Novel non-thermal Ablation Mechanics in the Laser Ablation of Silicon

TL;DR

The paper addresses how ultrafast laser irradiation induces non-thermal ablation in covalent silicon, a regime where bond weakening and transient electronic excitations alter traditional ablation pathways. It deploys an integrated framework combining the Thermal-Spike-Model with MD and an excitation-dependent MOD* potential to capture non-thermal bonding changes and carrier dynamics, enabling exploration of excitation-dependent phase behavior via thermodynamic integration. The main contributions include identifying three non-thermal ablation mechanisms (non-thermal evaporation, pre-shockwave non-thermal melting, and non-thermal void formation with spallation), demonstrating realistic-scale simulations with composed and direct approaches, and constructing preliminary electron-temperature–dependent phase diagrams that link excitation to structural instability. The work shows that MOD* yields ablation depths consistent with experiments and reveals how increasing electron temperature drives bond weakening and diamond-structure instability, providing a predictive, resource-efficient framework for covalent semiconductor laser ablation.

Abstract

We investigate the non-thermal material dynamics of strongly excited silicon during ultra-fast laser ablation. In contrast to metals, silicon shows strongly excitation-dependent interatomic bonding strengths, which gives rise to a number of unique material dynamics like non-thermal melting, Coulomb explosions and altered carrier heat conduction due to charge carrier confinement. In this study, we report novel non-thermal ablation mechanisms in the ultra-fast single shot laser ablation of silicon and perform large scale massive multi-parallel simulations on experimentally achievable length scales with atomistic resolution. For this, we model the ultra-fast carrier dynamics utilizing the Thermal-Spike-Model coupled to Molecular Dynamics simulations and include the accompanied excitation-dependent nonthermal bonding strength manipulation by application of the excitation-dependent modified Tersoff Potential. Further, we present first results on the systematic construction of the excitation-dependent phase diagram of silicon by thermodynamic integration.

Figures (8)

• Figure 1: Schematic presentation of simulation composition in two dimensions. In practice, all grids are three-dimensional. Implementation of periodic boundary conditions and communication schemes are not shown.
• Figure 2: Comparison of time resolved density histograms for $\sigma = 0.672$ J/cm$^2$ for the MOD and MOD* interaction potential.
• Figure 3: Ablation depth $x_\text{abl}$ depending on laser fluence $\sigma$. Blue triangles represent data obtained under the MOD* potential and yellow circles represent the data obtained under the MOD potential. Red squares are experimentally measured ablation depths by Zhang Zhang2013.
• Figure 4: Snapshots of the spatial density histograms at a simulation time of $t= 100$ ps after peak laser intensity using the MOD* as direct simulations (left D) on sample Si2D and composed from simulations on sample Si1D (right C) for varing laser fluences $\sigma$.
• Figure 5: Temperature-pressure phase diagrams of silicon at different electron temperatures.