The growth of epitaxial layers of graphene on silicon carbide substrate is a technologically very important process, however, the physics and chemistry of this process are still far from being well understood. In this work we study first stages of formation of buffer hydrocarbon structures on the (0001) Si-terminated surface of the 4H-SiC, which occur during the deposition of propane (C₃H₈) and benzene (C₆H₆) under thermal conditions corresponding to the growth in a chemical vapor deposition (CVD) reactor [1]. Our studies of the first stages of graphene growth through deposition of propane (benzene) precursor are performed by ab initio and classical molecular dynamics.

First, the mechanisms of C₃H₈ and C₆H₆ chemisorption were studied by means of DFT+GGA molecular dynamics (MD) with Nose–Hoover thermostat approximation. We performed calculations for a slab containing surface layers with lateral 3 x 3 surface unit cell and adsorbed molecules employing the SIESTA DFT-based package. The thermostat temperature was set close to the growth temperature of 1500 – 1800 K, and the time evolution of the system was followed up to 500 fs. Various starting positions of the C₃H₈ and C₆H₆ precursors were considered.

Performed MD simulations shed light on characteristic features of the hydrocarbon molecule adsorptive interactions with the 4H–SiC Si–terminated surface. In the case of propane, the following picture of the early growth stages emerges. At first, one of the C-H bonds in the propane molecule brakes owing to the interaction with a Si-site and one hydrogen atom is released, which neither binds with the substrate atoms nor with the propane molecule (owing to the access of the kinetic energy). The propane’s C atom in terminal or central position forms chemical bond to the surface Si–site. Then, adsorbed C₃H₇ moiety configuration behaves as dynamically stable evolving its vibrational states and stays chemically bonded to the surface. In the case of C₆H₆ precursor, we observe that thermal energy at the 1800K is large enough to cause braking of the stable benzene core at certain C atom site of the benzene ring. The resulting system: CH2=C=CH–CH=C=CH2 has linear molecular chain configuration and possess further dynamical stability during MD simulation. From the other side, the same chain molecule can be obtained from C₃H₃+C₃H₃ reaction between resonance-stabilized radicals [2]. The resulting chain includes all H atoms, which originally belonged to the benzene ring and created no bonding with the Si–clean 4H–SiC surface.

These MD simulations of the precursor adsorption, as well as the previous static DFT calculations [3], allow for tuning of the parameters in the empirical potentials employed in the classical MD [4]. Further, simulations of the growth process are performed within the classical MD and provide physical insight on larger length and time scales.

[1] W. Strupinski et al., Nano Lett. 11, 1786 (2011).

[2] Y. Georgievskii et al., Phys. Chem. Chem. Phys. 9, 4259 (2007).

[3] K. Tokar and J.A. Majewski, Acta Physica Polonica A 122, 1049 (2012).

[4] J. Tersoff, Phys. Rev. B 37, 6991 (1988); D. W. Brenner, Phys. Rev. B 42, 9458 (1990) and erratum Phys. Rev. B 46, 220 (1992).

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