PhD Thesis Defense: Shangda Li

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"Monolithic Integration of Group-IV Quantum and Photonic Materials on Si via Defect and Phase Engineering"

Abstract

The monolithic integration of tunable, CMOS-compatible group-IV materials, predominantly GeSn alloys, onto standard Si platforms is poised to revolutionize broadband infrared photonics and quantum device architectures. However, translating these theoretical advantages into functional devices is fundamentally bottlenecked by severe epitaxial lattice mismatches with Si, strict thermodynamic limitations, and the proliferation of non-radiative defects during fabrication.

This thesis advances the functional integration of group-IV materials by establishing an interconnected framework of phase, structural, and defect engineering. While the study initially explores the topological phases of Sn by demonstrating a scalable, seed-layer-driven integration of phase-pure α-Sn microdots for robust quantum applications, the primary research thrust focuses on overcoming the barriers to semiconducting GeSn alloy integration.

To resolve the foundational issue of heteroepitaxial strain, a novel mismatch alleviation strategy utilizing self-assembled SnO2 nanotemplates is introduced. By accommodating the lattice mismatch elastically, this architecture fundamentally alters microstructural evolution, replacing deleterious threading dislocations with benign, coherent planar defects to yield a 30-fold enhancement in room-temperature direct-gap photoluminescence.

Building upon this high-quality epitaxy, the critical downstream challenges of ion implantation damage recovery in ex situ doped GeSn layers are addressed. By mapping recrystallization kinetics and establishing a fundamental model for Sn surface segregation pathways, the study demonstrates that solid-phase continuous-wave laser annealing circumvents strict thermal constraints, achieving full lattice recovery without inducing Sn segregation. Finally, a highly scalable aqueous passivation methodology is established to neutralize residual surface and internal defects via atomic hydrogen incorporation, minimizing non-radiative recombination to further enable highly efficient group-IV optoelectronics.

Thesis Committee

• Jifeng Liu (chair), Eric R. Fossum
• Will Scheideler
• Ezra Bussmann (Sandia National Laboratories)

Contact

For more information, contact Thayer Registrar at thayer.registrar@dartmouth.edu.