Phonon-resolved and broad photoluminescence in strained Si1−xGex alloy MBE layers

In the photoluminescence (PL) spectra of Si_1?xGe_x multi-quantum wells (MQW) grown by conventional solid source molecular beam epitaxy (MBE), phonon-resolved, near-bandgap transitions due to shallow dopant bound exciton or free exciton recombination were observed when the well thickness was less than 40–100Å, depending on x. Increasing the Si_1?xGe_x well thickness caused the emergence of a broad, unresolved PL peak ～120 meV lower in energy than the expected bandgap energy. Interstitial-type platelets, less than 15Å in diameter, were measured by plan view transmission microscopy to occur in densities that correlated well with the intensity of the broad PL peak. A platelet density of ～10⁸ cm^?2 per well was sufficient to completely quench the phonon-resolved PL. Etching experiments revealed that within a given MQW, the platelet density is lowest in the first grown well and progressively increases in subsequent wells with increasing strain energy density, indicating that platelet formation is strictly a morphological phenomenon and suggesting that a strain relaxation mechanism is in effect before the onset of relaxation by misfit dislocation injection.