Influence of  SiC substrate orientation on epitaxial graphene quality studied by Raman spectroscopy

Grzegorz Kępisty 1Kacper Grodecki 2,3Włodzimierz Strupiński 2Andrzej Wysmołek 3Roman Stępniewski 3Jacek Baranowski 1,3

1. Warsaw University, Faculty of Physics, Hoża 69, Warszawa 00-681, Poland
2. Institute of Electronic Materials Technology (ITME), Wólczyńska 133, Warszawa 01-919, Poland
3. Warsaw University, Institute of Experimental Physics (IEP UW), Hoża 69, Warszawa 00-681, Poland

Abstract

    High temperature annealing, which induces sublimation of Si atoms from SiC substrates at high temperatures is one of the commonly used methods of graphene layers fabrication. It was shown that sublimation process can be successfully performed in standard CVD reactors used for SiC epitaxial growth. The final result of the process depends on the annealing temperature and time as well as any other parameters like the pressure of the noble gas (usualy argon) in the CVD reactor. In contrast to the fact that the influence of the argon pressure in the reactor on the growth rate of the graphene structures is well established [1], the role of the substrate orientation on the effectiveness of the process of SiC sublimation is still not well known.           In the case of epitaxy use of slightly misoriented substrates favors two dimensional growth modes, which is crucial in the case of flat and homogeneous epilayer growth. Thus the use of on-axis silicon carbide samples can’t be the optimal choice for this method. It could be argued that misorientation of the substrate could have positive influence on out-diffusion of silicon atoms during SiC decomposition.          In this communication the influence of SiC substrate orientation on epitaxial graphene is studied by Raman spectroscopy, which provides interesting information about strain state and homogeneity of the graphene structures. Here we focus our attention on the most prominent bands, so called the G and the 2D bands. The investigated samples were fabricated in the hot-wall CVD reactor Epigress VP508 at the temperature of 16000C. Two different argon pressures of 50 mbar and 100 mbar in the reactor were maintained during the growth process on the 4H-SiC substrates with different misorientation of 0, 4, and 8 degrees.           Micro-Raman scattering experiments were performed in back scattering geometry, using the 532nm line of a continuous wave Nd-YAG laser. The laser spot size on the sample was about 2mm in diameter. In order to collect information about strain and concentration homogeneity micro-Raman maps consisting of 530 points on a 2.3x2.2mm2 sample area were performed.           It was found that the average position of the 2D band, and thus biaxial strain present in graphene structures is not very sensitive to argon pressure on substrates with different misorientation. On the other hand, the 2D band average frequency increases with the misorientation angle. This is direct evidence that the average strain built in the graphene structures increases significantly with the misorientation angle. Similar behavior was found in the case of the 2D band broadening, for which the width is of about 30 cm-1 higher for 8o misoriented substrate, as compared to the on-axis samples. The observed broadening depends also on the pressure of argon during the epitaxial process.           The obtained results strongly suggest that the quality of graphene layers seems to decrease with misorientation of substrate. This provides additional information about mechanisms important in the growth of epitaxial layers by SiC sublimation. [1] A. Drabińska, K. Grodecki, W. Strupiński, R. Bożek, K. P. Korona, A. Wysmołek, R. Stępniewski, and J. M. Baranowski, Phys. Rev. B 81, 245410 (2010)       

 

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Presentation: Poster at Warsaw and Karlsruhe Nanotechnology Day, by Grzegorz Kępisty
See On-line Journal of Warsaw and Karlsruhe Nanotechnology Day

Submitted: 2011-09-17 19:08
Revised:   2011-09-18 00:01