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CVD of graphene on SiC
Institute of Electronic Materials Technology (ITME), Wólczyńska 133, Warszawa 01-919, Poland
|Graphene, the first available to us two-dimensional (2D) atomic crystal is a new material composed of one or more sheets of carbon atoms in which each carbon atom is covalently bound to its 3 neighbors (sp2 bonds) to form the honeycomb structure. For discovery of this material the Nobel Prize in Physics 2010 was awarded. The particular arrangement of the graphene layers called the Bernal stacking and the exceptional properties of the strong bonds sp2 which are stronger than carbon bonds in diamond greatly affects the electronic, mechanical and thermal properties of this material and its applications (e.g. RT mobility >200000cm2/Vs). The properties of epitaxial graphene are so compelling that it is currently recognized by the electronics industry as a possible alternative or even the successor to silicon for large scale integrated electronics and also suggest that graphene could be applied in other innovative devices or composites replacing traditional materials. Additionally, these extreme properties combined in one material could also enable development of new technologies as flexible electronics, transparent protective coatings and barrier films.
However, the true scientific and technological potential of graphene may not be known until very high quality material are created. This can only be achieved by facing such challenges as a graphitization with one atomic layer resolution and characterization of graphene morphology and electrical features in nano-technology, physics, chemistry and material science.
In order to improve graphene structure, in particular to increase the graphene flakes and thickness uniformity various growth directions were explored to obtain graphene growth conditions that retain essential features of a single graphene layer regardless of their number. The optimization of the growth conditions enabled to achieve atomic layer-by-layer control on the graphene growth. A principle issue in any future studies of the electrical properties of 2D graphite sheets is a precise measurement of the film thickness, defects (cracks, flakes borders, “puckers”) and strains.
Graphene deposited on a SiC has great potential for electronics applications, however, a major factor hindering the development of technology for the large-scale production of graphene-based nano-electronic devices is the lack of access to high-quality uniform graphene layers grown on large SiC substrates. Graphene produced by sublimating Si from SiC heated to high temperatures (1200-2000oC) is sensitive to the surface quality of the SiC substrates. Concurrently, the CVD epitaxial growth of graphene on metal substrates has lately received much attention. Unfortunately, epitaxial growth on metals suffers from the disadvantage that electronic applications require graphene on an insulating substrate, and although wafer-scale transfer is possible, it is a difficult process. A fundamental question thus arises: how can one reduce the dependence of graphene growth on substrate quality and simultaneously improve graphene layer uniformity? The differences between epitaxial graphene layers on 4H-SiC(0001) obtained by two different growing techniques have been investigated. The first method, commonly used is based on sublimation of Si from SiC(0001) surface at high temperature (S-EG). The second method has been developed by chemical vapor deposition technique (CVD-EG). In this paper, we report the CVD of epitaxial graphene (CVD-EG) on SiC substrates using propane gas as the carbon precursor. Graphene layers were grown using a commercially available horizontal CVD hot-wall reactor (Aixtron VP508), which is inductively heated with an rf generator. The epitaxial CVD of graphene relies critically on the creation of dynamic flow conditions in the reactor that simultaneously stop Si sublimation and enable the mass transport of propane to the SiC substrate. While protecting against Si sublimation, C deposition was enabled with one monolayer resolution by taking advantage of the high efficiency of kinetic processes at high T and low P.
The CVD-EG approach offers numerous potential benefits in comparison to S-EG, including the reduction of substrate surface influence on graphene thickness uniformity and the application of well-developed commercial epi-systems for SiC epitaxy. The proposed method permits the growth rate of graphene on the C-face of SiC(000-1) to be considerably lowered enabling the growth of 1ML, which is extremely difficult in the case of S-EG. Additionally, FLG can be grown on the Si-face SiC(0001) which, in comparison to max 2-3 ML of S-EG, creates greater research opportunities. Our approach also enables precise growth rate control by adjusting the mass transport of the carbon precursor in a similar way to the method used in MOCVD/CVD, as well as the passivation of the SiC substrate by any substances prior to graphene growth. Moreover, one can tune the reactor conditions to grow both CVD-EG and S-EG in the same system.
The growth mechanism is discussed and the hydrogen intercalation results are presented. To provide information at the atomic scale, samples were characterized by scanning tunneling microscopy (STM), micro-Raman spectroscopy, LEEM, LEED and transmission electron microscopy (TEM). The thickness of the graphene films were estimated also by ellipsometry, the position of the s and p electronic energy bands were evaluated by angle-resolved photo-emission spectroscopy (ARPES). ARPES data clearly show the expected linear dispersion for relativistic electrons near Dirac point. The transport parameters of the graphene samples were measured with the van der Pauw method and microwave resonance technique at room temperature. The electron density in several 1-2 ML graphene films grown in subsequent processes was typically 2-4x1012 cm-2, with a macroscopically averaged electron mobility inferred from Hall voltage in the range 1200-1800 cm2/Vs, demonstrating the high electronic quality of the CVD-EG layers on the wafer scale. The micro Raman maps have been created with 3mm light spot using 530 points measured on 2,3 x 2,3 mm2 area in the center of the sample. Histograms reveal that CVD growth of graphene produces less strained layers in comparison with S-EG.
Presentation: Invited oral at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, Topical Session 7, by Wlodek Strupinski
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17
Submitted: 2013-05-05 22:23 Revised: 2013-07-18 20:45