Laser Fluence Conditions Research Articles

Optimization of pulsed laser deposited ZnO thin-film growth parameters for thin-film transistors (TFT) application

In this work we present the optimization of zinc oxide (ZnO) film properties for a thin-film transistor (TFT) application. Thin films, 50±10 nm, of ZnO were deposited by Pulsed Laser Deposition (PLD) under a variety of growth conditions. The oxygen pressure, laser fluence, substrate temperature and annealing conditions were varied as a part of this study. Mobility and carrier concentration were the focus of the optimization. While room-temperature ZnO growths followed by air and oxygen annealing showed improvement in the (002) phase formation with a carrier concentration in the order of 1017–1018/cm3 with low mobility in the range of 0.01–0.1 cm2/V s, a Hall mobility of 8 cm2/V s and a carrier concentration of 5×1014/cm3 have been achieved on a relatively low temperature growth (250 °C) of ZnO. The low carrier concentration indicates that the number of defects have been reduced by a magnitude of nearly a 1000 as compared to the room-temperature annealed growths. Also, it was very clearly seen that for the (002) oriented films of ZnO a high mobility film is achieved.

Angular emission of ions and mass deposition from femtosecond and nanosecond laser-produced plasmas

We investigated the angular distribution of ions and atoms emanating from femto- and nanosecond laser-produced metal plasmas under similar laser fluence conditions. For producing plasmas, aluminum targets are ablated in vacuum employing pulses from a Ti:Sapphire ultrafast laser (40 fs, 800 nm) and an Nd:YAG laser (6 ns, 1064 nm). The angular distribution of ion emission as well as the kinetic energy distribution is characterized by a Faraday cup, while a quartz microbalance is used for evaluating deposited mass. The ion and deposited mass features showed that fs laser ablated plasmas produced higher kinetic energy and more mass per pulse than ns plumes over all angles. The ion flux and kinetic energy studies show fs laser plasmas produce narrower angular distribution while ns laser plasmas provide narrower energy distribution.

Distinct magnetic response of nanograined Zn1−xMnxO (x = 0, 0.02, 0.1) powders and thin films: Focus on the effect of the working atmosphere

Nanocrystalline Zn1−xMnxO (where x = 0, 0.02, 0.1) powders and thin films were prepared by a polymeric precursor method and pulsed laser deposition, respectively. The wet chemistry method was chosen to synthesize the powders in order to improve key parameters as purity and grain-size distribution. Optimization procedures, encompassing substrate temperature, laser fluence and background gas conditions were performed and are intended to improve the crystalline quality of the films. X-ray diffraction studies reveal no clear evidence (within the detection limit of the technique) of affectation by impurities generated during the synthesis. Films prepared under optimized conditions feature a mosaic spread <0.3° and nearly bulk-like c-axis lattice parameter (5.198 Å). The substitution of Mn2+ cations in the tetrahedral sites of the wurtzite structure of pristine ZnO is confirmed both X-ray diffraction and Raman spectroscopy. Magnetic measurements, in turn, show that the powders are paramagnetic for temperatures above 3 K, while the thin films, grown in vacuum, are ferromagnetic at room temperature. These results indicate that variations in the preparation parameters have a marked influence on the magnetic responses of the Zn1−xMnxO system.

High Quality Passivation Scheme Combined with Laser Doping from SiN:P and SiN:B Layer for Silicon Solar Cell

In this paper strategy combining both high passivation quality on n+ surface depleted emitter and P and B dopant source availability for further laser doping process from SiN:P and SiN:B layers is investigated. Passivation improvement is demonstrating by adding SiO2 based interfacial layer between n+ emitter and doped SiNx layer. Moreover efficient Excimer laser P and B doping is highlighted even through additional passivation layers. Surface concentration, profile shape and depth of both p+ and n+ doped regions could be tuned by using SiN:P or SiN:B layers, SiO2 interfacial layer type and laser fluence conditions. It is highlighted fluence ranges where the initial emitter can be fully compensated or over-compensated. Such processes could be helpful to simplify the complex fabrication of photovoltaic structures like IBC solar cells.

Computational and experimental study of nanosecond laser ablation of crystalline silicon

In this paper, a numerical model of nanosecond laser ablation of crystalline silicon has been established. Based on the highly nonlinear model of heat transfer and phase change in crystalline silicon after absorbing laser light, heat transfer equation is solved by using finite element method implemented in ANSYS. The simulation of ablation depth of crystalline silicon is obtained under different conditions of laser fluence and pulse overlap. Comparing with the ablation morphology obtained from SEM observations, the computational results and experimental data show good agreement.

Cation off-stoichiometric SrMnO3−δ thin film grown by pulsed laser deposition

The laser energy density (laser fluence) dependency of the Sr/Mn ratio was investigated for SrMnO3−δ (SMO) thin films grown by pulsed laser deposition (PLD). It was found that the Sr/Mn ratio showed a steep increase followed by a gradual increase as the laser fluence was increased. However, the Sr/Mn ratio always showed Mn-excess under the present laser fluence condition as long as stoichiometric SrMnO3 targets were used. In order to obtain cation stoichiometric SMO films, it was necessary to use Sr-excess SrMnO3 targets in addition with laser fluence tuning. The crystal quality of the SMO thin film was found to vary with the Sr/Mn ratio. In stoichiometric or Sr-excess SMO thin films, epitaxial thin films could be obtained, whereas Mn-excess thin films showed very low crystallinity. Sr-excess films were also found to have some extra SrO planes. In addition, they exhibited out-of-plane lattice expansion which electron energy loss spectroscopy analysis revealed was due to Mn vacancies. The variation of film growth was closely related to point defects due to excess cations included in growing thin films.

Nouvelle approche de détermination du seuil d’endommagement de tissus biologiques par laser femtoseconde : intérêt et application à la chirurgie de la cornée

Optimization of femtosecond laser characteristics in corneal surgery is still needed to improve clinical results. In this study, we describe an original characterization technique able to measure laser damage of corneal tissues precisely and to provide complementary physical results on the laser-matter interaction. A femtosecond laser was used to damage corneas not suitable for graft. The epithelium and the Bowman layer are exposed to a set of different single-shot fixed laser fluences. Optical microscopy can determine the probability of laser damage on the corneal surface. The high damage threshold (minimum fluence systematically damaging the cornea) roughly fixes the operating laser fluence conditions, while the low damage threshold sets the maximum laser fluence level preserving tissue integrity (safety level). We precisely evaluate the damage fluence threshold of a tissue, using a statistical approach coupled with optical microscopy analysis. This technique gives essential information on laser-tissue interaction with a high rate of confidence. For corneal epithelium and the Bowman layer, we determine the maximum laser fluence level preserving tissue integrity (safety level) and the minimum fluence level systematically damaging the tissue. High and low threshold fluences of epithelium and the Bowman layers are (5.6 ± 0.4 J/cm(2); 2.7 ± 0.1 J/cm(2)), and (7.1 ± 1.1 J/cm(2); 3.4 ± 0.1 J/cm(2)), respectively. These data constitute determinant parameters for clinical applications, since they determine a working window providing the minimal effective irradiation dose that is mandatory for the development of high-quality laser-cutting surgery processes with minimized side effects.

Solid-Phase Thermodynamic Interpretation of Ion Desorption in Matrix-Assisted Laser Desorption/Ionization

This work demonstrates a quantitative interpretation of ion desorption in matrix-assisted laser desorption/ionization (MALDI). The theoretical modeling incorporates transition state theory for the desorption of surface ions, assuming chemical and thermal equilibrium in the solid state prior to desorption. It is distinct from conventional models that assume chemical equilibrium in the gas phase. This solid-state thermodynamic interpretation was used to examine the desorption of pure 2,4,6-trihydroxyacetophenone (THAP) and of angiotensin I mixed with THAP. It successfully described the changes in ion yield with the effective temperature under various laser fluence and initial temperature conditions. The analysis also revealed the key role played by ion concentration in the modeling to provide the best fit of the model to observations. On the other hand, divergence of the ion beam with laser fluence was examined using an imaging detection method, and the signal saturation normally seen at high fluence was appropriately reduced by ion focusing. Simplified but deceptive theoretical interpretations were obtained when the analysis was conducted without adequate calibration of the instrument bias.

Laserspray Ionization on a Commercial Atmospheric Pressure-MALDI Mass Spectrometer Ion Source: Selecting Singly or Multiply Charged Ions

Multiply charged ions, similar to those obtained with electrospray ionization, are produced at atmospheric pressure (AP) using standard MALDI conditions of laser fluence and reflective geometry. Further, the charge state can be switched to singly charged ions nearly instantaneously by changing the voltage applied to the MALDI target plate. Under normal AP-MALDI operating conditions in which a voltage is applied to the target plate, primarily singly charged ions are observed, but at or near zero volts, highly charged ions are observed for peptides and proteins. Thus, switching between singly and multiply charged ions requires only manipulation of a single voltage. As in ESI, multiple charging, produced using the AP-MALDI source, allows compounds with molecular weights beyond the mass-to-charge limit of the mass spectrometer to be observed and improves the fragmentation relative to singly charged ions.

IR spectral characteristics of lithium hydride thin films

Lithium hydride thin films were deposited by pulsed laser deposition and structurally characterized by means of IR spectra. The pressure of H2 used as a reactive gas was changed and the substrate temperature, substrate- target distance, laser fluence, and other conditions were defined. The research result shows that the pressure of H2 is an important factor that affects the quality of lithium hydride films. With increase in hydrogen gas pressure, the H2 content in the films rises and peak maxima of the Li–H bond move to higher wave numbers.

Influence of laser–target interaction regime on composition and properties of surface layers grown by laser treatment of Ti plates

Surface laser treatment of commercially pure titanium plates was performed in air using two different Nd : YAG sources delivering pulses of 5 and 35 ns. The laser fluence conditions were set to obtain with each source either yellow or blue surface layers. Nuclear reaction analysis (NRA) was used to quantify the amount of light elements in the formed layers. Titanium oxinitrides, containing different amounts of oxygen and nitrogen, were mainly found, except in the case of long pulses and high laser fluence, which led to the growth of titanium dioxide. The structure of the layers was studied by x-ray diffraction and Raman spectroscopy. In addition, reflectance spectra showed the transition from a metal-like behaviour to an insulating TiO2-like behaviour as a function of the treatment conditions.Modelling of the laser–target interaction on the basis of the Semak model was performed to understand the different compositions and properties of the layers. Numerical calculations showed that vaporization dominates in the case of short pulses, whereas a liquid-ablation regime is achieved in the case of 35 ns long pulses.

Effect of Laser Surface Modification on the Crystallinity of Poly(L-Lactic Acid)

Crystallinity of semicrystalline polymers such as aliphatic homopolymer poly(L-lactic acid) (PLLA) affects their degradation and physical properties. In this paper, the effects of laser irradiation using the third harmonic of a Nd:YAG laser on the crystallinity, long-range order, and short-range conformations at the surface of PLLA films are investigated. The factors affecting the transformation are also studied. Detailed characterization of the effect of laser treatment is accomplished using microscopy, X-ray diffraction, and infrared spectroscopy. The cooling rates in the process and the spatial and temporal temperature profiles are numerically examined. The simulation results in conjunction with melting and crystallization kinetics of PLLA are used to understand the effect on sample crystallinity. The effects of laser fluence and annealing conditions on the crystallinity of the processed films are examined. Since degradation profiles depend on crystallinity, laser processing can potentially be used to achieve a modified spatially controlled polymer surface with promising applications such as controlled drug delivery.

Luminescent nanoclusters array fabricated by pulsed laser beam irradiation

Ordered luminescent nanoclusters array in the form of grating structures are fabricated on silicon (100) surface by Q-switched Nd:YAG laser beam irradiation of second harmonic wavelength (532nm) in vacuum. Blue-green photoluminescence (PL) spectrum from the ordered nanoclusters array exhibits two asymmetrical peaks at 2.58eV and 2.88eV in the blue-green region corresponding to the bimodal distribution of nano size clusters. The size of the nanoclusters is estimated from the three dimensional quantum-confined model incorporating Gaussian size distribution. When subjected to rapid thermal annealing at 710°C for 10min in N2 atmosphere there is an enhancement of the PL intensity without any change in the peak emission energy and broadening suggesting that the origin of PL is related to quantum confinement effect in Si nanocrystallite. The surface morphology of the irradiated surface varies considerable with the number of laser shots, laser fluence and ambient conditions.

Fundamental processes in nanosecond pulsed laser ablation of metals in vacuum

Velocity and kinetic energy distributions (VDs, KEDs) of metal ions generated by nanosecond (ns) pulsed laser ablation under high vacuum have been determined using an electrostatic analyzer plus time-of-flight coupled system. A number of metal and alloy targets, principally involving Fe and Ni, have been studied at different laser fluences. At low fluence, the ion distributions have been shown to fit single Maxwell-Boltzmann-Coulomb (MBC) distributions; for medium and higher fluences, each ion distribution is found to comprise that of the surviving ``precursor'' ion, itself, overlapped with sidebands which arise from ion-electron recombination and/or ionization. The so-called surviving ``precursor'' ion of a distribution is that which underwent no change of charge. The Coulomb velocities of the surviving ``precursor'' ions and those of the ion products resulting from ion-electron collisions have been compared. Ion velocities are correlated with the local electric field resulting from ejection of the photoelectrons following laser ablation. Under identical conditions of laser fluence, the ions are seen to experience an electrical field nearly independent of their charge. The transit times of ions in the plasma have been estimated to be of the order of $1\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$. An overall quantitative mechanism for metal ablation on this basis is presented, including the ejection time for photoelectrons and differences in ion distributions resulting from employing laser pulses in the nanosecond (ns) and femtosecond (fs) regimes.

Formation of periodic microstructures on multilayer dielectric gratings prior to total ablation

The damage morphology produced by high-power, short-pulse lasers on multilayer dielectric (MLD) gratings has been closely examined. An unusual ripple formation arises under specific laser fluence conditions and produces a bright diffractive effect. A single irradiation does not produce this morphology, proving that it is a cumulative effect requiring multiple laser shots on a test site. The period of this microstructure is found to be between 2.0 and 2.4 mum. The ripple orientation varies across the test site. Varying several experimental conditions such as pulse length, beam polarization and angle of incidence still produces this periodic microstructure, though not always efficiently. This morphology is not seen on MLD stacks or other homogeneous samples.

Silicon cluster formation in the laser ablation of SiO at 308 nm

Neutral silicon cluster formation in the laser (308 nm) ablation of silicon monoxide was investigated through the analysis of composition and dynamics of the ablation plume under different laser fluence conditions. The neutral species were ionized by a second laser (193 nm) and the positionized species detected by TOF-MS (time-of-flight mass spectrometry). At low laser fluences, plume composition is dominated by SiO; above 0.6 J/cm2 Si, SiO and Si2 have comparable intensity and Sin (n≤7) clusters are observed. Flow velocities and temperatures of the ejected species are nearly mass-independent, indicating that the plume dynamics are close to the strong expansion limit, implying a collisional regime. Through the relation between the estimated values of terminal flow velocity and surface temperature, uT2∝TS, it is found that, at low laser fluences, the surface temperature increases linearly with laser fluence, whereas, at the laser fluence at which Sin clusters are observed, the increase of temperature is below the linear dependence. The population distribution of the ejected Sin provides some indication of a formation mechanism based on condensation. Analogies between the ablation behavior of silicon monoxide and silicon targets are considered.

Inhomogeneity of calcium phosphate coatings deposited by laser ablation at high deposition rate

Calcium phosphate coatings were deposited with a KrF excimer laser onto titanium alloy to study their homogeneity. Deposition was performed at a high deposition rate under a water vapour atmosphere of 45 Pa and at a substrate temperature of 575 °C. Samples were also submitted to annealing under the same conditions of deposition for different times just after deposition. The effects of the annealing were also investigated. The morphology of the coatings was studied by scanning electron microscopy. Their structure and phase distribution was analysed by X-ray diffractometry and infrared and micro-Raman spectroscopies. Besides the non-uniform thickness, the results reveal an inhomogeneity in the spatial distribution of calcium phosphate phases in the coatings. The phase distribution can be almost completely correlated with the deposition rate. High deposition rates (0.5 nm/pulse) occurring in the centre of deposition results in the formation of amorphous calcium phosphate, while lower deposition rates favour the presence of hydroxyapatite and alpha tricalcium phosphate. At intermediate deposition rates, beta tricalcium phosphate is found, probably because the superimposed effect of energetic particles bombardment. The annealing process promotes the crystallisation of the amorphous material. The importance of the deposition rate in the phases obtained is stated after comparing these results with a previous work where homogeneous hydroxyapatite coatings were obtained under the same conditions of laser fluence, temperature and pressure, but at lower deposition rates.

Focus Fluctuations in Laser-Materials Interactions

In laser-materials processing, the maximization of workpiece quality depends in part on optimizing conditions of laser beam size and fluence. Taking as an example the case of misfocus in sharp threshold laser ablation, the author describes the quality variations in laser-material processing that can be caused by a moving workpiece that jitters in and out of the laser beam's focal plane. He also discusses the possibility of engineering specific materials for improved misfocus tolerance.

Bi 2Sr 2Ca n−1 Cu nO 2( n+2)+ δ thin films on c-axis oriented and vicinal substrates

Bi 2Sr 2Ca n−1 Cu n O 2( n+2)+ δ ( n=1,2,3) thin films are prepared on c-axis oriented and vicinal substrates by pulsed-laser deposition. Optimization of substrate temperature, laser fluence and post-annealing conditions produces single-phase, well-oriented and smooth films. The surface morphology, texture, crystallinity and stoichiometry of Bi 2Sr 2CaCu 2O 8+ δ (Bi-2212) thin films strongly depend on the fabrication parameters. On ( 0 0 1 ) SrTiO 3 substrates, c-axis oriented Bi-2212 films with J c (60 K)=2×10 6 A/cm 2 and T c0=82 K are obtained. Vicinal Bi-2212 films grown on miscut substrates reveal anisotropic resistivities independent of film thickness (20–300 nm), high J c anisotropy and strong in-plane and out-of-plane texture.

TEA CO2 laser-induced reaction of CH3NO2 with CF2HCl: A mechanistic study

Dissociation of nitromethane has been observed when a mixture of CF2HCl and CH3NO2 is irradiated using pulsed TEA CO2 laser at 9R (24) line (1081 cm-1), which is strongly absorbed by CF2HCl but not by CH3NO2. Under low laser fluence conditions, only nitromethane dissociates, whereas at high fluence CF2HCl also undergoes dissociation, showing that dissociation occurs via the vibrational energy transfer processes from the TEA CO2 laser-excited CF2HCl to CH3NO2. Time-resolved infrared fluorescence from vibrationally excited CF2HCl and CH3NO2 molecules as well as UV absorption of CF2 radicals are carried out to elucidate the dynamics of excitation/dissociation and the chemical reactions of the dissociation products.