Visualization of localized strain of a crystalline thin layer at the nanoscale by tip-enhanced Raman spectroscopy and microscopy

Since the carrier mobility in a strained silicon thin layer is enhanced compared to unstrained layers, strained silicon is finding tremendous attention as a promising material for ultralarge integrated electronic circuits assembled on one substrate. These strained substrates, however, suffer from nanoscale fluctuation of strain distribution, which can strongly affect the performance of devices. Raman spectroscopy is a powerful tool because the optical phonons in the Raman spectra are strongly influenced by strain. However, Raman efficiency of a thin layer is extremely small and is often eclipsed under the Raman scattering of the underlying buffer substrates. Also, the spatial resolution is restricted by the diffraction limits of the probing light. Here, in this article we demonstrate the use of surface enhancement in Raman scattering to overcome both these problems. In the first step, a strained silicon thin layer was covered with a silver layer to invoke surface-enhanced Raman spectroscopy (SERS), and it was found that SERS can effectively enhance the Raman signal originating from the strained silicon layer, so that it stands distinctly apart from the background signal originating from the buffer layer. In the next step, we demonstrate the utilization of the same mechanism for a point surface enhancement, rather than a large surface enhancement. This is done by utilizing a silver-coated sharp tip, just like SERS, but only from the sample region very close to the tip apex. This technique, known as tip-enhanced Raman spectroscopy (TERS), provides nanometric resolution in our measurement. We observed localized strains by utilizing TERS. The TERS spectra revealed clear nanoscale variation in the Raman wavenumber. Micro-Raman measurements, however, show only uniform features because of the averaging effect due to the diffraction limit of light. For further improvement of TERS on silicon materials, we discuss the utilization of shorter wavelength, specialized tip, tip-pressure effect, and depolarization configurations. Copyright (c) 2007 John Wiley & Sons, Ltd.