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)     

Influence of internal radiation on the heat transfer during growth of YAG single crystals by the Czochralski method

Heat and mass transfer taking place during growth of Y₃Al₅O₁₂ (YAG) crystals by the Czochralski method, including inner radiation, is analyzed numerically using a Finite Element Method. For inner radiative heat transfer through the crystal the band approximation model and real transmission characteristics, measured from obtained crystals, are used. The results reveal significant differences in temperature and melt flow for YAG crystals doped with different dopands influencing the optical properties of the crystals. When radiative heat transport through the crystal is taken into account the melt‐crystal interface shape is different from that when the radiative transport is not included. Its deflection remains constant over a wide range of crystal rotation rates until it finally rapidly changes in a narrow range of rotation rates. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature gradient controlled growth and optical properties of Er:BaY2F8 crystals

Single crystals of Erbium (Er) doped BaY₂F₈ have been obtained by the temperature gradient technique (TGT). No‐seed‐grown crystal of Er:BaY₂F₈, with the dimensions up to several centimeters, was obtained by self‐crystallization. The optimizations of various growth parameters were systemically investigated. The results indicated that the temperature gradient of 6‐7 K/mm and the cooling velocity less than 6 K/h were suitable for the crystal growth. The XRD data and the investigations on the growth striations by a stereo polarization microscope displayed that the [001] direction is the dominating direction for the crystal growth. The crystal grown by TGT often cracks along with the (100) plane, which is caused by the excessive decrease of the temperature during the crystal growth, for there is a rapid change in the thermal expansion curve of the BaY₂F₈ crystal in the temperature range from 800 °C to 900 °C. The spectral properties of Er:BaY₂F₈ single crystals have been studied and the effects of frequency up‐conversion of the crystals are reported. Spectral data suggest that the quality of Er:BaY₂F₈ crystal obtained by TGT method is good and the crystal has the potential application in laser devices. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The impact of pressure and temperature on growth rate and layer uniformity in the sublimation growth of AlN crystals

To effectively design a large furnace for producing large-size AlN crystals, a fully coupled compressible flow solver was developed to study the sublimation and mass transport processes in AlN crystal growth. Compressible effect, buoyancy effects, flow coupling between aluminum gas and nitrogen gas, and Stefan effect are included. Two sets of experimental data were used to validate the present solver. Simulation results showed that the distributions of Al and N₂ partial pressures are opposite along the axial direction due to constant total pressure and Stefan effect, with the Al and nitrogen partial pressures being highest at the source and seed crystals positions, respectively. The distributions of species inside the growth chamber are obviously two-dimensional, which can curve a flat crystal surface. Simulation results also showed that AlN crystal growth rate can be increased by reducing total pressure or by increasing seed temperature or by increasing source-seed temperature difference. High nitrogen pressure causes decrease in growth rate, but it is beneficial for obtaining uniform growth rate in the radial direction. Results of simulation also showed that there is an optimized temperature difference (40 °C) in the present furnace for obtaining good homogeneity of growth rate.     

A new piezoelectric single crystal obtained by Ge doping in the SiO2 structure

The interest of Si_1–xGe_xO₂ single crystals with alpha‐quartz structure is connected to improvement of electromechanical coefficients and rise of α – β phase transition of quartz one. Growth of an α‐Si_xGe_1–xO₂ crystal was realized by a hydrothermal method of temperature gradient in autoclaves, made from Cr–Ni alloys. Nutrient material was prepared from synthetic quartz as crashed rods and placed in the bottom of autoclaves. There was loaded GeO₂ powder additive in proportions to quartz nutrient. Single crystals were investigated by electron microprobe analysis, X‐ray diffraction and atomic force microscopy. The most important result, which was obtained during the investigations, is an experimental proof of growth of α‐Si_xGe_1–xO₂ single crystals under the hydrothermal conditions. The present results thus open the possibility to tune the piezoelectric properties of these materials by varying the chemical composition. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Improved blue photorefractive properties of near‐stoichiometric LiNbO3:Mn:Fe:Zr crystal

Near‐stoichiometric LiNbO₃ single crystal tri‐doped with ZrO₂, MnO and Fe₂O₃ was grown from Li‐riched melt by Czochralski method. The defect structures and composition of these crystals were analyzed by means of ultraviolet‐visible and infrared transmittance spectra. The appearance of 3466 cm^‐1 peak in infrared spectra showed that the crystal grown from Li‐riched melt was near stoichiometric. The photorefractive properties at the wavelength of 488 nm and 633 nm were investigated with two‐beam coupling experiment, respectively. The experimental results showed that the response speed and sensitivity were enhanced significantly and the high diffraction efficiency was obtained at 488 nm wavelength. This manifested that near‐stoichiometric LiNbO₃:Mn:Fe:Zr crystal was an excellent candidate for holographic storage. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bridgman growth and characterization of birefringent crystal NaNO3

With a bigger birefringence than calcite, sodium nitrate (NaNO₃) single crystal might have potential applications in fiber‐optic polarizer, opto‐isolators and polarizing prisms. However, it is difficult to grow large‐size NaNO₃ crystals for their thermal conductivity anisotropy and phase transition at 275°C. In this paper, crack‐free NaNO₃ crystals with size of Φ20×100 mm³ were prepared by Bridgman method with lower growth rate and lower interface temperature gradient. The dependence of the transmittance on the sample exposure time was measured and studied. The principal refractive indices of NaNO₃ crystal at the wavelengths 0.4730, 0.5320, 0.6328, 1.064 and 1.338 μm were measured by auto‐collimation method. From which, we calculated and obtained the Sellmeier's equation of NaNO₃ crystal. Moreover, the photoluminescence spectra were detected under the excitation at 240 nm, and NaNO₃ crystal presented its fluorescence around 416 nm.     

The role of inner and internal radiation on the melt growth of sapphire crystal

In this paper, for an inductively heated Czochralski furnace used to grow sapphire single crystal, influence of the inner (wall‐to‐wall) and crystal internal (bulk) radiation on the characteristics of the growth process such as temperature and flow fields, structure of heat transfer and crystal‐melt interface has been studied numerically using the 2D quasi‐steady state finite element method. The obtained results of global analysis demonstrate a strong dependence of thermal field, heat transport structure and crystal‐melt interface on both types of radiative heat transfer within the growth furnace.     

Growth,crystal structure and spectrum of a novel rare‐earth orthophosphate crystal: Yb:LuPO4

Yb: LuPO₄ crystals with the size up to 6×2×0.5mm³ were grown by the flux growth process using lead pyrophosphate Pb₂P₂O₇ as the high‐temperature solvent. The crystal structure of Yb: LuPO₄ crystals at room temperature was refined by using single crystal X‐ray diffraction data. Crystal structure analysis showed that Yb: LuPO₄ crystals possessed the tetragonal xenotime structure. The polarized absorption spectra of Yb: LuPO₄ were tested at room temperature. The results showed that the absorption spectral region of Yb: LuPO₄ crystal was well matched for pumping with readily available diode lasers.     

Flux growth and characterizations of NdPO4 single crystals

Neodymium phosphate single crystals, NdPO₄, have been grown by a flux growth method using Li₂CO₃-2MoO₃ as a flux. The as-grown crystals were characterized by X-ray powder diffraction(XRPD), differential thermal analysis (DTA) and thermogravimetric analysis (TG) techniques. The results show that the as-grown crystals were well crystallized. The crystal was stable over the temperature range from 26 to 1200 °C in N₂. The specific heat of NdPO₄ crystal at room temperature was 0.41 J/g °C. The absorption and the fluorescence spectra of NdPO₄ crystal were also measured at room temperature.     

Effective electro‐optic coefficient of (1–x)Pb(Zn1/3Nb2/3)O3–xPbTiO3 single crystals

Refractive indices and effective electro‐optic coefficient γ_c of (1–x)Pb(Zn_1/3Nb_2/3)O₃–xPbTiO₃ (PZN‐xPT, x = 0.05, 0.09 and 0.12) single crystals were measured at 532 nm wavelength. Orientation and temperature dependences of the electro‐optic coefficient were investigated. Large electro‐optic coefficient (γ_c = 470 pm/V) was observed in [001]‐poled PZN‐0.09PT crystal. More importantly, γ_c of tetragonal PZN‐0.12PT is almost unchanged in a temperature range −20 ∼ 80 °C. The γ_c of PZN‐xPT single crystals are much higher than that of widely used electro‐optic crystal LiNbO₃ (γ_c = 20 pm/V). These results show that PZN‐xPT single crystals are very promising materials for electro‐optic modulators in optical communications.     

Zur Züchtung von CaF2-UND CaF2(U)-einkristallen aus der dampfphase

Monocrystals of pure and uranium-doped CaF₂ have been grown from the vapour phase in graphite crucibles using a vertical moving gradient method with seed-selection. The growth technique and the properties of the vapour phase grown crystals are described. Evaporation temperatures: 1450–1550 °C, δT≌ 65 °C, growth temperatures: ca. 1300–1400 °C. Dopant: UF₄. Size of the crystals up to 18 mm in diameter and 50 mm in length. In accordance to the low colour center formation by X-raying the pure CaF₂ crystals contain only a small number of lattice defects. The uranium content and distribution in the doped crystals depends strongly on vaporization temperature and inner shape of the crucibles, the valence of the uranium in the CaF₂(U) crystals is mainly determined by the fluorine vapour pressure in the growth system. Tetravalent U can be incorporated in the presence of PbF₂ and trivalent U under high vacuum.     

Single‐crystal growth and characterization of mullite‐type orthorhombic Bi2M4O9 (M = Al3+, Ga3+, Fe3+)

Pure and homogeneous single crystals of orthorhombic mullite‐type Bi₂M₄O₉ (M = Al³⁺, Ga³⁺, Fe³⁺), and a mixed Bi₂Fe_1.7Ga_2.3O₉ crystal from an equimolar Ga/Fe composition were grown by the top seeded solution growth (TSSG) method. All these compounds melt incongruently in the range of about 800 and 1100 °C. In case of bismuth gallate and ferrate inclusion‐free crystals with dimensions up to several cubic centimeters can be grown. Limited solubility in Bi₂O₃ and the high steepness of the liquidus curve are the reasons for getting only small imperfect bismuth aluminate crystals. In contrast to ceramic materials preparation reported in literature, divalent calcium and strontium could not be incorporated into the mullite‐type structure during the melt growth process. Several fundamental physical properties like heat capacity, thermal expansion, heat conductivity, elastic constants, high‐pressure behavior and oxygen diffusivity were determined by different research groups using single‐crystalline samples from the as‐grown materials. Furthermore, the refractive indices of Bi₂Ga₄O₉ were measured in the range of 0.430 and 0.700 μm. Such as many other bismuth containing compounds the refractive indices of Bi₂Ga₄O₉ are larger than 2, and Bi₂Ga₄O₉ is an optic biaxial positive crystal. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical defects in barium—strontium niobate single crystals

The distributions of edge dislocations and residual mechanical stresses in Ba_xSr_1-xNb₂O₆ (BSN) crystals are investigated and the explanation of the nature of the “growth column” is proposed. The “growth column” is a defect zone going through all of the crystal and usually repeating in its cross-section the contour of the seed crystal. The “growth column” boundary is the closed contour with extremely high edge dislocation density. These dislocations are connected with thermal stresses due to seed-melt contact or abrupt crystal widening. Under proper crystal seeding and widening conditions one can obtain the BSN crystals with dislocation densities less than 10 cm⁻² and without the “growth column”. The method of chemico-mechanical polishing of BSN crystals not forming a defect layer on the surface of the crystals have been developed. The high temperature diffusion annealing is shown to eliminate the growth striae in BSN 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.     

Control of the Growth Optimum in Producing High-Quality CaF2 Crystals by an Improved Bridgman-Stockbarger Technique

The axial temperature distribution in an empty crucible and the determination of quenched interface within fixed crucible were investigated to find the relationships governing the growing process of CaF₂ by Bridgman-Stockbarger methods. A system of additional molybdenum reflective sheaths was placed into the furnace unit and its effect on the temperature field was estimated in order to achieve the growth optimum. A long liner into the diaphragm bore of the furnace and a number of thin rings slipped on the crucible stem were found capable to provide a reproducible optimum on each stage of growing in single and multicamera crucibles at a relatively low level of power supply. Single and in-group growth experiments were carried out using high density quenched ingots purified from oxygen contaminants through high temperature treatment of fluorspar. The properties of the grown crystals were found to depend on the degree of the fluorspar purification and on the improvement of the thermal conditions yielding either competitor or laser-grade CaF₂. In multicamera crucibles the crystals grown in the central compartment are of better characteristics than those in the peripheral ones.     

Synthesis and characterization of a new nonlinear optical single crystal: L‐Lysinium trifluoroacetate

Single crystals of a new L‐Lysine salt: L‐Lysinium trifluoroacetate {abbreviated as LLyTFA; [(NH₂)‐(CH₂)₄‐CH‐(NH₃)‐(COOH)]⁺ CF₃COO^‐} were grown by slow evaporation of an aqueous solution at room temperature. The grown crystals were subjected to single crystal X‐ray diffraction, FTIR and UV‐Vis‐NIR spectrum analyses. The UV‐Vis‐NIR spectrum shows that the absorption is very less in the whole of the region from ultraviolet to near IR. The Kurtz‐Perry powder SHG measurement using a Nd:YAG laser of wavelength 1064nm confirms the frequency doubling of the crystal and its powder SHG efficiency was measured as d_eff = 0.96 d_eff (KDP). (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and characterization of the nonlinear optical crystal: L‐arginine trifluoroacetate

Single crystal growth of the organic nonlinear optical crystal, L‐arginine trifluoroacetate (L‐Arg·CF₃COOH, abbreviated as LATF) is reported. Low temperature solution growth method is employed for the growth of bulk single crystals. The cell parameters are verified by single crystal diffraction. Fourier transform infrared (FT‐IR) and Fourier transform Raman analysis are used to confirm the presence of various functional groups in the grown crystal. The thermal properties of the grown crystals are studied by thermogravimetric analysis and differential scanning calorinetry analysis (TGA/DSC). Second harmonic generation (SHG) measurement confirms the NLO properties of the grown crystal. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal structure refinement from electron diffraction data

A procedure of crystal structure refinement from electron diffraction data is described. The electron diffraction data on polycrystalline films are processed taking into account possible overlap of reflections and two-beam interaction. The diffraction from individual single crystals in an electron microscope equipped with a precession attachment is described using the Bloch-wave method, which takes into account multibeam scattering, and a special approach taking into consideration the specific features of the diffraction geometry in the precession technique. Investigations were performed on LiF, NaF, CaF₂, and Si crystals. A method for reducing experimental data, which allows joint electron and X-ray diffraction study, is proposed.     

Characterization of large‐sized Nd:YAG single crystals grown by horizontal directional solidification

Nd³⁺‐doped Y₃Al₅O₁₂ single crystals have been grown by the horizontal directional solidification (HDS) method in different thermal zone. The Grashof (G_r), Prandtl (P_r), Marangoni (M_a) and Rayleigh (R_a) numbers of melt in HDS system have been discussed for our experimental system to understand the mechanism of melt flow patterns and concentration gradient of dopant. The concentration gradient of Nd³⁺ ions was explained with melt flow processes during crystal growth in different thermal zone, and results indicated that high growth temperature will be helpful for uniformity of dopant in HDS‐grown single crystal. The main microscopic growth defects such as bubbles and irregular inclusions in HDS‐grown Nd:YAG crystals were observed, and the causes were discussed as well. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)