Special silicon-germanium alloy holds promise for optical communications
Silicon has been the cornerstone of the electronics industry for over 50 years. However, silicon, germanium and their alloys, such as SiGe, share a crystal structure that makes them inefficient at emitting light. This limitation has profoundly influenced the development of the semiconductor industry. As a result, the industry evolved into two distinct areas. On one side, the electronics industry relies on silicon, which excels at advanced electronic processing but cannot provide a light source for communication. On the other, the communication industry depends on III-V semiconductors, which are excellent at emitting light but lack the ability to handle advanced electronic processing on the same chip.
From a cubic to a hexagonal structure
A major breakthrough in silicon technology, pioneered by the team working on the EU-funded Opto silicon(opens in new window) project, has been the development of hexagonal SiGe (hex-SiGe), a new material that can efficiently emit light. Unlike traditional SiGe, whose atoms are arranged in a cubic structure and struggles with light emission, hex-SiGe is a direct bandgap semiconductor ideal for optoelectronic applications. It introduces critical new capabilities to silicon technology, such as generating light for LEDs and lasers, amplifying light in optical systems and detecting light with high efficiency.
SiGe’s potential for light-based and laser devices
One of the key milestones was the demonstration of the first hex-Ge/SiGe quantum wells – tiny layers of material that can trap and control light –with sharp, clean interfaces. The team observed clear evidence of carrier confinement, meaning the material can effectively control electron and hole movement – essential for creating efficient light-emitting devices. The researchers also confirmed a type I band alignment, a property that renders these quantum wells ideal for creating lasers and quantum dot single-photon sources. “We demonstrated that hex-SiGe quantum wells can emit light with a nanosecond radiative lifetime, showing that the material is highly efficient at converting energy into light,” notes project coordinator Jos Haverkort. “By fine-tuning the composition of hex-SiGe alloys, we were able to adjust the emission wavelength to as low as 1.55 µm at very low temperatures (4K), a significant improvement from the previous 1.8 µm.” Another breakthrough has been the observation of a linear increase in confined light modes within a suspended nanowire. This finding provided clear evidence of stimulated emission – critical for creating lasers using hex-SiGe. “We measured an impressive optical gain of 545 cm^-1 in hex-SiGe, demonstrating its strong potential for amplifying light. We also studied how energy carriers lose energy, a process known as carrier cooling,” states Haverkort. “Unlike other materials, hex-SiGe lacks polar optical phonon interactions, which typically slow down this cooling process. Remarkably, the carrier cooling time in the material was found to be comparable to that of InGaAs/InP laser structures.”
Reducing crystal defects
Since hex-SiGe exists in a metastable crystal phase, the team also tackled challenges related to I3 stacking faults – thin layers of cubic SiGe occasionally forming within the hexagonal crystal. The team significantly expanded knowledge about these faults and developed methods to reduce their density.
Enabling next-generation optoelectronic devices
Opto silicon made important progress in integrating hex-SiGe onto silicon-on-insulator platforms, yet there is still more work to be done to fully develop the technology. The researchers will continue their work on the Bright Chips project, where the focus will be on advancing the growth techniques and achieving seamless planar integration of hex-SiGe.
