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

Hameed Naseem

Second Committee Member

Gregory Salamo

Third Committee Member

Zhong Chen

Fourth Committee Member

Matthew Leftwich

Keywords

GeSn, group-iv, laser, optoelectronics, SiGeSn

Abstract

Si photonics is a fast-developing technology that impacts many applications such as data centers, 5G, Lidar, and biological/chemical sensing. One of the merits of Si photonics is to integrate electronic and photonic components on a single chip to form a complex functional system that features compact, low-cost, high-performance, and reliability. Among all building blocks, the monolithic integration of lasers on Si encountered substantial challenges. Si and Ge, conventional epitaxial material on Si, are incompetent for light emission due to the indirect bandgap. The current solution compromises the hybrid integration of III-V lasers, which requires growing on separate smaller size substrates and bonded on Si wafers. The monolithic growth of III-V lasers suffers from high-density defects and the growth temperature incompatible with the complementary-metal-on-semiconductor (CMOS) process. Therefore, alternative solutions are of high interest to overcome such difficulties. SiGeSn is a Group-IV semiconductor that could achieve direct bandgap, monolithically grown on Si substrate, and CMOS process compatible. These advantages make SiGeSn rather promising towards the monolithic laser for Si photonics.

This dissertation presents the multiple efforts on developing the GeSn-based lasers towards the electrical injection. The development process starts with the bulk lasers by optically pumping. By incorporating Sn in the active region and leaving the threading dislocation away from the active region, the maximum operating temperature (Tmax) of the broad ridge laser reached 270 K with 20% Sn in the GeSn active region. The lasers with the multiple-quantum-well (MQW) as the gain region were studied for reducing the threshold. The results implied a sufficient gain volume was required to overcome the existing loss within the device. The laser structure with four wells exhibited lasing at temperatures up to 90 K. The introduction of the SiGeSn cap layer balance more optical field overlapping the MQW active region, leading to an increase of Tmax. By adding the quantum well number, the lasers showed improvement in the modal gain, eventually reducing the threshold and elevating the Tmax. The study of light-emitting diodes provides the insight of GeSn heterostructures before achieving the electrically injected GeSn lasers. The three developing structures including Ge/GeSn/Ge, GeSn homojunction, and GeSn/GeSn/SiGeSn heterostructures were designed for: (1) achieving direct bandgap in GeSn active region, (2) incorporating high Sn composition and maintaining strain relaxation in the active region, and (3) eliminating carrier leakage through the hole barriers. With the advances in the GeSn heterostructures, the layer structures were applied to the electrically injected lasers. The electrically injected lasing from GeSn was demonstrated at temperatures up to 100 K. The laser diode structures were further investigated by comparing the layer material and thickness, providing further suggestions on optimizing the laser design.

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