  We have investigated the transport and magnetic properties of group-IV
ferromagnetic semiconductor Ge1-xFex films (x = 1.0 and 2.3 %) with and without
boron doping grown by molecular beam epitaxy (MBE). In order to accurately
measure the transport properties of 100-nm-thick Ge1-xFex films, (001)-oriented
silicon-on-insulator (SOI) wafers with an ultra-thin Si body layer (~5 nm) were
used as substrates. Owing to the low Fe content, the hole concentration and
mobility in the Ge1-xFex films were exactly estimated by Hall measurements
because the anomalous Hall effect in these films was found to be negligibly
small. By boron doping, we increased the hole concentration in Ge1-xFex from
~1018 cm-3 to ~1020 cm-3 (x = 1.0%) and to ~1019 cm-3 (x = 2.3%), but no
correlation was observed between the hole concentration and magnetic
properties. This result presents a contrast to the hole-induced ferromagnetism
in III-V ferromagnetic semiconductors.
