Quantum dots (orange) form via self-assembly and nucleate randomly on the atomic steps of the semiconductor surface (blue).
Next to silicon, germanium (Ge) is the most widely used semiconductor material in the world. But while it’s great at conducting electricity, its inefficiency at turning light into electricity (or electricity into light) restricts the other applications for which it can be used.
Paul Simmonds, an assistant professor with a dual appointment to the departments of physics, and materials science and engineering, wondered if there was a way to fine-tune germanium’s physical properties, and thus improve its optoelectronic characteristics (how well it interfaces between electronics and light).
The Air Force Office of Scientific Research also was intrigued and funded a proposal titled “Optoelectronic Properties of Strain-Engineered Germanium Dots” with a three-year, $622,000 grant. Simmonds is working on the project through a sub-award administered through the University of California, Merced, and the University of California, Los Angeles. Boise State’s share of the award is $206,000.
“If we can turn Ge into an optoelectronic material, then other characteristics would make it attractive as a laser material,” Simmonds says. “It’s a bit like alchemy. We hope to change the fundamental properties of an element on the periodic table simply by stretching it a little.”
For years, scientists have tried putting germanium under tensile strain (stretching it at the atomic level) in order to improve its optoelectronic properties. But germanium is fragile, and crystalline imperfections cause it to break before enough tensile strain can be built up.
Simmonds and his research team have responded to the challenge by developing a new family of self-assembled nanomaterials capable of storing large amounts of tensile strain, without damage to the crystalline structure.
“Self-assembly has allowed us to develop a way for the materials to sustain high tensile strains without falling apart,” Simmonds says. “Instead of remaining flat, the atoms rearrange to form nanoscopic islands, like raindrops on the top of a car but about a million times smaller. The process of rearranging into 3D islands relieves a little of the strain and creates a window that allows us to have high tensile strain without breaking any atomic bonds. We’ve shown this works with other materials and now we want to try it with germanium.”
Doing so would help establish tensile self-assembly as a novel means by which to integrate dissimilar materials and demonstrate to the research community that nanostructure band engineering with tensile strain is an effective tool for discovering and designing materials for technological innovation.
While their work has real-world applications — creating direct band gap Ge nanostructures would be a critical breakthrough in optoelectronic materials research — Simmonds is excited that it’s also an opportunity to simply understand the world a little better.
Simmonds’s team will carry out the synthesis and structural characterization of the Ge nanostructures. Collaborators at UCLA and UC Merced, who are world-renowned experts in computational modeling and optical spectroscopy, will help analyze the materials created at Boise State’s Collaboratory for Epitaxy of Nanomaterials.
Source: Boise State University
