System Bits: Dec. 10

Lasers From Nano Wires
A few weeks ago, Semiconductor Engineering published a special report about silicon photonics and concentrated on the integration of the laser onto the silicon surface. Growing III-V materials on silicon is problematic because of the lattice mismatch, but researchers at the Technische Universität München (TUM) may have found a way around that problem. Thread-like semiconductor structures called nanowires, so thin that they are effectively one-dimensional, show potential as lasers.

These experimental nanowire lasers emit light in the near-infrared, approaching the “sweet spot” for fiber-optic communications. They can be grown directly on silicon, presenting opportunities for integrated photonics and optoelectronics. And they operate at room temperature, a prerequisite for real-world applications.

A unique advantage, according to Prof. Jonathan Finley, director of TUM’s Walter Schottky Institute, is that the nanowire geometry is “more forgiving than bulk crystals or films, allowing you to combine materials that you normally can’t combine.” Because the nanowires arise from a base only tens to hundreds of nanometers in diameter, they can be grown directly on silicon chips in a way that alleviates restrictions due to crystal lattice mismatch – thus yielding high-quality material with the potential for high performance.

A number of significant challenges remain, however. For example, laser emission from the TUM nanowires was stimulated by light – as were the nanowire lasers reported almost simultaneously by a team at the Australian National University – yet practical applications are likely to require electrically injected devices.

Ongoing research is directed toward better understanding the physical phenomena at work in such devices as well as toward creating electrically injected nanowire lasers, optimizing their performance, and integrating them with platforms for silicon photonics.

“At present very few labs in the world have the capability to grow nanowire materials and devices with the precision required,” says co-author Prof. Gerhard Abstreiter, founder of the Walter Schottky Institute and director of the TUM Institute for Advanced Study. “And yet,” he explains, “our processes and designs are compatible with industrial production methods for computing and communications. Experience shows that today’s hero experiment can become tomorrow’s commercial technology, and often does.”

Carbon Nanotubes Could Improve Packaging
When engineers design devices, they must often join together two materials that expand and contract at different rates as temperatures change. Such thermal differences can cause problems if, for instance, a semiconductor chip is plugged into a socket that can’t expand and contract rapidly enough to maintain an unbroken contact over time.

The potential for failure at such critical junctures has intensified as devices have shrunk to the nano scale, bringing subtle forces into play that tug at atoms and molecules, causing strains that are difficult to observe, much less avoid.

“Think about the heat sink for a microprocessor,” said Kenneth Goodson, professor and Bosch Chair of Mechanical Engineering at Stanford. “It is exposed to high heat fluxes for long periods of time, and repeated instances of heating and cooling.”

At present, materials such as solder and gels have been used at such junctions. But as electronics continue to shrink, more electrical power is pushed through smaller circuits, putting materials under ever increasing thermal stress.

These three images taken by a scanning electron microscope zoom in on experimental carbon nanotube structures. The image on the left visualizes CNTs at 50 micrometers, about half the width of a human hair. At this resolution the CNTs appear straight. But at a resolution of two micrometers, entanglements start to appear. At the scale of 500 nanometers the entanglements look clearer. (Goodson Lab)

The Stanford experiments and simulations were designed to reveal how to create carbon nanotube structures (CNTs) with the optimal blend of all three characteristics – strength, flexibility and heat conductivity – that are required in critical junctures where thermal stress is a fact of life.

To some degree, the findings of Yoonjin Won, who was then a doctoral student in mechanical engineering, represent a tradeoff. Denser, shorter CNT structures are stronger and more efficient at dissipating heat. But they are also more entangled and stiffer. Won’s experimental results showed that as CNT strands grew longer, they tended to grow straighter and were less tangled, which increased the flexibility of the structure, albeit with some acceptable losses in the other two parameters.

Because the ultimate goal of this work is to reveal how to optimize CNT structures for use as thermal transfer materials, the Stanford team built a computer simulation of the CNT assembly process with an eye toward understanding how the CNTs became bent and entangled despite efforts to grow them straight.

Taken together, the experimental results and computer simulation reinforce the findings that longer, less entangled CNTs would offer the best mixture of the desired characteristics strength, flexibility and heat transfer. But because of the van der Waals forces operating on these atom-thick carbon tubes, engineers will have to accept some bending and irregularity as they strive to create workable, though less than ideal, structures for dissipating heat.

“When you hear about nanotechnology, it’s usually about the superlatives, the strongest this, the thinnest,” Goodson said. “But we think the answers will lie in finding the right combinations of properties, something that’s strong and conducts heat like a metal but can flex and bend as well.”