Function Of Laser Energy Density Research Articles

High strain rate effect on tensile ductility and fracture of AM fabricated Inconel 718 with voided microstructures

The paper describes Electromagnetic Ring Expansion Tests (ERET) performed on Laser Melting Powder Bed Fusion (LPBF) Inconel 718 stress relieved test pieces, to establish the effect of a randomly dispersed spherically voided microstructure on tensile ductility, fracture, and fragmentation at high strain rate (10−3 < ε < 104 s−1). An empirical model to predict porosity type and growth rates as a function of laser energy density was established, to select the LPBF process parameters to fabricate test pieces under stable conduction and keyhole melting. The size, shape, distribution of macro and keyhole pores in the test pieces obtained for ERET testing were characterised. At high strain rate the number of ring fragments for the highest porosity doubled, accompanied by a reduction in true strain at maximum uniform elongation and fracture strain. The trend for reducing fracture strain with increasing porosity at high strain rate was described by a decaying power law. Overall, there was a significant positive strain rate effect on tensile ductility at lower porosities attributed strain rate hardening (Hart, 1967) [1]. Fracture surfaces containing the highest porosity identified four different void coalescence mechanisms that helped explain the influence of larger pores on the stress state in the alloy.

Experimental investigation of laser ablation of stone polycrystalline targets

We report the results of an experimental investigation of ablation of stone polycrystalline targets of complex multicomponent composition, which imitate the substance of asteroids. The targets were irradiated by nanosecond pulses of a neodymium laser at an energy density ΦL of up to 5 × 104 J cm−2. The experiments demonstrated the existence of two ablation regimes, with the boundary between them lying at ΦL ≈ 4000 J cm−2. The regime change is characterised by a change in the form of the dependence of the surface mass density of removed target material on the laser energy density and by the appearance of a minimum in the dependence of specific energy of destruction on ΦL. This is supposedly related to the passage from a one-dimensional plasma plume expansion to the three-dimensional one and the corresponding decrease in the efficiency of energy transfer from the laser beam to the target due to a lowering of laser-produced plasma density. Our experiments also showed the existence of a maximum in impulse coupling coefficient Cm as a function of laser energy density (Cm ≈ 6.3 × 10−5 N W−1 for ΦL = Φopt ≈ 25 J cm−2). Maxima were also recorded in the dependences of the ablation efficiency and average ablation flow velocity on ΦL. For ΦL > Φopt, the decrease in the function Cm(ΦL) turns out to be much steeper than for metals and polymer materials. The difference is supposedly due to the lower strength and lower plasticity of the polycrystalline stone targets.

Scalable laser powder bed fusion processing of nitinol shape memory alloy

Abstract

Effect of laser energy on the properties of nanostructured lead iodide film prepared via pulsed laser deposition technique

The effect of laser energy density on the optical, structural and electrical properties of lead iodide PbI2 film prepared by laser deposition technique was studied. X-ray diffraction XRD results revealed that the PbI2 films deposited at laser energy density smaller than 3.5 J/cm2 are polycrystalline in nature with preferred orientation along (001) plane, while the film deposited at 3.5 J/cm2 was amorphous. The thickness and deposition rate of the film was found to be increased with laser energy density. The optical energy gap of PbI2 film was in the range of (2.5–2.65) eV. SEM investigations showed that the deposited films are dense and have a homogeneous structure and the average grain size increases from 25 to 55 nm as laser energy density increases from 2 to 3.5 J/cm2. The film surface composed of aggregated grains and particulates. Energy dispersive x-ray illustrated the film deposited at 2 J/cm2 was stoichiometric and the films deposited at laser energy density >2 J/cm2 were off-stoichiometric. The electrical properties revealed that the deposited films were p-type and the electrical resistivity decreases from 9.2 × 104 to 1.49 × 104 Ω.cm when laser energy density increases from 2 to 3 J/cm2.The hole mobility of the film was found to increase with laser energy density. The dark and illuminated I–V characteristics of p-PbI2/n-Si heterojunctions were investigated as a function of laser energy density.

Calculated and Experimental Research of Sheet Resistances of Laser-Doped Silicon Solar Cells

The calculated and experimental research of sheet resistances of crystalline silicon solar cells by dry laser doping is investigated. The nonlinear numerical model on laser melting of crystalline silicon and liquid-phase diffusion of phosphorus atoms by dry laser doping is analyzed by the finite difference method implemented in MATLAB. The melting period and melting depth of crystalline silicon as a function of laser energy density is achieved. The effective liquid-phase diffusion of phosphorus atoms in melting silicon by dry laser doping is confirmed by the rapid decrease of sheet resistances in experimental measurement. The plateau of sheet resistances is reached at around 15Ω/□. The calculated sheet resistances as a function of laser energy density is obtained and the calculated results are in good agreement with the corresponding experimental measurement. Due to the successful verification by comparison between experimental measurement and calculated results, the simulation results could be used to optimize the virtual laser doping parameters.

Stoichiometry of LaAlO3 films grown on SrTiO3 by pulsed laser deposition

We have studied the stoichiometry of epitaxial LaAlO3 thin films on SrTiO3 substrate grown by pulsed laser deposition as a function of laser energy density and oxygen pressure during the film growth. Both x-ray diffraction (θ-2θ scan and reciprocal space mapping) and transmission electron microscopy (geometric phase analysis) revealed a change of lattice constant in the film with the distance from the substrate. Combined with composition analysis using x-ray fluorescence we found that the nominal unit-cell volume expanded when the LaAlO3 film was La-rich, but remained near the bulk value when the film was La-poor or stoichiometric. La excess was found in all the films deposited in oxygen pressures lower than 10−2 Torr. We conclude that the discussion of LaAlO3/SrTiO3 interfacial properties should include the effects of cation off-stoichiometry in the LaAlO3 films when the deposition is conducted under low oxygen pressures.

Flexible High-Performance Amorphous InGaZnO4 Thin-Film Transistors Utilizing Excimer Laser Annealing

We have investigated an excimer laser annealing (ELA) process for use in fabricating high-performance amorphous InGaZnO4 (IGZO) thin-film transistors (TFTs) on flexible plastic substrates. We numerically estimate the temperature increase of the IGZO film and substrate as a function of laser energy density. This is one of the most important measures for optimizing ELA conditions in order to apply plastic-based TFT fabrication. Because the optical absorption coefficient of IGZO film is three orders of magnitude higher than that of plastic substrates with respect to 308-nm laser light, it is possible to selectively increase the temperature of the IGZO film. The temperature of the IGZO film is estimated to increase to approximately 1500 °C at typical laser energy levels. Furthermore, incorporating a SiO2 buffer layer (some hundreds of nm) between the IGZO film and the plastic substrate is found to effectively suppress thermal damage to the substrate. We have experimentally investigated the properties of IGZO films irradiated with various excimer-laser energy densities. X-ray diffraction patterns and carrier densities of the IGZO films are found to significantly vary with laser energy density. We have used calculations and experimental results to optimize the ELA process; this has enabled us to produce high-performance IGZO-TFTs having a field-effect mobility of 15.6 cm2 V-1 s-1.

Fabrication of polycrystalline silicon thin films on glass substrates using fiber laser crystallization

Laser crystallization of amorphous silicon (a-Si), using a fiber laser of λ = 1064 nm wavelength, was investigated. a-Si films with 50 nm thickness deposited on glass were prepared by a plasma enhanced chemical vapor deposition. The infrared fundamental wave ( λ = 1064 nm) is not absorbed by amorphous silicon (a-Si) films. Thus, different types of capping layers (a-CeO x , a-SiN x , and a-SiO x ) with a desired refractive index, n and thickness, d were deposited on the a-Si surface. Crystallization was a function of laser energy density, and was performed using a fiber laser. The structural properties of the crystallized films were measured via Raman spectra, a scanning electron microscope (SEM), and an atomic force microscope (AFM). The relationship between film transmittance and crystallinity was discussed. As the laser energy density increased from 10–40 W, crystallinity increased from 0–90%. However, the higher laser density adversely affected surface roughness and uniformity of the grain size. We found that favorable crystallization and uniformity could be accomplished at the lower energy density of 30 W with a-SiO x as the capping layer.

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.

Plasma plume characteristics and properties of pulsed laser deposited diamond-like carbon films

Pulsed laser deposition is a unique technique for the deposition of hydrogen-free diamond-like carbon films. During deposition, amorphous carbon is evaporated from a solid target by a high-energy KrF laser, ionized, and ejected as a plasma plume. The plume expands outwards and deposits the target material on a substrate. The plasma properties of the plume determine the quality of the thin films deposited on the substrate. These plume properties include ion density, ion flow speed, electron temperature, and plume peaking parameter. In this research, a triple Langmuir probe is used to determine various plasma properties of the plume created from the pulsed laser ablation of amorphous graphite as a function of laser energy density and laser spot size on the target. A thin diamond-like carbon film is deposited and analyzed with electron energy-loss spectroscopy to determine the sp3/sp2 fraction. A special preparation technique was used to prepare the thin film for analysis to prevent the damage that may be caused by conventional ion milling techniques.

Hydrogen Free, High sp3 Content DLC Films Produced by Pulsed Laser Ablation of Amorphous Graphite

Pulsed laser deposition is a unique technique for the deposition of a wide variety of thin films. These films include magnetic and optical films, novel semiconductors, high temperature superconductors, and diamond-like carbon films. Amorphous Carbon material is evaporated from a solid target and ionized by a high-energy KrF laser and ejected as a plasma plume. The plume expands outwards and deposits the target material on a silicon substrate. Plasma and flow parameters in this plume determine the quality of the film deposited on the substrate. In these experiments, a quadruple Langmuir probe is used to determine various plasma properties in the plume as a function of laser energy density on target, laser spot size on the target and probe position from target. These properties include electron temperature, ion flow speed, ion kinetic energy, plume peaking parameter, and ion density. Film analysis was conducted by electron energy loss spectroscopy (EELS) indicating that DLC films have been successfully deposited with an sp3 concentration as exceeding 72%.

Characteristics of Laser-Ablated Plasma and Properties Of Diamond-Like Carbon Film In Pulsed Laser Deposition

AbstractCharacteristics of laser-ablated carbon plasma and properties of diamond-like carbon film in KrF pulsed laser deposition were investigated using laser-induced fluorescence (LIF) and optical emission spectroscopy. Two-dimensional LIF images of C2 (Swan band, d3Φg – a3Φu) and C3 (Comet Head System, A1Φu –X1Σg+) molecules were detected as a function of laser energy density by narrow band pass filters and an intensified CCD camera. C2 LIF intensity is found to be weaker in the central part of the plume than that at the periphery at incident energy greater than 6 J/cm2. It is conjectured that C2 molecules are dissociated by collision with energetic species in the central part of the ablation plume, and degrade the diamond-like property of deposited films.

Laser Induced Fluorescence Measurement Of Plasma Plume During Pulsed Laser Deposition Of Diamond-Like Carbon

AbstractDynamics of carbon ablation plasma plume during the preparation of diamond-like carbon films by KrF excimer pulsed laser deposition was investigated using laser induced fluorescence (LIF) and optical emission spectroscopy. LIF signal from C2molecule (Swan band, d 3Φg – a3Φu) was detected using a photomultiplier tube and an intensified CCD camera. Temporal evolution and spatial distribution of C2 molecules in the ablated plume were measured as a function of laser energy density and ablation area. LIF intensity is found to be weaker in the central part of the plume than that at the periphery at incident energy greater than 6 J/cm2. It is conjectured that some of C2molecules are dissociated by collision with energetic species in central part of the ablation plume. Dynamics of ablation plasma plume is strongly dependent on the size of ablated area.

Pulsed KrF laser annealing of RF sputtered ZnS:Mn thin films

Pulsed KrF laser annealing (PLA) of ZnS:Mn thin (800 nm) film phosphors has been investigated as an alternative to thermal annealing for the fabrication of electroluminescent devices. The influence of the surrounding gas pressure during exposure, the energy density (Ed) of the laser beam and the effect of double irradiation is reported. Luminescent properties as function of laser energy density (Ed) are determined via photoluminescent (PL) characterisation. Energy densities used vary from 53 to 777 mJ/cm 2. PL intensities are determined to be linearly dependent with Ed beyond a threshold of 150 mJ/cm 2 and maximum PL enhancement is a factor of 2.1 x. A thermal simulation of the PLA process suggest that PL improvement is proportional to deposited thermal energy and this in the solid state. The calculated melting threshold agrees well with previous work.

Detrapping and diffusion of H and D implanted in carbon studied by high temperature laser annealing and depth profiling

The depth profiles of H and D implanted at low energy (0.7 and 1.5 keV) in Highly Oriented Pyrolytic Graphite (HOPG) ( a-axis) and vitreous carbon (v-C), and then partially released by pulsed laser induced desorption were investigated in order to understand the H behaviour in carbon at high temperature. During the laser desorption experiment, the re-emitted gas quantity was measured as a function of laser energy density by mass spectrometry. Depth profiles of H and D remaining after partial desorption were also obtained by means of the elastic recoil detection (ERD)- E × B technique. Firstly, a significant difference between the re-emitted H 2 or D 2 gas quantity and the released amount deduced from the ERD profiles is observed. This could be explained by atomic H release or by far in-depth diffusion of molecules. Secondly, it is seen from the depth profiles of hydrogen in HOPG that no profile modification is observed below 1500 K. The desorption process is compatible with a local molecular recombination model with fast diffusion of the molecules. At higher temperatures, significant diffusion of atomic hydrogen allows it to be retrapped in the vacancy profile created during the implantation.

Pulsed Laser Etching of GaN and AIN Films

AbstractDue to limited success in wet etching of GaN and AIN, dry etching techniques have become more relevant for the processing of the GaN films. Here we demonstrate the results of an alternative dry etching process, namely, pulsed laser etching, for GaN and AIN. In this method, a KrF pulsed excimer laser (λ=248 nm, τ=30 ns) was used to etch epitaxial GaN and AIN films. The dependence of the etching characteristics on the laser energy density and the number of pulses has been studied. The etch depth showed a linear dependence on the number of pulses over a wide range of laser energy densities. The threshold intensity for GaN etching was determined to be 0.33 J/cm2. The etching rate was found to be a strong function of laser energy density. Above the threshold, the etch rate was found to be 300–1400 Å per pulse leading to etching rates of 0.1–1μm/sec depending upon the laser energy density and the pulse repetition rate. It is shown that the etching mechanism is based on laser induced absorption, decomposition and layer by layer removal of the GaN.

A Comparative Study of Residual Stresses and Microstructure in a-tC Films

ABSTRACTWe compare the microstructure of highly tetrahedrally-coordinated-amorphous carbon (atC) films prepared by pulsed laser deposition (PLD), measured using both small angle x-ray scattering (SAXS) and x-ray reflectivity, with other physical properties such as film stress and electrical resistivity. These properties are controlled by the film growth conditions and film thicknesses. Films prepared under vacuum conditions exhibit a shift in the measured mass density, as a function of laser energy density. The density for films approximately 600Å thick approach that of crystalline diamond. The measured densities for thicker, approximately 1000Å films, exhibit a smaller shift, and a lower density value. This shift correlates to observed changes in film stress and electrical resistivity. The small angle signal of the reflectivity spectra suggests the presence of layering, or in-plane density variations or a combination of both within the films.

Characterization of the Substrate Interface of Excimer Laser Crystallized Polycrystalline Silicon Thin Films

ABSTRACTExcimer laser crystallized Si thin films on fused silica substrates exhibit a peak in the average grain size as a function of laser energy density. The average grain size increases with increasing laser fluence until a maximum value , approximately 10 microns for a 100 nm thick Si film, is achieved. The peak in grain size is accompanied by a peak in the electron Hall mobility. Further increases in the laser fluence result in a decrease in the Si grain size and an increase in the intragranular defects. A small energy range of 40 mJ/cm2 exists in which this peak in grain size can be achieved. Cross section TEM has shown that when the peak laser fluence is exceeded, the fused silica substrate can be as rough as 17 nm. Atomic force microscopy. performed on the substrate surface after the Si has been etched off, also shows that the magnitude and spatial frequency of the roughness increases when the critical laser fluence is exceeded. This degradation of the interface may also produce sites for stacking faults to form during the solidification of the Si. This result and results of simulations of the temperature of the interface during crystallization suggests that the peak energy range exists after the complete melting of the Si thin film and before the silica substrate starts to soften.

Microstructure of pulsed laser irradiated Sn and Ge-50 at.%Sn thin films on Ge single crystal substrates

The microstructures produced from pulsed laser irradiation of evaporated thin films of Sn and Ge-50 at.%Sn on single-crystal Ge substrates have been investigated as a function of laser energy density and film thickness. In general, the irradiated samples exhibit a trilayer morphology consisting of an overlayer of equiaxed β-Sn and/or α-Ge in a β-Sn matrix, and epitaxial cellular and segregated-free α-Ge(Sn) alloy layers. The overlayer thickness decreases and the cellular layer thickness and the cell size both increase with increasing laser energy density. The maximum Sn content in the segregation-free α-Ge(Sn) alloy layers remains below 2 at.%Sn, but the maximum Sn content in the cellular α-Ge(Sn) alloy layers can reach up to 55 at.%Sn, estimated from the selected area diffraction pattern.

Pulsed UV laser irradiation effect for Sn +-implanted GaAs

The effect of pulsed UV laser irradiation on Sn +-implanted GaAs was investigated systematically by combining RBS, SIMS and Raman scattering characterization techniques. Implantation energies and doses were controlled so as to realize a flat doping profile about 800 Å thick to a dose from 10 17 to 10 21 cm −3. An ArF excimer laser ( λ = 193 nm) was used for the irradiation with a single pulse (∼20 ns) in the energy density range from 200 to 1400 mJ/cm 2. Strong segregation of Sn atoms toward the surface and partial movement of Sn atoms from an interstitial lattice site to substitutional sites were observed after the irradiation. Ordering of the disordered lattice was observed to proceed in the direction from the surface into the bulk in the same crystal orientation as the underlying layer. The behavior of Raman scattering spectra as a function of laser energy density was analyzed quantitatively based on a “spatial correlation” model. The analysis makes it possible to estimate an average particle size for the annealed crystal region in samples irradiated with a given energy density.