Light plays on the microchip

Scientists make silicon shine by using quantum effects

Here silicon shines already - but not in the diode, as it would be necessary for optoelectronics © SPL - Agency Focus
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The manufacturers of computer chips love silicon - but now it is bursting at the seams. The demand for ever higher computing capacity is enormous. Processors made of silicon would become smaller and faster if they could count on light. Scientists have now lit two different silicon diodes.

In the new issue of the science journal MaxPlanckResearch, the researchers explain why silicon chips may soon be able to juggle many more bits and bytes.

Some scientists hope that light can even remedy the lack of space on computer chips. Then photons should handle data instead of electrons and thus enable even more computing power in the smallest of spaces. Of course, the researchers do not want to do without silicon as the basis for microchips. In the meantime, the industry has mastered etching transistors made of this material too well. A research group at the Max Planck Institute for Microstructure Physics in Halle has now taught silicon to shine by exploiting quantum effects.

Energetic springboard

For a material to emit light, its electrons must first absorb energy, which can be offered to them in the form of electricity, for example. They jump from a lower level of energy into a higher - in a kind of energetic springboard. From there they fall back into the depths and in the best case release their surplus energy as light. But the electrons of silicon only arrive at the springboard by a detour. They find it difficult, so that silicon does not normally light up.

A trick to help the electrons of silicon on the jumps dominate at the Halle Max Planck Institute Peter Werner, Vadim Talalaev and their employees. For example, researchers have recently constructed a semiconducting LED. However, silicon is only one component of this: as if in a sandwich, the scientists alternately stacked nanometer-thin layers of silicon with a pinch of antimony and germanium on top of each other. The germanium-silicon superlattice lights up because the electrons of silicon in the neighboring germanium layers find matching holes into which they can fall with a trail of light. display

Electrons caught tunneling

On average, the layers of germanium can not even measure five nanometers, and those made of silicon can not measure much more so that the superlattice emits light. "The quantum effects that occur at the nanometer scale make this research interesting for us, " says Ulrich Gösele, director of the institute.

One of the effects is that the electrons of adjacent silicon layers tunnel through the separating germanium plane. They accomplish a piece of art that is only possible in quantum physics. They go through a wall. The tunnel effect first makes the silicon-germanium superlattice a useful light source. Although sandwiches from thicker layers of the two semiconductors also shine, but only very weakly, because the electrons reach the holes in the germanium only badly.

"A silicon diode with our efficiency would be enough for optoelectronics, " says Talalaev, referring among other things to the manufacturers of computer chips. "Now we're trying to make a laser out of it."

On the way to the silicon laser

Margit Zacharias, who recently headed a research group in G seles' department, is also trying to build a silicon laser. She relies on nanotechnology. Their silicon nanocrystals arrange themselves in a block of silicon dioxide into a pattern reminiscent of images of a transmission electron microscope to a Belgian waffle.

But actually the comparison fits better with a cherry cake, because the nanocrystals sit in the insulating layer of silicon dioxide like the cherries in the dough. Just a lot more ordered, so they in turn form a superlattice of quantum dots.

Oxide layers so far too thick

The size of the crystals is also crucial here. "Sometimes I imagine that the electrons and holes in the nanocrystals just can not stay out of the way, " says Zacharias. Since the distance is so small, they find each other easier and give off a flash of light when they meet. But the silicon dioxide as an insulator does not only trap the electrons and holes in the nanocrystals, it also prevents them from penetrating into the crystal from outside. This is unfavorable when electricity is to provide the energy to shine.

"The holes and electrons must tunnel through the silicon dioxide layer, " says Lorenzo Pavesi. He supports Zacharias from the University of Trento in northern Italy. "The oxide layers around the nanocrystals are still too thick." After all, Pavesi already has ideas to solve the problems. "How can we do that, of course, I can not tell, " he says: "But the odds are good."

(MaxPlanckResearch, 28.11.2007 - DLO)