Free Charge Carrier Properties in Two-Dimensional Materials and Monoclinic Oxides Studied by Optical Hall Effect

Authors

Sean Knight, University of Nebraska - LincolnFollow

First Advisor

Mathias Schubert

Date of this Version

8-2020

Document Type

Thesis

Citation

Knight, Sean "Free Charge Carrier Properties in Two-Dimensional Materials and Monoclinic Oxides Studied by Optical Hall Effect", PhD Dissertation, University of Nebraska-Lincoln (2020)

Comments

A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Electrical Engineering, Under the Supervision of Professor Mathias Schubert. Lincoln, Nebraska: August, 2020

Copyright 2020 Sean Knight

Abstract

In this dissertation, optical Hall effect (OHE) measurements are used to determine the free charge carrier properties of important two-dimensional materials and monoclinic oxides. Two-dimensional material systems have proven useful in high-frequency electronic devices due to their unique properties, such as high mobility, which arise from their two-dimensional nature. Monoclinic oxides exhibit many desirable characteristics, for example low-crystal symmetry which could lead to anisotropic carrier properties. Here, single-crystal monoclinic gallium oxide, an AlInN/GaN-based high-electron-mobility transistor (HEMT) structure, and epitaxial graphene are studied as examples. To characterize these material systems, the OHE measurement technique is employed. The OHE is a physical phenomenon where a change in a conducting sample’s optical response is observed when immersed in an external magnetic field (i.e. the optical analogue of the electrical Hall effect). To quantify this change in a sample’s optical response, generalized ellipsometry was employed for our OHE measurements. All necessary data is collected and analyzed with appropriate optical models, providing the free charge carrier properties of interest. To obtain the free charge carrier properties of the material systems studied here, OHE measurements were performed in two different spectral ranges: in the mid-infrared range for monoclinic gallium oxide, and in the terahertz (THz) range for the HEMT structure and epitaxial graphene. Measurements in the THz spectral range are made possible by exploiting Fabry-Pérot interferences inside THz-transparent substrates, as well as within an additional cavity external to the samples. Results for carrier concentration and mobility determined by OHE are in excellent agreement with previous electrical Hall effect characterizations. Results for effective mass are also in agreement with previous density functional theory calculations. Characterizations of thorium dioxide and uranium dioxide, as well as cupric oxide are also included. Since no free charge carriers were detected in these samples, these reports serve as introductions to infrared ellipsometry, THz ellipsometry, and the Fabry-Pérot enhancement techniques used here.

Advisor: Mathias Schubert