For better lasing, keeping one’s distance not always a bad thing

Albuquerque, N.M. — The term “quantum dots” has a nice ring to it, like “beanie babies” or a special version of connect-the-dots games that absorb the attention of children.

But while the tiny entities may sound cuddly, they find each other repulsive, and in that just-discovered knowledge may lie the secret of controlling their formation.

Effective control would turn assemblages of dots — each only a few thousand atoms in size compared with the trillions in a pencil dot — into the world’s most effective solid-state lasers, says principal investigator Jerry Floro.

He leads a research team using probes developed at the Department of Energy-funded (DOE) Sandia National Laboratories.

“We developed novel probes that uncovered a repulsion effect between quantum dots. This effect may completely govern the way they organize themselves. Understanding this self-organization is critical if we are to control dot characteristics for lasing devices,” says Floro.

The probes, one of which was just patented, have made unique real-time measurements of atoms clustering to form relatively large three-dimensional dots, called islands. Scientists for the first time observed the role of mutual repulsion in causing dots to change shape and self-organize as they grow.

Understanding these factors is an important step in basic science. The smaller the dot, the shorter the emission wavelength; the more tightly the dots are packed, the more intense the beam; and the more uniform their size, the more uniform the frequencies.

The ongoing work, details of which have been published in a series of articles in Physical Review Letters, was the subject of an invited talk by Floro, “Formation of Quantum Dots,” at this summer’s Gordon Conference on Thin Film and Crystal Growth Mechanisms, in Plymouth, N.H. Work was done in collaboration with researchers now at Brown University and the University of Illinois at Urbana-Champaign.

Buckling up to relieve stress

Quantum dots form when very thin semiconductor films buckle due to the stress of having lattice 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 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.