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) Si1-xGex. The search reveals a robust configurational motif-SiGe(2)Si(2)Ge(2)SiGen on (001) SixGe1-x substrate (x <= 0.4) presenting a direct and dipole-allowed gap resulting from an enhanced Gamma-X coupling at the band edges.