CMOS-compatible tensile-strained GeSn disk supports continuous lasing

March 25, 2020 //By Julien Happich
GeSn disk
With the aim to move electronics into photonics with faster computing speeds, researchers computing to optical a team of physicists at the Centre de Nanosciences et de Nanotechnologies (C2N), in collaboration with researchers at Germany's Forschungszentrum Jülich (FZJ) and STMicroelectronics, have implemented a new material engineering method to fabricate a laser microdisk in a strained germanium-tin (GeSn) alloy compatible with CMOS processes.

Their results published in the Nature Photonics journal under the title “Ultra-low-threshold continuous-wave and pulsed lasing in tensile-strained GeSn alloys” describes GeSn microdisk lasers fully encapsulated by a stressor layer made of dielectric Silicon Nitride (SiNx) to produce tensile strain. An indirect-bandgap semiconductor as-grown, the 300 nm-thick GeSn layer with 5.4 atomic percent of tin is transformed via tensile strain engineering into a direct-bandgap semiconductor that supports lasing. A specific microdisk cavity design was developed to allow high strain transfer from the stressor layer to the active region, remove the interface defects, and enhanced thermal cooling of the active region.


Scanning Electron Micrographs: (left) A layer of GeSn is transferred onto a silicon substrate and then structured as a microdisk to form an optical cavity. During the transfer, the defective layer in the GeSn, which was at the interface with the Ge/Si substrate, was removed by etching. The transfer also makes it possible to insert a stressed SiNx layer underneath the GeSn layer. An Aluminium layer was used to maintain the cavity while allowing excellent thermal cooling of the laser device through the substrate. (right) A final conformal deposition of a strained film on the microdisk allows to obtain an "all-around" configuration of the stress transfer from the SiNx to the GeSn. The GeSn is then under a tensile strain of 1.6% very homogeneously distributed in its active volume. Credit: C2N / M. El Kurdi & al.

With this approach, the researchers write, low Sn concentration enables improved defect engineering and the tensile strain delivers a low density of states at the valence band edge, which is the light hole band.


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