  Raman light scattering, low-temperature photoluminescence, light-scattering
tomography, and hydrogenation were used to investigate optical properties of
defects in strain-relaxed Si_{1-x}Ge_x (0.05 \le x \le 0.50) alloys. The
photoluminescence emission was characterized by typical zero-phonon,
phonon-assisted, and dislocations-related emissions, which are dependent on Ge
composition x. However, luminescence spectra exhibited above band-gap features,
which are likely associated with the presence of Si-rich regions in the alloys.
The results are correlated with light-scattering tomography, revealing the
presence of dislocations and Si precipitates. The excess peak at 519 cm^{-1} in
Ge-rich samples is supportive of this observation. At low Ge content, a
dislocation-related band (D2 line) at 14,204 {\AA} dominates D-band emission
for x < 0.25 while overall D-band emission intensity decreases with x.
Hydrogenation was found to enhance D-band emission, indicating a passivation of
nonradiative recombination centers inside dislocation cores. Si-Si, Si-Ge, and
Ge-Ge phonons (TO, TA, and LA), which are participating in luminescence
emission, evolve with increasing Ge content and Ge-Ge and Si-Ge TO lines
dominate the Raman spectrum to the detriment of the Si-Si TO phonon line. Raman
spectra reveal the presence of alloy fluctuations and possible presence of Ge
particles, particularly in Ge-rich samples.
