Light-Emitting Silicon: As Good as GaAs
Peter Singer, Editor-in-Chief -- Semiconductor International, 12/1/2002
Researchers have been trying to coax light out of silicon for years, and have been only modestly successful. They've shown that it's possible, but the light is of very low intensity, with little practical use. A new approach developed by STMicroelectronics , however, appears to have broken through this barrier. The new approach, which uses implanted rare-earth (RE) ions such as erbium and cerium, allows silicon-based light emitters to match the efficiency of traditional light-emitting compound semiconductor materials such as gallium arsenide.
The new technology — developed in Catania, Italy, by researchers from ST's Corporate Technology R&D organization — sets a world record for efficiency from silicon. It is based on an innovative structure in which ions of rare-earth metals such as erbium or cerium are implanted in a layer of silicon-rich oxide (SRO), which is silicon dioxide enriched with silicon nanocrystals 1-2 nm in diameter.
"We have achieved 10% external quantum efficiency, which is comparable to standard III-V LEDs," said Salvo Coffa, manager of the team responsible for the breakthrough. "Measurements of the emitted power vs. current driving the LED, on the same experimental apparatus, confirm that the efficiency is comparable. These results are two orders of magnitude better than any previous silicon-based light sources."
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| Light-emitting silicon uses cerium ions to create blue light and erbium ions to produce green. (Source: STMicroelectronics) |
The frequency of the emitted light depends on the choice of RE dopant, and ST has patented key techniques for implanting the RE ions into the silicon. "We have fabricated LEDs emitting at 1.54 µm (standard communication wavelength) using erbium incorporation, but also LEDs operating in the green (using terbium) and in the blue (using cerium)," Coffa reported.
Light is generated by the RE ions by "pumping" them in two different ways. RE ions embedded in a SiO2 matrix are pumped by hot electrons injected into the conduction band of the oxide. Also, RE ions embedded in a silicon-rich matrix are pumped by electron recombination into the silicon nanostructure, with energy transfer to RE ions sitting in the oxide but close to the nanostructures. The wider bandgap of the nanostructures (due to quantum confinement effect) inhibits the non-radiative de-excitation process that hampers room-temperature efficiency in RE-doped bulk silicon, Coffa explained.
The new device is compatible with standard processing techniques in that the only non-standard process is the rare-earth incorporation. Coffa said they have patented a simple modification in the source of a standard ion implanter that allows them to implant RE ions. The SRO oxide layer is deposited by PECVD.
According to ST, one of the first applications of the new technology is to build power control devices in which the control circuitry is electrically isolated from the power-switching transistors. Currently, electrical isolation, which is mandatory in many applications for safety reasons, can only be achieved by using external devices such as relays, transformers or discrete optocouplers, all of which involve additional cost, power consumption or bulk.
ST has patented a structure in which two circuits, built on the same chip but electrically separated from each other by SiO2, communicate via optical signals using integrated silicon light emitters and detectors. These devices will have numerous uses, including motor control, power supplies, solid-state relays and similar applications where the power circuit needs to handle much higher voltages than the control circuit. Engineering samples will be available by the end of 2002.
In the longer term, ST is investigating integrated optical data-transmission systems for use in advanced CMOS circuits where clock signals are distributed through the chip at the speed of light, as well as low-cost integrated devices for dense wavelength division multiplexing (DWDM) fiber-optic communication.
For additional information on emerging technologies, go to www.semiconductor.net/emerging.
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