Raman spectroscopy is a powerful analytical technique used to study vibrational, rotational, and other low-frequency modes in molecules and materials. It relies on the inelastic scattering of monochromatic light, typically from a laser source, to provide detailed information about molecular structure, chemical composition, and crystallinity. The technique is widely applied in chemistry, materials science, and semiconductor research due to its non-destructive nature and high specificity.

The theoretical foundation of Raman spectroscopy is rooted in the interaction of light with matter. When a photon interacts with a molecule, most of the scattered light is elastically scattered, known as Rayleigh scattering, where the energy of the photon remains unchanged. However, a small fraction of the scattered light undergoes inelastic scattering, resulting in a shift in energy. This shift corresponds to the vibrational or rotational energy levels of the molecule, producing the Raman effect. The energy difference between the incident and scattered photons is called the Raman shift, measured in wavenumbers (cm⁻¹).

Two types of Raman scattering are observed: Stokes and anti-Stokes scattering. Stokes scattering occurs when the molecule absorbs energy from the photon, transitioning to a higher vibrational state. The scattered photon has lower energy than the incident photon, resulting in a positive Raman shift. Anti-Stokes scattering occurs when the molecule is already in an excited vibrational state and transfers energy to the photon, resulting in a higher-energy scattered photon and a negative Raman shift. At room temperature, Stokes scattering is more intense than anti-Stokes scattering because most molecules reside in the ground vibrational state.

The selection rules governing Raman activity are derived from quantum mechanics. A vibrational mode is Raman-active if it induces a change in the polarizability of the molecule during vibration. This contrasts with infrared (IR) spectroscopy, where a mode is active if it changes the dipole moment. The polarizability tensor describes how the electron cloud of a molecule distorts under an applied electric field. Symmetric vibrations often produce strong Raman signals, while asymmetric vibrations may be weak or inactive.

Polarization effects play a crucial role in Raman spectroscopy. The intensity of Raman scattering depends on the orientation of the molecule relative to the incident laser polarization. By analyzing the depolarization ratio, which compares the scattered light intensity parallel and perpendicular to the incident polarization, information about molecular symmetry can be obtained. For example, totally symmetric vibrations exhibit low depolarization ratios, while non-totally symmetric vibrations show higher ratios.

The instrumentation for Raman spectroscopy consists of several key components. A laser provides the monochromatic light source, with common wavelengths including 532 nm, 785 nm, and 1064 nm. The choice of wavelength depends on the sample properties; shorter wavelengths offer higher scattering efficiency but may cause fluorescence, while longer wavelengths reduce fluorescence but require more sensitive detectors. The laser beam is focused onto the sample using a microscope objective or lens, and the scattered light is collected and directed into a spectrometer.

The spectrometer disperses the scattered light into its constituent wavelengths. Modern Raman systems use diffraction gratings with high groove densities to achieve fine spectral resolution. The dispersed light is then detected by a charge-coupled device (CCD) or an InGaAs array detector, depending on the wavelength range. CCD detectors are sensitive in the visible to near-infrared range, while InGaAs detectors extend into the short-wave infrared. To minimize stray light and improve signal-to-noise ratio, notch or edge filters are used to block the intense Rayleigh scattering.

Spontaneous Raman spectroscopy, the conventional form of the technique, measures naturally occurring inelastic scattering. The signal is inherently weak due to the low probability of Raman events, often requiring long integration times or high laser power. In contrast, coherent Raman techniques enhance the signal by exploiting nonlinear optical effects. Coherent anti-Stokes Raman spectroscopy (CARS) and stimulated Raman spectroscopy (SRS) are two prominent examples. CARS generates a strong signal at the anti-Stokes frequency through wave mixing, while SRS amplifies the Raman signal by using a second pump beam to stimulate transitions. These methods offer improved sensitivity and faster acquisition times but require more complex instrumentation.

Raman spectroscopy provides valuable insights into semiconductor materials. It can identify crystal phases, measure strain, and detect defects. For example, in silicon, the Raman peak position shifts under mechanical stress, allowing quantitative strain mapping. In compound semiconductors like gallium nitride, Raman spectra reveal phonon modes related to crystal quality and doping levels. The technique is also used to study two-dimensional materials such as graphene, where the G and 2D bands provide information about layer thickness and electronic properties.

The versatility of Raman spectroscopy makes it indispensable for material characterization. Its ability to probe vibrational modes without extensive sample preparation, combined with high spatial resolution in confocal systems, enables detailed analysis of heterogeneous samples. Advances in laser technology, detectors, and data processing continue to expand its applications, ensuring its relevance in both research and industrial settings. By understanding the fundamental principles and instrumentation, researchers can leverage Raman spectroscopy to uncover critical material properties and drive innovation in semiconductor science.