  Commonly, germanium crystals are grown after the Czochralski (CZ) method. The
crucible-free pedestal and floating zone (FZ) methods, which are widely used
for silicon growth, are hardly known to be investigated for germanium. The
germanium melt is more than twice as dense as liquid silicon, which could
destabilize a floating zone. Additionally, the lower melting point and the
related lower radiative heat loss is shown to reduce the stability especially
of the FZ process with the consequence of a screw-like crystal growth. We found
that the lower heat radiation of Ge can be compensated by the increased
convective cooling of a helium atmosphere instead of the argon ambient. Under
these conditions, the screw-like growth could be avoided. Unfortunately, the
helium cooling deteriorates the melting behavior of the feed rod. Spikes appear
along the open melt front, which touch on the induction coil. In order to
improve the melting behavior, we used a lamp as a second energy source as well
as a mixture of Ar and He. With this, we found a final solution for growing
stable crystals from germanium by using both gases in different parts of the
furnace. The experimental work is accompanied by the simulation of the
stationary temperature field. The commercially available software FEMAG-FZ is
used for axisymmetric calculations. Another tool for process development is the
lateral photo-voltage scanning (LPS), which can determine the shape of the
solid-liquid phase boundary by analyzing the growth striations in a lateral cut
of a grown crystal. In addition to improvements of the process, these
measurements can be compared with the calculated results and, hence, conduce to
validate the calculation.
