  Indium incorporation into wurtzite (0001)-oriented
In$_{x}$Al$_{y}$Ga$_{1-x-y}$N layers grown by plasma-assisted molecular beam
epitaxy was studied as a function of the growth temperature (565-635
$^{\circ}$C) and the AlN mole fraction (0.01 < y < 0.27). The layer
stoichiometry was determined by Rutherford backscattering spectrometry (RBS).
RBS shows that Indium incorporation decreased continuously with increasing
growth temperature due to thermally enhanced dissociation of In-N bonds and for
increasing AlN mole fractions. High resolution x-ray diffraction and
transmission electron microscopy (TEM) measurements did not show evidence of
phase separation. The mosaicity of the quaternary layers was found to be mainly
determined by the growth temperature and independent on alloy composition
within the range studied. However, depending on the AlN mole fraction,
nanometer-sized composition fluctuations were detected by TEM.
Photoluminescence spectra showed a single broad emission at room temperature,
with energy and bandwidth S- and W-shaped temperature dependences typical of
exciton localization by alloy inhomogeneities. Cathodo-luminescence
measurements demonstrated that the alloy inhomogeneities, responsible of
exciton localization, occur on a lateral length scale below 150 nm, which is
corroborated by TEM.
