New nanostructure could be the key to quantum electronics

Date:
October 12, 2021
Source:
Vienna University of Technology
Summary:
A novel electronic component could be an important key to the era
of quantum information technology: Using a tailored manufacturing
process, pure germanium is bonded with aluminum in a way that
atomically sharp interfaces are created.



FULL STORY ==========================================================================
A novel electronic component from TU Wien (Vienna) could be an important
key to the era of quantum information technology: Using a tailored manufacturing process, pure germanium is bonded with aluminium 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 aluminium 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 fabrication technologies. Instead, well established semiconductor fabrication
techniques 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 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. However, if it is used to produce components on
a nanometre scale, major problems arise: the material makes it extremely difficult to produce high-quality electrical contacts. This is related to
the high impact of even smallest impurities at the contact points that 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 Masiar Sistani.

Diffusing atoms The key is temperature: when nanometre-structured
germanium and aluminium are brought into contact and heated, the atoms
of both materials begin to diffuse into the neighbouring material --
but to very different extents: the germanium atoms move rapidly into
the aluminium, whereas aluminium hardly diffuses into the germanium 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," 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 aluminium."
Normally, aluminium is made up of tiny crystal grains, but this
novel fabrication method forms a perfect single crystal in which the
aluminium 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 aluminium, 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.

Quantum transport in Grenoble In order to take a closer look at the
properties of this monolithic metal- semiconductor heterostructure
of germanium and aluminium at low temperature, we collaborated with
Dr. Olivier Buisson and Dr. Ce'cile Naud from the quantum electronics
circuits group at Ne'el Institute -- CNRS-UGA in 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.

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 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 well-established semiconductor processing schemes. The novel structures not only have theoretically interesting quantum properties, but also opens up a technologically very realistic possibility of enabling further novel
and energy-saving devices.

========================================================================== Story Source: Materials provided by Vienna_University_of_Technology. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Jovian Delaforce, Masiar Sistani, Roman B. G. Kramer, Minh A. Luong,
Nicolas Roch, Walter M. Weber, Martien I. den Hertog, Eric
Robin, Cecile Naud, Alois Lugstein, Olivier Buisson. Al-Ge-Al
Nanowire Heterostructure: From Single‐Hole Quantum Dot to
Josephson Effect. Advanced Materials, 2021; 33 (39): 2101989 DOI:
10.1002/adma.202101989 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211012095024.htm

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