Formation of gallium nitride particles during the two-stage chemical vapor process

The effects of the reaction temperature in the first stage T_I on the formation and the luminescent property of both the seed and the grown particles were investigated in the region from 1050 to 1200 °C for the two-stage vapor-phase synthesis of GaN particles. The reaction efficiency of vaporized Ga and NH₃ to form the seed particles increased with increasing T_I up to about 1150 °C, where the maximum value of about 70% was obtained. Further raising T_I caused a decrease of the efficiency. The X-ray diffraction and the photoluminescence (PL) measurements indicated both of the crystal quality and the luminescent property of the seed particle were improved with increasing T_I. On the other hand, the PL intensity of the particles grown on the seed in the second stage decreased with increasing T_I. This difference in the dependence was explained in terms of the morphology of the grown particles. The mechanism of particle formation during these processes was also discussed based on the results.     

Numerical investigation of heat transport and fluid flow during the seeding process of oxide Czochralski crystal growth Part 2: rotating seed

In this paper, the role of seed rotation on the characteristics of the two‐dimensional temperature and flow field in the oxide Czochralski crystal growth system has been studied numerically for the seeding process. Based on the finite element method, a set of two‐dimensional quasi‐steady state numerical simulations were carried out to analyze the seed‐melt interface shape and heat transfer mechanism in a Czochralski furnace with different seed rotation rates: ω_seed = 5‐30 rpm. The results presented here demonstrate the important role played by the seed rotation for influencing the shape of the seed‐melt interface during the seeding process. The seed‐melt interface shape is quite sensitive to the convective heat transfer in the melt and gaseous domain. When the local flow close to the seed‐melt interface is formed mainly due to the natural convection and the Marangoni effect, the interface becomes convex towards the melt. When the local flow under the seed‐melt interface is of forced convection flow type (seed rotation), the interface becomes more concave towards the melt as the seed rotation rate (ω_seed) is increased. A linear variation of the interface deflection with respect to the seed rotation rate has been found, too. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Crystallisation of Aluminium Trihydroxide (Gibbsite) from Sodium Hydroxide Solution Onto Fine Seeds: Physico-chemical Studies on the Internal Structure and Agglomeration of Different Crystal Fractions

A batch of Bayer gibbsite crystals (particle size range 3 to 70 μm) was prepared by crystal growth onto fine seeds (1 to 3 μm) from sodium hydroxoaluminate/sodium hydroxid-solution. The internal structures and seed agglomeration of different fractions were examined by optical microscopy, infra-red spectrophotometry, thermal analysis and chemical dissolution analysis. These physico-chemical studies confirmed that crystal growth occurs with some seed agglomeration in the early stages, followed by growth onto the ‘agglomerates’. The agglomerated seeds contents (X) were as follows: for fraction DiD (l₀ = 6.3 μm), X = 0.44; for fraction Gi C (l₀ = 20 μm), X = 0.41; for fraction Gi B (l₀ = 38 μm), X = 0.31 and for fraction Gi A (l₀ = 55 μm), X = 0.24.     

Growth of Nd:Sr3Ga2Ge4O14 crystals by a modified Bridgman method

Bridgman growth of Nd:SGG (Sr₃Ga₂Ge₄O₁₄) crystals has been investigated for the first time. Pt crucible of ∅︁25mm×250mm with a seed well of ∅︁10mm×80 mm is used, and seed is SGG crystal of ∅︁10mm×50mm grown by Bridgman method in advance. The growth parameters are optimized as the furnace temperature is set to 1450∼1500°C, temperature gradient in the crystal‐melt interface is less than 25 K/cm and growth rate is less than 0.5mm/h. The Nd:SGG crystals with 25mm in diameter and 60mm in length are grown successfully from 1.5 to 8at% Nd³⁺ doped stoichiometric Sr₃Ga₂Ge₄O₁₄ melt. The distribution coefficient and concentration of Nd³⁺ in Nd:SGG crystals are obviously higher than those of Nd:YAG crystal. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Seeded batch crystallization of ammonium aluminum sulfate from aqueous solution

Seed crystals of ammonium aluminum sulfate ((NH₄)Al(SO₄)₂··12H₂O) were grown in aqueous solution by cooling. The temperature of a crystallizer was lowered with no control by circulating cooling water through the jacket. It fell in an exponential manner. The effects of seed amount and size on the product crystal size distribution were examined. The product crystals obtained were of narrow and uni-modal size distribution with suppressed secondary nucleation if seed crystals were loaded more than a critical value. The critical value was determined and well compared with previously reported values for other material systems. This crystallization technique does not need any prior knowledge of the kinetics of crystal growth and nucleation. It is simple and robust, and can be easily applied to an existing crystallizer without installing any additional control systems.     

Influence of SiO32‐ impurity on growth habit of potassium dihydrogen phosphate crystal

By altering the concentration of silicate (SiO₃^2‐) impurity in the solution, a series of potassium dihydrogen phosphate (KDP) crystals was obtained by the conventional temperature cooling and the rapid growth methods, respectively. It was observed that the presence of SiO₃^2‐ made KDP crystals tapering in conventional cooling method, while more SiO₃^2‐ induced inclusions at prismatic sectors in the rapid growth method. Laser‐polarization‐interference results showed that SiO₃^2‐ extended the dead zone and reduced the growth rate of (100) face of KDP crystals. The negative influence of SiO₃^2‐on the growth was considered absolutely similar to the effect of cations. It was also suggested that the stability of solution doped with SiO₃^2‐ was improved without seed crystals, while it was destructed with seed crystals. The inhibition mechanism was analyzed in terms of SiO₃^2‐ absorption on (100) face. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and characterization of l-Tartaric acid,an NLO material

Single crystals of l-Tartaric acid (C₄H₆O₆), an organic nonlinear optical (NLO) material, have been grown by hanging seed solution and submerged seed solution techniques at room temperature. The crystal system has been confirmed from the single crystal X-ray diffraction analysis. The crystalline perfection was evaluated using high-resolution X-ray diffractometry (HRXRD). From this analysis, it was found that the quality of the crystal is quite good. The functional groups were identified using FTIR spectroscopy. UV–vis–NIR spectrum showed the absence of absorption in the wavelength region of 220–900 nm. The second harmonic generation efficiency is 89% compared to that of standard potassium dihydrogen phosphate (KDP). The laser damage threshold value is much higher than that of KDP and closer to that of β-barium borate.     

Time-varying Temperature Profile CVD Methods for Growing CuInS2 and CdIn2S4 Crystals

A time-varying temperature profile method is reported for growing crystals in horizontal closed-tube arrangements by vapour phase iodine transport. After an initial stage, during which the temperature is linearly raised with time in the source region of the container, in such a way as to control the primary nucleation (a limited number of seed crystals can thus be produced), a second stage follows during which the source temperature is allowed to oscillate in order to favour the secondary nucleation (only the first grown seed crystals are then allowed to grow). A proper choice of the frequency and amplitude of the temperature cycle is shown to control the growth up to the obtainement of solitary crystals per run of CuInS₂ and CdIn₂S₄. The role of the slow growing faces in favouring good performances of the method is emphasized.     

Growth Kinetics of Barium Sulphate in Suspension

The growth rate of barium sulphate seed crystals from stirred solutions was studied conductometrically at 25°C by a stopped-flow technique. The supersaturation ranged from 3 × 10⁻⁷ to 3 × 10⁻⁸ mol BaSO₄/cm³. The seed crystals were grown in the system during the initial (steady-state) period of the experiment. Crystal size distributions were determined by optical microscopy. The growth rate of barium sulphate under the conditions of the experiments can be expressed by a quadratic function of supersaturation. The results, which suggest an interface rate-controlling mechanism, are discussed with respect to published data.     

Crystal growth of Ca3−xYxMn2Ge3O12 and phase relations in the PbO?B2O3?GeO2 based melts

The bulk crystals of Ca_3−xY_xMn₂Ge₃O₁₂ garnets were grown on a seed from the PbO B₂O₃ GeO₂ based melts, where the primary crystallization field of garnets was found. The saturation temperatures of the melts were determined before each growth run. The dependence of Y incorporation into the crystals versus the melt composition was also evaluated.     

Kinetics of crystal growth of mirabilite in aqueous supersaturated solutions

The crystallization of sodium sulfate decahydrate (Na₂SO₄·10H₂O, mirabilite) from supersaturated solutions was investigated using stable supersaturated solutions seeded with mirabilite seed crystals. The experiments were done in batch, stirred reactors in which the supersaturated solutions were prepared either by dissolution of sodium sulfate anhydrous at 32 °C followed by cooling to 18 or 20 °C or by mixing equal volumes of equimolar ammonium sulfate and sodium hydroxide solutions at 20 °C. Inoculation of the solutions supersaturated only with respect to mirabilite with seed crystals was accompanied with temperature increase of the thermostated solution. Despite the fact that crystal growth was initiated with seed crystals, the process started past the lapse of induction times inversely proportional to the solution supersaturation. The rates of crystal growth were measured both from the temperature rise and from the concentration–time profiles, which were linearly correlated. The measured crystal growth rates showed a parabolic dependence on supersaturation at low supersaturations. For higher values this dependence changed to linear, a behavior consistent with the BCF spiral crystal growth model. The morphology of the crystals growing at 20 °C showed typical prismatic habit, while at 18 °C when crystallized from cooled sodium sulfate solutions changes in the crystal habit to a leaf like morphology were observed.     

Effect of Ampoule Rotation on Growth of InGaAs Ternary Bulk Crystals from Solution

In_xGa_1-xAs (x = 0.045) ternary bulk crystals were grown on GaAs seeds from an In–Ga–As solution by the temperature-difference method modified to rotate a growth ampoule. The effect of ampoule rotation on the profiles of the composition and the growth rate were investigated. The In compositional profiles were uniform irrespective of the ampoule rotation. On the other hand, the growth rate at the center of the crystal increased from 40 μm/h at 0 rpm to 55 μm/h at 100 rpm. The profile of growth rate changed from concave to convex toward the seed due to the ampoule rotation. Flow patterns and compositional profiles in the solution were simulated by solving four equations: Navier-Stokes, continuity, energy, and solute diffusion. The ampoule rotation enhanced the transportation of As component from the GaAs feed toward the seed at the central region in the solution. This led to the increase of the growth rate.     

Über das Wachstumsverhalten Kleiner Impfkristalle von Ammoniumdihydrogenphosphat in einer modifizierten Zirkulationsapparatur

We have investigated the crystal growth of small seed crystals of NH₄H₂PO₄. The crystal growth was measured by a modified circulation apparatus. The measured values are compared with those by other investigators and the reasons of differences in crystallization discussed. The influence of cobalt nitrate in solution on crystallization velocity and the impurity in crystals was investigated. A new crystal holder in the apparatus is described.     

KTiOPO4(KTP) crystal growth from high-temperature solutions containing WO3

The process of seed growth of KTiOPO₄ crystals from three different high-temperature solutions containing WO₃, with definite compositions is reported. The low viscosity and relatively slight change of the solubility of the KTP with the variation of the temperature allow the growth of adequately large crystals, free from inclusions, in the case of two tested techniques – slow cooling and temperature gradient. Conditions to avoid defects in the seed region have been found. The relatively high growth rates allow the production of crystals in a 5-day growth run, that can be cut into samples oriented along synchronism for the second harmonic generation(SHG) with dimensions 8 × 8 × 7 mm³. SHG measurements reveal that the quality of the samples considered is comparable with KTP crystals grown from high-temperature nonstoichiometric solutions.     

Preparation of (NH4)2SO4 monocrystals

The preparation of (NH₄)₂SO₄ crystals has been studied, and the difficulties, especially the formation of numerous parasitical crystals and the irregular growth are discussed from a theoretical point of view and with regard to the performed investigations. Further on the habit of crystals and an acceptable procedure for the growing of homogeneous regular crystals are given. Likewise the effect of temperature, of P(H) of the crystallizing solution, the choice of seed crystals and the admixture of glycerine are reported.     

Growth of ZnGeP<Subscript>2</Subscript> crystals from melt

Some features of the growth of ZnGeP₂ single crystals by the Bridgman method have been considered. The ratio of the thermal-conductivity coefficients of the liquid and solid phases of ZnGeP₂ at the melting temperature was estimated to be 2.3. It is established that, in the case of ZnGeP₂ growth on a seed, the most favorable crystallographic directions are 〈100〉 and 〈001〉. It is shown that annealing and electron irradiation significantly decrease the optical absorption coefficient in the impurity absorption region.     

Kinetics of Mixed Crystal K2(SO4, CrO4) Growth from Aqueous Solution

The investigation of crystal growth kinetics and phase equilibria in the system K₂SO₄–K₂CrO₄–H₂O was carried out using the method for the determination of the saturation temperature and the growth rate by microscopic observation of the seed crystal behaviour. The kinetics of mixed crystal growth can be characterized by the expression V = 450.24σ^1,6,2, where σ is the value of effective supersaturation of the solution calculated by taking into account the specific features of the phase equilibria diagram. Use of this criterion allows the control of the rate of delta crystal growth from solutions of variable composition.     

Kinetic of potash alum crystallisation

Metastable zone width of potash alum in aqueous solutions has been measured as function of the cooling rate in absence as well as in presence of seed crystals. From these data, the effective nucleation rate was calculated. Growth rate of potash alum crystals has been measured in a fluidized bed. Overall growth rate coefficient, k_G, was then used for determination of the mass transfer coefficient, k_D, and the rate constant of the intergration step k_r. The dependence of k_D on hydrodynamic conditions is discussed.     

Geschwindigkeitsoszillationen beim Wachstum aus der Schmelze mit komplizierten Erstarrungsflächen

Basing on a formerly described model, the fluctuation of the growth velocity is computed for the growth of a crystal with rotating seed and a solidification interface, which has an maximum at r = r_c = 0. The distribution of the velocity and the pattern of the resulting striations is discussed.     

Growth of corundum single crystals by the top-seeded solution growth technique (TSSG)

Single crystals of corundum were grown by the top-seeded solution growth technique from a cryolite, Na₃AlF₆, solvent. The relationship between the growth rate (mg/h) of a crystal and the temperature difference (= supersaturation) or the rotation rate of a seed crystal was investigated, and optimum growth conditions for obtaining single crystals with good quality are discussed.