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PhD Thesis Defense: Shang Liu



10:00am - 12:00pm ET

Rm 232, Cummings Hall (Jackson Conf Rm)/Online

Optional ZOOM LINK

"Germanium-tin on silicon for integrated photonics and integrated quantum materials"


Group IV GeSn alloys are attracting attention due to their compatibility with the complementary metal-oxide-semiconductor (CMOS) process. On one hand, Ge-rich GeSn alloys with a tunable direct bandgap are well-suited to infrared (IR) photonic applications such as image sensors. On the other hand, Sn-rich GeSn alloys in diamond cubic α phase are topological quantum materials (TQM) holding potential for important quantum applications. However, directly growing GeSn on Si remains challenging due to the lattice mismatch. Regular epitaxial GeSn grown on a Ge buffer layer is not applicable to many photonic applications including CMOS image sensors (CIS) because the buffer layer prevents the direct transfer of photoelectrons from GeSn to Si storage wells. Meanwhile, recent theories predicted that short-range order (SRO) exists in GeSn and can impact its band structure, but characterizing SRO is difficult due to the requirement of atomic resolution. Therefore, to tackle these challenges, this thesis introduces innovative approaches for the growth, characterization, and integration of GeSn on Si, culminating in the prototype of the first GeSn/Si IR CIS.

We first demonstrate the direct growth of crystallized Ge-rich GeSn on Si by physical vapor deposition (PVD) and construct a meta-stable phase diagram for GeSn thin films to guide the design and fabrication of GeSn devices. Two novel methods based on a Ge seed layer or Ge doping are then presented to grow Sn-rich GeSn on Si, for integrated TQM. Next, a new algorithm based on Poisson statistics is developed to achieve three-dimensional nano-scale mapping of SRO in GeSn using atom probe tomography (APT). Surface termination and precursors are investigated to control SRO towards bandgap engineering. At last, photoresponse from a 32x32 GeSn/Si CIS pixel array at 1310-1854 nm wavelength is demonstrated under thermoelectric cooling at -60 or -65 oC for the first time using back-end-of-line (BEOL) processing, paving the path towards further optimization. Overall, these advancements in GeSn on Si bring exciting opportunities for integrated photonics and integrated quantum materials.

Thesis Committee

  • Prof. Jifeng Liu (chair)
  • Prof. Eric Fossum
  • Prof. Hui Fang
  • Prof. Tianshu Li (from George Washington University)


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