Nanotinous layers of semiconductors

A new method to make nanotinous layers of semiconductors fit together has resulted in both scientific discovery and development of a new type of power electronics transistors. The result, which has attracted much attention, is published in Applied Physics Letters.

Photo Courtesy: An important part of the work has been done in one of the world’s sharpest electron microscopes Arwen, at Linköping University. Photo by Magnus Johansson

The company is the result of a close collaboration between researchers at Linköping University and the company SweGaN, which is a spin-off from research in materials physics at LiU. The company manufactures custom electronics components in gallium nitride.

Can handle high effects in electric vehicles

Gallium nitride, GaN, is a semiconductor material used for low-power LEDs, but can also be of great importance in other applications, such as transistors, since the material can withstand higher temperatures and currents than many other semiconductor materials. These are important features for the development of electronics of the future, not least for electric vehicles.

A vapor of gallium nitride is allowed to condense on a wafer of silicon carbide, forming a thin coating. The method by which one crystalline material is grown on another is called epitaxy. The method is often used in the semiconductor industry because it offers great opportunities to control both the crystal structure and the chemical composition of the nanometer thick film. The combination of gallium nitride, GaN, and silicon carbide, SiC, both capable of high electric fields, make the circuits suitable for applications where high power is needed.

However, there is poor matching in the surfaces between the two crystalline materials, gallium nitride and silicon carbide. The atoms fall obliquely to each other and destroy the transistor. This has been solved by research and then by industry by adding an even thinner layer of aluminum nitride between the other two layers.

By chance, the developers at Swegan discovered that their transistors were capable of significantly higher field strengths than they had expected, but at first did not understand why. The answer was found at the atomic level and in a couple of critical intermediate surfaces inside the components.

Transmorphic epitaxy

In an article in Applied Physics Letters, researchers at LiU and Swegan now, with LiU researchers Lars Hultman and Jun Lu at the forefront, present an explanation of the phenomenon and a method that provides transistors with even higher capacity to withstand high voltages.

Lars Hultman

The researchers have found a new epitaxial growth mechanism that they call transmorphic epitaxy – that is, the elongation between the different layers is gradually absorbed over a pair of atomic layers. This means that they can grow the different layers, gallium nitride and aluminum nitride on silicon carbide, in such a way that they can control at the atomic level how the different layers end up in relation to each other in the material. They have also shown in the laboratory that the material can withstand high voltages, up to 1800V. If such a voltage were to be applied to a classical component made of silicon, it would strike sparks and destroy the transistor.

– That SweGaN can already market the invention is just to congratulate, it also shows good collaboration and technology transfer between research and society. Thanks to the close contact we have with the former colleagues who are now active in the company, our research results are quickly impacted even outside the academy, says Lars Hultman.

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