Quantum-confined biexcitons inSi1−xGexgrown on Si(001)

We report experimental evidence for the existence of three-dimensionally (3D) -confined biexcitons in a strain-relaxed ${\mathrm{Si}}_{0.7}$ ${\mathrm{Ge}}_{0.3}$ layer grown on a stepwise graded buffer on Si(001) by ultrahigh vacuum chemical vapor deposition. A calculation of the photoluminescence line shape based on a simple model is found to be in good agreement with experiment. From this theoretical fit we deduce a binding energy of 1.55 meV for the 3D-confined biexcitons. This binding energy is larger than the reported value of 1.36 meV for a free biexciton in Si, indicating a quantum-confinement effect.