Biaxial strain effects on adatom surface diffusion on tungsten from first principles

Abstract

Using first-principles electronic structure calculations, the energy barriers for diffusion mechanisms of adatoms on the tungsten (W) (001) and (110) surfaces under externally-applied biaxial strain fields are determined. Adatoms move either by hopping on the surface or by an exchange process with a surface atom, which is found to be completed in one step (direct exchange), or via the formation of a surface crowdion (crowdion-mediated exchange). As a result of the compact atomic stacking, hopping is found to be the major diffusion mechanism on the W(110) surface, irrespective of the surface strain state. On the other hand, the main diffusion mechanism on the less compact W(001) surface is found to be a competition between direct exchange and crowdion-mediated exchange, depending on the magnitude of the surface biaxial strain. Results of the model reveal that, if surface crowdions form, they will be highly mobile and migrate anisotropically. A microscopic explanation is presented by analyzing the charge density associated with surface crowdions. A "mechanism diagram"for atomic surface diffusion on the W(001) indicates that the diffusion direction and its rate can both be modulated by an applied biaxial strain. Migration volumes for the three mechanisms are calculated, and the significance of the results to the understanding of surface evolution under plasma or other energetic ion bombardment is highlighted.

Coexistence of two diffusion mechanisms: W on W(100)

“…4(a), which yields an activation energy of 1.60 ± 0.24 eV for an adatom exchange and a prefactor D 0 = 0.37 (×50.2 ±1 ) × 10 −4 cm 2 /s. The data agree perfectly with the activation energy of 1.55 eV for the exchange mechanism on an unreconstructed W(100) surface, obtained from our DFT calculation, and also are in good agreement with the findings of Chen and Ghoniem [15]. The activation energy for an adatom moving via exchange on the zigzag reconstructed surface is 2.05 eV for the [110] direction and 2.11 eV for the [1−10] direction.…”

“…For both surfaces, DFT confirmed that the leading mechanism is adatom exchange. The energetic sequence between exchange and hopping agrees with the previous findings of Chen and Ghoniem [15] performed on an unreconstructed surface.…”

“…However, according to Chen and Ghoniem [15], the crowdion mechanism has lower activation energy than exchange, therefore the (1 × 1) map should be observed at lower temperature than c(2 × 2), which does not agree with the obtained experimental results. The estimated value of activation energy for an adatom jump is in good agreement with the results of our DFT calculations for adatom jumping on an unreconstructed W(100) surface, which is 2.27 eV, also agrees with the value provided by Chen and Ghoniem [15]. The calculated activation energy for adatom jump on the zigzag reconstructed surface is 2.49 eV for the short bridge and 2.48 eV for the long bridge.…”

“…In their research, only the jump mechanism was taken into account. Recently, Chen and Ghoniem [15] used the more reliable VASP and found the crowdion as a leading mechanism of motion. We investigated the mechanism and energetics of a tungsten atom diffusing on a W(100) surface using FIM.…”

Machine-learning interatomic potential for radiation damage and defects in tungsten

“…In order to further test the GAP for surface properties, we carried out NEB calculations for the main migration paths of adatoms on the (1 1 0) and (1 0 0) surfaces (adatom-hopping between adjacent ground state adsorption sites, and the exchange migration as discussed in e.g. [79]). The tests revealed that while the GAP reproduces the correct stable adsorption sites, the migration paths are systematically underestimated by about 20-30% compared to the DFT values from [79].…”