First Principles Study on Hydrogen Intercalation into Buffer Layer Grown on SiC(0001) Surface
Authors: Nara Jun , Yamasaki Takahiro , Ohno Takahisa 
Published: February 2020

Graphene has attracted much attention because it has quite high electron mobility due to the characteristic electronic properties such as Dirac cone and then has been expected as a future electronic device material to contribute to the low energy consumption, which could help combat the global warming. Among graphene fabrication methods suggested so far, thermal decomposition of SiC substrate, in which graphene sheets are formed after Si atom sublimation from SiC surface at high temperature, has been intensively studied. The C-atom layer directly grown on the SiC substrate is not graphene but so-called buffer layer (BL), which has similar honeycomb structure to graphene but does not have the graphene's characteristic electronic properties such as the Dirac cone, because BL is covalently bonded to the SiC surface and lacks the hexagonal symmetry. To utilize BL grown on SiC substrate as graphene, it is necessary to anneal it under hydrogen (H) ambient to intercalate H atoms between BL and SiC substrate, so that the graphene’s characteristic properties is recovered. However, the lack of knowledge on the intercalation mechanism make it difficult to control this process to obtain high quality graphene.
In this paper, we report our recent studies on the interaction between H atoms and BL grown on SiC substrate with the periodicity of (6√3x6√3)R30˚ by using first-principles density functional calculations [1]. We first investigated the adsorption process of H2 molecule on BL. The total adsorption energy for two H atoms is 1.6 eV, then H2 molecules prefer to dissociatively adsorb on BL. The activation energy of this process is 0.9 eV and that of the reverse process is 2.5 eV. Considering the fact that H atoms on Si(001) surface desorb from the surface at around 780K with the activation energy of about 2.5eV, we conjecture that H atoms on BL would also desorb from it at the temperature at which the experiments are conducted (above 870K). The H atom adsorption energy ranges between 0.94 eV and -0.35 eV depending on the adsorption sites. Adsorption energies are evaluated based on a H2 molecule in vacuum, and the positive values mean stable states, while negative ones mean unstable states. This variety comes from the C atom bonding states. On the C atom with π bonds, H atom is quite stable with a large adsorption energy, while on the C atom without π bonds due to the bonding to a substrate Si atom, H atom is not stable (sometimes it is endothermic). For the H atom diffusion process, it is found that the activation energy ranges from 1.7 eV to 2.3 eV depending on the diffusion path. This variety comes from the initial and the final states of H atom. If the initial and the final states are stable (on C atom with π bonds), the activation barrier is small like 1.7 eV, while if one of the two is unstable (on C atom without π bonds), the activation energy is large like 2.3 eV. We also investigate the penetration process of H atoms through BL. In the final state, H atom stays on a Si atom of the SiC substrate below BL. The final state is 0.8 eV higher in energy than the initial state where H atom stays on BL. So, H atoms prefers to stay on BL rather than penetrate through BL. The activation barrier of this penetration process is about 4.9 eV, which is quite larger than that of H2 desorption process described above, meaning that H atoms prefer to desorb from BL rather than penetrate through BL. We also study the behaviour of H atoms or molecules below BL. It is found that H molecules put between BL and SiC surfaces easily dissociate into two H atoms, which then make covalent bonds to Si dangling bonds. Surprisingly, there is no energy barrier for the adsorption of H molecule onto Si dangling bonds on a SiC(0001) surface. This is quite contrastive to the adsorption of H molecule onto BL. To investigate the diffusion of H atoms below BL, we performed first-principles molecular dynamics simulation in NVT condition (T=1500K). Even in a short simulation time of three picoseconds, we observed several H atom hoppings from one Si dangling bond to another, meaning that H atom easily diffuse below BL.
Acknowledgments: This research was partially supported by MEXT within the priority issue 6 of the FLAGSHIP2020. Earth Simulator of JAMSTEC and NIMS Numerical Materials Simulator were used for this study.
[1] PHASE code: https://azuma.nims.go.jp/.