  We have grown the ferromagnetic semiconductor GaMnAs containing up to 10% Mn
by migration enhanced epitaxy at a substrate temperature of 150^oC. The
alternate supply of As2 molecules and Ga and Mn atoms made it possible to grow
single crystalline GaMnAs layers at very low substrate temperature, at which
conventional molecular beam epitaxial growth under excess As supply is not
possible due to As condensation. Secondary ion mass spectroscopy and X-ray
diffraction measurements confirmed a higher Mn content in the films grown by
this method in comparison to the GaMnAs layers grown by low temperature
molecular beam epitaxy. The lattice constant of hypothetical zinc-blende
structure MnAs is determined to be 5.9 \AA, which deviates somewhat from
previously reported values. This deviation is ascribed to growth-condition
dependent density of point defects. It is stressed that this effect must be
taken into account for any assessment of Mn content from X-ray diffraction
data.
  Magnetization measurements showed an onset of ferromagnetic ordering around
75 K for the GaMnAs layer with 10% Mn. This means that the trend of falling
Curie temperatures with increasing Mn concentrations above 5.5% is broken.
  We tentatively assign this to the variation of the carrier concentration,
including contributions from donor and acceptor centers formed by antisite
defects and Mn doping, and increased density of magnetically active Mn ions.
