Micro-Raman and micro-transmission imaging of epitaxial graphene grown on the Si and C faces of 6H-SiC
© Tiberj et al; licensee Springer. 2011
Received: 10 September 2010
Accepted: 29 July 2011
Published: 29 July 2011
Micro-Raman and micro-transmission imaging experiments have been done on epitaxial graphene grown on the C- and Si-faces of on-axis 6H-SiC substrates. On the C-face it is shown that the SiC sublimation process results in the growth of long and isolated graphene ribbons (up to 600 μ m) that are strain-relaxed and lightly p-type doped. In this case, combining the results of micro-Raman spectroscopy with micro-transmission measurements, we were able to ascertain that uniform monolayer ribbons were grown and found also Bernal stacked and misoriented bilayer ribbons. On the Si-face, the situation is completely different. A full graphene coverage of the SiC surface is achieved but anisotropic growth still occurs, because of the step-bunched SiC surface reconstruction. While in the middle of reconstructed terraces thin graphene stacks (up to 5 layers) are grown, thicker graphene stripes appear at step edges. In both the cases, the strong interaction between the graphene layers and the underlying SiC substrate induces a high compressive thermal strain and n-type doping.
Since the first report by C.V. Raman in 1928 [1, 2], Raman spectroscopy has become increasingly popular in materials science and, especially, in semiconductor physics and microelectronics. Basically, Raman scattering probes the inelastic scattering of a monochromatic light (photons) by the lattice vibrations (phonons) in a solid. Since in a crystalline solid, the phonons are very sensitive to the internal and external perturbations, like doping and stress, the frequency of the scattered light (photons) is a local probe of the perturbation experienced (or not) by the medium. For a more detailed introduction and description of Raman and micro-Raman spectroscopy in materials science (and especially in semiconductors) please refer to [3–5].
Today numerous applications exist that cover the whole development of modern electronic and optoelectronic devices. They run from basic inspection of as-grown semiconductors to advanced device inspection tools. For instance, Raman mapping enables to check the crystalline quality [6, 7], the composition [8, 9], the doping level [10–13], or the uniformity of as-grown semiconductor materials. Along this line one on the most popular applications in microelectronics is strain measurements, either at the device or at the full wafer scale [9, 14–17]. Raman measurements can also be used for final device inspection, through the temperature mapping of operating devices like FETs, lasers, and actuators [18–21]. In this case, thanks to the use of recent turnkey Raman systems, one can perform fast mapping with spatial resolution down to 300 nm. Combining with the use of several laser wavelengths, one can also probe the in-depth profile of multilayer systems and device. All together, these features confirm the unique versatiliy and potentialities of micro-Raman imaging in microelectronics. As a consequence, and because of its contactless and nondestructive nature, micro-Raman spectrosocopy (μ RS) has become an attractive characterization tool in industrial clean-room facilities.
In this field, graphene is a new comer. Because of its outstanding electronic, thermal, optical, and mechanical properties [22–24] it can be considered as a promising candidate for future carbon-based electronics . However, and because of the so-called Kohn anomaly (which is nothing but the failure of the usual adiabatic Born-Oppenheimer approximation in zero-gap semiconductors ) it is also a perfect example to illustrate all the applications of μ RS that have been mentioned before. Raman spectroscopy on few layers graphene (FLG) not only can evaluate the crystalline quality but also, the thickness, the stacking order of graphene sheets, the doping level, and finally the residual strain.
In this study, we review some recent Raman imaging results collected on epitaxial graphene grown on the C- and Si-faces of 6H-SiC substrates [27–29]. In the first section, we briefly describe the growth techniques and the experimental set-up used for micro-Raman and micro-transmission imaging. In the second section, we discuss results collected on self-organized graphene ribbons grown on the C-face of 6H-SiC substrates and we show how thicknesses, stacking order, and a rough estimate of doping level can be obtained. Finally, in the third section, FLG grown on the Si-face are investigated and we show how the compressive stress experienced by such FLGs can be estimated.
All samples were grown using the processes described in [27–29]. We used 1 × 1 cm2 pieces of on-axis 6H-SiC substrates cut from, either, the Si-terminated (0001) face or the C-terminated face of 6H-SiC wafers. Before cutting, polishing was done by Novasic to get Epiready® morphology . Then, a sacrificial oxide was thermally grown and chemically etched in HF to remove any trace of sub-surface damage from the polishing process. Finally, standard RCA treatments were done to remove any trace of surface contamination. All treatments were clean-room compatible and similar to the one used for SiC before thermal oxidation or post-implantation annealing. In this way, atomically flat surfaces were systematically obtained.
For sublimation, we used a high temperature furnace from Jipelec  previously dedicated to post-implantation annealing. It was rf-induction heated and fitted with a turbo-molecular pump. The vacuum limit reached in this way was 10-6 Torr. Before sublimation, the samples were heated at 1150°C for 10 min to remove any trace of native oxide. During the growth, the samples were covered by a graphite cap to increase the C and Si partial pressures over the SiC surface. Such graphite coverage lowers the Si out-diffusion process during the growth and enables to perform FLG growth at higher temperature. This promotes better SiC surface reconstruction.
In this way, on the C-face of 6H-SiC SiC substrates after 15 min annealing at 1700°C in a secondary vacuum, the growth of long (self-organized) graphene ribbons can be reached . These ribbons are 5-μ m wide and 150-μ m long, but a longer 1700°C annealing results in longer ribbons (up to 600-μ m) with the same width. The width does not depend on the annealing time because the ribbons fully occupy a single terrace of the heavily reconstructed (step-bunched) SiC surface .
On the Si-face, to increase the FLG anisotropy, a modified growth process was used. The growth was done at 1750°C for 20 min under argon with a graphite cap covering the sample . In this way, a full graphene coverage was obtained with a similar (pronounced) step-like morphology of the SiC substrate. The average terrace width was again 5 -μ m and the average step height 10 nm. Optical microscopy showed that the terraces had a remarkable homogeneity of width and orientation over a scale of 1 cm2.
Coupling micro-Raman spectroscopy with micro-transmission measurements
Raman spectra were collected at room temperature, using a Jobin-Yvon Horiba T64000 spectrometer operated in the confocal mode. The 514-nm line of an Ar-Kr ion laser was used for excitation. With a × 100 microscope objective, the spot diameter was about approximately 1 μ m with, typically 1-mW power focussed on the sample. To combine micro-Raman spectroscopy with micro-transmission experiments, a low noise photodiode was inserted between the SiC substrate and the XYZ piezoelectric stage. For details, see . In this way, it was possible to measure at the same time (using the same laser beam as probe) the power transmitted through the sample and the associated micro-Raman spectrum. The true FLG's spectra were obtained by subtracting the SiC reference signal from the experimental results.
In this equation, n = 2.68 is the SiC refractive index, N is the number of graphene layers, and σ is the optical conductivity of a single (isolated) graphene sheet that was confirmed experimentally . The relative (theoretical) extinction of a monolayer and a bilayer graphene on top of a SiC substrate is then 1.23 and 2.44%, respectively.
Raman imaging of isolated graphene ribbons grown on the C-face
Let us now consider everything more in detail. The first (extinction) map shows that both ribbons have an excellent thickness uniformity. However, and because the relative extinction is different, it shows also that the left one is thicker than the right one. Concerning the absolute values, from these extinction maps complemented by additional point by point measurements, we find that the thinner (right) ribbon corresponds with relative extinction values η ranging from 1.2 to 1.4%. This shows that we deal with a true epitaxial monolayer graphene (MLG) ribbon. To ascertain this result, a Raman spectrum collected at the center of ribbon is shown in Figure 2 (lower spectrum). It is clearly similar to the one reported in the literature  for monolayers graphene exfoliated on top of an oxidized silicon substrate and all spectra collected on the same ribbon exhibited the same Raman fingerprint.
In these equations, γ ep = 21 meV is proportional to the electron-phonon scattering rate , v 0 = 106 ms-1 is the electron velocity. The 0.06 factor is deduced from the function f(r S )  by using the effective dielectric constant ε eff = 5.33 for our graphene layer comprised between air and 6H SiC (ε 6H-SiC = 9.66 ). The absolute value of the Fermi level was then evaluated for all data points collected on the right ribbon (see Figure 1), giving absolute values between 200 and 350 meV. It corresponds to a doping level between 3 × 1012 and 9 × 1012 cm-2 with an average of 6 × 1012 cm-2. We have not checked directly on the same ribbon but transport measurements performed on few similar ones  gave a hole concentration of 5 × 1012 cm-2. According to the work of  such concentration is also in excellent agreement with the G and 2D band positions.
Concerning the second ribbon (on the left side), as already said we found a twice larger relative extinction. Ranging from 2.6 to 2.8%, this indicates a bilayer system. The relative extinction and the G band intensity both indicate that this ribbon is a bilayer with an excellent thickness uniformity. On the contrary, the 2D band intensity map in Figure 1 reveals sharp variations. Basically, the ribbon can be divided into three different domains, the top and bottom part having a less intense 2D band intensity than the middle one. Since these variations are correlated with shifts of the 2D and G bands positions, we assume that there are some doping level fluctuations. Indeed, it has been recently demonstrated that the G Raman band depends strongly on its electrostatic environment [38, 39]. If the top graphene sheet has a different doping level than the bottom one, the doping difference changes the Raman shift and intensity of the G band. It also breaks the inversion symmetry and activates antisymmetric modes (that are usually Raman inactive). This results in a splitting of the G band which was observed experimentally [40–42]. Our interpretation is strengthened by the two Raman spectra shown in Figure 3. In both cases, the 2D band exhibits the characteristic shape of AB (Bernal) stacking  but the G band is different. On the upper spectrum a single G peak is observed while on the lower one (collected in the bottom part of the ribbon) a clear G band splitting shows that both layers are not evenly doped.
Bernal stacking is not that usual for epitaxial graphene grown on the C-face of SiC substrates. Most of the time the graphene planes are slightly misoriented, corresponding to turbostratic stacking. In this work, we also found misoriented ribbons (not shown). The presence of rotational stacking faults between the two (or more) successive graphene planes results in Raman spectra similar to the monolayer one as shown in Figure 2. The line shape is not modified. Simply the intensity increases as the number of graphene layers increases. In Figure 2, we mentioned the relative extinction measured on these ribbons. We found 2.5 and 9.7% that corresponds, respectively, to a bilayer and a 7 or 8 misoriented layers stack. These spectra correspond to FLG where all graphene sheets are disoriented with respect to each other. Therefore, the fact that the 2D band has a single Lorentzian shape can definitively not be used as a proof to assert the monolayer character of FLG flakes. The combination of μ RS with micro-transmission measurements appears then as a most necessary tool to discriminate (without any ambiguity) between true MLG and misoriented multilayers. Of course, to perform such reproducible intensity measurements, any laser power fluctuation has to be carefully corrected. In this work, this was done using an additional low noise photodiode that measured continuously the laser power during the Raman map acquisitions.
Raman imaging of FLG grown on the Si face
From these results, we confirm that there is a strong difference between graphene grown on the C-face and graphene grown on the Si-face of SiC substrates. On the Si-face graphene strongly interacts with the underlying SiC lattice. This interaction leads to the formation of the so-called buffer layer, which is covalently bound to the SiC lattice [45, 46]. This buffer layer interacts also with the graphene layers that are grown subsequently. It induces a downward shift (E D = -0.4 eV) of the K point corresponding to a n-type doping. This interaction causes also deviations from the linear band dispersion leading to a parabolic dispersion with an apparent gap of approximately 0.25 eV. This strong interaction is also responsible for the thermal stress experienced by these graphene layers. This is no longer true on the C-face, on which it has been shown that graphene interacts very weakly with the underlying substrate [47, 48]. It is the weakness of this interaction that explains why rotational stacking faults can easily occur and why the graphene sheets can relax the thermal stress by forming wrinkles or pleats.
Reviewing recent Raman imaging experiments performed on epitaxial graphene grown on the C and Si face of 6H SiC substrates, we have shown the benefits of combining Raman spectroscopy with micro-transmissions measurements. Provided the relative extinction of FLGs can be obtained, this enables to determine (without any ambiguity) the thickness, homogeneity, and stacking order (Bernal or turbostratic) of FLGs. On the C-face of SiC substrates we have shown that long, self-ordered, graphene ribbons can be grown. These ribbons have excellent crystalline quality and are strain relaxed. They are up to 600-μ m long and 5-μ m wide. They are mainly monolayers and Bernal stacked bilayers but turbostratic bi and multilayer areas have also been found. Finally for monolayers graphene, we also illustrated how the absolute value of the Fermi level can be found, in good agreement with electrical results.
On the Si face, on the opposite, a full graphene coverage of the SiC surface has been found. The surface is still heavily step-bunched but a high compressive thermal strain and n-type doping has been observed. It confirms that on the Si face a strong interaction exists between the graphene layers and the underlying SiC substrate. FLG on the Si-face exhibits Bernal stacking with thickness inhomogeneity. Thin (5 layers) FLGs were grown in the middle of terraces, while thicker graphene stripes grew close to the step edges. In the vicinity of these steps disordered graphite pits and crystalline Si clusters were found. There might be a link between the presence of these defects, the thickness inhomogeneity and the clear electrical anisotropy that has been recently evidenced by magnetoresistance experiments.
We greatly acknowledge for partial support the EC through the RTN "ManSiC" Project (Grant MRTN-CT-2006-035735) and the French ANR through the Graph-SiC Project (Grant No. ANR-07-Blanc-0161). Finally, one of us (N.C.) acknowledges the Spanish Government for a Grant "Juan de la Cierva 2006" during his stay in Barcelona.
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