Our present chip expertise is essentially primarily based on silicon. Solely in very particular elements a small quantity of germanium is added. However there are good causes to make use of increased germanium contents sooner or later: The compound semiconductor silicon-germanium has decisive benefits over in the present day’s silicon expertise when it comes to vitality effectivity and achievable clock frequencies.
The principle downside right here is to determine contacts between metallic and semiconductor on a nanoscale in a dependable method. That is way more tough with a excessive proportion of germanium than with silicon. The staff at TU Wien, nonetheless, along with analysis groups from Linz and Thun (Switzerland), has now proven that this downside might be solved — with contacts product of crystalline aluminium of extraordinarily top quality and a classy silicon germanium layer system. This permits completely different fascinating contact properties — particularly for optoelectronic and quantum elements.
The issue with oxygen
“Each semiconductor layer is routinely contaminated in typical processes; this merely can’t be prevented on the atomic stage,” says Masiar Sistani from the Institute for Stable State Electronics at TU Wien. At first, it’s oxygen atoms that accumulate in a short time on the floor of the supplies — an oxide layer is fashioned.
With silicon, nonetheless, this isn’t an issue: silicon at all times kinds precisely the identical type of oxide. “With germanium, nonetheless, issues are way more difficult,” explains Masiar Sistani. “On this case, there’s a complete vary of various oxides that may type. However that implies that completely different nanoelectronic gadgets can have very completely different floor compositions and due to this fact completely different digital properties.”
In case you now wish to join a metallic contact to those elements, you could have an issue: Even should you attempt very exhausting to supply all these elements in precisely the identical method, there are nonetheless inevitably large variations — and that makes the fabric advanced to deal with to be used within the semiconductor trade.
“Reproducibility is an enormous downside,” says Prof. Walter Weber, the top of the Institute for Stable State Electronics, TU Wien. “In case you use germanium-rich silicon germanium, you possibly can’t make sure that the digital part, after you’ve got put contacts on it, will actually have the traits you want.” Consequently, this materials is barely used to a restricted extent in chip manufacturing.
That may be a pity, as a result of silicon germanium would have decisive benefits: “The cost provider focus is increased, particularly constructive cost carriers, the so-called “holes,” can transfer way more effectively on this materials than in silicon. The fabric would due to this fact enable a lot increased clock frequencies with elevated vitality effectivity than our present silicon chips,” says Lukas Wind, a doctoral pupil in Walter Weber’s analysis group.
The “excellent” interface
Now, nonetheless, the analysis staff has been in a position to present how the issue might be solved: They discovered a way to create excellent interfaces between aluminium contacts and silicon germanium elements on an atomic scale. In a primary step, a layer system is produced with a skinny silicon layer and the precise materials from which the digital elements are to be made — the silicon-germanium.
By heating the construction in a managed method, a contact can now be created between the aluminium and the silicon: At round 500 levels Celsius, distinctive diffusion happens, the atoms can depart their place and begin to migrate. Silicon and germanium atoms transfer into the aluminium contact comparatively shortly, and aluminium fills the vacated area.
“The diffusion dynamics within the layer system used thus create an interface between aluminium and the silicon germanium with a particularly skinny silicon layer in between,” explains Masiar Sistani. By way of this manufacturing course of, oxygen atoms by no means have the chance to get to this atomically sharp and extremely pure interface.
“Our experiments present that these contact factors might be produced in a dependable and simply reproducible method,” says Walter Weber. “The technological methods it’s worthwhile to do that are already getting used within the chip trade in the present day. So, this isn’t only a laboratory experiment, however a course of that could possibly be used comparatively shortly within the chip trade.
The decisive benefit of the offered manufacturing course of is that high-quality contacts might be produced whatever the silicon-germanium composition. “We’re satisfied that the offered abrupt, sturdy and dependable metal-semiconductor contacts are extremely fascinating for a wide range of new nanoelectronic, optoelectronic and quantum gadgets,” says Walter Weber.
