Band structure of germanium carbides for direct bandgap silicon photonics

Abstract

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.

Gas Source Techniques for Molecular Beam Epitaxy of Highly Mismatched Ge Alloys

“…A decrease in the direct bandgap of 24 meV is consistent with theoretical predictions and very close to the photoreflectance results [17]. Additional growth with %C close to 1 at % should be performed to confirm the theoretical predictions for a direct bandgap and confirm the measured decrease in bandgap is due to C and not the Franz-Keldysh effect.…”

“…Although not 100% pure, this is a significant improvement from the previous sample (refer to Figure 3), and the remaining background compound is not expected to interfere with semiconductor growth [17]. improvement from the previous sample (refer to Figure 3), and the remaining background compound is not expected to interfere with semiconductor growth [17]. …”

“…To the authors' knowledge, this was the first demonstration of a decrease in direct bandgap in a Ge:C sample with no defects detectable by electron-dispersive X-ray spectroscopy in scanning transmission electron microscopy (STEM-EDX), nuclear reaction analysis Rutherford backscattering spectroscopy (NRA-RBS), or Raman spectroscopy [17]. It is noteworthy that a significant reduction in direct bandgap was achieved even though the carbon and defect concentrations were both below the detectable limits of these common techniques.…”

“…As shown in Figure 7, crystal quality is very high with no stacking faults or dislocations detected. Another confirmation of the crystal quality is the lack of detectable carbon clusters by HRTEM, Raman spectroscopy, and NRA-RBS [17]. Due to the lack of evidence of C clusters, it is believed that the C atoms are incorporating as part of the Ge4C structure of the original (H3Ge)4C molecule.…”

“…Part 2 describes the synthesis of our carbon precursor gas source, 4GeMe, including modifications to the previous reports of the synthesis. Undesirable carbon-carbon bonds are avoided using a carbon precursor based on a Ge4C core, in which each carbon atom is already fully surrounded by Ge [17,18]. This is distinct from the more common methylgermanes with Ge surrounded by multiple C atoms.…”