Full length articleAn investigation of thin Zn films on 4H-SiC(0001)graphene
Introduction
Graphene is the most promising electronic material discovered in the past 15 years. This material exhibits a number of interesting properties such as high electron mobility at room temperature, remarkable optical transparency, high thermal conductivity and exceptional mechanical properties (e.g. Young's modulus up to 2.4 TPa) [1,2]. Moreover, the peculiar band structure of graphene makes it different from any other material. The conduction and valence bands in graphene meet in six single points (Dirac points) at the corners of the Brillouin Zone [3]. This unique Dirac cone structure is the reason for naming graphene a “zero-gap semiconductor”. A crucial goal in ongoing research is to characterise various graphene growth techniques along with their potential application (e.g. flexible electronics, optoelectronics, bio-sensing, nanocomposites, energy storage devices) [4]. One of the most well-known technique to obtain graphene is high temperature thermal decomposition of silicon carbide.
Silicon carbide is a semiconductor characterised by a wide band gap (>3 eV), high electric breakdown field, high-saturation electron velocity and tolerance to high-temperature and radioactive field [[5], [6], [7]]. The fundamental structural unit in SiC is a covalently bonded tetrahedron of four C atoms with a single Si atom at the centre. In each layer the silicon (or carbon) atoms have a close-packed hexagonal arrangement. By repeating 2D layer along the c-axis it is possible to form many specific polytypes of SiC crystal (the most popular are 3C, 4H and 6H) [8].
The first stage of graphitisation is the (6√3 × 6√3)R30° reconstruction at 1400 K. Originally, it was interpreted as a surface-graphene layer but further research and analysis have shown that this is a so-called “buffer layer” - a carbon layer in a honeycomb lattice (like graphene) but without the same electronic properties and band structure characterised by Dirac cone. The specific properties of graphene develop only after a new layer on top of this “buffer layer” is grown [9,10].
It detail the graphitisation process is as follows: after heating the sample at relatively high temperatures 1470–1670 K Si evaporates and the C atoms, which are still on the surface, diffuse, aggregate and form uniform graphene layers [11]. The control of graphene thickness is realised within 50 K heating “window” starting from the buffer layer forming at ~1470 K and graphene trilayer graphene at ~1670 K [10]. This evolution in morphology is well characterised with LEED since buffer layer is measured from strong 6 × 6 spots surrounding the fundamental spots and weaker G(10) spots. As thicker graphene develops the 6 × 6 fade in intensity, the G(10) spots become stronger and SiC spots weaken reaching very low intensity with trilayer graphene formation.
Another important issue is to find the way to control graphene physical properties. One of the possibilities is the intercalation. This is the process in which atoms are placed between the graphene and the substrate [12,13]. In case of SiCgraphene system intercalation can effectively decouple the buffer layer from the SiC substrate [14,15]. However, it is still not clear what determines the optimal conditions (temperature, annealing time, deposited amount) needed for a given element to intercalate beneath the graphene.
In this work, the growth of thin Zn films on 4H-SiC(0001)graphene and results for system annealing at various temperatures, in order to obtain Zn intercalation, are presented. Several transition metals have been already used for intercalation which are more strongly bound and require higher intercalation temperatures. Zn as a more weakly bound transition metal offers the possibility for the intercalation kinetics to be easier and at a lower temperature. Zinc is a diamagnetic metal characterised by good electrical conductivity. It has a hexagonal structure, similar to SiC and has relatively low melting (693 K) and boiling (1180 K) points, because of what is often not considered to be transition metals. Due to its properties it seems to be interesting to investigate thin zinc films on 4H-SiC(0001)graphene.
The properties of the Zn deposited films, chemical composition and surface structure were studied using X-ray Photoelectron Spectroscopy (XPS) and Low Energy Electron Diffraction (LEED). Combining the information from XPS that gives the amount of Zn still on the surface and which diffraction spots still present as a function of temperature, it can be deduced whether Zn is on top of the surface or being intercalated underneath graphene. Moreover, DFT calculations were performed in order to obtain complementary results related to the phase stability and bonding.
Section snippets
Experimental
The substrates used in the experiment were N-doped 4H-SiC (0001) crystals (MTI Corporation) with surface roughness <10 Å and resistivity approximately 1–5·10−4 Ω·m. Typical size of samples was 25 mm2. After inserting into the vacuum chamber the substrates were degassed at 880 K for a few hours and then flashed up to 1430 K to remove contamination. For surface graphitisation, the well-known and well established process (described in Introduction) was applied. The samples were flashed up to
Clean 4H-SiC(0001) surface
In the first stage of experiments, the clean substrate surface characterisation has been made. Fig. 1 presents the results for 4H-SiC(0001) surface after the cleaning procedure and graphitisation process. The XPS spectra exhibit characteristic peaks for silicon, carbon and oxygen elements (Fig. 1a, b and c respectively). The estimated atomic concentration (based on presented formula (1)) for measured surface layer (in case of XPS this layer extends over 3× electron mean free path ≈ 4.5 nm [19])
Conclusions
In this work, Zn thin films deposited on 4H-SiC(0001)graphene surface were investigated with respect to metal growth and the intercalation process. The graphene has been first grown on the Si-face terminated sample by the “thermal decomposition process” and then Zn was deposited. The XPS analysis indicates no significant chemical changes in the surface region after Zn adsorption at RT and after thermal annealing, but it confirms the presence of adsorbate even after heating up to 1080 K.
Acknowledgment
This research was funded by Uniwersytet Wrocławski grant, no. 1426/M/IFD/15. We acknowledge provision of computer time from the Interdisciplinary Centre for Mathematical and Computational Modelling (ICM) of Warsaw University (Project No. G4423). Part of this work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy 318 Sciences, Materials Science and Engineering Division, 319 under Contract No. DEAC0207CH11358 (MCT).
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