Raman spectroscopy is a powerful, non-destructive analytical technique widely used for semiconductor characterization. It provides critical insights into phonon modes, strain distribution, crystallinity, and electronic properties through inelastic light scattering. The method is particularly valuable for studying emerging materials such as 2D semiconductors and heterostructures, where subtle structural and electronic changes significantly influence device performance.
Phonon modes are fundamental vibrational excitations in a crystal lattice, and their behavior is directly probed by Raman spectroscopy. In semiconductors, optical phonons dominate the Raman spectrum due to their strong interaction with incident light. For example, in silicon, the triply degenerate optical phonon mode at the Brillouin zone center appears as a sharp peak near 520 cm⁻¹. In compound semiconductors like GaAs, longitudinal optical (LO) and transverse optical (TO) phonons exhibit distinct peaks, with LO modes often showing stronger intensity due to Fröhlich interaction. In polar materials, the LO-TO splitting arises from the macroscopic electric field associated with longitudinal vibrations.
Strain analysis is another key application of Raman spectroscopy. Phonon frequencies are sensitive to lattice deformations, making Raman shifts an effective strain gauge. Compressive strain typically increases phonon frequencies, while tensile strain reduces them. In silicon, a biaxial compressive strain of 1 GPa can induce a Raman shift of approximately 4 cm⁻¹. For III-V semiconductors like GaN, the E₂ (high) mode shifts linearly with strain, enabling quantitative mapping of stress distributions in heterostructures and devices. Strain assessment is particularly important in epitaxial films, where lattice mismatch with substrates can lead to defects or relaxation.
Crystallinity assessment relies on the linewidth and intensity of Raman peaks. High-quality single crystals exhibit narrow, symmetric peaks, while disordered or polycrystalline materials show broadening and asymmetry. In nanocrystalline silicon, the Raman peak broadens and shifts to lower wavenumbers due to phonon confinement effects. Amorphous semiconductors, such as a-Si, display a broad feature around 480 cm⁻¹, reflecting the loss of long-range order. The crystallinity information obtained from Raman spectroscopy complements microscopy techniques by providing statistical data over larger sample areas.
Resonance Raman effects occur when the excitation laser energy matches electronic transitions in the material, greatly enhancing specific phonon modes. This effect is particularly pronounced in direct bandgap semiconductors and low-dimensional systems. For instance, in monolayer MoS₂, using a 532 nm laser (close to its excitonic transition) significantly enhances the A₁g and E₂g modes compared to off-resonance conditions. Resonance Raman spectroscopy also enables selective probing of different layers in heterostructures or regions with varying bandgaps, offering layer-specific structural and electronic information.
Polarization dependence adds another dimension to Raman analysis. The intensity of phonon modes varies with the relative orientation of the crystal axes and the polarization of incident/scattered light. In anisotropic materials like black phosphorus, the A₁g mode intensity is maximized when the laser polarization aligns with the armchair direction, while the B₂g mode is stronger along the zigzag direction. Polarized Raman measurements thus provide crystallographic orientation mapping, essential for understanding grain boundaries and anisotropic properties in 2D materials.
Applications in 2D materials highlight the versatility of Raman spectroscopy. Graphene exhibits a characteristic G band (~1580 cm⁻¹) and 2D band (~2700 cm⁻¹), where the 2D band lineshape reveals layer thickness and stacking order. In transition metal dichalcogenides (TMDCs) like WS₂, the frequency difference between E₂g and A₁g modes correlates with layer number due to interlayer coupling. Heterostructures of 2D materials, such as graphene-hBN or MoS₂-WSe₂, exhibit modified phonon modes due to interfacial interactions, enabling studies of charge transfer and strain engineering.
Raman spectroscopy also plays a crucial role in defect characterization. In hexagonal boron nitride (hBN), local defects activate otherwise Raman-inactive modes, providing insights into point defects and grain boundaries. In carbon nanotubes, the radial breathing mode (RBM) frequencies are inversely proportional to tube diameter, allowing diameter distribution analysis. Defect-induced D bands in graphene and related materials quantify disorder levels, critical for optimizing electronic and mechanical properties.
Recent advances in tip-enhanced Raman spectroscopy (TERS) and spatially resolved mapping have pushed the technique’s resolution to the nanometer scale. Hyperspectral Raman imaging enables correlative analysis of composition, strain, and defects across device structures, essential for failure analysis and process optimization in semiconductor manufacturing.
In summary, Raman spectroscopy is indispensable for semiconductor research, offering detailed information on phonon dynamics, strain, crystallinity, and defects. Its non-destructive nature, coupled with advancements in sensitivity and resolution, ensures its continued relevance in studying conventional and emerging semiconductor systems. The technique’s ability to probe electronic and vibrational properties simultaneously makes it uniquely suited for advancing materials science and device engineering.