti3+ ion distribution in several slices cut perpendicular to the growth direction show that the concentration is higher at the periphery of the crystal as compared to the central part of the ingot. Numerical modeling is applied to investigate heat transfer, melt convection and species transport during the Kyropoulos growth process. The transient simulation shows an unsteady convection generated by the strong interaction between the flow and the thermal field. The distribution of Ti in the melt is nearly uniform due to the intense convective mixing. The radial distribution of titanium depends mainly on the shape of the crystal-melt interface. The measured U-shaped concentration profile is explained by accounting the conical shape of the interface. High curvatures of the growth interface observed in experiments are explained by the increased thermal conductivity of the sapphire crystal, which is participating to the radiative heat transfer. The interface curvature depends on the absorption coefficient, which is higher for Ti-doped sapphire crystals than for undoped crystals. Therefore, the initial Ti concentration in the melt should be decreased in order to reduce the absorption coefficient and the interface curvature. The comparison of numerically computed titanium distribution in the crystals to experimental measurements shows a good agreement.

Development of Auto-Seeding System Using Image Processing Technology in the Sapphire Crystal Growth Process via the Kyropoulos Method

The Kyropoulos (Ky) and Czochralski (Cz) methods of crystal growth are used for large-diameter single crystals. The seeding process in these methods must induce initial crystallization by initiating contact between the seed crystals and the surface of the melted material. In the Ky and Cz methods, the seeding process lays the foundation for ingot growth during the entire growth process. When any defect occurs in this process, it is likely to spread to the entire ingot. In this paper, a vision system was constructed for auto seeding and for observing the surface of the melt in the Ky method. An algorithm was developed to detect the time when the internal convection of the melt is stabilized by observing the shape of the spoke pattern on the melt material surface. Then, the vision system and algorithm were applied to the growth furnace, and the possibility of process automation was examined for sapphire growth. To confirm that the convection of the melt was stabilized, the position of the island (i.e., the center of a spoke pattern) was detected using the vision system and image processing. When the observed coordinates for the center of the island were compared with the coordinates detected from the image processing algorithm, there was an average error of 1.87 mm (based on an image with 1024 × 768 pixels).

Synchrotron X-ray diffraction imaging studies of dislocations in Kyropoulos grown Ti doped sapphire crystal

In this study, X-ray diffraction and X-ray topography, using synchrotron radiation source, were used to analyse the nature of defects in a sapphire single crystal sample grown by Kyropoulos method. Qualitative and quantitative analysis were carried out on the results of the topography experiments. The dislocation density was found to be around 103–104 dislocations/cm2 indicating a crystal of good crystalline quality. Also, the variation of dislocation density with respect to the position on the sample was observed and discussed.

Comparison between numerical modeling and experimental measurements of the interface shape in Kyropoulos growth of Ti-doped sapphire crystals

Numerical modeling is applied to investigate the factors affecting the shape of the crystal-melt interface during the growth of Ti-doped sapphire crystals by using the Kyropoulos method. Numerical results are compared to experimental visualization of the growth interface in the case of ingots grown in crucibles of 15cm in diameter. The transient computations of the heat transfer and melt convection show that the interface curvature depends on the internal radiative effect in the sapphire crystal. The effective thermal conductivity increases significantly in the case of Ti-doped crystals, leading to conical shapes of the interface with large curvatures. The growth interface is less curved in the case of non-doped sapphire crystals which have a smaller absorption coefficient. The convection driven by buoyancy and Marangoni effects has also a strong effect on the interface shape. The intensity of the Marangoni flow increases significantly during the shouldering stage of the growth, leading to a more curved interface with a convex-concave shape. The comparison between numerically computed interface deflection and the experimental results shows a good agreement. According to present numerical analysis, the formation of a plateau and the temporal concave shape of the crystal are related to unfavorable thermal conditions at the beginning of the growth process.

Spectroscopic properties of Bi2ZnOB2O6 single crystals doped with Pr3+ ions: Absorption and luminescence investigations

Nonlinear optical Bi2ZnOB2O6 single crystals doped with Pr3+ ions were grown using the Kyropoulos method. The absorption and luminescence properties of these new systems were investigated for the first time. The crystals are characterized by the large values of nonlinear optical coefficients. Effective luminescence of the Pr3+ ions makes this system an excellent candidate for the near-infrared (NIR) and/or ultraviolet (UV) to visible (VIS) laser converters. Based on the obtained experimental spectroscopic data, detailed analysis of the absorption and luminescence spectra was performed using the conventional Judd–Ofelt theory. Those transitions, which can be potentially used for laser applications of the Pr3+ ion, have been identified. In addition to the intensity parameters Ω2, Ω4, Ω6 the branching ratios and radiative lifetimes were estimated for all possible transitions in the studied spectral region.

A conical shape-evolution model for the control of sapphire crystal growth in Kyropoulos (KY) method

Generally, the process of sapphire crystal growing in the KY furnace has to be monitored in order to reduce the power of heater continuously as the crystal grows to occupy the crucible. However, it has been difficult to control the heater power as the weight and diameter of crystal change because the solid-liquid interface could not be easily located from on-site measurement. Hence, first, the model was developed to locate the crystal-melt interface and to estimate its dynamic feature during the crystal growth by incorporating the mass and the force balances. This model was applied to analyze the data obtained from the industry-scale crystal growth experiment (for the production of the sapphire crystals in the KY method). Finally, the shape and the dynamic feature of crystallizing front are analytically verified by generating some bands of small bubbles along the crystal-melt interface called a comb-pattern experiment.

Crystal growth of sapphire for substrates for high‐brightness, light emitting diodes

Sapphire has emerged as the preferred substrate for high‐brightness, light emitting diodes (HB‐LEDs) made with a GaN epilayer. While the growth of most crystalline materials for electronic and laser applications normally converges onto a single, preferred method—Czochralski growth for silicon, lithium niobate and YAG (yttrium aluminum garnet), vertical gradient freeze (VGF) and Liquid Encapsulated Czochralski (LEC) for GaAs crystals (depending upon current density and device application), vapor transport for SiC, for example—a wide range of processes are employed for sapphire growth. These include Czochralski, Kyropoulos, EFG (Edge‐defined, Film‐fed Growth), Bagdasarov, classical Bridgman and several variants of Bridgman such as HEM (Heat Exchanger Method) and CHES (Controlled Heat Extraction System).This variety of approaches to grow crystals of what is basically the same material reflects a balance between acceptable defect levels, the costs of downstream fabrication steps such as wire sawing, and the wafer diameters demanded by the market. Each process has advantages and disadvantages. The Kyropoulos method, for example, probably yields crystals with the lowest defect levels, while EFG eliminates the wire sawing stage of wafer fabrication which is a major cost factor. Large sapphire crystals grown by HEM are frequently oxygen deficient, and a‐axis Czochralski crystals have higher dislocation densities than Kyropoulos boules.Facet formation on the c‐plane is an important criterion in selecting the growth methodology for sapphire. However, we will show that melt grown sapphire crystals have a curved interface that passes through other facet planes, and this can impact the growth process.In this paper we will summarize the various crystal growth methods currently employed to grow sapphire, discuss the strengths and weaknesses of each approach and also discuss the often overlooked thermodynamic aspects of this high‐temperature crystal growth process. We will attempt to show the long‐term trends in the evolution of the various growth processes and predict the ultimate winner of the contest to supply substrates.

Characterization of low angle grain boundary in large sapphire crystal grown by the Kyropoulos method

Low angle grain boundaries (LAGB) are one of the most commonly seen defects in sapphire crystals. In this paper, we have studied the origin, topography and orientation of LAGB in large sapphire crystals grown by the Kyropoulos method through etching, polaroscopy, optical microscopy as well as synchrotron white-beam X-ray topography. The results show that the LAGB starts mainly from the atomic layers mismatch caused by environmental fluctuations. The misorientation angle of the grain boundaries is 2–3°, and the grain boundaries are distributed around 〈101¯0〉 direction.

Influence of temperature-dependent thermophysical properties of sapphire on the modeling of Kyropoulos cooling process

Sapphire is widely used as substrates in semiconductor industry. Kyropoulos (Ky) method is the most popular technique to grow large-size and substrate-quality sapphire single crystals. Computational modeling is a powerful tool to analyze the growth conditions for the improvement of this crystal growth. for simulation, thermophysical properties of sapphire are usually set constant, which may increase the deviation between experiments and modeling results. In this paper, temperature-dependent properties of sapphire, i.e., heat capacity, thermal conductivity, and thermal expansion coefficient, are summarized and adopted in the simulation studies. Their effects on the predicted results are discussed for the cooling process, during which large temperature change in the crystal leads to big differences of the material properties. Comparative analysis tells that temperature difference and thermal stress can be more than 200K and 4MPa for the cases with or without consideration of the temperature-dependence of the properties. Consequently, it is necessary to consider the impact of material properties on the accuracy of numerical simulation during the sapphire single crystal growth.

Vibrational properties of nonlinear optical Bi2ZnOB2O6 single crystals doped with Pr3+: μ-Raman investigations

Nonlinear optical Bi2ZnOB2O6 single crystals doped with Pr3+ ions were grown using the Kyropoulos method. The crystals are characterized by the large values of nonlinear optical coefficients as well as the effective luminescence of exciting Pr3+ ions, which makes them an excellent candidate for NIR to VIS laser converters. In this study the vibrational properties of Bi2ZnOB2O6 single crystals doped with Pr3+ ions were investigated for the first time with the use of μ-Raman spectroscopy. On the basis of Raman spectra, the modes symmetry was analyzed and the modes assignment was made to characteristic molecular groups BO3, BO4, ZnO4 and BiO6. The Raman scattering data showed that the NLO properties of Bi2ZnOB2O6:Pr3+ crystals correspond to the low frequency lattice modes associated with BO3 and BO4 groups as well as Bi3+and Zn2+ ions.

Growth of large diameter ZnTe single crystals by the double crucible liquid encapsulated pulling method

AbstractIn our work, large diameter ZnTe single crystals were grown by various melt growth methods. By a liquid encapsulated vertical gradient freezing method, crystals with large grains and low dislocation density were obtained. By a liquid encapsulated Kyropoulos method, 80 mm diameter crystals were successfully grown from ZnTe seed crystals. However, crystals grown by the both methods had large strain and cracks owing to difference in coefficient of thermal expansion between an encapsulant and ZnTe. Therefore, a new crystal growth method was developed. Two crucibles were used in the new method. It was named double crucible liquid encapsulated pulling method. Crystals up to 100 mm in diameter and 40 mm in length were reproducibly obtained. Both <100> and <110> crystals were successfully grown. The etch pit density of the grown crystal was lower than 10,000 cm–2 and any large strain or cracks were not observed. In order to reduce Te inclusions, wafer annealing was examined. Most Te inclusions were eliminated with suitable Zn pressure and annealing temperature. Optical transparency of the ZnTe crystal increased after annealing. This is because scatter of the incident light by Te inclusions decreased. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Multifunctional Bi2ZnOB2O6 single crystals for second and third order nonlinear optical applications

Bi2ZnOB2O6 nonlinear optical single crystals were grown by means of the Kyropoulos method from stoichiometric melt. The second and third harmonic generation (SHG/THG) of Bi2ZnOB2O6 crystals were investigated by the SHG/THG Maker fringes technique. Moreover, SHG microscopy studies were carried out providing two-dimensional SHG images as a function of the incident laser polarization. The high nonlinear optical efficiency combined with the possibility to grow high quality crystals make Bi2ZnOB2O6 an excellent candidate for photonic applications.

Role of internal radiation at the different growth stages of sapphire by Kyropoulos method

Internal radiation is investigated using discrete ordinates (DO) model for the Kyropoulos growth of large-size LED-grade sapphire crystals. Its effects on the solid/liquid interface shape, melt convection, temperature and thermal stress are presented for the different growth stages characterized by the crystal length. The results are validated by observing cracked locations and remelting regions of the grown ingot. Internal radiation makes temperature distribution uniform by increasing heat transfer ability of the material, weakens natural and Marangoni convections in the melt, and reduces stress level in the crystal. It is also found that a slightly distorted interface is predicted affected by internal radiation, which is consistent with experimental observation. More reasonable flow patterns in the melt can be achieved taking into account the internal radiation. Natural convection at the early growth stage and Marangoni convection at the late growth stage are responsible for the interface and surface remelting. By analyzing thermal stresses, it is further concluded that the excessive stress is not the major cause of crystal cracking during Ky-sapphire growth. The maximum von Mises stress is always below the critical value predicted by other researchers. Consequently, the locally polycrystalline growth or the three-dimensionally non-uniform heating conditions are the most possible factors of crystal cracking.

Crystal cracking analysis and three‐dimensional effects during Kyropoulos sapphire growth

Kyropoulos (Ky) method is the most suitable technique for the growth of high‐quality sapphire single crystal for substrate applications. Cracks are often observed in the grown ingot that significantly reduces industrial productivity. In the paper, cracking causes are analyzed by examining crystal shape, thermal stress and three‐dimensional effects during the stable growth of sapphire crystal. It is found that locally induced thermal stress around the shoulder of the crystal is the largest. However, thermal stress is not fully responsible for the cracks, since the predicted stress level is lower than a critical value regarding crystal cracking. Polycrystalline growth or/and other crystal defects must be another factor that degrades the critical value and makes the crystal more fragile. Simulation results further show that crystal shape has less effect on the thermal stress level, although experiments have shown that crack‐free crystals usually have smooth surfaces. The initial cracking position in the ingot predicted in simulation agrees well with experimental observation after considering crystal defects in a qualitative discussion. From the view of three‐dimensional simulation, the variation of heating condition during growth may result in high thermal stress locally that leads to the cracks at one side of the crystal. Additionally, three‐dimensionally unexpected temperature drop of the heater may be responsible for the sticking‐to‐crucible phenomenon at the shoulder region of the grown crystal.

전산해석을 통한 키로플러스 사파이어 단결정 성장공정의 유동 및 remelting 현상 분석

사파이어(<TEX>$Al_2O_3$</TEX>) 단결정 웨이퍼는 청색 LED(light emitting diode) 제작을 위한 핵심 소재로 사용되고 있으며, 사파이어 단결정의 품질에 따라 LED의 성능이 크게 좌우하게 된다. 여러 가지 사파이어 단결정 제조방법 중 키로플러스(Kyropoulos)법은 도가니 직경에 근접한 크기로 잉곳 생산이 가능하며, 내부 전위밀도가 낮아 고품질의 대구경 사파이어 잉곳 제작이 가능하다. 키로플러스법 공정에서 용융 알루미나의 유동은 seed의 성장 형태, 도가니 및 단열재의 형상에 영향을 받으며, 유동양상에 따라 단결정 사파이어 잉곳의 품질이 좌우된다. 특히 온도구배는 hot-zone 내부의 히터 구조와 밀접한 관련이 있으므로 본 연구에서는 도가니 단위표면적당 하부와 측면 히터의 발열비율에 따른 CFD(computational fluid dynamics) 해석을 실시하고, 해석결과를 토대로 각각 용융 알루미나의 유동 및 remelting 현상에 대해 분석하였으며, 이상적인 히터 발열비율을 도출하였다. Sapphire wafers are used as an important substrate for the production of blue LED (light emitting diode) and the LED's performance largely depends on the quality of the sapphire single crystals. There are several crystal growth methods for sapphire crystals and Kyropoulos method is an efficient way to grow large diameter and high-quality sapphire single crystals with low dislocation density. During Kyropoulos growth, the convection of molten melt is largely influenced by the hot zone geometry such as crucible shape, heater and refractory arrangements. In this study, CFD (computational fluid dynamics) simulations were performed according to the bottom/side ratios (per unit of the crucible surface area) of heaters. And, based on the results of analysis, the molten alumina flows and remelting phenomena were analyzed.