矽鍺錫量子井雷射及電致吸收光調變器

Abstract

光電積體電路可以提供高速，低損耗，晶片型光子網路，可應用於通信系統以及晶片間的訊號傳輸。高性能的矽基雷射及光調變器是其中不可或缺的元件: 雷射用以產生同調性光源，而光調變器用來高速調變信號。然而，因為矽是間接能隙材料，所以矽的發光效率非常低。再者，矽的線性電光效應以及電致吸收效應也很低，所以很難利用矽來達成高速調變。所以在實現高性能矽基雷射及光調變器之前，這種高速光電積體電路還無法製作出來。本文的主旨即在利用新穎的矽鍺錫材料系統來實現矽基雷射及光調變器。

矽鍺錫材料矽統是一種用來發展矽光技術的新平台。隨著近年來低溫成長磊晶技術的進步，高品質的矽鍺錫合金已經可以合成了。本文首先研究矽鍺錫材料系統的電子特性，光電特性，以及機械特性來探索這種新材料在光電領域的新應用。

在四族材料之中，鍺是一種可來製作矽基雷射及光調變器的高潛力材料。鍺是一種準直接能隙半導體材料，意味著將鍺變成直接能隙材料的可能性。本文提出二個利用矽鍺錫材料系統來達成直接能隙材料的方法來增強其發光效率，進而發展矽基雷射:(1)具有張應變及n型摻雜的鍺/矽鍺錫量子井，發光波長在1550奈米附近，以及(2)應力平衡的鍺錫/矽鍺錫量子井，發光波長在中紅外線範圍。我們發展了一套完整理論來分析此二種雷射結構，包括電子能帶結構，載子分布，與極化有關的光增益，與極化有關的光侷限因子，以及雷射閥值分析。分析的結果顯示我們所設計的雷射結構可以達成雷射輸出。相對於矽的弱線性電光效應以及光電致吸收效應，鍺具有強健的弗朗茲-凱爾迪什效應以及量子史塔克效應，可用來製作電致吸收光調變器。本文提出及分析二種結構來製作可操作於1550奈米波長的電致吸收量子井光調變器:張應變鍺/矽鍺錫量子井以及無應力鍺錫/矽鍺錫量子井，分析結果顯示此二種結構的調變波長落在常用的1550奈米通訊波長。這些結構對於實現光電積體電路有很大的幫助。

Compact electronic-photonic integrated circuits are one of the promising technology to enable on-chip, low-power, high-speed optical networks for telecommunications and inter/intra-chip interconnections. High-performance Si-based lasers and modulators are critical to achieve the goal: the former generate coherent light while the later optically encode light signals at the optical transmitter end. However, the light-emitting efficiency in Si is very low due to its indirect bandgap in nature, and fast modulation in Si is challenging because of the lack of efficient linear electro-optical and electroabsorption effects. Thus, high-performance electronic-photonic integrated circuits are not possible until the realization of electrically-pumped Si-based lasers and highspeed modulators. This dissertation focuses on the research of using novel SiGeSn material system for efficient Si-based electrical-pumped lasers and electroabsorption modulators.

SiGeSn material system represents a new promising platform to develop Si photonics. With the significant advances in the growth of SiGeSn material system by low-temperature UHV-CVD, high-quality GeSn, SiSn, and SiGeSn alloys are able to be grown. To explore possible electronic and photonic devices based on the novel SiGeSn material system, we first study the fundamental properties of Si, Ge, and α-Sn as well as their compounds, in cluding the electronic, optical, and mechanical properties. Among the group-IV semiconductors, Ge is a potential material for efficient electrically-pumped Si-based lasers and high-speed modulators. Ge is a quasi direct-bandgap semiconductor, suggesting the possibility of transforming it into a direct bandgap semiconductor for photonic active applications. We propose two approaches to crate direct-bandgap semiconductors based on the SiGeSn material system for developing Si-based lasers: tensile-strained, n-type doped Ge/SiGeSn quantum wells, and strain-balanced GeSn/SiGeSn multiple-quantum-well. We develop a theoretical model for laser analysis, including the strained electronic band structure, carrier occupation, polarization-dependent optical gain, polarization-dependent optical confinement factor, and threshold analysis. Our calculations indicate lasing action in the two designed lasers is possible.

While Si has no linear electro-optical and very weak electroabsorption effects for efficient, high-speed modulators, Ge is suitable for Si-based modulators because it possesses significant Franz-Keldysh and quantum-confined Stark effects. We propose and analyze two structures for electroabsorption waveguide modulators operating at 1550 nm wavelength based on the quantumconfined Stark effect: tensile-strain Ge/SiGeSn quantum wells and strainfree GeSn/SiGeSn quantum wells. A theoretical model for describing the quantum-confined Stark effect is present. With adequate design of the quantumwell materials and waveguide geometry, it is possible to achieve effective modulation at 1550 nm wavelength using the two designed electroabsorption modulators for electronic-photonic integrated circuits.