Cryogenically Enhanced Laser-Induced Amorphous Phase Transitions in Crystalline Silicon

TL;DR

This study demonstrates that cryogenic temperatures significantly enhance ultrafast laser-induced amorphization of crystalline silicon, revealing new mechanisms for silicon microstructuring at low temperatures.

Contribution

It is the first detailed investigation of silicon amorphization under ultrafast laser irradiation at cryogenic temperatures, combining experimental and modeling approaches.

Findings

Amorphization is more pronounced at lower temperatures.

Cryogenic conditions alter surface morphology and melt redistribution.

Simulations show reduced phonon populations and modified absorption promote amorphous freezing.

Abstract

Amorphization of silicon is crucial to applications in photonics, microelectronics and solar cell technologies. Ultrafast lasers have been used to generate amorphous silicon from crystalline silicon using rapid nonthermal melting and solidification in room temperature. As material temperature can affect cooling rates significantly, adding temperature control in ultrafast laser modification of silicon may allow a new degree of freedom in ultrafast laser modification. In this work, we investigate the role of cryogenic temperature in governing ultrafast damage pathways via single-shot femtosecond laser irradiation of silicon from room temperature down to 24K at 1030nm. Across this temperature range, we observe a pronounced enhancement of amorphization at lower temperatures, revealed through optical microscopy, Raman spectroscopy, and Kelvin probe force microscopy (KPFM). Raman analysis identifies this ring as an amorphous surface layer, while complementary AFM and SEM imaging show temperature-dependent changes in surface morphology, including localized melt redistribution and refrozen material. To elucidate the physical origins of this behavior, we implement a carrier dependent two-temperature model (nTTM). The simulations reproduce the experimentally observed trends and indicate that reduced phonon population, modified absorption pathways, and altered lattice relaxation dynamics at cryogenic temperatures collectively promote amorphous freezing over recrystallization. This study represents the first detailed examination of silicon under ultrafast irradiation below the liquid-nitrogen regime and reveals temperature-governed mechanisms relevant for advanced silicon microstructuring.