Growth and evaluation of lanthanoids orthoniobates single crystals processed by a miniature pedestal growth technique

Optimized conditions for the growth of lanthanoids orthoniobates (LnNbO₄, Ln = lanthanide elements) single crystal minirods by a floating zone technique were investigated. Adequate atmospheres and pulling to feeding speed ratios to grow these materials were determined. Emphasis is given to the study of LaNbO₄ because of their more favorable growth conditions and crystalline quality. This material can be efficiently doped with rare earth elements such as erbium. It grows with high crystallinity and its preferential growth direction is [1 1 0]. A preliminary evaluation of optical properties of Er³⁺‐doped LaNbO₄ single crystal under the Judd‐Ofelt formalism indicates spectral parameters Ω_t close and even larger than for Er³⁺ ions in YVO₄. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Singular ring‐shaped distribution of Nd in NdxY1‐xVO4 crystals grown by floating zone method

A singular ring‐shaped distribution of high Nd concentration was observed in Nd‐doped YVO₄ single crystals grown by the floating zone (FZ) method. The ring‐shaped distribution appeared 500‐1000 μm inside from the rim of the crystals. Results of growth experiments by the anisotropic heating floating zone (AHFZ) method showed that the Nd concentration was high at the high‐temperature side of the grown crystals. We found a small concave projection at a part of the convex solid‐liquid interface by quenching the molten zone during growth. The cause of the singular ring‐shaped distribution of the Nd‐rich area was discussed in relation with the concave projection at the interface and the convection in the molten zone. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and magneto‐optical characteristics of cerium‐doped terbium gallium garnet by the floating zone method

Cerium‐doped terbium gallium garnet single crystal having a large Verdet constant was grown by floating zone (FZ) method, which was suitable for the use in optical devices. The lattice parameters and the X‐ray rocking curve measurement of the crystal was determined by X‐ray diffraction analysis. The Verdet constant of the crystal (B = 0.55 T) at the wavelength of 632.8 nm was −165.8 rad m⁻¹ T⁻¹ at room temperature, 23.7% larger than that of pure TGG (−134.0 rad m⁻¹ T⁻¹). The performance of the high optical quality and excellent magneto‐optical properties of the crystal shows the great potential of using this new method to meet the increasing demand of VI‐NIR Faraday rotators.     

Research on the multi‐crystalline structure in sapphire grown by Kyropoulos technique

The macroscopic distribution of multi‐crystalline structure (MCS) in a sapphire boule grown by Kyropoulos technique was characterized using polariscope. The morphology and texture of subgrains from , and orientations were investigated using Synchrotron radiation white‐beam topography technique. The morphology of subgrains was found to be diverse from crystallographic orientation. The relative angles between subgrains are smaller than 1°. The defect structure on a basal‐plane specimen from the lower part of the “shoulder” was examined by X‐ray rocking curve and chemical etching technique. It was found that the defect concentration changes drastically near MCS center. The nature of spatial distribution of MCS and the defect distribution near MCS in a grown boule was discussed in detail.     

Influence of the atmosphere on the growth of LiYF4 single crystal fibers by the micro‐pulling‐down method

Micro‐pulling‐down growth of LiYF₄ single crystal fibers have been performed under different gas atmospheres using stoichiometric LiYF₄ single crystal pieces from prior Czochralski experiments as starting material. Completely transparent and phase pure LiYF₄ single crystal fibers could be obtained after evacuation of the recipient to 2×10^‐6 mbar and subsequent filling with pure (99.995%) CF₄ gas. Using a gas mixture of 5% CF₄ in Argon or pure 5N Argon leads to the formation of micro crystallites of oxofluorides on the surface. Evacuating only to 3 × 10^‐3 mbar leads, independently of atmosphere, to completely white fibers that are heavily contaminated with oxofluorides. DSC measurements of the completely transparent fiber grown under pure CF₄ atmosphere reveal congruent melting behavior. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Formation of longitudinal aggregation of inclusions in bulk sapphire and yttrium‐aluminum garnet grown by horizontal directed crystallization method

It is shown that the formation of longitudinal aggregation of inclusions in bulk sapphire and yttrium‐aluminum garnet (YAG) grown by the method of HDC is caused by local accumulation of impurities, disturbance of morphological stability of the crystallization front and capture of inclusions and impurities in the nodal region of the melt two‐vortex convection. Studied is the influence of thermal and geometrical parameters of the melt and the shape of the crystallization front on the conditions of the formation of the capture of inclusions.     

Synthesis and single crystal growth of SnS by the Bridgman‐Stockbarger technique

SnS is a promising candidate as PV absorber material according to the material properties and the Loferski diagram, but despite the numerous publications on this material, the intrinsic material properties are widely unknown and the theoretical possible values for efficiency are still far away from those achieved in reality. Due to the fact that this material is mostly grown as thin film material, bulk research is rare. The material synthesis and the melt growth of tin monosulfide (SnS) by using Bridgman‐Stockbarger technique have been investigated in this study. After first growth experiments produced polycrystalline SnS, a significant reduction of the growth velocity lead to samples with a high amount of single crystalline material. These samples were investigated in detail regarding the structural and optical properties by using XRD/HRXRD, chemical etching and photoluminescence.     

Study of the hydrodynamic instabilities in a differentially heated horizontal circular cylinder corresponding to a Bridgman growth configuration

We are interested in determining the origin of the instabilities occurring in a metallic liquid (Prandtl number Pr=0.026) contained in horizontal circular cylinders heated from the end-walls. Our approach by direct numerical simulation (DNS) allows the determination of the transition thresholds for different aspect ratios varying from 1.5 to 10 as well as a precise characterization of the nature and structure of the new flow regimes close to the thresholds. In order to understand the mechanisms of flow transition, fluctuating energy analyses close to the threshold have been performed. The main contributions have been determined and localized in the cavity: shear has been found as the main instability factor but the way it acts is different according to the aspect ratio.     

Axial macrosegregation in Ga‐doped germanium grown by the vertical gradient freeze technique with a rotating magnetic field

The impact of a rotating magnetic field (RMF) on the axial segregation in Vertical Gradient Freeze (VGF) grown, Ga doped germanium is investigated. Growth experiments were performed using the VGF‐RMF as well as the conventional VGF technique. Carrier concentration profiles characterising the Ga segregation were measured by the Spreading Resistance method and calibrated using Hall values of carrier concentration and mobility. The Ga concentration rises more gradually under RMF action, i.e., the dopant segregation is significantly reduced by the rotating field. This effect is attributed to a better mixing of the melt. Numerical results on the flow velocity confirm this explanation. The RMF induced flow is much more intense than the natural buoyant convection due to the radial temperature gradient and leads to a pronounced decrease of the effective partition coefficient k_eff. In the early stages of growth a k_eff value close to k₀ was obtained, i.e., the gallium was almost homogeneously distributed within the melt. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Compositional variation in 0.65 PbMg2/3Nb1/3O3 ‐0.35 PbTiO3 single crystals grown by high temperature solution growth technique

Single crystals of lead magnesium niobate titanate, 0.65 PbMg_2/3Nb_1/3O₃ (PMN) ‐0.35PbTiO₃ (PT) were grown using flux method near morphotropic phase boundary (MPB) composition. The crystals grown at the centre of the platinum crucible were found to PT deficient compared to those grown near the walls of the crucibles. A variation of ∼3.8 mol% in PT concentration was found in the crystals grown at the wall and at the centre of the crucible. The difference in the chemical composition of crystals grown at the centre and the near the wall of the crucible was observed by X‐ray diffraction, EDXRF, dielectric and thermal measurements. The presence of PT rich and deficient crystals is explained in terms of the segregation coefficient of PT in PMN. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Factors affecting the formation of misoriented domains in 6H‐SiC single crystals grown by PVT method

Misoriented domains (MDs) are common defects in 6H‐SiC single crystals. We performed an experimental study on the formation of MDs in 2‐inch 6H‐SiC single crystals. Micro‐Raman spectroscopy revealed that the polytype of MDs was mainly 4H‐SiC. By changing growth conditions, it was found that the MDs' formation was closely related to growth rate and the position of highest temperature relative to growth interface. When the growth rate of ingots was relatively high the MDs were more likely to form. Furthermore, the nearer growth interface the position of highest temperature was, the larger the size of the MDs. Based on our experimental findings we suggested that the MDs' formation and the polytype switching from 6H‐ to 4H‐SiC were due to too large axial and/or radial temperature gradients.The results would be helpful to improve the quality of SiC single crystals grown by PVT technique. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Problems in the growth of Ce3+‐doped Li6Gd(BO3)3 crystals by Czochralski method

Cerium‐doped lithium gadolinium borate (Li₆Gd(BO₃)₃:Ce) single crystal has been grown by the Czochralski method. Some problems like macro defects and concave growth face were found. The macro defects are suggested to be caused by constitutional supercooling and concave growth face is caused by interface inversion. These problems can be resolved or eliminated through controlling the growth parameters strictly. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mathematical modeling of the multi‐run process of crystal pulling from the melt by EFG (Stepanov) technique in dependence on the angle of inclination of the working edges of the dies

A mathematical model for the determination of melt hydrodynamics, impurity concentrations and thermal stresses in the multi‐run process of the growth of sapphire ribbons by EFG (Stepanov) technique with inclinated working surfaces of the dies is considered. The mathematical model deals with thermal conductivity equation, Navier‐Stokes equation, diffusion equation, capillary Laplace equation. This problem has been solved by the finite‐element method. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An experimental and CFD study on gas flow field distribution in the growth process of multi‐walled carbon nanotube arrays by thermal chemical vapor deposition

Multi‐walled carbon nanotube arrays (MWCNTAs) were grown by thermal chemical vapor deposition (TCVD) in a horizontal furnace reactor. The scanning electron microscopy (SEM) results show that MWCNTAs grown on the bottom and the central of the quartz tube are different in one experiment. Moreover, the MWCNTAs grown on the central position are more aligned and longer than those on the bottom. A computational fluid dynamics (CFD) model was employed to investigate the gas flow field impact on the MWCNTAs growth. The results show that gas circulations appear after carrier gas and carbon source are injected into the quartz tube. Because of the existence of gas circulations, the gas flow field at the central of the quartz tube is more stable, which is conducive to the growth of MWCNTAs. The CFD simulation results match well with the experimental data.