Raman spectroscopy

Jul 12, 2022

Raman spectroscopy is an optical method for measuring the vibrational properties of materials based on inelastic Raman scattering of light. When a laser beam illuminates a sample, most photons are scattered elastically (Rayleigh scattering), but a small fraction exchange energy with optical phonons in the material. This inelastic process shifts the energy of the scattered photons either downward (Stokes shift) or upward (anti-Stokes shift) relative to the incident laser. The resulting Raman spectrum, plotted as scattered intensity versus frequency shift, reveals the frequencies of optical phonon modes and provides information about the crystal structure, composition, and quality of the material.

Unlike Brillouin light scattering, which probes acoustic phonons in the gigahertz range, Raman spectroscopy accesses optical phonons in the terahertz range, typically from tens to hundreds of wavenumbers (cm⁻¹). In silicon and other semiconductor nanostructures, the Raman peak position and linewidth are sensitive to strain, phonon confinement, and crystalline disorder. Phonon confinement in nanostructures causes a redshift and asymmetric broadening of the Raman peak compared to bulk silicon. Strain shifts the peak position, allowing Raman spectroscopy to map stress distributions in thin films and patterned devices. These features make Raman spectroscopy a powerful non-contact diagnostic tool for characterizing the structural quality of fabricated nanostructures.

Raman spectra of Si structures

The figure above shows examples of Raman spectra measured on various silicon structures. In phononic crystal research, Raman spectroscopy complements BLS measurements by probing a different frequency range. While BLS reveals how periodic patterning modifies the acoustic phonon dispersion at the gigahertz scale, Raman spectroscopy can verify that patterning does not significantly alter the terahertz optical phonon modes of the constituent material, as phonon interference effects at the nanoscale are frequency-dependent and eventually break down at sufficiently high frequencies.

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