Molecular dynamics simulations of the sputtering of boron and boron oxide surfaces

Abstract

We employ classical molecular dynamics (MD) simulations to study processes governed by particle-surface interactions. The interatomic potential energy functions are described by a neural network potential (NNP) trained on an extensive set of density functional theory (DFT) calculations in a semi-iterative fashion. Potential construction and simulation set up follow the Behler-Parrinello approach. As a specific example we investigate sputtering, reflection, and adsorption phenomena occurring on boron and boron oxide surfaces under the impact of deuterium atoms, systems that reflect recent developments in materials science. Besides the frequent use of boron surfaces as an oxygen-gathering material in technical applications, boron-based compounds will be used in future fusion devices. Understanding their interaction with energetic plasma particles is essential, yet their stability and sputtering behavior at the atomic level have remained largely unexplored. From our simulations, we analyzed sputtering yields, adsorption, and reflection events on both boron and boron oxide surfaces. The production simulations included about 750 atoms and covered a range of incident energies and impact angles. Increasing the deuterium impact energy generally leads to an increase in sputtering yields but with distinct energy-dependent trends. The sputtering yield of boron from boron oxide surfaces remains significantly lower than from surfaces of pristine boron. We use an analytical approach to estimate an effective surface binding energy. The work presented here, especially the various energy- and angle dependent rates, can be used to create a parametric model of the B and B₂O₃ surfaces under the impact of hot particles.

Fig. 1. Correlation between DFT-calculated and NNP-predicted atomic energies (left) and forces (right) for the boron–deuterium dataset, accompanied by histograms illustrating the distribution of their values. The structures shown below and at the bottom scatter points provide representative examples corresponding to the distributions presented above.

Fig. 2. Simulation cells used in the non-cumulative MD simulations. Left: B(100); right: B₂O₃(001).

Fig. 3. Sputtering yields of boron from a boron surface under bombardment by deuterium (a) and boron atoms (b). Triangles represent our NNP simulations. 0° means perpendicular impact.

Fig. 4. Sputtering yields of boron atoms (a) and oxygen atoms (b). For D colored triangles indicate different incident angles. The Eckstein curve given for reference shows yields for pure boron surfaces only.

Fig. 5. Deuterium atoms after collision with a boron surface: probabilities of reflection (left), adsorption (middle), and retention (right).

Fig. 6. Energy distributions of sputtered boron atoms. The shaded areas represent the normalized yield. The solid lines are lognormal distributions fitted to the data.

Fig. 7. Sputtering yields of B, O and B–O from B₂O₃ surfaces under deuterium bombardment.