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Beyond the Bologna and Cheese Metaphor

A visit with Meera Chandrasekhar, Professor of Physics

By LuAnne Roth
Published: - Topics: physics semiconductors heterostructures design quantum confined heterostructures
  1. Research Initiatives

Meera Chandrasekhar, Professor of Physics at MU, describes herself as “a condensed matter experimentalist,” that is, a physicist who studies a class of materials called condensed matter systems (formerly known as “solids”). Within this class are three types of materials: insulators (Styrofoam, plastic, and rubber), which do not allow electricity to flow; conductors (metals), which do allow electricity to flow; and semiconductors, which “have conductivities in between that of insulators and conductors.” Chandrasekhar has spent most of her research career seeking to understand the special properties of this “in between” class of materials, and she speaks lovingly about how these semiconductors are unusual by virtue of their limited electrical conductivity and their particular response to light. While conductors mostly reflect light and nonconductors allow light to pass right through, “semiconductors have a very special property because of the energy levels in the solids” and because of the way in which they absorb light. From deep red all the way around the color wheel to ultraviolet, “depending upon the material, they will absorb the light,” she explains.

“If you pick a certain semiconductor (let’s say cadmium sulfide), it has what is called a ‘bandgap,’ a specific property where light that is red just goes right through it, but light that is yellow or green gets absorbed. So it’s got a cut-off color, below which all the light goes right through, and above which everything gets absorbed. That comes from the way the energy bands of the material are configured.” Not all materials have this property, which makes semiconductors rather special.

On one level, because of this specific property semiconductors can be fashioned for such devices as light-emitting diodes, the bright red indicator lights that have become ubiquitous on everything from electrical equipment to flashlights. On another level, it turns out that “you can put very small amounts of impurities in these materials (parts per million or even per billion), in a very controlled fashion,” in order to change their conductivity. This renders the semiconductors very useful for electronic devices. Some of Chandrasekhar’s earliest work in the 1970s focused on silicon as a semiconductor. Today, of course, “the whole computer and electronic industry depends on silicon – the parent material out of which every integrated [computer] chip is made.”

Figuring out how to make semiconductors and control what was in them resulted in the explosion of the transistor industry in the early 1950s; yet there was only a small group of semiconductors available with natural properties of energy, and so it was “hard to make them, hard to ‘dope’ them, and hard to fashion them into different kinds of devices” – limitations scientists sought to surmount. In the early 1970s, however, “there was a very beautiful breakthrough” when “people realized that if you took two semiconductors and alternatively layered them, creating something called heterostructures, then you could tune those energy levels.” That is, scientists alternated layers of very thin slices of different semiconductor materials (as tiny as 2 to 50 nanometers) made of, for example, gallium arsenide and aluminum gallium arsenide. “So it’s like a multiple stacked bologna and cheese sandwich,” Chandrasekhar offers by way of a culinary metaphor.

“What happens when one puts such thin layers together is that the materials behave in a somewhat different manner because of the electrons that give conduction. As an electron travels around, it keeps bumping into stuff. So the behavior of the electron gets defined not just by all the other stuff around it, but also by the fact that it is bumping into the edges.” Quantum mechanics helps to explain different kinds of behaviors that occur when working with such small scales. In a very thin heterostructure sandwich, “all the quantum effects that occur in the material start to become amplified.” Not only do you get different kinds of behavior, but you can even tune the energy levels (for example, allow red light to be absorbed rather than the original infrared). “You can now create a new kind of device that is sensitive to more colors than the original bulk material’s typical color.” That property gives rise to a lot of practical applications in the electronic industry, such as red and green indicator lights.

All of this background information is necessary to appreciate the important contributions Chandrasekhar has made to this field. That is, once people realized that these “bologna and cheese” heterostructures could be reliably constructed, a whole bunch of new questions arose. For instance, she notes, “if you have two materials together, exactly where do the energy levels lie? What’s really going on inside them?” It is important for engineers who design electrical devices to know every parameter in order to tightly tune their manufacturing. This is where Chandrasekhar’s research comes into play. “We don’t really ‘grow’ the devices…or even the materials,” she stipulates. “All the work we do is on studying the properties of these devices: how to control them, what drives them, how far you can be off and still be within your range” – research crucial for engineers designing new technology.

Not long after arriving at MU as an assistant professor of physics nearly three decades ago, Chandrasekhar turned her attention toward the study of these heterostructure materials, developing an innovative method with which to study them. Of course, the process was complex. “It turns out that if you want to study the energy distributions for these materials, you couldn’t do it by just using a direct measurement because the direct measure only revealed the difference between the top and the bottom (the gap), but not where the top or bottom really were relative to everything else. We found that if we applied [hydrostatic] pressure on the system – take the whole sample and squeeze it [like a smashed bologna and cheese sandwich] – all the atoms and molecules came closer to each other, and all the electrons became more energetic, and then the gap became bigger and bigger.” Chandrasekhar recounts the moment of this dramatic discovery: “It turns out there’s not just one gap; there are two of them, one that opens up and another that closes down. And by knowing where the two of them crossed, we could figure out the absolute positions of those energy levels under normal circumstances.” In regard to this method, Chandrasekhar remarks: “There are a whole bunch of things that change just by squeezing molecules together.” This procedure has become a commonly used method of studying energy levels. Various kinds of innovative bologna-and-cheese heterostructure sandwiches are now being made and studied in a whole new field of physics called quantum confined heterostructures.

A lot of Chandrasekhar’s work, as a matter of fact, has involved using pressure as a tool to do different kinds of physics, and “it’s been a wonderful tool,” she says. She used it for the high-temperature superconductivity work her lab did in the early 1990s, and the technique is now being employed with organic materials, the area of her most recent research. “The wonderful thing about organic materials is that the sky is the limit in terms of how you build them because carbon bonds with carbon. You can’t do that with inorganics because the bonding doesn’t allow it.” Chandrasekhar continues to utilize pressure to investigate these organic materials in hopes that they may someday compete against inorganic semiconductors in the electron industry.

  1. Educational Initiatives for Students

Beyond her research, Chandrasekhar is passionate about education at both the university level, where she teaches, and at the elementary and secondary levels. Over the years she has spearheaded a number of hands-on physical science programs for K-12 students and teachers. Some of these programs are geared especially towards female students, who appear to lose interest in science around the middle-school years. “There’s a leakage in the pipeline,” she explains, as potential female scientists seem to drop out at every stage. Thus Chandrasekhar began collaborating with key people from the Columbia Public Schools and acquired funding from the National Science Foundation in order to develop Exploring Physics, a hands-on programs to get female students (and their teachers) interested in physics – “so that students don’t think of it as a bunch of formulae or as a place where they’ll get scared and flunk out.”

Other programs Chandrasekhar has developed include the Summer Teacher Institute, the Saturday Scientist program, and the Newton Science Academy. Overall, the programs are designed to be fun, with illustrated concepts and hands-on activities to familiarize students with the equipment. “We get them hooked, basically,” she laughs, perhaps thinking back to her own experience of getting hooked on physics. This program is still being used by the Columbia Public Schools. “One program cannot make a difference,” she argues. “There needs to be a whole series of things layered on it, so that at every stage there is encouragement, there is interest and confidence-building, and people are learning science at the same time.”

Chandrasekhar’s newest program, “Physics First,” results from collaboration with the State of Missouri and the Columbia Public Schools. The present secondary education system does not offer physics until the senior year (which means that many students end up not taking it). Yet fields as diverse as engineering and the health professions require a full year of physics. As a result, students who choose those career paths end up having to take physics in college, and that “is not a good place to take your first physics course,” Chandrasekhar advises. “Physics First” seeks to reverse the sequence in which science is taught" – physics in ninth grade, chemistry in tenth, biology in eleventh, and an advanced placement (AP) science course in the last year. She explains that this new system makes more sense “because this is the way the complexity of science is organized. Physics is a very fundamental science. It tells you about the basics of how things work; chemistry applies a lot of the principles of physics to molecules, structures, and so on; and biology depends very heavily on chemistry.” By arguing for a reversal of the existing system, which was initiated in the 1890s, Physics First advocates argue that the time for change has come. The Columbia Public Schools began the program this year, and Meera Chandrasekhar, for one, is watching with excitement to see what comes out of it.