Pulsed laser ablation of SiC in a nitrogen atmosphere: formation of CN

Optical emission lines from the plasma generated by a laser ablation process have been investigated to gather information on the nature of the chemical species present. In particular, the experiments were carried out during the laser ablation of a ceramic sintered SiC target, both in vacuum and in presence of controlled nitrogen atmosphere. Time integrated and spatially resolved emission spectra are dominated by the atomic emission lines from silicon and carbon species, either neutral, or singly ionized. When the ablation process was carried out in a nitrogen gas background direct evidence of the formation of the CN molecular specie was found. Fast photography imaging of the expanding plume revealed the formation of a shock wave at nitrogen pressure above 13.3 Pa, with the consequent heating of the shocked region and enhancement of the kinetics of ionization and excitation. Since the C₂ specie was absent, a CN formation mechanism involving atomic carbon and nitrogen in the presence of a shock wave is suggested. PACS 52.38.Mf; 52.50.Jm, 47.40.-x     

Deposition of NbC thin films by pulsed laser ablation

Niobium carbide thin films were prepared by pulsed laser ablation of a stoichiometric NbC target. XeCl (308 nm, 30 ns) and Nd:YAG (266 nm, 5 ns) lasers operating at a repetition rate of 10 Hz were used. Films were deposited on Si (100) substrates at room temperature either in vacuum or in an argon atmosphere (2᎒^-1 mbar). Different laser fluences (2, 4 and 6 J/cm²) and different numbers of pulses (1᎒⁴, 2᎒⁴ and 4᎒⁴) were tested. For the first time, NbC films were prepared through a clean procedure without the addition of a hydrocarbon atmosphere. The phase constitution of the films, unit cell size, mean crystallite dimensions and preferred orientation are determined as a function of deposition conditions by X-ray diffraction. Complementary morphological and structural analysis of the films were performed by scanning electron microscopy, atomic force microscopy and Rutherford backscattering spectroscopy.     

Spectroscopy of laser-produced cerium plasma for remote isotope analysis of nuclear fuel

A cerium oxide sample was ablated by 2nd harmonic radiation of Nd:YAG laser at a power density of 0.1 GW/cm². Time evolution of the ablation plume was investigated by laser absorption time-of-flight (TOF) measurement. It was found that the ablated ionic plume in vacuum consisted of two components having different velocities whereas the ablated neutral atoms had mainly a single component. The flow velocity perpendicular to the sample surface in vacuum was determined to be 3.5 km/s for neutral atoms, and 4.7 km/s and 9.3 km/s for singly charged ions. From the detailed plume evolution in ambient atmosphere with several pressures we obtained some experimental conditions suitable for isotope analysis of atomic cerium.     

Shadowgraphic and emission imaging spectroscopic studies of the laser ablation of graphite in an Ar gas atmosphere

Laser ablation of graphite in an Ar atmosphere at 560 Torr was done using a nanosecond-pulse Nd:YAG laser (1064 nm) at a fluence of 12 J/cm². Dynamics in the ejection of carbon species and in their confinement near the graphite surface (<1 mm) due to their numerous collisions with Ar atoms were investigated by shadowgraphy, emission imaging, and emission spectroscopy at delay times of 0.01-100 7s following the laser irradiation. A shock wave was generated, and temporally and spatially dependent emissions from Ar⁺ and Ar were observed in addition to those from carbon species (C, C⁺, and C₂) and the Bremsstrahlung radiation from a hot plasma. We suggest that the dissipation of the kinetic and thermal energies of the carbon species, their backward motion, and their collisions with each other lead to the formation of clusters and particles through the interaction with Ar atoms.     

Propagation of LaMnO3 laser ablation plume in oxygen gas

The propagation of LaMnO₃ laser ablation plume in oxygen background has been investigated using fast photography of overall visible plume emission and time-resolved optical emission spectroscopy. The plume expansion was studied with ambient oxygen pressures ranging from vacuum level to 100 Pa. Free-expansion, splitting, sharpening and stopping of the plume were observed at different pressures and time delays after the laser pulse. Time-resolved optical emission spectroscopy showed that oxides are mainly formed through reaction of the atomic species ablated from the target with oxygen in the gas-phase. These reactions mainly affect the content of lanthanum oxide in the plume, while emission of manganese oxide is barely observed in all the range of pressure investigated.     

Nanocrystalline growth and diagnostics of TiC and TiB2 hard coatings by pulsed laser deposition

Nanocrystalline coatings of TiC and TiB₂ were grown by pulsed laser deposition on Si(100) and on X155 steel at low substrate temperatures ranging from 40 °C to 650 °C. A pulsed KrF excimer laser was used with the deposition chamber at a base pressure of 10^-6 mbar. The morphology and structure of the films, studied with SEM, XRD, and TEM, showed that nanocrystalline films with a fine morphology of TiC and TiB₂ were deposited with a grain size of 10 nm-70 nm at all substrate temperatures. The growth of the polycrystalline coatings possessed a columnar morphology with a 𘜄¢ preferred orientation. The hardness of the coatings was determined to be 40 GPa and the elastic modulus, 240 GPa. The composition and the kinetics of the plume produced during the pulsed laser deposition of TiC and TiB₂ was studied under film growth conditions. The mass analysis of ions of the ejected material was performed by time-of-flight mass spectroscopy (TOF-MS) and showed the presence of Ti⁺ and C⁺ during TiC ablation and B⁺, B₂⁺, and Ti⁺ during TiB₂ ablation. The kinetic energies (KE) of the ions depended on the laser fluence which was between 0.5 eV and 340 eV. The kinetic energy and the evolution of the plasma was studied with a streak camera. The velocity of the plasma was of the order of 10⁶ cm/sec and was linearly dependent on the energy fluence of the laser. The emission spectroscopy of the plasma plume confirmed the atomic neutral and single excited species of Ti. These results show that coating growth basically occurs by the recombination of the ionic species at the surface of the substrate.     

Vacuum versus gas environment for the synthesis of nanocomposite films by pulsed-laser deposition

Nanocomposite films formed by Cu nanocrystals (NCs) with sizes <10 nm embedded in an amorphous Al₂O₃ host have been grown by alternate pulsed-laser deposition both in vacuum and in a buffer gas (Ar) up to pressures of 0.1 Torr. The dimensions, dimension distributions, and shape of the NC produced in vacuum and in Ar up to pressures of 5᎒^-3 Torr follow a similar trend as a function of the Cu areal density. This allows us to conclude that the nucleation and growth of the NC are dominated by processes occurring at the substrate surface rather than in the gas phase. For Ar pressures ̓᎒^-2 Torr, the anisotropy of the NC is enhanced, the deposition rate decreases abruptly and a significant amount of the buffer gas is incorporated into the host, thus leading to the formation of a porous material.     

Time-resolved shock-wave photography above 193-nm excimer laser-ablated graphite surface

Highly oriented pyrolytic graphite (HOPG) was ablated by a 193-nm ArF excimer laser in air. The fluence was varied in the range 1-25 J/cm². Every laser shot hit a pristine graphite surface. The emerging shock wave was recorded by a nanosecond-resolution photographic arrangement. The velocity of the shock wave as a function of time and laser fluence was measured. The amount of energy that generates the shock wave was determined and found to be about 5-7% of the incident laser energy. The shock wave is already present 10-15 ns after the maximum of the incident laser pulse. These facts imply that, even if high-energy (10-100 eV) ions, atoms, or clusters leave the surface, a layer several 10 nm thick has to be removed during this short period. The temperature of the shock front is ~2500-4000 K, as derived from the measured velocities. Measuring the ablation depth by atomic force microscopy as a function of fluence revealed that the single-shot ablation threshold is 1.4ǂ.2 J/cm², and the effective absorption coefficient is ~1.5᎒⁵ cm^-1.     

Al2O3 ceramics under high-fluence irradiation: plasma plume dynamics through space- and time-resolved optical emission spectroscopy

The expansion in vacuum of the plume generated by the UV ablation of a LiYF₄ crystal was analysed as a function of several parameters: distance from the target along the plume axis, laser fluency and angular dislocation with respect to the plume axis. The study was carried out by the optical time of flight technique. Time-resolved signals of the optical emission of the neutral as well as ionised species in the plume were recorded and analysed for different experimental situations. The most probable velocity for each species was calculated and confirmed by the Maxwell-Boltzmann distribution fits of the relative emission temporal profiles. An angular distribution of the ablated species could also be provided.     

A comparison of the electron component within laser-ablated titanium plumes formed by UV and visible lasers

A Langmuir probe was used as a diagnostic of the temporally evolving electron number densities within a low-temperature laser-ablated titanium plasma expanding in vacuum. Measurements were made following ablation by a KrF excimer laser (248 nm, F=30 ns) and a frequency-doubled Nd:YAG laser (532 nm, F=7.5 ns) for laser power densities between 85 MW cm^-2 and 1130 MW cm^-2 on target. Electron number density data were obtained from the saturation electron current region of the probe (I/V) characteristic. Peak electron number densities in the range 1.5᎒¹⁰ cm^-3 to 1.5᎒¹³ cm^-3 were measured, at a distance of 5 cm along the target normal, for the laser power range investigated. Above ablation threshold the temporally integrated electron flux increased linearly with incident power density for both ablation wavelengths. The ablation thresholds, in terms of peak power density within the laser spot on the target, were found to be 85ᆨ MW cm^-2 for KrF ablation and 300ᇆ MW cm^-2for 2P YAG ablation.     

An investigation of neutral and ion number densities within laser-produced titanium plasmas in vacuum and ambient environments

Optical absorption spectroscopy in combination with laser-induced fluorescence imaging is applied to determine spatially and temporally resolved number densities within laser-produced titanium plasmas, expanding into vacuum and low-pressure nitrogen. Contour mapping of species number density and subsequent volumetric integration to yield the total number of absorbing species in the plume are demonstrated for Ti I expanding into vacuum. The results obtained indicate that for an incident KrF energy density of ~4 J cm^-2 the total plume content is >10¹⁷ Ti neutrals and ions. The ground-state neutral and ground-state ion yields are both observed to increase linearly with laser fluence above thresholds of ~2.2 J cm^-2 and ~3.7 J cm^-2, respectively. Reduction in absorption linewidths and spatial widening of the corresponding LIF images, observed for plume expansion in the presence of low-pressure ambient gases, reflects the reduction in species velocities and randomisation of the velocity distributions of plume species with increasing ambient pressure.     

193-nm laser ablation of CVD diamond and graphite in vacuum: plume analysis and film properties

The pulsed laser ablation of chemical vapor deposition (CVD) diamond and graphite samples in vacuum has been investigated by the use of an ArF excimer laser operating at 5=193 nm. The composition and propagation of both ablation plumes has been probed via wavelength and spatially and temporally resolved measurements of the plume emission and found to be very similar. Electronically excited C atoms and C⁺ and C²⁺ ions are identified among the ablated material. Plume expansion velocities are estimated from time-gated imaging of specific C and C⁺ emissions. Langmuir probe measurements provide further insight into the propagation of the charged components in both ablation plumes. Diamond-like carbon (DLC) films grown by 193-nm laser ablation of both target materials on Si substrates maintained at room temperature have been investigated by laser Raman spectroscopy (325 nm and 488 nm excitation) and by both optical and scanning electron microscopy, and their field emission characteristics investigated. Again, similarities outweigh the differences, but DLC films grown from ablation of the diamond target appear to show steeper I/V dependencies once above the threshold voltage for field emission.     

Transparent conducting AZO and ITO films produced by pulsed laser ablation at 355 nm

Thin films of aluminium-doped zinc oxide (AZO) and indium tin oxide (ITO) were deposited on glass substrates by laser ablation in an oxygen environment. The electrical and optical properties of films grown at various oxygen pressures were compared. With no substrate heating, highly transparent and conducting films were obtained with oxygen pressures between 15 and 23 mTorr for both materials. We obtained a specific resistivity of 1.8᎒^-3 Q cm for AZO and 1.1᎒^-3 Q cm for ITO. By heating the substrate to 160 °C or 200 °C, the resistivity was further reduced to 1.1᎒^-3 Q cm for AZO and 3.9᎒^-4 Q cm for ITO. The average transmission of visible light (450-750 nm) was between 82% and 98% in most cases. The results suggest that AZO is a promising alternative to ITO.     

Dual-beam ablation of fused silica by multiwavelength excitation process using KrF excimer and F2 lasers

A new technique of dual-beam laser ablation of fused silica by multiwavelength excitation process using a 248-nm KrF excimer laser (ablation beam) coupled with a 157-nm F₂ laser (excitation beam) in dry nitrogen atmosphere is reported. The dual-beam laser ablation greatly reduced debris deposition and, thus, significantly improved the ablation quality compared with single-beam ablation of the KrF laser. High-quality ablation can be achieved at the delay times of KrF excimer laser irradiation shorter than 10 ns due to a large excited-state absorption. The ablation rate can reach up to 80 nm/pulse at the fluence of 4.0 J/cm² for the 248-nm laser and 60 mJ/cm² for the F₂ laser. The ablation threshold and effective absorption coefficient of KrF excimer laser are estimated to be 1.4 J/cm² and 1.2᎒⁵ cm^-1, respectively.     

Pulsed-laser deposition of Ta-doped PZT ferroelectric films for memory applications using conductive oxide La0.25Sr0.75CoO3 and SrRuO3 electrodes

New methods for fabricating highly 𘚡¢-oriented and complete 𘜏¢-textured Pb(Ta_0.05Zr_0.48Ti_0.47)O₃ (PTZT) films on Pt/TiO₂/SiO₂/Si(001) substrates by pulsed-laser deposition have been developed using conductive oxide La_0.25Sr_0.75CoO₃ and SrRuO₃ electrodes. The 𘚡¢-preferred orientated PTZT ferroelectric capacitor was not subjected to loss of its polarization after 1᎒¹⁰ switching cycles at an applied voltage of 5 V and a frequency of 1 MHz, and the 𘜏¢-textured PTZT film capacitor retains 94.7% of its polarization after 1.5᎒¹⁰ switching cycles at 5 V and 50 kHz. The PTZT capacitors using these conductive oxide electrodes have low leakage current dominated by Schottky field emission mechanism.     

Carbon-plasma produced in vacuum by 532 nm-3 ns laser pulses ablation

A study of VIS laser ablation of graphite, in vacuum, by using 3 ns Nd:YAG laser radiation is reported. Nanosecond pulsed ablation gives an emission mass spectrum attributable to C_n neutral and charged particles. Mass quadrupole spectroscopy, associated to electrostatic ion deflection, allows estimation of the velocity distributions of several of these emitting species within the plume as a function of the incident laser fluence. Time gated plume imaging and microscopy measurements have been used to study the plasma composition and the deposition of thin carbon films. The multi-component structure of the plume emission is rationalized in terms of charge state, ions temperature and neutrals temperature. A special regard is given to the ion acceleration process occurring inside the plasma due to the high electrical field generated in the non-equilibrium plasma conditions. The use of nanosecond laser pulses, at fluences below 10 J/cm², produces interesting C-atomic emission effects, as a high ablation yield, a high fractional ionization of the plasma and presence of nanostructures deposited on near substrates.     

Examination of photoproducts in the ablation plume of doped PMMA

Optical emission and laser-induced fluorescence spectroscopies (OES and LIF) are employed to examine the plume ejected into vacuum upon UV pulsed laser ablation (248 nm, 20 ns, and 266 nm, 5 ns) of poly(methyl methacrylate) PMMA films doped with photostable compounds naphthalene (NapH) and phenanthrene (PhenH), and their photolabile iodide derivatives iodonaphthalene (NapI) and iodophenanthrene (PhenI). Spontaneous emissions observed in the irradiation of NapI and PhenI sensitised films can be assigned to excited products resulting from photodissociation of the dopants and to excited C₂, CH, and CN radicals. The presence in the plume of ground state products is revealed by LIF upon excitation at 266 nm. Measurements of the dependence of the spontaneous and LIF emissions on distance to the surface, ablation fluence, and flight velocities of species are discussed with reference to the mechanistic implications derived from the measurements. PACS 52.38.Mf; 61.82.Pv; 82.50     

Study of Mn laser ablation in methane atmosphere

Laser ablation of Mn target in vacuum and in the presence of CH₄ was studied under 308 nm laser irradiation. Time-resolved emission using gated detection and scanning monochromator and absorption using the cavity ring-down spectroscopy were used to study vaporized plume. In the CH₄ atmosphere we observed transitions identified as C₂ and MnH bands, while these spectral features were not detected in emission spectra. This is a clear evidence of importance in combining both spectroscopic techniques in laser vaporized plume study.     

Application of time-resolved digital holographic microscopy in studies of early femtosecond laser ablation

We studied evolution of femtosecond laser ablation by employing novel method of time-resolved off-axis digital holographic microscopy. Phase and amplitude profiles of early shock front and ablation plume dynamics of irradiated tempered steel were reconstructed from the digital holograms. In order to gain additional information, digital holographic microscopy was combined with plasma emission imaging. By using both techniques simultaneously we studied material response to multi-pulse irradiation, shock wave propagation, ablation plume formation and plasma emission. The significant changes in ablation performance were observed when using multi-pulse irradiation if compared to widely investigated single-shot regime.