Xiuling Li


Xiuling Li
Xiuling Li

Q: What’s your area of expertise?
A: Nanostructured semiconductor materials and devices.

Q: Give a quick synopsis of your career and education.
A: I received my bachelor’s degree from Peking University in China, and my PhD degree from UCLA. Then I had a postdoc position at Cal Tech first, followed by another one at Illinois. Then I went to a local start-up company called Epiworks and worked there for six years. About four and a half years ago I came back and started as an assistant professor at ECE.

Q: So you’ve been a researcher at this university since 1998, and a faculty member since 2007. What do you enjoy most about being here?
A: If I have to name one, I would say the environment. You don’t have to walk very far to find the ideal collaborators. In terms of the willingness to collaborate and the level of expertise, it is unmatched around here, and that makes interdisciplinary work really productive.

Q: When you studied at Peking University and UCLA, you majored in chemistry. Why did you decide to become an engineer?
A: I wanted to do some more applied research, and I was fortunate to have had the opportunity to do so. The knowledge base is different from field to field, but the critical thinking process and problem solving skills are not that different.

Q: How did you get interested in semiconductors and nanostructures?
A: I have always been fascinated by semiconductor. The ability to control the conductivity of semiconductor by adding dopants is a very useful property that insulators and metals do not have. Silicon is an elemental semiconductor, and the workhorse of the microelectronics industry, and now solar cells.

My research involves both Si and III-V compound semiconductors such as GaAs, InP, and GaN.  The beauty of compound semiconductor is that you can tune the bandgap easily and have heterojunctions. You can have materials of different compositions and put them right next to each other, and you change the band structure. It’s more complex than elemental semiconductor, and that’s where the fun is.
In nanotechnology, the goal is to make things smaller, faster, better, and cheaper. When you make things small, several things happen. First, you can pack them denser. That’s where you get speed. And for the same speed, you don’t have to pay more. So that’s the cheaper side. On the other hand, not only is the geometrical factor becoming smaller, the physics changes as well. When you go to a size that is small enough, quantum mechanical principles come in. And when you have new physics, you can have new functionalities and better performance, as well as new challenges, and that’s why nanotechnology is so fascinating.

Q: What’s a recent research accomplishment you’re proud of?
A:  Since our research is about innovations on materials and structure that lead to new and better devices, the first accomplishment I would say is the planar nanowire growth method we developed.  Normally, when you grow nanowires on a substrate, they grow either in the surface-normal direction or at an angle from the surface, which is not compatible with existing planar processing technology for microelectronics. Our innovation is that we grow laterally. We can have the wire propagate in-plane in a self-aligned fashion along a single crystal orientation.

We call this selective lateral epitaxy. The selectivity comes from the metal catalyst. We position the catalysts where we want nanowires to grow. Epitaxy—this is a concept in crystal growth. It has to extend the substrate crystal structure. Because the epitaxy occurs along the surface laterally, we can change doping type and composition as the wire grows. So we can form axial p-n junctions and heterojunctions in plane and in situ.

Using this innovative growth method, my students have successfully demonstrated all kinds of transistors including MESFET, MOSFET, and HEMT with the planar nanowire as channels, with very good performance and uniformity. Other accomplishments I am proud of include the strain-induced self-rolled up tube based nanophotonics work which offers a curved platform for new physics and new devices, as well as a unique semiconductor etching method we have been developing for high aspect ratio nanostructures.

Q: What do you enjoy most about teaching?
A: Students, the interaction with students. I have been teaching a senior elective, ECE 444: IC Device Theory and Fabrication. In this class, students not only learn about fabrication theory in lecture, they also have to go in the lab, take a plain silicon wafer, and make it all the way into integrated circuits. Because this is a senior elective, these seniors are out there looking for jobs. Some of them come back with job offers and tell me things like, “I went in for an interview, and the interviewer was impressed I actually know so much about the fab.” I get great satisfaction knowing that I am making a difference.

Q: You were a research associate here and then you worked at EpiWorks for six years. What made you come back and take a faculty position?
A: Well, I guess it’s about passion and impact. When I was talking to friends about this opportunity for a faculty position at lunch one day, my friend said, “You’ve got to take the job.” I asked why, and he said, “Your eyes sparkle when you talk about these things.” I needed the academic freedom an institution such as Illinois offers to explore longer term research ideas. I was excited, and still am, about the potential impact my research and teaching could have on the advancement of the whole society.

Having said that, I’m glad I had the opportunity to work at EpiWorks. In production, you think about issues that university research hardly ever considers: uniformity and repeatability. But because of my industry background, when I do research on nanostructures, I tend to consider approaches that are potentially compatible and manufacturable.

Q: What kind of roles do students play in your research?
A: When you buy a house, they say, “Location, location, and location.” When you want to run a research group, it’s “students, students, and students.”

They are the ones who make things happen, at every level.

Q: Over the years, you’ve received a number of awards for your work. What recognition is most meaningful to you?
A: I received the NSF CAREER award the first year I got here. That was an important boost to get my research program started. The DARPA Young Faculty Award and recently the ONR Young Faculty Award are all meaningful to me. All of these are very competitive; and I’m fortunate to get these recognitions along with the research funding that comes with these awards; and of course the deliverables.

Q: What does the future hold?
A: The future of nanotechnology is definitely bright if we can address the manufacturability challenge. The scientific and engineering approaches we explore certainly use manufacturability as one of the guiding principles. The foundation of our research is material and structure innovation and our current applications include nanoelectronics, sensing, solar cells, thermoelectrics etc. I believe as long as our research is driven by the needs of our era, it will have the staying power.

Q: What technology are you working on that you are most excited about completing?
A: I can name one – MacEtch. It stands for metal assisted chemical etching. Normally there are two types of etching: one is wet etching, one is dry etching. Wet etching etches as deep as it is wide, that is isotropic. Dry etch can go straight down—anisotropic—but could damage your surface.

MacEtch is wet etching, but it’s directional. So it goes against the conventional concept that we teach in textbooks. The trick here is the metal catalyst. With this technology we can make high aspect ratio semiconductor structures that could not be achieved previously, in a simple, efficient, and noninvasive manner. This is a technology that is pretty well developed thanks to joint efforts from our collaborators. I could envision this technology be applied soon to more efficient solar cells, thermoelectric modules, among other applications.