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Acoustic Waves Allow LED "nanoseismology"

The piezoelectric response of a GaN film to terahertz radiation can give simpler measurements of film thickness, according to results from Lawrence Livermore National Laboratory.

US researchers have mimicked the operation of certain kinds of microphones with GaN and terahertz radiation in a method that could speed up and simplify LED metrology.

In microphones, the pressure of acoustic waves on material with different piezoelectric responses generates current and, says Michael Armstrong of Lawrence Livermore National Laboratory, electromagnetic radiation as well.

Terahertz frequency radiation "“ the highest-frequency acoustic wave possible "“ causes a similar effect, and in doing so can retrieve information about the piezoelectric material's structure.

In a paper published in Nature Physics on March 15, the Livermore team focused terahertz radiation on a GaN/AlN system that then emits another terahertz signal. The delay between the two signals depends upon the thickness of the heterostructure, and hence the method might eventually benefit the semiconductor industry, and in particular LED manufacturers.

“Methods for determining thin-film thicknesses, such as X-ray and optical ellipsometry, have some limitations," Armstrong told compoundsemiconductor.net. He points out that the ease of using ellipsometric metrology can vary given the specific film structure.

“Use of our technique for thickness measurements has the potential to be faster, less expensive and it will work on an arbitrary structure, although this will require some more technology development." Armstrong does concede, however, that the materials to be measured must have a piezoelectric response.

Armstrong and his colleagues demonstrated their “nanoseismological" approach for the first time using a specially-produced epistructure from GaN-on-silicon experts Nitronex.

The Durham, North Carolina, company grew the structure on an atomically flat silicon (111) substrate. Nitronex deposited a proprietary buffer layer on top of this, followed by a 400 nm GaN layer, a 10 nm AlN layer, another 1 µm GaN layer and various thicknesses of aluminum layers.

Thanks to the combination of reasonable piezoelectric response and index of refraction at terahertz frequencies, GaN is particularly well placed to benefit from nanoseismology.

As well as the potential to use this approach for materials characterization, the Livermore team hopes to exploit the system's terahertz frequency emission.

“We hope to generate terahertz radiation without using an ultrafast laser to create a terahertz acoustic wave, which could be a compact and practical device," Armstrong said.

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