Enhanced Morphological Stability in Sb-Doped Ge
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UCTION
THE morphological stability of solid/liquid (s/l) interfaces is an extremely complex phenomenon. Fluid flow, heat and mass transfer, surface energy, s/l interface orientation, and kinetics all affect the stability of an interface. Since the introduction of the constitutional supercooling (CS) criterion in 1953,[1] there have been many attempts to identify the conditions under which instability will occur. Mullins and Sekerka (MS) examined the one-dimensional steady-state problem for continuous growth of a single-phase binary alloy, and they proposed a criterion based on perturbation theory that included a conductivity-weighted temperature gradient and accounted for capillarity.[2] Since then, researchers have extended the MS approach to examine the time-dependent stability of interfaces, nonlinear effects, geometrical effects on stability, rapid solidification, convection and forced flow effects near the interface, multicomponent systems, and kinetic effects, among other issues.[3] While experimental evidence supporting many of these theories exists, additional work investigating kinetic effects is still needed. This particular area is important to the semiconductor industry, because semiconductors are facet-forming materials with large anisotropic kinetics. The experimental investigation of interfacial morphological stability is limited by the ability to accurately determine the conditions near the s/l interface during solidification. Crystal growth techniques such as the Czochralski, vertical Bridgman, and float zone methods[4] supply heat radially to the melt, leading to radial ANDREW DEAL, Material Scientist, is with GE Global Research, Niskayuna, NY. ERCAN BALIKCI, Assistant Professor, is with the Department of Mechanical Engineering, Bogazici University, Istanbul, Turkey. REZA ABBASCHIAN, Dean and Professor, is with the Bourns College of Engineering, University of California Riverside, Riverside, CA. Contact e-mail: [email protected] Manuscript submitted February 2, 2006. 100—VOLUME 38A, JANUARY 2007
temperature gradients across the s/l interface that are difficult to measure experimentally without disturbing the interface. Furthermore, natural convection can cause augmented segregation of the solute. Thus, it is difficult to accurately determine the thermal and solutal fields at the onset of an instability during growth. Recently, a technique called axial heat processing (AHP), also known as ‘‘axial heat flux close to the phase interface,’’ was developed to better control the thermal and fluid flow conditions near the interface. The AHP is similar to the vertical Bridgman method, but it incorporates a baffle immersed in the melt near the s/l interface.[5,6] Experiments using AHP have demonstrated that a baffle reduces the amount of natural convection by effectively reducing the melt height.[7, 8] Additionally, a conductive or heated baffle reduces the radial temperature gradients inherent to other methods of crystal growth, which further lowers the level of convection in the melt. Moreover, when thermocouples are incorporated
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