  We present a theoretical investigation of space separated electron and hole
distributions, which consists in the self-consistent solving of the Schrodinger
equations for electrons and holes and the Poisson equation. The results are
illustrated for the GaN/Al0.3Ga0.7N quantum well. The optical gain spectrum in
a [0001]-oriented GaN/Al0.3Ga0.7N quantum well in the ultraviolet region is
calculated. It is found that both the matrix elements of optical transitions
from the heavy hole band and the optical gain spectrum have only the strict x
(or y) light polarization. We present studies of the influence of the
confinement of wave functions on the optical gain which implicitly depends on
the built-in electric field calculated to be 2.3 MV/cm. Whereas the structures
with narrow well widths exhibit the usual development of the light gain maximum
almost without shifting the spectral region, a significant blueshift of the
gain maximum is found with increase in the plasma density for wider quantum
wells. This blueshift is ascribed to the interplay between the screening of a
strain-induced piezoelectric field and the bandstructure. A large Sommerfeld or
Coulomb enhancement is present in the quantum well.
