ice structures slightly different in size from those of the material upon which the films are grown.
Just a few percent difference in lattice size creates stresses (or pressures) in a film that are ten times larger than those present in the deepest oceans of Earth.
These huge pressures, as new layers are deposited, force the initially flat film to separate into dots and then pop up into the third dimension to relieve stress, rather than continue to grow against resistance in two dimensions. This extra dimension, combined with the extremely small size of the dots, gives them different properties from when the material was in its original flat film shape.
Semiconductor quantum dots have the potential to produce laser light output at wavelengths where, in a manner of speaking, no flat film has gone before, depending on the size of the dots.
But while crude collections of quantum dots have been grown and set lasing, knowledge of the conditions needed to influence the dots so that they form more regularly in size, shape, and pattern -- thus improving control of their lasing frequency and intensity -- has been only a dream.
Techniques of physical etching -- even nanolithography -- can't be used to make them, because the dots are so small that they do not manifest the continuous nature of a solid. "People have made crude devices," says Floro. "But to make them reliably, we need to understand the ground rules."
The collaboration between Sandia and Brown University makes use of optical and stress measurements to observe dot formation as it happens on silicon germanium.
Stress in the film causes the substrate to bend, which the researchers measure by bouncing laser beams off the sample. When the dots form and change shape, the stress changes and so does the amount of bend in the substrate. So, mapping the substrate as it bends reveals when dots first form and how their shapes evolve.
Says Floro, "Tiny dots cause detectable bending in a substrate that is ten thousand times thicker than the dots themselves."
The conventional 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 too