IV.21 - Silicon LEDs?

Light emission from silicon is quite a challenging prospect, since silicon is an indirect bandgap material (i.e. when an electron crosses the bandgap and emits a photon, it must change its energy level, obviously, as well as its momentum. A direct bandgap material does not require a change in momentum. This means that photons are much more likely to be emitted in a direct bandgap material.)

However, people are founding ways around this problem. The following text is from the UK's Defense Research Agency web site:

• Four years ago the DRA announced the invention of a special form of highly porous silicon that emitted visible light with high efficiency at room temperature when irradiated with ultraviolet light. This invention attracted worldwide excitement. It opened up the possibility of a revolutionary new technology, silicon opto-microelectronics, combining the computational power of silicon integrated circuits with the cormmunication power of light. At present optical communication and opto-electronics cannot be done with silicon; a different group of semiconductors is required, the III-V's, and this adds a lot of complexity and cost. Being able to do everything in silicon would be a giant breakthrough.

  The material which DRA invented four years ago emitted visible light when ultraviolet light was shone on to it. The next step has been to achieve electroluminescence - the emission of lights when an electrical current is passed through the material.

  The DRA scientists in Electronics Sector had already achieved electroluminescence with a useful 0.1% efficiency using liquid electrolyte as a contact to the porous silicon. But to make a silicon opto-microelectronic circuit requires an all-solid device. This has now been achieved with an electroluminescence efficiency comparable to that obtained previously with the liquid contacts. The team is now working on improving the all-solid device and demonstrating that it can be integrated on to silicon circuitry.