Lights On for Silicon Photonics

An inter­national team led by Giacomo Scalari and Jérôme Faist from the Institute for Quantum Electronics present an important step towards new silicon pho­tonics device. They report electro­luminescence from a semi­conductor structure based on silicon-​germanium (SiGe), a material that is compatible with standard fabrication processes used for silicon devices. Moreover, the emission they observed is in the terahertz frequency band, which is of high current interest with a view to a variety of applications.

Scanning transmission electron microscopy (STEM) images of one of the Ge/SiGe heterostructures at different magnifications. The SiGe layers appear darker. (Source: U. Roma Tre, De Seta Group)

The main reason why silicon cannot be used directly for building a laser following to the GaAs template has to do with the different nature of their band gaps, which is direct in the latter but indirect in the former. In a nutshell, in GaAs electrons recom­bine with holes across the bandgap producing light; in silicon, they produce heat. Laser action in silicon therefore requires another path. And exploring a fresh approach is what doctoral researcher David Stark and his colleagues are doing. They work towards a silicon-​based quantum cascade laser (QCL). QCLs achieve light emission not by electron–hole recom­bination across the bandgap, but by letting electrons tunnel through repeated stacks of precisely engineered semi­conductor structures, during which process photons are emitted.

The QCL paradigm has been demonstrated in a number of materials but never in silicon-​based ones, despite promising predictions. Turning these predictions into reality is the focus of an inter­disciplinary project funded by the Euro­pean Commission, bringing together a team of leading experts in growing highest-​quality semiconductor materials at the Università Roma Tre, charac­terising them at the Leibniz-​Institute for innovative micro­electronics in Frankfurt (Oder) and fabri­cating them into devices at the University of Glasgow. The ETH group of Scalari and Faist is responsible for performing the measure­ments on the devices, but also for the design of the laser, with numerical and theoretical support from partners in the company Nextnano in Munich and at the Universities of Pisa and Rome.

With this bundled knowledge and expertise, the team designed and built devices with a unit structure made of SiGe and pure germanium, less than 100 nano­meters in height, which repeats 51 times. From these hetero­structures, fabricated with essen­tially atomically precision, Stark and co-​workers detected electro­luminescence, as predicted, with the spectral features of the emerging light agreeing well with calculations. Further confi­dence that the devices work as intended came from a comparison with a GaAs-​based structure that was fabri­cated with identical device geometry.

Whereas the emission from the Ge/SiGe structure is still significantly lower than for its GaAs-​based counter­part, these results clearly signal that the team is on the right track. The next step will be now to assemble similar Ge/SiGe structures according to a laser design that the team developed. The ultimate goal is to reach room-​tempera­ture operation of a silicon-​based QCL.

Such an achieve­ment would be significant in several respects. Not only would it, at long last, realize a laser on a silicon substrate, thereby bringing a boost to silicon photonics. The emission of the structures is in the tera­hertz region, for which currently compact light sources are widely missing. Silicon-​based QCLs, with their potential versa­tility and reduced fabri­cation cost, could be a boon for the large-​scale use of terahertz radiation in existing and new fields of application, from medical imaging to wireless communi­cation. (Source: ETHZ)

Reference: D. Stark et al.: THz intersubband electroluminescence from n-type Ge/SiGe quantum cascade structures, Appl. Phys. Lett. 118, 101101 (2021); DOI: 10.1063/5.0041327

Link: Project Flash, Horizon 2020, Brussels, BelgiumQuantum Optoelectronics Group, ETH Zurich, Zurich, Switzerland

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