Rachid Belkhou

Observation of thermally activated domain wall transformations

The spin structure of head-to-head domain walls in Ni80Fe20 structures is studied using high-resolution photoemission electron microscopy. The quantitative phase diagram is extracted from these measurements and found to exhibit two phase boundaries between vortex and transverse domain walls. The results are compared with available theoretical predictions and micromagnetic simulations and differences to the experiment are explained, taking into account thermal excitations. Temperature-dependent measurements show a thermally activated transformation of transverse to vortex domain walls in 7 nm thick and 730 nm wide structures at a transition temperature between 260 °C and 310 °C, which corresponds to a nucleation barrier height for a vortex wall between 6.7×10−21J and 8.0×10−21J.

Epitaxial Graphene on 4H-SiC(0001) Grown under Nitrogen Flux: Evidence of Low Nitrogen Doping and High Charge Transfer

Nitrogen doping of graphene is of great interest for both fundamental research to explore the effect of dopants on a 2D electrical conductor and applications such as lithium storage, composites, and nanoelectronic devices. Here, we report on the modifications of the electronic properties of epitaxial graphene thanks to the introduction, during the growth, of nitrogen-atom substitution in the carbon honeycomb lattice. High-resolution transmission microscopy and low-energy electron microscopy investigations indicate that the nitrogen-doped graphene is uniform at large scale. The substitution of nitrogen atoms in the graphene planes was confirmed by high-resolution X-ray photoelectron spectroscopy, which reveals several atomic configurations for the nitrogen atoms: graphitic-like, pyridine-like, and pyrrolic-like. Angle-resolved photoemission measurements show that the N-doped graphene exhibits large n-type carrier concentrations of 2.6 × 10(13) cm(-2), about 4 times more than what is found for pristine graphene, grown under similar pressure conditions. Our experiments demonstrate that a small amount of dopants (<1%) can significantly tune the electronic properties of graphene by shifting the Dirac cone about 0.3 eV toward higher binding energies with respect to the π band of pristine graphene, which is a key feature for envisioning applications in nanoelectronics.

Structural coherency of epitaxial graphene on 3C-SiC(111) epilayers on Si(111)

Graphene has emerged as a promising nanoelectronic material in electronic devices applications and studying two-dimensional electron gases with relativistic dispersion near Dirac point. Nonetheless, the control of the preparation conditions for homogeneous large-area graphene layers is difficult. Here, we illustrate evidence for high structural and electronic quality epitaxial graphene on 3C-SiC(111). Morphology and electronic structure of the graphene layers have been analyzed with low energy electron microscopy and angle resolved photoemission spectroscopy. Using scanning tunneling microscopy and scanning transmission electron microscopy, we show that graphene exhibits remarkably continuity of step edges suggesting the possibility of growing large scale graphene layer

Large-Area and High-Quality Epitaxial Graphene on Off-Axis SiC Wafers

The growth of large and uniform graphene layers remains very challenging to this day due to the close correlation between the electronic and transport properties and the layer morphology. Here, we report the synthesis of uniform large-scale mono- and bilayers of graphene on off-axis 6H-SiC(0001) substrates. The originality of our approach consists of the fine control of the growth mode of the graphene by precise control of the Si sublimation rate. Moreover, we take advantage of the presence of nanofacets on the off-axis substrate to grow a large and uniform graphene with good long-range order. We believe that our approach represents a significant step toward the scalable synthesis of graphene films with high structural qualities and fine thickness control, in order to develop graphene-based electronic devices.

Epitaxial graphene on single domain 3C-SiC(100) thin films grown on off-axis Si(100)

The current process of growing graphene by thermal decomposition of 3C-SiC(100) on silicon is technologically attractive. Here, we study epitaxial graphene on single domain 3C-SiC films on off-axis Si(100). The structural and electronic properties of such graphene layers are explored by atomic force microscopy, x-ray photoelectron spectroscopy, and Raman spectroscopy. Using low energy electron diffraction, we show that graphene exhibits single planar domains. Near-edge x-ray absorption fine structure is used to characterize the sample, which confirms that the graphene layers present sp2 hybridization and are homogeneously parallel to the substrate surface.

Tailoring of the structural and magnetic properties of MnAs films grown on GaAs—Strain and annealing effects

MnAs films were deposited by molecular-beam epitaxy on GaAs(001) and GaAs(111)B surfaces. Imaging of the temperature-dependent magnetic structure by x-ray magnetic circular dichroism photoemission electron microscopy, and the comparison with magnetization measurements by superconducting quantum interference device (SQUID) magnetometry, is used to study the impact of the different strain state of MnAs/GaAs(001) and of MnAs/GaAs(111)B films on the phase transition between ferromagnetic α-MnAs and paramagnetic β-MnAs, the spatial distribution of the two structural and magnetic phases, and the transition temperature. For the isotropically strained MnAs/GaAs(111)B films, the phase coexistence range is much wider than for the anisotropically strained MnAs/GaAs(001) films. The characteristic change of the saturation magnetization with film thickness is found to be a universal property of films with different epitaxial orientation, if at least one MnAs⟨112¯0⟩ direction is in the film plane. For MnAs/GaAs(001) fil...

Formation of a surface alloy by annealing of Pt/Cu(11

The dissolution of Pt/Cu(111) and the kinetic blocking of the dissolution process which results in the formation of a surface alloy have been studied. We use AES, PES, XPD, and LEED to characterise the non-classical dissolution behaviour. Chemical shifts on the Pt 4f72 and the Cu 3p core levels are measured during dissolution. The top layer of the surface alloy is Cu3Pt for an annealing temperature of 315°C. For the higher annealing temperature of 350°C the Pt concentration is smaller. The alloy composition is therefore temperature dependent and the formation of a relatively stable alloy in local equilibrium over the surface layers is determined by the segregation and ordering behaviour. The chemical ordering of the surface alloy and its electronic structure remains to be verified.

Atomically Sharp Interface in an h-BN-epitaxial graphene van der Waals Heterostructure

Stacking various two-dimensional atomic crystals is a feasible approach to creating unique multilayered van der Waals heterostructures with tailored properties. Herein for the first time, we present a controlled preparation of large-area h-BN/graphene heterostructures via a simple chemical deposition of h-BN layers on epitaxial graphene/SiC(0001). Van der Waals forces, which are responsible for the cohesion of the multilayer system, give rise to an abrupt interface without interdiffusion between graphene and h-BN, as shown by X-ray Photoemission Spectroscopy (XPS) and direct observation using scanning and High-Resolution Transmission Electron Microscopy (STEM/HRTEM). The electronic properties of graphene, such as the Dirac cone, remain intact and no significant charge transfer i.e. doping, is observed. These results are supported by Density Functional Theory (DFT) calculations. We demonstrate that the h-BN capped graphene allows the fabrication of vdW heterostructures without altering the electronic properties of graphene.

360° domain wall generation in the soft layer of magnetic tunnel junctions

High spatial resolution x-ray photoemission electron microscopy technique has been used to study the influence of the dipolar coupling taking place between the NiFe and the Co ferromagnetic electrodes of micron sized, elliptical shaped magnetic tunnel junctions. The chemical selectivity of this technique allows us to observe independently the magnetic domain structure in each ferromagnetic electrode. The combination of this powerful imaging technique with micromagnetic simulations allows us to evidence that a 360° domain wall can be stabilized in the NiFe soft layer. In this letter, we discuss the origin and the formation conditions of those 360° domain walls evidenced experimentally and numerically.

Direct view at colossal permittivity in donor-acceptor (Nb, In) co-doped rutile TiO2

Topical observations of colossal permittivity (CP) with low dielectric loss in donor-acceptor cations co-doped rutile TiO2 have opened up several possibilities in microelectronics and energy-storage devices. Yet, the precise origin of the CP behavior, knowledge of which is essential to empower the device integration suitably, is highly disputed in the literature. From spectromicroscopic approach besides dielectric measurements, we explore that microscopic electronic inhomogeneities along with the nano-scale phase boundaries and the low temperature polaronic relaxation are mostly responsible for such a dielectric behavior, rather than electron-pinned defect-dipoles/grain-boundary effects as usually proposed. Donor-acceptor co-doping results in a controlled carrier-hopping inevitably influencing the dielectric loss while invariably upholding the CP value.