12 November 2014

Aachen-based research develops first nanoscale optical analysis of GaN

The Fraunhofer Institute for Laser Technology ILT of Aachen, Germany has worked with RWTH Aachen University’s Institute of Physics (IA) to develop an analysis technology that, for the first time it is claimed, allows the structural and electronic properties of gallium nitride (GaN) and GaN composites to be studied optically at the nanometer level.

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Optical analysis on the nanoscale

The resolution of conventional optical microscopes reaches its physical limits when confronted with objects on the nanometer scale. Because of the light source employed, structures in the nanometer range cannot be brought into focus, ruling out optical analysis techniques. However, near-field microscopy can circumvent this fundamental limitation and penetrate the nanometer domain to provide an optical view. This places extremely high demands on the light source used.

Laser system for using near-field microscopy on GaN

In collaboration with fellow researchers from the Chair for Experimental Physics at RWTH Aachen University, scientists at Fraunhofer ILT have spent the past few years developing a broadband tunable laser system that is geared towards the particular requirements of semiconductor analysis.

The wavelength can be adjusted to the material under inspection, which enables the new system to investigate a wide range of materials. In contrast to the solutions available on the market to date and those employed in R&D, it is claimed, the Aachen system enables much faster spectroscopic analyses. It also opens up access to material systems that were beyond the capacities of previous systems, including GaN and GaN composites, it is reckoned.

Picture: Near-field microscope with a fragment of GaN wafer.

Using the new analysis system, last year the researchers obtained an optical 2D image showing tensions in the crystal structure of undoped GaN wafers for the first time. Computer simulations helped to quantify the exact extent of the tension. Recently the technique was also applied to a variety of doped GaN layers within complex structures. This is the first time that an optical technique has been available for studying the structural and electronic properties of GaN and GaN composites on the nanoscale, it is reckoned.

Cost-effective, precise and non-destructive

Near-field microscopy offers cost and quality benefits over standard analysis techniques, say the researchers. The structural properties of thin GaN layers are currently studied using transmission electron microscopy (TEM), but the costs incurred are extremely high due partly to the laborious sample preparation process. In contrast, near-field analysis can usually be conducted without any preparation.

Another benefit concerns secondary-ion mass spectrometry (SIMS), which is used to study the electronic properties. Although this technique can be used to determine electronic properties along an axis at the nanometer level, it is not yet possible to laterally ascertain the concentration of doping atoms at a comparable resolution. The technique also damages the samples. In contrast, near-field microscopy offers nanoscale resolution in all dimensions. It is also a completely non-destructive technique and can be implemented under normal conditions.

Potential applications

Near-field microscopy is suitable for a range of applications, say the researchers. For example, when used in close consultation with the developers of new semiconductor components, the method can help to optimize process parameters in a targeted way. The analysis also aids the understanding of physical processes from a very early stage in development, particularly at the interfaces between individual layers.

The researchers reckon that these findings can shape subsequent development stages significantly. In high-frequency and high-power electronics too, GaN is becoming increasingly common as a component due to its physical properties. Near-field microscopic analysis techniques are well suited to researching these materials, it is concluded.

Tags: GaN

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