Optoelectronics

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.

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)

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