Screens. We see them everywhere nowadays: on phones, laptops, televisions, medical devices, and other equipment. Less obvious are the people working behind the scenes to improve them. Kathryn Roach, an MS degree candidate, is one of them. She is part of Michael O. Thompson’s Materials Science and Engineering research team.
Most screens are made of numerous tiny dots or pixels, each emitting its own color. These pixels are powered by thin film transistor technology, and they are typically made using silicon.
New Materials for Better Screens and More Stable Devices
Indium Gallium Zinc Oxide transistors (IGZO) have started to appear in very recent products. Transcending the typical tradeoff between performance and battery life, materials such as IGZO improve both at the same time. “It has a higher charge carrier mobility than amorphous silicon,” Roach says. This fact, along with the low off current, means that less energy goes towards powering the display. At the same time, IGZO can unlock higher pixel density, giving higher display resolutions.
The majority of research on IGZO has been done on its amorphous phase, which Roach is studying. However, Roach is also working on the c-axis aligned crystalline phase, or CAAC phase, which has received less attention from current studies. “We don’t know a lot about it yet, but we’ve been studying its structure and growth, and found that it produces more stable devices,” says Roach.
Roach is studying how different processing conditions affect the properties of IGZO transistors. Annealing is an example of an important step in the manufacture of transistors, where a material such as IGZO is heated in a very controlled and specific manner to modify its properties. With the CAAC phase, one of Roach’s analyses is on how varying the annealing conditions such as temperature and ambient environment affect the electronic properties of IGZO. This includes determining how species such as oxygen vacancies diffuse through a material under different annealing conditions. Doing so helps model the resulting charge carrier mobility and carrier concentration, allowing for finer tuning of those properties.
Roach is also studying the etch rate of IGZO. Etching is the process of chemically removing material in a specific pattern, another important step in developing semiconductor devices. However, previous methods of measuring this etch rate have yielded inconsistent results, she says. Thus, with new materials, new methods of analysis are explored. “One of the most exciting things is that we have started using quartz crystal monitor measurements or QCM,” says Roach. This QCM method promises more reliable measurements of in-situ etch rates of IGZO, even after annealing.
To see how important QCM is, we must first understand that etching and annealing are not entirely separate processes; the results of the annealing process affects the etch rate of the material. Annealing is often done in an oxygen-deficient environment. This oxygen deficiency gives rise to oxygen vacancies that act as charge carriers in the target material. Interestingly, the oxygen vacancy concentration is not uniform throughout the material. Rather, it varies, starting from the surface of the material in which the oxygen vacancies first diffuse.
Indium Gallium Zinc Oxide transistors (IGZO) have started to appear in very recent products. Transcending the typical tradeoff between performance and battery life, materials such as IGZO improve both at the same time.
Roach and her team think that this may explain the difference in etch rates; however, they are still considering other factors. This is important to understand because if the properties of a thin-film transistor measured on the surface are different from its actual bulk properties, the device under manufactured will be significantly different in performance. In this context, the QCM method is especially promising because it is capable of measuring properties throughout the material.
From Undergrad to Grad—How a Research Interest Became a Passion
Roach’s work in research began when she was still an undergraduate at Cornell. “Originally I was introduced to the Thompson group by my friend, who is also studying Materials Science and Engineering.” Roach has stayed ever since and has learned to deal with the unexpected nature of research. “Research can destroy expectations. You think it’s going to go one way, and then you get this whole new result which leads you to a whole new research path. Even what I’m doing now is not what I was originally researching, but we kept getting weird results and we decided to take a deeper look.”
Cornell has certainly helped her progress. “I absolutely love research at Cornell. We have access to so many amazing resources. I wouldn’t be able to do my research without for example the Cornell NanoScale Science and Technology Facility,” she says.
Beyond facilities, she also enjoys the atmosphere prevalent throughout Cornell’s research scene. “I find the people very friendly and collaborative. For the QCM method, I have to use different labs and work with different graduate students, and they have been very helpful and friendly about it. The professors are also always willing to help with questions.” With knowledge permeating rapidly through the university, it is perhaps no surprise that Roach decided to carry on her work here at Cornell.