* Sciport *

Welcome to the world of the nanoscale, where nothing seems to behave like it does up here in the everyday world. And nobody seems to know exactly what the rules are and what to expect. Frankly, it's probably not a place you'd want to visit if it weren't so important.

At least, it's important to anyone involved in or affected by such technologies as computing, telecommunications, and electrical powergeneration.

Technology's race to the bottom is being driven by the continuing miniaturization of computer systems. As computer components shrink down to the nanoscale (a nanometer is about four atoms long and about 1/1000th the diameter of a human cell), small, formerly trivial particles can have catastrophic effects.

Take phonons. They're the smallest unit (quantum) of sound, just like photons are the smallest quantum of light. They produce the vibrations that we experience as heat.

Usually we don't worry about phonons-they only travel at the speed of sound and carry only a fraction of the energy of a photon. Compared to the electrical effects of, say, an ion strike by a cosmic ray-which can shut down a computer system if it knocks out the right transistor-the heat produced by radiation has previously seemed to exert a negligible effect on the performance of integrated circuits.

But phonons become more problematic as integrated circuits get smaller, particularly in power devices, direct energy-conversion devices and telecommunications switches.

Vanderbilt Assistant Professor of Mechanical Engineering and Electrical Engineering Greg Walker has studied various types of transistors used in power and telecommunications systems and has found that the previously overlooked thermal effects from phonon transport can have a significant impact on the devices' performance.

His research on phonon production and transport from an ion striking electronic devices showed that highly scaled structures are susceptible to thermally induced failure, regardless of the strength of the cosmic ray's electronic impact.

"As energy carried by phonons propagates through semiconductor materials, it will leave a path that experiences extreme temperatures," Professor Walker says. "Our research has shown that thermal energy production may be significant enough to affect nanoscale microelectronics devices."

Professor Walker is working with the Vanderbilt Institute of Nanoscale Science and Engineering (VINSE). VINSE is an interdisciplinary center involved in nanoscale theoretical and experimental research. VINSE conducts a wide range of projects in nano-optics; nano-bio processes; spintronics; nanoscale electronics; nanocrystal fabrication, characterization, and integration; and radiation effects and defect studies. The Institute recently opened a new nano science and engineering laboratory complex and this fall launched a National Science Foundation (NSF)-sponsored Integrative Graduate Education and Research Traineeship (IGERT) Ph.D. program.

Professor Walker is one of the professors involved in the new NSF IGERT doctoral program, but he will spend little of his research time in the new nanolab.

Instead you'll find him at the computer, developing models and simulations.

"What I'm working on is the theoretical understanding of phonon behavior," Professor Walker says. "We need a more precise way of evaluating thermal effects when predicting the performance of semiconductor devices as they reach nanoscale proportions."

Professor Walker hopes that this work will help microelectronics designers develop materials and fabrication techniques that will protect the equipment from, or compensate for, thermal effects.

"Vanderbilt's Institute for Space and Defense Electronics (ISDE) is the best research center in the nation for the study of radiation effects in integrated circuits," he notes. "I hope that my colleagues at ISDE will be able to take the models we are developing to augment the research they are doing on radiation effects."

The model Professor Walker and his associates have developed produces simulations of nanoscale energy transport throughout the entire thermodynamic cycle.

"Current research efforts to integrate models of electronic and thermal transport are limited, so our work represents the first attempt to couple models that include phonon dispersion, polarization and multiple scattering mechanisms with electronic simulation," Professor Walker says.

The problem with traditional models of the dynamics of heat produced by electrons scattering throughout semiconductor materials is that these models are based on assumptions that the material itself is in a state of thermodynamic equilibrium. But radiation, particularly from an ion strike, creates non-equilibrium in the system. Power devices are also non-equilibrium systems because they involve large currents, high switching speeds and reduced sizes.

"You can't model these phenomena as thermal diffusion processes," Professor Walker says. "We need a new fundamental physics for non-equilibrium transport of phonons."

In addition to his work on radiation effects, Professor Walker and his associates have focused their research on direct energy-conversion devices and metal oxide semiconductor field effect transistors (MOSFETs), which are used in power electronics, telecommunications switches, and "smart chip" integrated circuits.

Professor Walker analyzed the thermal effects in MOSFET devices by first running a commercial simulation that models the behavior of electrons. Professor Walker then fed these results into a new model he has developed to analyze nonequilibrium thermal effects.

This process revealed that the energy carried by the phonons can result in debilitating "hot spots" in the devices. "The confinement of thermal energy causes a higher rate of interaction with electrons, which ultimately causes reduction of electrical current," Professor Walker says. "The findings lead us to believe that devices with similar or smaller dimensions with short time-scale features are susceptible to performance aberrations or failures."

This information not only will help microelectronics engineers design new nanoscale devices, but perhaps will explain computer failures that have heretofore been inexplicable.

Direct energy-conversion devices, specifically thermoelectric refrigerators and electrical generators, can be used in a wide variety of applications such as power plant bottoming or topping cycles and scaled cooling situations. By understanding and leveraging nonequilibrium phonon transport in these solid state devices, efficiencies can be improved, which will ultimately reduce the world's dependence on fossil fuels.

"We don't have all the answers, but understanding that electrical characteristics of devices are closely coupled with thermal nonequilibrium effects is a big step in the right direction," Professor Walker says.