A unique electrical component developed at TU Wien (Vienna) might be a cornerstone to the quantum information era: Pure germanium is bonded to aluminum in such a way that atomically sharp interfaces are formed using a custom manufacturing method. As a consequence, a monolithic metal-semiconductor-metal heterostructure is formed.
This structure has unusual properties, which are most noticeable at low temperatures. Not only does the aluminum become superconducting, but this feature is also transmitted to the nearby germanium semiconductor, which may be controlled selectively using electric fields. As a result, it’s well-suited to complicated quantum technology applications like processing quantum bits.
Germanium is a material which will definitely play an important role in semiconductor technology for the development of faster and more energy-efficient components.Dr. Masiar Sistani
One advantage of this technique is that it does not necessitate the development of wholly new fabrication methods. To allow germanium-based quantum electronics, well-established semiconductor manufacturing processes may be employed instead. The findings have now been published in the Advanced Materials journal.
Germanium: difficult to form high-quality contacts
“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. Masiar Sistani from the Institute for Solid State Electronics at TU Wien.
When employed to build nanoscale components, however, substantial issues arise: the material makes producing high-quality electrical connections exceedingly difficult. This is due to the enormous influence of even the tiniest contaminants at the contact sites, which modify the electrical characteristics dramatically.
“We have therefore set ourselves the task of developing a new manufacturing method that enables reliable and reproducible contact properties,” says Masiar Sistani.
When nanometre-structured germanium and aluminum are brought into contact and heated, the atoms of both materials begin to diffuse into the neighboring material, but to very different degrees: germanium atoms diffuse rapidly into the aluminum, whereas aluminum atoms barely diffuse into the germanium.
“Thus, if you connect two aluminum 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 aluminum can then easily penetrate,” explains Masiar Sistani. “In the end, only a few nanometre areas in the middle of the nanowire consists of germanium, the rest has been filled up by aluminium.”
Normally, aluminum is produced comprised of small crystal grains, but this new production process creates a flawless single crystal with a consistent pattern of aluminum atoms. A totally clean and atomically sharp transition forms between germanium and aluminum, as observed under the transmission electron microscope, with no disordered zone in between. No oxides may develop at the boundary layer, unlike in traditional procedures when electrical contacts are put to a semiconductor, such as by evaporating a metal.
Quantum transport in Grenoble
We worked with Dr. Olivier Buisson and Dr. Cécile Naud from the quantum electronics circuits department at the Néel Institute CNRS-UGA in Grenoble to investigate the characteristics of this monolithic metal-semiconductor heterostructure of germanium and aluminum at low temperatures. It turned out that the novel structure indeed has quite remarkable properties:
“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,” explains Masiar Sistani.
This novel heterostructure offers a variety of benefits: The structure offers great physical qualities for quantum technologies, such as high carrier mobility and excellent manipulability with electric fields, and it also has the benefit of being compatible with existing microelectronics technology: Germanium is already used in current chip architectures and the temperatures required for heterostructure formation are compatible with well-established semiconductor processing schemes.
The innovative structures not only offer theoretically fascinating quantum features, but they also bring up a practically feasible potential of allowing more unique and energy-saving gadgets.