  The interplay between disorder and spin polarization in a GaMnAs thin layer
results into spin-polarized impurity hole bands. A figure of merit is defined
to label the hole state as being extended or localized. The calculation leads
to a phase diagram determining the metallic or non-metallic character of the
sample. It is shown that samples with the highest figures of merit have a ratio
between the extended hole density and the Mn concentration near 0.2, in
agreement with the ratio of 0.1-0.25 known to occur among samples produced with
the highest Curie temperatures. Both the non-metal-to-metal and the
metal-to-non-metal transitions experimentally observed in the ferromagnetic
regime are obtained, as the Mn concentration increases. An explanation is given
for the occurrence of a maximal Curie temperature in ferromagnetic GaMnAs
samples.
