Genetic-algorithm discovery of a direct-gap and optically allowed superstructure from indirect-gap Si and Ge semiconductors.

Combining two indirect-gap materials-with different electronic and optical gaps-to create a direct gap material represents an ongoing theoretical challenge with potentially rewarding practical implications, such as optoelectronics integration on a single wafer. We provide an unexpected solution to this classic problem, by spatially melding two indirect-gap materials (Si and Ge) into one strongly dipole-allowed direct-gap material. We leverage a combination of genetic algorithms with a pseudopotential Hamiltonian to search through the astronomic number of variants of Si(n)/Ge(m)/…/Si(p)/Ge(q) superstructures grown on (001) Si(1-x)Ge(x). The search reveals a robust configurational motif-SiGe(2)Si(2)Ge(2)SiGe(n) on (001) Si(x)Ge(1-x) substrate (x≤0.4) presenting a direct and dipole-allowed gap resulting from an enhanced Γ-X coupling at the band edges.