Mohamed Chaker

Direct evaluation of the sp3 content in diamond-like-carbon films by XPS

The analysis of the X-ray photoelectron spectra (XPS) of the C 1s core level of pulsed laser deposited diamond-like carbon thin films, obtained at different laser intensities is presented. These spectra are deconvoluted into two different contributions, at 284.4 eV and 285.2 eV, which are respectively attributed to sp2 and sp3 hybridized carbon atoms. From the deconvoluted spectra, the sp3 content in the films is evaluated. It is found to increase from 33% to about 60% as laser intensity is varied from 0.9×108 to 7.1×108 W/cm2. These measurements have been compared to those obtained by the analysis of the C KLL X-ray excited Auger electron spectra. The two methods provide the same qualitative variation of the sp3 content with laser intensity. However, the XPS of the C 1s core level yields systematically higher sp3 content values. These differences are attributed to the presence of an sp2 rich outer layer on the surface of the DLC films, as confirmed by angle-resolved XPS. The analysis of the C 1s peak is shown to be a very simple and direct method to evaluate the sp3 content in unhydrogenated DLC thin films.

Temporal characterization of femtosecond laser pulses induced plasma for spectrochemical analysis of aluminum alloys

Abstract This paper reports studies on time-resolved space-integrated laser induced breakdown spectroscopy (LIBS) of plasmas produced by ultrashort laser pulses at atmospheric pressure, on aluminum alloy targets. The temporal behavior of specific ion and neutral emission lines of Al, Mg and Fe has been characterized. The results show a faster decay of continuum and spectral lines, and a shorter plasma lifetime than in the case of longer laser pulses. Spectroscopic diagnostics were used to determine the time-resolved electron density, as well as the excitation and ionization temperatures. In comparison with plasmas produced by ns laser pulses, the plasma generated by ultrashort pulses exhibits a faster thermalization. Analytical performances of fs-LIBS were also evaluated. Linear calibration curves for minor elements (Mg, Fe, Si, Mn, Cu) presented in aluminum alloys were obtained. The limits of detection are in the parts per million (ppm) range and are element-dependent.

A photoinduced metal-like phase of monoclinic VO2 revealed by ultrafast electron diffraction

How to make vanadium dioxide metallic At about 70°C, the material vanadium dioxide (VO2) switches from being a semiconductor to a metal. The switch happens so fast that it may be useful in electronic devices, but it is not clear whether the switch is primarily caused by enhanced interactions between electrons or by a change in the crystal structure. Morrison et al. shone laser light on a sample of VO2, initially in a semiconducting state. They used electron diffraction to monitor the changes in the materials crystal structure and simultaneously measured its optical properties to monitor the electronic state. For certain laser powers, VO2 switched to a long-lived metallic state even though it preserved its initial crystal structure. Science, this issue p. 445 Simultaneous measurements of structural and optical properties are used to study optically excited vanadium dioxide. The complex interplay among several active degrees of freedom (charge, lattice, orbital, and spin) is thought to determine the electronic properties of many oxides. We report on combined ultrafast electron diffraction and infrared transmissivity experiments in which we directly monitored and separated the lattice and charge density reorganizations that are associated with the optically induced semiconductor-metal transition in vanadium dioxide (VO2). By photoexciting the monoclinic semiconducting phase, we were able to induce a transition to a metastable state that retained the periodic lattice distortion characteristic of the semiconductor but also acquired metal-like mid-infrared optical properties. Our results demonstrate that ultrafast electron diffraction is capable of following details of both lattice and electronic structural dynamics on the ultrafast time scale.

Effects of Ti–W codoping on the optical and electrical switching of vanadium dioxide thin films grown by a reactive pulsed laser deposition

Thin films of thermochromic VO2, V1−xWxO2 and V1−x−yWxTiyO2 (x=0.014, and y=0.12) were synthesized onto quartz substrates using a reactive pulsed laser deposition technique. The films were then characterized by x-ray diffraction and x-ray photoelectron spectroscopy. The W and Ti dopant effects on the semiconductor-to-metal phase transition of VO2 were investigated by measuring the temperature dependence of their electrical resistivity and their infrared transmittance. Remarkably strong effects of Ti–W codoping were observed on both the optical and electrical properties of V1−x−yWxTiyO2 films. The IR transmittance was improved, while the transition temperature could be varied from 36°C for W-doped VO2 film to 60°C for Ti–W codoped VO2 film. In addition, at room temperature, a higher temperature coefficient of resistance of 5.12%∕°C is achieved. Finally, both optical and electrical hysteresis are completely suppressed by Ti–W codoping the VO2 films.

Hardness and Young's modulus of amorphous a -SiC thin films determined by nanoindentation and bulge tests

Due to its interesting mechanical properties, silicon carbide is an excellent material for many applications. In this paper, we report on the mechanical properties of amorphous hydrogenated or hydrogen-free silicon carbide thin films deposited by using different deposition techniques, namely plasma enhanced chemical vapor deposition (PECVD), laser ablation deposition (LAD), and triode sputtering deposition (TSD). a -Si x C 1− x : H PECVD, a -SiC LAD, and a -SiC TSD thin films and corresponding free-standing membranes were mechanically investigated by using nanoindentation and bulge techniques, respectively. Hardness ( H ), Youngs modulus ( E ), and Poissons ratio ( v ) of the studied silicon carbide thin films were determined. It is shown that for hydrogenated a -Si x C 1− x : H PECVD films, both hardness and Youngs modulus are dependent on the film composition. The nearly stoichiometric a -SiC: H films present higher H and E values than the Si-rich a -Si x C 1−x : H films. For hydrogen-free a -SiC films, the hardness and Youngs modulus were as high as about 30 GPa and 240 GPa, respectively. Hydrogen-free a -SiC films present both hardness and Youngs modulus values higher by about 50% than those of hydrogenated a -SiC: H PECVD films. By using the FTIR absorption spectroscopy, we estimated the Si-C bond densities ( N SiC ) from the Si-C stretching absorption band (centered around 780 cm −1 ), and were thus able to correlate the observed mechanical behavior of a -SiC films to their microstructure. We indeed point out a constant-plus-linear variation of the hardness and Youngs modulus upon the Si-C bond density, over the N SiC investigated range [(4–18) × 10 22 bond · cm −3 ], regardless of the film composition or the deposition technique.

X-RAY PHOTOELECTRON SPECTROSCOPY OF CARBON NITRIDE FILMS DEPOSITED BY GRAPHITE LASER ABLATION IN A NITROGEN POSTDISCHARGE

Carbon nitride thin films have been deposited on silicon substrates, using a newly developed surface wave discharge/pulsed laser deposition system. Nitrogen incorporation in the films is examined by x‐ray photoelectron spectroscopy (XPS). It shows that interaction between the laser ablated carbon species and nitrogen atoms from the surface‐wave N2 plasma enhances the incorporation of N in the carbon nitride layers, for example, up to 19% at a deposition pressure of 2 mTorr. Increasing the deposition temperature decreases nitrogen incorporation and changes the local chemical environment of nitrogen atoms.

Remarkably enhanced photocatalytic activity of laser ablated Au nanoparticle decorated BiFeO3 nanowires under visible-light

Hybrid photocatalysts consisting of single crystalline BiFeO3 nanowires and laser ablated Au nanoparticles were synthesized by a functionalization-step-free solution process. The 1.0 wt% Au nanoparticle decorated BiFeO3 nanowires exhibit ~30 times higher photocatalytic activity for water oxidation than that exhibited by the parent wires during the first 4 h.

Ultrafast x‐ray sources*

Time‐resolved spectroscopy (with a 2 psec temporal resolution) of plasmas produced by the interaction between solid targets and a high contrast subpicosecond table top terawatt (T3) laser at 1016 W/cm2, is used to study the basic processes which control the x‐ray pulse duration. Short x‐ray pulses have been obtained by spectral selection or by plasma gradient scalelength control. Time‐dependent calculations of the atomic physics [Phys. Fluids B 4, 2007, 1992] coupled to a Fokker–Planck code [Phys. Rev. Lett. 53, 1461, 1984] indicate that it is essential to take into account the non‐Maxwellian character of the electron distribution for a quantitative analysis of the experimental results.

Effect of rapid thermal annealing on both the stress and the bonding states of a-SiC:H films

The stress evolution of plasma enhanced chemical vapor deposition a‐SiC:H films was studied by increasing the annealing temperature from 300 to 850 °C. A large stress range from −1 GPa compressive to 1 GPa tensile was investigated. Infrared absorption, x‐ray photoelectron spectroscopy, and elastic recoil detection analysis techniques were used to follow the Si‐C, Si‐H, and C‐H absorption band evolutions, the Si2p and C1s chemical bondings, and the a‐SiC:H film hydrogen content variations with the annealing temperatures, respectively. It is pointed out that the compressive stress relaxation is due to the hydrogenated bond (Si—H and C—H) dissociation, whereas the tensile stress is caused by additional Si—C bond formation. At high annealing temperatures, a total hydrogen content decrease is clearly observed. This total hydrogen loss is interpreted in terms of hydrogen molecule formation and outerdiffusion. The results are discussed and a quantitative model correlating the intrinsic stress variation to the Si...

Properties of TiC thin films grown by pulsed laser deposition

Abstract Titanium carbide thin films have been deposited on both (100) silicon and fused silica substrates by pulsed laser ablating a polycrystalline TiC target. At a KrF excimer laser intensity of about 8×10 8 W/cm 2 , the pulsed laser deposition (PLD) of TiC films was investigated at substrate deposition temperatures ranging from 25 to 600°C. The structure, surface composition, electrical resistivity, stress, work function and morphology of the PLD TiC films were characterized as a function of the deposition temperature. While all the deposited TiC films are polycrystalline with a preferred (111) orientation, both the magnitude of their compressive stress and their resistivity were found to decrease gradually as a function of the deposition temperature, from −7 to ∼−3 GPa and from 140 to 80 μΩ cm, respectively. The X-ray photoelectron spectroscopy (XPS) analysis revealed that oxygen (at an average level of ∼10 at.%) is incorporated into the TiC films where it substitutes for C. The surface composition of the films is found to be stoichiometric with an average Ti/(C+O) ratio of 0.98±0.06. The deposited TiC films exhibited low work functions in the (3.74–3.94) eV range. Their surface morphology is characterized by a very smooth surface on which some particulates are present. The density of these particulates (of which typical lateral dimension is about 1 μm and height is about 100 nm) is of the order of 2 per 100 μm 2 . The properties of these PLD TiC films are discussed in view of their use as electron injecting electrode materials for organic electroluminescent devices.