  We present first-principles calculations of AlGaN/GaN superlattice,
clarifying the microscopic origin of the light emission and revealing the
effect of local polarization within the quantum well. Profile of energy band
and distributions of electrons and holes demonstrate the existence of a main
active site in the well responsible for the main band-edge light emission. This
site appears at the position where the local polarization becomes zero. With
charge injection, the calculated optical spectra show that the broadening of
the band gap at the active site leads to the blueshift of emission wavelength.
