LPE growth of GaAs by yo-yo solute feeding method

The effect of gravity on both dissolution and growth of GaAs in the Ga---As system has been investigated using a horizontal “sandwich” system consisting of a substrate-solution-substrate configuration. Remarkable differences were observed in growth and dissolution between upper and lower substrates. These phenomena were attributed to the solutal convection driven by a concentration gradient. Based on these facts, a layer with a thickness of about 80 μm was successively grown by the yo-yo solute feeding method with 8 yo-yo repetition times between 700 and 650°C.     

Mass transport in chalcogenide electrolyte films – materials and applications

Certain metals can be added to thin films of chalcogenide glasses by photodissolution to create materials with unique morphology and properties. When Ag is combined in this fashion with Ge–Se or Ge–S glasses, the resulting ternary contains a dispersed nanocrystalline Ag₂S(e) phase that has large quantities of mobile metal ions. The presence of these ions allows the ternaries to act as solid electrolytes. If an anode which has an oxidizable form of the ionic metal and an inert cathode are applied in contact with such a phase-separated electrolyte, an ion current can flow under an applied bias in excess of a few hundred millivolt. Electrons from the cathode reduce the excess metal due to the ion flux and an electrodeposit forms on or in the electrolyte. Utilizing this effect, we developed programmable metallization cell (PMC) technology which offers new functionality for such materials. Based on mass transport driven by electrochemical processes, PMC technology may be applied in solid state electronics, integrated optics, microelectromechanical systems (MEMS), and microfluidics. This paper is a review of the unique materials aspects of thin film solid electrolytes formed by photodissolution of metal into a chalcogenide base glass and the demonstrated applications of this technology.     

Diffusion-controlled distribution of solute in Sn-1% Bi specimens solidified by the submerged heater method

Two 1.35 kg, 5.8 cm diameter Sn-1% Bi specimens were solidified using the submerged heater method (SHM). The distribution of solute was found to be uniform axially, indicating that diffusion-controlled steady-state segregation, though to be unattainable on earth because of the gravity induced interference, was achieved.     

Predicting the rate of chemical transport using the flux function method

A systematic approach to the selection of independent variables for a theoretical analysis of the rate of chemical transport is discussed. The methodology is based on the use of the transport flux function, which enables optimum conditions to be found on the basis of a minimum number of the variables most likely to have the greatest effect on the transport rate. A more precise quantitative comparison between experimental and theoretical results can be made by taking into account the effects of convection, uncertainties in thermochemical data, and differences in the diffusion properties of vapour species. The latter effect can lead to a redistribution of components in the vapour, and this may have important consequences in certain CVT processes. The theoretical methodology is exemplified by consideration of transport of CoP₃ in the Co-P-Br system, FeP in the Fe-P-I system, and YIG in the Y-Fe-O-Cl and Y-Fe-O-HCl systems. The complex behaviour revealed would probably have been difficult to rationalize on the basis of experimental studies alone.     

Mass spectrometric study on chemical transport of VO2

The analysis of gas in the chemical transport reaction of VO₂ using TeCl₄ as a transport agent was carried out with a quadrupole mass spectrometer. In the transport reaction from 600 to 500°C, it was found that the oxygen and vanadium of VO₂ were transported in the form of VOCl₃ and TeOCl₂ gases; the transport reaction was $\text{VO}{}_2\text{+}\text{3}\text{2}\text{TeCl}{}_4\text{= VOCl}{}_3\text{+ TeOCl}{}_2\text{+}\text{1}\text{2}\text{TeCl}{}_2$. The transport reaction from 900°C to 800°C was assumed to be $\text{VO}{}_2\text{+}\text{3}\text{2}\text{TeCl}{}_2\text{= VOCl}{}_3\text{+}\text{1}\text{2}\text{O}{}_2\text{+}\text{3}\text{4}\text{TeCl}{}_2$. In the transport at high temperature, the oxygen partial pressure estimated from the mass spectrum was considerably higher than that in equilibrium with VO₂ phase. In this paper a study of the chemical transport of the system VO₂-TeCl₄ is presented.     

Investigation of the transport processes in tetramethoxysilane gel by the moiré fringes method

Moiré fringes method have been used in order to study the mass transport processes in tetramethoxysilane (TMS) gel in respect of crystal growth experiments. Diffusion coefficients of potassium dihydrogen phosphate (KDP) solution and ethanol in pure and soaked TMS gel have been measured.     

Mass transport effect within the boundary layers in solution crystal growth: Holographic study of KTP and KDP crystal growth

Using holographic phase-contrast interferometric microphotography, we have carried out real-time investigations of the mass transport processes taking place during the high temperature solution growth of KTiOPO₄ (KTP) crystal and low-temperature solution growth of KH₂PO₄ (KDP) crystal. Our experiments demonstrate that a mere diffusion boundary layer is not existing. The mass transport process within the boundary layers is a result of the coupled effect of diffusion and convection actions, no matter whether it is high-temperature or low-temperature solution growth. Under free convection state, the influence of bulk supersaturation on the thickness of solute boundary layer exists in the two different regions. The solute concentration distribution within the layer is an exponential function of the position.     

Terahertz spectroscopy of polar solute molecules in non-polar solvents

Low frequency spectra of liquid solutions of non-polar solvents were measured in the frequency range of 15–80 cm⁻¹ at room temperature by terahertz time-domain spectroscopy to investigate the solute–solvent interaction. The extinction coefficient of the solutions with polar solutes decreases as a function of frequency, whereas that of a non-polar solute increases. It is concluded that the observed spectral change of the polar solute is caused by fast reorientational dynamics due to librational motion of solutes.     

Influence of thermal stabilization on the solute concentration of the melt in directional solidification

Experiments on Al–25 at%Ni peritectic alloy consisting of melting followed by thermal stabilization ranging from 0 to 2 h were carried out in a Bridgman-type furnace. Temperature distribution, microstructure evolution and solute concentration in the mushy zone are characterized. An analytical model is proposed to evaluate the Ni concentration of the melt after thermal stabilization. Effect of temperature gradient and volume fraction of liquid phase in the mushy zone on the Ni concentration of the melt is discussed. The steady state Ni concentration of the melt is inappropriately below the initial Ni concentration of the sample. The deviation increases with decreasing temperature gradient. Finally, the influence of thermal stabilization on the solute concentration of the melt is discussed based on a comparison of Al–Ni peritectic alloys with Al–Ni hyper-eutectic alloys and Al–Cu hypo-eutectic alloys.     

Crystal growth of SnO2 by chemical transport method

Single crystals of SnO₂ were grown by the chemical transport method from the powder prepared by heating hydrous stannic oxide. The transport reactions were carried out when iodine and sulfur were simultaneously used as transporting agent, and fine crystals of a few millimeters in dimensions were grown after a ten-day transport; the transport proceeded from 1100°C (charge) to 900°C part in a silica ampoule. The crystals were identified as SnO₂ (cassiterite) by the Weissenberg method. Thermodynamic calculations led to the equilibrium $\text{SnO}{}_2\text{(s) + I}{}_2\text{(g)}\text{+}\text{1}\text{2}\text{S}{}_2\text{(g)}\text{?}\text{SnI}{}_2\text{(g) + SO}{}_2\text{(g)}$ for this transport reaction.     

Stoichiometric single crystal growth of AgGaS2 by iodine transport method and characterization

High quality single crystals of ternary AgGaS₂ (AGS) semiconductor with chalcopyrite structure have been grown by chemical vapor transport (CVT) technique using iodine as a transporting agent at different growth zone temperatures. The powder X‐ray diffraction and single crystal X‐ray diffraction studies indicate that the as‐grown AGS crystals belong to the tetragonal (chalcopyrite) system with (112) plane as the dominant peak. The full width at half maximum (FWHM) of the X‐ray rocking curve for the as‐grown AGS single crystal is 5 arcsec. The energy dispersive X‐ray analysis (EDAX) and optical transmission spectra of as‐grown AGS single crystals grown at different conditions show the almost same composition and band gap (2.65 eV). Photoluminescence (PL) spectra of as‐grown AGS single crystals show prominent band edge emission at 2.61 eV. The resistivity of the as‐grown AGS single crystal has been measured. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effective convection coefficient for porous interface and solute segregation

Convective heat and mass transfer coefficients are used to calculate the rate at which convection sweeps heat and mass away from the interface. Convection in melt growth is driven by various forces, and the resulting convoluted flows are laminar or turbulent. Furthermore, cross-flow through the “porous” interface has a profound effect on convection. Thus, a general effective coefficient (h_eff ), which accounts for: (i) uniform flow “suction” through the porous interface, (ii) forced and/or natural convection, (iii) laminar or turbulent flow, and (iv) finite Schmidt numbers, is derived. Focusing next on solute segregation, mass conservation is used to derive a simple equation for k_eff (effective segregation coefficient) as a function of h_eff. Here, h_eff is an input, which provides the rate at which convection sweeps the rejected solute away from the interface.From the general expression, even simpler expressions are developed for restricted range of conditions, e.g., Czochralski growth under forced laminar convection (no natural convection or turbulence). h_eff utilizes numerous established correlations, all developed for impermeable solids.     

The influence of solute distribution on the high nucleation density of Al crystals in amorphous aluminum alloys

An extension of a previous model that describes the role of solute atoms on glass formability leads to the conclusion that solute distribution plays an important role in the formation and stability of amorphous metals. A random distribution of solutes is shown to produce local solute-depleted regions (on the size scale of the mean inter-solute spacing) that provide preferred sites for the formation of crystalline nuclei. The possibility that these solute-depleted regions are responsible for the exceptionally high number density of critical Al nuclei is explored for three Al-Y binary alloys using a computer simulation. Up to 10⁷ Y atoms were placed at random locations in the system, and the number of solute-free regions were counted as a function of the size of these regions. The experimentally observed number density of critical nuclei (∼3 × 10²¹ m⁻³) is reproduced for a critical nucleus about 5 Al atoms in diameter, containing ∼60 Al atoms in an fcc array. Good agreement with previous suggestions of the size of a critical nucleus (about 6 atoms in diameter, containing about 100 atoms) support the conclusion that the current model provides a reasonable physical explanation for the quenched-in features responsible for the exceptionally high nucleation density in some amorphous Al alloys.     

Epitaxial growth of CuInSe2 single crystal by halogen transport method

CuInSe₂ single crystals were epitaxially grown on (001) GaP, (001) GaAs, and (110) GaP by the halogen transport method. The orientation relationships in the growth on the (001) and (110) faces were (a) [001]_CuInSe2 [001]_sub and [100]_CuInSe2 [100]_sub (c-axis orientation growth), and (b) [110]_CuInSe2 [110]_sub and [001]_CuInSe2 [001]_sub, respectively. On (001) InP, the orientation relationships between the layer and substrate consist of two sets: (c) [100]_CuInSe2 [001]_sub and [001]_CuInSe2 [010]_sub, and (d) [100]_CuInSe2 [001]_sub and [001]_CuInSe2 [ 00]_sub (a-axis orientation growth). The above results, i.e., c-axis growth on (001) GaP and (001) GaAs and a-axis growth on (001) InP, could be explained by a criterion of the minimum lattice mismatch between grown layers and substrates. A series of growth experiments on (001) GaAs indicated that appropriate gas etching of the substrate surface and growth temperature were required for obtaining twin-free single-crystal epitaxial CuInSe₂.     

The influence of the terminal alkyl chain on the orientational order of solute molecules dissolved in a nematic matrix

The orientational order of five homologous 4-n-butyl 4-n-alkyloxyazobenzenes in the same nematic matrix has been measured. It was found that the orientational order of the solute molecules increases with increasing length of the terminal alkyl chain.     

Solute segregation in a lid driven cavity: Effect of the flow on the boundary layer thickness and solute segregation

Our objective in the present work is to study the effect of convective flows, ranging from laminar to fully turbulent, on solute segregation in directional solidification configurations. To do so, numerical simulations performed in a model 2D lid driven cavity; the problem parameters, apart from the species molecular diffusion coefficient, are the lid and growth velocities. Purely diffusive to fully convective mass transport conditions are modelled in our parametric study. In parallel, a scaling analysis aiming at the determination of the solute boundary layer thickness is proposed. The results show that a single non-dimensional number, based on the interface stress, is able to capture the physics of the solute transport phenomena.     

The effect of solute on ultrasonic grain refinement of magnesium alloys

An experimental study has been conducted to assess the structural refinement of magnesium and its alloys by ultrasonic irradiation during solidification. It is shown that (i) ultrasonic irradiation leads to significant refinement only in the presence of adequate solute, which is alloy dependent; (ii) the attendant grain density increases linearly with increase in solute content at a given irradiation level; (iii) increasing the solute content at a low irradiation level above the cavitation threshold is more effective than substantially increasing the irradiation intensity; and (iv) the difference in the grain size between two ultrasonicated magnesium alloys is mainly determined by the solute content rather than the irradiation intensity. In view of these, the effect of ultrasonic irradiation on solute redistribution in a solidifying magnesium alloy seems rather limited even at a substantial intensity level such as 1700 W cm⁻². The implications of these findings are discussed and a mechanism is proposed to account for the experimental observations.