Since 2008, epitaxial graphene growth has been developed in terms of homogeneity and scale by using a 1 ATM argon pressure at high temperature (>1650°C). Until now, it still remains challenging to obtain films with different and controlled characteristics such as the number of graphene layers or the doping by tuning the growth parameters. Here, we optimized the epitaxial growth of monolayer graphene (1LG) on 4H-SiC (0001) under a low argon pressure of 10 mbar. This intermediate pressure allows growing a continued 1LG in a short process time ~1h30. First, we discuss the initial growth stages from buffer layer (BL) to 1LG as a function of annealing temperature (same heating rate). The combined Raman spectroscopy and atomic force microscopy analyses show that a BL, fully covering the Si-face of SiC, forms as the first step of growth. Subsequently, 1LG starts to grow at step edges and continue to cover the terraces with a step-flow growth mechanism, as demonstrated. Eventually, reproducible syntheses of 1LG films were achieved by this temperature-controlled growth process. Then, we investigate the structural and electronic properties of the 1LG films. The integrated intensity of G-band in Raman spectra normalized with respect to a HOPG reference, AG/AG-HOPG, of each spectrum in Raman map of our continued graphene film is very close to the experimental value reported for a 1LG, demonstrating the good homogeneity. Atomically resolved scanning tunnelling microscopy (STM) evidences a (6x6) superstructure, indicative of 1LG covering a reconstructed interface layer. Regarding the transport measurement, quantum Hall plateau values observed in our graphene layers confirmed both continuity and thickness of the 1LG film8. Moreover, we estimated by transport measurement and STM analysis a hole concentration of pHall~10^10 - 10^11 cm^-2 at 1.7 K in helium atmosphere. We ascribe this unusual doping type to an atmosphere contamination effect since we regain the typical n-doped graphene under vacuum.

Raman spectroscopy is commonly used to determine the number of layers of few-layer graphene (FLG)samples. In this work, we focus on the criteria based on the G-band integrated intensity and on the laseroptical contrast. Limitations due to stacking order are discussed and lead to the conclusion that it isnecessary to combine Raman and optical contrast to avoid misinterpretation. Both methods enable todistinguish unambiguously between single layer graphene and multilayer graphene. However, neithereach method separately nor the combination of the two enable a determination of the number of layers forall possible stacking orientations. Importantly, since the two methods always significantly disagree whenthey fail, the comparison of the values deduced by each method allows to discriminate if the determinednumber of layers can be specified or not. Other important parameters (substrate, laser wavelength,objective numerical aperture) are discussed to define a reliable method to determine the number ofgraphene layers in FLG and its domain of validity. The proposed method which combines Raman andoptical contrast measurements, carried out with a 532 nm laser and using a 100x objective with anumerical aperture of 0.9, allows the determination of the number of layers for (up to 5) FLG on thefollowing substrates: (i) glass (soda lime glass or similar with refractive index between 1.50 and 1.55) and(ii) oxidized silicon (SiO2 on silicon, with a SiO2 thickness of 90 ± 5 nm). The method is however limited tohigh quality graphene and FLG with small defect density and low residue.

Thermal decomposition of silicon carbide (SiC) provides transfer free and wafer-scale homogeneous graphene forming on a semi-insulating substrate. Large-area monolayer graphene growth was initially developed at the atmospheric pressure (around 900 mbar) in an argon ambient and at high temperature (>1650°C). However, it remains challenging to obtain films with different and controlled characteristics such as the number of graphene layers or the doping by tuning the growth parameters. In this work, we optimized the epitaxial growth of monolayer graphene (1LG) on 4H-SiC (0001) under a low argon pressure of 10 mbar. This intermediate pressure allows growing a continued 1LG in a short process time of ~1h30. Here, we discuss the initial growth stages from buffer layer to 1LG as a function of annealing temperature (same heating rate). The combined Raman spectroscopy and atomic force microscopy (AFM) analyses show that a buffer layer, fully covering the Si-face of SiC, forms as the first step of growth (1600°C). Subsequently, 1LG starts to grow at step edges (1700°C) and continue to cover the buffer layer with a step-flow growth mechanism, as shown in the literature. Eventually, continued large-scale graphene films were achieved at 1750°C with a reproducible process. These graphene films were characterised by Raman, STM and electrical measurement. The integrated intensity of G-band in Raman spectra normalized with respect to a HOPG reference, AG/AG-HOPG, of each spectrum in Raman map is very close to the experimental value reported for a 1LG. Atomically resolved scanning tunnelling microscopy evidences a (6x6) superstructure, indicative of 1LG covering a reconstructed interface layer. Moreover, quantum Hall plateau values observed in our graphene layers confirmed both continuity and thickness of the 1LG film. The carrier density of pHall~1010 - 1011 cm-2 in 1LG films can be achieved at 1.7K in helium atmosphere. Thus, our as-grown 1LG graphene films with low p-type carrier density provide perspectives for further studies and applications on graphene.

Silicon carbide (SiC) sublimation is the most promising option to achieve transfer-free graphene at the wafer-scale. We investigated the initial growth stages from the buffer layer to monolayer graphene on SiC(0001) as a function of annealing temperature at low argon pressure (10 mbar). A buffer layer, fully covering the SiC substrate, forms when the substrate is annealed at 1600°C. Graphene formation starts from the step edges of the SiC substrate at higher temperature (1700°C). The spatial homogeneity of the monolayer graphene was observed at 1750°C, as characterized by Raman spectroscopy and magneto-transport. Raman spectroscopy mapping indicated an AG-graphene/AG-HOPG ratio of around 3.3%, which is very close to the experimental value reported for a graphene monolayer. Transport measurements fromroom temperature down to 1.7 K indicated slightly p-doped samples (p~10^10cm-2) and confirmed both continuity and thickness of the monolayer graphene film. Successive growth processes have confirmed the reproducibility and homogeneity of these monolayer films.

Graphene, known as one carbon layer material, holds attractive properties due to its hexagonal lattice, also called honeycomb structure. Since the seminal work on mechanically exfoliated few layer graphene, more growth processes were explored: Chemical Vapor Deposition (CVD) on metals or on silicon carbide (SiC); chemical reduction of graphene oxides and SiC sublimation. In contrast to the other graphene growth techniques, thermal decomposition of SiC provides wafer-scale homogeneous graphene spontaneously forming on semi-insulating substrate. SiC sublimation is the most promising option to achieve the transfer free and wafer-scale graphene. Furthermore, graphene/SiC is compatible with lithography techniques for further applications; thereby epitaxial graphene on SiC is a potential candidate for nanoelectronics. Until now, monolayer graphene growth by SiC sublimation at a pressure closed to the atmospheric pressure (around 900 mbar) and at high temperature (>1650°C) is well known . However, to obtain films with different and controlled characteristics such as the number of graphene layers or the type of doping by controlling the growth parameters remains challenging. We will present the initial growth stages from buffer layer to monolayer graphene on SiC (0001) as a function of the temperature at low pressure (10 mbar). A reproducible synthesis of low p-type doped monolayer graphene was optimized (few 10^11 cm-2). The p-type doping are obtained here on the bare samples without any post-growth process such as lithography. A prototypal HTA-100 furnace developed by Annealsys Company has allowed the synthesis of these original samples. All the samples were characterized by Raman spectroscopy and Atomic Force Microscopy (AFM). In addition, transport measurements (at room temperature and low temperature with high magnetic field) were carried out in the case of continuous monolayer graphene films showing especially characteristics of Quantum Hall Effect.

Counting number of graphene layers on copper ? (OK for SiO2/Si, SiC, glass)Graphene source in Montpellier by CVD on copper and on SiC and by SiC sublimation (development of prototype furnace)

Raman spectroscopy (RS) of graphene-related materials (GRM) is being considered as a fast, versatile, powerful and non-destructive characterization technique. Indeed, RS is sensitive to the number of layers, their stacking order, the nature and density of defects, the charge carrier density and in-plane strain variations. However, the positions, linewidths, profiles, intensities of the graphene/MLG Raman bands are not only affected by all these perturbations but also depends on the uniformity across the probed area (laser spot size and field depth) and on the substrate (through optical interference effects, dielectric screening…). An accurate interpretation of Raman spectra becomes then extremely complex and deserves the combined use of complementary diagnosis. We recently developed a unique expertise in terms graphene/MLG characterization. This know-how is based on instrumental developments (including laser power, transmission, reflection and Raman signal simultaneous monitoring), as well as the development of data treatment (including specific algorithms to subtract the substrate Raman background), interpretation tools and a detailed modeling of the Raman scattering and optical properties of graphene/MLG on a wide set of substrates (including SiC, Cu, SiO2/Si,…). Special care was paid to define protocols that ensure a high reproducibility and repeatability of calibrated measurements. In this contribution, we apply this tool for counting the number of layers of any kind of graphene samples. We define and validate standard procedures for GRM characterization on different substrates.