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Selective area growth of GeSn for infrared photonic devices

Published on 23 April 2024
The capability to integrate active and passive optoelectronic devices on a Si wafer is an essential paradigm in the quest for the development of low-cost and energy-efficient data communication, imaging, and quantum sensing technologies. To this end, a monolithic all-group IV semiconductor platform is at reach using direct band gap GeSn semiconductors.1 Over the last decade, tremendous progress was made in the epitaxial growth of GeSn, where a direct band gap material (i.e. high efficiency for the optical emission) is obtained at Sn contents of >9 at.%. Prototypes of GeSn photodetectors, lasers, and LEDs have been fabricated from the short-wave infrared (SWIR: 1.5-3 μm) to mid-wave infrared (MWIR: 3-8 μm) wavelengths.2,3 The main bottleneck with the GeSn technology is, however, the large number of structural defects that severely reduces the efficiency of the photonic devices made using GeSn. This prevents a wide scale adoption of GeSn photonics in favor of conventional, yet expensive III-V and II-VI semiconductor technologies.

The growth of GeSn is commonly performed on Si using Ge as an interlayer in a chemical vapor deposition (CVD) reactor, hence with an industrial- compatible fabrication process. However, the lattice-mismatch between GeSn and the Ge/Si substrate leads to compressive strain in GeSn and plastic strain relaxation results in structural defects. Defects are a source of nonradiative recombination and largely ​contribute to the dark current of GeSn photonic devices, in turn strongly reducing efficiency.
This thesis will overcome these challenges and develop the selective area growth (SAG) of defect-free GeSn p-i-n diodes from nanometer-size openings that are patterned into an oxide mask layer on Si. The SAG has proven to be a highly valuable approach for the integration of defect-free III-V semiconductors on Si,4 with similar results that are now being explored in Ge.5 The GeSn growth will be selectively confined in very small regions of the patterned oxide/Si wafer. In SAG reducing the lateral dimensions of the patterned oxide windows will strongly decrease the defect density in the epitaxially-grown GeSn layer through dislocation filtering. The unmatched crystalline quality of SAG GeSn will boost the efficiency of infrared optoelectronic devices and thus establish a robust, scalable monolithic infrared photonics platform using group IV semiconductor materials. Photodetector devices made of SAG GeSn p-i-n diodes will be fabricated as a template system to demonstrate the effectiveness of SAG compared to the existing GeSn technology based on unpatterned GeSn samples.2
Competences to be acquired
(i) Epitaxial growth of Sn-based group IV semiconductors using CVD and molecular beam epitaxy (MBE) tools.
(ii) Structural characterization of the SAG materials down to the atomic-level.
(iii) Fabrication of infrared photonic devices in a cleanroom facility.
(iv) Optoelectronic characterization of materials and devices.

Required skills
Background in solid-state physics and materials science, interest in performing experiments in the lab, working in a collaborative team, and contributing to international collaborations. ​

Starting date
By the 1st October 2024.

PhD funding
Thesis funded by Initiatives de Recherche à Grenoble Alpes (IRGA).

Dr. Simone Assali (CEA-IRIG/PHELIQS, Grenoble).

Further reading
1. Monolithic infrared silicon photonics: The rise of (Si)GeSn semiconductors.
2. High-Bandwidth Extended-SWIR GeSn Photodetectors on Silicon.
3. Electrically injected GeSn​ lasers with peak wavelength up to 2.7 μm.
4. GaAs nano-ridge laser diodes fully fabricated in a 300 mm CMOS pilot line.
​5. Coherent Hole Transport in Selective Area Grown Ge Nanowire Networks.

To apply for this PhD position, send your application (including CV) by e-mail to: Simone ASSALI.