Strain-free $Ge∕GeSiSn$ quantum cascade lasers based on $L$-valley intersubband transitions

The authors propose a $Ge∕Ge0.76Si0.19Sn0.05$ quantum cascade laser using intersubband transitions at $L$ valleys of the conduction band which has a “clean” offset of $150meV$ situated below other energy valleys $(Γ,X)$⁠. The entire structure is strain-free because the lattice-matched Ge and $Ge0.76Si0.19Sn0.05$ layers are to be grown on a relaxed Ge buffer layer on a Si substrate. Longer lifetimes due to the weaker scattering of nonpolar optical phonons reduce the threshold current and potentially lead to room temperature operation.