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Now a professor in the Department of Materials Science and Engineering, Oscar Dubon is also an alum of the program.

Nanotechnology promises to revolutionize computer technology, enabling the development of processors thousands of times faster than today's speediest chips. The key is replacing the basic building blocks of integrated circuits with nanoscale counterparts so that many more of the components can be packed onto the same size chip. Controlling how and where nanostructures are placed on the silicon wafer is no easy task though. UC Berkeley materials scientist Oscar Dubon Jr. recently won a prestigious Presidential Early Career Award for Scientists and Engineers for his work in this area.

"My research is about modifying the properties of semiconductors," Dubon says. "One way to do that is chemically, but the other is structurally."

An atomic force microscopy image of germanium nanorods grown on a gold-patterned silicon substrate.

Dubon explores both avenues, but the research that earned him by the award focuses on the latter approach. Traditionally, integrated circuits are fabricated using optical lithography. In a series of steps, light is shined through a stencil-like mask to pattern features on a wafer of silicon. Dubon also uses this process, but in lieu of light, he blasts metallic vapor through the stencil.

Dubon and his collaborators--Materials Science and Engineering graduate student Jeremy Robinson and LBNL staff scientist J. Alexander Liddle - start with a traditional silicon wafer. Next, they evaporate gold or other metal through a stencil mask onto the wafer. Once the thin film of metal forms the desired pattern on the silicon, the researchers use a special high-temperature reactor at Berkeley Lab to deposit germanium onto the substrate. Most of the germanium collects in spots where the gold is not. The material then grows into three-dimensional "islands", with shapes that vary depending on the metal used in the patterning and the orientation of the silicon substrate.

Using different metals (gold at left and tin at right) for patterning affects the shape of the islands.

"One of the big challenges with growing nanostructures is not just controlling the size, but also the location," he says. 'How do you lay out millions of structures in a nicely-controlled fashion? The metal patterning enables us to organize the arrangements of the islands on the silicon surface with a high throughput."

Jeremy Robinson uses the reactor at LBL used to deposit germanium onto a silicon substrate.

Already, Dubon and his collaborators have grown arrays of a half-million pyramid and rod-shaped islands. Each rod is about one micron long, 30 nanometers tall, and 80 nanometers wide. (A nanometer is one billionth of a meter.) One long-term goal is to add a gate and electrical leads to a rod, forming a miniscule nanotransistor. Potentially, more traditional forms of lithography could then be used to connect groups of the devices together and link them with other components.

"Accessing the electronic properties of the islands could we hope lead to nanoelectronic applications," Dubon says.

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Currently, the researchers are studying how the metal patterns affect the shape and arrangement of the islands. The ability to predictably and repeatedly drive the island formation depends on a deep understanding of the germanium growth processes, Dubon says.

"By looking at how the islands evolve, we should be able to scale down the size of our structures even more," he says. "Honestly though, we're surprised at how well the technique seems to work."

Oscar Dubon's home page

Lawrence Berkeley National Laboratory Materials Sciences Division

"Materials science researcher receives early career award" by Sarah Yang (Media Relations, 13 June 2005)

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