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Periodic Reporting for period 2 - SiLAS (SiliconLaser)

Reporting period: 2019-01-01 to 2021-06-30

The electronic industry is currently lacking an efficient light emitter enabling chip-to-chip or core-to-core optical communication within a processor chip. The SiLAS project aims to demonstrate efficient light emission from SiGe alloys which are compatible with the electronics industry. The key point is that we study SiGe alloys within a hexagonal crystal phase, which are predicted to feature a direct bandgap, which is a key point for demonstrating efficient light emission. The project will also study approaches for CMOS compatible fabrication. The final objective of the project is to demonstrate a hexagonal SiGe nanolaser.
The primary objective of this project is to provide a pathway for green ICT, in which energy consumption is considerable reduced by replacing copper wiring by optical interconnects which are powered by silicon compatible light emitters. A silicon compatible light source for silicon photonic integrated circuits will not only find applications for optical interconnects, but might also provide a lightsource for silicon photonics. As a final possible application, we like to mention an integrated SiGe lightsource for disposable sensors. When the expensive III/V laser can be replaced by a SiGe lightsource, these sensors might find applications for medical diagnostics, remote sensing and food safety.
Major achievements after 2 years:
• It has been unambiguously determined that hexagonal silicon-germanium (Hex-SiGe) is a direct bandgap semiconductor for Ge-compositions above approximately ~70%.

• We strongly improved the crystal quality of our Hex-SiGe nanowire shells.

• We demonstrated tuning of the direct bandgap emission between 3.5 µm and < 2.0 µm. The experimental results are perfectly in agreement with our theoretical calculations.

• We experimentally obtained substantial evidence for direct bandgap emission.

• We realized room temperature emission of Hex-Ge.

• Preliminary results indicate amplified spontaneous emission in Hex-Ge

Major achievements at the end of the project:

• Calculation of the bandstructure of hex-SiGe

• Direct bandgap photoluminescence between 1.8 µm and 4 µm by tuning the Ge-composition

• Observation of a subnanosecond radiative lifetime of hex-SiGe

• Defect study showing that the dominant I3 defects do not provide a state within the bandgap

• Improvement of the material quality: Reaching the radiative limit at 300K !

• Growth of a planar layer of hexagonal GaAs

• First indications of lasing for hex-SiGe in a nanowire-microstadium cavity
This is the first project ever heading towards optical quality hexagonal SiGe. The project intends to first demonstrate direct bandgap SiGe, followed by the demonstration of a hexagonal SiGe nanoLED, an optically pumped SiGe nanolaser and finally an electrically pumped SiGe nanolaser. The long-term objective of this project is to merge the electronics industry based on Si with the telecommunication industry based on III/V compound semiconductors into a new industry, solely based on both cubic and hexagonal SiGe.

We published a paper in Nature showing efficient light emission from hex-SiGe. The paper has been elected as the 2020 breakthrough of the year by Physics World. Moreover, we observed the first indications for lasing shortly before the end of the project. The results from the Nature paper indicate that a low threshold hex-SiGe laser operating at room temperature should be feasible. However, a low threshold room temperature hex-SiGe laser has still to be demonstrated, thus still providing some risks for generating economic impact.

These results show that hex-SiGe is indeed capable to meet the most important requirement to become the first silicon-based light emitter. The SiLAS project thus removed the most important fundamental obstacles for hex-SiGe to become an important light emitter in silicon photonics and silicon electronics. The remaining obstacle is to integrated hex-SiGe in silicon technology. Results by IBM in this project show promising results by using Template Assisted Selective Epitaxy. Now the fundamental obstacles have been removed, we expect many more efforts to integrate Hex-SiGe within Si-technology.

We also published a paper in Science which demonstrated detection of THz radiation using pairs of antenna coupled semiconductor nanowires. This device has the potential to improve THz spectroscopy and imaging by providing polarisation information. The monolithic device avoids polarisation crosstalk and is very compact. University of Oxford in investigating to commercialize the device.
Tunability of the emission of Hex-SiGe
Hexagonal SiGe nanowire shells