Noninvasive strain mapping approach boasts nanoscale spatial resolution, no sample prep

January 12, 2009--Researchers at CIC nanoGUNE (San Sebastian, Spain) and the Max Planck Institutes of Biochemistry and Plasma Physics (Munich, Germany) say they have produced nanoscale-resolved infrared maps of strain fields in semiconductors non-invasively. Their approach, based on near-field microscopy, promises to enable new ways of analyzing mechanical properties of high-performance materials, and contact-free mapping of local conductivity in strain-engineered electronic devices.

January 12, 2008--Researchers at CIC nanoGUNE (San Sebastian, Spain) and the Max Planck Institutes of Biochemistry and Plasma Physics (Munich, Germany) say they have accomplished non-invasive, nanoscale-resolved infrared mapping of strain fields in semiconductors. Their approach, based on near-field microscopy, is described in Nature Nanotechnology. It promises to enable new ways of analyzing mechanical properties of high-performance materials, and contact-free mapping of local conductivity in strain-engineered electronic devices.

Visualizing strain at length scales below 100 nm is key because strain determines the mechanical and electrical properties of high-performance ceramics and electronic devices, respectively. But accomplishing this non-invasively has been a challenge.

A promising route for highly sensitive and non-invasive mapping of nanoscale material properties is scattering-type Scanning Near-field Optical Microscopy (s-SNOM). Part of the team had pioneered this technique over the last decade, enabling chemical recognition of nanostructures and mapping of local conductivity in industrial semiconductor nanodevices. The technique makes use of extreme light concentration at the sharp tip of an Atomic Force Microscope (AFM), yielding nanoscale resolved images at visible, infrared and terahertz frequencies. The s-SNOM thus breaks the diffraction barrier throughout the electromagnetic spectrum and with its 20 nm resolving power matches the needs of modern nanoscience and technology.

Evidence that the microscopy technique is capable of mapping local strain and cracks of nanoscale dimensions was produced by pressing a sharp diamond tip into the surface of a Silicon Carbide crystal. With the near-field microscope the researchers were able to visualize the nanoscopic strain field around the depression and the generation of nanocracks.

"Compared to other methods such as electron microscopy, our technique offers the advantage of non-invasive imaging without the need of special sample preparation" says Andreas Huber who performed the experiments within his Ph.D. project. "Specific applications of technological interest could be the detection of nanocracks before they reach critical dimensions, e.g. in ceramics or Micro-Electro-Mechanical Systems (MEMS), and the study of crack propagation", says Alexander Ziegler.

The researchers also demonstrated that s-SNOM offers the intriguing possibility of mapping free-carrier properties such as density and mobility in strained silicon. By controlled straining of silicon, the properties of the free carriers can be designed, which is essential to further shrink and speed-up future computer chips. For both development and quality control, the quantitative and reliable mapping of the carrier mobility is strongly demanded but hitherto no tool has been available. "Our results thus promise interesting applications of s-SNOM in semiconductor science and technology such as the quantitative analysis of the local carrier properties in strain-engineered electronic nanodevices" says Rainer Hillenbrand, leader of the Nano-Photonics Group at MPI and the Nanooptics Laboratory at nanoGUNE.

For more information, see the MPI Nano-Photonics Group's website, and read the paper, Infrared nanoscopy of strained semiconductors, in Nature Nanotechnology.

Posted by Barbara G. Goode, barbarag@pennwell.com.

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