Jean Roch Huntzinger

Selective epitaxial growth of graphene on SiC

We present a method of selective epitaxial growth of few layers graphene (FLG) on a “prepatterned” silicon carbide (SiC) substrate. The methods involves, successively, the sputtering of a thin aluminium nitride (AlN) layer on top of a monocrystalline SiC substrate and, then, patterning it with e-beam lithography and wet etching. The sublimation of few atomic layers of Si from the SiC substrate occurs only through the selectively etched AlN layer. The presence of the Raman G-band at ∼1582cm−1 in the AlN-free areas is used to validate the concept. It gives absolute evidence of selective FLG growth.

Early stage formation of graphene on the C face of 6H-SiC

An investigation of the early stage formation of graphene on the C face of 6H-silicon carbide (SiC) is presented. We show that the sublimation of few atomic layers of Si out of the SiC substrate is not homogeneous. In good agreement with the results of theoretical calculations it starts from defective sites, mainly dislocations that define nearly circular graphene layers, which have a pyramidal, volcanolike shape with a center chimney where the original defect was located. At higher temperatures, complete conversion occurs but, again, it is not homogeneous. Within the sample surface, the intensity of the Raman bands evidences inhomogeneous thickness.An investigation of the early stage formation of graphene on the C face of 6H-silicon carbide (SiC) is presented. We show that the sublimation of few atomic layers of Si out of the SiC substrate is not homogeneous. In good agreement with the results of theoretical calculations it starts from defective sites, mainly dislocations that define nearly circular graphene layers, which have a pyramidal, volcanolike shape with a center chimney where the original defect was located. At higher temperatures, complete conversion occurs but, again, it is not homogeneous. Within the sample surface, the intensity of the Raman bands evidences inhomogeneous thickness.

X-Ray Diffraction and Raman Spectroscopy Study of Strain in Graphene Films Grown on 6H-SiC(0001) Using Propane-Hydrogen-Argon CVD

We have grown graphene films on 6H-SiC(0001) using propane CVD and evidenced the strong impact of the hydrogen/argon mixture used as the carrier gas on the graphene/SiC interface and on the orientation of graphene layers. By studying a set of samples grown with different hydrogen/argon mixture using Raman spectroscopy and grazing incidence X-ray diffraction, we evidence the links between graphene/SiC interface and strain in graphene.

Uniformity of Epitaxial Graphene on On-Axis and Off-Axis SiC Probed by Raman Spectroscopy and Nanoscale Current Mapping

Epitaxial graphene was grown on the surface of on-axis and off-axis SiC (0001) by solid state graphitization at high temperatures (2000 °C) in Ar ambient. The effect of the miscut angle on the lateral uniformity of the few layers of graphene (FLG) was investigated by combined application of micro-Raman spectroscopy and Torsion Resonance Conductive Atomic Force Microscopy, the latter method enabling a quantification of the FLG coverage on SiC with submicrometer lateral resolution. While the on-axis samples result in uniform coverage by thin (~ 3 monolayers) FLG, the coverage for off-axis samples is much less uniform, following closely the step bunching morphology of the SiC surface.

Surface loving and surface avoiding modes

We theoretically study the propagation of sound waves in GaAs/AlAs superlattices focusing on periodic modes in the vicinity of the band gaps. Based on analytical and numerical calculations, we show that these modes are the product of a quickly oscillating function times a slowly varying envelope function. We carefully study the phase of the envelope function compared to the surface of a semi-infinite superlattice. Especially, the dephasing of the superlattice compared to its surface is a key parameter. We exhibit two kind of modes: Surface Avoiding and Surface Loving Modes whose envelope functions have their minima and respectively maxima in the vicinity of the surface. We finally consider the observability of such modes. While Surface Avoiding Modes have experimentally been observed [Phys. Rev. Lett. 97, 1224301 (2006)], we show that Surface Loving Modes are likely to be observable and we discuss the achievement of such experiments. The proposed approach could be easily transposed to other types of wave propagation in unidimensional semi-infinite periodic structures as photonic Bragg mirror.

Differences between Graphene Grown on Si-Face and C-Face

Epitaxial graphene growth is significantly different depending on the polarity of the 6H-SiC surface: Si- or C-face. On the Si-face, a uniform coverage of few layers on the whole sample can be obtained, but with electrical properties disturbed by the presence of a Carbon-rich buffer layer at the interface. On the contrary, on the C-face, we demonstrated that almost free-standing very large monolayers of graphene can be obtained by covering the sample with a graphitic cap during the growth.

Growth of Few Graphene Layers on 6H, 4H and 3C-SiC Substrates

We report a comparative investigation of few layers graphene grown on 6H, 4H and 3C-SiC substrates. We show that the size of the graphitic domains depends more on the <0001> SiC surface orientation than the polytypism.

Scattering of light by sound on a nanoscale

We report on light scattering experiments (Raman-Brillouin) in semiconductor quantum wells and quantum dots nanostructures. All measurements were performed under resonant excitation of the optical transitions involving confined electronic states. The scattered light was detected in the very low-frequency range around the Rayleigh line. We observe strong oscillations of the scattered intensity. Their period and relative amplitudes depend on the sample characteristics (size, density and spatial distribution of nano-objects). We show that such signal originates from interference effects due to the interaction between sound waves and the excited electronic density. By comparing simulated and measured spectra, we are able to extract, from the experiments, sample characteristics such as average size and size distribution of quantum dots. This optical sensing technique, namely Raman interferometry, is similar to the well-known X-ray diffraction technique, in the sense that it allows imaging of electronic states in the reciprocal space. Moreover, we show that Raman interferometry is a surface sensitive technique. By using quantum dots and quantum wells as Thz acoustic-detectors we are able to measure the reflection of sound waves at the sample surface. The surface characteristics (nano-scale roughness and oxidation) can be addressed using this method.