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Excelitas Technologies Corp. - Novem LB 10/22

Group IV Laser Materials Prepare to Board Silicon Photonics Train

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A team of scientists led by Moustafa El Kurdi from the Center for Nanosciences and Nanotechnologies, University of Paris-Saclay and collaborators from the Alternative Energies and Atomic Energy Commission (CEA) in France have developed a technology to enable laser integration into CMOS chips. The work could lower the cost of production and ease integration into the silicon chip fabrication chain.

Specifically, the team developed a germanium tin on insulator (GeSnOI) technology that combines defects, strain, electronic-band, modal, and thermal engineering. The combination enabled the team to demonstrate a GeSn laser on a versatile photonic platform with improved performances.

Low-cost and CMOS-compatible Si-based photonic technologies have enabled significant advances over the past few decades, particularly for datacom applications and high- speed optical links. However, a major bottleneck of monolithically integrated silicon photonic circuits is the lack of CMOS-compatible lasers. Up to now, III-V lasers are the most standard and reliable light source on an integrated platform. Yet the CMOS-incompatible processes of these lasers increase the cost of production and complicate integration. Group IV GeSn semiconductors offer an attractive alternative due to their simple integration into the silicon chip fabrication chain.

Still, these lasers face issues of high threshold power and low lasing temperature. These hinder their integration into full CMOS-compatible photonic chips.

Since the first GeSn laser demonstration in 2015, research has focused on a GeSn laser designed on the basis of an as-grown GeSn layer on Ge strain-relaxed-buffer on silicon. The lattice mismatch between GeSn and Ge induces a compressive strain, however, which degrades the optical gain properties of the GeSn alloys and can even change the GeSn alloys’ band structure from direct to indirect, which eliminates gain properties.

As a result, mainstream approaches centered on growing thick GeSn layers above their critical thickness for plastic relaxation, which negatively affects lasing, and allows a residual compressive strain to remain. Designs compensated by increasing the concentration of Sn, which improved the maximum lasing temperature, but led to more GeSn-Ge interface defect. This in turn caused higher excitation thresholds of the order of MW/cm2.

Today, the further increase of Sn concentration in GeSn proves challenging because the equilibrium solubility of Sn in Ge is only 1%. Therefore, GeSn lasers based on as-grown layer suffer from the bottleneck on material growth as well as laser performances.

In the current approach, the researchers fabricated a germanium tin-silicon nitride-aluminum (GeSn-SiN-Al) stack using bonding processes on silicon wafer. The GeSnOI layer was then patterned into microdisk laser cavities.

The team demonstrated that its GeSnOI technology tackles lattice mismatch interface defects, compressive/tensile strain engineering, thermal management and optical confinement all together. Using this technology, the team developed a GeSn laser with a lower threshold, higher maximum lasing temperature, and stronger lasing intensity.

The GeSn-Ge interface defects of GeSnOI stack are removable through a simple top-etching process after the transfer and bonding processes, resulting in better active layer quality and higher optical gain, the team said. The improved gain enables 60x-higher laser intensity, a 55 K enhancement in the maximum lasing temperature, and lower threshold in the GeSnOI-based laser compared to conventional as-grown GeSn approaches.

The low index of the SiN layer provides strong optical confinement in GeSn layer without the need to undercut it. The SiN layer is also used as a stressor layer that enables the transfer of tensile strain to the GeSn active cavity to overcome residual compressive strain issues.

Using this technology, the team developed a GeSn laser with a lower threshold, a higher maximum lasing temperature, and a stronger lasing intensity.

The versatile GeSnOI platform paves the way for a multifunctional planar GeSn laser. For example, through deposition and partial etching of the top SiN strain layer, scientists can control tuning of the lasing wavelength. With the help of Al circular grating, they can redirect the laser emission from in-plane to out-plane, the researchers said.

The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-021-00675-7).

Photonics Spectra
Feb 2022
lasersGermanium tinalloysmid-infraredResearch & TechnologyeducationEuropeCEAuniversity of paris-saclayCMOSsilicon laserssiliconsilicon and germaniumsilicon photonicsGeSndatacomCommunicationslaser communicationssemiconductor lasersTechnology News

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