Papers by Author: Samira Abdelli-Messaci

Abstract: Plume expansion dynamics of an ablated graphite target by the third harmonic of Nd-YAG laser of 355 nm wavelength has been investigated using an ICCD camera fast imaging. This study was carried out into vacuum and methane (CH4) atmosphere at different pressures. Into vacuum, the two dimensional spatio-temporal evolution of plasma plume was performed at a laser irradiance range of 5.5x108 - 6x109 W/cm2. The plasma mean velocity has been estimated and was found to increase with laser power density increases until it reaches a maximum value of 107 cm/s. In presence of gas, the plasma plume dynamics was studied at a fixed irradiance of 4.3x 109 W/cm2 and from 0.05 to 5 mbar CH4 pressures. The plasma spatio-temporal evolution was found to be influenced by the gas pressure. In earlier time the expansion was almost linear independently of the background gas pressure used. However, as time evolves, the plume is decelerated, then presented a stationary behaviour at 1 and 5 mbar. Furthermore, the light emission from the plume very close to the target surface exists until a few microseconds into vacuum and CH4 atmosphere and then vanishes. It appears again around 10 µs following by the emission of particulates as bright incandescent tracks.

Abstract: In this contribution we study the effect of the laser fluence on the stoichiometry, morphology and density of Sm1-xNdxNiO3 thin films. The latter were grown by a KrF excimer laser (λ = 248 nm, τ = 25 ns) ablation of a rotating target onto unheated (100) silicon substrates for 9000 pulses at different laser fluences into vacuum. The target used was a mixture of samarium, neodymium and nickel oxides. The relative ratio of neodymium (x = 0.45) is set to have a transition temperature close to room temperature (TMI = 310 K). The target-substrate distance was maintained at 4 cm. The composition and the morphology of the deposited layers were analysed by energy dispersion X-ray spectroscopy (EDX) and scanning electron microscope (SEM) respectively. It was found that films properties depend strongly on the laser fluence. The EDX measurements revealed that the laser fluence must be higher than 1 Jcm-2 for a congruent evaporation. However, even at this condition, the films were deficiency in oxygen. The morphology study showed that the films surface was widely contaminated by droplets for fluences superior to 2 Jcm-2. Also, it was found that by increasing laser fluence the films density increases and reach a plateau at 1.3 Jcm-2. According to all those elements, the laser fluence was set to be in the range of 1.3 – 2 Jcm-2.

Abstract: Carbon nitride films were synthesized by pulsed laser ablation of graphite target under nitrogen ambience. The third harmonic of a pulsed Nd-YAG laser of 355 nm wavelength and 7 ns pulse duration was focused onto a rotating target at an incidence angle of 45°. The laser fluence at the target surface was set at 30 J/cm2. The carbon nitride films were deposited on (100) silicon substrate kept at room temperature and placed at a distance of 40 mm from the target surface. The CNx films were grown under N2 gas in the pressure range of 5×10-3 to 4×10-1 mbar. The deposited films composition was investigated by different techniques RBS, NRA and AES. We found an N/C ratio equal to 0.4 in the pressure range cited above.

Abstract: In this work we report the optical characteristics of carbon nitride films produced by a KrF excimer laser ablation technique. The ablated materials were collected on different wafers, glass and porous silicon for different N2 pressures (0.1-0.5mbar). The thin films were synthesized at room temperature. The deposited thin films were characterized by spectrophotometry, ellipsometry, scanning electron microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). The results show that the optical band gap deduced from optical transmission spectra in the ultraviolet- visible – near infrared range increases with deposition time and with nitrogen pressures increasing. SEM observation indicates that the CNx film is granular. Finally, FTIR spectra reveal carbon nitride absorption bands which do not seem change considerably with N2 pressures.

Abstract: The perovskites RNiO3 (R rare earth ≠ La) are classified as a phase transition metal-insulator. The transition temperature is modulated by the size of the rare earth. The use of compound R1-xR'xNiO3 can vary transition temperature on a wide thermal range depending on the concentration of the two rare earths. The Sm1-xNdxNiO3 (x = 0.45) thin layers have been carried out on (100) silicon substrates by KrF laser ablation (λ = 248 nm, 25ns) at two different fluences 2 and 3 Jcm-2. The oxygen pressure and the target-substrate distance have been maintained at 0.2 mbar and 4 cm respectively. The deposition temperature has been set at 500 ° C. The obtained layers were characterized by X-ray diffraction, atomic force microscopy and Rutherford back scattering diagnostics. The resistivity Measurements were carried out by the conventional four-probe method. The XRD spectra revealed the presence of an ideal cubic perovskite phase. The RBS analysis showed that the deposited layers are rich in oxygen. A correlation between the morphology properties of the deposited layers and the plasma dynamics studied by fast imaging has been found.