 laboratory approach, by contrast, has been to artificially stutter dot formation into separate time intervals and bring intermediate results to powerful microscopes to observe formations at each stage.
As more film layers are deposited, the dots grow closer and closer, and, because they repel each other, they are forced to become more uniform in size, line up in orderly fashion, and change their shape. Some dots are even "eaten" by their neighbors in an attempt to reduce the overcrowding, a process known as coarsening.
Working with larger entities
But how does one "see" dots so small? The researchers were clever and used bigger dots, called islands, thousands rather than hundreds of angstroms in size, made of silicon germanium, since the larger ones could be more easily examined. The researchers earlier showed that larger semiconductor entities in groups interact the same way as smaller semiconductor entities.
According to Dennis Deppe, an electrical engineering professor at the University of Texas at Austin working in the field but unconnected with the Sandia-Brown effort, "Many of the same basic growth phenomena are seen in different material systems. So it is possible to learn some important physical principles concerning nucleation and dot formation in one semiconductor system [and have some of them] carry over to another."
"We directly measure the kinetics of nucleation, coarsening, self-organization, and phase transformations within growing island arrays," says Floro. "All these processes are explained within a unified model that works with ensembles of islands rather than individual islands in isolation."
In terms of actual formation, the process characteristically went like this: ten atomic layers of film would form smoothly. As more layers were deposited, the film broke up into tiny pyramid-shaped islands. With more layers, the pyramids self-organized and coarsened, and then became dome-shaped islands.
Quantum dots as diffraction gratings
But there's more. The researchers, not content with one novel tool to examine dots, realized they had another.
Floro, along with Bob Hwang (a California-based Sandia researcher), Ray Twesten at the University of Illinois at Urbana-Champaign, Eric Chason (a former Sandia researcher) and Ben Freund of Brown University, made measurements that treat dots as the originators of light-interference patterns. Since light's direction and intensity varies depending on the size, shape, and spacing of the islands, the results offer information in real time to determine what is happening to the tiny islands as temperatures, material compositions, and stresses change.
"We realized that if we could produce islands more than 1,000 angstroms across, the spacing between islands was like that of a diffraction grating," says Floro. "Combined with our real-time stress observations, this allowed us to measure stress, shape, and size simultaneously instead of having to stop the process, take the dots out, and measure them. A key ingredient was our ability to show that the basic physics of the large islands mimics that of the much smaller dots."
Observing the process of dots going from one shape to another to relieve stress provided deep insight into the physics governing island formation. "It showed us what controls dot evolution, and how process conditions like temperature and strain enhance or suppress dots."
Silicon germanium is not a good laser emitter, but it is simple enough to derive the applicable physics. "We next need to find out next how much of the physics learned in silicon germanium will apply to real laser materials like indium gallium arsenide," says Floro. "If we can understand the physics, we can make better quantum dots."
The DOE Office of Basic Energy Sciences, Division of Materials Sciences, funds the work.
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