Electroluminescence at Terahertz Frequencies Brings Silicon QCLs into Focus

An international research team based at the Institute for Quantum Electronics at ETH Zurich generated electrical light from a silicon-germanium (SiGe) semiconductor structure. That material is compatible with standard silicon device fabrication processes — giving the development meaning as a potential alternate approach to building a laser on silicon.

Specifically, the advancement counters the physical processes that generate light in gallium arsenide (GaAs), on which diode lasers (used commonly in barcode scanners and laser pointers) are often based. Those physical processes are not highly amenable to silicon.

The electroluminescence that the team of Giacomo Scalari and Jérôme Faist achieved is in the terahertz range. This band, situated between microwave electronics and infrared optics, is applicable to a variety of applications ranging from spectroscopy to QCL lasing, and photodetection and datacom/telecom.

The primary cause of silicon’s inability to be used directly in the construction of a laser based on a GaAs platform involves the differing nature of the materials’ bandgaps. Where GaAs demonstrates a direct bandgap structure, silicon’s bandgaps are characterized as indirect; electrons of GaAs recombine with holes across the bandgap to produce light, while in silicon they produce heat.

As electrons tunnel through the Ge/SiGe heterostructure, they emit light, currently at two slightly different frequencies, due to suboptimal injection in the upper state of the radiative transition. Courtesy of ETH Zurich/David Stark.

To overcome the limitation, ETH researchers led by David Stark launched development of a silicon-based quantum cascade laser (QCL). That type of laser emits light by allowing electrons to tunnel through stacks of semiconductor structures, rather than by electron-hole recombination across the plane of the bandgap.

Growing very high-quality semiconductor materials is one necessity to achieving a functional QCL using a silicon base. Characterizing and fabricating them into the device are separate steps. The European Commission is funding the interdisciplinary project that involves individual groups from Università Roma Tre (Italy, responsible for growing the semiconductor materials), Leibniz-Institut für Innovative Mikroelektronik (Germany, responsible for characterizing the semiconductor materials), and the University of Glasgow (Scotland, responsible for fabricating the characterized semiconductor material into devices).

The ETH group (Scalari and Faist) is performing device measurements and is responsible for designing the laser. German semiconductor nanodevice software company nextnano and the universities of Pisa and Rome are supporting the ETH Zurich researchers.

The ETH Zurich group designed and built devices with a silicon-germanium (SiGe) and pure germanium (Ge) unit structure. The structure was less than 100 nm in height, which repeated 51 times. Stark and co-workers used the detected electroluminescence from the designed heterostructures as predicted; the spectral features of the produced light, they said, were in line with their calculations. Further, when the researchers compared their structure to a GaAs-based structure fabricated with the same device geometry, the new device functioned similarly. Still, the researchers said, emission from the Ge/SiGe remained significantly lower than that from the GaAs-based counterpart.

One of the Ge/SiGe heterostructures at different magnifications. The SiGe layers appear darker. Courtesy of Università Roma Tre, De Seta Group.

The next step will involve assemblage of similar Ge/SiGe structures that are precisely targeted to fit the specifications of the group’s laser design.

Room-temperature operation of a silicon-based QCL is the goal at this stage of the design and development process, according to a press release from ETH Zurich. However, the achievement of room-temperature operation would be significant given that the emission of the new silicon-based structure is in the terahertz region — a band in which compact and practical light sources are desired, and in which few currently exist.

A silicon-based QCL, given its potential versatility and reduced fabrication cost, could enable the large-scale use of terahertz radiation in existing and new fields of application. These span medical imaging and wireless communications.

The research was published in Applied Physics Letters (www.doi.org/10.1063/5.0041327).