Direct bandgap silicon is the holy grail of the semiconductor industry since it would allow the monolithic integration of opto-electronic functionalities onto an all-silicon platform. Bulk silicon is by itself, however, an indirect bandgap semiconductor with only poor light emission properties, inhibiting the realization of functional opto-electronic and photonic devices. But recently, new hexagonal alloys of silicon-germanium (hexSi-Ge) with direct bandgap have become available, providing entirely new perspectives for Si-based optoelectronics. Besides the direct character of their bandgap, optical emission occurs in the technologically important 1.5 - 4.0μm infrared range, that spans both the low-loss region for optical fibres and the molecular fingerprint regions. This provides much scope for opto-electronic integration on a monolithic silicon platform, as well as novel applications for integrated nanoscale sensors.  

Hex-Si-Rei (Hexagonal SiGe for Integrated Optoelectronics), a project consortium between the Walter Schottky Institut (WSI) at TUM and TU/e (Eindhoven Technical University), aims to take these novel materials to the next level. Specifically, our goal is to demonstrate proof-of-principle integration of hex-SiGe into silicon-on-insulator (SOI) nano-photonic circuits by combining the methods developed at TU/e to grow defect-free hex-SiGe on SOI with photonic engineering and ultrafast optical spectroscopy at TUM. Hereby, the team will develop optically pumped hex-SiGe nanoscale laser sources integrated onto a silicon photonic circuit, and benchmark their performance with other non-Si based photonic materials. We further explore novel resonator-cavity concepts to tailor the gain properties, and study the ultrafast gain dynamics via advanced pump-probe spectroscopy methods. Ultimately, our vision is direct light coupling and high-speed modulation of hex-SiGe based light in underlying photonic circuits.