Optically Pumped Terahertz Amplitude Modulation in Type-II Ge QD/Si Heterostructures Grown via Molecular Beam Epitaxy

Abstract

This article explores group-IV germanium (Ge) quantum dots (QDs) on silicon-on-insulator (SOI) grown by molecular beam epitaxy (MBE) in order to explore their optical behavior in the terahertz (THz) regime. In this work, Ge QDs, pumped by an above bandgap near–infrared wavelength, exhibit THz amplitude modulation in the frequency range of 0.1–1.0 THz. The epitaxial Ge QDs outperform the reference SOI (170 nm top Si) substrate in THz amplitude modulation due to higher carrier generation in weakly confined dots compared to their bulk counterpart. This is further corroborated using a theoretical model based on the nonequilibrium Green’s function (NEGF) method. This model enables the calculation of photocarrier generated (PCG) and their confinement in the Ge QD region. Our model also reroutes the calculation from PCG to the corresponding plasma frequency and hence to refractive index and THz photoconductivity. Moreover, the photogenerated confined holes’ accumulation at the Ge QDs/Si interface is elevated after optical illumination, leading to decreased THz photoconductivity. This augmentation in THz photoconductivity contributes to a significant enhancement of THz modulation depth of ∼77% at Ge QDs/Si interfaces compared to bare SOI at 0.1 THz.