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

TL;DR

This study demonstrates that optically pumped Ge quantum dots on silicon can significantly modulate terahertz signals, with a theoretical model explaining carrier dynamics and interface effects, leading to enhanced THz modulation.

Contribution

It introduces a novel approach using Ge QDs grown by MBE on SOI for THz modulation and provides a theoretical framework based on NEGF to understand carrier behavior.

Findings

Ge QDs exhibit 77% THz modulation depth at 0.1 THz.

Theoretical NEGF model accurately predicts carrier generation and confinement.

Optical illumination increases hole accumulation at Ge QD-Si interface, reducing THz conductivity.

Abstract

This article exploits group-IV germanium (Ge) quantum dots (QDs) on Silicon-on-Insulator (SOI) grown by molecular beam epitaxy (MBE) in order to explore its optical behaviour 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 reference SOI substrate in THz amplitude modulation owing to higher carrier generation in weakly confined dots compared to its bulk counterpart. This is further corroborated using theoretical model based on the non-equilibrium Green's function (NEGF) method. This model enables the calculation of photo carriers generated (PCG) and their confinement in the Ge QD region. Our model also reroutes the calculation from PCG to corresponding plasma frequency and hence to refractive index and THz photo-conductivity. Moreover, the photo-generated confined holes accumulation at the Ge QDs-Si interface is elevated after optical illumination, leading to a decreased THz photo-conductivity. This augmentation in THz photo-conductivity contributes to a significant enhancement of THz modulation depth ~77% at Ge QDs-Si interfaces compared to bare SOI at 0.1 THz.