Optoelectronics
Peter Singer -- Semiconductor International, 7/1/2003
Electrons are fast, but photons are even faster, moving as they do at the speed of light. This speed, coupled with the huge amounts of data that can be carried by lightwaves — about an order of magnitude greater than electrical signals — make light desirable for lots of applications, none more obvious than high-speed, broadband communication. This is one of the most promising applications of optoelectronics (or photonics, if you prefer). Data is converted to light signals and transmitted vast distances through fiber-optic cables. Upon arrival at its destination, it is then converted back into electrical signals that are processed by conventional electronics. The challenge lies in the proverbial "last mile" to the home or most offices, where it is not yet cost-effective to install a fiber-optic cable; instead, signals are transmitted over traditional copper wires.
Since silicon is very poor at generating light and is not particularly sensitive to light, most semiconductor devices that deal with light are based on III-V materials, such as gallium arsenide (GaAs). About the only exception to this are military devices such as night-vision goggles that are based on II-VI materials, including cadmium telluride (CdTe). Scientists are working on ways to generate and detect light using silicon, and recent results look promising. A new approach developed by STMicroelectronics, which uses implanted rare-earth ions such as erbium and cerium, allows silicon-based light emitters to match the efficiency of traditional light-emitting compound semiconductor materials such as GaAs. The new technology, announced late last year, sets a world record for efficiency from silicon. The rare-earth metals are implanted in a layer of silicon-rich oxide (SRO), which is SiO2 enriched with silicon nanocrystals of 1-2 nm in diameter.
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Recently, scientists were able to generate light from silicon with the same efficiency as GaAs. Cerium ions are used to create blue light. (Source: STMicroelectronics)
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In broadband communication applications, semiconductor lasers produce light on one end, while photodetectors convert the data back to electrical signals on the other. It's desirable to keep the signal in light form as long as possible, since it is so efficient. Various optoelectronic devices have been developed to manipulate lightwave data, including DWDMs (dense wavelength division multiplexers) and optical switches and routers. Such optical networking products are generically known as planar lightwave circuits (PLCs), but they also include optical MEMS (microelectromechanical systems) that employ tiny mirrors to route optical signals. As in ICs, the goal in PLCs is to integrate more functionality in a single device. Research in this area, however, has come to a standstill because of dismal market conditions. The greatest challenge with these small devices is really in packaging, particularly when it comes to properly lining up the fiber-optic cable with the package.
Photons are also poised to make their way onto the chip. At first, it's likely they will be used primarily for clocking on high-speed microprocessors. Here, the light will be generated off the chip and then piped on and distributed throughout the chip with optical waveguides made of silicon and SiO2. Eventually, it's possible that the light will be generated on-chip using semiconductor lasers, and detected using III-V photodetectors. Although this would appear to require the marriage of silicon and III-Vs, that doesn't seem as impossible as it once did, thanks to recent advancement in GaAs-on-silicon.
Of course, while broadband and on-chip communication are the most intriguing applications of optoelectronic/photonic devices, it's important to realize that by far the greatest percentage of devices actually sold are fairly simple things such as light-emitting diodes (LEDs). The exciting news here is that high-brightness LEDs have certainly reached a critical mass-market stage with booming applications in traffic lights, signage, automobiles, very large video screens, etc. This sector has enjoyed robust growth of more than 50% per year over the past several years. The advantages of LEDs in traffic lights have been very well documented, and the momentum for switching to LED traffic lights is so great that there is a possibility of lawmakers making it illegal to use an incandescent bulb in a traffic signal. In the case of very large video screens such as those found in sports arenas, LED screens are replacing conventional CRTs in large part because of superior resolution. The past two years have seen enormous capacity expansion for LED plants all over the world, but particularly in Asia. As prices tumble, we are sure to see even more LEDs in our day-to-day lives. Recently, white LEDs have started appearing in consumer products and will continue to get significant attention in R&D. Of course, the Holy Grail is for LEDs to challenge the stranglehold of incandescent and fluorescent lamps in the general lighting area.1
III-V Epi Wafers: Why They're so Important
03/13/2010When Electrons and Photons Converge
03/13/2010Can We Afford High-k for III-Vs?
03/13/2010GaAs-on-Silicon, Finally!
09/30/2001
