Compact optical interconnects require efficient lasers and modulators compatible with silicon. Ab initio modeling of Ge1−xCx (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 Ge0.998C0.002 shows a bandgap reduction supporting these results. Growth of Ge0.998C0.002 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.

Dilute germanium carbides (Ge 1-x C x ) offer a direct bandgap for compact silicon photonics, but widely varying results on its properties have been reported. This work uses ab initio simulations with HSE06 hybrid density functionals and spin-orbit coupling to study the Ge 1-x C x band structure in the absence of defects. Contrary to Vegard's law, the conduction band minimum at k=0 is consistently found to decrease with increasing C content, while L and X valleys change much more slowly. A vanishing bandgap was observed for all alloys with x>0.017. Conduction bands deviate from a constant-potential band anticrossing model except near the center of the Brillouin zone.

We demonstrate nearly-spherical, strain-free, self-assembled Ge quantum dots (QDs) fully encapsulated by AlAs, grown on (100) GaAs by molecular beam epitaxy (MBE). The QDs were formed without a wetting layer using a high temperature, in-situ anneal. Subsequent AlAs overgrowth was free from threading dislocations and anti-phase domains. The straddling band alignment for Ge in AlAs promises strong and tunable confinement for both electrons and holes. The reflection high-energy electron diffraction (RHEED) pattern changed from 2×3 to 2×5 with anneal, which can be explained by surface reconstructions based on the electron-counting model. Keywords:Ge quantum dots, molecular beam epitaxy, surface reconstruction, carrier confinement Germanium quantum dots (QDs) have interesting properties such as a large exciton binding energy, long radiative lifetime, and strong size dependence of radiative lifetime, 1 which could be utilized for optical devices. In particular, as an indirect bandgap material, Ge can offer long minority carrier lifetimes, ranging from µs to ms.

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