Static magnetic fields in semiconductor floating-zone growth

Heat and mass transfer in semiconductor float-zone processing are strongly influenced by convective flows in the zone, originating from sources such a s buoyancy convection, thermocapillary (Marangoni) convection, differential rotation, or radio frequency heating. Because semiconductor melts are cond ucting, flows can be damped by the use of static magnetic fields to influen ce the interface shape and the segregation of dopants and impurities. An im portant objective is often the suppression of time-dependent flows and the ensuing dopant striations. In RF-heated Si-FZ-crystals, fields up to 0.5Tes la show some flattening of the interface curvature and a reduction of stria tion amplitudes. In radiation-heated (small-scale) Si-FZ crystals, fields o f 0.2-0.5Tesla already suppress the majority of the dopant striations. The uniformity of the radial segregation is often compromised by using a magnet ic field, due to the directional nature of the damping. Transverse fields l ead to an asymmetric interface shape and thus require crystal rotation (res ulting in rotational dopant striations) to achieve a radially symmetric int erface, whereas axial fields introduce a coring effect. A complete suppress ion of dopant striations and a reduction of the coring to insignificant val ues, combined with a shift of the axial segregation profile towards a more diffusion-limited case, are possible with axial static fields in excess of 1Tesla. Strong static magnetic fields, however, can also lead to the appear ance of thermoelectromagnetic convection, caused by the interaction of ther moelectric currents with the magnetic field.