Pulsed KrF Excimer Laser Research Articles

Effects of crystallinity on laser-induced voltage effect from Zn0.9Co0.1O thin film

Co-doped ZnO epilayer films were grown by pulsed laser deposition (PLD) on vicinal cut silicon and sapphire substrates. Changes in deposition time were observed as a moderate effect on the quality of the films, and the influence of the thickness on thermoelectric signals from Zn0.9Co0.1O thin films were discussed. The effect of one of the main deposition parameters, the deposition time, on the crystallinity and electron mobility properties of the Zn0.9Co0.1O thin films grown on sapphire was investigated by means of X-ray diffraction (XRD) and laser-induced voltage (LIV) effect. It shown that the XRD rocking curve full-width half-maximun (FWHM) decreased as time increasing, and the LIV signals were observed along the tilting angle of the substrate orientation when the pulsed KrF excimer laser of 248 nm were irradiated on the films. When the films illuminated in pulse lasers, the highest signals occurred in the films with best crystalline quality, and the signals were higher in the films grown on sapphire than those on silicon substrates. It suggested that the electrical resistivity and electron mobility have close relations with not only the crystallinity but also with the interface of the thin films.

Laser-induced front side etching of CaF2 crystals with KrF excimer laser

The laser-induced front side etching (LIFE) of amorphous materials like fused silica was manifold studied and the LIFE process was sufficient optimized for the fabrication of well-defined etching trenches with a very low surface roughness. The LIFE process is an indirect laser-induced ablation process, the – for the used laser wavelength – transparent substrate was covered by a highly absorbing material and the absorbing process causes a transfer of the laser energy into the substrate and, finally, to an ablation process of the substrate surface.However, the structuring of crystalline materials like CaF2 is a great challenge for the LIFE process. The properties of CaF2(111) and CaF2(001) surfaces etched by KrF excimer laser pulses (pulse duration Δtp=25ns, wavelength λ=248nm) were analysed by white light interferometry (WLI) as well as scanning electron microscopy (SEM). The surface morphologies of laser etched CaF2 surfaces depend on the laser parameters and on the crystal orientation and are frequently characterized by microcracks and flake spallation. The most probable reasons therefore are laser-induced thermal stress or laser-induced shock waves.

Effects of substrate material on carbon films grown by laser molecular beam epitaxy

The carbon thin films were grown on different substrates with different buffer layers by laser molecular beam epitaxy (LMBE) with a high purity graphite carbon target. A UV pulsed KrF excimer laser with a wavelength of 248nm was used as laser source. The structure, surface morphology and other properties of the carbon thin films were characterized by Raman spectroscopy, transmission electron microscopy (TEM), selected area electron diffraction (SAED) and atomic force microscopy (AFM). The results show that the properties of the carbon thin films and the formation of the graphene film have a strong relation to the structure and properties of the substrate. The substrate with a hexagonal wurtzite structure which is similar to the hexagonal honeycomb structure of the carbon atoms arranged in the graphene is more beneficial for the formation of the graphene thin film. In our experiment conditions, the carbon films grown on sapphire substrates with different buffer layers have an ordered structure and a smooth surface, and form high quality tri-layer graphene films.

Dynamical process of KrF pulsed excimer laser crystallization of ultrathin amorphous silicon films to form Si nano-dots

Molecular dynamics (MD) simulations based on the Tersoff potential have been developed to study the laser-induced crystallization of amorphous silicon (a-Si) film with ultrathin thickness to form size-controllable Si nano-dots. The influences of laser fluence and a-Si film thickness on the crystallization process were discussed. Classic nucleation theory was used to explain the results of the MD simulations. The constrain effect of a-Si films thickness on the formation of Si nano-dots was evaluated accordingly.

Epitaxial growth and properties of cubic Zn0.7Mg0.3O films on TiN-buffered Si(100) substrates by in situ pulsed laser deposition

A novel cubic Zn0.7Mg0.3O film on silicon substrate is conducted by KrF excimer pulsed-laser ablation system. By introducing a thin TiN buffer, layer-by-layer growth of cubic Zn0.7Mg0.3O film epilayer has been realized. The overall growth process was monitored in situ by reflection high-energy electron diffraction (RHEED) method. It was found that the crystallinity and surface morphology of the Zn0.7Mg0.3O films were strongly affected by the TiN buffer layer. The Zn0.7Mg0.3O film obtained at an optimal buffer layer exhibited high quality and good surface. For the metal-insulator-metal (MIM) structure of Pt/Zn0.7Mg0.3O (200 nm)/TiN (20 nm)/Si (400 μm) prepared at the optimal growth conditions achieved a very low leak current density of ∼10−6 A cm−2 at an electric field of 9 × 105 V cm−1 and the permittivity (ɛr) of about 8.1, agreed well with that of acquired MgO film and MgO single crystal.

Effect of hydrogen on surface texturing and crystallization of a-Si:H thin film irradiated by excimer laser

Hydrogenated amorphous silicon (a-Si:H) thin films have been considered for use in solar cell applications because of their significantly reduced cost compared to crystalline bulk silicon. However, their overall efficiency and stability are lower than that of their bulk crystalline counterpart. Limited work has been performed on simultaneously solving the efficiency and stability issues of a-Si:H. Previous work has shown that surface texturing and crystallization on a-Si:H thin film can be achieved through a single-step laser processing, which can potentially alleviate the disadvantages of a-Si:H in solar cell applications. In this study, hydrogenated and dehydrogenated amorphous silicon thin films deposited on glass substrates were irradiated by KrF excimer laser pulses and the effect of hydrogen on surface morphologies and microstructures is discussed. Sharp spikes are focused only on hydrogenated films, and the large-grained and fine-grained regions caused by two crystallization processes are also induced by presence of hydrogen. Enhanced light absorptance is observed due to light trapping based on surface geometry changes of a-Si:H films, while the formation of a mixture of nanocrystalline silicon and original amorphous silicon after crystallization suggests that the overall material stability can potentially improve. The relationship between crystallinity, fluence and number of pulses is also investigated. Furthermore, a step-by-step crystallization process is introduced to prevent the hydrogen from diffusing out in order to reduce the defect density, and the relationship between residue hydrogen concentration, fluence and step width is discussed. Finally, the combined effects show that the single-step process of surface texturing and step-by-step crystallization induced by excimer laser processing are promising for a-Si:H thin-film solar cell applications.

Femtosecond laser printing of living cells using absorbing film-assisted laser-induced forward transfer

The applicability of a femtosecond KrF laser in absorbing film-assisted, laser-induced forward transfer of living cells was studied. The absorbing materials were 50-nm-thick metal films and biomaterials (gelatine, Matrigel, each 50 μm thick, and polyhydroxybutyrate, 2 μm). The used cell types were human neuroblastoma, chronic myeloid leukemia, and osteogenic sarcoma cell lines, and primary astroglial rat cells. Pulses of a 500-fs KrF excimer laser focused onto the absorbing layer in a 250-μm diameter spot with 225 mJ/cm2 fluence were used to transfer the cells to the acceptor plate placed at 0.6 mm distance, which was a glass slide either pure or covered with biomaterials. While the low-absorptivity biomaterial absorbing layers proved to be ineffective in transfer of cells, when applied on the surface of acceptor plate, the wet gelatine and Matrigel layers successfully ameliorated the impact of the cells, which otherwise did not survive the arrival onto a hard surface. The best short- and long-term survival rate was between 65% and 70% for neuroblastoma and astroglial cells. The long-term survival of the transferred osteosarcoma cells was low, while the myeloid leukemia cells did not tolerate the procedure under the applied experimental conditions.

Enhanced Secondary Electron Emission in Nanoscale Thin Metal Containing MgO Film: Laser Irradiation on Creation of F Centers

A number of oxygen vacancies were found in the MgO film when a nanoscale-metal embedded MgO film (MgO/nanoscale metal film/MgO) was exposed to a pulsed KrF excimer laser. This was due to the interfacial reaction between embedding nanoscale metal and MgO film. In a case of Nb embedding metal, the Nb2O5 crystallites were evidenced to proceed the reaction at the MgO/Nb interface; 5MgO + 2Nb → Nb2O5 + 5Mg + 5VO··. The oxygen vacancies formed the F centers in the MgO bandgap, resulting in high cathodoluminescence intensity at the 431 and 526 nm region, corresponding to F+ and F centers. The F centers in MgO played a role in increasing secondary electron emission coefficient (γ). As several kinds of nanoscale metals for creating F centers (Cr, Ti, V, In, Ta, and Nb) were embedded into MgO film, the γ values were found to be inversely linear with the ΔGf for the formation of metal oxides. The Nb embedded MgO film showed the highest γ value of ∼0.232 compared with normal MgO film (0.034).

Characterization and antibacterial functions of Ag–TiO2 and W–TiO2 nanostructured thin films prepared by sol-gel/laser-induced technique

A novel sol-gel/laser-induced technique (SGLIT) has been developed to form nanocrystalline titanium dioxide (TiO2) based thin films with an improved antibacterial per- formance. TiO2 precursor films loaded with W +6 and Ag +2 ions (W-TiO2, Ag-TiO2) were prepared separately by sol- gel method and spin-coated on microscopic glass slides. As- dried films were subjected to KrF excimer laser pulses at optimized parameters to generate mesoporous anatase and rutile phases at room temperature. The anatase phase was obtained after irradiation with 10 laser pulses only at 75- 85 mJ/cm 2 fluence in W-TiO2 films. However, higher num- ber of laser pulses and higher W +6 content favored the for- mation of rutile. Whereas Ag-TiO2 films exhibited anatase up to 200 laser pulses at the same fluence. The films were characterized by using XRD, FEG-SEM, TEM and UV-Vis spectrophotometer to investigate the crystallographic struc- ture, phase transformation, surface morphology, film thick- ness and the optical properties. A crystallite size of approx- imately 20 nm was achieved from the anatase prepared by SGLIT. The films exhibited an enhanced antibacterial func- tion against E-Coli cells under the UV excitation.

Laser energy density, structure and properties of pulsed-laser deposited zinc oxide films

Zinc oxide thin films were deposited on soda lime glass substrates by pulsed laser deposition in an oxygen-reactive atmosphere at 20 Pa and a constant substrate temperature at 300 °C. A pulsed KrF excimer laser, operated at 248 nm with pulse duration 10 ns, was used to ablate the ceramic zinc oxide target. The structure, the optical and electrical properties of the as-deposited films were studied in dependence of the laser energy density in the 1.2–2.8 J/cm 2 range, with the aid of X-ray Diffraction, Atomic Force Microscope, Transmission Spectroscopy techniques, and the Van der Pauw method, respectively. The results indicated that the structural and optical properties of the zinc oxide films were improved by increasing the laser energy density of the ablating laser. The surface roughness of the zinc oxide film increased with the decrease of laser energy density and both the optical bang gap and the electrical resistivity of the film were significantly affected by the laser energy density.

Micro-nano scale ripples on metallic glass induced by laser pulse

A Zr47.7Cu31Ni9Al12.3 bulk metallic glass was irradiated directly by KrF excimer laser pulses with wavelength 248 nm and duration 10 ns. Scanning electronic microscope photographs indicated that many ripples in micro-nano scale would be generated on the edge of the irradiated area under the action of the higher intensity laser pulse. Detailed observation demonstrated that the ripples exhibited fluidity and became closer and closer out from interior. Theoretical analysis revealed the formation mechanism of the ripples, including melting, subsequent propagation of capillary waves and final solidification.

Metal and Metal-oxide Nanoparticle Synthesis by Laser Ablation of Aqueous Aerosols

ABSTRACTWe present a scalable, continuous manufacturing method of nanoparticle production based on laser ablation of an aerosol generated from an aqueous precursor solution. A Collison nebulizer is used to generate a mist of ~10 μm diameter water droplets containing dissolved transition metal salts, suspended in 1 atmosphere of buffer gas. Water from the droplets quickly evaporates, leaving solid particles ~2 μm in diameter for a typical solution concentration. These microparticles are then ablated by a pulsed KrF excimer laser (10 ns, λ = 248 nm, 2 J/cm2 at focus). Ablation results in plasma breakdown of the microparticle and photothermal decomposition of the precursor material. Following ablation, nanoparticles 5-20 nm in diameter are formed and collected. For AgNO3 ablated in He gas, metal Ag nanoparticles were produced. For Cu(NO3)2 ablated in He, crystalline Cu2O nanoparticles were produced. For Ni(NO3)2 ablated in He, crystalline NiO nanoparticles were produced. A combination of AgNO3 and Cu(NO3)2 ablated in a reducing atmosphere of 10% H2 and 90% He yielded Ag-Cu alloy nanoparticles. In contrast to conventional wet-chemical synthesis processes, our nanoparticles are formed ‘bare,’ without surfactants or organic material contaminating the surface. Owing to their small size and high free surface area, nanoparticles produced by this process are ideally suited for applications that include catalysis and facilitated transport membranes.

Metals and metal oxides particles produced by pulsed laser ablation under high vacuum

A pulsed KrF Excimer laser (248 nm, 15 mJ) was utilized to synthesize different particles on (111) Si substrate under vacuum \sim 10^{-6} torr using Aluminum (Al), Platinum (Pt), Tungsten (W), Molybdenum (Mo), Cadmium Oxide (CdO), and Yttrium Oxide (Y_2O_3) targets. Scanning electron microscope (SEM) was used to study particle size distribution on silicon substrate. The size of individual metal particles was found to be ranging from 71 nm to 2 \mu m whereas metal oxides particles were found in the range of 71 nm to 1.1 \mu m. The particles below 100 nm are formed by gas phase condensation of ions, atoms and molecules whereas particles greater than 100 nm size results either from the ejection of liquid droplets from the melt surface or stresses induced by laser on target surface. The advantage of particle formation due to laser ablation in high vacuum is pure particles production.

Effects of annealing atmosphere on thermoelectric signals from ZnO films

Zinc oxide epilayer films were grown on vicinal cut sapphire substrates by pulsed laser deposition with in situ annealing oxygen pressures varied from 0 to 10 × 10 3 Pa. The best crystalline quality was obtained for ZnO layer with annealing oxygen pressure of 6 × 10 3 Pa. Laser induced thermoelectric voltage (LITV) were observed along the tilting angle orientation of the substrate when the pulsed KrF excimer laser of 248 nm were irradiated on the films. The largest LITV signal was measured for the film grown at 6 × 10 3 Pa annealing oxygen pressure. According to the measured LITV signals, Seebeck anisotropy was evaluated and was found to range from 3 to 12 μV/K for ZnO films annealed at different oxygen pressures from 2 to 10 × 10 3 Pa. It is suggested that oxygen ambient plays an important role in the electronic properties of the ZnO films.

Effect of 4MeV electron beam irradiation on carbon films

Pulsed KrF excimer laser is used to deposit tetrahedral amorphous carbon (ta-C) thin films on Si (1 1 1) single crystal substrates at room temperature under vacuum ∼10 −6 mbars. The pristine deposited films are then irradiated by 4 MeV electron beam at doses varying from 1000 to 4000 cGy. Analysis through AFM illustrates that the irradiation of electron has induced cluster formation on the film surface and increased the surface roughness. Optical properties ( n, α and Tauc optical band gap) measured by spectroscopic ellipsometry and electrical resistivity measured by four probe technique are found to depend strongly on electron dose. High electron doses cause significant alteration in order/disorder or sp 2 states in the film which is the main cause of modifying band gap in the carbon films. The electrical conductivity of the films also increases by increasing electron dose which is due to tunneling of charge carriers through neighbouring conductive chains. The present electron irradiation process at varying electron doses proved to be successful to modulate the optical and electrical properties of carbon films.

Comparing soft X-ray pulses induced by different metallic plasmas

In this work, we report on the characterization of pulsed soft X-rays emitted from laser-produced plasma by different targets. The measurements were performed in a vacuum stainless-steel chamber at 10−7 mbar. The plasma was generated by a pulsed KrF excimer laser of 248 nm laser wavelength, operating with a single shot regime and having pulses of 23 ns full width at half maximum. The beam was focused on pure Si, Cu and Ta targets. The laser energies were 40, 80 and 120 mJ. The beam was focused on a spot of about 0.01 cm2 with a resulting irradiance of 1.7, 3.5 and 5.2×108 W cm−2. The detector used was a very sensitive Faraday cup which, when properly biased, was able to record time-resolved signals of X-rays and to estimate their energy. The X-ray energy values found were compared with the ion temperature of the target plasma, fitting the time-resolved ion current signals by the shifted Maxwell–Boltzmann velocity distribution.

The transverse laser induced thermoelectric voltages in step flow growth (1 — x)Pb(Mg1/3Nb2/3)O3—xPbTiO3 thin films

This paper reports that the transverse laser induced thermoelectric voltages (LITV) are observed for the first time in the step flow growth (1 — x)Pb(Mg1/3Nb2/3)O3—xPbTiO3 (PMN-PT, x = 0.20, 0.33, 0.50) thin films deposited on vicinal-cut strontium titanate single crystal substrates. Because lead magnesium niobate-lead titanate is a solid solution of lead magnesium niobate (PMN) and lead titanate (PT), there are two types of signals. One is wide with a time response of a microsecond, and the other superimposed with the wide signal is narrow with a time response of a nanosecond. The transverse LITV signals depend on the ratio of PMN to PT drastically. Under the irradiation of 28-ns pulsed KrF excimer laser with the 248-nm wavelength, the largest induced voltage is observed in the 0.50Pb(Mg1/3Nb2/3)O3—0.50 PbTiO3 films. Moreover, the effects of film thickness, substrates, and tilt angles of substrates are also investigated.

Electrochemical Characteristics of HA Film on the Ti Alloy Using Pulsed Laser Deposition

In this study, we have investigated the surface morphology of hydroxyapatite (HA) coated Ti alloy surface using pulsed laser plating. The HA (tooth ash) films were grown by pulsed KrF excimer laser, film surfaces were analyzed for topology, chemical composition, crystal structure and electrochemical behavior. The Ti-6Al-4V alloy showed α and β phase, Cp-Ti showed α phase and the HA coated surface showed HA and Ti alloy peaks. The HA coating layer was formed with 1-2㎛droplets and grain-like particles, particles which were smaller than the HA target particle, and the composition of the HA coatings were composed of Ca and P. From the electrochemical test, the pitting potential (1580 mV) of HA coated Ti-6Al-4V alloy was higher than those of Cp-Ti (1060 mV) and HA coated Cp-Ti (1350 mV). The HA coated samples showed a lower current density than non-HA coated samples, whereas, the polarization resistance of HA coated samples showed a high value compared to non-HA coated samples.

The effect of laser irradiation on ZnO thin films

The effect of KrF pulsed excimer laser irradiation on intrinsic defects，ultra-violet (UV) emission and surface morphology of ZnO thin films was investigated，and also the origin of room temperature UV emission was discussed in detail. It was found that，the KrF laser can break the Zn—O bonds; therefore，the concentration of VO (or Zni) defects increases，leading to the decrease of resistivity and increase of carrier concentration. By adjusting the laser energy densities，the donor defect concentration can be controlled in a wide range. Simultaneously，under the heat of laser，the melting grains connect with each other，resulting in the great decrease of surface roughness. Room temperature UV emission of ZnO film is composed of contribution from free-exciton (FX) recombination and its longitudinal-optical phonon replica (FX-LO), the defect density determines the relative strengths of FX to FX-LO emission intensities，which strongly affect the peak position and intensity of UV emission of ZnO film. This investigation indicates that the laser irradiation is an effective technique to modulate the exciton emission by controlling the defect density，which is important for the application of high performance of UV emitting optoelectronic devices.

GaN/metal/Si heterostructure fabricated by metal bonding and laser lift-off

A process methodology has been adopted to transfer GaN thin films grown on sapphire substrates to Si substrates using metal bonding and laser lift-off techniques. After bonding, a single KrF (248 nm) excimer laser pulse was directed through the transparent sapphire substrates followed by low-temperature heat treatment to remove the substrates. The influence of bonding temperature and energy density of the excimer laser on the structure and optical properties of GaN films were investigated systemically. Atomic force microscopy, X-ray diffraction and photoluminescence measurements showed that (1) the quality of the GaN film was higher at a lower bonding temperature and lower energy density; (2) the threshold of the energy density of the excimer laser lift-off GaN was 300 mJ/cm2. The root-mean-square roughness of the transferred GaN surface was about 50 nm at a bonding temperature of 400 °C.