Nanosecond Pulsed Laser Deposition Research Articles

Structural and morphological characterization of TiO 2 nanostructured films grown by nanosecond pulsed laser deposition

TiO 2 has attracted a lot of attention due to its photocatalytic properties and its potential applications in environmental purification and self cleaning coatings, as well as for its high optical transmittance in the visible–IR spectral range, high chemical stability and mechanical resistance. In this paper, we report on the growth of TiO 2 nanocrystalline films on Si (1 0 0) substrates by pulsed laser deposition (PLD). Rutile sintered targets were irradiated by KrF excimer laser ( λ = 248 nm, pulse duration ∼30 ns) in a controlled oxygen environment and at constant substrate temperature of 650 °C. The structural and morphological properties of the films have been studied for different deposition parameters, such as oxygen partial pressure (0.05–5 Pa) and laser fluence (2– 4 J/cm 2). X-ray diffraction (XRD) shows the formation of both rutile and anatase phases; however, it is observed that the anatase phase is suppressed at the highest laser fluences. X-ray photoelectron spectroscopy (XPS) measurements were performed to determine the stoichiometry of the grown films. The surface morphology of the deposits, studied by scanning electron (SEM) and atomic force (AFM) microscopies, has revealed nanostructured films. The dimensions and density of the nanoparticles observed at the surface depend on the partial pressure of oxygen during growth. The smallest particles of about 40 nm diameter were obtained for the highest pressures of inlet gas.

Influence of deposition parameters on composition and refractive index of femtosecond and nanosecond pulsed laser deposited gallium lanthanum oxysulphide

The influence of the deposition parameters on femtosecond and nanosecond pulsed laser deposited gallium lanthanum oxysulphide (GLSO) glass films has been investigated. A comparison between films deposited by femtosecond and nanosecond pulsed laser deposition (PLD) shows that the compositional range of each ablation regime varies significantly; in particular, femtosecond PLD shows a unique potential for selective fabrication of films with a high lanthanum content well outside the conventional glass-melting region. We demonstrate how manipulation of the PLD growth parameters can influence the stoichiometric transfer of the PLD process, leading to films with compositions that differ significantly from the GLSO target material. We also reveal how the refractive index of as-deposited films is dependent upon the composition and briefly discuss the thermal properties of bulk GLSO material of various compositions which indicate the potential for films grown by PLD to be used in optical data-storage device applications.

Raman spectroscopy of Bi‐Te thin films

AbstractThe deposition of micro‐ and nanocrystalline bismuth telluride thin films with tailored structure and composition is of interest in view of improving the well‐known material thermoelectric properties. Only a few works exist that discuss Raman scattering of Bi2Te3 crystals and films, while a Raman characterization of other phases, i.e. other lesser known compounds of the Bi‐Te system, such as tsumoite (BiTe) and pilsenite (Bi4Te3), is still completely lacking. We here present a Raman investigation of Bi‐Te polycrystalline thin films with controlled structure (stoichiometry and growth orientation), morphology and phase composition, produced by nanosecond pulsed laser deposition. Interpretation of Raman spectra from Bi‐Te films was supported by scanning electron microscopy, energy dispersive spectroscopy (EDS) and X‐Ray diffraction measurements, together with the predictions of the group theory. In this way, the first Raman characterization of Bi‐rich phases (namely BiTe and Bi4Te3) has been obtained. For Bi‐Te compositions characterized by a high Bi or Te content, Raman spectra reveal that segregation of elemental Bi or Te occurs. Copyright © 2008 John Wiley & Sons, Ltd.

A new model for studying the plasma plume expansion property during nanosecond pulsed laser deposition

A physical model is proposed to study the laser-induced plasma dynamics expansion during irradiation of material by a high-intensity nanosecond pulsed laser beam. Based on a consideration of the plasma ionization effect and local conservations of mass, momentum and energy, combined with the assumption that plasma can be viewed as a compressible ideal fluid and a high temperature–high pressure ideal gas, we developed a new dynamics expansion mechanism for plasma produced by pulsed laser radiation. A set of new plasma expansion dynamics equations based on our model are first deduced. Then, taking the example of the carbon target, using the finite difference method, the plasma flow dynamics into a vacuum, such as plasma spacial number density distribution, plasma number density and velocity evolvement in radial and axial directions, are studied in detail. The results show that there are some main factors affecting the velocity distribution of the plasma, including the temperature of the plasma, the ionization fraction of the plasma and the dynamic source provided by the vaporized material. The velocity uz in the longitudinal direction is different from ur in the transverse direction because of huge initial eject velocity and evaporated dynamic source. The velocities of the plasma calculated by this model are found to be in better agreement with the experimental results derived from the work of Sanz et al (1985 Plasma Phys. Control. Fusion 27 329) compared with the conventional model.

A comparative study of gadolinium gallium garnet growth by femtosecond and nanosecond pulsed laser deposition

The growth of epitaxial Nd:Gd 3Ga 5O 12 (GGG) on Y 3Al 5O 12 (YAG) by femtosecond pulsed laser deposition is reported. We have used a Ti:sapphire laser at a wavelength of 800 nm and pulse length of 130 fs, operating at a repetition rate of 1 kHz. The film properties have been studied systematically as a function of the deposition parameters of laser fluence, spot-size, oxygen pressure, target-substrate distance and temperature. Scanning electron microscopy, atomic force microscopy and X-ray diffractometry were used to characterise the surface structure and crystallinity of the films. X-ray diffraction analysis shows that epitaxial growth has occurred. A comparison between the ion velocities produced by nanosecond and femtosecond laser ablation of the GGG target material has been investigated by the Langmuir probe technique. The results indicate a large difference in the plasma characteristics between femtosecond and nanosecond ablation, with ion velocities up to eight times faster observed in the femtosecond case.

Pulsed laser deposition of nanoparticle films of Au

Nanosecond pulsed laser deposition (PLD) has been used to grow nanoparticle films of Au on Si and sapphire substrates. The equivalent solid density thickness was measured with a quartz crystal monitor and the ion flux was measured with a time-of-flight Langmuir probe. The ion signal yields the ion energy distribution. The angular distribution of deposited material and the ablated mass per pulse were also measured. These values are incorporated into an isentropic plasma expansion model for a better description of the expansion of the ablated material. Atomic force microscopy and UV/vis optical spectroscopy were used to characterise the films. Atomic force microscopy shows that in the equivalent thickness range 0.5–5 nm the deposited material is nanostructured and the surface coverage increases with increasing equivalent thickness. The optical absorption spectra show the expected surface plasmon resonance, which shifts to longer wavelengths and increases in magnitude as the equivalent thickness is increased.

Femtosecond pulsed laser ablation deposition of tantalum carbide

In this work a frequency-doubled Nd:glass laser with a pulse duration of 250 fs has been used to ablate a TaC target and to deposit thin films on silicon. The results have been compared with those previously obtained by nanosecond pulsed laser deposition and evidence of large differences in the plasma characteristics has been revealed. In particular, in the femtosecond and nanosecond plumes the energy and the velocity of neutral and ionized particles are very different. The features of femtosecond ablation include the delayed emission from the target of large and slow particles. The characteristics of the femtosecond plasma are clearly related to the morphology and composition of the deposited films and the results show a nanostructure consisting of a large number of spherical particles, with a mean diameter of about 50 nm, with a stoichiometry corresponding to Ta 2C. To explain these features, an ablation–deposition mechanism, related to the ejection of hot particles from the target, is proposed.

Ionization effect to plasma expansion study during nanosecond pulsed laser deposition

The dynamic characteristic and effects of plasma play an important role in film growth process of pulsed laser deposition (PLD). Based on numerical hydrodynamic modeling, supposed the laser radiation and partial ionization of the plasma as a dynamic source, we deduce a set of new plasma expansion dynamics equations. Based on which, as an example of carbon target, using finite difference method, the plasma flow dynamics evolvement in vacuum is investigated. Our results show the dynamic partial ionization increases the expansion in all directions, which changes into a new dynamic source for plasma expansion. In the axial direction, because of the collisional interactions between particles, the plume density peak is in the vicinity of the target surface and the acceleration of plasma occurs mainly near the target surface too. In the transverse direction, the plume peak is not near the target, but at the surface. The space expansion distance is far less than the axial direction because there is no high initial velocity component in this direction. The predictions of the plasma expansion action based on the proposed dynamics source assumption are found to be in agreement with the experimental observation.

Pulsed laser deposition of tungsten and tungsten oxide thin films with tailored structure at the nano- and mesoscale

Nanostructured thin films synthesized by assembling atoms or clusters present a structure characterized by a modulation at the nanoscale and by a large effective area, which can be exploited for the tailoring of specific structural or electronic properties. These systems are appealing for functional applications, e.g. in sensing and catalysis. We have investigated the deposition of tungsten and tungsten oxide thin films with a wide range of morphologies by exploiting nanosecond pulsed laser deposition (PLD) in an inert background atmosphere (He, Ar and Kr). We show that the non-dimensional ratio of the target-to-substrate distance to the time integrated visible plume length, which depends on the gas mass and pressure and on the substrate position, permits to select morphologies ranging from a compact structure with a density similar to bulk, to a film with an open, low density foam-like mesostructure and a high fraction of voids.

Thin-film CdS formed with pulsed-laser deposition towards optical and hybrid device applications

The paper shows various optical and optoelectronic experiments carried out on thin-film CdS on glass. The films have been prepared with nanosecond pulsed-laser deposition (PLD) on glass by using ultraviolet and infrared Nd:YAG laser emissions. The discussed topics contain the successful realization of green light emitting lasers, the properties of ultrafast two-photon excited emission, PLD controlled CdS dichroism, and organic/inorganic hybrid devices. Based on the presented results several future investigations are proposed.

Nanostructured tungsten oxide with controlled properties: Synthesis and Raman characterization

Tungsten oxide thin films are interesting for their electrochromic and gas sensing properties. For technological applications, the control of morphology (effective surface, porosity) and structure (crystallinity, stoichiometry) at the nanoscale is of paramount interest to deliver distinctive properties. Nanostructured tungsten and tungsten oxide films have been synthesized by nanosecond pulsed laser deposition (PLD) starting from a W metallic target. Both inert and reactive buffer gases (He, Ar, dry air) have been exploited to induce cluster formation and/or oxidation and to vary the deposition energies of ablated species. A wide range of morphologies extending from compact and smooth, up to extremely porous and spongy-like structures have been obtained. The structure, the oxide phase and the degree of crystallinity of the films have been investigated by Raman spectroscopy. Films deposited in the presence of inert buffer gases reveal a spontaneous ex-situ oxidation, when exposed to ambient atmosphere, which is strongly related to the nanostructure. Instead, deposition in a varying pressure of dry air permits to grow tungsten oxide films and to tune their structure from amorphous to nano- and microcrystalline with different coexisting oxide phases.

Optical properties of high-density amorphous carbon films grown by nanosecond and femtosecond pulsed laser ablation

High-density tetrahedral amorphous carbon (ta-C) films have been prepared by nanosecond (17 ns) and femtosecond (150 fs) pulsed laser deposition (PLD) using fluences and repetition rates compatible with fast and homogeneous growth over large areas. Their optical properties were measured by spectroscopic ellipsometry from 1.0 to 4.7 eV and analyzed using a multi-layer Tauc-Lorentz model. In spite of very different ablation mechanisms, both PLD techniques produce high density bulk layers as revealed by a refractive index (n at 2 eV) of 2.7±0.1 for both fs-PLD and ns-PLD. Films are covered by a few nm-thick sp2-rich top layer which is denser and thicker in femtosecond PLD as compared to nanosecond PLD. The respective roles of low and high energies in the kinetic energy distribution of the incident carbon species are discussed in terms of densification and sp3↦sp2 configurational relaxation predicted by the subplantation growth model. The significantly higher optical gap found in the ns-PLD films is attributed to the larger contribution of energetic species with kinetic energies Ec≥200 eV, as revealed by time-of-flight optical studies.

Vaporization effect studying on high-power nanosecond pulsed laser deposition

The vaporization effect generated by high-power nanosecond pulsed laser ablation was studied in detail based on the heat flux equation which accounting for the energy loss of vaporization. The vaporization velocity and temperature evolvement with different laser fluence, and the effect of vaporization on ablation depth during and after the laser irradiation were emphatically analyzed. The results show that the vaporization strongly affects the surface temperature and whole ablation depth in the pulsed laser ablation. Under the same experimental conditions, the numerical results calculated with our modified ablation depth calculation formula are more in agreement with the experimental datum.

Target ablation characteristics of thin films during nanosecond pulsed laser deposition in the ablation process

The whole ablation process of target during the pulsed laser preparation of thin films is studied in this paper. An ablation model of targets in which the vapo rization is taken account is present based on the superheated theory. Different heat flux equations for different stages are then established. Finally, as usin g Si as the target, a finite difference method is employed to simulate the space _ and time_dependence of temperature in the target. Vaporization velocity and va porization thickness evolutions with different laser fluence are investigated. T he dependence of solid_liquid interface location S(t) on time which takes in to account the melting relaxation time is derived too. The results show that th e vaporization strongly affects the surface temperature in the pulsed laser abla tion. When the laser intensity reaches near the phase explosion energy threshold , the vaporization velocity and ablation depth will evidently decrease because o f the gas dynamic effects. This result is more appropriate than that obtained in previous works.

Microstructure evolution of Al–Mg–B thin films by thermal annealing

The growth of Al–Mg–B thin films on SiO2/Si(100) substrates was performed by nanosecond pulsed laser deposition at three different substrate temperatures (300 K, 573 K, and 873 K). The as-deposited films were then annealed at 1173 K or 1273 K for 2 h. X-ray photoelectron spectroscopy, x-ray diffraction (XRD), and atomic force microscope were employed to investigate the effects of processing conditions on the composition, microstructure evolution, and surface morphology of the Al–Mg–B films. The substrate temperatures were found to affect the composition of as-deposited films in that the Mg content decreases and C content increases at higher substrate temperatures, in particular for the 873 K-deposited film. XRD results show that the as-deposited films were amorphous, and this structure may be stable up to 1173 K. Annealing at 1273 K was found to fully crystallize the room temperature and 573 K-deposited Al–Mg–B films with the formation of the polycrystalline orthorhombic AlMgB14 phase, accompanied by the development of a pronounced (011) preferred orientation. Nevertheless, high C incorporation in the 873 K-deposited Al–Mg–B film inhibits the crystallization and the amorphous structure remains stable even during 1273 K annealing. The presence of Si in the room-temperature-deposited 1273 K-annealed film due to the interdiffusion between the substrate and film leads to the formation of an additional tetragonal α-FeSi2 phase, which is thought to cause the surface cracking and microstructural instability observed in this film.

Microstructure and nanomechanical properties of Al–Mg–B–Ti films synthesized by pulsed laser deposition

Thin films were deposited from an ultra-hard, nanocomposite AlMgB 14/TiB 2 target on Si (1 0 0) by nanosecond (ns) and femtosecond (fs) pulsed laser deposition (PLD) techniques. X-ray diffraction and transmission electron microscopy were used to determine the film structure; scanning electron microscopy was employed to characterize the film surface topography; and X-ray photoelectron spectroscopy was performed to examine the film compositions and chemical bonding states of constituents. The mechanical properties of thin films such as hardness and elastic modulus were determined by nanoindentation tests. Results showed that the as-deposited films were amorphous, possessing a complex oxide glass structure, and this structure was stable up to 1223 K. The preferential reaction of Al, Mg and B with O inhibits the formation of crystalline AlMgB 14. The absence of BB bonds and nanocrystalline structure leads to nanohardness values for as-deposited films as low as approximately 10 GPa. The intrinsic hardness of the film could not be evaluated exactly because of surface roughness and substrate effects. It was found that the density of particulates in the films was drastically reduced by the use of a fs-pulsed laser source, suggesting that fs-PLD is a potential method for decreasing surface roughness.

Chemical analysis of a-CN x thin films synthesized by nanosecond and femtosecond pulsed laser deposition

An ArF excimer laser (22 ns, 193 nm) and a hybrid dye/excimer laser system (500 fs, 248 nm) are used to deposit amorphous carbon nitride films at room temperature by ablation of a graphite target in nitrogen atmosphere. The chemical composition and structure of the films is characterized by X-ray photoelectron spectroscopy. In the nanosecond case, the nitrogen content increases with reactive gas pressure up to 45 atomic %, while in the subpicosecond case it remains below 7 at. %. When processed with nanosecond pulses, the films’ nitrogen content steeply increases with fluence up to a maximum. The target-to-substrate distance has only minor influence on the amount of nitrogen incorporated into the films. The dependence of the carbon–carbon and carbon–nitrogen bond configurations on the processing parameters is also given.

Chemical analysis of a-CNx thin films synthesized by nanosecond and femtosecond pulsed laser deposition

Chemical analysis of a-CNx thin films synthesized by nanosecond and femtosecond pulsed laser deposition