Out of the Lab: Erbium-Doped Chips?
Thus far, the showstopper in silicon integration has been the fact that bulk silicon doesn't emit light unless it is supercooled to temperatures close to absolute zero, which is hardly practicable.
Now scientists at ST's research center in Catania, Sicily, claim to have overcome this problem. In fact, they are claiming that the silicon-based devices they've made have an efficiency that matches that of gallium arsenide and indium phosphide -- the most popular materials for making lasers and LEDs (light-emitting diodes).
To put a figure on this, ST scientists are claiming an external quantum efficiency of 10 to 20 percent. That means for every electron that goes into the system, 10 to 20 percent are converted into useful photons. A range is given because the efficiency largely depends on the packaging of the device (see Scientists Try a Solar Sell).
Previous "breakthroughs" in light-emitting silicon only achieved efficiencies an order of magnitude lower than this. Last year, for example, it was widely reported that researchers at the University of Surrey in the U.K. had created a silicon LED with an external quantum efficiency of 0.2 percent (see Out of the Lab: Light From Silicon).
However, ST may be stretching the truth a little by claiming to get light out of silicon itself. What it's actually done is produce a layer of silicon-rich oxide (SRO), which it has doped with erbium ions. The result is a material similar to erbium-doped fiber, which is compatible with standard silicon processing techniques. Light is really produced by erbium ions, not silicon.
Salvo Coffa, manager of the research team responsible for the work, says that light is produced from silicon in the sense that the electrical current needed to power the device is applied to silicon. Inside the device, the electrons travel to the erbium ions, which then give up energy as photons.
It's worth pointing out that other companies -- notably those working on erbium-doped waveguide amplifiers (EDWAs) -- have also come up with ways of producing erbium-doped layers of glass on a silicon substrate. But there's one important difference: These other companies can only get electrons into the erbium by pumping it with light from another laser. ST's main breakthrough was finding a way of injecting electrons into the material simply by applying an electric current.
Glass is normally an electrical insulator -- hence the need for optical pumping. However, silicon-rich oxide does conduct electricity. As the name suggests, SRO is basically glass that contains more silicon than is expected from the chemical formula SiO2. It consists of small agglomerates of silicon inside a matrix of glass, which conducts electricity by a process called Fowler-Nordheim tunneling, an explanation of which is a bit beyond the scope of this article (and the author -- so go look it up!).
The most interesting thing about all this is that ST's work could be close to commercialization. In the next couple of months, it expects to ready engineering samples of a device containing two separate electrical circuits that communicate using light. The device would be used in applications where the two circuits must be electrically isolated to prevent crosstalk, such as power control circuits. The chip integrates an LED, a waveguide, and a detector, as well as electronics.
The devices are being produced on the same pilot line that ST uses to develop new MOSFET (metal-oxide semiconductor field-effect transistor) and bipolar integrated circuits, the company says.
Beyond this initial product, key application areas are likely to be high-speed digital circuits and optoelectronics for telecommunications. In its press release, STMicroelectronics says it is investigating advanced CMOS circuits where clock signals are distributed through the chip at the speed of light, as well as low-cost integrated devices for DWDM transmission. Another intriguing possibility is the development of new types of optical amplifier that are electrically, as opposed to optically, pumped.
Coffa declined to give details of future applications, saying the industry would have to wait for product announcements. His company has applied for patents on the technology and plans to present technical papers in due course.
— Pauline Rigby, Senior Editor, Light Reading