Analysis of turbulent flow in silicon melts by optical temperature measurement

Turbulent convection in silicon melts plays a decisive role for the transport of heat and oxygen during the silicon Czochralski crystal growth. Information on the state of turbulence is usually derived from the frequency behaviour of temperature fluctuations. Until now, temperature fluctuations were mostly detected by thermocouples, which are protected from the chemically aggressive melt by silica quartz tubes. The major disadvantage of this technique is the damping effect on amplitudes of temperature fluctuations at frequencies higher than 0.5 Hz. This drew back significantly the information available on the state of turbulence. A method to overcome this disadvantage is an optical technique of measuring temperatures that provides sampling rates up to 100 Hz. In this contribution, we present optical temperature measurements using silica rods as waveguides in large-diameter Czochralski silicon melts. The detected temperature fluctuations give slopes of power spectra that are approximately proportional to f−4 for higher frequencies, in the case of the smaller crucible of 14 inch diameter. This indicates a soft turbulence state where melt flow is stabilised by rotational forces. On the other hand, a hard turbulence state of melt flow, indicated by an f−5/3 behaviour of the power spectra, was found for the large crucible with 32 inch diameter. In contrast to the smaller crucible, we found no changes of the state of turbulence by increasing the crucible rotation rate. The results are discussed with respect to its consequences on the properties of silicon crystals grown by the Czochralski method from large-scale melts.