Greta M. Zenner
Mold or nanotechnology?
Bin Yang grows quantum dots (the arrows point to them)
as part of his nanotechnology research.
At first glance, Bin Yang's research looks like he left it in the fridge a little too long. However, upon closer examination, and after talking with the University of Wisconsin-Madison chemistry graduate student, it becomes clear that what looks like spots of mold are actually artificial atoms. As Bin explains, these artificial atoms, called quantum dots, can be used to make better microelectronics, including finely-tuned lasers, infrared photo-detectors, and, maybe one day, quantum computers.
Bin grows these quantum dots, or QDs, himself. Or rather, he and his advisor, Professor Max Lagally, developed a technique that makes the QDs grow themselves - or "self assemble" - kind of like mini mold. This particular self-assembly technique has exciting potential applications in the microcircuitry business, one of the main reasons Bin chose it as the focus of his research.
Bin working on a recent presentation on quantum dots
The conventional approach to making microcircuits like those in computers is expensive, time-consuming, and limited in terms of how small the circuit features can be. QDs, says Bin, are quicker, cheaper, and much smaller. In fact, the graduate student's QDs, which are less than 100 nanometers in diameter, are about half the size of the smallest circuit features made using traditional methods. Approximately 500 times smaller than the diameter of a human hair, QDs hold the future of circuitry and computing in their tiny, island-like existence.
To make these artificial atoms useful for microelectronics, Bin faced the challenge of developing a way to grow them in an ordered fashion. "If you can organize QDs the way you want, you can design structures using QDs as building blocks," explains Bin. Order, certainty, and control are necessities for growing QDs that are useful for microcircuits.
The top image represents the square wave pattern on the Si, and the bottom one the rounded wave pattern.
Given that his interests lie in microcircuitry, Bin ensures these precision growing conditions by using a lattice-mismatched system, a methodology that takes advantage of the phenomenon that occurs when you try to stack atoms of different sizes. Bin studies the two elements germanium (Ge) and silicon (Si), with Ge being the bigger of the two.
To set up his experiment, Bin uses a conventional microelectronics method to make patterns of "square waves," as he describes them, on a Si surface. Then he heats the patterned Si to 1200 degrees celsius to round the square edges into curved waves. Bin's final step in creating a lattice-mismatched system is to deposit several layers of Ge, each only one atom thick, on top of the rounded Si waves. The large Ge atoms try to align themselves with the small Si ones and stack up in neat columns, but with little success. Instead, strain is created.
Bin checking the heat during the growth of his QDs
To understand a lattice-mismatched system and why strain results from them, imagine two sets of building blocks. One set has big blocks, the other small ones. If you build a tightly packed square out of the smaller blocks and stack a layer with an equal number of big blocks on top of it, trying to match each big block with a smaller one, something starts to happen. The two layers can't align with each other. If you attempt to add more layers of big blocks, your creation will likely fall apart.
Similarly, on the nanoscale, strain builds up when the Ge atoms try to stack up like the foundational Si layers. After three or four layers of Ge, the strain becomes too great and something has to give. However, unlike your block construction, which fell apart, the Ge atoms do something really exciting - they spontaneously form a tiny 3-D island, or a QD.
A computer image of a GE QD
Although Bin and other researchers find self-assembly of QDs interesting in its own right, they are also excited about its potential applications in the microelectronics industry. This new way of growing these atomic islands results in QDs of uniform size and spacing and allows scientists to predict where they will appear. "QDs grown using this self-assembly method have very good size uniformity and spatial ordering," explains Bin. This is important for building microcircuits.
One of the reasons Bin can accurately predict where the atomic pyramids will appear is because they grow on the ridges on the patterned surface, where they have more room. "The atoms are trying to relax and expand because of the strain," explains Bin. "On the ridges they have more space and are less crowded."
Bin made a "UW" in the silicon and grew QDs on its ridges. The little bumps on the edges are QDs, visible on the enlarged section just above.
Once they understood why this happens, Bin, Lagally, and their collaborator Feng Liu, a materials science professor at the University of Utah, developed a mathematical formula that allows them to predict where the dots will grow. According to Bin, the formula also applies to research beyond his. "I can also apply this model to others' work, too." This means that Bin's efforts have broader significance for research in QDs and microelectronics.
However, not all QDs are made using the self-assembly system Bin developed. For example, says Bin, they can also be made by using simple chemical reactions in solution. How researchers make their QDs depends on what they are going to do with them. Bin designed his methodology with a specific use in mind. "I use [the GeSi lattice-mismatch approach] because of the need for better circuitry options for microelectronics," he explains. Other scientists make QDs in a variety of sizes for a variety of applications, such as biological probes and tags or quantum dot lasers, depending on the elements and the technique they use.
As scientists and engineers strive to make electronics smaller and smaller, the future will rely on the quantum dot research of Bin and others. These tiny artificial atoms are indeed a far cry from the moldy "science experiments" conducted in fridges around the world.
1. How did you become interested in nanotechnology? Did you have an epiphany, or did your interest gradually grow? Please explain.
"I still remember the excitement when I read books about physics history in the 1930s. I always have thought that physics is cool, so I thought I would like to be part of it. I like to explore how things happen and to make new stuff. Now it is the era of nanotechnology. As professor Richard P. Feynman said [in his 1959 speech], 'There's plenty of room at the bottom!' There are so many exciting things in the nanoworld."
2. What do you plan to do when you finish your degree?
"I may do research in some research lab or university. I think it will be fun."
3. What advice would you give to someone (a college or high school student) who is interested in pursuing nanotechnology research?
"Nanotechnology involves so many disciplines, so keeping your eyes open is always good. Read some stories by science fiction authors; they are good at describing the future world. Or visit science museums or attend some science camps."
Exploring the Nanoworld | MRSEC Nanostructured Interfaces
Copyright © 2008 The Board of Regents of the University of Wisconsin System.
This material is based upon work supported by the National Science Foundation under DMR Grant No. 0520527
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.