Floating-zone growth of silicon in magnetic fields - II. Strong static axial fields

Silicon floating-zone experiments in axial magnetic fields up to 5 T have been performed and the macroscopic and microscopic segregation have been determined. Although the axial segregation is shifted towards the diffusive case with the application of higher fields, a pure diffusion-controlled regime cannot be attained even with fields of 5 T. The deterioration of the radial profiles experienced with smaller fields, due to the separation of the flow field in a quiescent center and a periphery mixed by thermocapillary (Marangoni) convection, can be reduced considerably with higher fields. A complete suppression of dopant striations caused by time-dependent thermocapillary convection is possible with fields of several Tesla. However, the use of high axial magnetic fields can sometimes lead to the appearance of a new type of pronounced dopant striations of oscillatory nature. These striations can be attributed to thermoelectromagnetic convection, caused by the interaction of thermoelectric currents with the magnetic field, and often suggest an annular flow pattern. The fact, that the direction and magnitude of thermocurrents depend on the interface shape, temperature field, and composition, explains the variety of striation patterns obtained, as well as the possibility to grow striation-free crystals. (C) 1998 Elsevier Science B.V. All rights reserved.