Disk-driven flows and interface shape in vertical Bridgman growth with a baffle

Abstract

In vertical Bridgman (VB) systems, the shape of the S-L interface greatly influences the yield and perfection of single crystal, because of the continuous contact with the crucible. The melt flows and the shape of the S-L interface are difficult to modify and control.

Baffles are flow-directing or obstructing devices. In VB melts, the baffles are disk shaped, and positioned horizontally above the solid-liquid (S-L) interface. The role of the baffle is to: i) minimize the thermally-driven convection ii) control/reduce the axial heat transfer to the S-L interface and iii) generate the disk-driven flows. Furthermore, the baffle acts as a partition, splitting the melt into: the growth melt below the baffle and the feeding melt above the baffle.

Forced convection is a practical alternative to the less feasible and reliable option of completely eliminating thermally-driven unsteady flows. In the Czochralski (CZ) process, the flow driven by crystal rotation is a key control parameter which the VB process lacks. Baffle rotation brings the CZ-like flow into the VB process. The disk-driven flows are optimal for various scientific and engineering applications because the laminar boundary layers at the disk surface are steady and have uniform thickness.

In VB melts, the thermal conductivity of the baffle and its rotation rate dominate the interface shape and thus the yield and perfection of single crystals. Under the rotating baffle, the effects of natural convection can be made negligible in production size melts.