Abrupt Raman Line-Shape Evolution and Quantifying Short-Range Order Parameter: Laser-Driven Nanocrystallization in Amorphous Germanium

Abstract

Amorphous solids, categorized as disordered materials, show order in a very short range, and distance of this short range is never quantified despite its importance in understanding various electronic properties. Here, a careful analysis of the Raman spectrum obtained from an amorphous germanium (a-Ge) film, prepared using the pulsed-DC sputtering method, has been carried out not only to estimate the extent of short-range order but also to propose a method to quantify the short-range order therein. The a-Ge film was exposed to high (CW) laser power to locally crystallize, and an a-Ge to nanocrystalline Ge (n-Ge) transformation was evident through an abrupt Raman line-shape evolution. The broad Raman spectrum centered at 269 cm–1, from the as-deposited a-Ge film, is replaced by an asymmetric Raman line-shape centered at 298 cm–1 on crystallization. The latter is red-shifted and asymmetrically broadened with respect to its bulk counterpart, which ascertains the presence of n-Ge. The line-shape was analyzed within the framework of the phonon confinement model (PCM), modified PCM, and bond polarizability model (BPM) to estimate the size of nanocrystallites in the laser-annealed region. Comparative analysis reveals a strong agreement between experimental data and theoretical predictions, reinforcing the validity of these models. The laser-induced crystallization has been explained by using a nucleation and growth model and represented schematically. © 2025 Elsevier B.V., All rights reserved.