(528g) Stabilizing the Detached Bridgman Process Via Model-Based, Nonlinear Control
Detached Bridgman growth represents a radical change over existing industrial crystallization technology. In this process, first observed in space-based growth, the melt dewets the wall, allowing the crystal to pull away from the ampoule. Detached growth eliminates deleterious interactions between the growing crystal and the ampoule, dramatically improving crystal quality. However, the promising early results of microgravity experiments have been difficult to advance in terrestrial growth systems due to a number of instabilities that can be manifest during growth. From the point of view of industrial production, the successful development of the detached Bridgman process would be transformative, especially for the growth of compound semiconductors sensitive to the effects of thermal stress on dislocation levels, such as cadmium zinc telluride.
Much experimental work on detached systems has focussed on suitably modifying melt-ampoule wetting conditions following the original analysis of Duffar et al. , who showed that inherent stability of the process could only be obtained when the sum of the growth and ampoule wetting angles is greater than 180 degrees. Since the growth angle for most semiconductor crystals is small (typically less than 15 degrees), achieving this objective is difficult and requires strongly non-wetting interactions between the melt and ampoule.
In this presentation, we challenge the conventional wisdom of detached crystal growth and claim that suitable process control can be used to stabilize dewetted systems that would otherwise be unstable by the sum-of-the-angles criterion of Duffar et al. . We demonstrate a proof of concept via computational modeling of the dynamics of various control scenarios for detached growth. Conventional, closed-loop PI control that adjusts the pressure across the meniscus by measuring gap size fails to stabilize the system. However, a model-based, nonlinear controller is shown to stabilize the system and produce growth with constant gap width. Such an approach may allow for the reliable industrial practice of this promising growth technology.
 T. Duffar, P. Boiton, P. Dusserre, J. Abadie, "Crucible de-wetting during Bridgman growth in microgravity. II. Smooth crucibles," J. Crystal Growth 179 (1997) 397-409.
This work has been supported in part by the Department of Energy, National Nuclear Security Administration, under Award Numbers DE-FG52-06NA27498 and DE-FG52-08NA28768, the content of which does not necessarily reflect the position or policy of the United States Government, and no official endorsement should be inferred.