Sowing strain reaps ordered dots

By Eric Smalley, Technology Research News

Picture small fault lines lurking beneath plateaus that rise at regular intervals above a microscopic desert landscape and you have a pretty good idea of a fabrication process that promises precise placement of quantum dots. Quantum dots are tiny specks of semiconductor that behave like oversized atoms by corralling electrons.

Quantum dots have unique electrical properties that make them suitable for use as light sources for tiny lasers, and they could also form the basis of quantum computers or other novel electronic devices. A major hurdle to using quantum dots for electronic devices has been the difficulty of getting them to form where researchers want them.

A process developed at the University of California at Santa Barbara deliberately adds stress to specific places in a layer of material in order to grow quantum dots in two-dimensional and three-dimensional patterns. The added stress primes the material for forming quantum dots.

The process uses layers of semiconducting materials beginning with a layer in which the researchers engineer strain at regular intervals. The top layer includes raised areas positioned above the added strain. If the layer with the added strain is at least 20 nanometers thick, quantum dots form only on the raised surfaces.

"The stress propagates all the way to the surface," said Pierre Petroff, a professor of materials science and electrical and computer engineering at UC Santa Barbara.

Because the researchers can control the position of the added strain, the resulting quantum dot patterns can be made with specific spacings and orientations.

"There's been quite a lot of work on controlled growth of dots using pattern substrates, but what's different about this work is the use of a stressor layer to control the patterning," said Gregory L. Snider, an associate professor of electrical engineering at Notre Dame University. "The result is probably the nicest arrays of dots that I've seen."

The technique can be used to make three-dimensional patterns by simply adding layers, said Petroff. "Because then the quantum dots you have created in the first layer serve as subsurface stressors in the next layer," he said.

The quantum dots range from 27 to 45 nanometers in diameter and four to 10 nanometers in height, and are about 10 nanometers apart. A nanometer is about 10 carbon atoms long. Each raised area on the surface includes 3 to 4 quantum dots. The researchers produced patterns with raised areas about 170 nanometers in diameter with about 250 nanometers between the centers of each raised area.

The researchers are studying the properties of these patterns, said Petroff. The first application the researchers are aiming for is a single-photon emitter, he said. Single-photon sources are critical components in research efforts to produce quantum cryptographic systems that theoretically could provide completely secure communications.

The process could be used to manufacture patterns of quantum dots now. However, one drawback to the quantum dot patterning technique is that it produces quantum dots that operate only at extremely low temperatures, Petroff said. While the low-temperature requirement will likely not prove much of a barrier to researchers working on quantum cryptography and quantum computers, it could make it difficult to commercialize the process, he said.

The researchers are exploring other semiconducting materials in an effort to use the technique to produce quantum dots that operate at room temperature, said Petroff.

Petroff's research colleagues were Hao Lee, JoAnn A. Johnson, Ming Y. He and Jim S. Speck. They published the research in the January 1, 200 1 issue of Applied Physics Letters. The research was funded by the Army Research Office in the Air Force Office of Scientific Research.

Timeline:   Now
Funding:   Government
TRN Categories:   Semiconductors and Materials
Story Type:   News
Related Elements:  Technical paper, "Strain-engineered self-assembled semiconductor quantum dot lattices," Applied Physics Letters, January 1, 2001


January 17, 2001

Page One

Light spins resin rotors

Crossed nanowires make Lilliputian LEDs

Simulation sizes up Web structure

Quantum bit hangs tough

Sowing strain reaps ordered dots


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