Researchers in Germany have managed to make the old semiconductor element germanium into a superconductive material. The findings, published in the journal Physical Review Letters, have implications for nanoelectronics and the development of novel computers.
The study was partly funded by the EuroMagNET ('A coordinated approach to access, experimental development and scientific exploitation of European large infrastructures for high magnetic fields') project, which was funded with EUR 3.68 million under the 'Infrastructures' Thematic area of the Sixth Framework Programme (FP6).
Semiconductors carry electric current and are used to make electronic devices ranging from transistor radios to computer chips and solar panels. Superconductivity, first observed in 1911, is a quantum mechanical phenomenon and describes the ability of a material to carry current extremely quickly, without any electrical resistance. This usually happens at very low temperatures (slightly above -273 degrees Celsius, or 0 Kelvin) or under very high pressure.
The researchers were looking for an element that could offer predictable, reliable electronic properties. It would need to be chemically pure and have a flawless crystal structure because impurities or faults, even on the smallest scale, can have a huge impact on a material's conductivity. Because of this, special methods are currently used to grow and purify the crystal.
Silicon and germanium are known 'pure' semiconductors, or elements that can be transformed into conducting materials after foreign atoms are added to their crystal structure in a process called 'doping'. In the current research, carried out at the Dresden-Rossendorf Research Centre (FZD) in Germany, samples of germanium were doped with about six gallium atoms per 100 germanium atoms. They chose gallium because it is more soluble than boron in germanium (boron has been used with silicon in past studies).
The result was a superconducting 'doped' germanium layer about 60 nanometres thick. Follow-up experiments established that the superconductivity of germanium could be reproduced, and that the temperature at which the element starts to become superconductive can be raised.
However, the process of doping damages the lattice structure of the germanium crystal, so it needs to be repaired if it is to be of any use in manufacturing. To do this, the researchers used a flash-lamp annealing facility, which can repair the destroyed crystal lattice by rapidly heating the sample surface without disturbing the distribution of the 'dopant' atoms.
The new material has great potential, not least because it becomes superconductive at a temperature above absolute zero: the gallium-doped germanium samples became superconducting at about 0.5 Kelvin. Encouragingly, the researchers expect this temperature to increase in future collaborative experiments, which will fine-tune the ion implantation and annealing processes.
The results are surprising because germanium has not been considered as promising a material as silicon or diamond; although used in the first generation of transistors, it was soon replaced by silicon. The renewed interest in germanium has a lot to do with the ever-decreasing size of transistors and microchips: extremely thin oxide layers are needed for transistors, and silicon oxide does not work well on such a small scale. Using germanium in computer chips could offer faster processes while still being suitable for further miniaturisation in micro- and nano-electronic applications.