Matthew Hilfiker

Ph.D. candidate, Electrical and Computer Engineering

Title: Critical Point Spectroscopic Ellipsometry Analysis of Ultrawide Bandgap Materials
Advisors: Dr. Mathias M. Schubert

I received my bachelors from UNL in electrical engineering. I knew I desired a PhD because I wanted a degree that reflected my aspirations to perform state-of-the-art research. Originally, I wasn’t sure where I wanted to get my PhD from until I met Prof. Mathias Schubert. His lab focuses on spectroscopic ellipsometry, an optical characterization method with wide ranging applications from chip-manufacturing to biology. Ellipsometry can be a difficult technique to apply as it requires a model-based approach to unravel specific material properties. I found this has led many other researchers to not fully understand the capability of ellipsometry, where significant job opportunities existed. The University of Nebraska has a deep history of ellipsometry and I was attracted to the possibility of working for one of the top ellipsometry labs, which could open many future careers in places such as Silicon Valley tech companies or working in the defense industry.

My work focused on ultrawide bandgap semiconductors for next-generation high-power device electronics. Current materials are unable to handle the power demands of future high-power electronic machines including vehicles, airplanes, or submarines. Specifically, I concentrated on gallium oxide, a promising material due to its rapid crystal growth expansion, high mobility, and low defect density. Furthermore, aluminum gallium oxide alloys can be grown to engineer the bandgap across a wide spectral range for the desired application. Before I started my PhD work, the bandgap of such materials, a highly critical parameter for device design, largely remained theoretical and highly uncertain. Difficulty arises trying to determine such information as these materials are commonly of lower-symmetry and therefore there exists an optical anisotropy. Spectroscopic ellipsometry is a perfect technique to handle these challenges as it is both polarization-based and highly-sensitive. Additionally, unlike onset of absorption measurements, spectroscopic ellipsometry has the ability to determine electronic transition energies, excitonic contributions, and the full dielectric function. I had the pleasure of directly collaborating with many other research groups who could grow high-quality crystals for us to analyze including Prof. Debdeep Jena at Cornell University, Prof. James Speck at University of California Santa Barbara, and Dr. Zbigniew Galazka from the Leibniz Institute for Crystal Growth in Berlin, Germany. I loved that my PhD program allowed me to develop these connections and work closely with some of the best crystal growers in the world. Through this research I was able to successfully be the first to use spectroscopic ellipsometry for determination of the bandgap of such ultrawide bandgap materials.

During my PhD I was fortunate to be included on 15 publications where I gained experience working on many different applications of ellipsometry. The department awarded me the Milton E. Mohr fellowship, which I was truly honored to be recognized by them. I had the benefit of being able to travel to multiple conferences to present our work, including the international spectroscopic ellipsometry conference in Barcelona, Spain. It was an incredible experience to present and learn from some of the best researchers in my field and see the impact of our research on others. My research also led me to spending a semester working in Dresden, Germany with our collaborators. For someone born and raised in Lincoln Nebraska I had never imagined having the capability to live in Europe during my PhD, but the experience gave me a great perspective on a different work environment and culture. One of the most impactful parts of my PhD was my teaching experience. The electrical engineering department provided the opportunity for me to teach a lab class for my first couple semesters. Since I got my bachelors in this department from UNL, I remember being on the opposite side as a student in these same classes. It had a great impact on me to develop meaningful relationships with my students and to help mentor them in their first years of the program. Additionally, our lab commonly employed many undergraduates that I had the pleasure of mentoring and further developing my teaching skills.

Finally, the most important thing I took away from my PhD is the relationships I was able to form with the other graduate students. When I joined the PhD program, I didn’t know many other graduate students. However, that dramatically shifted as our lab fostered a team-first approach and we collaborated with many other research labs. I will never take for granted the Nebraska nice spirit my colleagues showed me during my education.