20 June 2008—Researchers at the University of California, San Diego, say they’ve made an experimental integrated circuit that operates using excitons—basically energized electrons bound to positively charged counterparts called holes, which form when light hits a semiconductor. The group’s results were detailed 19 June in the online version of Science . If the technology could be commercialized, it could speed computing and communications by better integrating electronic circuits and optical data communications.
Computers process signals using electrons, but most data communication outside the computer happens via photons. Because light cannot be easily manipulated with electric fields, the light signals are usually first converted to electrical signals by a photodiode, processed in the computer, and then converted to light again using a laser and other optical components for transport—an inefficient scheme. Excitons, however, directly interact with electric fields, eliminating the need for all the conversion steps and making for potentially faster and more energy-efficient computing.
When the photon signal enters the chip, it instantly transforms into excitons. The exciton signal is processed by the circuit and automatically transforms back to photons at the output. ”You start with the photonic signal and end with the photonic signal,” says Leonid Butov, a physics professor at UC San Diego and author of the study.
The device Butov and his colleagues built consisted of a number of exciton transistors on a specialized gallium arsenide chip capable of electronically steering the exciton signal through any of three paths. The device is the first excitonic integrated circuit ever constructed. Excitons are tricky to use, because they are extremely unstable, ordinarily living only for tens of picoseconds. Butov and his group were able to extend the life of the exciton by keeping the electron and its hole separate. Their chip contains thin layers of semiconductor called quantum wells. One well confines the holes, and the other, just nanometers away, the electrons. ”Now [the exciton] lives on the order of 100 nanoseconds,” says Butov. ”A lifetime long enough to perform all the necessary operations.”
Butov had to keep the electron and its hole separate, but they still had to be close enough to travel as a pair. An exciton whose electron is not bound to its hole is a neutral particle without a built-in dipole moment, making it difficult to manipulate with voltage signals. But an exciton where the electron and hole are separate but still travel together creates a dipole—one end positively charged, the other negatively charged. Because of the dipole, Butov could control the exciton with voltage.
Pierre Petroff, a professor of materials and electrical & computer engineering at the University of California, Santa Barbara, says the study could be an important breakthrough. One problem, though, is that Butov’s chip runs only at extremely cold temperatures—about 40 Kelvin (233 C). But, if it can be made to run at higher temperatures, it would have the advantage of being extremely energy efficient and very fast, Petroff says.
Mainly because of the cold temperatures required, excitonic integrated circuits are far from being ready for commercial use. Making a chip that can function at a higher temperature requires using a different semiconductor material than gallium arsenide. In gallium arsenide, at temperatures above 40 Kelvin electrons and holes won’t stay bound together in excitons. But other semiconductors have exciton-binding energies that could allow for higher-temperature exciton ICs. Butov says the next steps are to find semiconductor materials to build a chip that runs at room temperature and also to make a more complex circuit.