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Register a new userThe (3$times$3)-SiC-($bar{1}bar{1}bar{1}$) Reconstruction: Atomic Structure of the Graphene Precursor Surface from a Large-Scale First-Principles Structure Search
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Silicon carbide (SiC) is an excellent substrate for growth and manipulation of large scale, high quality epitaxial graphene. On the carbon face (the ($bar{1}bar{1}bar{1}$) or $(000bar{1}$) face, depending on the polytype), the onset of graphene growth is intertwined with the formation of several competing surface phases, among them a (3$times$3) precursor phase suspected to hinder the onset of controlled, near-equilibrium growth of graphene. Despite more than two decades of research, the precise atomic structure of this phase is still unclear. We present a new model of the (3$times$3)-SiC-($bar{1}bar{1}bar{1}$) reconstruction, derived from an {it ab initio} random structure search based on density functional theory including van der Waals effects. The structure consists of a simple pattern of five Si adatoms in bridging and on-top positions on an underlying, C-terminated substrate layer, leaving one C atom per (3$times$3) unit cell formally unsaturated. Simulated scanning tunneling microscopy (STM) images are in excellent agreement with previously reported experimental STM images.
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We address the stability of the surface phases that occur on the C-side of 3C-SiC($bar{1} bar{1} bar{1}$) at the onset of graphene formation. In this growth range, experimental reports reveal a coexistence of several surface phases. This coexistence can be explained by a Si-rich model for the unknown (3$times$3) reconstruction, the known (2$times$2)$_{C}$ adatom phase, and the graphene covered (2$times$2)$_{C}$ phase. By constructing an $ab$ $initio$ surface phase diagram using a van der Waals corrected density functional, we show that the formation of a well defined interface structure like the buffer-layer on the Si side is blocked by Si-rich surface reconstructions.
We have performed electronic state calculations to clarify the initial stage of the oxidation of the Si- and C-faces in 4H-SiC based on the density-functional theory. We investigate how each Si and C atomic site is oxidized on C- and Si-face, and explore most probable reaction pathways, corresponding energy barriers, and possible defects generated during the oxidation. We have found that carbon annihilation process is different between on Si- and on C-face, and this difference causes different defects in interface; In C-face case, (1), carbon atoms are dissociated directly from the substrate as CO molecules. (2), after CO dissociation, 3-fold coordinated oxygen atoms (called Y-lid) are observed at the interface. (3), high density of C-dangling bonds can remain at the interface. In Si-face case, (1), C atoms inevitably form carbon nano clusters (composed of a few atoms) in interface to reduce the number of dangling bonds there. Moreover, we have found that the carbon nano clusters are composed of not only single but also double chemical bonds. (2), We have observed that CO molecules are dissociated from the carbon nano clusters in MD simulations. Furthermore, we have investigated whether H$_2$ and NO molecules react with the defects found in this study.
We perform density functional theory calculations for the determination of the structural and electronic properties of epitaxial graphene on 4H-SiC(000$bar{1}$). Using commensurate supercells that minimize non-physical stresses we show that, in contrast with Si-face epitaxial films, the first graphene layer that forms on the C-face of SiC is purely metallic with its $pi$-bands partially preserved. Typical free-standing characteristics are fully recovered with a second graphene layer. We moreover discuss on the magnetic properties of the interface and the absence of Fermi-level pinning effects that could allow for a plausible device operation starting from the off-state.
The c(6x2) is a reconstruction of the SrTiO3(001) surface that is formed between 1050-1100oC in oxidizing annealing conditions. This work proposes a model for the atomic structure for the c(6x2) obtained through a combination of results from transmission electron diffraction, surface x-ray diffraction, direct methods analysis, computational combinational screening, and density functional theory. As it is formed at high temperatures, the surface is complex and can be described as a short-range ordered phase featuring microscopic domains composed of four main structural motifs. Additionally, non-periodic TiO2 units are present on the surface. Simulated scanning tunneling microscopy images based on the electronic structure calculations are consistent with experimental images.
We present a systematic study of the atomic and electronic structure of the Si(111)-(5x2)-Au reconstruction using first-principles electronic structure calculations based on the density functional theory. We analyze the structural models proposed by Marks and Plass [Phys. Rev. Lett.75, 2172 (1995)], those proposed recently by Erwin [Phys. Rev. Lett.91, 206101 (2003)], and a completely new structure that was found during our structural optimizations. We study in detail the energetics and the structural and electronic properties of the different models. For the two most stable models, we also calculate the change in the surface energy as a function of the content of silicon adatoms for a realistic range of concentrations. Our new model is the energetically most favorable in the range of low adatom concentrations, while Erwins 5x2 model becomes favorable for larger adatom concentrations. The crossing between the surface energies of both structures is found close to 1/2 adatoms per 5x2 unit cell, i.e. near the maximum adatom coverage observed in the experiments. Both models, the new structure and Erwins 5x2 model, seem to provide a good description of many of the available experimental data, particularly of the angle-resolved photoemission measurements.
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