Visualizing phonon anharmonicity and confinement in nano- and non-crystalline germanium using Raman spectro-microscopy

Abstract

The effect of laser flux and temperature on the semiconducting properties of low-dimensional semiconductors helps to achieve innovative technological progress. This study provides a comprehensive analysis and understanding of the thermal response on the phonons present in amorphous germanium (a-Ge) and nano-crystalline germanium (n-Ge) through Raman spectroscopy and Raman microscopy. The a-Ge film was prepared using sputtering method and the n-Ge sample was obtained using the laser annealing process. Raman spectra and thermal Raman map from n-Ge show expected softening of the (confined) phonon modes and peak broadening with increasing temperature which is consistent with anharmonic theory revealing the presence of two- and three-phonon decay process therein. Two-phonon decay dominates at low temperatures, while three-phonon decay prevails at elevated temperatures. In the case of a-Ge, phonon softening is observed due to thermal expansion and anharmonicity present, but the temperature-dependent behavior of width (phonon lifetime) deviates significantly from the predictions of Balkanski’s theory due to the presence of disorder in the amorphous phase. Further the line width variation is irregular and random due to multiple phonon participation and non-compliance of momentum selection rule (i.e., they have poorly defined momentum) with different lifetimes in the Raman scattering process. © The Author(s), under exclusive licence to EDP Sciences, SIF and Springer-Verlag GmbH Germany, part of Springer Nature 2026.