Pulsed KrF Excimer Laser Research Articles

Excimer Laser Crystallized (Pb,La)(Zr,Ti)O3 Thin Films

A KrF pulsed excimer laser (248 nm) was utilized to crystallize sputtered La‐modified Pb(Zr,Ti)O3 (3:30:70) (PLZT) films on LaNiO3‐coated silicon substrates. The film surface was irradiated with defocused laser pulses in an oxygen ambient at various substrate temperatures. Polycrystalline, phase pure perovskite PLZT thin films were produced for substrate temperatures of 250°C and higher. The dielectric constant and loss tangent values of laser‐assisted crystallized (10 min exposure at 10 Hz using a substrate temperature of 400°C) PLZT thin films at 10 kHz were 406 and 0.027; in comparison, rapid thermal annealed films (annealed at 700°C for 1 min) showed values of 400 and 0.021, respectively. Laser crystallized films exhibited a remanent polarization value of 14 μC/cm2 with a coercive field |(E+c+E−c)|/2 of 95 kV/cm.

Direct fabrication of surface relief gratings in chalcogenide glasses by excimer laser interference lithography

In this paper we study the possibility to realise surface relief gratings in thin chalcogenide glass films by holographic exposure using a pulsed KrF excimer laser. Gratings with a period of 540 nm and depths of 100–300 nm were patterned at the surface of 1 μm thick films. Due to coupling of an incident near-infrared laser beam to waveguide modes a resonance-like polarisation dependent decline of transmission was observed at specific incidence angles. Just one laser pulse with a fluence of 12 mJ/cm2 per beam was sufficient to achieve the required grating parameters in sulphide glasses with low Tg.

Thermal analysis of carbon nanotube film irradiated by a pulsed laser

The prepared multi-walled carbon nanotubes (MWCNTs) film was mounted on the holder and the film surface was flashed with a single pulse of Nd:YAG laser (λ=532nm) or a pulsed KrF excimer laser (λ=248nm) in the air. The threshold energy density for surface damage was measured by inspecting the surface morphology. The dynamics of pulsed nanosecond laser heating process was simulated by the solution of the one-dimensional heat conduction equation. The finite element method (FEM) was applied to solve the equation. The damage threshold energy density for MWCNTs films flashed by a pulsed Nd:YAG laser were in the range 650mJ/cm2 and 790mJ/cm2. At the laser fluence of 790mJ/cm2 with Nd:YAG laser, the surface reached the maximum temperature 913°C at 13ns. Moreover, the Raman spectroscopy of MWCNTs films before and after irradiation were measured. Graphitization occurred on the sample after irradiation over the threshold energy of surface damage.

Pulsed-laser deposition of nanostructured Pd/C thin films: A new entry into metal-supported catalysts for hydrogen producing reactions

Carbon-supported palladium thin films were synthesized at room temperature by KrF excimer pulsed laser deposition by using oriented pyrolytic graphite and Pd target. By changing the experimental conditions, a range of structures, from smooth to porous cluster-assembled carbon films were obtained prior to deposition of the Pd films. High amount of sp 2 coordinated graphitic carbon embedded with large degree of disorder is observed in these carbon films. Surface morphology was studied by using scanning electron microscopy, while compositional analysis was performed by using energy dispersive spectroscopy. Structural analysis was performed using Raman spectroscopy and Fourier transform infrared spectroscopy. The importance of morphology and chemical structure of carbon films on their catalytic activity was investigated by using the hydrolysis of aqueous NaBH 4 as a reference process. Pd/C film produced by pulsed laser deposition are here demonstrated to be a better catalyst than Pd/C powder because of the high content of sp 2 bonding carbon atoms arranged into the porous and irregular surface of the carbon film.

The mechanical strength of fiber Bragg gratings under controlled UV laser conditions

Several factors influencing the mechanical strength of fiber Bragg grating (FBG) sensorsduring their fabrication were examined and FBG sensors were made under controlled UV laserconditions. A chemical stripping technique to remove the acrylate coating of the fiber andH2 loading treatment to increase the photosensitivity of the optical fibers at a wavelengthrange of 240–250 nm showed negligible impact on the mechanical strength of the fiberoptic sensor. The UV laser beam width exposed on the fiber was controlled up to60 µm so as to reduce the damaged areas induced from laser exposure. Also the pulsefluence and the total dose of a pulsed KrF excimer laser were varied in order toinvestigate their influence on the mechanical strength. Finally FBG sensors withdifferent reflectivities were fabricated with only slight strength degradation suchthat they could be used in a structure with 2.54% measurement range of strain.

Effects of laser sources on the reverse-bias leakages of laser lift-off GaN-based light-emitting diodes

The KrF pulsed excimer laser (248nm) and the frequency-tripled neodymium doped yttrium aluminum garnet laser (355nm) have been used to separate GaN thin films from sapphire substrates and transfer to bond other substrate. However, these processes would increase the dislocation density, resulting in an increase of the leakage current. In this study, the effects of these two laser sources on the reverse-bias leakages of InGaN–GaN light-emitting diodes were studied.

Effects of Laser Sources on the Reverse-Bias Leakages of Laser Lift-Off GaN-Based LEDs

The KrF pulsed excimer laser (248 nm) and the frequency- tripled Nd YAG laser (355nm) were used to separate GaN thin films from sapphire substrates. The effect of these two laser sources on the reverse-bias leakages of InGaN-GaN light- emitting diodes (LEDs) were studied.

Thin epitaxial GaN films ablated by a pulsed KrF excimer laser

We have studied the laser-induced ablation thresholds in heteroepitaxial gallium nitride (GaN) layers grown on sapphire substrates. The relationship between the laser-ablated GaN film thickness and the flux density of VUV laser radiation with a wavelength of 248 nm has been established. The photo-and thermoablation thresholds have been observed at a radiation intensity of 252 and 520 mJ/cm2, respectively. The enthalpy of GaN dissociation estimated from an analysis of thermoablation data is 150 kJ/mol, in agreement with the published reference values. The experimental results were implemented in the technology of blue light-emitting diodes (LEDs), which allowed the LED output yield to be significantly increased.

Rare Earth Doped Gallium Gadolinium Orthogallate Films Prepared by Pulsed Laser Deposition

The decomposition of Gd3Ga5O12 single crystal target induced by a KrF excimer laser pulse during the deposition process was observed. This phenomenon was studied ex situ by EDX analyse of ablated target and in situ by Optical Emission Spectroscopy of the laser plasma. The decomposition process depends mainly on the absorption coefficient of the target on the corresponding laser wavelength and oxygen partial pressure in the deposition chamber during laser ablation. Taking into account the differences between absorption coefficients of Pr:Gd3Ga5O12 and Yb:Gd3Ga5O12 targets the Pr:GdGaO3 and Yb:GdGaO3 thin films were successfully fabricated at different oxygen pressures of 1 Pa and 2.5 Pa, respectively. The structural properties of the fabricated films were studied by RBS and XRD.

Comparative interaction mechanisms for different laser systems with selected materials on titanium alloys

This paper describes further research into, and modelling of, the interaction mechanisms of various laser types with materials, including synthetic oil and silicone grease. A Q-switched Nd:YAG pulsed laser, a CW CO 2 laser, and a pulsed KrF excimer laser ( λ = 248 nm) were used in the study. In general, the materials studied were very absorptive in the ultraviolet, less absorptive in the infrared, and least absorptive in the near-infrared. For the excimer, photo-ablation takes place, while for the other two wavelengths thermal vapourization dominates. In the case of silicone grease, full volatilization is only obtained using the excimer. For the other two wavelengths, this is not the case, with a sticky residue remaining after treatment. Interaction with synthetic oil with its lower boiling point can be obtained for all three laser types. With the Nd:YAG the dominant heating mechanism is conduction from the substrate and a baking effect is observed with strong interaction between the contaminant and the substrate. For the two pulsed lasers, oxidation is substantially less, while for the continuous wave CO 2, the interaction time is sufficiently long for oxidation and secondary reactions to take place if care is not taken to work below the reaction threshold.

Carbon nitride films of uniform thickness by inverse PLD

In the present contribution we report on a target–substrate arrangement to considerably improve the homogeneity of films produced in the static-substrate inverse pulsed laser deposition (IPLD) configuration. Carbon nitride films were grown in a modified IPLD geometry, by holding a silicon substrate in the target plane and jointly rotating it with the target at 5.5 rpm. The graphite target was ablated by KrF excimer laser pulses in nitrogen atmosphere of 5 Pa pressure. The laser fluence was kept at a constant value (∼7 J/cm 2) in each experiment. Typically a 1 cm × 1 cm large silicon substrate was fixed to the target within the ablated track of approx. 10 mm radius. The thickness distribution of the films was measured by profilometry on masked steps. The maximum relative deviation of the film thickness was ±6% and typically around ±2.5%. The homogeneity of the refractive index and the extinction coefficient was found to be ±0.55% and ±3.58%, respectively. An additional benefit of this co-rotating IPLD geometry is its appealing simplicity, i.e. that the target and substrate are rotated by the same motor.

Tunneling recombination processes in PbWO4 crystals

AbstractTime‐resolved emission and excitation spectra and luminescence decay kinetics were studied at 160‐300 K for undoped and Mo‐doped PbWO4 crystals under XeCl (4.02 eV), N2 (3.67 eV) and KrF (5.0 eV) pulsed excimer laser excitation. The G(II) emission was found to be responsible for the slow (µs‐ms) luminescence decay. Under excitation or after irradiation in the exciton region (Eexc = 4.02 eV), this emission accompanies the monomolecular tunneling recombination in genetic pairs of electron and hole centers produced at the photo‐thermally stimulated decay of localized excitons, which occurs without release of free charge carriers. Under excitation or after irradiation in the host lattice (Eexc = 5.0 eV) and defect‐related (Eexc = 3.67 eV) regions, the bimolecular tunneling recombination takes place in chaotic pairs of the electron and hole centers created at the trapping of optically released free charge carriers. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Reactive pulsed laser deposition of gold nitride thin films

We report on the growth and characterization of gold nitride thin films on Si 〈1 0 0〉 substrates at room temperature by reactive pulsed laser ablation. A pure (99.95%) Au target was ablated with KrF excimer laser pulses in nitrogen containing atmosphere (N 2 or NH 3). The gas ambient pressure was varied in the range 0.1–100 Pa. The morphology of the films was studied by using optical, scanning electron and atomic force microscopy, evidencing compact films with RMS roughness in the range 3.6–35.1 nm, depending on the deposition pressure. Rutherford backscattering spectrometry and energy dispersion spectroscopy (EDS) were used to detect the nitrogen concentration into the films. The EDS nitrogen peak does not decrease in intensity after 2 h annealing at 250 °C. Film resistivity was measured using a four-point probe and resulted in the (4–20) × 10 −8 Ω m range, depending on the ambient pressure, to be compared with the value 2.6 × 10 −8 Ω m of a pure gold film. Indentation and scratch measurements gave microhardness values of 2–3 GPa and the Young's modulus close to 100 GPa. X-ray photoemission spectra clearly showed the N 1s peak around 400 eV and displaced with respect to N 2 phase. All these measurements point to the formation of the gold nitride phase.

Laser induced backside dry etching of transparent materials

Laser induced backside wet etching (LIBWE) is a well-known procedure in the micromachining of transparent materials. We developed a modified technique in which the absorbing liquid of LIBWE was substituted by an absorbing solid thin film (laser induced backside dry etching, LIBDE). Fused silica plate having 1 mm thickness was coated by a 100 nm thick silver layer. The metal film was irradiated through the quartz plate by single pulses of a nanosecond KrF excimer laser. The applied fluence was varied in the range of 90–4030 mJ/cm 2, the illuminated area was 1.05 mm 2. At the appropriate fluences it was found that the silver was removed from the irradiated spots and here the fused silica was etched, simultaneously. The dependence of the etch rate (which corresponds to the etched depth, since this is a single-pulse technology) on the fluence was investigated. The results showed a linear relation between etching depth (0–600 nm/pulse) and applied fluence in the range of 0.2 (the etching threshold fluence) to 4 J/cm 2. With its higher etching rate, the LIBDE procedure is much more effective than LIBWE. The morphology of the etched holes was studied by an atomic force microscope. A thermal model was developed for the description of the etching mechanism which was in a good agreement with the experimental results.

Nanoscopic photodeposited structures analyzed by an evanescent optical method

The evanescent field propagating in waveguides was used to evaluate the profile and growth rate of laser photodeposited a-Se. A pulsed KrF excimer laser was used for deposition. The differential-evanescent light leaking image, was used to analyze the nanostructures in the deposited zones. The relation between the emerging light intensity of the evanescent wave and the optical light intensity propagating in the waveguide was connected to an effective range of the evanescent wave leaking power into the deposited material. The technique provides the nanometric profiles of the ultra-thin photodeposited structures.

Growth of Transparent Conductive Al-Doped ZnO Thin Films and Device Applications

High-purity transparent conductive Al-doped ZnO (AZO) films were grown by KrF excimer pulsed laser deposition. We used ultraviolet and X-ray photoelectron spectroscopes to directly measure the absolute values of the vacuum work function of AZO films. The structure and electrical and optical properties of the as-grown AZO films were studied using X-ray diffraction, room temperature Hall effect measurement and spectro photometer, respectively. Finally, organic light emitting diodes (OLED) were fabricated on these AZO films. OLED device measurement showed that the current of the OLED with AZO was clearly increased. Our AZO thin films showed a higher conductivity (ρ=1.33×10-4 Ω cm, Rs=10.1 Ω/sq) than conventional indium tin oxide films.

Gas Sensing Properties of Metal Doped WO3 Thin Film Sensors Prepared by Pulsed Laser Deposition and DC Sputtering Process

Tungsten trioxide (WO3) thin films gas sensors were prepared by the KrF excimer pulsed laser deposition (PLD) method. The films were prepared on the quartz glass, silicon and also on the Al2O3 sensor substrates with platinum interdigitated electrodes. The effect of doping of the platinum (Pt), palladium (Pd) or gold (Au) on the WO3 thin film was also investigated. These metals were doped to the WO3 thin film by the DC sputtering process during the PLD. The substrate temperature and the oxygen pressure were 400 °C and 100 mTorr, respectively, during the deposition. The films were characterized by atomic force microscopy (AFM) and X-ray diffraction (XRD). The sensitivity of the prepared sensors to 60 ppm NO gas was examined using the two terminal resistance method in a chamber at atmospheric pressure and operating temperatures of 25–350 °C. The sensitivity of the WO3 thin films doped with Pt, Pd, or Au was found to be higher than that of the undoped WO3 thin film.

Photoinduced reflectivity oscillation in LiNbO3 crystals irradiated by a KrF laser

A study on LiNbO3 crystal irradiation by pulsed KrF excimer laser beam is presented. The interaction with the 248nm laser light modifies the material properties so that, when the irradiation is switched off, a time-periodic variation in the material reflectivity, depending on the irradiation conditions, was observed. This phenomenon can be explained in terms of the electro-optic effect induced by the buildup of internal electric fields since the compositional characterization, performed by the secondary ion mass spectrometry, showed no modification in the element concentration and the high resolution x-ray diffraction did not detect any structural deformation within the crystal.

Optical emission in magnetically confined laser-induced breakdown spectroscopy

Magnetically confined laser-induced breakdown spectroscopy was investigated by studying the optical emission from laser-induced plasma plumes expanding across an external transverse magnetic field. KrF excimer laser pulses with a pulse duration of 23ns and a wavelength of 248nm were used to produce plasmas from Al, Cu, and Co targets. Various optical emission lines obtained from Al and Cu targets show an obvious enhancement in the intensity of optical emission when a magnetic field of ∼0.8T is applied, while the optical emission lines from Co targets show a decrease in the optical emission intensity. The enhancement factors of optical emission lines were measured to be around 2 for the Al and Mn (impurity) lines from Al targets, and 6–8 for Cu lines from Cu targets. Temporal evolution of the optical emission lines from the Al samples shows a maximum enhancement in emission intensity at time delays of 8–20μs after the incident laser pulse, while from the Cu targets it shows a continuous enhancement at time delays of 3–20μs after the pulse. The enhancement in the optical emission from the Al and Cu plasmas was presumably due to the increase in the effective plasma density as a result of magnetic confinement. The decrease in the emission intensity from the Co plasmas was suggested to be due to the decrease of effective plasma density as a result of the magnetic force.

Laser-induced shock wave stimulated doping of CdTe crystals

Action of a laser-induced plane shock wave has been considered as the mechanism of doping of CdTe surface region with In. CdTe crystals coated with a relatively thick In film were subjected to irradiation with KrF excimer laser pulses. The In film was not completely evaporated under irradiation and it served further as an electrode in the fabrication of nuclear radiation detectors. Dopant atoms, implicated by laser-induced stress and shock waves, penetrated into CdTe. An In-enriched region was formed and a built-in p-n junction arose at the depth where a stress wave was converted to a shock wave.