Direct time correspondence between crystal growth behavior and the thermal characteristics of the melt under destabilizing gradients was established by means of "time markers" which were introduced into the growing crystals and simultaneously registered on the continuous recording of the thermal behavior of the melt. Employing Te-doped InSb, it was found that as solidification progressed, the melt exhibited successively turbulent convection, oscillatory thermal instabilities, and, finally, thermal stability. During turbulent convection, the crystals underwent pronounced transient back-melting and the average microscopic growth rate was found to be independent of, and about 20 times greater than, the average macroscopic growth rate; this microscopic growth rate was controlled by the convection characteristics of the melt and the thermal gradients in the solid. With the onset of oscillatory thermal instabilities, back-melting became less pronounced and then ceased. In this region the average microscopic and macroscopic growth rates were equal and the crystals exhibited fluctuations in dopant concentration with a periodicity identical to that of the thermal oscillations in the melt. Finally, under stabilized thermal conditions, the microscopic and macroscopic growth rates were identical and no localized fluctuations in dopant concentration could be detected. The Rayleigh numbers of the melt during a growth experiment ranged from 3 • l0 S to 0. In view of the fact that the vertical thermal gradient changed only by a factor of about two during the entire growth, the wide range of Rayleigh numbers was the result of the changes in melt height. In the turbulent convection region, the Rayleigh numbers ranged from about 3 X l0 s to about4 X 103 , in the oscillatory region from 3 X 103to 2 • 103, and in the region of thermal stability from l0 s to 0.
A B S T R A C TIt was established that ideal, diffusion-controlled, steady-state segregation, never accomplished on earth, was achieved during the growth of Te-doped InSb crystals in Skylab. Surface tension effects led to nonwetting conditions u n d e r which free surface solidification took place in confined geometry. It was further found that, under forced contact conditions, surface tension effects led to the formation of surface ridges (not previously observed on earth) which isolated the growth system from its container. In addition, it was possi~ble, for the first time, to identify unambiguously: the origin of segregation discontinuities associated with facet growth, the mode of nucleation and propagation of rotational t w i n boundaries, and the specific effect of mechanical-shock perturbations on segregation. The results obtained prove the advantageous conditions provided by outer space. Thus, fundamental data on solidification thought to be unattainable on earth because of gravityinduced interference are now within reach.Structural and compositional control during solidification of materials is impeded by gravity-induced effects in the melt. Thermal gradients necessary for crystal growth lead, in the presence of gravitational forces, to thermal convection which in general causes uncontrolled variations in the solidification rate and in diffusion boundary layer thickness; such variations lead directly to periodic and/or random microscopic and macroscopic segregation inhomogeneities. F u rthermore, in the presence of gravity, establishing steep thermal gradients, frequently required to prevent constitutional supercooling, is often impossible and consequently interface breakdown is unavoidable.Gravity effects are, thus, primarily responsible for the present lack of reliable solidification data and the existing gap between theory and experiment. Consequently, crystal growth and associated segregation phenomena are still based on empiricism, and the properties and performance of solids are not at their theoretical limits.Gravity-free conditions made accessible through the space program provide a unique opportunity to obtain reliable crystal growth data and, therefore, to advance our quantitative understanding of solidification processes; in addition this program makes possible the exploration and assessment of the potential of outer space for materials processing.Indium antimonide was chosen for the presently reported Skylab experiment because its relatively low melting point (525~ made the experiment compatible with the available electrical power. In addition, chemical etching, the only high-resolution technique available, at the time, for the study of segregation inhomogeneities on a microscale, had been developed on InSb to its most advanced level.The experiments performed during the Skylab-III and -IV missions included the growth of undoped, tellurium-doped, and tin-doped indium antimonide. The present report is concerned primarily with results obtained on tellurium-doped InSb. ObjectivesThe objectives of...
On the basis of mass-transport principles. a theoretical model of electroepitaxial growth-current· controlled liquid-phase epitaxy-was developed which defines the contribution of the Peltier effect (at the solid-solution interface) and that of solute electromigration to the overall growth process. According to the model. the contribution of electromigration to growth is dominant in the absence of convection in the solution, whereas the contribution of the Peltier effect can be dominant in the presence of convection. On the basis of the model, expressions were derived which relate quantitatively the growth velocity to growth parameters. The model was found to be in excellent agreement with extensive experimental data on the electroepitaxial growth of GaAs from a Ga·As solution.
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