Quantum-confined direct band transitions in tensile strained Ge/SiGe quantum wells on silicon substrates

We directly demonstrate quantum-confined direct band transitions in the tensile strained Ge/SiGe multiple quantum wells grown on silicon substrates by room temperature photoluminescence. The tensile strained Ge/SiGe multiple quantum wells with various thicknesses of Ge well layers are grown on silicon substrates with a low temperature Ge buffer layer by ultrahigh vacuum chemical vapor deposition. The strain status, crystallographic, and surface morphology are systematically characterized by high-resolution transmission electron microscopy, atomic force microscopy, x-ray diffraction, and Raman spectroscopy. It is indicated that the photoluminescence peak energy of the tensile strained Ge/SiGe quantum wells shifts to higher energy with the reduction in thickness of Ge well layers. This blue shift of the luminescence peak energy can be quantitatively explained by the direct band transitions due to the quantum confinement effect at the Gamma point of the conduction band.