  Spin accumulation induced in p-type germanium from Fe/MgO tunnel contacts is
studied as a function of hole concentration p (10^16 - 10^19 cm-3). For all p,
the contacts are free of rectification and Schottky barrier, guaranteeing spin
injection into the Ge and preventing spin accumulation enhancement by two-step
tunneling via interface states. The observed spin accumulation is smallest for
nondegenerate doping (p ~ 10^16 cm-3) and increases for heavily doped Ge. This
trend is opposite to what is expected from spin injection and diffusion theory.
For heavily doped Ge, the observed spin accumulation is orders of magnitude
larger than predicted.
