Date of Graduation

12-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Microelectronics-Photonics (PhD)

Degree Level

Graduate

Department

Microelectronics-Photonics

Advisor

Shui-Qing Yu

Committee Member

Gregory Salamo

Second Committee Member

Hammed Naseem

Third Committee Member

Zhong Chen

Fourth Committee Member

Mathew Leftwich

Keywords

GeSn, Quantum Wells, Semiconductor Materials, Si Photonics, SiGeSn, Strain Engineering

Abstract

Silicon (Si)-based optoelectronics have gained traction due to its primed versatility at developing light-based technologies. Si, however, features indirect bandgap characteristics and suffers relegated optical properties compared to its III-V counterparts. III-Vs have also been hybridized to Si platforms but the resulting technologies are expensive and incompatible with standard complementary-metal-oxide-semiconductor processes. Germanium (Ge), on the other hand, have been engineered to behave like direct bandgap material through tensile strain interventions but are well short of attaining extensive wavelength coverage. To create a competitive material that evades these challenges, transitional amounts of Sn can be incorporated into Ge matrix to form direct bandgap GeSn alloys that have led to the increasing possibility of engineering a suite of low-cost, light emission sources that applies to a wide range of infrared photonics and optoelectronics systems. Hence, the importance of studying the structural and optical properties of these GeSn heterostructures cannot be overemphasized.

The first part of this dissertation investigates the structural and optical properties of SiGeSn/GeSn/SiGeSn quantum wells (QWs) where the photoluminescence (PL) behaviors of thick (22 nm in well) and thin (9 nm in well) GeSn QW samples are compared. Using PL results from two excitation lasers (532 nm and 1550 nm lasers) as well as studying their respective optical transitions, the result reveals that the thicker well sample shows i) a more direct bandgap outcome in addition to a much lower ground energy Г valley; ii) a higher carrier density within the well, and iii) an increased barrier height coupled with improved carrier confinement. All of these resulted in a significantly enhanced emission that allows for the first-ever estimation of GeSn QWs quantum efficiency (QE) while also suggesting a path towards efficient mid-infrared devices.

To further improve the carrier confinement while also reducing the carrier leakage in the thicker well design, a SiGeSn/GeSn/GeSn/SiGeSn separate confinement heterostructure (SCH) is introduced. The sample is characterized and the optical properties are compared with the previously reported 9 nm and 22 nm well non-SCH samples. Based on the optical transition analysis, the SCH QW also shows significantly higher carrier confinement compared to reference samples. In addition to these studies, an attempt is made to investigate advanced quantum well structures through an all-inclusive structural and optical study of SiGeSn/GeSn/SiGeSn multi-quantum wells (MQWs). The resulting analysis shows evidence of intermixing diffusion during growth.

The second part of this work provides insights into the behavior of annealed GeSn bulk samples near the indirect-to-direct transition point. The study attempts to provide connections between the strain, composition, and defect densities before and after annealing. The result reveals the impact of annealing on a sample may either i) lower the strain giving rise to an increased PL while reducing the energy separation or ii) introduce misfit dislocation/ surface roughness leading to an affected or decreased PL.

Finally, this work also explores the low-temperature capability of our in-house plasma-enhanced ultra-high vacuum chemical vapor deposition system through the growth of Si-on-Ge epitaxy and pressure-dependent growth of GeSn bulk heterostructures.

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