Faculty Profile: Paul Thibado

The Surface Physics of Semiconductor Growth

Paul Thibado joined our department in August 1996. He received B.S. degrees in both physics and mathematics from San Diego State University in 1990, and a Ph.D. in physics from the University of Pennsylvania in 1994. He has co-authored more than 20 refereed journal articles and a chapter in a book on semiconductors, given six invited talks, and presented his work at more than 40 conferences. One of his research results was honored on the 1996 American Physical Society (APS) calendar.

Dr. Thibado describes his research as follows:

In 1982, researchers at the IBM Zurich Research Laboratory in Switzerland announced the development STM, which exploited quantum mechanical tunneling to resolve individual atoms at the surface of a material. This was a giant step in high magnification microscopy. Every university, most colleges, and many high schools now has at least one of these instruments. When I began graduate school at the University of Pennsylvania, I joined a surface physics group and had my first exposure to STM. By the time I left Penn, I had designed and built three STM systems, developed a deep appreciation for the tunneling process, and made a contribution to the understanding of how metal atoms grow on semiconducting surfaces.

After finishing my dissertation, I received a National Research Council Post-Doctoral Fellowship to work at the Naval Research Laboratory NRL. I had the benefit of being part of an effort that brought together four NRL divisions: Condensed matter, electronics, materials science, and chemistry. This type of research was unprecedented. The goal was to combine into a single facility two exceptionally powerful scientific instruments: STM and MBE. MBE is a form of vapor phase epitaxy (growth) which allows the artificial fabrication of semiconductor structures with deposition control at the level of a single atomic plane. The effort combines the highest precision in semiconductor growth with the highest precision in semiconductor characterization. No research effort prior to this had combined these two techniques to study the particular semiconductor system we were studying for the Navy. Consequently, we were uncertain of what we would uncover and what research issues we would address. After about six months of customizing, the first results were obtained. Thereafter, we uncovered more exciting projects than we could ever possibly pursue.

The year I began seeking a faculty position happened to be the same year that the University of Arkansas was starting a new program to combine a new MBE chamber and a new STM chamber into a single multi-chamber facility! At the time I interviewed, there were only a few people in the world who had attempted this type of research and even fewer who were successful. Furthermore, there was no other university offering a new MBE-STM facility to a new faculty member. Needless to say, the University of Arkansas and I were a great match! Our MBE-STM effort at Arkansas is unique in that it is the only such system which will grow semiconductors containing phosphide. The phosphide-based semiconductors are a member of the III-V compound semiconductors (since they contain elements from the III and V columns of the periodic table). I have extensive experience within the III-V semiconductors in general, but no experience in the III-P compounds. When Greg Salamo, University Professor of Physics, mentioned to me that his interest was in the III-P compounds, I conducted a literature search to determine what characteristics are unique to these compounds. Much to my surprise, the fastest transistors in the world are made from the III-P compounds. They are of primary interest to the military, but a commercial interest is developing in portable and high-speed communications such as direct broadcast satellite TV, cellular phones, and global positioning systems. In addition, InP-based materials are the only suitable system for optical fiber applications. These materials can emit and absorb laser light at the 1.3 and 1.55 m wavelengths, which correspond to the low-loss and low-dispersion regions of conventional optical fibers.

Historically, artificially fabricated semiconductor structures were introduced more than 20 years ago with some spectacular consequences, both in basic physics discoveries and in commercial applications. Quantum well lasers, for example, are now found in practically every compact disc (CD) player. The quantum well laser is a prime example of what has been termed "first generation quantum devices," a device which reproduces the function of a similar conventional bulk device, but with higher performance specifications.

My long-term goals are to develop "second generation quantum devices," which are still in the research stage. These structures are multi-functional. A single structure can accomplish a task which would normally require as many as ten conventional devices. A factor of ten reduction in the number of components naturally leads to a significant increase in speed, as well as a reduction in power consumption. For this class of devices, one designs semiconductor structures that take advantage of the wave nature of the electron to produce electrical or optical properties that could not be duplicated using conventional electronics (without a large number of components).

These novel quantum structures may have as few as 10 atomic layers of one type of semiconductor sandwiched between two other types of semiconductors. Consequently, these systems are very different from bulk grown equilibrium structures, which would randomly mix the different semiconductor layers together. These materials must be artificially fabricated with precise composition control of each atomic layer. Structural flaws, such as a missing atom or a wrong atom type, near the interface of two adjacent materials are thought to be the weak link in developing second generation devices. Identifying the origins of these structural or compositional defects can be quite difficult because the disorder may be driven by atomic-scale phenomena and typically occurs during the fabrication process itself. By combining MBE semiconductor growth with STM atomic-resolution characterization during the growth process, we can uncover the fundamental parameters governing growth dynamics, such as atom diffusion rates and potential energy barrier heights for diffusion. We now have an opportunity to identify origins of atomic-scale disorder and begin to seek solutions to these problems.

One of the most significant factors associated with the MBE-STM facility is the creation of a broad range of exciting scientific career opportunities for our physics graduates. Not only can our graduates study the fundamental atomic processes of semiconductor growth, but they can also obtain hands-on skills in semiconductor growth and processing that industry demands in today's competitive job market. In addition, the possibility of designing new semiconductor structures with market potential is now a reality, which may ultimately lead to the origination of high-technology businesses in the Fayetteville area--a goal strongly supported by the department, the university, and the state.*