Laser sintering of perovskite-oxide and metal coatings by the sol gel process

This paper presents a novel laser technique for the formation of metal Ag and perovskite-oxide La_xSr_1-xCoO₃ coatings. Metallic Ag and La_xSr_1-xCoO₃ are conductive materials with applications as resistors and as electrodes in the microelectronics area. Suitable precursors in the form of sol gels are placed on substrates and are subsequently sintered by high-power laser irradiation. The Ag precursor is an aqueous sol of nanosized particles, while for the La_0.8Sr_0.2CoO₃, a sol gel type precursor is used. Substrates (e.g., fused silica) are coated with the precursor solutions by spinning and are dried to stable solid layers. The coatings are cured and sintered to a defined pattern by means of a 3-kW CO₂ laser beam scanned over the whole substrate surface. The microstructure of the coatings was examined by the use of scanning electron microscopy and X-ray diffraction, and the electrical properties were measured by the four-point resistivity method. The La_0.8Sr_0.2CoO₃ coatings had a perovskite cubic structure with a lattice constant of 0.383 nm. The resistivity of the coatings was 30 mQ cm, and the temperature dependence of the resistivity was 1.8 mQ cm/°C. Metallic Ag coatings with a thickness of 100-170 nm were obtained with a resistivity of 20 7Q cm. This new technology is promising for the growth of three-dimensional (3-D) structures and multilayers, especially because it can be introduced in industrial scale production.     

Laser surface cleaning, de-rusting, de-painting and de-oxidizing

Many materials have been tested as substrates and surface products. Typically ferrous (Carbon Steels and Stainless Steels) and non-ferrous (Al and Cu metals and its alloys) materials have been employed. Some epoxy, polyurethane, polyester and acrylic paints with different thickness and colour have been tested. Many types of surface rust and oxide on different bulk material have been undertaken to test. Similarly, some different types of oils and greases commonly used in industry to prevent oxidation, have been studied. Different types of laser sources have been employed: an axial fast flow, 1.5 kW CO₂ c.w. and pulsed laser source emitting a 10.6 7m beam; and a portable Nd:YAG laser, Q-switchedand normal-mode source: 1st harmonic 1.064 7m (6 ns pulse duration), 2nd harmonic 532 nm (120 7s duration pulse, 1 J max per pulse) wavelengths, multi-articulated seven mirror beam guiding device, 20 Hz repetition rate. This provides shots with 600 mJ maximum energy per pulse and 100 MW peak power per pulse with very low beam divergence: 0.5 mrad at full angle.     

Si nanocrystallites in SiO2 with intense visible photoluminescence synthesized from SiOx films deposited by laser ablation

We observed very intense and highly reproducible photoluminescence (PL) spectra for SiO_x films obtained by laser ablation of Si targets in 50-mTorr oxygen gas followed by proper annealing. It was found that the PL peak continuously changes from 1.4 eV at the center of the samples to 1.8 eV at the sample edge. The optimum values of the oxygen component in SiO_x was x=1.3-1.4 and the optimum annealing temperature was 1000 °C for intense PL. From transmission electron microscopy images of annealed films, Si nanocrystallites are found to be formed in the matrix of SiO₂ grown from the SiO_x and have diameters of 2-3 nm. These indicate that a high density of Si nanocrystallites with diameters of 2-3 nm in the SiO₂ phase are probably responsible for the PL and that the Si nanostructure is well formed from the as-deposited, metastable SiO_x (x=1.3-1.4) films by annealing at 1000 °C.     

Surface Erosion of GaN Bombarded by Highly Charged 208Pbq+-Ions

Surface change of gallium nitride specimens after bombardment by highly charged Pbq^＋-ions （q = 25, 35） at room temperature is studied by means of atomic force microscopy. The experimental results reveal that the surface of GaN specimens is significantly etched and erased. An unambiguous step-up is observed. The erosion depth not only strongly depends on the charge state of ions, but also is related to the incident angle of Pbq^＋-ions and the ion dose. The erosion depth of the specimens in 60° incidence （tilted incidence） is significantly deeper than that of the normal incidence. The erosion behaviour of specimens has little dependence on the kinetic energy of ion （Ek = 360, 700 keV）. On the other hand, surface roughness of the irradiated area is obviously decreased due to erosion compared with the un-irradiated area. A fiat terrace is formed.     

On the validity of an approximation for the deviation of a light beam. by a prism

According to many textbooks on optics, the deviation angle of a prism with the apex angle (or prism angle), %, and the refractive index, n, with respect to the surrounding medium can be approximated by '_appr=(n-1)%, if %€?/2 and the angle of incidence !₁€?/2. The validity of this approximation is investigated considering the relative error of the deviation. It is shown that this approximation can be extended to angles of incidence !₁=n% with very fair accuracy and even to larger prism angles %<?/4 in the range of angles of incidence near the minimum of deviation depending on the acceptable relative error.     

Small angle neutron scattering study of pore microstructure in ceria compacts

Ceria powders were prepared by gel combustion process using cerium nitrate and hitherto unexplored amino acids such as aspartic acid, arginine and valine as fuels. The powders have been characterized by X-ray and laser diffraction. Cold pressed compacts of these powders have been sintered at 1250°C for 2 h. Internal pore microstructure of the sintered compacts has been investigated by small angle neutron scattering (SANS) over a scattering wave vector q range of 0.003–0.17 nm⁻¹. The SANS profiles indicate surface fractal morphology of the pore space with fractal dimensionality lying between 2.70 and 2.76.      

Interaction between laser beam and target in pulsed laser deposition: laser fluence and ambient gas effects

The most prominent phenomena on the target surface induced by laser irradiation in pulsed laser deposition is the formation of conical morphologies in the irradiated area. The conical morphologies formed under different laser fluences and the ambient oxygen pressures in KrF laser ablation of Pb(Zr_0.53Ti_0.47)O₃ were studied in detail by using scanning electron microscopy. The results indicate that, depending on the melting extent of the irradiated surface, there are two kinds of cones: one type with well-defined cone tip and cone body and the other having only a cone tip. Pb is very deficient in cone tip. The gas ambient pressure plays a remarkable role in the laser-target interaction. These results are interpreted by employing the impurity shielding mechanism and the surface instability mechanism.     

Piezoelectric Pb0.7La0.2TiO3 prepared by pulsed laser deposition on (100)InP

La-modified PbTiO₃ (PLT) thin films have been deposited by pulsed laser deposition on (100)InP substrates. The nominal target composition was selected to optimize piezoelectric properties of the material. It is shown that PLT deposition on as-received InP produces amorphous PLT films because of the presence of a native oxide on the substrate. PLT films deposited on bare InP have poor adhesion as a result of the surface reoxidation of the substrate due to the high oxygen pressure required for the deposition of stoichiometric PLT. To prevent substrate oxidation, several buffer oxides (CeO₂, ZrO₂, SrO, Y-stabilized ZrO₂, MgO, and SrTiO₃) have been grown in vacuum on (100)InP. Highest-quality heteroepitaxy was found with Y-stabilized ZrO₂ (YSZ), being 𘜄¢{100} YSZ𘜄¢{100} InP oriented. The PLT deposited on this buffer layer is oriented with the [101] direction perpendicular to the substrate surface plane.     

Large-area pulsed laser deposition of YBCO thin films: homogeneity and surface

YBCO thin films and buffer layers are deposited by a special PLD setup with an 8-cm line focus on cylindrical targets and substrate scanning perpendicular to it. Different kinds of substrates (SrTiO₃, MgO, LaAlO₃, Y-ZrO₂and sapphire) as large as 7᎜ cm² were coated with YBCO. Two important aspects of the presented PLD setup will be discussed in detail: the method of substrate heating and the variation of the angle between the incident laser beam and the target surface ("wobbling"). The surface of the target material has been investigated by SEM. The influence of target "wobbling" on the time stability of the plasma will be discussed. The homogeneity of the deposited YBCO films with respect to structural and electrical properties has been investigated by XRD, RBS/channeling, and spatially resolved inductive measurements of T_c and j_c. The values of j_c on 7᎜ cm²in situ buffered sapphire substrates are 2.0 MA/cm² at (77 K, 0 T) with a j_c variation of less than ᆨ%.     

Space- and time-resolved measurements of temperatures and electron densities of plasmas formed during laser ablation of metallic samples

Temperature and electron-density profiles in a laser-produced plasma have been measured at various distances from the sample surface and different delay times from the laser pulse. The plasma is produced with a Nd:YAG laser focused on a low-alloy steel sample in air at atmospheric pressure. The determination of the parameters is made starting from the distinct emission spectra emitted by the plasma along a direction parallel to the sample surface and measured simultaneously by a CCD detector. The experimental relative error is 1.5% for temperatures and 4.5% for electron densities. A small spatial variation of the plasma temperature ((T󾉨 K) is obtained except for the outer regions, were the intensity is weak. A higher spatial variation is obtained for the electron density, especially at initial times ((N_e᜖¹⁷ cm^-3 at t=3 7s).     

Microstructure and electrical properties of textured Sr0.51Ba0.48La0.01Nb2O6 thin films

Sr_0.51Ba_0.48La_0.01Nb₂O₆ (SBLN) thin films were prepared on platinized silicon substrates by pulsed laser deposition (PLD) combined with annealing technique. The preferred orientation, surface morphology, composition, and interfacial properties of the SBLN thin films were characterized by X-ray diffraction, atomic force microscopy, transmission electron microscopy, X-ray energy dispersive spectroscopy, and automatic spreading resistance measurement. The ferroelectric properties were confirmed by P - E hysteresis loops. The frequency variation of the dielectric constant was measured as well.     

Low-temperature growth of epitaxial ZnO films on (001) sapphire by ultraviolet- assisted pulsed laser deposition

ZnO thin films have been grown on thin Si₃N₄ membranes and (001) sapphire substrates by an ultraviolet-assisted pulsed laser deposition (UVPLD) technique. The microstructure of the films grown on Si₃N₄ membranes, investigated by transmission electron microscopy, showed that crystalline and textured films can be grown by UVPLD at a substrate temperature of only 100 °C. For deposition temperatures higher than 400 °C, ZnO films grown on sapphire substrates were found to be epitaxial by Rutherford backscattering (RBS) and X-ray diffraction measurements. The minimum yield of channeling RBS spectra recorded from films deposited at 550 °C was around 2% and the FWHM of the rocking curve for the (002) diffraction peak was 0.17°; these values are similar to those recorded from ZnO layers grown by conventional PLD at 750 °C.     

Laser Molecular Beam Epitaxy of Multilayer Heterostructure SrNb0.05Ti0.95O3/La0.9Sr0.1MnO3 in 10000 Unit-Cell Layers

Ten thousands of unit-cell multilayer heterosturctures, [SrNb0.05 Ti0.95O3/La0.9 Sr0.1MnO3]3 （SNTO/LSMO）, have been epitaxial grown on SrTiO3 （001） substrates by laser molecular beam epitaxy. The monitor of insitu. reflection high-energy electron diffraction demonstrates that the heterosturctures are layer-by-layer epitaxial growth. Atomic force microscope observation indicates that the surface of the heterosturcture is atomically smooth. The measurements of cross-sectional low magnification and high-resolution transmission electron microscopy as well as the corresponding selected area electron diffraction reveal that the interfaces are of perfect orientation, and the epitaxial crystalline structure shows the orientation relation of SNTO（001）//LSMO（001）, and SNTO[100]//LSMO[100].     

Excimer laser-assisted chemical process for formation of hydrophobic surface of Si (001)

Silicon (Si) wettability is one of the important parameters in the development of Si-based biosensing and lab-on-chip devices. We report on UV laser induced hydrophobicity of Si (001) wafers immersed in methanol during the irradiation with an ArF excimer laser. The irradiation with 800 pulses of the laser operating at 65 mJ/cm² allowed to significantly increase the hydrophobicity of investigated samples as characterized by the static contact angle change from 77° to 103°. Owing to the irradiation with relatively low laser fluence, no measurable change in surface morphology of the irradiated samples has been observed with atomic force microscopy measurements. The nature of the hydrophobic surface of investigated samples is consistent with X-ray photoelectron spectroscopy analysis that indicates formation of Si–O–CH₃ bonds on the surface of the laser-irradiated material.     

High-quality epitaxial LaNiO3 thin films on SrTiO3(100) and LaAlO3(100)

LaNiO₃ thin films have been deposited by pulsed laser deposition on SrTiO₃(100) and LaAlO₃(100) substrates. The processing conditions have been investigated in order to optimize electrical resistivity, crystal quality, and surface morphology. Excellent properties are achieved at moderate substrate temperature and relatively low oxygen pressure, without the need for annealing. Thickness exerts an important influence on electrical transport, as the electrical resistivity increases quickly in films thicker than a few tens of nanometer. The surface of the films on LaAlO₃ is very flat in all the studied thickness range, but the films on SrTiO₃ develop a pattern of boundaries and even cracks as the thickness is higher. Below the critical thickness, high-quality epitaxial films with very smooth surface and low electrical resistivity are obtained under the optimum conditions of substrate temperature and oxygen pressure. The optimum processing conditions are different depending on the substrate, and control is especially critical in films deposited on SrTiO₃.     

Structure Stability of LaAlO3 Thin Films on Si Substrates

A series of amorphous and single-crystalline LaAlO3 （LAO） thin films are fabricated by laser molecular-beam epitaxy technique on Si substrates under various conditions of deposition. The structure stability of the LAO films annealed in high temperature and various ambients is studied by x-ray diffraction as well as high-resolution transmission electron microscopy. The results show that the epitaxial LAO films have very good stability, and the structures of amorphous LAO thin films depend strongly on the conditions of deposition and post-annealing. The results reveal that the formation of LAO composition during the deposition is very important for the structure stability of LAO thin films.     

Sintering and electrical conductivity of gadolinia-doped ceria

Bulk specimens of Ce_0.9Gd_0.1O_2-δ prepared with powders within a range of specific surface area were sintered in oxidizing, inert, and reducing atmospheres. The aim of this work is to investigate the effects of the sintering atmosphere on the microstructure and grain and grain boundary conductivities of the solid electrolyte. The lattice parameter determined by Rietveld refinement is 0.5420(1) nm, and the microstrain was found negligible in the powder materials. Specimens sintered in the Ar/4 % H₂ mixture display larger average grain sizes independent on the particle size of the starting powders. The grain and grain boundary conductivities of specimens sintered under reducing atmosphere are remarkably lower than those sintered under oxidizing and inert atmospheres. The activation energy (～0.90 eV) for total electrical conductivity remains unchanged with both the initial particle size and the sintering atmosphere.     

Total second harmonic emission from a laser produced plasma

The total and specular light emitted at 2ω by a laser produced plasma (τ = 30 ps, λ = 1 μm) have been measured versus intensity (10¹²-10¹⁴ W/cm²), polarization and angle of incidence of the laser beam. The possibility for second harmonic to arise from resonant absorption, through a rippled critical surface, or from parametric decay instability is discussed.     

Medium energy electron diffraction on aluminum

Measurements of the specular beam intensity in the medium range of energy (645 eV and 980 eV) are presented for the aluminum (001) and (110) surfaces. They have been performed with a LEED goniometer allowing to achieve high accuracy in the determination of the angular parameters of the diffraction experiment. The experimental results are displayed in the form of iso-intensity maps in (θ, φ) coordinates at a constant primary beam energy. θ denotes the colatitude angle of incidence and is varied in the range 30° to 80°. φ denotes the azimuth and is varied between 0° and 360°. Sections of the maps at constant angle of incidence are known as rotation diagrams. A model calculation ignoring intra-layer multiple scattering, but incorporating inter-layer multiple scattering processes shows the prominent role of the latter, in many cases, and the non-negligible contribution of the former in other cases. Sections of the maps at constant azimuth are called incidence-profiles. They are very sensitive to the azimuth and very hard to compute. Using the preceding model calculation, average incidence profiles have been computed, using as input parameters the refractive and absorptive potentials and the surface normal relaxation parameter obtained by a pseudo-kinematic analysis of experimental average incidence profiles as explained in another paper. The peak position and width have been compared to those obtained from the pseudo-kinematic calculation. From the result, it is shown that if averaging is performed according to a specific set of restrictive rules, the pseudokinematic analysis of the data has a semi-quantitative meaning.     

A novel route to aligned nanotubes and nanofibres using laser-patterned catalytic substrates

We describe the generation of aligned carbon nanotube bundles and films by pyrolysis of solid organic precursors (for example 2-amino-4,6-dichloro-s-triazine, s-triaminotriazine) at 950-1050 °C over laser-patterned thin metal (Fe, Co, Ni) films, deposited on silica substrates. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies reveal that surface roughness of the laser-etched catalytic substrates plays a key role in achieving control of nanotube growth. We believe that, during the etching process, the energised (ablated) metal clusters condense and recrystallise evenly, possibly as the metal oxide, within the edges or surface of the eroded regions. During pyrolysis these catalytic particles, embedded in the silica substrates, are responsible for carbon agglomeration and subsequent tube axial growth, suggesting that nanotube alignment strongly depends upon the etching conditions (for example laser power, pulse duration, and focal distance). The pyrolysed products (usually nanotubes or nanofibres) were characterised by SEM, high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDX). Samples containing only small amounts of amorphous carbon and other carbonaceous particles are notably absent. We observe that the degree of graphitisation is dependent upon the catalyst and the organic precursor. Interestingly, a nitrogen content З% was detected within the nanofibres, which exhibit corrugated graphite-like morphologies. This pyrolytic method may be used to advantage in generating aligned heteroatomic nanostructures such as B_xC_yN_z systems.