🔬 physics

Atomistic dynamics of early oxidation of Si upon ultra-short pulsed laser heating

This study combines reactive molecular dynamics simulations with experimental validation to demonstrate that the early oxidation of silicon under ultra-short pulsed laser heating is governed by a transition from negligible growth below the melting point to rapid, diffusion-controlled oxide formation within the transient molten silicon layer.

Original authors: Tatiana E. ITINA, Ilemona S. OMEJE, Magali Gregoire, Jean-Gabriel Mattei, Wissal Benali, Pol Sopena, David GROJO

Published 2026-09-02

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Tatiana E. ITINA, Ilemona S. OMEJE, Magali Gregoire, Jean-Gabriel Mattei, Wissal Benali, Pol Sopena, David GROJO

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Silicon is the quiet foundation of the modern world, the material that makes up the tiny chips inside our phones, computers, and solar panels. To make these devices work reliably, engineers often coat the silicon with a very thin layer of rust, known as silicon dioxide. This coating acts as a shield, protecting the delicate electronics from damage and helping them conduct electricity properly. Usually, this protective layer is grown slowly in a hot oven, a process that can take hours. However, scientists have discovered that if they blast the silicon with an incredibly fast pulse of laser light, a similar layer can form in a fraction of a second. The mystery has been how this happens so quickly. The laser heats the surface so fast that the silicon doesn't just get hot; it briefly turns into a liquid, and the question is whether this fleeting moment of melting is the key to the rapid growth of the protective coating.

A team of researchers from France has now peeled back the layers of this mystery by combining powerful computer simulations with direct observation. They wanted to understand exactly what happens to silicon atoms when they are hit by an ultra-short laser pulse and how oxygen from the air rushes in to form that protective skin. Using a sophisticated computer model that tracks the movement of individual atoms, they recreated the split-second events that occur when a laser heats a silicon surface. They found that the behavior of the silicon changes dramatically depending on whether the surface stays solid or turns into a liquid. When the temperature stays below the melting point, the oxygen atoms are like visitors stuck in a crowded room; they cannot move easily through the rigid crystal structure of the solid silicon, so almost no coating forms. The process is too slow to matter in the tiny window of time the laser is active.

The story changes completely the moment the silicon melts. The researchers observed that once the surface turns into a liquid, the oxygen atoms suddenly find a highway. In the liquid state, the silicon atoms are no longer locked in place, allowing the oxygen to dive deep into the material with incredible speed. The simulations showed that the thickness of the new oxide layer is directly tied to how deep the liquid silicon goes. If the laser creates a deeper pool of molten silicon, the oxygen travels further, and the resulting coating is thicker. This happens so fast that a layer roughly two and a half nanometers thick can form in just a few hundred picoseconds, a timeframe so short it is measured in trillionths of a second. The team also noted that if there is not enough oxygen available, the growth is limited by the supply, but once there is plenty, the speed is limited only by how fast the oxygen can swim through the molten silicon before it cools down and hardens again.

To make sure their computer models matched reality, the researchers performed real-world experiments using a deep-ultraviolet laser to treat silicon wafers. They then sliced these samples into incredibly thin sheets and examined them with a high-powered electron microscope, a tool that can see individual atoms. The images confirmed their predictions perfectly. They found a continuous layer of silicon dioxide sitting on top of a zone of amorphous silicon, which is silicon that has lost its orderly crystal structure and become disordered, much like glass. This amorphous layer sat directly on top of the solid, crystalline silicon of the original wafer. The measurements showed a protective oxide layer about 2.3 nanometers thick, a number that aligned closely with what the simulations had forecast.

The most significant takeaway from this work is that the rapid formation of this oxide layer is not a mysterious, non-thermal trick of the laser. Instead, it is a straightforward physical process driven by heat. The laser creates a brief, transient pool of liquid silicon, and it is the enhanced ability of oxygen to move through this liquid that allows the coating to grow so fast. The researchers ruled out the idea that the laser must be doing something exotic or non-thermal to force the reaction. Instead, they demonstrated that the simple act of melting the silicon, even for a fleeting moment, is sufficient to explain the rapid oxidation. This understanding provides a clear physical explanation for why these ultra-thin layers form so quickly and confirms that the depth of the molten region is the primary factor controlling how thick the protective coating becomes.

Technical Summary: Atomistic Dynamics of Early Oxidation of Si upon Ultra-Short Pulsed Laser Heating

Problem Statement
Silicon dioxide is a critical material in microelectronics, photovoltaics, and photoelectrochemistry, serving as a passivation layer, diffusion barrier, and dielectric. While conventional thermal oxidation (furnace-based) is well-understood and described by models like Deal–Grove, the mechanisms governing oxide formation under ultra-short pulsed laser heating (femtosecond regime) remain poorly understood. In these non-equilibrium conditions, characterized by ultrafast energy deposition, rapid electron excitation, and transient surface melting, experimental data indicates that ultrathin oxide layers (<25 nm) grow significantly faster than classical models predict. Existing literature lacks a comprehensive physical explanation for these "early oxidation" phenomena, particularly regarding the transition from solid-state to liquid-state transport mechanisms and the specific role of transient melting in oxide growth kinetics.

Methodology
The study employs a combined approach of reactive molecular dynamics (MD) simulations and high-resolution experimental characterization:

Computational Modeling: The authors utilized Reactive Force Field (ReaxFF) molecular dynamics simulations implemented in LAMMPS to model the interaction between silicon and oxygen under ultra-short pulsed heating.
- System Setup: A crystalline Si(100) slab was constructed with periodic boundary conditions in-plane and a free surface. Oxygen ( $O_2$ ) molecules were introduced in a vacuum gap above the surface.
- Thermal Protocol: To mimic femtosecond laser heating, the system was heated linearly from 300 K to a target maximum temperature ( $T_{max}$ ) over 5 ps (representing electron-phonon coupling time), held briefly, and then cooled linearly back to 300 K over 50 ps.
- Parameters: Simulations covered $T_{max}$ ranges of 400–3000 K and varying oxygen concentrations. The effective energy deposition depth was calculated considering carrier diffusion, extending beyond the optical penetration depth.
Experimental Validation: Complementary experiments were conducted using deep-ultraviolet (258 nm) femtosecond laser irradiation on Si(100) substrates.
- Characterization: High-Resolution Transmission Electron Microscopy (HR-TEM) and Scanning Transmission Electron Microscopy-Electron Energy Loss Spectroscopy (STEM-EELS) were used to analyze the cross-sectional morphology and chemical composition of the laser-treated regions.

Key Results
The simulations and experiments reveal two distinct oxidation regimes governed by the maximum surface temperature relative to the silicon melting point ( $T_m \approx 1687$ K):

Below Melting Point ( $T < T_m$ ): Oxidation is extremely limited. The low mobility of oxygen in crystalline silicon, combined with the short duration of the thermal transient (picoseconds), prevents significant diffusion. Consequently, only a negligible oxide layer forms.
Above Melting Point ( $T > T_m$ ): A dramatic transition occurs. The formation of a liquid silicon layer enhances oxygen diffusivity by several orders of magnitude (from $\sim 10^{-15}$ $\sim 1 0^{- 15}$ cm $^2$ $^{2}$ /s in solid Si to $\sim 10^{-5}$ $\sim 1 0^{- 5}$ cm $^2$ $^{2}$ /s in liquid Si).
- Oxide Growth: The oxide layer thickness correlates directly with the depth of the molten layer. The simulations predict the formation of a continuous oxide layer of approximately 2.5 nm within hundreds of picoseconds.
- Kinetic Transitions: The study identifies a transition from oxygen-limited oxidation (at low oxygen concentrations) to diffusion-limited oxidation (at higher concentrations). Once a continuous oxide layer forms, growth is controlled by oxygen transport within the transient molten silicon and the finite lifetime of the molten state.
- Structural Evolution: A layered morphology emerges: a top silicon oxide layer ( $\sim$ 2.5 nm), an underlying amorphous silicon (a-Si) layer ( $\sim$ 12 nm), and the crystalline substrate. This amorphization is attributed to bond rearrangement in the subsurface region where temperatures were sufficient to melt but oxygen concentration was insufficient for full oxidation.

Experimental Corroboration
The experimental results validate the computational findings. HR-TEM and STEM-EELS analysis of laser-treated samples confirmed the formation of a continuous surface oxide layer approximately 2.3 nm thick, situated directly atop an amorphous silicon region, which in turn rests on the crystalline bulk substrate. The EELS fine-structure analysis distinctly identified three phases: Si-O (oxide), a-Si, and c-Si, with minimal inter-diffusion between layers.

Significance and Claims
The authors claim that this work provides a physical explanation for the rapid formation of ultrathin oxide layers under femtosecond laser irradiation without invoking non-thermal mechanisms, plasma-assisted reactions, or extended post-irradiation oxidation.

Dominant Mechanism: The study establishes that thermally activated diffusion within the transient molten phase is the dominant mechanism governing early oxidation at temperatures below 3000 K.
Correlation: A key contribution is the established correlation between the oxide layer thickness and the depth of the transient molten layer, identifying melt depth as the primary governing parameter for early oxide formation.
Modeling Validity: The results demonstrate that classical diffusion arguments, when applied to the liquid phase with appropriate timescales, are sufficient to explain the observed oxidation depths, challenging the necessity of complex non-thermal models for this specific regime.

The work bridges the gap between atomistic simulations and experimental observations, offering a coherent framework for understanding silicon oxidation kinetics in ultrafast laser processing.

More like this