Fig. 8

a Physical model of quasi-direct-bandgap emission with a three-level system built in the Si-Ge layers structure, in which the rise of X valley in nanosilicon and L valley in nanogermanium forms the pumping levels, and the Γ valley of Ge direct-gap band or the localized state near Si edge band becomes emission level with inverse population. b Structures of three levels in the Si-Ge layer quantum system, in which the states of Si-Ge QDs are higher to be pumping levels for population inversion in the Γ valley of Ge