Papers by Author: Tahar Kerdja

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: The formation of Ti and Si nanoparticles by laser ablation of metallic targets in liquid environment is reported. The use of Second Harmonic Generation (SHG) pulsed Nd-YAG laser (532 nm, 15ns, 20 Hz repetition rate) allows a good rate of nanoparticles (NPs) formation. The NPs were characterized by UV-Visible transmission spectrometry and (SEM) scanning electron microscopy. The results show spherical Ti NPs and squared Si NPs. The optical band gap energies of the colloidal solutions were estimated for the two targets as function of the laser fluences.

Abstract: In thin film deposition by pulsed laser ablation (PLD), the mass ablation rate depends on laser energy, on the pulse duration and on the thermodynamic properties of the ablated materials. In order to optimize the PLD technique and the films quality, the evolution of the amount of the ejected materials with laser irradiance, the SEM images of the laser impacts on the target and the ion yield in the vapour plume, were used. This allows us to predict the different mechanisms that are responsible to mass ablation according to laser irradiance which was ranging from 1.5108W/cm2 to 5.51010 W/cm2. Three diagnostics devices have been used: A quartz microbalance placed in front of the target, where the maximum of materials ejection occurs, a Scanning Electron Microscope (SEM) was used to show the impact morphology evolution with the laser irradiance and a charge collector, biased at negative voltage, was used to measure the ions yield and ions kinetic energy. The results show the evolution from normal evaporation mechanism at moderate laser irradiance to phase explosion mechanism at higher laser irradiance. Laser irradiance threshold for phase explosion onset is well determined by microbalance measurement, SEM micrographic pictures and the laser breakdown in the vapour plume was determined by the charge collector.

Abstract: The growth of thin films by laser ablation involves very complex physical processes. The quality of the layer and stoechiometry of the deposits depend on key parameters like the ion energy and their angular distribution. The evolution of ions number and energy, and the angular distributions in regards to the incident laser energy, have been studied by the mean of a charges collector. We present the polar diagrams of energy and number of ions collected by irradiating a silicon target using an excimer laser at different energies.

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: In this paper we present the study of a Schottky diode gas sensing by using porous SiC films with palladium as a catalytic metal. The Schottky diodes were used for the first time for hydrocarbon (C2H6) gas sensing. The properties of the porous SiC films formed by electrochemical method were investigated by scanning electron microscopy (SEM). The electrical measurements were made at room temperature (295 K) in different ambient. The effect of the porous surface structure was investigated by evaluating electrical parameters such as the ideality factor (n), barrier height (Bp) and series resistance (Rs). The porous layer significantly affects the electrical properties of the Schottky diodes. Analysis of current-voltage (I-V) characteristics showed that the forward current might be described by a classical thermal emission theory. The ideality factor determined by the I–V characteristics was found to be dependent on the SiC thickness. For a thinner SiC layer (0.16 µm), the electrical parameters n was found around 1.135, 0.7041 eV for a barrier height and 45  for a series resistance, but for a thicker one (1.6 µm) n, Bp and Rs were 1.368, 0.7756 eV and 130 , respectively. The low value of the series resistance obtained using Cheung’s method clearly indicated the high performance of the Schottky diode for thinner SiC layer. This effect showed the uniformity of the SiC layer. Finally, sensitivity around 66 % and selectivity of the sensors were reached by using the PSC layer at low voltages below 0.5 Volt.

Abstract: In this work, the combined diagnostic measurements of ablated material by scanning electron microscopy of the craters and ion time of flight analysis have been employed to study the interaction of third harmonic Nd-Yag laser with silicon and titanium target. Evidence for the transition from normal vaporization to phase explosion has been obtained, showing an abrupt increase in the mass ablation rate beyond a laser intensity threshold and round-shaped micrometer-sized cavities found on the irradiated areas. Ion time of flight analysis is used to estimate the surface temperature under near threshold ablation conditions.

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.