This photo shows the transmission electron microscope the researchers used to help discover the novel epitaxial growth mechanism. Photo: Magnus Johansson.
This photo shows the transmission electron microscope the researchers used to help discover the novel epitaxial growth mechanism. Photo: Magnus Johansson.

A new method to fit together layers of semiconductors as thin as a few nanometers has resulted in not only a scientific discovery but also a novel type of transistor for high-power electronic devices. This result, reported in a paper in Applied Physics Letters, has already aroused huge interest.

The achievement is the result of a close collaboration between scientists at Linköping University in Sweden and SweGaN, a spin-off company from materials science research at Linköping University. The company manufactures tailored electronic components from gallium nitride (GaN).

GaN is a semiconductor currently used for efficient light-emitting diodes. It may, however, also be useful in other applications, such as transistors, since it can withstand higher temperatures and current strengths than many other semiconductors. These are important properties for future electronic components, not least for those used in electric vehicles.

To produce a GaN transistor, GaN vapor is allowed to condense onto a wafer of silicon carbide (SiC), forming a thin coating. This method, in which one crystalline material is grown on a substrate of another, is known as ‘epitaxy’. It is regularly used in the semiconductor industry, since it provides great freedom in determining both the crystal structure and the chemical composition of the resulting nanometer-thick film.

The combination of GaN and SiC (both of which can withstand strong electric fields) ensures that the resulting circuits are suitable for applications in which high powers are needed. But the fit at the surface between the two crystalline materials is poor. Their atoms end up mismatched with each other, which can lead the transistor to fail. This problem has been addressed by research that subsequently led to a commercial solution, in which an even thinner layer of aluminium nitride is placed between the GaN and SiC layers.

The engineers at SweGaN noticed by chance that these transistors could cope with significantly higher field strengths than they had expected, and they could not initially understand why. The answer could be found at the atomic level – in a couple of critical intermediate surfaces inside the components.

In the Applied Physics Letters paper, the scientists at Linköping University and SweGaN, led by Linköping University’s Lars Hultman and Jun Lu, present an explanation of this phenomenon, and describe a method for manufacturing transistors with an even greater ability to withstand high voltages.

The scientists have discovered a previously unknown epitaxial growth mechanism that they term ‘transmorphic epitaxial growth’, which causes the strain between the different layers to be gradually absorbed across a couple of layers of atoms. This means the scientists can now grow the GaN and aluminium nitride layers on SiC while controlling at the atomic level how the layers are related to each other. In the laboratory, they have shown that this transistor is able to withstand high voltages, up to 1800V. If such a voltage were placed across a classic silicon-based component, sparks would start flying and the transistor would be destroyed.

"We congratulate SweGaN as they start to market the invention," says Hultman. "It shows efficient collaboration and the utilization of research results in society. Due to the close contact we have with our previous colleagues who are now working for the company, our research rapidly has an impact also outside of the academic world."

This story is adapted from material from Linköping University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.