# On-Chip Strained Germanium Lasers: A Review

## TL;DR

This review discusses strained germanium nanowires as a potential solution for on-chip lasers in photonic circuits.

## Contribution

The paper provides a comprehensive review of strain engineering methods and properties of strained germanium nanowires for on-chip lasers.

## Key findings

- Strain engineering can transform germanium's bandgap from indirect to direct, enabling efficient light emission.
- Nanowire diameter, crystalline orientation, and strain direction significantly affect the bandgap transformation.
- Recent strain engineering methods and application cases are summarized to guide future research.

## Abstract

The 100 GHz-class ultrafast photonic integrated circuit (PIC) positions itself as a promising technology in the post-Moore era, when the bandwidth limit of metallic interconnections constrains current electronic integrated circuits. Nevertheless, the lack of an effective on-chip, CMOS-compatible laser source challenges the ongoing development of PIC. Germanium straintronics facilitate bandgap transformation from indirect to direct, thereby enabling effective band-to-band radiative recombination. Some parameters, such as nanowire diameters or crystalline orientation and strain direction, have a profound effect on the bandgap transformation of Ge nanowires. In this review, we will discuss changes in the fundamental physical properties of Ge nanowires under strain, including mechanical, electronic, optical, and thermal properties. Subsequently, we summarize common methods for strain engineering, as well as novel approaches that have emerged in recent years. Some notable application cases reported in the last few decades will be discussed in detail. This review may fill knowledge gaps and provide a solid background for forthcoming investigations of on-chip strained Ge lasers.