Tag Archives: aluminium

Key to quantum electronics could be germanium-bonded aluminium

Leave a reply

I have not seen aluminum called aluminium in quite some time. (I’ve always had a fondness for that extra syllable.) I first saw notice of this work from Austria in an October 11, 2021 news item on Nanowerk,

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 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 technologies. Instead, mature and well established semiconductor fabrication techniqueses can be used to enable germanium-based quantum electronics.

An October 6, 2021 Technical University of Vienna (TU Wien) press release (also on EurekAlert but published October 12, 2021), which originated the news item, delves into the technical details and the importance of temperature,

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. Cécile Naud from the quantum electronics circuits group at Néel Institute – CNRS-UGA [Centre National de la Recherche Scientifique; Université Grenoble Alpes] 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.

Here’s a link to and a citation for the paper,

Al–Ge–Al Nanowire Heterostructure: From Single-Hole Quantum Dot to Josephson Effect by 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. Advanced Materials Volume 33, Issue 39 October 1, 2021 2101989 DOI: https://doi.org/10.1002/adma.202101989 First published [online]: 08 August 2021

This paper is behind a paywall.

How do nanoscale crystals make volcanoes explode?

Leave a reply

This research may have the answer as to why a supposedly peaceful volcano will suddenly explode violently. From a September 24, 2020 University of Bayreuth press release (also on EurekAlert),

Tiny crystals, ten thousand times thinner than a human hair, can cause explosive volcanic eruptions. This surprising connection has recently been discovered by a German-British research team led by Dr. Danilo Di Genova from the Bavarian Research Institute of Experimental Geochemistry & Geophysics (BGI) at the University of Bayreuth. The crystals increase the viscosity of the underground magma. As a result, a build-up of rising gases occurs. The continuously rising pressure finally discharges in massive eruptions. The scientists present the results of their nanogeoscientific research in the journal “Science Advances“.

“Exactly what causes the sudden and violent eruption of apparently peaceful volcanoes has always been a mystery in geology research. Nanogeoscience research has now allowed us to find an explanation. Tiny crystal grains containing mostly iron, silicon, and aluminium are the first link in a chain of cause and effect that can end in catastrophe for people living in the vicinity of a volcano. The most powerful volcanic eruption in human history was Mount Tambora in Indonesia in 1815”, says Dr. Danilo Di Genova. For the recently published study, he worked closely with scientists from the University of Bristol, the Clausthal University of Technology, and two European synchrotron radiation facilities.

Because of their diameter of a few nanometres, the crystals are also known as nanolites. Using spectroscopic and electron microscopy methods, the researchers have detected traces of these particles, invisible to the eye, in the ashes of active volcanoes. In the BGI’s laboratory, they were then able to describe these crystals and finally to demonstrate how they influence the properties of volcanic magma. The investigations focused on magma of low silicon oxide content cooling to form basalt on the earth’s surface after a volcanic eruption. Low silica magma is known for its low viscosity: It forms a thin lava that flows quickly and easily. The situation is different, however, if it contains a large number of nanolites. This makes the magma viscous – and far less permeable to gases rising from the earth’s interior. Instead of continuously escaping from the volcanic cone, the gases in the depths of the volcano become trapped in the hot magma. As a result, the magma is subjected to increasing pressure until it is finally ejected explosively from the volcano.

“Constant light plumes of smoke above a volcanic cone need not necessarily be interpreted as a sign of an imminent dangerous eruption. Conversely, however, the inactivity of apparently peaceful volcanoes can be deceptive. Rock analyses, written and archaeological sources suggest, for example, that people in the vicinity of Vesuvius were surprised by an extremely violent eruption of the volcano in 79 AD. Numerous fatalities and severe damage to buildings were the result”, says Di Genova. In his further research, the Bayreuth scientist hopes to use high-pressure facilites and computer simulation to model the geochemical processes that lead to such unexpected violent eruptions. The aim is to better understand these processes and thus also to reduce the risks for the population in the vicinity of volcanoes.

The researchers have included a nanocrystal image to illustrate their work,

Caption: A transmission electron microscopy image of a nano crystal (ca 25 nm in diameter) in a basaltic magma from Mt. Etna (Italy). The nano crystal is enriched in iron (Fe) and it was produced in a laboratory during at BGI. Credit Image: Nobuyoshi Miyajima.

Here’s a link to and a citation for the paper,

In situ observation of nanolite growth in volcanic melt: A driving force for explosive eruptions by Danilo Di Genova, Richard A. Brooker, Heidy M. Mader, James W. E. Drewitt, Alessandro Longo, Joachim Deubener, Daniel R. Neuville, Sara Fanara, Olga Shebanova, Simone Anzellini, Fabio Arzilli, Emily C. Bamber, Louis Hennet, Giuseppe La Spina and Nobuyoshi Miyajima. Science Advances DOI: 10.1126/sciadv.abb0413 Vol. 6, no. 39, eabb0413 Published: 23 Sep 2020

This paper appears to be open access.