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In an article recently published in the open-access journal Light: Science and Applications, the researchers built a specific GeSnOI (GeSnOI) stack on insulator with the help of a stress layer. They also proved that GeSnOI is suitable for monolithic integration of planar group IV semiconductor lasers on a photonic platform in the near-infrared and mid-infrared wavelength range.
Research: GeSnOI mid-infrared laser technology. Image source: Aumm graphixphoto/Shutterstock.com
Cost-effective, CMOS-compatible silicon-based photonics technologies, such as silicon-on-insulator (SOI), have achieved advanced on-chip optical processing in the near-infrared range, especially in high-speed detection and optical signal modulation. However, the lack of monolithic integrated Group IV lasers is a major disadvantage.
For decades, researchers have been working to integrate III-V compounds with high laser performance to improve silicon photonics technology. Despite the high manufacturing cost and complex integration in Si CMOS compatible manufacturing, III-V lasers are by far the most reliable light source on silicon.
GeSn semiconductor alloy is compatible with low-cost and large-scale silicon processing and manufacturing tools for direct band gaps with tin content greater than 7%, making it an ideal choice for low-cost lasers.
In addition, compared with Ge, GeSn alloy has a narrower band gap, so it can transfer photon wavelengths from near-infrared to mid-infrared, which is very suitable for a wide range of applications such as gas monitoring, biochemical detection, and thermal imaging. The integration of GeSn on silicon paves the way for new applications of silicon photonics.
Design and manufacture of GeSnOI plane cavity. a Schematic diagram of GeSn grown on a stack of Ge SRB and GeSnOI and the TE and TM mode distributions of restricted light waves with a wavelength of 2.4 µm. More details are given in SI (Figure S3). b X-TEM image of GeSn and GeSnOI stacks grown on Ge SRB. c Scanning electron microscope (SEM) images of GeSn and GeSnOI and the omni-directional strained microdisk cavity. The cross-sectional schematic diagram of the GeSnOI mesa completely covered with SiN can be found on the left side of the in-plane strain change of the bottom SEM image (d). Through finite element modeling (FEM) analysis, GeSn and GeSnOI microdisks have 7 due to different Μm diameter due to design and processing technology. Here, we plot the strain relative to the compressive strain after initial residual growth of -0.5% of the GeSn layer. The SiN layer is calculated with the initial compressive stress that can be relaxed, thereby causing a change in normal strain. Image source: Wang, B et al., Application of Light Science
The researchers combined a Ge0.9Sn0.1 alloy layer on a Ge strain relaxation buffer on a Si(001) substrate, and fabricated a microdisk resonator on a mesa structure. The growth layers are analyzed using X-ray diffraction reciprocal space mapping, and these layers have a residual compressive strain of -0.5%. The author also performed whispering gallery mode (WGM) analysis and scattering on Al gratings based on the non-periodic Fourier mode method.
After combining the GeSn layer, a simple etching step is used to eliminate the dense misfit dislocation array near the GeSn/Ge interface, thereby improving the quality of the active layer, and further improving the carrier injection efficiency. These bonding structures also open up the possibility of having a GeSn layer on top of the aluminum layer, which helps to better dissipate heat.
Optical analysis of strain. a Raman spectrum of LO phonon vibration in the overlay, Ge SRB stack, overlay GeSnOI stack, GeSn-on-Ge SRB microdisk, GeSnOI mesa and 10.5% grown GeSn on the omnidirectional GeSnOI mesa. The diameter of the disc and the table is 7 µm. b The photoluminescence spectrum of the GeSn layer, as in (a) pumped with cw light. c GeSn direct and indirect band gap energy as a function of strain. For GeSn 10.5% grown on Ge SRB stacks and GeSn-on-Ge SRB microdisks, the direct band gap energy from the PL measurement is shown as a blue circle. The red circle shows the PL energy covering the GeSnOI stack, the GeSnOI mesa and the omnidirectional GeSnOI mesa (from left to right). The solid line is the direct band gap (green) and indirect band gap (grey) energy calculated as a function of strain using the model described in the reference. The figure above is a schematic diagram of the band structure of GeSn under compressive and tensile strain. Image source: Wang, B et al., Application of Light Science
Compared with the traditional floating microdisk resonator produced from the growth layer, the GeSnOI microdisk mesa shows a significantly higher optical gain. They further demonstrated the vertical out-coupling of the in-plane radiation of the improved disk WGM, and the vertical out-coupling efficiency is close to 30%.
This work shows that the GeSnOI stack produced by the bonding of the GeSn active layer solves all the major limitations mentioned above, such as compression/tensile strain management, lattice mismatch interface defect management, and optical limitations.
The team demonstrated these by systematically comparing two structures with equivalent optical constraints-a suspended microdisk cavity generated from a GeSn layer on Ge SRB and another cavity fabricated using the GeSnOI method with a specific SiN stress film as an insulating layer And a simple disc-shaped table as a cavity.
The SiN layer used for tensile strain engineering provides high refractive index contrast with GeSn, which is required for optical confinement. Disk cavities glued to the substrate without any insufficient etching can reduce their diameter to 3 µm, which is impossible for the grown GeSn microdisks. Without affecting the mechanical and thermal robustness, the lowest diameter that the grown GeSn microdisks can reach is 5 µm.
GeSnOI mesa and grown GeSn microdisk laser. a Light-light (LL) curve of a 7 µm diameter GeSn microdisk and GeSnOI mesa, with 360 nm thick GeSn (b) and c PL spectra under 75 K pulse pumping. Various pulse excitation powers of GeSnOI at 75 K, respectively It is the countertop and GeSn-MD. The excitation level is given by the peak power density. Image source: Wang, B et al., Application of Light Science
All in all, compared to suspended GeSn microdisks, the microdisk-shaped mesa etched in the bonded GeSnOI stack has many advantages. The GeSnOI disc-shaped mesa has no interface defects seen in the suspended GeSn-microdisk at the GeSn/Ge interface of the base.
The bonding structure in the GeSnOI disc-shaped mesa is not under-etched, and it has an aluminum layer used as a heat sink for better thermal management.
Compared with the underetched structure suspended in air, the optical confinement of the optical mode in the active GeSn layer is equally good in the GeSnOI mesa. In addition, the complete encapsulation of GeSn mesa with SiN insulator layer provides a high level of uniformly distributed tensile strain, which is not feasible for suspended GeSn microdisks.
The main advantage of the GeSnOI platform is that it can mix active laser structures with passive SiN circuits ranging from the near-infrared to the mid-infrared.
This represents a new standard for infrared group IV photonics, and the integration of group III-V lasers is no longer required. Therefore, the GeSnOI method is a valuable resource for the development of silicon-based mid-infrared photonics, which combines complex lightwave engineering with integrated sources in a photonic platform.
Wang, B., Sakat, E., Herth, E. etc. GeSnOI mid-infrared laser technology. Light Science Applications 10, 232 (2021). https://www.nature.com/articles/s41377-021-00675-7
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Bismay is a technical writer living in Bhubaneswar, India. His academic background is engineering, and he has extensive experience in content writing, journal review, and mechanical design. Bismay holds a master's degree in materials engineering and a bachelor's degree in mechanical engineering, and is passionate about science, technology and engineering. Outside of work, he likes online games and cooking.
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