F. J. Ferrer

Graphene growth by molecular beam epitaxy on the carbon-face of SiC

Graphene layers have been grown by molecular beam epitaxy (MBE) on the (0001¯) C-face of SiC and have been characterized by atomic force microscopy, low energy electron diffraction (LEED), and UV photoelectron spectroscopy. Contrary to the graphitization process, the step-terrace structure of SiC is fully preserved during the MBE growth. LEED patterns show multiple orientation domains which are characteristic of graphene on SiC (0001¯), indicating non-Bernal rotated graphene planes. Well-defined Dirac cones, typical of single-layer graphene, have been observed in the valence band for few graphene layers by synchrotron spectroscopy, confirming the electronic decoupling of graphene layers.

Electrochromic behavior of WxSiyOz thin films prepared by reactive magnetron sputtering at normal and glancing angles

This work reports the synthesis at room temperature of transparent and colored W(x)Si(y)O(z) thin films by magnetron sputtering (MS) from a single cathode. The films were characterized by a large set of techniques including X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectrometry (RBS), Fourier transform infrared (FT-IR), and Raman spectroscopies. Their optical properties were determined by the analysis of the transmission and reflection spectra. It was found that both the relative amount of tungsten in the W-Si MS target and the ratio O(2)/Ar in the plasma gas were critical parameters to control the blue coloration of the films. The long-term stability of the color, attributed to the formation of a high concentration of W(5+) and W(4+) species, has been related with the formation of W-O-Si bond linkages in an amorphous network. At normal geometry (i.e., substrate surface parallel to the target) the films were rather compact, whereas they were very porous and had less tungsten content when deposited in a glancing angle configuration. In this case, they presented outstanding electrochromic properties characterized by a fast response, a high coloration, a complete reversibility after more than one thousand cycles and a relatively very low refractive index in the bleached state.

A new bottom-up methodology to produce silicon layers with a closed porosity nanostructure and reduced refractive index

A new approach is presented to produce amorphous porous silicon coatings (a-pSi) with closed porosity by magnetron sputtering of a silicon target. It is shown how the use of He as the process gas at moderated power (50-150 W RF) promotes the formation of closed nanometric pores during the growth of the silicon films. The use of oblique-angle deposition demonstrates the possibility of aligning and orientating the pores in one direction. The control of the deposition power allows the control of the pore size distribution. The films have been characterized by a variety of techniques, including scanning and transmission electron microscopy, electron energy loss spectroscopy, Rutherford back scattering and x-ray photoelectron spectroscopy, showing the incorporation of He into the films (most probably inside the closed pores) and limited surface oxidation of the silicon coating. The ellipsometry measurements show a significant decrease in the refractive index of porous coatings (n(500 nm) = 3.75) in comparison to dense coatings (n(500 nm) = 4.75). The capability of the method to prepare coatings with a tailored refractive index is therefore demonstrated. The versatility of the methodology is shown in this paper by preparing intrinsic or doped silicon and also depositing (under DC or RF discharge) a-pSi films on a variety of substrates, including flexible materials, with good chemical and mechanical stability. The fabrication of multilayers of silicon films of controlled refractive index in a simple (one-target chamber) deposition methodology is also presented.

Microscopic and macroscopic dielectric description of mixed oxide thin films

Compact Si–Ti–O and Si–Zr–O mixed oxide thin films are studied by optical characterization (refractive index, band gap energy) and local probes (Auger parameter obtained by x-ray photoelectron spectroscopy). Interpretation of the obtained results is discussed in the framework of the classical dielectric theory that correlates the macroscopic refractive index to the microscopic electronic polarizability of each particular ion in the compound through the Lorentz-Lorenz relationship. Quantum mechanical cluster calculations have also been performed to support the correlations obtained between the experimental findings.

Atomic scale flattening, step formation and graphitization blocking on 6H- and 4H-SiC{0 0 0 1} surfaces under Si flux

Atomically flat terraces separated by almost parallel steps have been obtained on 6H- and 4H-SiC{0 0 0 1} surfaces after extended annealing under Si flux in the 1050?1100 ?C temperature range for both the Si- and C-face. The height of the steps is one-half the unit cell for both faces and polytypes. Graphitization is avoided because the incoming Si atoms compensate for the outgoing sublimated Si.

Nanostructured Ti thin films by magnetron sputtering at oblique angles

The growth of Ti thin films by the magnetron sputtering technique at oblique angles and at room temperature is analysed from both experimental and theoretical points of view. Unlike other materials deposited in similar conditions, the nanostructure development of the Ti layers exhibits an anomalous behaviour when varying both the angle of incidence of the deposition flux and the deposition pressure. At low pressures, a sharp transition from compact to isolated, vertically aligned, nanocolumns is obtained when the angle of incidence surpasses a critical threshold. Remarkably, this transition also occurs when solely increasing the deposition pressure under certain conditions. By the characterization of the Ti layers, the realization of fundamental experiments and the use of a simple growth model, we demonstrate that surface mobilization processes associated to a highly directed momentum distribution and the relatively high kinetic energy of sputtered atoms are responsible for this behaviour.

Microstructural and Chemical Characterization of Nanostructured TiAlSiN Coatings with Nanoscale Resolution

Nanoscale resolution electron microscopy analysis combined with ion beam assisted techniques are presented here, to give answers to full characterization of morphology, growth mode, phase formation, and compositional distribution in nanocomposite TiAlSiN coatings deposited under different energetic conditions. Samples were prepared by magnetron sputtering, and the effects of substrate temperature and bias were investigated. The nanocomposite microstructure was demonstrated by the formation of a face-centered cubic (Ti,Al)N phase, obtained by substitution of Al in the cubic titanium nitride (c-TiN) phase, and an amorphous matrix at the column boundary regions mainly composed of Si, N (and O for the samples with higher oxygen contents). Oxygen impurities, predicted as the principal responsible for the degradation of properties, were identified, particularly in nonbiased samples and confirmed to occupy preferentially nitrogen positions at the column boundaries, being mainly associated to silicon forming oxynitride phases. It has been found that the columnar growth mode is not the most adequate to improve mechanical properties. Only the combination of moderate bias and additional substrate heating was able to reduce the oxygen content and eliminate the columnar microstructure leading to the nanocomposite structure with higher hardness (>30 GPa).

Initial stages of graphitization on SiC(000-1), as studied by phase atomic force microscopy

The initial stages of graphitization on 4H- and 6H-SiC (000-1) under ultrahigh vacuum at temperatures of 1125–1175°C have been studied by atomic force microscopy (AFM), X-ray photoemission spectroscopy and reflected high energy electron diffraction. A progressive coverage of the surface by graphene has been observed depending on the time and temperature of annealing. Graphene growth mainly starts from the step edges, although it sometimes nucleates in the middle of a SiC terrace. Comparison of the topographic and phase AFM images shows that the latter are the most efficient for identifying graphene before complete coverage of the surface.

Size and shape of supported zirconia nanoparticles determined by x-ray photoelectron spectroscopy

In this paper we want to go a step forward by comparing the results obtained with the peak shape analysis methodology with the evolution of electronic parameters of Zr i.e., binding energy and Auger parameter 6‐8 of characteristic electron photoemitted peaks as a function of the amount of deposited zirconia moieties. In previous publications we have shown that these parameters change with the size of the particles and that this variation may provide interesting clues for the electronic interactions that develop at the interface between the deposited phase and the substrate. 9‐13 In the

Energy-Sensitive Ion- and Cathode-Luminescent Radiation-Beam Monitors Based on Multilayer Thin-Film Designs

A multilayer luminescent design concept is presented to develop energy-sensitive radiation-beam monitors on the basis of colorimetric analysis. Each luminescent layer within the stack consists of rare-earth-doped transparent oxides of optical quality and a characteristic luminescent emission under excitation with electron or ion beams. For a given type of particle beam (electron, protons, α particles, etc.), its penetration depth and therefore its energy loss at a particular buried layer within the multilayer stack depend on the energy of the initial beam. The intensity of the luminescent response of each layer is proportional to the energy deposited by the radiation beam within the layer, so characteristic color emission will be achieved if different phosphors are considered in the layers of the luminescent stack. Phosphor doping, emission efficiency, layer thickness, and multilayer structure design are key parameters relevant to achieving a broad colorimetric response. Two case examples are designed and fabricated to illustrate the capabilities of these new types of detector to evaluate the kinetic energy of either electron beams of a few kilo-electron volts or α particles of a few mega-electron volts.