Photoluminescence from trapped excitons in Si1-xGex/Si quantum well structures

Photoluminescence spectra from Si1-xGex/Si quantum well structures grown at high temperatures are investigated. The luminescence properties are found to be very different from those of free excitons. To describe correctly the spectral lineshape, the radiative recombination from excitons trapped on the local potential fluctuations in Si1-xGex quantum wells must be considered. At low sample temperatures, the luminescence is mainly from the trapped excitons. With increasing temperature, capped excitons are thermally activated into free excitons and luminescence peaks shift to higher energies. By comparing measured spectra with the calculated spectra, the trap density and the trap energy are derived. The origin of the trap and its relation with the crystal growth are discussed.