Increasing Laser Energy Density Research Articles

Analysis of 532nm long pulse laser-induced thermal decomposition damage to GaAs by semi-analytical method

Considering the fact that the GaAs has the characteristics of thermal decomposition, the thermal decomposition damage to GaAs surface, induced by a 532 nm wavelength long pulse laser with a millisecond pulse width is studied by the heat conduction theoretical and semi-analytical method. First, the calculation models of two-dimensional axisymmetric transient temperature field and the surface thermal decomposition damage threshold for long pulse laser irradiation of GaAs are established, and the transient temperature fields and the thermal decomposition damage thresholds in GaAs with different absorption rates are simulated. The results show that the higher absorption rate causes the higher temperature rise on the surface of material, but the required decomposition damage energy density is lower. With the increase of laser energy density, the decomposition damage occurs more early. This paper has guiding significance and practical value for investigating the interaction between long pulse laser and GaAs and its damage mechanism.

Investigation of 1064-nm laser damage mechanism of neutral density filter

The typical neutral density filter is a metal film plated on a K9 glass to achieve the effective absorption of laser. Its lower damage threshold severely restricts its application to high energy laser systems. Experimental study on damage morphology and damage mechanism of filter in a higher laser energy density is carried out. The variation characteristics of damage morphology are as follows: with the increase of laser energy density, damage spots first appear on the filter, then develop into cracks, and the cracks grow gradually longer and eventually connect into linear and block forms, resulting in a large area of film dropping off. A model of defect absorption leading to film damage on neutral density filter is established. And temperature and stress distributions on the film surface are calculated, separately. The inhomogeneous temperature rise on film surface leads to radial, hoop and axial thermal stress distributions. Theoretical analysis shows that cracks along the radial direction are caused by hoop stress. When laser energy density is larger than about 2.2 J/cm2, impurity particle radius is larger than 140 nm and the distance between impurity particles is less than 10 m. A large number of cracks can connect together to cause a large area of film to drop off.

Pulsed Neodymium-Doped Yttrium Aluminum Garnet Laser Heating of Multiwalled Carbon Nanotube Film

A multiwalled carbon nanotube (MWCNT) film surface was irradiated with a single pulse from a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser. When an MWCNT film is irradiated with a pulsed Nd:YAG laser of 23.7 mJ/cm2, the calculated maximum surface temperature becomes 304 °C. The surface temperature increases from 595 to 2968 °C with an increase in laser power from 59.4 to 469 mJ/cm2. The MWCNT film is examined by increasing the intensity of the two characteristic Raman shifts ID (defect-induced mode: D-band) and IG (graphite-induced mode: G-band) to clarify the effect of pulsed Nd:YAG laser heating. Moreover, a new original image analytical method is developed to characterize CNTs from scanning electron microscopy (SEM) images. This method is useful for measuring the diameter of as-grown CNTs and those after annealing with a pulsed Nd:YAG laser. The number of detected diameters between 40 and 60 nm increases with an increase in laser energy density from 181 to 268 mJ/cm2.

Nickel Silicide Formation using Pulsed Laser Annealing for nMOSFET Performance Improvement

The formation of a uniform, high tensile stress and low silicide/Si interfacial resistance nickel silicide in nMOSFET by introducing pulsed laser annealing (PLA) is reported. This annealing approach facilitated the phase transformation of nickel silicide to Si-rich NiSix compounds using a low-thermal-budget process, improves the silicide/Si interface regularity and avoids familiar (111) NiSi2 facet formation at a laser energy of 1.5 J cm−2. By increasing laser energy density up to 2.3 J cm−2, the device performance and statistics junction leakage distribution were degraded due to the increased sheet resistance of silicide layer and the destroyed silicide/Si interface morphology. When the PLA with a laser energy density of 1.5 J cm−2 was employed for nickel silicidation on the p-type Schottky diodes, a 0.16 eV hole Schottky barrier height (SBH) increase from 0.52 to 0.68 eV was observed. In addition, the application of PLA for source/drain silicidation of nMOSFETs demonstrated an 8% enhancement in Ion−Ioff characteristic relative to that obtained through the conventional two-step rapid thermal annealing (RTA). This PLA method holds promise as a potential replacement for current nickel silicide annealing approaches toward extremely scaled-down transistors.

Microstructure and mechanical properties of selective laser melted magnesium

The effects of laser processing parameters on the microstructure and mechanical properties of selective laser-melted magnesium were investigated. The results show that the microstructure characteristics of the laser-melted samples are dependent on the grain size of SLM magnesium. The grains in the molten zone coarsen as the laser energy density increases. In addition, the average hardness values of the molten zone decreases significantly with an increase of the laser energy densities and then decreased slowly at a relatively high laser energy density irrespective of mode of irradiation. The hardness value was obtained from 0.59 to 0.95 GPa and corresponding elastic modulus ranging from 27 to 33 GPa. The present selective laser-melted magnesium parts are promising for biomedical applications since the mechanical properties are more closely matched with human bone than other metallic biomaterials.

Thickness dependent self limiting 1-D tin oxidenanowire arrays by nanosecond pulsed laser irradiation

Fast, sensitive and discriminating detection of hydrogen at room temperature is crucial for storage, transportation, and distribution of hydrogen as an energy source. One dimensional nanowires of SnO2 are potential candidates for improved H2 sensor performance. The single directional conducting continuous nanowires can decrease electrical noise, and their large active surface area could improve the response and recovery time of the sensor. In this work we discuss synthesis and characterization of nanowire arrays made using nanosecond ultraviolet wavelength (266 nm) laser interference processing of ultrathin SnO2 films on SiO2 substrates. The laser energy was chosen to be above the melting point of the films. The results show that the final nanowire formation is dominated by preferential evaporation as compared to thermocapillary flow. The nanowire height (and hence wire aspect ratio) increased with increasing initial film thickness h0 and with increasing laser energy density Eo. Furthermore, a self-limiting effect was observed where-in the wire formation ceased at a specific final remaining thickness of SnO2 that was almost independent of h0 for a given Eo. To understand these effects, finite element modeling of the nanoscale laser heating was performed. This showed that the temperature rise under laser heating was a strong non-monotonic function of film thickness. As a result, the preferential evaporation rate varies as wire formation occurs, eventually leading to a shut-off of evaporation at a characteristic thickness. This results in the stoppage of wire formation. This combination of nanosecond pulsed laser experiments and thermal modeling shows that several unique synthesis approaches can be utilized to control the nanowire characteristics.

Laser-ablated nanofunctional polymers for the formulation of slow-release powders for dry powder inhalers: physicochemical characterization and slow-release characteristics

Recently, dry powder inhalation (DPI) powders coated with nanometre-thin layers of biodegradable polymers, prepared using pulse laser deposition (PLD), have been evaluated as a slow-release formulation for DPI use, with the goal of improving pulmonary selectivity. This paper describes evaluation of the chemical stability of one potential polymer, poly lactic acid (PLA), during the ablation process, the resulting respirable properties and potential cytotoxicity of coated glucocorticoid powders, and the resulting sustained-release characteristics of PLA-coated glucocorticoids creating using PLD. Triamcinolone acetonide (TA) and budesonide (BUD) were used as two model glucocorticoids to determine pulmonary targeting (PT) in-vivo. The chemical stability of PLA was determined at various laser energy densities. The respirable fraction and the cytotoxicity of the micronized particles of TA and BUD, coated using optimum laser energy density, were determined. In-vitro dissolution profiles were generated for the coated/uncoated formulations and an ex-vivo receptor binding assay was used to determine PT in rats. Increasing laser energy density led to decreases in molecular weight and film density, and increases in degradation products, roughness and thickness of the film. The mean dissolution time of coated formulations of BUD was longer (4 h) than with the less lipophilic TA (2 h). This correlated well with a more pronounced pulmonary selectivity observed for coated BUD ex-vivo. Stability and the physical properties of the film correlated with the laser energy density. We observed a direct relationship between the dissolution rate of the uncoated and coated formulation and the degree of PT; however, physiochemical properties of the drug (e.g. lipophilicity) may also contribute to the improved PT.

Analysis of the Microstructure and Thermal Shock Resistance of Laser Glazed Nanostructured Zirconia TBCs

Nanostructured zirconia thermal barrier coatings (TBCs) have been prepared by atmospheric plasma spraying using the reconstituted nanosized yttria partially stabilized zirconia powder. Field emission scanning electron microscope was applied to examine the microstructure of the resulting TBCs. The results showed that the TBCs exhibited a unique, complex structure including nonmelted or partially melted nanosized particles and columnar grains. A CO2 continuous wave laser beam has been applied to laser glaze the nanostructured zirconia TBCs. The effect of laser energy density on the microstructure and thermal shock resistance of the as-glazed coatings has been systematically investigated. SEM observation indicated that the microstructure of the as-glazed coatings was very different from the microstructure of the as-sprayed nanostructured TBCs. It changed from single columnar grain to a combination of columnar grains in the fracture surface and equiaxed grains on the surface with increasing laser energy density. Thermal shock resistance tests have showed that laser glazing can double the lifetime of TBCs. The failure of the as-glazed coatings was mainly due to the thermal stress caused by the thermal expansion coefficient mismatch between the ceramic coat and metallic substrate.

Characterization on pulsed laser deposited nanocrystalline ZnO thin films

Nanocrystalline zinc oxide thin films were deposited on glass and silicon substrates by using pulsed laser deposition at different laser energy densities (1.5, 2, and 3 J/cm 2). The film thickness, surface roughness, composition, optical and structural properties of the deposited films were studied using an α-step surface profilometer, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), optical transmittance, and X-ray diffraction (XRD), respectively. The film thickness was calculated as 244 nm. AFM analysis shows that the root-mean-square roughness increases with increasing laser energy density. XPS analysis shows that the interaction of zinc with oxygen atoms is greatly increased at high laser energy density. In the optical transmittance spectra, a shift of the absorption edge towards higher wavelength region confirms that the optical band gap increases with an increase in laser energy density. The particle size of the deposited films was measured by XRD, it is found to be in the range from 7.87 to 11.81 nm. It reveals that the particle size increases with an increase in laser energy density.

Characterization of iron oxide films prepared by laser irradiation in oxygen atmosphere

Iron oxide films were produced by pulsed laser irradiation of iron metal in pure oxygen gas at 0.1 MPa pressure. A XeCl excimer laser was used for this purpose. Different energy densities were applied for the laser irradiation in order to investigate the laser–plasma–metal interaction and the phase formation mechanisms. The oxygen depth profiles as obtained by Rutherford backscattering spectrometry show an increase in oxygen content with increasing laser energy density. Different magnetic and non-magnetic oxide phases were formed, as revealed by x-ray diffraction and Mössbauer analysis. A possible mechanism for the formation of oxides is presented. Subsequent irradiation of the oxide film with the laser beam in an Ar atmosphere reduces the oxide layer back to pure α-Fe.

FE simulation of laser curve bending of sheet metals

Laser bending is a highly flexible sheet metal forming technique. For the production of complex shaped or spatially curved parts, it is more convenient and efficient to apply curved irradiation paths instead of linear paths. In this paper, a finite element model of heat flux based on scanning path described with B-spline curve was built. Then, FE simulation of laser beam scanning on the forming sheet metals was carried out. And transient temperature fields, displacement fields, stress fields and strain fields were investigated. The proposed model can be used to forecast laser curve bending of sheet metal conveniently. The simulated results show that: (1) there is a good agreement between the FE simulations based on the proposed model and the experiments; (2) the peak temperatures of the upper surface increase when the laser power or the path curvature increases, but decrease when the laser spot diameter or the scanning velocity increases. The peak temperatures increase roughly when the laser energy density increases; (3) the laser curve bending produces a significant change of distortion under the same conditions of deformation. The warped curvature increases when the laser energy density or the path curvature increases.

Numerical modeling and experimental investigation of the superficial layer of SKD61 steel during laser surface hardening

Laser heating of SKD61 steel usually causes the formation of a melting layer on the steel surface, which is of poor thermal conductivity and diffusivity. The modified Ashby‐Easterling heat‐transfer equation was used to simulate the temperature distribution for laser surface hardening of SKD61 steel. The phase transformation temperatures of SKD61 in the quenched and as‐received conditions were compared with each other for different laser energy densities. When the laser was focused on the steel, the temperature of the SKD61 was raised. Due to the effect of superheating, the critical phase transformation temperature in laser hardening became higher than the austenized temperature (1010°C) in traditional quenching. However, the critical phase transformation temperature of SKD61 decreased with increasing laser energy density.

Low Temperature Preparation of Pb(Zr0.52Ti0.48)O3 Thin Films Using Pulsed Excimer Laser Annealing Process

In-Situ Laser annealing of Pb(Zr 0.52 Ti 0.48 )O 3 (PZT) thin films (0.6 mm) prepared by metalorganic decomposition (MOD) process on Pt/SiO 2 /Si substrates were investigated. The annealing of the films is done by simultaneously heating the substrate to 400°C and laser annealing at room temperature using KrF excimer laser (248 nm). The properties of in-situ laser annealed films were compared with the RTA annealed films. It is observed that in-situ annealed films show better density and smaller grain formation than RTA annealed films. The properties of in-situ laser annealed films were studied as a function of laser repetition frequency; laser duration time and laser energy density. Good crystallized films were obtained for laser repetition frequency of 4∼6 Hz. It is observed that the laser duration time has no effect on the crystallization of the PZT films However, the increase in laser energy density for laser annealing is found to decrease the grain size. The optimum laser energy density for in-situ laser annealing is found to be 120 mJ/cm 2 . The ferroelectric properties in-situ laser annealed PZT films were compared with respect to laser energy density. In the present studies the in-situ laser annealing with laser energy of 125 mJ/cm 2 annealed for 120 s has shown better remnant polarization than those reported in literature. †Present address: Directorate of Advanced Composites, advanced systems Laboratory, Kanchanbagh P.O., Hyderabad-500058.

UV crystallization of poly-si using a CeO2 seed layer on plastic substrate for microelectronics applications

We are reporting the crystallization of amorphous silicon (a-Si) film using XeCl excimer laser annealing. Polycarbonate (PC) plastic substrate was used to deposit poly-Si film at low temperatures. We deposited the a-Si films at a room temperature using an inductive coupled plasma chemical vapor deposition (ICPCVD). Then it was crystallized as a function of laser energy density using a XeCl excimer laser. As the laser energy density increases, crystallinity value decreases from 72.78 to 81.72%. The relationship between the film transmittance and crystallinity was analyzed as a function of process pressure. In another process, CeO2 seed layer was grown by sputtering method. a-Si film was deposited over this layer using ICPCVD and then it was crystallized using the XeCl excimer laser. It was observed that the crystallization could be accomplished at lower energy density with a seed layer than that without such layer. In this paper, we present the crystallization properties of a-Si on the plastic substrate.

Effects of KrF (248 nm) excimer laser irradiation on electrical and optical properties of GaN:Mg

The electrical and optical characteristics of GaN:Mg irradiated by a pulsed KrF (248 nm) excimer laser have been studied. When an as-grown Mg-doped GaN film was irradiated by an excimer laser at an energy density of 590 mJ/cm2 in a nitrogen atmosphere, the hole concentration was drastically increased up to 4.42×1017 cm−3. Furthermore, a GaN:Mg thin film, which was treated by laser irradiation following a conventional rapid thermal annealing process, showed a very high hole concentration of 9.42×1017 cm−3. The GaN:Mg samples, which were activated in a nitrogen ambient by the KrF excimer laser irradiation, showed two photoluminescence peaks at 2.95 eV and 2.7 eV. The intensities of both photoluminescence peaks were increased with increasing laser energy density and number of pulses. The changes in photoluminescence peaks depending on the laser energy density further suggest that the pulsed KrF excimer laser irradiation dissociates the Mg–H complexes and allows the hydrogens to diffuse out, thus significantly enhancing the p-type conductivity of GaN:Mg.

The Shallow Implantation of Bismuth During the Growth of Bismuth Nanocrystals in Al2O3 by Pulsed Laser Deposition

AbstractThe effect of the laser energy density used to deposit Bi onto amorphous aluminum oxide (a-Al2O3) on the growth of Bi nanocrystals has been investigated using transmission electron microscopy of cross section samples. The laser energy density on the Bi target was varied by one order of magnitude (0.4 to 5 J cm-2). Across the range of energy densities, in addition to the Bi nanocrystals nucleated on the a-Al2O3 surface, a dark and apparently continuous layer appears below the nanocrystals. Energy dispersive X-ray analysis on the layer have shown it is Bi rich. The separation from the Bi layer to the bottom of the nanocrystals on top is consistent with the implantation range of Bi species in a-Al2O3. As the laser energy density increases, the implantation range has been measured to increase. The early stages of the Bi growth have been analyzed in order to determine how the Bi implanted layer develops.

Correlation Between Electrical Characteristics and Oxide/Polysilicon Interface Morphology for Excimer-Laser-Annealed Poly-Si TFTs

This work investigates the correlation between electrical characteristics and gate-oxide/polysilicon interface morphology for excimer-laser-annealed (ELA) poly-Si thin-film transistors (TFTs). The main feature of ELA poly-Si films is protrusion at grain boundaries that makes the film surface appear very rough. The surface roughness increases with increasing laser energy density and causes degradation of off-current and reliability for the ELA poly-Si TFTs. This degradation of the off-current is attributed to the lower channel resistance due to the increase in crystallinity of the poly-Si layer and the enhancement of localized electric field arising from the protrusions at the grain boundaries. In addition, the increase of localized electric field also degrades device reliability. Passivation of gate oxide/poly-Si channel by -plasma treatment was found to be favorable in improving the performance and reliability of the ELA poly-Si TFTs. © 2002 The Electrochemical Society. All rights reserved.

Mechanical properties of pulsed laser-deposited hydroxyapatite thin films for applications in biomedical implants

ABSTRACTWe have obtained nanostructured hydroxyapatite thin films on titanium alloy substrates by pulsed laser deposition. Deposition was carried out using a KrF excimer laser (248 nm) with the energy density of 4 – 7 J/cm2 at substrate temperatures in the 550°C - 650°C range. The crystallinity of the coatings was probed by X-ray diffraction. Phase transitions from hydroxyapatite to other calcium phosphate compounds were observed with varying the substrate temperature during the growth process. Scanning electron microscopy revealed thin films made up of partially sintered nanoscale grains. The average size of nanoscale grains increased significantly with film thickness, suggesting a growth mechanism involving the coalescence of nanoscale grains. As the laser energy density increases, the hydroxyapatite crystallites in the coatings are oriented preferentially along the c-axis perpendicular to the substrate. Mechanical properties of the highly c-axis oriented coatings such as hardness and Young's modulus were studied by using nanoindentation technique.

Formation of TiB 2 whiskers in laser clad Fe–Ti–B coatings

In order to reduce the high brittleness of laser clad Fe–B coatings, the composite Fe–Ti–B coating was developed by laser cladding with a powder mixture of B 4C and Fe–Ti alloy. The microstructure, hardness and cracking resistance of these coatings were investigated in this paper. The experimental work enables the following findings to be reached: (i) the needle-like TiB 2 whiskers could be synthesized in situ using the laser cladding method in the composite Fe–Ti–B coating. (ii) The size, shape and volume fractions of TiB 2 whiskers in the composite coating are greatly dependent upon powder compositions (contents of B, Ti elements and the ratio of B/Ti elements) and the laser energy density used. The average ratio of width/length for the synthesized TiB 2 whiskers increases with increasing laser energy density and ratio of B/Ti elements in this coating. (iii) Both the hardness and cracking resistance of the Fe–B laser clad coating could be improved by the formation of TiB 2 whiskers.

Origin of a-axis outgrowth in pulsed-laser-depositedYBa2Cu3O7-δ thin films from modified melt-textured growntargets

YBa2Cu3O7-δ (YBCO) thin films were deposited usingpulsed laser deposition (PLD) from modified melt-textured grown targets. Asthe laser energy density was increased, the surface of the films wascovered with enhanced a-axis outgrowths. In order to determine the origin ofthese formations, the microstructures of films deposited at 2 and4 J cm-2 were investigated using x-ray diffraction, transmission electronmicroscopy and high-resolution electron microscopy. It was found that asignificant number of Y2O3 inclusions were formed during the growth of c-axis-oriented films at 4 J cm-2. These inclusions formed nucleationsites for the a-axis outgrowths. The formation of Y2O3 inclusions wasattributed to the Y-rich targets. In addition, the non-equilibriumcharacteristics of PLD and the enhanced ablation of Y-rich phases attributedto the increase of laser energy density led to these Y2O3 inclusions. Asfor the a-axis outgrowth nucleation, it is considered that, due to theunstable growth conditions with a high flux density of incident vapour speciesand the strain induced by the surrounding c-axis films, the Y2O3inclusions would prefer the nucleation of a-axis grains.