Strain-induced enhancement of plasma dispersion effect and free-carrier absorption in SiGe optical modulators

TL;DR

This paper demonstrates that applying compressive strain to SiGe significantly enhances plasma dispersion and free-carrier absorption effects, doubling modulation efficiency in Si-based optical modulators at telecommunication wavelengths.

Contribution

It provides the first experimental demonstration of strain-induced enhancement of free-carrier effects in SiGe, improving modulation efficiency for CMOS-compatible optical devices.

Findings

Strain reduces hole effective mass in SiGe, enhancing free-carrier effects.

Strained SiGe modulators show over twice the modulation efficiency of unstrained Si.

The method is compatible with existing CMOS fabrication processes.

Abstract

The plasma dispersion effect and free-carrier absorption are widely used for changing refractive index and absorption coefficient in Si-based optical modulators. However, these free-carrier effects in Si are not large enough for making the footprint of the Si modulators small. Here, we have theoretically and experimentally investigated the enhancement of the plasma dispersion effect and free-carrier absorption by strain-induced mass modulation in silicon-germanium (SiGe). The application of compressive strain to SiGe reduces the conductivity hole mass, resulting in the enhanced free-carrier effects. Thus, the strained SiGe-based optical modulator exhibits more than twice modulation efficiency as large as that of the Si modulator. To the best of our knowledge, it is the first demonstration of the enhanced free-carrier effects in strained SiGe at the near-infrared telecommunication wavelength. The strain-induced enhancement technology for the free-carrier effects is expected to boost the modulation efficiency of the most Si-based optical modulators thanks to high complementary metal-oxide-semiconductor (CMOS) compatibility.