  The pumping intensity (I) dependence of the photoluminescence (PL) spectra of
perfectly laterally two-dimensionally ordered SiGe quantum dots on Si(001)
substrates was studied. The PL results from recombinations of holes localized
in the SiGe quantum dots and electrons localized due to the strain field in the
surrounding Si matrix. The analysis of the spectra revealed several distinct
bands, attributed to phonon-assisted recombination and no-phonon recombination
of the excitonic ground state and of the excited excitonic states, which all
exhibit a linear I dependence of the PL intensity. At approximately I>3W/cm^2,
additional bands with a nearly quadratic I dependence appear in the PL spectra,
resulting from biexcitonic transitions. These emerging PL contributions shift
the composite no-phonon PL band of the SiGe quantum dots to higher energies.
The experimentally obtained energies of the no-phonon transitions are in good
agreement with the exciton and biexciton energies calculated using the envelope
function approximation and the configuration interaction method.
