Axial Solute Transport during Bridgman Growth of BiSbTe3‐Mixed Crystals ‐ a comparison of analytical and numerical results

2D/3D‐transient finite‐element computer simulations of heat and mass transport including convection have been performed for a Bridgman configuration close to real growth conditions. The results for the axial distribution of the excessive tellurium in BiSbTe₃ semiconductor crystals grown from the melt are compared with the predictions of analytical segregation models.It is shown that Favier's model can be successfully applied for quantitatively estimating model parameters of segregation. Finally, the transition from normal gravity to microgravity conditions is discussed.     

Thermal analysis,powder diffraction and spectroscopic investigation of the solid phases of 1‐chloro‐2,2‐dimethylpropane

By the substitution of one methyl group in Neopentane C(CH₃)₄ with a methylchlorid group the globular sphere is expanded in 1‐chloro‐2,2‐dimethylpropane (CH₃)₃C(CH₂Cl). With the aid of DSC measurements, X‐ray powder diffraction, ¹H NMR and ³⁵Cl NQR measurements two orientationally disordered (plastic) phases were established. The low temperature structure was solved and a structural model of phase II is proposed. A mechanism to describe the reorientation in the different phases is given as well as a connectivity scheme between the phases is discussed. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermodynamic assessment of the Cu‐Mg‐Si system in its copper‐rich region

A thermodynamic assessment of the ternary Cu‐Mg‐Si system was made in its copper‐rich region in relation with the development of a thermodynamic database for Cu‐Mg alloys. The adjustable parameters of the binary end‐systems (Cu‐Mg, Cu‐Si and Mg‐Si) were taken from the literature and those of the Cu‐Mg‐Si system were optimized using experimental thermodynamic and topological data. For the sake of simplicity, the solution phases were described with the substitutional solution model and the intermetallic compounds were treated as plain semi‐stoichiometric phases ((A,B)_pC_q‐type). The assessment developed is applicable for magnesium contents up to 20 wt% (i.e. magnesium mole fractions ≈0.4) and silicon contents up to 16 wt% (i.e. silicon mole fractions ≈0.3). (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental analysis of Mn segregation in Bridgman‐grown gallium antimonide: Dependency on the ampoule radius

This work considered the segregation of manganese in vertical Bridgman‐grown GaSb crystals for different ampoule diameters. Experimental data of the impurity distribution were obtained from atomic absorption spectrometry and also Hall measurements. It was demonstrated that the radial segregation is more pronounced in the case of thick ampoule diameters. Furthermore the manganese effective segregation coefficient, resistivity, carrier mobility and density were obtained in each case. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Comments and reconciliation of the Ni–Bi‐system thermodynamic reassessments

The binary Ni‐Bi alloys as well as multicomponent materials comprising systems like Ni‐Bi‐Sn, Ni‐Bi‐Sb etc. are recently considered as prospective for design of lead‐free solders. For that reason various authors focused their attention to the Ni‐Bi system thermodynamics. The studies identified five phases in the above system. Two of them (i.e., (Ni) and liquid), were treated as disordered substitutional solutions. The phase NiBi₃ is stoichiometric and a standard two‐sublattice model was repeatedly applied to describe its Gibbs free energy, while for the nonstoichiometric NiBi phase, a three‐sublattice model was retained. The pure bismuth phase ((Bi)) is stoichiometric as well. The results are discussed and compared to literature thermochemical and topological data. In liquid state, strong repulsions between nickel and bismuth atoms are anticipated in the nickel rich‐side, but no data about a liquid phase miscibility gap is available yet. Therefore, the stability of the liquid phase has to be reassured, in spite of positive deviations from ideal behaviour. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of a Czochralski grown silicon crystal doped with 1020 cm‐3 germanium

A dislocation‐free silicon single crystal doped with 10²⁰ cm^‐3 germanium (Ge) has been grown using the Czochralski (CZ) growth technique. The Ge concentration in the seed‐end and tang‐end of the crystal was 8×10¹⁹cm^‐3and 1.6×10²⁰ cm^‐3, respectively. The effective segregation coefficient of Ge, the distribution of flow pattern defects (FPDs) and the wafer warpage have been characterized. Both the effective segregation coefficient and the equilibrium segregation coefficient of Ge in silicon were evaluated. Then, the density of FPDs was traced from seed‐end to tang‐end of the ingot, a suppression of FPDs by Ge doping was shown. That is probably because the Ge atoms consume free vacancies and thus a higher density of smaller voids is formed. Furthermore, the mechanical strength of wafers has also been characterized by batch warpage analysis. The warpage in the seed‐end was larger than that in the tang‐end of the ingot, showing that the mechanical strength of wafers is enhanced by Ge doping. Such improvement is interpreted by an enhanced dislocation pinning effect associated with the enhanced nucleation of grown‐in oxygen precipitates in the Ge‐doped silicon wafers. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An Improved Model for Diffusion Induced Segregation with Elasticity

An advanced mathematical model for describing diffusion induced segregation (DIS) in the case of a (Zn,Fe)S single crystal is presented. Generalising an existing model, by the use of linear elasticity the Ginzburg‐Landau potential now depends explicitly on the local strain. As a consequence, the diffusion process can be resolved with higher precision and the shape of the transition layer between sphalerite and chalcopyrite is now captured correctly. Finite element calculations in 2‐d are presented and the new simulations are compared with results of the old model.     

Complex boron redistribution kinetics in strongly doped polycrystalline‐silicon/nitrogen‐doped‐silicon thin bi‐layers

We have investigated the complex behaviour of boron (B) redistribution process via silicon thin bi‐layers interface. It concerns the instantaneous kinetics of B transfer, trapping, clustering and segregation during the thermal B activation annealing. The used silicon bi‐layers have been obtained by low pressure chemical vapor deposition (LPCVD) method at 480 °C, by using in‐situ nitrogen‐doped‐silicon (NiDoS) layer and strongly B doped polycrystalline‐silicon (P⁺) layer. To avoid long‐range B redistributions, thermal annealing was carried out at relatively low‐temperatures (600 °C and 700 °C) for various times ranging between 30 min and 2 h. To investigate the experimental secondary ion mass spectroscopy (SIMS) doping profiles, a redistribution model well adapted to the particular structure of two thin layers and to the effects of strong‐concentrations has been established. The good adjustment of the simulated profiles with the experimental SIMS profiles allowed a fundamental understanding about the instantaneous physical phenomena giving and disturbing the complex B redistribution profiles‐shoulders. The increasing kinetics of the B peak concentration near the bi‐layers interface is well reproduced by the established model.     

Optimization of control parameters of cadmium zinc telluride Bridgman single crystal growth

The temperature gradient within a furnace chamber and the crucible pull rate are the key control parameters for cadmium zinc telluride Bridgman single crystal growth. Their effects on the heat and mass transfer in front of the solid‐liquid interface and the solute segregation in the grown crystal were investigated with numerical modeling. With an increase of the temperature gradient, the convection intensity in the melt in front of the solid‐liquid interface increases almost proportionally to the temperature gradient. The interface concavity decreases rapidly at faster crucible pull rates, while it increases at slow pull rates. Moreover, the solute concentration gradient in the melt in front of the solid‐liquid interface decreases significantly, as does the radial solute segregation in the grown crystal. In general, a decrease of the pull rate leads to a strong decrease of the concavity of the solid‐liquid interface and of the radial solute segregation in the grown crystal, while the axial solute segregation in the grown crystal increases slightly. A combination of a low crucible pull rate with a medium temperature gradient within the furnace chamber will make the radial solute segregation of the grown crystal vanish. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Columnar grain growth pattern with fluid flowing in molten pool

A coupled model was used to simulate columnar grain growth in TIG (tungsten inert‐gas) molten pool of nickel base alloy. The cellular automaton algorithm for dendritic growth is incorporated with solute transport model to take fluid flow into consideration. The results indicate that shear flow changes the solute distribution at the S/L (Solid/Liquid) interface, leading to asymmetrical growth of columnar grains. The dendrite arms on the upstream side grow fast, while the growth of dendrite arms on the downstream side is much delayed. However, dendrite arms on both sides are not as well‐developed as the grain growth without flow. With inlet flow velocity increasing, the phenomenon becomes more obvious. In addition, shear flow also results in more severe coring segregation. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dendritic grain growth simulation in weld molten pool based on CA‐FD model

The finite difference model is coupled with a cellular automaton (CA) model to simulate dendrite growth in TIG (tungsten inert‐gas) weld molten pool of nickel‐based alloys. Based on the macro finite difference computation of the thermal field of the whole weldment, the micro CA model is used to simulate grain formation process in 2‐dimension area within weld molten pool. The results illustrate that the complicated thermal field and solute field can lead to complex grain morphologies in weld molten pool. The primary dendrite spacing changes during welding solidification process and the final primary dendrite spacing is decided by the thermal field, solute field and other parameters. And it is indicated that the grain boundary segregation has important relationship with welding speed. The grain boundary segregations become more severe as the welding speed increases when the other parameters are kept unchanged. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Model of the transformation of several basis structures into intermetallic crystal structure types

A model is presented that states that all intermetallic crystal structure types can be hypothetically derived from few basis structures. These hypothetical basis structures consist of different atomic layer stackings and conform to the principles of simplicity and high symmetry. The transition from the basis structure to the real structure takes place by the replacement of formerly occupied atomic sites by interstices followed by a lateral relaxation of the remaining atoms. This leads to, in most cases, a lowering of symmetry and packing density, as well as an alteration of stoichiometry. The requirement of the transition to the known real structure depends on the individual “nature” of the atoms of the different elements in the compound. The connection between basis structures and crystalline non‐equilibrium phases is highlighted. Non‐equilibrium phases, a topic of modern research, can be regarded as “frozen‐in” basis structures. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Liquid Crystal Polymers: Conformation of Deuterated Parts by SANS

Comb like liquid crystal polymers are polymers on each monomer is grafted a hanging liquid crystal molecule. The bulk state has the classical liquid crystal phases (nematic, smectic); what is the conformation of a chain in these phases is the problem to solve with the help of small angle neutron scattering. The values of the global sizes R _|| and R _‖ of a chain (in the direction parallel and perpendicular to the director) are discussed as a function of the chemical species and the temperature. Strange results are discussed in terms of segregation of labelled parts.     

Quantification of the In‐distribution in embedded InGaAs quantum dots by transmission electron microscopy

The In‐concentration in InGaAs quantum dots located within a GaAs matrix was determined with the composition evaluation by lattice fringe analysis (CELFA) technique. However, the results obtained with this method cannot account for the three‐dimensional shape of quantum dots and their embedding in GaAs. A correction procedure was developed that takes into consideration the shape of the quantum dots and the TEM sample thickness and quantum‐dot size. After correction, In‐concentration profiles show an increasing In‐content towards the top of the quantum dots which is consistent with the effect of In‐segregation and earlier studies using other experimental techniques. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of structural perfection and faceting in highly Er‐doped Yb3Al5O12 crystals

The undoped Yb₃Al₅O₁₂ (YbAG) single crystals and doped with 1.5, 10 and 30 at% erbium were grown by the Czochralski method. The YbAG crystals offer efficient emission of laser beam of 2.94 µm and 1.55 µm important for practical applications in communication and medicine. The crystals were investigated by various synchrotron X‐ray diffraction methods including white beam topographic methods, monochromatic beam topography and recording of the rocking curves. The experiments were performed at E2 and F1 experimental stations in HASYLAB. The investigations proved a good crystallographic perfection of the crystals, in most cases revealing only segregation fringes and growth facets. The crystallographic identification of the facets was performed together with direct evaluation of growth front radius from the transmission section topographs. Relative lattice parameter changes connected with erbium segregation were found to be less than 2 × 10^‐5 inside the segregation fringes and 8 × 10^‐5 in the facets. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

ACRT forced convection and its effects on solute segregation and heat and mass transfer during single crystal growth

A numerical simulation study was carried out for CdZnTe vertical Bridgman method crystal growth with the accelerated crucible rotation technique (ACRT). The convection, heat and mass transfer in front of the solid‐liquid interface, and their effects on the solute segregation of the grown crystal can be characterized with the following. ACRT brings about a periodic forced convection in the melt, of which the intensity and the incidence are far above the ones of the natural convection without ACRT. This forced convection is of multiformity due to the changes of the ACRT parameters. It can result in the increases of both the solid‐liquid interface concavity and the temperature gradient of the melt in front of the solid‐liquid interface, of which magnitudes vary from a little to many times as the ACRT wave parameters change. It also enhances the mass transfer in the melt in a great deal, almost results in the complete uniformity of the solute distribution in the melt. With suitable wave parameters, ACRT forced convection decreases the radial solute segregation of the crystal in a great deal, even makes it disappear completely. However, it increases both the axial solute segregation and the radial one notably with bad wave parameters. An excellent single crystal could be gotten, of which the most part is with no segregation, by adjusting both the ACRT wave parameters and the crystal growth control parameters, e.g. the initial temperature of the melt, the temperature gradient, and the crucible withdrawal rate. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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)     

Inhomogeneity of composition in near‐stoichiometric LiNbO3 single crystal grown from Li rich melt

A near‐stoichiometric LiNbO₃ single crystal has been grown by the Czochralski technique from a melt of 58.5 mol% Li₂O. Its composition homogeneity was assessed by measuring the UV absorption edge. It was found that the maximum composition difference is about 0.03 mol% in the radial direction and 0.05 mol% in the axial direction. Differential scanning calorimetry (DSC) analysis was performed on the powder from the synthesized raw material and the frozen melt after crystal growth. The analytical results indicate that, during crystal growth, the magnitude of lithium volatilization from the melt surface is more than the degree of segregation from the crystal. The volatilized lithium diffuses into the crystal to compensate for the lithium segregation in the LiNbO₃ crystal. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermodynamic and lamella models relationship for the eutectic system benzoic acid – cinnamic acid

The present investigation reveals the relationship between excess thermodynamic functions and the growth habits of the eutectic phases from the melt by continuous melt‐growth technique. Excess thermodynamic functions computed for different compositions of the benzoic acid – cinnamic acid eutectic system have been found consistent with the criteria of spontaneity and Planck formulation, and their reliability has been ascertained by the application of Guggenheim lattice theory. The results on the kinetics of anisotropic growth of the eutectic phases from the melt, evidentially evince the dislocation mechanism. Evidences have been obtained for a parabolic variation of mechanical strength with growth velocity of the eutectic material grown anisotropically from the melt at different intervals, which offer supporting complement to the dislocation mechanism governing the dependence of growth velocity on supercooling ΔT in the solidus – liquidus interface in a form : V = k(ΔT)². A moderate anisotropic growth region has been explored by unique results of strength properties and microscopic results as well, to growing a layer of lamellae in a unidirectional lamina. An anisotropic eutectic composite lamellae lamina developed by moderate growth velocity (7.3 × 10^‐8m³s^‐1), is of greater interest offering optimum hardness, approximately varying between three‐and eight fold average increase in different modes of the mechanical strength in comparison to its isotropic growth carried out in an ice‐bath (∼273K), and manifold superior to its constituent phases irrespective of the growth mode. The directional lamina of uniform microstructural parameter lamellae, indicates that there is a perfect lamella‐ matrix equilibrium for which excess thermodynamic functions do vanish. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)