Band structure of germanium carbides for direct bandgap silicon photonics

Compact optical interconnects require efficient lasers and modulators compatible with silicon. Ab initio modeling of Ge_1-xC_x (x = 0.78%) using density functional theory with HSE06 hybrid functionals predicts a splitting of the conduction band at Γ and a strongly direct bandgap, consistent with band anticrossing. Photoreflectance of Ge_0.998C_0.002 shows a bandgap reduction supporting these results. Growth of Ge_0.998C_0.002 using tetrakis(germyl)methane as the C source shows no signs of C-C bonds, C clusters, or extended defects, suggesting highly substitutional incorporation of C. Optical gain and modulation are predicted to rival III-V materials due to a larger electron population in the direct valley, reduced intervalley scattering, suppressed Auger recombination, and increased overlap integral for a stronger fundamental optical transition.