Optical and structural characterization of epitaxial graphene on vicinal 6H-SiC(0001)–Si by spectroscopic ellipsometry, Auger spectroscopy, and STM

Abstract

The authors report results of spectroscopic ellipsometry (SE) measurements in the near-IR, visible, and near-UV spectral ranges using a Woollam dual rotating-compensator ellipsometer, analyzing data in terms of both epitaxial graphene and interface contributions. The SiC samples were cleaned by standard methods of CMP and HF etching prior to mounting in UHV and growing epitaxial graphene by thermal annealing at ∼1400 °C. Most samples were vicinally cut 3.5° off (0001) toward [11−20]. STM measurements show that the initial regular step edges were replaced by somewhat irregular edges after graphene growth. From growth-temperature and Auger data the authors estimate that the graphene is ∼3–4 ML thick. The authors find significant differences among the spectral features of the interface “buffer” layer and those of graphene. Specifically, the hyperbolic-exciton peak reported previously at ∼4.5 eV in graphene shifts to a similarly shaped peak at ∼4 eV in the interface buffer layer. The authors attribute this shift to a significant component of sp3 bonded carbon in the buffer, which occurs in addition to the sp2 bonded carbon that is present in the graphene layer. SE data in the terahertz range obtained by Hoffman et al. [Thin Solid Films 519, 2593 (2011)] show that the mobility values of graphene grown on the carbon face of SiC vary with proximity to the substrate. This leads to the question as to whether an interface layer at the Si face has properties (i.e., dielectric function/complex refractive index) that are different from and/or affect those of the graphene layers.

Determination of a refractive index and an extinction coefficient of standard production of CVD-graphene

“…13 It is thus crucial for the inclusion of graphene at an industrial optoelectronic level that its transparency and optical behaviour are well understood. From the bibliography, it is possible to infer that the optical behaviour of graphene differs significantly depending on the production technique, whether it has been exfoliated [14][15][16] or deposited through chemical vapour deposition; [17][18][19] furthermore, that the diverse substrates used like metals, 17 semiconductors 14 and insulators, 9,14,16,20,21 may they be transparent 18 or opaque, put in an additional variable to make a proper comparison of graphene's optical behaviour. Finally, for the case of transferred graphene, a layer of transferred residues seems to form between the substrate and graphene 18 which affects the optical response and should be deducted in order to obtain the results for pristine graphene.…”

Optical Constants of Chemical Vapor Deposited Graphene for Photonic Applications

“…A precise method for determining refractive indices and extinction coefficients is spectroscopic ellipsometry, which allows for extracting the dielectric function in a broad wavelength range directly from the raw data [ 16 , 17 ]. The previous works report ellipsometric studies of the optical constants of exfoliated graphene [ 18 , 19 , 20 , 21 , 22 ], epitaxial graphene [ 23 , 24 , 25 ], and CVD graphene [ 26 , 27 , 28 , 29 , 30 , 31 ], also transferred onto various substrates such as optical glass [ 26 ], silicon oxide [ 18 , 20 , 31 ], or fused silica [ 18 , 19 , 29 , 30 ]. However, the measured dielectric functions show more than 20% differences, caused not only by the graphene production technique, the effect of the substrate, or the quality of graphene but also by the use of different ellipsometric models and respective initial assumptions.…”

The optical properties of transferred graphene and the dielectrics grown on it obtained by ellipsometry

“…As a result, TG has been divided for analysis, as a real graphene and the impurities on it. The same approach was followed in the work of Nelson et al [23] to analyse different properties of multilayer graphene and a buffer layer beneath of graphene layer grown on SiC. Therefore, the dielectric constants of the material remained to depend on the predefined monolayer thickness obtained from multilayer graphitic material.…”