Quantum technologies will change the world. Calculation capacities of computers, the performance of simulations and the precision of sensor systems and optics will be raised to a whole new level. This requires materials that reliably enable the desired quantum effects. “The basis of today’s microelectronics is silicon, a very powerful material platform into which quantum technologies are sought to be integrated,” says Prof. Thomas Schröder, Director of the IKZ.
A promising approach for this is silicon germanium (SiGe). This material has been used in high-frequency technology since the 1990s and for the last fifteen years more and more in silicon photonics. For the application in quantum technologies, however, the quality of the material used so far is not sufficient. The reason is that it contains various isotopes that differ to some extent in their mass and thus partly have non-compensated nuclear spins. The only crystals of sufficiently high quality are isotopically pure Si and Ge crystals made of silicon-28 and germanium-72.
The IKZ, which is proficient in growing these high-purity crystals, has now concluded a supply agreement with Dr. Eberl MBE-Komponenten GmbH for these high-potential semiconductor materials. “We have entered into this agreement, which includes strict export controls, to once again fulfil our mission of not only researching and developing new, innovative materials, but also producing them in sufficient quantities to make them available to other researchers”, emphasizes Thomas Schröder.
As the IKZ does not maintain a customer network to offer its products worldwide, Dr. Eberl MBE-Komponenten GmbH will take over this part. Isotopically pure silicon and germanium crystals are more expensive by a factor of 1000 than the “normal” elements. In addition, the company has world-leading equipment technologies at its disposal to evaporate this material on chips in a highly defined manner, thus avoiding costly material losses.
For quantum computing, the electrons in semiconductors must be able to reach a certain state: two electrons each must be quantum mechanically entangled with one another. According to the laws of quantum mechanics, this, however, will only work if they do not have the same spin. To maintain the entangled state for a sufficiently long time, no disturbances must occur in the semiconductor, for example due to isotopes with a nuclear spin. Therefore, only atoms with an even number of nucleons are suitable – such as silicon-28 and germanium-72.
For the production of isotopically pure silicon-28, atoms of the element, which in its natural form consists of approx. 92.2% silicon-28, 4.68% silicon-29 and 3.1% silicon-30, are centrifuged in huge plants and collected separately on the basis of their atomic mass. Also natural germanium is “sorted” in this way. At the IKZ, Si and Ge crystals are then grown from the high-purity material, which are subsequently purified and freed from impurities, e.g. due to process gases. “We have mastered this art because we have been researching the subject for a long time, jointly with Germany’s national metrology institute Physikalisch-Technische Bundesanstalt (PTB) for the worldwide redefinition of the kilogram. It is based on the perfectly polished isotopically pure silicon 28 sphere weighing exactly one kilogram, which we have pursued together with PTB. It can thus replace – with great public appeal – the platinum-iridium block in the Louvre in Paris which serves as the reference standard for the kilogram,” Schröder explains and laughs.
“On the basis of isotopically pure Si and Ge materials, our company can offer a comprehensive service for our customers in the field of quantum technologies worldwide,” emphasizes Eberl. The medium-sized company Dr. Eberl MBE-Komponenten GmbH, based in Weil der Stadt, near Stuttgart, specializes in the development and production of ultra-high vacuum systems (molecular beam epitaxy systems) for epitaxial material synthesis. This enables the production of semiconductors for nano- and optoelectronics, thin solar cells, quantum materials, oxide films or nanostructures, such as those made of graphene. With this company, founded in 1990, the IKZ has been cooperating in research for many years, jointly developing, specifying and patenting such systems.
