By Colleen Kearney Rich
It’s probably happened to you: you use your laptop or smartphone a little too much one day, and suddenly you realize you need to recharge it—as soon as possible.
Mason electrical engineer Qiliang Li has been working to solve this problem. In fact, he foresees a future where a person might only need to recharge his or her electronic devices once a week. He believes the answer lies in nanotechnology.
“It is called local memory,” says Li of the central processing unit (CPU) that keeps all the applications open, running, and on track, remembering where the user was in an e-mail or online. It is different from the kind of computer memory that one can buy and add to the computer, and it is more expensive.
“The CPU is the heart of the computer. All computers depend on the CPU,” he says. “In a CPU, half the space is occupied by this local memory, which is the most important kind of memory.”
And Li believes that memory needs to be fast enough and energy efficient. “You don’t want the computer to consume a lot of energy,” says Li, who is an assistant professor in the Department of Electrical and Computer Engineering in the Volgenau School of Engineering.
Li and his team are working with nanowire-based logic and memory, a technology that uses tiny silicon wires, less than 20 nanometers in diameter. These nanowires form the basis of memory that is nonvolatile, meaning that it can hold its contents even when the power is switched off.
According to the National Institute of Standards and Technology (NIST), a collaborator in Li’s research, such nanowire devices are being studied as the possible basis for next-generation computer memory because they hold the promise to store information faster and at lower voltage.
Before Li came to Mason in 2007, he was a research scientist in the Semiconductor Electronics Division of NIST, where he was involved in the fabrication, characterization, and simulation of complementary metal–oxide–semiconductor and nanoelectronics materials and devices. His relationship with the U.S. Department of Commerce continues to this day. In fact, four of the graduate students supervised by Li work at the NIST lab through this ongoing collaboration.
Li has used nanowire in previous research. In 2011, Li and Dimitris Ioannou, also from Volgenau’s Department of Electrical and Computer Engineering, and two former PhD students, Yang Yang and Xiaoxiao Zhu, were issued a patent for the nanowire field effect junction diode.
One application of this silicon-based diode is its use as a semiconductor device that produces alternating current (AC) voltage from sunlight; current semiconductor junctions can only produce direct current (DC) voltage. Producing AC directly improves the conversion of sunlight by eliminating costly power losses associated with converting DC to AC.
This diode expands the possibilities for solar cells and related applications. The device is also relatively inexpensive and can be used for memory devices, switches, and variable transistors.
“The nanowire is a good static control device for energy-efficient electronics,” says Li, who received a National Science Foundation CAREER Award in 2009. “This is one of most attractive strategies for future electronics. We need really functional mobile devices that consume much less power.”
Li is also a Virginia Microelectronics Consortium Professor. Since it was created in 1997, the Virginia Microelectronics Consortium has fostered education and research in support of the microelectronics industry throughout Virginia. Consortium members include seven universities and two microelectronics companies. One of the principal activities of the group is to support a summer research internship program where students from member universities spend a summer at another Virginia university or one of the industry members, gaining valuable experience in microelectronics. Li is one of six professors throughout the state who holds this appointment.
Li and his graduate students “grow” the nanowire in a lab in the Nguyen Engineering Building on Mason’s Fairfax Campus, where the students gain hands-on experience learning to fabricate devices in a cleanroom.
“It is somewhat like growing mushrooms,” he says, but it is more complicated than that. The silicon atoms in gas phase are absorbed by gold nanoparticles catalysts. The gold and silicon form a liquid-solid interface that enhances the absorption of silicon and the formation of silicon nanowire under the gold nanoparticles. This growth method is called the vapor-liquid-solid method.
Li says training the students in the fabrication techniques can take up to seven months, but it is well worth the effort. His PhD students have been able to immediately go into research and development at large technology companies after graduation.
Li’s students work in two different labs on campus. In the Microelectronics Fabrication Laboratory, which Li manages along with Volgenau colleague Rao Mulpuri, there are facilities for processing solid state materials and fabricating microelectronic devices. The other lab, the Semiconductor Characterization Lab, has equipment for measuring the output of semiconductor devices, such as metal-oxide-semiconductor field effect transistors and solar cells. This lab is managed by Li and Ioannou and also provides vacuum storage chambers for the graduate students’ projects.
In addition to his ongoing research on nanowire logic and memory devices, Li is looking toward the next important topic in nanoelectronics. With the assistance of one of his graduate students, Li is exploring the field of spintronics, an emerging technology that exploits the electron’s spin, instead of charge, for information storage and processing.
Spintronics devices will have high energy efficiency and fast processing speeds.
“[This research] is going to take a lot of time and effort,” he says, “but it will completely change current technology if successful.”