Our current chip technology is largely silicon-based. Only in very special components is a small amount of germanium added. But there are good reasons to use higher germanium contents in the future: the silicon-germanium semiconductor compound has decisive advantages over current silicon technology in terms of energy efficiency and frequencies achievable clocks.
The main problem here is to make contacts between the metal and the nanoscale semiconductor reliably. This is much more difficult with a high proportion of germanium than with silicon. The team from TU Wien, however, together with research teams from Linz and Thun (Switzerland), have now shown that this problem can be solved – with very high quality crystalline aluminum contacts and a sophisticated system of silicon-germanium layers. This allows different interesting contact properties, especially for optoelectronic and quantum components.
The oxygen problem
“Every semiconductor layer is automatically contaminated in conventional processes; this simply cannot be avoided at the atomic level,” says Masiar Sistani of the Institute for Semiconductor Electronics at TU Wien. These are above all oxygen atoms which accumulate very quickly on the surface of the materials: an oxide layer is formed.
With silicon, however, this is not a problem: silicon always forms exactly the same type of oxide. “With germanium, on the other hand, things are much more complicated”, explains Masiar Sistani. “In this case, there is a whole range of different oxides that can form. But this means that different nanoelectronic devices can have very different surface compositions and therefore different electronic properties.”
If you now want to connect a metal contact to these components, you have a problem: even if you strive to produce all these components in exactly the same way, there are still inevitably massive differences — and this makes the material complex to handle for use in the semiconductor industry.
“Reproducibility is a big problem,” says Professor Walter Weber, director of the Institute for Semiconductor Electronics, TU Wien. “If you’re using germanium-rich silicon-germanium, you can’t be sure that the electronic component, after putting contacts into it, will really have the characteristics you need.” Therefore, this material is only used to a limited extent in the production of chips.
It’s a shame, because silicon-germanium would have decisive advantages: “The concentration of charge carriers is higher, especially the positive charge carriers, the so-called ‘holes’, can move much more efficiently in this material than in silicon. The material would therefore allow much higher clock frequencies with increased power efficiency than our current silicon chips,” says Lukas Wind, PhD student in Walter Weber’s research group.
The “perfect” interface
Now, however, the research team has been able to show how the problem can be solved: they have found a method to create perfect interfaces between aluminum contacts and silicon-germanium components on an atomic scale. First, a layer system is made with a thin layer of silicon and the material from which electronic components are to be made, silicon-germanium.
By heating the structure in a controlled manner, a contact can now be created between the aluminum and the silicon: Around 500 degrees Celsius, a characteristic diffusion occurs, the atoms can leave their place and start to migrate. The silicon and germanium atoms come into contact with the aluminum relatively quickly, and the aluminum fills the vacated space.
“The diffusion dynamics in the layer system used thus creates an interface between aluminum and silicon-germanium with an extremely thin layer of silicon between the two”, explains Masiar Sistani. Thanks to this manufacturing process, the oxygen atoms never have the possibility of reaching this atomically sharp and very pure interface.
“Our experiments show that these touchpoints can be produced reliably and easily reproducibly,” says Walter Weber. “The technology systems you need to do this are already in use in the chip industry today, so it’s not just a lab experiment, but a process that could be used relatively quickly. in the chip industry.
The decisive advantage of the presented manufacturing method is that high quality contacts can be produced regardless of the silicon-germanium composition. “We are convinced that the abrupt, robust and reliable metal-semiconductor contacts presented are very interesting for a variety of new nanoelectronic, optoelectronic and quantum devices”, says Walter Weber.
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