A.G. Ostrogorsky

Suppression of cracks in InxGa1−xSb crystals through forced convection in the melt

It was demonstrated that forced convection or mixing in the melt during directional solidification of bulk InxGa1−xSb (0<x<0.1) ternary alloys significantly reduces cracks in the crystals. In this study, the enhanced mixing in the melt was generated by a rotating submerged baffle. The resultant improvement in spatial compositional homogeneity lowers the strain gradient or chemical stresses; thus eliminating cracks. The results presented are generally beneficial and should also improve the crystalline quality of other mixed alloys.

Normal and zone solidification using the submerged heater method

Abstract Using the submerged heater method, steady-state axial segregation may be reached by normal freezing and zone solidification. With solvent-solute systems having equilibrium segregation coefficient k = 0.35 to 0.5, diffusion-controlled steady-state was achieved by normal freezing. Steady-state segregation of dopants with k ≪0.1 was reached by doping the shallow melt zone below the submerged heater (zone leveling). These experimental findings are explained using a recently developed analytical model of axial segregation.

Melt growth of quasi-binary (GaSb)1-x(InAs)x crystals

A new class of III-V quasi-binary [A III B V ] 1-x [C III D V ] x semiconductor alloys has been synthesized and bulk crystals grown from the melt for the first time. The present investigation is focused on (GaSb) 1-x (InAs) x (0 < x < 0.05) due to its importance for thermophotovoltaic applications. The structural properties of this melt-grown quasi-binary alloy are found to be significantly different from the conventional quaternary compound Ga 1-x In x As y Sb 1-y with composition x = y. Synthesis and growth procedures are discussed.

Forced convection in vertical Bridgman configuration with the submerged heater

Abstract Ga-doped Ge single crystals were grown in vertical Bridgman configuration, using the submerged heater method (SHM). When used without rotation, the submerged heater drastically reduces convection at the solid-liquid interface. When the submerged heater is set in to rotation or oscillatory rotation, it acts as a centrifugal viscous pump, inducing forced convection (radial-inward flow) along the interface. The flow produced by a rotation and oscillatory rotation of the submerged heater was visualized using a 1 : 1 scale model. The vigorous mixing produced by the oscillatory rotation creates a nearly perfectly stirred melt, and yields a uniform lateral distribution of the dopant. The crystals were free of unintentionally produced striae.

Directional Solidification Using a Baffle in Microgravity

Abstract Numerical simulations were performed to optimize the conditions and parameters for directional solidification of Te-doped GaSb in reduced gravity ranging from 10 −3 to 10 −5 g 0 . Our key goal was to quantify the velocity and concentration fields with and without a baffle present in the melt. The effect of the distance of the baffle from the solid–liquid interface was investigated. When the baffle is placed 0.5 cm from the solid–liquid interface, acceleration of 10 −3 g 0 does not cause significant interference with segregation. Furthermore, the flow between the baffle and the interface (low Reynolds number “creeping” flow) does not depend on fluid properties (viscosity).

Visualization of convection in Czochralski melts using salts under realistic thermal boundary conditions

NaCl-CaCl 2 was used to visualize convection in Czochralski melts under realistic high-temperature boundary conditions. The side view of the NaCl-CaCl 2 melt at T = 600°C was obtained by rapidly lowering the tubular heater, in 10s intervals. The Prandtl number of the melt is Pr = 0.58. Streak photography revealed asymmetric, unsteady, convection. Temperature fluctuations were present in almost all flow regimes.

Interface shape in the vertical Bridgman configuration with and without the submerged heater

The shape of the solid-liquid interface in a multi-zone furnace was studied experimentally and numerically. Gallium-doped germanium single crystals were grown with and without the submerged heater (SH). With the SH, the direction of the heat flux was controlled, and the convective heat transfer at the interface was eliminated. As a result, the interface becomes convex with respect to the liquid. During growth with the rotating SH, a natural convection counter-flow is generated by the centrifugal forces imposed by the rotating heater. This forced flow causes vigorous mixing in the melt near the interface which allows significantly higher growth rates while maintaining a planar interface.

Model of Tellurium- and Zinc-Doped Indium Antimonide Solidification in Space

Numerical simulations were performed to determine the effect of residual microaccelerations on the distribution of dopants (Te and Zn) during solidification of InSb in space. A moving geometry model was developed and used to account for the reduction in melt size during growth. The model demonstrates that diffusion controlled segregation in doped InSb can be obtained at 10 -5 g 0 gravity for the considered growth parameters. The results for the moving geometry and semi-infinite melt domain models are compared to each other and to the analytical correlations for the case of diffusion-controlled segregation.