Non-adiabatic ab initio molecular dynamics of supersonic beam epitaxy of silicon carbide at room temperature

Abstract

In this work, we investigate the processes leading to the room-temperature growth of silicon carbide thin films by supersonic molecular beam epitaxy technique. We present experimental data showing that the collision of fullerene on a silicon surface induces strong chemical-physical perturbations and, for sufficient velocity, disruption of molecular bonds, and cage breaking with formation of nanostructures with different stoichiometric character. We show that in these out-ofequilibrium conditions, it is necessary to go beyond the standard implementations of density functional theory, as ab initio methods based on the Born-Oppenheimer approximation fail to capture the excited-state dynamics. In particular, we analyse the Si-C 60 collision within the non-adiabatic nuclear dynamics framework, where stochastic hops occur between adiabatic surfaces calculated with time-dependent density functional theory. This theoretical description of the C 60 impact on the Si surface is in good agreement with our experimental findings.

2D Material Armors Showing Superior Impact Strength of Few Layers

“…Notable exceptions have been reported in modeling analogous problems, in which intermediate kinetic energy regimes (around tens of eV) were used to achieve the epitaxial growth of silicon carbide [27][28][29] and graphene 30,31 via buckyball beams impacting on silicon or metallic substrates.…”

The Interaction of C60 on Si(111) 7 × 7 Studied by Supersonic Molecular Beams: Interplay between Precursor Kinetic Energy and Substrate Temperature in Surface Activated Processes

“…Experimental results of C 60 supersonic impact on Si(111) 7 × 7 at 500°C show a picture completely different from what was observed using the same deposition technique at higher and lower substrate temperatures, evidencing an unexpected mechanism of interaction between the precursors and the surface. From previous experiments (Verucchi et al, 2002(Verucchi et al, , 2012, as well as theoretical calculations (Taioli et al, 2013), one finds a clear indication of a kinetically driven growth mechanism leading to sub-or thick SiC layer formation already at room temperature, with temperature factor not playing a major role. Raising the silicon substrate temperature to 500°C should generally result in a higher silicon-carbon reactivity, with increasing carbide percentage over physisorbed fullerene.…”

“…In this regard, it is clear that DFTB is missing a comprehensive explanation of the cage break. The reason of this discrepancy was already identified and thoroughly discussed in Taioli et al (2013). In that analysis, non-adiabatic effects and, specifically, the failure of the Born-Oppenheimer approximation was found responsible for this important discrepancy between simulation and experiments to determine cage rupture KE threshold.…”

“…Peak positions do not change as the fullerene KE increases. Previous experiments (Verucchi et al, 2012;Taioli et al, 2013) on the impact of supersonic fullerene on silicon at room and higher temperatures have shown that the molecule can undergo different paths, going from physisorption with intact cages to chemisorption, cage rupture and formation of new compounds, such as SiC, whose intensity increases with the precursor KE. The main peak at higher BE, P1, reflects the presence of unperturbed and physisorbed fullerenes, while the peaks at 284.1 and 283.2 eV can be identified as related, respectively, to chemisorbed species and to the formation of SiC occurring after cage breaking.…”

“…However, classical molecular dynamics is not suitable for reactive problems out of thermal equilibrium, where bonds break and form as in our case. At variance, a DFTB-based approach has been recently applied to study high KE impact dynamics of fullerenes on graphite (Galli and Mauri, 1994) and room temperature growth of SiC (Taioli et al, 2013), displaying an accuracy comparable to density-functional theory (DFT).…”