In a lab in Catania, Italy, on a fine day in May 2001, white-coated researchers connected probes to a sliver of semiconductor, turned on the current, and smiled as bright green light poured from the device. Sure, by then the world was already awash in green, blue, and purple light-emitting diodes, all of them fabricated from gallium nitride and other exotic compound semiconductors. But in that lab demo four years ago, the green glow came not from gallium nitride but from silicon. And at the time, most people in the semiconductor industry would have told you that silicon was pretty much worthless at turning electricity into light.
Despite its bad reputation in optoelectronics, silicon is arguably the most important and intensively studied material known to humankind. In the five decades since the invention of the silicon transistor, electronics and integrated circuits made from the material have comprehensively transformed the world, from the way we work and communicate to how we shop and entertain ourselves.
Silicon makes up the microprocessors, memory, and other chips and devices that constitute more than 85 percent of semiconductors sold--worth US $213 billion last year worldwide. But less than $14 billion was spent on optoelectronics, including the lasers that drive data through the optical fibers that crisscross the planet and the countless LEDs that flash from video billboards and on streetlights that tell you when it's safe to cross an intersection.
Silicon's absence from critical optical applications has long bothered semiconductor specialists. If photons could be easily coaxed from silicon, we could do marvelous things. Imagine plugging your office PC into an optical-fiber local area network and pulling files from a distant server at tens of gigabits per second--enormous, high-definition video files popping onto the screen instantaneously. Optical fibers linking the microchips within a PC would accelerate its computing speed as bandwidth bottlenecks from its motherboard's copper wiring disappeared.
The key to that vision is the fabrication of efficient, electrically driven light sources that work at room temperature and are produced using materials and processes compatible with the manufacturing methods currently used to make ordinary silicon memory and microprocessor chips.
Fiber-optic links are now reserved mostly for long-distance telecommunications. But with huge multimedia files hopping from computer to computer, that kind of bandwidth is increasingly needed everywhere, from local networks right down to the links between chips inside computers. And even in the long-distance links, the benefits of fully integrated optics and electronics would be enormous.
At either end of a fiber-optic link are electronics that route the data down the right path and allow countless conversations and data channels to occupy a single line. With lasers built right into the silicon, the electronics could be more closely and efficiently integrated and could cost a lot less.
Compared with the optoelectronics that drive data across continents, the silicon systems that channel, distribute, and store the torrents of bits are cheap. That's because the worldwide microelectronics industry has cumulatively invested trillions of dollars in building up an industrial infrastructure devoted to designing and manufacturing silicon-based microelectronics in high volumes at low cost. The cost savings go right down to the level of raw materials.
Silicon is one of the most common elements on earth, and a silicon wafer, essentially made from sand, costs just pennies per square centimeter. But lasers and LEDs are made of exotic substances called III-V semiconductors--from their columns on the periodic table of the elements. These materials, which include gallium arsenide and indium phosphide, cost anywhere from 30 to 200 times as much as silicon. What's worse, much of the manufacturing infrastructure and knowledge about how to make integrated circuits in silicon is useless for making chips from III-V semiconductors.
To unite the worlds of microprocessors and lasers, we need cheap, integrated optoelectronics made from silicon. And the heart of such optoelectronics will be a laser. Although a silicon laser was always considered a long shot, academic institutions, research labs, and semiconductor companies have spent millions of dollars in pursuit of it. And now, at last, some are close. Intel Corp., in Santa Clara, Calif., for example, announced in January that it had found a way to power a silicon-based laser with a conventional one. The technique allows engineers to integrate the silicon laser on the same chip with such standard computing fare as logic circuits and memory cells, as well as critical optical components such as the modulators that encode electronic bits onto the light beam. But the scheme does not eliminate the rather expensive III-V semiconductor laser; it just makes it cheaper to use. To be rid of the costly III-V compounds altogether, you'd need a silicon chip that turns electricity directly into laser light.