A novel electronic component from TU Wien (Vienna) could be an important key
to the era of quantum information technology: Using a special manufacturing
process, pure germanium is bonded with aluminum in a way that atomically
sharp interfaces are created. This results in a so-called monolithic
metal-semiconductor-metal heterostructure.
This structure shows unique effects that are particularly evident at low
temperatures. The aluminum becomes superconducting—but not only that, this
property is also transferred to the adjacent germanium semiconductor and can
be specifically controlled with electric fields. This makes it excellently
suited for complex applications in quantum technology, such as processing
quantum bits. A particular advantage is that using this approach, it is not
necessary to develop completely new technologies. Instead, mature and well
established semiconductor fabrication techniqueses can be used to enable
germanium-based quantum electronics. The results have now been published in
the journal Advanced Materials.
Germanium: difficult to form high-quality contacts
"Germanium is a material which is acknowledged to 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. "However, if one intends to use it to
produce components on a nanometre scale, you run into a major problem: it is
extremely difficult to produce high-quality electrical contacts, because
even the smallest impurities at the contact points can have a major impact
on the electrical properties. We have therefore set ourselves the task of
developing a new manufacturing method that enables reliable and reproducible
contact properties."
Traveling atoms
The key to this is temperature: 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
extents: the germanium atoms move rapidly into the aluminum, whereas
aluminum hardly diffuses into the germanium at all. "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 area
in the middle of the nanowire consists of germanium, the rest has been
filled up by aluminum."
Normally, aluminum made up of tiny crystal grains, but this novel
fabrication method forms a perfect single crystal in which the aluminum
atoms are arranged in a uniform pattern. As can be seen under the
transmission electron microscope, a perfectly clean and atomically sharp
transition is formed between germanium and aluminum, with no disordered
region in between. In contrast to conventional methods where electrical
contacts are applied to a semiconductor, for example by evaporating a metal,
no oxides can form at the boundary layer.
Feasability check in Grenoble
In order to take a closer look at the properties of this monolithic
metal-semiconductor heterostructure of germanium and aluminum, Masiar
Sistani collaborated with Prof. Olivier Buisson's quantum engineering group
at the University of Grenoble. 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," reports Dr. Masiar
Sistani. "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."
This new heterostructure combines a whole range of advantages: The structure
has excellent physical properties needed for quantum technologies, such as
high carrier mobility and excellent manipulability with electric fields, and
it has the additional advantage of fitting well with already established
microelectronics technologies: Germanium is already used in current chip
architectures and the temperatures required for heterostructure formation
are compatible with mature semiconductor processing schemes. "We have
developed a structure that not only has theoretically interesting quantum
properties, but also opens up a technologically very realistic possibility
of enabling further novel and energy-saving devices," says Dr. Masiar
Sistani.
Reference:
Jovian Delaforce et al, Al–Ge–Al Nanowire Heterostructure: From Single‐Hole
Quantum Dot to Josephson Effect, Advanced Materials (2021).
DOI: 10.1002/adma.202101989
Tags:
Physics