We are almost there. Although the green glowing device my group built in 2001 in our laboratory—which is part of STMicroelectronics NV, the Geneva-based semiconductor giant—did not emit light that was coherent, collimated, and monochromatic (it wasn't a laser, in other words), as a light emitter, it did match the efficiency of conventional LEDs fabricated from III-V semiconductors. Since then, we've been working to make our LEDs more laserlike, and we believe an electrically powered silicon laser—with all that means for computing and communications—is finally within reach.

Silicon is a lousy light emitter. To understand why, you need to know something about its electronic energy structure. In a typical semiconductor, the regular, repeating arrangement of atoms in its crystalline form results in distinct bands of closely spaced energy levels; these are the allowable energy states of the crystal's electrons. In between those bands are gaps where electrons cannot exist. For most practical purposes, only two bands really matter: the valence band, which contains the energy levels normally occupied by electrons, and the band immediately above it [see illustration, "Mind the Gap"]. The upper band is called the conduction band, because electrons energetic enough to reach it become mobile and free to accelerate under the influence of an electric field, thereby constituting an electric current. The difference in energy between the top of the valence band and the bottom of the conduction band is known as the band gap.

Normally, electrons occupy the valence band, but give them the right dose of heat, light, or voltage, and they will jump to the conduction band, leaving behind something called a hole, which is basically the absence of an electron in the crystal lattice. However, this electron/hole pair—an exciton—is a fleeting thing; sooner or later, the electron falls back to the valence band and recombines with a hole. Because energy is always conserved, this recombination of an electron and a hole is accompanied by the emission of a particle, preferably a photon, whose energy matches the difference between the conduction band and the valence band—the bandgap energy.

Energy, however, is not the whole story. Electrons also have momentum, and when an electron/hole pair is created—or destroyed by recombination—both energy and momentum are conserved. In direct-bandgap semiconductors, such as gallium arsenide, it happens that the maximum energy in the valence band and the minimum energy in the conduction band occur at the same value of electron momentum. With these direct-bandgap materials, an electron that has been excited into the conduction band can easily fall back to the valence band through the creation of a photon whose energy exactly matches the bandgap energy. Photons lack momentum, so it's a straight swap: all the energy of the bandgap jump goes into the photon.

That is essentially how any ordinary III-V light-emitting diode works. The key component of an LED is a p-n junction, a division in a semiconductor that separates a region rich in conduction-band electrons (n-type material) from one that is rich in valence-band holes (p-type material). Applying a negative voltage to the n-type side pushes the electrons across the junction and into the holes, and vice versa. They recombine and emit photons. The ratio of generated photons to the electrons injected across the junction is called the quantum efficiency, a key measure of how well a light emitter is working. For high-performance III-V LEDs, the efficiency is around 10 percent.

A III-V diode laser is based on essentially the same principles, but it requires a few extra features. The active area around the junction where the electrons and holes recombine is made smaller, to concentrate the recombination, and the opposite ends of the recombination region are made reflective. Photons bounce between the reflective ends, colliding with atoms and stimulating the emission of additional photons that are in phase with the others in the region. In a laser, the concentrated, active region bounded by the reflective ends is known as the resonant cavity.

Things are not so simple for silicon and many other semiconductor materials. The main problem is that their crystal structure results in what's called an indirect band gap. The minimum energy in the conduction band and the maximum energy in the valence band occur at different values of electron momentum. That means an electron in the conduction band can recombine with a hole in the valence band to produce a photon only if a source of momentum of just the right magnitude, such as a vibration in the crystal lattice—a phonon—is present. The probability that a phonon with just the right amount of momentum will meet an electron/hole pair in a silicon crystal is not very good. In fact, the occurrence of a photon-generating transition in a III-V material is thousands of times more likely than that of such a transition in silicon.

So in silicon, few excited electrons generate photons, most recombinations result in heat rather than light, and the quantum efficiency is terrible.

Faced with those problems, researchers have been pushing two strategies in their quest to get light out of silicon. One scheme is based on a curious effect called quantum confinement. That occurs when an electron/hole pair is physically restricted to a small area, typically less than 30 square nanometers, or 300 times the size of a typical atom. Embedding nanocrystals of silicon within an insulating silicon dioxide layer is one way to make such quantum cages. Within a nanocrystal, the energy levels of the valence and conduction bands differ significantly from those in bulk crystal. In general, the smaller the nanocrystal, the bigger the band gap, opening up the possibility of tuning a device's optical properties by fine control of the nanocrystal's growth during the manufacturing process. Best of all, quantum confinement reduces silicon's momentum problem, increasing the probability that injected electrons will produce photons.

The other idea scientists have pursued is to sidestep silicon's bandgap problems by having another material, embedded within the silicon device, emit the light. That's done by seeding the silicon with lanthanide rare-earth-element ions, which tend to give off light when electrically excited. Some of these, those with atomic numbers from 58 (cerium) to 71 (lutetium), form a group with similar chemical characteristics. The elements' particular electronic configuration is such that if you put them in another material (silicon or silicon dioxide, say), their electronic properties are not much influenced by the host material's quirks (say, low light emission).