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We have grown the ferromagnetic semiconductor GaMnAs containing up to 10% Mnby migration enhanced epitaxy at a substrate temperature of 150^oC. Thealternate supply of As2 molecules and Ga and Mn atoms made it possible to growsingle crystalline GaMnAs layers at very low substrate temperature, at whichconventional molecular beam epitaxial growth under excess As supply is notpossible due to As condensation. Secondary ion mass spectroscopy and X-raydiffraction measurements confirmed a higher Mn content in the films grown bythis method in comparison to the GaMnAs layers grown by low temperaturemolecular beam epitaxy. The lattice constant of hypothetical zinc-blendestructure MnAs is determined to be 5.9 \AA, which deviates somewhat frompreviously reported values. This deviation is ascribed to growth-conditiondependent density of point defects. It is stressed that this effect must betaken into account for any assessment of Mn content from X-ray diffractiondata.  Magnetization measurements showed an onset of ferromagnetic ordering around75 K for the GaMnAs layer with 10% Mn. This means that the trend of fallingCurie 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 antisitedefects and Mn doping, and increased density of magnetically active Mn ions.Comment: No LaTeX source; gzipped postscript text + 3 gzipped postscript  figure

Structural and magnetic properties of GaMnAs layers with high Mn-content grown by migration-enhanced epitaxy on GaAs(100) substrates