KrF Laser Pulses Research Articles

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.

Microstructural evolution and electrical properties of La0.7Ca0.3MnO3 thin films grown on SrTiO3 substrates by excimer-laser assisted metal-organic deposition

Thin films of La0.7Ca0.3MnO3 were successfully grown epitaxially on (100) single-crystal SrTiO3 substrates by excimer-laser assisted metal-organic deposition. Initial amorphous LCMO thin films were obtained by metal-organic deposition at 500 °C. Crystallization and epitaxial growth of the films was achieved using a KrF pulsed laser irradiation while the film/substrate samples were kept at 500 °C. High resolution transmission electron microscopy observations on cross-sections demonstrate the formation mechanism of the epitaxial films. The crystallization process starts at the LCMO/STO interface and grows by increasing the number of laser shots. A fully crystallized film was obtained after 5 min of irradiation. The film/substrate interface was found to be sharp and abrupt. The temperature dependence of the resistance R(T) shows various behaviors, starting from insulating to semiconducting and metal–insulator transition material during the formation of the manganite film. The oxygen content was also improved by increasing the irradiation time. Promising values of the temperature coefficient of resistance were obtained from these manganite films for prospect integration in silicon based microbolometric devices.

Study on pulsed laser ablation and deposition of ZnO thin films by L-MBE

ZnO, as a wide-band gap semiconductor, has recently become a new research focus in the field of ultraviolet optoelectronic semiconductors. Laser molecular beam epitaxy (L-MBE) is quite useful for the unit cell layer-by-layer epitaxial growth of zinc oxide thin films from the sintered ceramic target. The ZnO ceramic target with high purity was ablated by KrF laser pulses in an ultra high vacuum to deposit ZnO thin film during the process of L-MBE. It is found that the deposition rate of ZnO thin film by L-MBE is much lower than that by conventional pulsed laser deposition (PLD). Based on the experimental phenomena in the ZnO thin film growth process and the thermal-controlling mechanism of the nanosecond (ns) pulsed laser ablation of ZnO ceramic target, the suggested effective ablating time during the pulse duration can explain the very low deposition rate of the ZnO film by L-MBE. The unique dynamic mechanism for growing ZnO thin film is analyzed. Both the high energy of the deposition species and the low growth rate of the film are really beneficial for the L-MBE growth of the ZnO thin film with high crystallinity at low temperature.

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 TiCN Films Prepared by Hybrid Magnetron-Laser Deposition

Titanium carbonitride thin films were prepared by hybrid deposition arrangement combining DC magnetron sputtering and KrF pulsed laser deposition. Carbon and titanium species were simultaneously deposited on the same rotated Si substrate. Films were fabricated at room substrate temperature in argon–nitrogen ambient atmosphere, for laser fluence of 15 J · cm−2 and magnetron power of 150 W. Film properties were modified by RF discharge held between the target and substrate. Films were studied by XRD and glow discharge optical emission spectroscopy (GDOES). Mechanical and electronic properties (XPS) were also measured.

Negative pressure model for surface foaming of collagen and other biopolymer films by KrF laser ablation

A single KrF laser pulse of energy larger than 0.5 J/cm2 is enough to create a microfoam layer on the surface of a collagen film and other related biopolymers. This is a new result that can be of interest for many new applications. The target material is excited in the radiation absorption depth of ∼17 µm and expands into a foam layer whose new surface is ∼5 µm above the original one. The estimated surface transient temperature of ∼83°C at threshold fluence does not account satisfactorily for this fast foaming process but consideration of the bipolar pressure variation ∼±200 bar, i.e. laser induced acoustic wave suggests that a cold homogeneous boiling is induced by the tensile part of the pressure wave in the laser excited volume. The classical nucleation theory predicts a spontaneous dense and homogeneous bubble formation when the pressure is negative in the inviscid liquid. These results constitute new examples of laser induced fast expulsion of liquid due to the hydrodynamic pressure wave which can also be considered as resulting from the surface acceleration/deceleration sequence.

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)

Finite element simulation of pulsed laser ablation of titanium carbide

In the present paper, a 2D finite element model based on the heat-conduction equation and on the Hertz-Knudsen equation for vaporization was developed and used to simulate the ablation of TiC by Nd:YAG and KrF pulsed laser radiation. The calculations were performed for fluences of 8 and 10 J/cm 2, which according to experimental results obtained previously, correspond to large increases of the ablation rate. The calculated maximum surface temperature of the target for both lasers is higher than the estimated value of TiC critical temperature, corroborating the hypothesis that the increase of the ablation rate is explained by the explosive boiling mechanism.

Ultraviolet Photoelectric Effect in ZrO2 Single Crystals

Nanosecond photoelectric effect is observed in a ZrO2 singlecrystal at ambient temperature for the first time. The rise time is20 ns and the full width at half maximum is about 30 ns for thephotovoltaic pulse when the wafer surface of the ZrO2single crystal is irradiated by 248 nm KrF laser pulses. Theexperimental results show that ZrO2 single crystals may bea potential candidate in UV photodetectors.

Tunable synchrotron radiation from high intensity laser–cluster interaction

A tunable source of synchrotron radiation based on the interaction of a high intensity laser with small Xe clusters is studied. The synchrotron radiation yield for clusters with initial radii of 20 and 50 Å using a KrF laser (248 nm) with a pulse duration of 83.3 fs and laser intensity varying from 1017 to 1020 W cm−2 was examined theoretically. Clusters irradiated by an intense femtosecond laser pulse produce x-rays more efficiently than solids or gases, because the electrons ionized by and oscillating in the electromagnetic field of the laser absorb more energy and radiate together. This paper calculates the conversion efficiency for a ∼100 fs KrF laser pulse around 1019 W cm−2 on Xe clusters with nm sizes to be a few percent.

Time-resolved measurement of atomic emission enhancement by fs–ns dual-pulsed laser-induced breakdown spectroscopy

Time-resolved measurement of atomic emission enhancement is performed by using a 500-fs KrF laser pulse incident upon a high density supersonic O2 gas jet, synchronized with an orthogonal ns frequency-doubled Nd:YAG laser pulse. The ultra-short pulse serves as an igniter of the gas jet, and the subsequent ns-laser pulse significantly enhances the atomic emission. Analysis shows that the contributions to the enhancement effect are made mainly by the bremsstrahlung radiation and cascade ionization.

Modeling of X-ray resonance lines emitted from laser produced copper plasma

In this study, x-rays emitted from Ne-like and F-like copper plasmas have been modeled by modifying the original atomic and hydrodynamic EHYBRID code. X-ray resonance lines between 8 A and 13 A, and X-ray intensities from free-free and free-bound emission with the Si-, Al-, Mg-, Na-, Ne- and F-like ions emitted from copper plasmas created by KrF laser pulses have been modeled using single pulse configurations. Laser energy to x-ray energy conversion efficiencies have been calculated. For example; 0.195 % x-ray conversion efficiency is calculated from copper plasma created using a 249 nm, 25 ns pulse duration KrF laser pulse focused to 2 x 10 13 W/cm 2 in good agreement with published experimental measurements. For example, x-ray energy conversion efficiency for 2 x 10 13 W/cm 2 and 18 ns pulse duration is calculated as 1.29 % over 2π sr.

Space charge effects in Faraday cup ion detectors

Faraday cup ion detectors are widely used in the investigation of laser-produced plasmas. However, it is well known that these detectors are susceptible to space charge distortion of the ion signal. Although an analytical model describing the threshold condition for distortion was developed in 1970 by Green, this model has never been tested experimentally. In the present investigation, it is found that the analytic model derived by Green appears to be inadequate when compared in detail to experiments performed using KrF (248 nm, 15 ns) laser pulses incident on a graphite target at an intensity of 1.7/spl times/10/sup 10/ W/cm/sup 2/. By comparing the results of particle-in-cell (PIC) simulations with these experiments, it has been discovered that electrons play an important role in the space charge distortion phenomenon.

Observation of Atomic Emission Enhancement by fs-ns Dual-PulseLaser-Induced Breakdown Spectroscopy

An experiment of a 500-fs KrF laser pulse incident upona high density supersonic O2 gas jet synchronously with an nsfrequency-doubled Nd:YAG laser pulse is performed in orthogonalconfiguration. Significant atomic emission enhancement of overforty-fold is observed with an optical multi-channel analyser. Theenhancement effect is probably attributed to the different ionizationmechanisms between fs and ns laser pulses.

Two-photon-absorption of frequency converter crystals at 248 nm

The two-photon-absorption coefficient of KDP, BBO, LTB, and CLBO crystals has been determined from the measurement of the intensity dependent transmission through long samples. The intensity of the sub-picosecond KrF excimer laser pulses on the samples was varied from 0.2–80 GW/cm2. The linear absorption of the samples was determined by using a low intensity, long pulse KrF laser. The first-principle simulations to the experimental data show a TPA value of 0.48 cm/GW for KDP, 0.5 cm/GW (o-ray) and 0.34 cm/GW (e-ray) in BBO, 0.22 cm/GW in LTB and 0.53 cm/GW in CLBO.

KrF laser deposition combined with magnetron sputtering to grow titanium–carbide layers

Titanium–carbide films were grown using a magnetron assisted pulse laser deposition combining KrF pulsed laser deposition and DC magnetron sputtering (PLDMS). Plasma streams produced by magnetron and laser ablation were intersected on the substrate surface. Films' properties were characterized by GDOES, AFM, XRD, XPS as well as by Raman spectroscopy. The adhesion and microhardness were also studied. Crystalline TiC films were fabricated at room temperature.

CN emission spectroscopy study of carbon plasma in nitrogen environment

Spectroscopic emission diagnostics of a carbon plasma created by an excimer KrF laser pulse at three laser fluences (12, 25 and 32 J/cm 2) is performed under nitrogen ambient at pressures of 0.5 and 1 mbar. By following the time evolution of the radical CN spectral emission profiles, we notice, at a certain distance from the target surface, the existence of twin peaks for the time of flight distribution. This double structure depends on laser fluence and gas pressure parameters. The first peak moves forward in relation with the plasma expansion whereas the second peak moves backward and it is attributed to CN species undergoing oscillations or reflected shocks.

Formation of a dense nanocrystalline Si array on an insulating layer by laser irradiation of ultrathin amorphous Si films

A new approach to achieve a dense nanocrystalline Si array on an insulating layer was demonstrated. It was found that a single layer of nanocrystalline Si array can be formed by using KrF pulsed excimer laser irradiation on ultrathin hydrogenated amorphous silicon films (4–20 nm) followed by thermal annealing. The area density of the nanocrystalline Si formed is as high as 1011 cm−2, the lateral size is around 10 nm, and the height is about 2–4 nm when a suitable laser irradiation fluence was used. By controlling the laser irradiation fluence and the initial a-Si:H film thickness, the grain size, density and crystallization fraction can be changed accordingly.

Preparation and characterization of pulsed laser deposition (PLD) SiC films

Si K-edge XAFS was used to characterize a stoichiometric SiC film prepared by pulsed KrF laser deposition. The film was deposited on a p-type Si(1 0 0) wafer at a substrate temperature of 250 °C in high vacuum with a laser fluence of ∼5 J/cm2. The results reveal that the film contains mainly a SiC phase with an amorphous structure in which the Si atoms are bonded to C atoms in its first shell similar to that of crystalline SiC powder but with significant disorder.

Pulsed laser deposition of Er 3+-doped oxyfluoride thin films

Modern optical telecommunication requires materials having a broad emission band centered on 1.5 μm to use as optical amplifier in WDM systems. Oxyfluoride glass demonstrate characteristics to satisfy this demand. Thin films of Er 3+-doped oxyfluoride glass have been produced by Pulsed laser deposition technique using a KrF pulsed excimer laser and low-pressure (1–10 Pa) oxygen atmosphere. The optical transmission, the dispersion of refractive index and extinction coefficient, the guided optical modes and the film thickness have been investigated by different techniques. The best result was found at an oxygen pressure of 10 Pa, in which case the refractive index of the bulk and film were very close.