  In the present work, we have prepared Mn-doped Ge using different annealing
approaches after Mn ion implantation, and obtained samples with hole
concentrations ranging from 10^18 to 2.1x10^20 cm^-3, the latter being the
highest reported so far. Based on the magnetotransport properties of Mn doped
Ge, we argue that the hole concentration is a decisive parameter in
establishing carrier-mediated ferromagnetism in magnetic Ge.
