Design and modeling of group IV (Si)GeSn lasers

Marzban, Bahareh; Witzens, Jeremy (Thesis advisor); Vescan, Andrei (Thesis advisor)

Aachen : RWTH Aachen University (2021, 2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

Motivation, Goal and Task of the DissertationData communications, mid-infrared (IR) optical sensing and imaging technologies, to name a few, benefit immensely from scalable, silicon (Si) compatible, cost-effective optoelectronic devices. In view of this, demonstrating a monolithically integrated light source on Si remains the most pressing issue yet to beachieved. Towards its realization, the ternary (Si)GeSn material system is one of the leading contenders and has been the subject of extensive investigations in the last decade. This material system provides two degrees of freedom in engineering the band structure, namely strain and composition. Utilizing these, lasing from direct bandgap GeSn has not only been demonstrated at cryogenic temperatures but also at room temperature. Quantum engineered heterostructures, and device quality epilayers were also realized. Although advances have been made, this family of semiconductors still faces serious obstacles. These include the growth of high-quality crystalline layers with the desired strain and compositions and control over the defect density. Moreover, given the relatively short history of this alloy family, device design and modeling is very challenging due to the high uncertainty associated to its material properties. In this context, the goal of this thesis has been to model existingexperimental device results, with a thorough comparison to experiments allowing the validation of modeling assumptions and methodologies, as well as to conceive and model new (Si)GeSn laser device concepts. Major Scientific ContributionsWe present comprehensive descriptive modeling of an optically pumped (Si)GeSn multiple quantum well (MQW) under-etched microdisk laser. To this end, we initially evaluate the strain relaxation via mechanical modeling of the under-etch. This information is used in combination with k.p theory to calculate the band structure. We then adapt carrier transport, optical gain and loss models to this material system, using accumulated experimental data gathered in the last few years together with the calculated band structure. Subsequently, we utilize experimentally observed lasing thresholds and the maximum lasing temperature to verify the modeling assumptions. The benchmarked model is further used to better understand the limitations and the performance of the studied laser. Using gained insights into the material platform and its lasing properties, we introduce a novel design concept of a waveguide coupled microdisk laser. In the future, the verified assumptions and presented modeling flow can be employed for further predictive modeling of electrically pumped (Si)GeSn lasers.

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