Physicists make electron gas visible

Electronic structure of a boundary layer in a solid body clarified

Solid body of two oxide materials, at the boundary layer of which an electron gas has formed (green area). Physicists at the University of Würzburg have for the first time precisely determined the extent and density of the electron gas. This was achieved with the technique of X-ray-induced photoemission spectroscopy. © Götz Berner
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X-ray radiation makes hidden structures visible, such as human bones. Würzburg physicists have now for the first time also elucidated the electronic structure of a boundary layer in a solid. It is a so-called electron gas.

Why are such boundary layers being explored? "They are important for the functionality of modern electronic components such as transistors, " says Michael Sing from the Chair of Experimental Physics at the University of Würzburg. Anyone who wants to improve or redevelop such components should therefore know the properties of boundary layers very well.

Electron gas superconducting at very low temperatures

The Würzburg physicists have analyzed a very special boundary layer with colleagues from the University of Augsburg and the Swiss Paul Scherrer Institute: They applied a few layers of lanthanum aluminate to a substrate made of strontium titanate. They were interested in the area where the two materials meet.

Why these materials? Both are good insulators, but still carry electrical current when brought together. "At the boundary between them, a conductive layer forms, a so-called electron gas, which even becomes superconducting at very low temperatures, which means that the electric current is transported lossless, " explains Sing. In addition, the conductivity of the layer can be switched on and off. That makes the materials for future applications very interesting, the researchers.

Density and thickness measured

The conductive layer between the two materials was already proven in 2004. But now the scientists have determined their density and their thickness with high precision for the first time - both are decisive parameters for the electronic properties of conductive layers. display

Result: The conductive electrons occur only in a single layer of strontium titanate, directly at the inner interface to the aluminate. "With this structure, it may be possible in the future to downsize components such as computer chips even further because the electrically conductive layer is so thin - it only consists of one atomic layer, " says Sing.

Perspective: components for aggressive environments

In addition, the two materials may be suitable as an alternative to silicon, currently the most important source material for the semiconductor industry. Because components based on silicon have disadvantages according to the W rzburg physicists: At temperatures above 200 degrees Celsius and also at temperatures below freezing they do not work properly.

The situation is different with so-called oxide ceramics - this material group also includes lanthanum aluminate and strontium titanate. According to Sing, oxide ceramics can also be used in aggressive environments, such as in waste incineration plants or in outer space. In places where either very high or very low temperatures prevail.

Next goal: Analyze the functional component

The next goal of the W rzburg physicists is to analyze the electrically conductive boundary layer in a functioning component. They want to use a field effect transistor made of lanthanum aluminate and strontium titanate. From the experiments, they hope for even more knowledge about the processes that occur when switching a current in such a layered structure.

The measuring method

Their experiments describe the researchers in the journal "Physical Review Letters". They have used a modern variant of so-called X-ray induced photoemission spectroscopy. The method is based on the long-known photoelectric effect: electrons absorb X-rays, thereby absorbing a lot of energy and being accelerated. Because of their high speed, they are now capable of penetrating several atomic layers in a solid body and leaving it through its surface.

There the fast electrons are detected and their velocities are measured. This allows conclusions to be drawn as to which atom type they originate from and in which state of charge the atoms are located. Variating the radiated X-ray energy and thus the extent to which the electrons emerge from the solid, one can create an electronic and chemical depth profile and reconstruct an image of the structure under investigation, explains Sing.

(idw - University W rzburg, 02.07.2009 - DLO)