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02 Aug 2006
A holy grail of the silicon semiconductor industry is to develop a silicon-based laser, which is exactly what this MIT-based project intends to achieve.
MIT's Microphotonics Center, in Cambridge, Massachusetts, has this week commenced a research project into silicon-based lasers and nanophotonics, funded by the US Government's Department of Defense (DoD).
The project, initially funded with $3.6 million over a three-year term, is likely to be extended for an optional additional two years, for a total of $6 million. The multi-university research initiative (MURI) programs include research into 26 topic areas of associated basic science and engineering, including silicon-based lasers and nanophotonics.
The MIT project, titled Electrically Pumped Silicon Based Lasers for Chip-Scale Nanophotonic Systems, is led by Lionel C Kimerling, director of MIT's Materials Processing Center and Microphotonics Center. It includes collaborators from eight leading research universities: Boston, Caltech, Cornell, Lehigh, Stanford, Delaware, Rochester and MIT. The project also involves some international collaborators from the universities of Toronto and McMasters (Canada), Catania and Trento (Italy), and FOM (Netherlands).
Jürgen Michel, principal investigator at the Microphotonics Center, explained the objectives of the research to optics.org.
"People have tried unsuccessfully to get electrically pumped laser light emission from silicon for 15 to 20 years. There have been optically pumped Raman lasers based on silicon (such those as developed by Intel) but electrical pumping has not yet been achieved."
"If you could put a micron-size silicon laser on a silicon chip then there are several new things that you could do; you could connect the cores in your chip optically, increasing speed and efficiency. By the same technique you could connect to the memory section of the chip. The effect would be faster on-chip communications and lower power demands.
"You could also use the silicon-based laser to connect the chip to other devices outside. Such a laser would open a whole new area of chip design when you can use silicon based photonics. Optical devices, including lasers could be processed together with transistors on the same chip, using the same toolset."
However, the development of a silicon-based laser will not be easy. Silicon has an indirect bandgap, and so lacks the efficient band-to-band transition for light emission that make direct bandgap semiconductors such as GaAs or InP ideal laser materials.
The research partners are considering two approaches to make an electrically pumped laser on silicon. The first aims to use nanocrystalline silicon in combination with erbium to produce a 1550 nm source. This will be based in a dielectric matrix such as SiO2 or Si3N4.
Such an environment can be an efficient sensitizer for erbium, Michel explains. "We trap electron-hole pairs in the nanocrystal. Upon recombination, the energy will be efficiently transferred to an erbium ion that then will emit light at 1550 nm. A key is to use optical resonators to enhance the light emission. The big question is whether we can achieve successful resonator emission at room temperature."
The second approach is to use a germanium layer deposited on silicon as the active laser material. In this case, the germanium is modified to act as a direct bandgap semiconductor, which could create a high-power light source in the milliwatt range. "An advantage with this approach is that we could integrate the device into a fiber-optic network," he adds.
"Either way, these devices will be integrated into a CMOS process. We want to integrate these optical devices on the microchip; we want to be able to make millions of them."
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