To get information about the transport mechanisms of impurities or dopants in the liquid zone for the crucibleless zone melting method of crystal growth the boron distribution was studied in Ge-single crystals using neutron induced autoradiography on solid state nuclear track detectors. The boron distribution found for a cover-free surface of the zone was proved to be caused by non-stationary thermocapillary (Marangoni) convection. The Marangoni convection could be suppressed by a viscous oxide layer on the zone.