Fabrication, Characterization and Modification of Epitaxial Graphene on 6H-SiC(0001) Substrate

6H-SiC(0001)外延石墨烯的制備、表徵及改性研究

Student thesis: Doctoral Thesis

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Author(s)

  • Tingwei HU

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date25 Sep 2015

Abstract

Since discovered in 2004, graphene, a single layer sheet of sp2-bonded carbon atoms arranged in honeycomb structure, has been extensively studied due to its unique properties and potential applications. Presently, several techniques have been developed for the fabrication of graphene. Mechanical exfoliation is the typical simple one to produce high-quality graphene, but the production efficiency is rather low. Although oxidation and reduction of graphite can be used to make large-scale graphene sheets, unwanted defects are commonly observed. Graphene sheets prepared by chemical vapor deposition (CVD) on metal substrates (Cu or Ni) can be in industrial large-scale, but transfer process is needed for application, leading to complication of the fabrication. Hence, scientists continue exploring the way to fabricate large-scale high-quality epitaxial graphene (EG) directly on insulators by thermal decomposition of silicon carbide (SiC), which has already received considerable attention in scientific community. However, more detailed studies are essentially needed to comprehensively understand the formation process of EG on SiC substrate, including, the atomic structure of SiC buffer layer, the detailed growth mechanism and edge properties of EG, the fabrication of large-scale uniform ordered EG and the interaction between metal and EG surface. In this thesis, EG are produced in ultra-high vacuum (UHV) system through thermal decomposition of 6H-SiC(0001) at high annealing temperature and, in-situ scanning tunneling microscopy/spectroscopy (STM/STS), reflective high energy electron diffraction (RHEED) and Raman scatterring are mainly adopted to characterize the structures and properties of EG. It provides us groundwork for the high quality fabrication and application of EG. The main researches are included as following:

1) The formation mechanism of EG is addressed from the atomic viewpoint. Before EG come into being, three main surface reconstructions of SiC can be observed with gradually increasing annealing temperature: 3x3, ( √3x√3)R30°, 6x6. And, 6x6 reconstruction is knowns as the SiC buffer layer, which is the initial state of graphene coalescence and directly grows from ( √3x√3)R30° structure. It is found that the short-range atomic arrangement of SiC buffer layer undergoes order-disorder-order evolution but the long-range period is maintained during thermal annealing. Taking the orderly arranged triangular silicon clusters as template, graphene nucleates and grows gradually. The atomic structure of successive EG layers on SiC substrate can be imaged by STM, and the local density of states of EG can be detected by differential conductance of STS spectra (dI/dV), which is acquired at each point in a STM topographic image.

2) According to the epitaxial relationship between SiC buffer layer and EG lattice, a new simple approach referring to the basic vector of 6x6 reconstruction as well as fast Fourier transform (FFT) is suggested to judge the edge orientation of EG. This method is much lower cost and higher efficiency than polarized Raman scattering and atomically resolved STM. It is illustrated that armchair orientation is preferred in both monolayer and multi-layer regions,which is consistent with the basic vector of 6x6 structure as well as the <1120> closed-packed direction of SiC.

3) At a smooth armchair edge, incident and scattered electrons interference with each other, leading to √3x√3 patterns, and the period is about 0.37nm, which is closed to the electron wavelength around Fermi level. The distinct patterns, such as, dumbbells and horseshoes, are ascribed to the phase shift of the reflected Bloch waves. Meanwhile, atomically-resolved graphene lattice instead of √3x√3 patterns appears near the ridged armchair edges. A general model based on direct reflection and diffuse reflection is proposed to describe the underlying mechanism. Similar to light interference, when the electron wave encounters a rough edge, diffuse reflection occurs and coherent conditions between the incident and reflected waves cannot be satisfied and no √3x√3 patterns are observed. These findings provide better understanding on the structural stability and electronic states of EG edges..

4) Five kinds of annealing processes are conducted to fabricate EG. By thermal and flash annealing in UHV, rough EG surface are usually obtained, and the thickness of EG by thermal annealing at Si flux exhibits more-than-one layered nature. Flash annealing at Si and Pb atmosphere is an effective approach to realize the fabrication of large-scale uniform ordered single layer graphene (SLG). It is much more superior than the former methods. The synergetic effects rendered by flash annealing and atom atmosphere are evidenced. A model is postulated to understand the growth process and corresponding mechanism. Nearly three top bilayers on SiC are decomposed due to the fast heating and cooling processes, and the evaporation of Si atoms from SiC is retarded by atom atmosphere, resulting in confined sublimation.

5) Three phenomena of metals on EG are demonstrated, including selective growth of metal islands (Pb, Ag and In) on EG/SiC multi-domains, self-diffusion phenomenon of metal particles (Ag and In) and the effect of the bombardment of metal (Ag) plasma. Because of the dangling bonds of Si clusters, metal atoms have higher absorption energy and diffusion activation energy on SiC buffer layer than those on graphene domains, and thus prefer to nucleate on SiC buffer layer. Long straight trenches on EG due to self-slipping of metal particles in vacuum annealing condition is observed. The metal trenches are mainly along with zigzag or armchair directions. Meanwhile, EG films can be scratched and erased by the sliding metal particles, which has the simlar effect of etching. After the bombardment of Ag plasma, EG films will change into DLC, which can be evidenced by Raman spectra. These findings help us to understand more abtout the mutual interactions between metal and EG.

In brief, the formation mechanism of EG as well as the detailed configuration of SiC buffer layer is addressed from the atomic viewpoint. According to the epitaxial relationship between SiC buffer layer and EG lattice, a new simple approach is suggested to judge the edge orientation of EG. Armchair orientation is preferred in both monolayer and multi-layer regions, and the distinct quantum interference (QI) induced patterns with √3x√3 period are ascribed to the phase shift of the reflected Bloch waves. Large-scale uniform ordered SLG can be produced by flash annealing in atom (Si or Pb) atmosphere. Three phenomena of metals on EG are demonstrated for better understanding of the interactions between EG surface and metals. All these results in this thesis provide us a further insight into the fabrication, characterization and modification of EG, which will facilitate the applications of EG-based electronic devices in the future.