What is the difference between superconductors and semiconductors

From semiconductor to superconductor

Treated correctly, the basic material of modern electronics loses its electrical resistance. Silicon becomes a superconductor

The main reason why silicon is such a useful material is its lack of conductivity. In the purest state, its crystals lack free charge carriers, so they have an insulating effect. But woe to introduce impurities of a certain kind. Doping, which is completely legal in electronics, for example with phosphorus or boron, creates regions in a very controllable way that contain selected free charge carriers (namely electrons or holes). Now all you have to do is create transitions between such islands and the transistor is born - the basic building block of all electronics.

The engineers had previously considered it to be inevitable that - like any other material at ambient temperature up to now - it is not infinitely conductive. Silicon does not become a superconductor even at the lowest temperatures, regardless of whether it is pure or doped. At least if you "contaminate" silicon as weakly as is usual in semiconductor technology.

Simply adding a few more foreign atoms to the Si crystal lattice than usual is not that easy - in and of itself, this is a positive property of the material. A team of mainly French researchers has now succeeded in doing this anyway - they have incorporated boron atoms in the single-digit percentage range in the silicon. The fact that they are now reporting on this in the current issue of the science magazine Nature (doi: 10.1038 / nature05340) is due to a surprising property of the crystals modified in this way: they become superconductors if it's cold enough.

The scientists describe the process of mixing so many boron atoms with their silicon brothers in Nature. In so-called gas immersion laser doping, a silicon wafer is first chemically absorbed by gaseous boron chloride. Then the material is bombarded several times with high-energy laser pulses. This causes the silicon to melt briefly - just long enough for boron atoms to penetrate its crystal lattice. The crystal lattice is deformed a lot as a result, but it is retained. The high concentration of holes gives the samples treated in this way metallic properties.

It gets exciting, however, when you cool the heavily boron-doped silicon. Right, up to 150 millikelvin. At these temperatures, measurements by Etienne Bustaret's scientists show that the material develops into a superconductor. The exact transition point depends on the boron concentration. The new property cannot be caused solely by the addition of boron: Elementary boron only becomes superconducting at very high pressures of around 160 gigapascals. Measurements show that boron-doped silicon is a type II superconductor (PDF file).

For practical use this is positive news because the superconductivity is retained even if the external magnetic field penetrates the conductor. However, the very low transition temperatures still prevent any practical application today. There is also no telling whether this will change in the future.

The discovery is particularly important because it shows that silicon can actually become a superconductor - Si transistors that operate without loss are therefore not unimaginable. However, you will first have to find another doping material that leads to a higher critical temperature. (Matthias Graebner)

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