Pulsed KrF Excimer Laser Research Articles

Periodic structure with a periodicity of 2–3.5 μm on crystalline TiO2 induced by unpolarized KrF excimer lasers

Laser-induced periodic surface structures (LIPSS) were processed on the TiO2 bulk surface under the irradiation of 248 nm unpolarized KrF excimer laser pulses in air. Spatial LIPSS periods ranging from 2 to 3.5 μm are ascribed to the capillary wave. These microstructures were analyzed at different laser pulse numbers with the laser energy from 192 to 164 mJ. The scanning electron microscopy results indicated eventually stripes that have been disrupted as the increase in the laser pulse numbers, which is reasonably explained by the energy accumulating effect. In addition, investigations were concentrated on the surface modifications at pre-focal plane, focal plane and post-focal plane in the same defocusing amount. Compared with condition at pre-focal plane, in addition to the plasma produced at target, the air was also breakdown for the situation of post-focal plane. So it was reasonable that stripes appeared at pre-focal plane but not at post-focal plane.

Formation of copper tin sulfide films by pulsed laser deposition at 248 and 355 nm

The influence of the laser wavelength on the deposition of copper tin sulfide (CTS) and SnS-rich CTS with a 248-nm KrF excimer laser (pulse length τ = 20 ns) and a 355-nm frequency-tripled Nd:YAG laser (τ = 6 ns) was investigated. A comparative study of the two UV wavelengths shows that the CTS film growth rate per pulse was three to four times lower with the 248-nm laser than the 355-nm laser. SnS-rich CTS is more efficiently ablated than pure CTS. Films deposited at high fluence have submicron and micrometer size droplets, and the size and area density of the droplets do not vary significantly from 248 to 355 nm deposition. Irradiation at low fluence resulted in a non-stoichiometric material transfer with significant Cu deficiency in the as-deposited films. We discuss the transition from a non-stoichiometric material transfer at low fluence to a nearly stoichiometric ablation at high fluence based on a transition from a dominant evaporation regime to an ablation regime.

Pulsed KrF excimer laser dopant activation in nanocrystal silicon in a silicon dioxide matrix

We demonstrate that a pulsed KrF excimer laser (λ = 248 nm, τ = 22 ns) can be used as a post-furnace annealing method to greatly increase the electrically active doping concentration in nanocrystal silicon (ncSi) embedded in SiO2. The application of a single laser pulse of 202 mJ/cm2 improves the electrically active doping concentration by more than one order of magnitude while also improving the conductivity. It is confirmed that there is no film ablation or significant change in ncSi structure by atomic force microscopy and micro-Raman spectroscopy. We propose that the increase in free-carrier concentration is the result of interstitial P/B dopant activation, which are initially inside the Si crystallites. Evidence of mobility limited carrier transport and degenerate doping in the ncSi are measured with temperature-dependent conductivity.

Effect of KrF excimer laser irradiation on the surface changes and photoelectric properties of ZnO single crystal

In this paper, the effect of KrF pulsed excimer laser irradiation on the structural, surface morphology, photoluminescence and electrical properties of ZnO single crystal was investigated. Compared to the as-grown sample, at an irradiation energy density of 257 mJ/cm2, the ZnO single crystal exhibits a series of phenomenon: XRD and Raman results show that the crystallization of ZnO quality change slightly, resistivity is decreased by two orders of magnitude, carrier concentration is increased by one order of magnitude. After laser irradiation, the surface shows some strip lines and no cracks. Formula calculation and simulation results show that the stripes are not caused by surface melting. We speculate that these stripes are caused by the precipitation of ZnO material inside to the surface. Due to the reduction of oxygen vacancies, UV emission has been enhanced and visible emission has been declined after irradiation. After the laser irradiation, the visible light of ZnO surface can be regulated. The experimental results show that KrF laser irradiation could effectively improve the optical and electrical properties of ZnO single crystal, which is important for the application of high performance of emitting optoelectronic devices.

Surface and morphological features of laser-irradiated silicon under vacuum, nitrogen and ethanol

Laser-induced surface and structural modification of silicon (Si) has been investigated under three different environments of vacuum, nitrogen (100Torr) and ethanol. The interaction of 1000 pulses of KrF (λ≈248nm, τ≈18ns, repetition rate≈30Hz) Excimer laser at two different fluences of 2.8J/cm2 and 4J/cm2 resulted in formation of various kinds of features such as laser induced periodic surface structures (LIPSS), spikes, columns, cones and cracks. Surface morphology has been observed by Scanning Electron Microscope (SEM). Whereas, structural modification of irradiated targets is explored by Raman spectroscopy. SEM analysis exhibits a non-uniform distribution of micro-scale pillars and spikes at the central ablated regime of silicon irradiated at low laser fluence of 2.8J/cm2 under vacuum. Whereas cones, pits, cavities and ripples like features are seen at the boundaries. At higher fluence of 4J/cm2, laser induced periodic structures as well as micro-columns are observed. In the case of ablation in nitrogen environment, melting, splashing, self-organized granular structures and cracks along with redeposition are observed at lower fluence. Such types of small scaled structures in nitrogen are attributed to confinement and shielding effects of nitrogen plasma. Whereas, a crater with multiple ablative layers is formed in the case of ablation at higher fluence. Significantly different surface morphology of Si is observed in the case of ablation in ethanol. It reveals the formation of cavities along with small scale pores and less redeposition. These results reveal that the growth of surface and morphological features of irradiated Si are strongly dependent upon the laser fluence as well as environmental conditions. The difference in surface morphology is attributable to cooling, confinement and shielding effects as well as difference in plasma temperature, density and pressure of environmental media that corresponds to different energy deposition to the target surface. Raman spectroscopy shows that no new bands are identified in case of ablation in all three environments. However, a significant Raman shift is observed which is attributed to laser-induced stresses.

Synthesis and properties of Ag/ZnO core/shell nanostructures prepared by excimer laser ablation in liquid

Ag/ZnO core/shell nanostructure was synthesised by a 248-nm KrF excimer pulsed laser ablation in a liquid solution for the first time. It was found that the surface plasma resonance absorption of the Ag/ZnO core/shell nanostructures can be tuned by the thickness of the ZnO shell, which is in agreement with the finite difference in the time domain simulation. Furthermore, the ultraviolet emission spectrum of the Ag/ZnO core/shell nanostructures was stronger and blue-shifted compared with that of pure ZnO nanoparticles. This interesting photoluminescent phenomenon is analysed in detail and a possible explanation is proposed.

Effect of laser parameters on the measurement of U/Nd ratio using pulsed laser deposition followed by isotopic dilution mass spectrometry

Nd-YAG laser pulses of 1064, 532, 266nm in wavelength and of 8ns and 100ps in pulse width as well as KrF excimer laser pulses (248nm) of 30ns pulse width were employed to deposit thin films of the simulated spent nuclear fuel pellets. The deposited thin films were dissolved in nitric acid and analyzed for residual heavy element (U) to burn-up monitor (Nd) ratio using isotopic dilution mass spectrometry (employing the TIMS technique) to determine the burn-up in these simulated fuel pellets. While the use of ns-laser pulses of 1064nm wavelength resulted in high elemental fractionation with enhanced Nd concentration compared to the target composition, the 266nm ps-laser pulses produce films with least elemental fractionation in the film and hence is best suited for this analytical application. Due to the very small amount of material in the deposited film and the amenability of pulsed laser deposition system for use in radioactive shielded hot cells, this analytical technique is suitable for sampling highly radioactive materials.

Growth of centimeter-scale atomically thin MoS2 films by pulsed laser deposition

We are reporting the growth of single layer and few-layer MoS2 films on single crystal sapphire substrates using a pulsed-laser deposition technique. A pulsed KrF excimer laser (wavelength: 248 nm; pulse width: 25 ns) was used to ablate a polycrystalline MoS2 target. The material thus ablated was deposited on a single crystal sapphire (0001) substrate kept at 700 °C in an ambient vacuum of 10−6 Torr. Detailed characterization of the films was performed using atomic force microscopy (AFM), Raman spectroscopy, UV-Vis spectroscopy, and photoluminescence (PL) measurements. The ablation of the MoS2 target by 50 laser pulses (energy density: 1.5 J/cm2) was found to result in the formation of a monolayer of MoS2 as shown by AFM results. In the Raman spectrum, A1g and E12g peaks were observed at 404.6 cm−1 and 384.5 cm−1 with a spacing of 20.1 cm−1, confirming the monolayer thickness of the film. The UV-Vis absorption spectrum exhibited two exciton absorption bands at 672 nm (1.85 eV) and 615 nm (2.02 eV), with a...

Blue inorganic light emitting diode on flexible polyimide substrate using laser lift-off process.

The fabrication process for the blue GaN inorganic light emitting diode (ILED) on flexible polyimide (PI) substrate by laser lift off (LLO) method was demonstrated. The GaN epi-structure was grown on patterned sapphire wafer. GaN samples were temporary bonded with polyimide substrate by flexible silver epoxy. Separation of the whole GaN LED film from GaN/sapphire wafer was accomplished using a single KrF excimer (248 nm) laser pulse directed through the transparent sapphire wafer. Device fabrication was carried out on both rigid silicon and flexible polyimide substrate, and I-V performance for both devices was measured. The optimized LLO process for the whole GaN LED film transfer would be applicable in flexible LED applications without compromising electrical properties.

Optical response of anisotropic silver nanostructures on polarized light

Treatment of absorbing polymer (polyethyleneterephthalate) with pulse KrF excimer laser causes formation of laser induced periodic surface structures (LIPSS). Silver nanowires were prepared on the surface of laser treated polymer using vacuum evaporation at glancing angle geometry (70°). The wires were observed with Scanning Electron Microscope equipped with Focused Ion Beam module (FIB-SEM). Relations between size and spacing of wires and morphology of polymer template were observed from those images. Optical response of prepared wires with different cross-sections and spatial parameters was investigated. The effect of polarizer on measured UV–vis absorption spectra showed anisotropy in optical properties of 1D metal nanostructures. The excitation of Localized Surface Plasmon Resonance (LSPR) was achieved only with an appropriate arrangement of the wire axis to the plane of polarization. Influence of wire width on position of absorption band was detected. Wire thickness was found to be a crucial parameter resulting in plasmon peak intensity.

Probing on Growth and Characterizations of SnFe2O4 Epitaxial Thin Films on MgAl2O4 Substrate

Epitaxial tin ferrite (SnFe2O4) thin films were grown using KrF excimer (248 nm) pulsed laser deposition technique under different growth conditions. Highly epitaxial thin films were obtained at growth temperature of 650 ˚C. The quality and epitaxial nature of the films were examined by X-ray diffraction technique. Furthermore, the phi scans of the film and substrate exhibit four folds symmetry which indicates a cube-on-cube epitaxial growth of the film on MgAl2O4 substrate. Moreover, the magnetic force microscopy measurement shows domains with cluster-like structure which is associated with ferromagnetic phase at room temperature. The coercive field and remnant magnetization of the films decrease with increase in temperature. These high quality ingenious magnetic films could be potentially used in data storage devices.

The effect of a UV preionization pulse on short-wave radiation output from a laser-produced-plasma source with a Xe gas-jet target

Experiments aimed to raise the emissivity of a laser-produced plasma source with a Xe gas target in the far-UV spectral range are described. In these experiments, the main pulse of the IR Nd:YAG laser was preceded by a pre-ionization pulse of a UV KrF excimer laser. The consequences of applying the prepulse and its influence on the short-wavelength emission intensity were traced up to main-pulse delays of about 5 μs with respect to the prepulse. It is supposed that the main mechanism by which the prepulse affects the evolution of the plasma and its emission intensity is related to the density waves excited in the gas target by this pulse.

Laser Ablation of Carbon Fiber Reinforced Plastics: Laser-Ionization TOF Mass Spectrometric Study

Neutral fragments from a nanosecond KrF excimer laser-ablated carbon fiber reinforced plastic (CFRP, carbon fiber/epoxy resin composite) were investigated by laser-ionization time-of-flight (TOF) mass spectrometry in order to explore the laser ablation dynamics of CFRP. The neutrals were detected after the post-ionization by an ArF excimer laser pulse at variable delay times, td, relative to the ablation KrF excimer laser pulse. Depending on the ablation fluence, TOF mass spectra and laser-etched surface morphology drastically changed. At the laser fluence of 0.3 J·cm -2 , the resins were preferentially ablated. The TOF mass spectra contained a main peak at 36 amu assigned to C3 + in early delay times, and then the main peak changed to 39 amu assigned to C3H3 + . At 1.0 and 1.5 J·cm -2 , both of carbon fibers and resins were ablated. The most prominent peak was C3 + independent of the delay time. The delay-time distribution of C3 + exhibited a single peak at 0.3 J·cm -2 and then changed to two peaks at 1.5 J·cm -2 . The fits with shifted Maxwell-Boltzmann distribution gave good agreement, suggesting transient maximum CFRP temperature of 4700, 6400, and 7100 K at 0.3, 1.0 and 1.5 J·cm -2 , respectively.

Effects of excimer laser annealing on electrical properties of ZnO polycrystalline films deposited by sputtering

KrF excimer pulsed laser with wavelength of 248 nm was used to anneal the ZnO polycrystalline films deposited on silica glass substrates by radio frequency reactive sputtering. The laser pulse duration was 20 ns and energy density was 267 mJ/cm2. After laser annealing, the resistance of the ZnO film measured by four-probe method decreased sharply. The resistance of samples under UV illumination at 254 nm decreased too. Moreover, the ratio of dark resistance to photoresistance reduced, this was not good for enhancing the sensitivity of the UV detector fabricated by the ZnO film. From the results of Hall Effect measurement, it was found that the carrier concentration of the samples increased and mobility of the samples decreased. The reason may be that melting and recrystallization induced by laser irradiation brought some defects into the films for large energy density. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:906–910, 2014

Reducing the incubation effects for rear side laser etching of fused silica

Laser-induced back side wet and dry etching (LIBWE and LIBDE) were developed for precise etching of transparent materials. However, LIBWE and LIBDE feature characteristics such as incubation effects and etching depth saturation, respectively, that can be unfavourable for applications in ultra-precision machining. Therefore, the techniques of LIBDE and LIBWE were combined in such a manner that the dielectric material was supplied with a thin-film-modified surface before etching by LIBWE.With this goal fused silica samples were covered with a thin chromium film for surface modification before LIBWE etching in acetone with 25ns KrF excimer laser pulses. Etching with the first pulse was observed. At laser fluences adequate for hydrocarbon LIBWE (1J/cm2) the etching rate for LIBDE is much higher than for LIBWE. In consequence, the etching rate decreases with increasing pulse number up to ten. The surface morphology depends very strong on the laser fluence and the pulse number. Smooth etchings were achieved at high fluences and low pulse numbers. However, uneven, wavy etched surfaces and micron pattern formation were observed for moderate and low laser fluences.

Surface properties of a laser-irradiated Al 6061 alloy

The surface of Al 6061 alloy samples was irradiated with a pulsed KrF excimer laser at ambient conditions. The effect of irradiation on mechanical and electrochemical properties of the material was studied as a function of laser beam pulse width, power density and irradiation frequency. The results indicated that the structural properties of the alloy were improved by increasing the laser energy density of the ablating laser. The surface roughness of the alloy increased with the increasing laser energy density up to the point of plasma detachment from the sample surface, at approximately 200 MW/cm2; further energy increase lead to a reduction of surface roughness due to the smoothing effect of excess molten material on the irradiated profile. Both the optical bang gap and the electrical resistivity of the film were significantly affected by the laser energy density.

Design of experiments for highly reproducible pulsed laser deposition of YBa2Cu3O7−δ

Pulsed laser deposition (PLD) is a very versatile tool to deposit thin films from a large variety of materials. Pulsed KrF excimer laser radiation at 248 nm wavelength can be employed to evaporate most oxide materials. Special focus has been put on transferring the stoichiometry of the target to the thin film, which is very important for complex oxides such as the cuprate high-temperature superconductors. For the multilayer deposition of superconducting devices highly reproducible thin films with controlled morphological and electronic properties are required. We report on methods of experimental design for the optimized deposition of YBa2Cu3O7−δ thin films on single crystalline oxide substrates.

Crystal Structures and Surface Morphologies of LaGaO<sub>3</sub>-Based Epitaxial Thin Films Grown by a Pulse Laser Deposition Method

High-quality La0.84Sr0.16Ga0.26Mg0.74O3-δ (LSGM) epitaxial thin films were successfully grown on (100)-SrTiO3 (STO) substrates at a temperature of 800 °C by a pulsed laser deposition (PLD) method with KrF excimer laser pulses at an ozone pressure of 1.3 × 103 Pa. X-ray diffraction rocking curve measurements showed that the LSGM films had a full-width at half-maximum (FWHM) value of 0.11 °for out-of-plane 002 reflection, which was smaller than that reported for LaGaO3 films grown by atomic layer deposition methods (0.18 o). The reciprocal spaces mapping of 103 refraction showed that the LSGM films had a slightly larger lattice parameter a (out-of-plane) of 0.393 nm than a// (in-plane) of 0.391 nm.

Enhanced photocatalytic efficiency in zirconia buffered n-NiO/p-NiO single crystalline heterostructures by nanosecond laser treatment

We report the formation of NiO based single crystalline p-n junctions with enhanced photocatalytic activity induced by pulsed laser irradiation. The NiO epilayers were grown on Si(001) substrates buffered with cubic yttria-stabilized zirconia (c-YSZ) by using pulsed laser deposition. The NiO/c-YSZ/Si heterostructures were subsequently laser treated by 5 pulses of KrF excimer laser (pulse duration = 25 × 10−9 s) at lower energies. Microstructural studies, conducted by X-ray diffraction (θ-2θ and φ techniques) and high resolution transmission electron microscope, showed a cube-on-cube epitaxial relationship at the c-YSZ/Si interface; the epitaxial relationship across the NiO/c-YSZ interface was established as NiO⟨111 ⟩||c-YSZ⟨001⟩ and in-plane NiO⟨110⟩||c-YSZ⟨100⟩. Electron microscopy studies showed that the interface between the laser annealed and the pristine region as well as the NiO/c-YSZ interface was atomically sharp and crystallographically continuous. The formation of point defects, namely oxygen vacancies and NiO, due to the coupling of the laser photons with the NiO epilayers was confirmed by XPS. The p-type electrical characteristics of the pristine NiO epilayers turned to an n-type behavior and the electrical conductivity was increased by one order of magnitude after laser treatment. Photocatalytic activity of the pristine (p-NiO/c-YSZ/Si) and the laser-annealed (n-NiO/p-NiO/c-YSZ/Si) heterostructures were assessed by measuring the decomposition rate of 4-chlorophenol under UV light. The photocatalytic reaction rate constants were determined to be 0.0059 and 0.0092 min−1 for the as-deposited and the laser-treated samples, respectively. The enhanced photocatalytic efficiency was attributed to the suppressed charge carrier recombination in the NiO based p-n junctions and higher electrical conductivity. Besides, the oxygen vacancies ease the adsorption of 4-chlorophenol, hydroxyl, and water molecules to the surface. Thus, n-NiO/p-NiO single crystalline catalysts can be introduced as a potent candidate to remediate the environmental pollution.

Improvement of the electrical properties of nanocrystalline silicon films by the KrF pulsed excimer laser irradiation method

The effects of laser fluence on the microstructure and electrical properties of nanocrystalline silicon films were investigated on samples prepared by a plasma-enhanced chemical vapor deposition technique. It was found that the content of hydrogen in the films could be reduced gradually by increasing the laser fluence, accompanied by the formation of nc-Si. The evolution of hydrogen is responsible for a change in the optical band gap and dark conductivity. By controlling the laser fluence at 1.0 J cm−2, a dark conductivity as high as 5.9 × 10−3 S cm−1 could be obtained. Based on measurements of the temperature-dependent conductivity, the carrier transport processes are discussed. The effusion of hydrogen and the increase of crystallinity are thought to be contributed to the high dark conductivity and low conductivity activation energy.