Efficient Silicon LED Demonstrated

Nadya Anscombe

Researchers at STMicroelectronics in Catania, Italy, have reported the development of a silicon LED with what they claim is a world-record emission efficiency. To make the device, Salvo Coffa and his colleagues used ions of rare-earth metals such as erbium or cerium implanted in a layer of silicon-rich oxide -- silicon dioxide enriched with silicon nanocrystals of 1 to 2 nm in diameter.

Silicon nanocrystals in a silicon dioxide host doped with rare-earth metals form the basis of an LED from STMicroelectronics. The company hopes to develop an electrically pumped silicon laser based on the technology by the end of the year.

"Our device has an internal quantum efficiency of 50 percent and an external efficiency of 10 percent," Coffa said. "These quantum efficiencies are about 100 times better than has previously been possible in silicon and are, for the first time, comparable to those obtained from GaAs and other compound semiconductors."

The frequency of the emitted light depends on the choice of rare-earth dopant -- 1.54 µm for erbium -- and STMicroelectronics has patented techniques for implanting the rare-earth ions in the silicon. The company also has patented a structure in which two circuits, built on the same chip but electrically separated from each other by insulating silicon dioxide, communicate via optical signals using integrated silicon light emitters and detectors. These devices will have numerous applications, including motor control, power supplies, solid-state relays and similar applications where the power circuit has to handle much higher voltages than the control circuit. In the longer term, ST is investigating optical data transmission systems as well as low-cost integrated devices for dense wavelength division multiplexing.

Although many applications use an LED as a light source, silicon LEDs have slow switching times, and some applications need faster modulation. Coffa is confident that building a laser can solve this.

"We hope to have an optically pumped laser by July 2003 and an electrically pumped laser by the end of 2003," he said.