A novel electronic component from TU Wien (Vienna) could play a key role in the development of quantum information technology. Through a tailored manufacturing process, pure germanium is bonded with aluminium to enable the creation of atomically sharp interfaces.
The research detailing this new process was published in Advanced Materials.
Developing the New Approach
What comes out of this is a monolithic metal-semiconductor-metal heterostructure, which shows unique effects at low temperatures. At these low temperatures, the aluminium becomes superconducting, and this property is transferred to the adjacent germanium semiconductor. This also enables it to be specifically controlled with electric fields.
These characteristics make it especially useful for complex applications in quantum technology. In particular, it can be used to process quantum bits. The approach does not require the development of entirely new fabrication technologies since existing semiconductor fabrication techniques can be used to enable germanium-based quantum electronics.
Dr. Masiar Sistani is from the Institute for Solid State Electronics at TU Wien.
“Germanium is a material which will definitely play an important role in semiconductor technology for the development of faster and more energy-efficient components,” says Dr. Sistani.
Interface between the two materials. (Image: TU Wien)
Addressing Challenges
Major problems arise if it is used to produce components on a nanometre scale. In particular, the material makes it difficult to produce high-quality electrical contacts due to the high impact of small impurities at the contact points, which can significantly alter the electrical properties.
“We have therefore set ourselves the task of developing a new manufacturing method that enables reliable and reproducible contact properties,” says Dr. Sistani.
The key to this approach is temperature. When nanometre-structured germanium and aluminium make contact and are heated, the atoms of both materials start to diffuse into the other material. However, it happens to different extents.
Germanium atoms move rapidly into the aluminium, while the latter barely diffuses at all.
“Thus, if you connect two aluminium contacts to a thin germanium nanowire and raise the temperature to 350 degrees Celsius, the germanium atoms diffuse off the edge of the nanowire. This creates empty spaces into which the aluminium can then easily penetrate,” says Dr. Sistani. “In the end, only a few nanometre area in the middle of the nanowire consists of germanium, the rest has been filled up by aluminium.”
The new fabrication method forms a single perfect crystal in which the aluminium atoms are arranged in a uniform pattern. This is different from normal aluminium, which consists of tiny crystal grains. This enables an atomically sharp transition between germanium and aluminium.
“Not only were we able to demonstrate superconductivity in pure, undoped germanium for the first time, we were also able to show that this structure can be switched between quite different operating states using electric fields. Such a germanium quantum dot device can not only be superconducting but also completely insulating, or it can behave like a Josephson transistor, an important basic element of quantum electronic circuits,” says Dr. Sistani.
Besides its theoretical applications, these novel structures could have a big impact on future quantum devices.
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