Solutal convection around growing protein crystals and diffusional purification in Space

Convective and diffusional mass transport to an isolated crystal growing from solution, with slow linear interface kinetics, is considered analytically as a generic scaling model. We focus on the interface kinetics which is slow as compared to the diffusion mass transport which is typical of protein crystal growth. Independently, full-scale numerical solutions of transport equations around a cylindrical crystal, at the center of the bottom of a cylindrical cell filled with the solution, are found. The two approaches give results that agree over a wide range of parameters, providing dimensionless relationships that allow predictions of the contribution of convection and diffusion to mass transport. Requirements for microgravity level in Space experiments to achieve diffusional mass transport are estimated on the basis of these relationships. Coefficients of impurity distribution between a growing crystal and its solution, under the influences of convection and diffusion around the crystal, were numerically evaluated as functions of time. The results provide further support for the hypothesis concerning the role of the impurity depletion zone in the purification of a growing crystal. They also reveal that in general, the impurity distribution within the crystal is not homogeneous due to convection. The effects of various factors on growth kinetics and crystal purity are considered.     

Inversion domains and parallel growth in ammonothermally grown GaN crystals

Macro-defects such as twins, inversion domains, crevices, and columnar growth occasionally appear in ammonothermally grown GaN crystals. Twinning mechanisms and parallel growth are proposed to explain the formation of these defects. As a polar crystal with wurtzite structure, GaN can have several different kinds of twins depending on the polarity arrangement of each individual twin. Inversion domains are formed in one of the twinning mechanisms. Parallel growth is used to explain the formation of pits on the nitrogen face and the columnar growth on the gallium face. Etching in hot H₃PO₄ is used to reveal the polarities and defects of GaN crystals when they are indistinguishable. Optical microscopy, scanning electron microscope, and cathodoluminescence are also employed to study these defects. In addition, seed quality, avoidance of macro-defect formation, and impurity effects are also discussed.     

Effect of lattice oxygen on optical absorption, radiation hardness and thermally stimulated luminescence of BaF2 crystal

The influence of the presence of BaO impurity on the optical absorption, radiation hardness and thermally stimulated luminescence of BaF₂ has been investigated. The presence of oxygen impurity gives rise to three absorption bands in the UV region, peaking around 220, 280 and 335 nm. Further, the impurity is found to be detrimental to crystal hardness against ionizing radiations. The thermally stimulated luminescence (TSL) has been studied from gamma-irradiated crystals containing different fractions of BaO impurity. Four prominent glow peaks around 100°C (peak I), 150°C (peak II), 220°C (peak III) and 290°C (peak IV) are observed for crystals containing BaO impurity concentrations lower than 0.5% (by wt). For crystals containing higher impurity concentrations, two additional peaks around 75°C and 260°C are also observed. The kinetics of TSL emission is observed to be of first order, implying that the absorption and the emission centers responsible for TSL are the same. The normalized TSL output for peak I is found to vary linearly with the concentration of oxide impurity. This fact can be utilized to detect the presence of minute amounts of oxygen in BaF₂ lattice, which is crucial to the growth of crystals exhibiting high radiation hardness.     

Ga segregation during Czochralski-Si crystal growth with Ge codoping

The segregation of Ga during the growth of Czochralski-Si crystals with Ge codoping was investigated. The effective segregation coefficient of Ga in Ga/Ge-codoped Si crystal growth was nearly constant over a wide Ge concentration range, even at high Ge concentrations of about 10²¹ cm⁻³. In contrast, the effective segregation coefficient increased at high B concentrations in Ga/B-codoped CZ-Si crystal growth. The segregation behavior of Ga in Ga/Ge- and Ga/B-codoped CZ-Si crystal growth was theoretically compared. The difference in the segregation coefficients of Ga as a function of the codoped impurity (Ge or B) between the two Si crystals was attributed to a difference in the excess enthalpy due to impurity incorporation into the Si crystal between Ga–Ge pairs and Ga–B pairs     

Crystallization of zinc lactate in presence of malic acid

The influence of malic acid, which acts as an impurity on the cooling crystallization of zinc lactate is investigated in this paper by monitoring the relative supersaturation and the number of crystals during crystallization. The presence of malic acid increases the solution solubility and makes the metastable zone wider; it also changes the habit of the crystal. The purity of the final products is shown to be influenced by the amount and size of seed crystals, cooling rate, seeding temperature and final temperature, but appears to depend mainly on the particle size and level of supersaturation. Residual supersaturation thresholds are observed that depend on the final temperature. A model is proposed to predict the steady-state supersaturation value from the final temperature at a given impurity concentration. This model is based on Kubota and Gibbs equations.     

Growth and characterization of KY(WO4)2 crystals

Single-crystal growth of KY(WO₄)₂ (KYW) by top-seeded solution growth technique has been investigated. The effects of seed orientation, temperature gradient experienced by the growing crystal and rate of crystallization on crystal quality are reported. The best results are obtained when the growth is seeded along the 0 1 0 direction. Minute deviations from this growth direction are found to be detrimental to crystal perfection. The differential thermal analysis shows that the amount of super-cooling required for dissolution and crystallization of KYW in the flux is only 5° and this promotes an easy formation of tiny crystallites in the solution. Consequently, the crystal rotation and the solution cooling rates are found to have pronounced effects on the growth of KYW crystal.     

Evidence of free carrier concentration gradient along the c-axis for undoped GaN single crystals

Results of measurements of infrared reflectivity and micro-Raman scattering on the undoped GaN high pressure grown single crystals are reported. These crystals have usually a high electron concentration due to unintentional doping by oxygen. We show, by the shift of the plasma edge (infrared reflectivity measurements), that the free electron concentration is always higher on the (0 0 0  )N face of the GaN single crystal than on the (0 0 0 1)Ga face. In order to determine the profile of the free carrier concentration, we performed transverse micro-Raman scattering measurements along the (0 0 0 1) c-axis of the crystal with spatial resolution of 1 μm. Micro-Raman experiments give a quantitative information on the free carrier concentration via the longitudinal optical phonon–plasmon (LPP) coupling modes. Thus, by studying the behavior of the LPP mode along the c-axis, we found the presence of a gradient of free electrons. We suppose that this gradient of electrons is due to the gradient of the main electron donor, in undoped GaN single crystals, i.e. oxygen impurity. We propose a growth model which explains qualitatively the incorporation of oxygen during the growth of GaN crystal under high pressure of nitrogen.     

Growth of AlN single crystalline boules

We have obtained high-quality, crack-free AlN wafers using a convex thermal field inside the growth chamber. Free-standing AlN boules of 15 mm in height and 15 mm in diameter were grown. The carbon concentration was found to be similar in all parts of the boule (∼8×10¹⁸ cm⁻³) while the initial O concentration was higher (∼1×10¹⁹ cm⁻³) and slightly decreased during growth. It was found that O incorporated differently on different crystallographic faces. High resolution XRD showed a continuous improvement in crystal quality as a function of boule length. The full width at half maximum (FWHM) of the double crystal rocking curves decreased from 78 in at the beginning of growth to 13 in at the growth end. To the best of our knowledge, this is the first report on impurity incorporation on different crystallographic facets obtained from the same boule.     

Nd:YVO4 crystal growth by Czochralski technique with a submerged plate

This paper is to investigate the growth of Nd:YVO₄ (yttrium vanadate) crystal by the modified Czochralski technique with a submerged plate. Numerical studies are performed to examine melt convection and heat transfer during Nd:YVO₄ growth. The attention is paid to study the effects of initial elevation of the submerged plate, crystal diameter, and melt level on melt inclusions. It is found that the increase in crystal rotation rate and crystal diameter, and the decrease in melt level will increase the axial temperature gradient at the edge and in the center of the crystal, and change the interface shape from convex to flat. The experiments are also carried out to confirm the feasibility of the proposed new technique for controlling melt inclusions in Nd:YVO₄ crystal growth.     

Growth kinetics of CaF2 in a pH-stat fluidized-bed crystallizer

In this study, experiments were first conducted in a stirred vessel to establish the metastable region of calcium fluoride, using a pH-stat apparatus for controlling the pH value of solution. The effects of pH value on the solubility and metastable region were observed. Then, operating variables, including supersaturation, superficial velocity, pH value, seed size, and types of seed and solution, that influenced the crystal growth of the same system were investigated in a fluidized-bed crystallizer. The crystal growth rates were determined from the consumption rates of fluoride ions. The growth kinetics was explored by using the two-step growth model. The effectiveness factor was used to judge whether a controlling step was existing. The kinetics of crystal growth in a lean bed and a dense bed was also discussed.     

Model development for deactivation of bisphenol-A adduct particles during crystallization under the influence of impurity

A deactivation mechanism was developed to present the influence of 4-tert-butylphenol as a sample impurity on the bisphenol-A (BPA) adduct particles during the crystallization process. 4-tert-butylphenol is an organic sample impurity generally present in the reaction mixture of the industrial production of BPA. Kinetic parameters of growth, nucleation, agglomeration, and deactivation were estimated using the technique of model fitting to experimental data. The population and mass balances were used to model the adductive crystallization of BPA.     

Crystal structure, carrier concentration and IR-sensitivity of PbTe thin films doped with Ga by two different methods

The comparison of the results of chemical composition, crystal structure, electronic properties and infrared photoconductivity investigations of PbTe/Si and PbTe/SiO₂/Si heterostructures doped with Ga atoms by two different techniques is presented in this work. One of these techniques is principally based on the vapour-phase doping procedure of PbTe/Si and PbTe/SiO₂/Si heterostructures, which were previously formed by the modified “hot wall” technique. The second method of PbTe(Ga)/Si and PbTe(Ga)/SiO₂/Si heterostructure preparation is based upon the fabrication of lead telluride films, which have been doped with Ga atoms in the layer condensation process directly. The lattice parameter and charge carrier density evolutions with the Ga impurity concentration show principally the different character of PbTe(Ga)/Si films prepared by these techniques. It has been proposed that complicated amphoteric (donor or acceptor) behaviour of Ga atoms may be explained by different mechanisms of substitution or implantation of impurity atoms in the crystal structure of lead telluride.     

Crystal growth rate limited by step length — the case of oxygen-deficient TiO2 exposed to oxygen

We study how an oxygen-deficient crystal of TiO₂ crystal grows when exposed to O₂. While the O flux is external to the crystal, the Ti flux necessary for growth comes from internal (bulk) interstitials (Phys. Rev. Lett. 76 (1996) 791). We address where the reaction between O and Ti to form new crystal takes place in the regime of pure step flow (i.e., surface steps advancing without new-layers nucleating). The detailed partitioning of the growth flux among individual surface steps is studied using low-energy electron microscopy for two geometries on the (110) surface—an array of islands on a terrace and an island stack generated from a dislocation source. For both geometries, the areas of islands larger than the critical size grow at rates strictly proportional to their perimeter length, independent of the local step configuration. In addition, we find that the growth rate is proportional to the O₂ pressure. The step flow represents a simple limiting case of crystal growth (Phil. Trans. R. Soc. A. 243 (1951) 299)—only the growth species near a step edge becomes incorporated into the crystal. That is, only Ti and O reactions near the step edge lead to crystal growth. This case is in marked contrast to crystal growth controlled by species attaching to terraces and diffusing to steps, for which the growth rates depend upon the local step environment. Indeed, simulating the island array as if the growth flux was partitioned among the individual islands by concentration gradients (i.e., diffusion-controlled growth) totally failed to reproduce the experimental rates.     

Kinetics of residual doping in 4H-SiC epitaxial layers grown in vacuum

Investigation on residual Al, B, and N co-doping of 4H-SiC epitaxial layers is reported. The layers were produced by sublimation epitaxy in Ta growth cell environment at different growth temperatures and characterized by secondary ion mass spectrometry. The vapor interaction with Ta was considered through calculations of cohesive energies of several Si-, Al-, B-, and N-containing vapor molecules and also of diatomic Ta–X molecules. An analysis of kinetic mechanisms responsible for impurity incorporation is performed. Among residuals, B exhibits a stronger incorporation dependence on temperature and growth at lower temperatures can favor B decrease in the layers. Under the growth conditions in this study (Ta environment and presence of attendant Al and N), B incorporation is assisted by Si₂C vapor molecule. Boron tends to occupy carbon sites at higher temperatures, i.e. higher growth rates.     

AlN bulk single crystal growth on 6H-SiC substrates by sublimation method

Large and thick AlN bulk single crystals up to 43 mm in diameter and 10 mm in thickness have been successfully grown on 6H-SiC (0 0 0 1) substrates by the sublimation method using a TaC crucible. Raman spectrum indicates that the polytype of the grown AlN single crystals is a Wurtzite-2H type structure, and the crystals do not include any impurity phases. The quality at the top of the crystal improves as crystal thickness increases along the 〈0 0 0 1〉 direction during growth: a low etch pit density (7×10⁴ cm⁻²) and a small full width at half maximum for a 0002 X-ray rocking curve (58 arcsec) have been achieved at a thickness of ∼8 mm. The possible mechanism behind the improvement in the AlN crystal quality is also discussed.     

Impurity induced periodic mesostructures in Sb-doped SnO2 nanowires

Impurity doping on semiconductor nanowires formed via vapor–liquid–solid (VLS) mechanism has been investigated with the intention being to control the transport properties. Here we demonstrate that an addition of excess impurity dopants induces a mesostructure of long range periodic arched-shape in Sb-doped SnO₂ nanowires. The microstructural and composition analysis demonstrated the importance of the presence of impurities at the growth interface during VLS growth rather than the dopant incorporation into nanowires, indicating kinetically induced mechanisms.     

Solvent effects on the growth kinetics of subtilisin crystals

The effects of salts on subtilisin crystallization were investigated. Three salts—NaCl, NaNO₃ and NaSCN—were selected to study the effects of different anions on growth kinetics of three subtilisin mutants—Properase^®, Purafect^® and Purafect^®OX. The effectiveness of salts in decreasing the solubility of Properase^® and Purafect^® subtilisin followed the reverse order of the Hofmeister series: SCN⁻>NO₃⁻>Cl⁻. The average length and diameter of crystals were measured during crystallization. The nature of salt changed the length/diameter ratio of crystals, indicating the changes in the relative growth rate of different crystal faces. The required supersaturation, (c−s)/s, for a given growth rate increased in the order of NaCl, NaNO₃ and NaSCN. The observed trend in required supersaturation indicates a kinetic effect and was counter to the trend for the solubility data. A rationale is provided based on the influence of ion binding and kinetics on the energetics of crystal growth and growth rate is correlated to the molar Gibbs free energy of hydration of the anion.     

Surface growth mechanisms and structural faulting in the growth of large single and spherulitic titanosilicate ETS-4 crystals

Morphological, surface and crystallographic analyses of titanosilicate ETS-4 products, with diverse habits ranging from spherulitic particles composed of submicron crystallites to large single crystals, are presented. Pole figures revealed that crystal surfaces with a-, b- and c- axes corresponded to 110, 010 and 001 directions, respectively. Thus, technologically important 8-membered ring pores and titania chains in ETS-4 run along the b-axis of single crystals and terminate at the smallest crystal face. Height of the spiral growth steps observed on 1 0 0 and 0 0 1 surfaces corresponded to the interplanar spacings associated with their crystallographic orientation, and is equivalent to the thickness of building units that form the ETS-4 framework. Data suggest that the more viscous synthesis mixtures, with a large driving force for growth, increased the two- and three-dimensional nucleation, while limiting the transport of nutrients to the growth surface. These conditions increase the tendency for stacking fault formation on 1 0 0 surfaces and small angle branching, which eventually results in spherulitic growth. The growth of high quality ETS-4 single crystals (from less viscous synthesis mixtures) occurred at lower surface nucleation rates. Data suggest that these high quality, large crystals grew due to one-dimensional nucleation at spiral hillocks, and indicate that under these conditions un-faulted growth is preferred.     

Edge-defined film-fed growth of Yb:YVO4 single crystal plates

The applicability of the edge-defined film-fed growth (EFG) technique for Yb_xY_(1−x)VO₄ (x=0.05, 0.1 and 1) was approved by successful growth of crystals up to 80 mm in length as the thin plates. Low-angle grain boundaries and the crystal coloration as main defects were found. Optimal seed orientation was suggested on the strength of vanadate crystal plate morphology. Optical properties, chemical composition and the crystalline quality were investigated.     

Total pressure‐controlled PVT SiC growth for polytype stability during using 2D nucleation theory

A total pressure‐controlled physical vapor transport growth method that stabilizes SiC polytype is proposed. The supersaturation of carbon during SiC growth changed as a function of the growth time due to changes in the temperature difference between the surfaces of the source and the grown crystal. Supersaturation also varied as a function of the pressure inside the furnace. Therefore, modification of the pressure as a function of growth time allowed for constant supersaturation during growth. The supersaturation was calculated based on classical thermodynamic nucleation theory using data for heat and species of Si₂C and SiC₂ transfer in a furnace obtained from a global model. Based on this analysis, a method for polytype‐stabilized SiC growth was proposed that involves decreasing the pressure as a function of growth time. The 4H‐SiC prepared using this pressure‐controlled method was more stable than that of 4H‐SiC formed using the conventional constant‐pressure method.