Thermal Soret diffusion in the liquid phase epitaxial growth of binary III–V compounds

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Steady-state liquid phase epitaxial nucleation and growth conditions have been achieved in the growth of III–V compounds that yield terrace and miniscus line-free layers and uniform submicron thickness control. A theoretical analysis of transient thermodynamic transport through super- and undersaturated melts appears to show that the thermal Soret effect is a very strong coupling force between solute and heat transport. When nucleation and growth are initiated at near steady-state equilibrium liquidus conditions, at the substrate interface with a temperature gradient, melt-back and spontaneous nucleation are minimized. This appears to indicate that near the interface localized nonuniformities exist in supersaturated melts that yield unstable conditions for nucleation and growth. The Soret thermal diffusion coefficient has been evaluated in terms of the solute and solvent masses and the temperature dependence of the diffusion coefficient. At a noninteracting interface the steady-state solute concentration is always greater than the equilibrium liquidus concentration with a positive temperature gradient to the III–V source. The increased concentration is a result due to zero net solute flux at this interface where equal but opposite driving forces are developed as steady-state conditions are approached. At a very critical time during melt saturation the equilibrium liquidus concentration is reached when the substrate should be brought into contact with the melt to avoid melt-back and spontaneous nucleation. The critical time determined in this analysis appears to be in agreement with the experimental results.