  Room temperature photoluminescence and optical absorption spectra have been
measured in wurtzite In$_{x}$Al$_{y}$Ga$_{1-x-y}$N (x $\approx$ 0.06, 0.02 < y
< 0.27) layers grown by molecular beam epitaxy. Photoluminescence spectra show
both an enhancement of the integrated intensity and an increasing Stokes shift
with the Al content. Both effects arise from an Al-enhanced exciton
localization revealed by the S- and W-shaped temperature dependences of the
photoluminescence emission energy and bandwidth respectively. Present results
point to these materials as a promising choice for the active region in
efficient light emitters. An In-related bowing parameter of 1.6 eV was derived
from optical absorption data.
