  The Fermi level position in (Ga,Mn)N has been determined from the
period-analysis of GaN-related Franz-Keldysh oscillation obtained by
contactless electroreflectance in a series of carefully prepared by molecular
beam epitaxy GaN/Ga1-xMnxN/GaN(template) bilayers of various Mn concentration
x. It is shown that the Fermi level in (Ga,Mn)N is strongly pinned in the
middle of the band gap and the thickness of the depletion layer is negligibly
small. For x > 0.1% the Fermi level is located about 1.25 - 1.55 eV above the
valence band, that is very close to, but visibly below the Mn-related Mn2+/Mn3+
impurity band. The accumulated data allows us to estimate the Mn-related band
offsets at the (Ga,Mn)N/GaN interface. It is found that most of the band gap
change in (Ga,Mn)N takes place in the valence band on the absolute scale and
amounts to -0.028+-0.008 eV/% Mn. The strong Fermi level pinning in the middle
of the band gap, no carrier conductivity within the Mn-related impurity band,
and a good homogeneity enable a novel functionality of (Ga,Mn)N as a
semi-insulating buffer layers for applications in GaN-based heterostuctures.
