PhD Thesis Defense: Alejandra Vanessa Cuervo Covian

For info on how to attend this videoconference, please email alejandra.v.cuervo.covian.TH@dartmouth.edu

"Phase Stability and Optical Properties of GeSn Nanostructures"

Abstract

The alloys of Sn and Ge have many different applications. Materials and nanomaterials utilizing these alloys have applications in areas that range from telecommunications to MWIR detection, lasing, photonic integration towards optoelectronic ICs, and quantum computing, just to mention a few. Moreover, Sn-rich Ge alloys have attracted much attention since they are a potentially new family of quantum materials, including topological insulators and Dirac semimetals. In this thesis, we discuss the phase stability and the material properties of GeSn nanostructures. Furthermore, we explore novel materials with high at % Sn by breaking the solubility limit of Sn on Ge of < 1% at. Sn, using various techniques.

First, we will present an overview of band engineered Ge and GeSn films crystallized at low temperature, highlighting the good quality of the films. The films present large grains, high contents of Sn (up to ~14%, effectively achieving a direct bandgap) and high thermal stability. We proceed to discuss a more novel approach of integrating more Sn into Ge via Nanodot Induced Composition Enhancement. Using this method, we demonstrate GeSn with 26 at % Sn and with photoluminescence at λ=3-4 µm at room temperature. We also investigate SnGe alloys (GeSn with a high content at %Sn) with Ge contents of 10% and 5% Ge. For the case of 5% nominal Ge concentration, we observe the first instance of a reversed phase transformation of beta to alpha Sn at high temperature. To have a better understanding of the material system, we highlight some of our efforts investigating the effects of local ordering to achieve a high concentration of Sn. To do this, we study the tendency of Sn to cluster or de-cluster, and we analyze the Raman spectra of GeSn thin films with different compositions. Finally, we present some PL studies done on other material systems that can potentially be hybrid-integrated with silicon photonics, including one for an AlGaAs Coupled Multiple Quantum Well and one for a GaSb core optical fiber. Throughout the whole presentation, material characterization techniques are demonstrated to correlate the microstructure of materials with their physical and optical properties.

Thesis Committee

• Jifeng Liu, PhD (Chair)
• Christopher G. Levey, PhD
• William J. Scheideler, PhD
• Tianshu Li, PhD

Contact

For more information, contact Daryl Laware at daryl.a.laware@dartmouth.edu.