Quantitative Raman nanoprofilometry of ultrathin laser-induced amorphous layers on silicon and germanium

Quantitative determination of nanometer-scale amorphous layer thickness is essential for the characterization of semiconductor structures produced by processes such as ion irradiation, thin-film deposition, and laser irradiation. Here, we present a Raman-based approach for the quantitative determination and spatial mapping of amorphous layer thickness in crystalline silicon and germanium. Unlike most Raman methodologies that rely on the relatively weak signal of the amorphous phase, the proposed method exploits the attenuation of the strong Raman peak originating from the underlying crystalline material, providing a high signal-to-noise observable over a broad thickness range. By combining experimental Raman measurements with optical modelling based on the transfer matrix method, the attenuation factor of the crystalline Raman signal is directly correlated with amorphous layer thickness. The method is validated using amorphous silicon and germanium layers produced by femtosecond laser irradiation. Thickness values obtained from Raman measurements are validated against optical interference measurements based on Newton rings, revealing consistent thickness trends across all samples, with remaining discrepancies likely arising from the different sensitivities of the two methods to the optical constants used in the modelling. Beyond point measurements, the approach enables thickness mapping with sub-micrometer lateral resolution, as demonstrated in periodically structured surfaces fabricated by direct laser interference patterning. The influence of optical constants and Raman excitation wavelength on sensitivity and measurable thickness range is also analysed, providing practical guidelines for implementation. The proposed methodology establishes Raman spectroscopy as a quantitative tool for thickness determination and spatial profiling of amorphous layers in crystalline semiconductors, offering broad applicability across different amorphization routes and particular relevance to silicon and germanium photonic technologies based on crystalline-amorphous phase engineering.