Melting Threshold Research Articles

The p–n junction formation in Hg1−xCdxTe by laser annealing method

Abstract The formation of p–n junctions in Hg 1− x Cd x Te is still an open research task. In this paper, laser treatment of n-type Hg 1− x Cd x Te samples resulting in the formation of a p–n junction is studied. The YAG:Nd 3+ laser with pulse duration of 250 μs or 40 ns was used. The energy density of laser beam was below the threshold of sample surface melting. The interpretation of the results is based on a model of defects formation related to interstitial mercury diffusion following laser irradiation. The Hall measurements clearly point out to a simple p–n junction. The resistance of samples shows the long time relaxation described by the 1/2 power law, which is attributable to the defect diffusion processes, but not to the changes in the electron–hole systems.

Chamber wall materials response to pulsed ions at power-plant level fluences

Candidate dry-wall materials for the reactor chambers of future laser-driven Inertial Fusion Energy (IFE) power plants have been exposed to ion pulses from RHEPP-1, located at Sandia National Laboratories. These pulses simulate the MeV-level ion pulses with fluences of up to 20 J/cm 2 that can be expected to impinge on the first wall of such future plants. Various forms of tungsten and tungsten alloy were subjected to up to 1600 pulses, usually while being heated to 600 °C. Other metals were exposed as well. Thresholds for roughening and material removal, and evolution of surface morphology were measured and compared with code predictions for materials response. Powder-metallurgy (PM) tungsten is observed to undergo surface roughening and subsurface crack formation that evolves over hundreds of pulses, and which can occur both below and above the melt threshold. This roughening is worse than for other metals, and worse than for either tungsten alloyed with rhenium (W25Re), or for CVD and single-crystal forms of tungsten. Carbon, particularly the form used in composite material, appears to suffer material loss well below its sublimation point. Some engineered materials were also investigated. It appears that some modification to PM tungsten is required for its successful use in a reactor environment.

Laser treatment of white China surface

Abstract The surface of gloss fired porcelain with and without raw glaze coating was radiated by a CO 2 laser working at 10.6 μm, a choice resulted from spectroscopic studies of suspensions made of China. The shine of the untreated sample was defined as the distribution of micro-droplets on the surface. The surface alterations due to laser heating were classified by the diameter of the completely melted surface, the ring of the surface at the threshold of melting, and the size of microscopic cracks. The diameter of the laser treated area was in the range of 3 mm, while the incident laser power and the duration of laser heating were varied between 1 and 10 W and 1–8 min, respectively. The different stages of surface modifications were attributed primarily to the irradiating laser power and proved to be rather insensitive to the duration of the treatment. We have found a range of parameters under which the white China surface coated with raw glaze and followed by laser induced melting exhibited very similar characteristics to the untreated porcelain. This technique seems prosperous for laser assisted reparation of small surface defects of unique China samples after the firing process.

Jumping Nanodroplets

Flat gold nanostructures on inert substrates like glass or graphite were illuminated by single intensive laser pulses with fluences above the gold melting threshold. The liquid structures produced in this way are far from their equilibrium shape, and a dewetting process sets in. On a time scale of a few nanoseconds, the liquid contracted toward a sphere. During this contraction, the center of mass moved upward, which could lead to detachment of droplets from the surface due to inertia. The resulting velocities were on the order of 10 meters per second for droplets with radii in the range of 100 nanometers.

Effect of pulsed laser action on hole-energy spectrum ofGe∕Siself-assembled quantum dots

Space-charge spectroscopy has been used to study the hole energy spectrum of an array of Ge quantum dots (QD's) coherently embedded in a Si matrix and subjected to a ruby laser $(\ensuremath{\lambda}=694\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ nanosecond irradiation ex situ. The laser energy density in a single pulse was near the melting threshold of the Si surface. The number of laser pulses was varied from 1 to 10, and the duration of each pulse was $80\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$. From the capacitance-voltage characteristics, temperature- and frequency-dependent admittance measurements, the energies of holes confined in Ge QD's were determined. The pulsed laser annealing was found to result in a deepening of the hole energy level relative to the bulk Si valence band edge and in a decrease of the hole energy dispersion. After the treatment with ten laser pulses, the spread of the hole energies due to varying sizes of the QD's within the ensemble was reduced by a factor of about 2. The obtained results give evidence for a substantial reduction of the QD's size dispersion and for a narrowing distribution of the hole energy levels stimulated by nanosecond laser irradiation. A possible explanation of the improved uniformity of QD's sizes involves dissolving small size Ge QD's in a Si matrix by pulsed laser melting of the Ge nanoclusters and their subsequent intermixing with surrounding solid Si.

Formation of a ripple pattern at a water/silicon interface using an oscillating bubble

A single femtosecond laser pulse was irradiated at a water/silicon interface, and the processed surface was investigated. Rings surrounded by ripples were found within the irradiated spot. The diameter of the rings ranged from 500 nm to 10 μm. It is proposed that acoustic waves, caused by the oscillating motion of bubbles near the water/silicon interface, deformed the melting silicon surface. In the present work, a pulse (pulse width: 150 fs) was tightly focused in water to induce optical breakdown, and a bubble was generated at an arbitrary spot. When the power density was below the ablation threshold and above the melting threshold at the silicon surface and set above the breakdown threshold at the focus in water, a pattern was generated at a specific place and with a specific size.

Thermal Shock and Thermal Fatigue of Ferroelectric Thin Film due to Pulsed Laser Heating

Thermal shock and thermal fatigue of ferroelectric (FE) thin films were investigated by the pulsed laser tests. The power density was gradually increasing in the single pulsed laser heating test which simulated a thermal shock, the part melting threshold of Pb(Zr0.52Ti0.48)O3 (PZT) thin films was found by scanning electron microscopy (SEM). After thermal shock resulted the highest temperature below Curie point at the surface of PZT thin film, X-ray diffraction (XRD), SEM and RT66A standard ferroelectrics analyzer were used to study the microstructure, crystal grain sizes, and ferroelectric failure behavior. It was found that XRD peak of PZT thin film after laser beam heating was stronger than that before laser beam heating, crystal grain sizes decreased, and the ferroelectric properties were degraded. However there was no crack observed by SEM, until PZT thin films were melted. The fined grain effects on ferroelectric properties and XRD patterns of PZT thin film, depolarization due to the single pulsed laser heating were discussed respectively. The pulsed cycles with a certain power density were gradually increasing in the repetition pulsed laser heating test. It was interesting to find that the cracks will initiate and propagate due to the thermal fatigue induced by the repetition pulsed laser. The possible origins of the thermal fatigue cracks were also discussed.

Thermoelastic modeling of microbump and nanojet formation on nanosize gold films under femtosecond laser irradiation

A model of elasto-plastic flow combined with the two-temperature model in a two-dimensional approximation has been developed to describe microbump and nanojet formation observed recently on nanosize gold films irradiated by femtosecond laser pulses. It has been shown that the effect of microbump and nanojet formation is conditioned by the elastic characteristics, yield stress, and other properties that are unique for gold. The numerical results are in excellent agreement with the experimental data with respect to a number of parameters, particularly the threshold fluence for nanojet formation at the tops of microbumps, which occurs nearby and above the melting threshold. The analysis of properties for a number of materials is performed and some other materials are discussed as possible candidates for nanotexturing of thin films by femtosecond laser pulses.

Sub-micrometer adhesion modulation on polymer surfaces containing gratings produced by two-beam interference

Grating-like structures having a period of 416 nm were produced on the surface of poly-carbonate films by two-beam interference realized by the fourth harmonic of Nd:Yag laser. The period of the structures was half of that the applied master grating, the ratio of the width of the valleys to the period was tuned by the intensity, the depth of the modulation was increased by the number of laser pulses. Pulsed force mode atomic force microscopy was applied to study the topography and the adhesion on structured surfaces with sub-micrometer resolution. The adhesion modulation caused by the topography was calculated along line cross-sections of the AFM pictures taking into account the tip and surface geometry. The separation of the effects of the topography and the laser-induced material changes proved that the adhesion is increased at the areas illuminated by laser beam having a fluence above the melting threshold. The laser-induced material changes cause an additional adhesion increase at the valleys of the structure. It was shown that the adherence of albumin results in dense packing on poly-carbonate surface parts having sub-micrometer periodic adhesion modulation.

High-pressure phase diagram of the exp-6 model: The case of Xe

We investigated numerically the high-temperature--high-pressure phase diagram of xenon as modeled through the exp-6 interaction potential, which is thought to provide a reliable description of the thermal behavior of rare gases under extreme conditions. We performed a series of extensive NVT Monte Carlo simulations which, in conjunction with exact computation of the solid free energy by the Frenkel-Ladd method, allowed us to precisely locate the freezing and melting thresholds at each temperature. We find that, under isothermal compression, the exp-6 fluid freezes directly into a fcc solid; however, above 4500 K, an intermediate bcc phase becomes stable in a narrow range of pressures. The chemical potential of the hcp phase never significantly differs from that of the fcc solid of equal $T$ and $P$, though the former is found to be slightly greater than the latter. We discuss our results in the light of previous numerical studies of the same model system and of the experimental data available for xenon.

Microdroplet deposition by laser-induced forward transfer

Laser-induced forward transfer was used to deposit aluminum and nickel microdroplets onto a substrate using a Q-switched neodymium:Yttrium-aluminum-garnet laser. The droplets have diameters of a few microns, much smaller than the laser spot diameter, and are transferred at fluences slightly above the melting threshold. Scanning electron microscopy shows that the original donor film is deformed after laser irradiation, such that the film protrudes outward from the center of the laser spot. The film expands during laser heating, but is constrained until the melt interface reaches the free surface. When this occurs, the film is no longer constrained, allowing the melt to rapidly expand, forming the protrusions from which droplets are ejected.

Suppression of dewetting phenomena during excimer laser melting of thin metal films on SiO 2

Pulsed excimer laser irradiation has been used to fully melt 200 nm films of elemental Au and Ni on SiO 2 substrates. With the use of a capping layer of SiO 2 and line irradiation via projection optics, the typical liquid-phase dewetting processes associated with these metals on SiO 2 has been suppressed. In a series of experiments varying line widths and fluence, a process region is revealed immediately above the complete melting threshold for which the films remain continuous and smooth after melting and resolidification. Simple energetic arguments for mechanisms leading to initiation of dewetting support these observations, and a gas-mediated model is proposed to describe the process conditions that are necessary for the suppression of dewetting.

Simulation of Crustal Melt Segregation Through Cellular Automata: Insight on Steady and Non-steady State Effects Under Deformation

We present a numerical model of melt segregation under variable deformation regimes (pure and/or simple shear) and strain rates. The model is adapted from a cellular automaton. The Lagrangian description requires some initial melt content and obeys two thresholds. One allows melt connection and subsequent melt motion. The other rules melt escape. The geometry of the flow is that of pure and/or simple shear. Deformation is a necessary melt-segregating factor, with contraction being more efficient than simple shear. The ratio of escaping melt to the initial melt content defines the coefficient of melt extraction. It varies linearly with initial melt content, thresholds and deformation, which are tested separately. During each run, transient states of the melt-extraction coefficient were characterized by exponential spikes whose amplitude depends on the initial melt content, thresholds and deformation. Departures in melt extraction from average volume are damped when initial conditions are continuously changed, reflecting a memory effect. We suggest that melt extraction relates to a pinning-depinning process. Melt extraction being discontinuous and irregular with time, we suggest that detailed chemistry does not properly document the melt evolution in source rocks (migmatites).

Finite elements analysis of heteroepitaxial SiGe layers grown by excimer laser

In this work, the finite elements analysis using ANSYS ® (8.0) of the heteroepitaxial SiGe alloy formation induced by excimer lasers is presented. The numerical simulation of the temperature distribution induced by KrF excimer laser (energy densities 0.50 < Φ< 0.55 J/cm 2) on thin amorphous Ge films (10 nm thick) deposited on Si〈1 0 0〉 substrates is obtained. An acceptable agreement between the numerical simulations and the experimental results is found. The melting depth is also evaluated and the laser energy density threshold for the partial melting of the Si substrate is estimated. It allows us to determine the optimum conditions to achieve high quality epitaxy. For both the cases, the temperature profile versus time on the top of the Ge film and at the Ge/Si interface are obtained.

Enhanced nonlinear photoemission near the melting threshold of Pb(111)

A photoemission electron microscope activated by a 100 ps Nd : YAG laser was used to observe the spatial distribution of photoemitted electrons from Pb(111) by linear photoemission, using pulses with λ = 266 nm, and nonlinear photoemission with λ = 1064 nm. For excitation with λ = 1064 nm, the images show localized enhanced photoemission forming the previously observed hot spots, which are attributed to structural defects on the surface. Considerable enhancement of nonlinear photoemission was observed near the laser melting threshold of Pb producing intense bright regions. These bright regions increase rapidly in size and intensity as the melting threshold is approached and are interpreted as areas of local surface melting.

Channels of energy redistribution in short-pulse laser interactions with metal targets

The kinetics and channels of laser energy redistribution in a target irradiated by a short, 1 ps, laser pulse is investigated in computer simulations performed with a model that combines molecular dynamics (MD) simulations with a continuum description of the laser excitation and relaxation of the conduction band electrons, based on the two-temperature model (TTM). The energy transferred from the excited electrons to the lattice splits into several parts, namely the energy of the thermal motion of the atoms, the energy of collective atomic motions associated with the relaxation of laser-induced stresses, the energy carried away from the surface region of the target by a stress wave, the energy of quasi-static anisotropic stresses, and, at laser fluences above the melting threshold, the energy transferred to the latent heat of melting and then released upon recrystallization. The presence of the non-thermal channels of energy redistribution (stress wave and quasi-static stresses), not accounted for in the conventional TTM model, can have important implications for interpretation of experimental results on the kinetics of thermal and mechanical relaxation of a target irradiated by a short laser pulse as well as on the characteristics of laser-induced phase transformations. The fraction of the non-thermal energy in the total laser energy partitioning increases with increasing laser fluence.

Preliminary study into the effects of YAG laser processing of titanium 6Al–4V alloy for potential aerospace component cleaning application

This paper outlines some essential gas turbine aeroengine component manufacture cleaning requirements. It describes the preliminary results of work conducted into the effects of laser processing Ti–6A–4V alloy for cleaning of aerospace components, including determination of the melt and surface coupling thresholds for the Nd:YAG ( λ = 1.064 μm) system in use. The results of laser processing as a preparation for electron beam welding and diffusion bonding are presented. Finally, the direction of future work towards the development of an effective process for laser cleaning titanium aero-engine components is discussed. The results demonstrated the sensitivity of the system in use to gaseous contamination, producing acceptable electron beam welded joints, but unacceptable diffusion bonded joints.

Laser-Induced Growth of Titanium Nitride Microcolumns on Biased Titanium Targets

Titanium targets with a bias voltage ranging from −500 to +500 V were submitted to multipulse high repetition rate Nd:yttrium aluminum garnet (YAG; λ = 1.064 μm, τ ∼ 300 ns, ν = 30 kHz) laser irradiations in nitrogen at intensity values below the single-pulse melting threshold. The morphology of the TiN structures formed under the cumulative action of the laser pulses on the surface of the unbiased and biased targets was investigated by profilometry and scanning electron microscopy. Under these irradiation conditions, a specific columnar surface microrelief developed. The height of the microcolumns reached about 10–15 μm, and their diameter about 1–2 μm. The development of TiN microcolumns was enhanced by the applied bias voltage. The enhancement in the negative biased samples was stronger than that in the positive biased ones.

Simulation of tokamak armour erosion and plasma contamination at intense transient heat fluxes in ITER

For ITER, the potential material damage of plasma facing tungsten-, CFC-, or beryllium components during transient processes such as ELMs or mitigated disruptions are simulated numerically using the MHD code FOREV-2D and the melt motion code MEMOS-1.5D for a heat deposition in the range of 0.5–3 MJ/m2 on the time scale of 0.1–1 ms. Such loads can cause significant evaporation at the target surface and a contamination of the SOL by the ions of evaporated material. Results are presented on carbon plasma dynamics in toroidal geometry and on radiation fluxes from the SOL carbon ions obtained with FOREV-2D. The validation of MEMOS-1.5D against the plasma gun tokamak simulators MK-200UG and QSPA-Kh50, based on the tungsten melting threshold, is described. Simulations with MEMOS-1.5D for a beryllium first wall that provide important details about the melt motion dynamics and typical features of the damage are reported.

Metallic thin films heated by pulsed lasers. Numerical simulation of the thermal field and the melting kinetics

This modeling is especially applied to the pulsed laser induced heating and melting of a metallic film deposited on a substrate. Study of the thermal field over a surface is usually performed by considering the assumption of ‘semi-infinite medium’. However, a thin film deposited on a rough substrate surface induces bad thermal contacts commonly known as ‘thermal contact resistance’. This interfacial thermal resistance affects the melting kinetics mainly when the film thickness (Z) is small comparatively to the heat diffusion length (ZT). In this work the heat conduction equation and related boundary conditions are resolved by using the implicit finite differences method. The heat source (i.e. the laser intensity) is treated as a surface boundary layer. The thermal contact resistance is introduced in the computation procedure when the heat wave propagation reaches the thin film/substrate interface. It is then possible to calculate the critical temperatures and the melting threshold fluence for high and low contact resistance values. Under these conditions, the temperature profile and melting depth are plotted considering different thickness.. Finally, for 750 mJ/cm² excimer laser fluence and 0.1 cm²/s thin film apparent diffusivity results show that for Z/ZT higher than 0.5, there is no sensitive effect of the thermal contact resistance on the melting kinetics.