Chad A. Stephenson

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 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.

Ge quantum dots encapsulated by AlAs grown by molecular beam epitaxy on GaAs without extended defects

We demonstrate nearly spherical, strain-free, self-assembled Ge quantum dots (QDs) fully encapsulated by AlAs, grown on (100) GaAs by molecular beam epitaxy. The QDs were formed without a wetting layer using a high temperature, in situ anneal. Subsequent AlAs overgrowth was free from anti-phase domains and threading dislocations in cross section transmission electron microscopy. The straddling band alignment for Ge in AlAs promises strong and tunable confinement for both electrons and holes. The reflection high-energy electron diffraction pattern changed from 2 × 3 to 2 × 5 with anneal, which can be explained by surface reconstructions based on the electron-counting model.

Stability of Tensile-Strained Ge Studied by Transmission Electron Microscopy

The sensitivity of strained Ge to damage after irradiation with 300kV electron-beam was studied. A time-dependent damage analysis under TEM irradiation on two 0.5 μm-width waveguides, one with 1 GPa stress and a 20 nm protection layer, the other with 2 GPa stress but no protection layer, was performed. The interface for highly-strained waveguides was severely damaged by e-beam within mere seconds, and the damage propagated into the deeper region with time. For the weakly strained sample, the weak damage was confined within 2 nm of the interface and did not propagate. Highly strained Ge interfaces were susceptible to damage, and the dislocations propagated deep into the waveguide.

Band structure of germanium carbides for direct bandgap photonics

Ab-initio simulations of dilute germanium carbides (Ge:C) using hybrid functionals predict a direct bandgap with <;1%C. Growth of dilute Ge:C shows reduced direct gap consistent with the model, with no structural defects detected. Ge:C may enable lasers and compact modulators on Si.

Analysis and design of core-shell upconverting nanostructures

Core-shell upconverting nanostructures (CSUNs) improve upconversion efficiency by blocking midgap recombination. Consecutive 3-level systems absorb strongly but preserve long carrier lifetime in their final states. Germanium has a nearly-direct bandgap ideal for a CSUN core: strong optical absorption yet slow recombination. Electrons in the indirect conduction band valley absorb a second photon to escape the shell and reach the host, unable to return. Our model indicates that free carrier absorption (FCA) from L to continuum is the limiting step. Surface plasmon resonances (SPR) in Ge-AlGaAs may selectively enhance this absorption. CSUNs would help preserve current matching at dawn and dusk.