Massive-scale molecular dynamics of ion-irradiated III-V compound semiconductors at the onset of nanopatterning

Large-scale atomistic computational modeling is critical to developing a fundamental understanding of the driving mechanisms behind ion beam-induced pattern formation on III-V compound semiconductor surfaces. Existing theoretical efforts fail to account for the existence of compositional depth profiles, as observed in experiments, which evolve over time and contribute to the development of lateral compositional instabilities. Another critical knowledge gap in existing theories is the a priori selection of ion-driven compositional mechanisms, so that different models predict the same surface morphology for radically different surface compositions. Atomistic simulations can elucidate key ion-induced compositional mechanisms leading to nonequilibrium compositional depth profiles. Such simulations must be performed on large length scales to enable connection of depth compositional mechanisms to lateral instabilities which ultimately drive patterning. To address these essential knowledge gaps, a 100 × 100 nm2 GaSb surface was constructed for molecular dynamics simulations with altered composition based on experimental data at the pattern threshold fluence. The surface was then bombarded with 500 eV Kr+ ions to a differential fluence of 8.4 × 1013 cm−2. In Ga-enriched (Sb-enriched) regions, clusters of Ga (Sb) are observed to form. While the Ga clusters remain amorphous, Sb clusters rapidly crystallize. The crystallization is not directly ion-induced but rather an intrinsic material behavior of pure Sb in response to the ion-induced compositional instability in the surface. To elucidate the ion-driven compositional changes leading to this state, 25 × 25 nm2 “pristine” 50-50 GaSb crystalline surfaces were irradiated with 500 eV Ne+, Ar+, and Kr+ to fluences on the order of 1016 cm−2. The ion-induced displacement cascade tends to lead to formation of Sb nanoclusters, while Ga rarely forms clusters but readily bonds to the Sb clusters. While initially the clustering behavior is the same for all ion species, for continued Kr+ irradiation the clusters become less prevalent within the material surface.