Volume 2, Issue 8 October 2002

2002
2001

The science-fiction fantasy of nanotechnology — building novel structures, devices, and materials at the atomic or molecular scale — is becoming a reality. For the great potential of nanoscience and nanotechnology to be fully realized, however, research efforts must cross many disciplines, from electrical engineering, mechanical engineering, materials science, and computer science to bioengineering, chemistry, and physics. Nowhere is this cross-disciplinary approach fostered more than at UC Berkeley. Each month, Lab Notes is proud to present the work of nanotechnology researchers from the College of Engineering and our collaborators across the campus. LED There Be Light by David Pescovitz At the Hearst Memorial Mining Building rededication reception, the Weber Group demonstrated this ring of white LEDs. (Click for larger image.) Peg Skorpinski photo Bright ideas in nanotechnology at UC Berkeley are helping usher in a new age of environmentally-friendly, inexpensive, and innovative forms of illumination. Based on technology similar to the ubiquitous red Light Emitting Diodes (LEDs) found everywhere from alarm clocks to car stereos, the new generation of solid-state lighting devices are set to replace traditional incandescent bulbs for many applications. "The incandescent bulbs we use everyday to light our rooms go back a century to Thomas Edison," says materials science professor Eicke R. Weber, who's leading Berkeley's solid-state lighting efforts. It's time for a change. By 2020, solid-state lighting devices could cut electricity used for illumination by 50 percent and keep 28 million metric tons of carbon emissions out of the atmosphere each year, according to a U.S. Department of Energy study. Incandescent and fluorescent bulbs would have remained unchallenged if it weren't for researchers at the Nichia Corporation in Japan who in 1991 developed LEDs based on gallium nitride (GaN). This particular semiconductor is capable of emitting blue light, which combined with other colors or shined on yellow-emitting phosphors produces the holy grail of white light. Already, these GaN-based high-brightness LEDs can easily be spotted in efficient and ultra-bright green traffic lights and some special-purpose flashlights. Professor Eicke Weber and his students also research new materials to improve photovoltaics (PV), or solar cells. (Click for larger image.) Peg Skorpinski photo "Although we have these LEDs on the market, we don’t understand fundamental issues in how they work," Weber says. This lack of certainty compounds the challenge of increasing the devices' brightness and efficiency — essential if LEDs are to have a chance at replacing tried-and-true incandescent and fluorescent technology. Will we be lighting up our homes with new and nano-improved LEDs? We want to hear from you... The difficulties lie deep in the atomic structure of the semiconductor, Weber explains. To build a GaN-based LED, two thin layers of the semiconducting material laced, or doped, with different impurities that result in specific electrical properties are sandwiched together. One layer has a surplus of electrons while the other is rife with positive charge-carriers, also known as holes. In between the two is a so-called quantum well, containing a precise amount of the element Indium. Passing current through the device forces the electrons and holes into the quantum well between the layers where they recombine and emit photons. Weber and his team use techniques like electron microscopy and laser spectroscopy — enabled by UC Berkeley and the Lawrence Berkeley Laboratory's state-of-the-art arsenal of nanotechnological tools — to better understand how the structure and chemistry of the quantum wells may affect the efficiency and brightness of the light generated by the LEDs. "We can analyze the key parameters and go back to the growers of the semiconductors and suggest changes, enabling them to fabricate improved devices," says Weber, who collaborates on the research with industrial partner Lumileds. Within this decade, he says, GaN-based LED will be ready to be used for general illumination. "If you create white light by combining red, green, and blue LEDs, you could just as easily twist a knob next to your light switch and change the color and mood of your room," Weber says. Professor Eicke R. Weber's Home Page GaN-based LED research of the Weber Group Lumileds Lab Notes is published online by the Public Affairs Office of the UC Berkeley College of Engineering. The Lab Notes mission is to illuminate groundbreaking research underway today at the College of Engineering that will dramatically change our lives tomorrow. Editor, Director of Public Affairs: Teresa Moore Writer, Researcher: David Pescovitz Designer: Robyn Altman Subscribe or send comments to the Engineering Public Affairs Office: lab-notes@coe.berkeley.edu. © 2002 UC Regents. Updated 9/30/02.