Melting Threshold Research Articles

Modeling and experiments on diffusion and activation of phosphorus in germanium

We report on phosphorus diffusion and activation related phenomena in germanium. We have used both conventional thermal processing and laser annealing by pulsed nanosecond Nd:YAG laser. Chemical profiles were obtained by secondary-ion-mass spectroscopy, sheet resistance was estimated by the van der Pauw method, and structural defects were monitored by transmission electron microscopy. Our study covers the temperature range from 440 to 750 °C, and we were able to efficiently simulate the dopant profiles within that temperature range, taking into account a quadratic dependence of the P diffusion coefficient on the free electron concentration. To achieve that we have taken into account dopant activation dependence on temperature as well as dopant pile-up near the surface and dopant loss owing to outdiffusion during the annealing. A combined laser thermal treatment above the melting threshold prior to conventional annealing allowed the elimination of the implantation damage, so we could perceive the influence of defects on both transient dopant diffusion and outdiffusion.

Surface micro/nanostructuring of titanium under stationary and non-stationary femtosecond laser irradiation

In this paper the surface topography of titanium samples irradiated by femtosecond laser pulses is described. When the fluence is about 0.5 J/cm 2 periodic ripples with a period of about 700 nm are formed. For fluences between 0.5 and 2 J/cm 2, a microcolumnar surface texture develops in the center of the irradiated spots and ripples are formed in the periphery of the spots. When experiments are performed with a non-stationary sample, the microcolumns exhibit ripples similar to those observed when the radiation fluence is about 0.5 J/cm 2 and in the outer regions of the irradiated areas for fluences between 0.5 and 2 J/cm 2. Since the energy distribution in the transverse cross-section of the laser beam is Gaussian, we conclude that the ripples form when the microcolumns are subjected to fluences near the melting threshold of the material at the trailing edge of the moving laser beam.

Ultrafast Laser-Induced Elastodynamics in Single Crystalline Silicon Part II: Near-Field Response

The multi-time scale ultrafast laser model of axisymmetric geometry presented in Part I is validated with computed carrier densities and melting thresholds that agree well with published physical data. Transport phenomena initiated by femtoseconds heating including the spatial and temporal evolutions of electron and lattice temperatures and electron-hole carrier density are highly localized in both time and space. The temporal and spatial scales associated with the generation of thermal stress waves are significantly larger at tenths of nanoseconds and microns, respectively. Ultrashort laser pulse induced transverse stress waves are highly dispersive and characteristically of broadband, low amplitude, and extremely high frequency and power density contents. Near-field responses that precede the development of a full-blown plate wave are also localized in space with a power density magnitude on the order of 1013 Watts per cubic meters in volume.

A Comparative Study of Two-Temperature and Boltzmann Transport Models for Electron-Phonon Nonequilibrium

Ultrashort-pulsed laser irradiation on metals creates a thermal nonequilibrium between electrons and the phonons. Previous computational studies used the two-temperature model and its variants to model this nonequilibrium. However, when the laser pulse duration is smaller than the relaxation time of the energy carriers or when the carriers mean free path is larger than the material dimension, these macroscopic models fail to capture the physics accurately. In this article, the nonequilibrium between energy carriers caused by a laser pulse interaction with a metal film is modeled via a numerical solution of the Boltzmann transport model (BTM) for electrons and phonons. A comparative assessment of the two-temperature model and its variants is carried out relative to the BTM. The higher order Runge-Kutta discontinuous Galerkin (RKDG) method is used for numerical discretization of the models. In this study, the gold film thickness is varied between 2–2000 nm, and the laser pulse duration and fluence are varied between 5 fs to 10 ps and 10–2000 J/m2, respectively. It is found that BTM shows the best agreement with the experimental data compared to the two-temperature models for the electron and phonon temperature profiles and the melting threshold fluence.

Formation of absorbing heterogeneous plasma layer by femtosecond laser-induced melting and ablation of silicon

The experimental study of absorption in silicon in infrared and visible spectral ranges, where the photon energy is less or more than the bandgap width, is performed by means of the ultrafast interferometry technique. The exactly solvable model in the electromagnetic of heterogeneous lossy plasma layer was developed. The density of carriers, their frequency of collisions, absorbing depth of the probing waves, real and imaginary parts of dielectric function of nonuniform layer and their spatial gradients are determined from the reflectance data by means of this model subject to the pump fluence. The heterogeneity-induced effects are visualized due to comparison of obtained plasma parameters with those calculated in the framework of homogeneous plasma model It is shown that in the intensity range near thresholds of melting and ablation the absorption, occurring in both cases mainly within a thin (∼10 nm) absorbing layer (similarly to metals), is due to free carrier intraband absorption.

Damage to preheated tungsten targets after multiple plasma impacts simulating ITER ELMs

The behavior of a preheated at 650 °C tungsten targets under repetitive ELM-like plasma pulses is studied in simulation experiments with the quasi-stationary plasma accelerator QSPA Kh-50. The targets have been exposed up to 350 pulses of the duration 0.25 ms and the surface heat loads either 0.45 MJ/m 2 or 0.75 MJ/m 2, which is below and above the melting threshold, respectively. The development of surface morphology of the exposed targets as well as cracking and swelling at the surface is discussed. First comparisons of obtained experimental results with corresponding numerical simulations of the code PEGASUS-3D are presented.

Characterization of third-harmonic generation from femtosecond laser-excited silicon

Third-harmonic generation (THG) from silicon crystals excited by femtosecond pulses has been studied. THG as a function of probe pulse delay from linear and circularly polarized probe pulses was examined. Both polarizations exhibit a fast, 300 fs drop in THG when the Si is pumped by pulses with fluence above the melting threshold. This drop for both polarizations indicates that electronic effects dominate THG in laser-melted Si since structural effects should principally influence THG from linearly polarized probes, suggesting that care needs to be exercised in interpreting THG signal as an ultrafast probe of crystalline structure in semiconductors.

Surface energy balance and melt thresholds over 11 years at Taylor Glacier, Antarctica

In the McMurdo Dry Valleys, Victoria Land, Antarctica, melting of glacial ice is the primary source of water to streams, lakes, and associated ecosystems. To understand geochemical fluxes and ecological responses to past and future climates requires a physically based energy balance model. We applied a one‐dimensional model to one site on Taylor Glacier using 11 years of daily meteorological data and seasonal ablation measurements. Inclusion of transmission of solar radiation into the ice was necessary to accurately model summer ablation and ice temperatures. Results showed good correspondence between calculated and measured ablation and ice temperatures over the 11 years. Ablation (∼18 cm a−1) was dominated by sublimation with very few occurrences of melt (42 days during 11 years). Results also indicated that above freezing air temperatures did not necessarily result in melt and, in turn, melt occurred during subfreezing air temperatures under some conditions. For air temperatures near freezing, low wind speed was critically important for melt initiation. According to the model, subsurface melt, away from rocks and sediment in the ice, occurred three times more frequently than surface melt; occurs no deeper than 50 cm below the glacier surface; and was small, never exceeding 8% by mass. The magnitude of subsurface melting and the energy balance indicate that Taylor Glacier ice is intermediate in optical properties between snow and blue ice.

Multiple pulse thermal damage thresholds of materials for x-ray free electron laser optics investigated with an ultraviolet laser

Optical elements to be used for x-ray free electron lasers (XFELs) must withstand multiple high-fluence pulses. We have used an ultraviolet laser to study the damage of two candidate materials, crystalline Si and B4C-coated Si, emulating the temperature profile expected to occur in optics exposed to XFEL pulses. We found that the damage threshold for 105 pulses is ∼20% to 70% lower than the melting threshold.

Numerical and experimental analysis on green laser crystallization of amorphous silicon thin films

The effect of laser fluence on the crystallization of amorphous silicon irradiated by a frequency-doubled Nd:YAG laser is studied both theoretically and experimentally. An effective numerical model is set up to predict the melting threshold and the optimized laser fluence for the crystallization of 200-nm-thick amorphous silicon. The variation of the temperature distribution with time and the melt depth is analyzed. Besides the model, the Raman spectra of thin films treated with different fluences are measured to confirm the phase transition and to determine the optimized fluence. The calculating results accord well with those obtained from the experimental data in this research.

Diffusion and activation of phosphorus in germanium

In this work we investigate the diffusion and the activation behavior of implanted phosphorus in Ge. We used both conventional thermal processing as well as laser annealing by pulsed ns Nd–YAG laser. Chemical profiles were obtained by secondary-ion-mass spectrometry (SIMS) and sheet resistance was estimated by Van der Pauw method. These measurements demonstrated a box-shaped dopant profile for both conventional and laser annealed samples which are in agreement with other research reports indicating enhanced dopant diffusivity. From these experiments and critical comparison with other studies we conclude about the value of the intrinsic diffusion coefficient and we discuss the validity of the doubly charged vacancy model in simulating our experiments. To more accurately account for these parameters we have also implemented a pileup and a segregation model to simulate the dopant loss due to outdiffusion of phosphorus during the annealing process. In order to understand the influence of defects on transient dopant diffusion as well as on outdiffusion we have also annealed P implanted Ge prior to conventional annealing with laser above melting threshold to eliminate ion implantation defects as these are monitored by transmission electron microscopy.

Pulsed laser annealing of semiconductor structures for functional devices

AbstractWe review our progress on the laser processing of semiconductor microstructures and nanostructures for functional devices. Fundamental research conducted to understand the melt‐mediated phase transformations induced by nanosecond laser irradiation in thin semiconductor films is presented. A detailed experimental study analyzed the physical mechanisms of the explosive crystallization in amorphous germanium that produces large area self‐sustained crystal growth. The double laser crystallization method that combines a nanosecond laser pulse and a modulated microsecond laser beam was shown to produce ultra‐large grain polycrystalline silicon, enabling fabrication of thin film transistor devices of high performance from amorphous silicon films. The crystal growth process was imaged by temporally resolved photography. Non‐melt Excimer Laser annealing of thin silicon‐on‐insulator films for dopant activation was demonstrated. Applying multiple laser pulses below the melting threshold effected solid phase annealing of the single crystalline silicon films. Semiconductor nanowires are one‐dimensional nanostructures that have displayed the potential to be used with low‐cost flexible plastic substrates for applications such as large‐area displays and sensor arrays. The excimer laser annealing of silicon nanowires is demonstrated as an alternative to conventional thermal annealing for dopant activation. The optical absorption of the nanowires is discussed and the effect of parameters such as fluence and number of pulses is investigated. The interaction of laser pulses with silicon nanowires is investigated through numerical simulations. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Behaviour of melted tungsten plasma facing components under ITER-like transient heat loads: Simulations and experiments

Tungsten in the form of macrobrush structure is foreseen as one of the two candidate materials for the ITER divertor and the dome. Melting and thus melt motion and melt splashing are expected to be main mechanisms of metallic target damage that determine the lifetime of ITER plasma facing components. Experiments carried out at the plasma gun facility QSPA-T for ELM-like heat loads demonstrated a significant erosion of frontal and lateral brush edges, which was confirmed by further numerical simulations. In the experiments and numerical simulations a threshold of brush edge melting was determined. In this paper most important mechanisms of melt splashing and melt bridge formation under ITER transient heat loads are analyzed. Approximate criteria for droplet ejection are obtained and the range of transient events without significant droplet injection is calculated. The critical radius of brush edges rounding which prevents the bridge formation at the macrobrush edges is determined.

Ablation by ultrashort laser pulses: Atomistic and thermodynamic analysis of the processes at the ablation threshold

Ultrafast laser irradiation of solids may ablate material off the surface. We study this process for thin films using molecular-dynamics simulation and thermodynamic analysis. Both metals and Lennard-Jones (LJ) materials are studied. We find that despite the large difference in thermodynamical properties between these two classes of materials---e.g., for aluminum versus LJ the ratio ${T}_{c}/{T}_{\text{tr}}$ of critical to triple-point temperature differs by more than a factor of 4---the values of the ablation threshold energy ${E}_{\text{abl}}$ normalized to the cohesion energy, ${ϵ}_{\text{abl}}={E}_{\text{abl}}/{E}_{\text{coh}}$, are surprisingly universal: all are near 0.3 with $\ifmmode\pm\else\textpm\fi{}30%$ scattering. The difference in the ratio ${T}_{c}/{T}_{\text{tr}}$ means that for metals the melting threshold ${ϵ}_{m}$ is low, ${ϵ}_{m}l{ϵ}_{\text{abl}}$, while for LJ it is high, ${ϵ}_{m}g{ϵ}_{\text{abl}}$. This thermodynamical consideration gives a simple explanation for the difference between metals and LJ. It explains why despite the universality in ${ϵ}_{\text{abl}}$, metals thermomechanically ablate always from the liquid state. This is opposite to LJ materials, which (near threshold) ablate from the solid state. Furthermore, we find that immediately below the ablation threshold, the formation of large voids (cavitation) in the irradiated material leads to a strong temporary expansion on a very slow time scale. This feature is easily distinguished from the acoustic oscillations governing the material response at smaller intensities, on the one hand, and the ablation occurring at larger intensities, on the other hand. This finding allows us to explain the puzzle of huge surface excursions found in experiments at near-threshold laser irradiation.

Generation of hardened steel surfaces with adjustable roughness by means of a pulsed electron beam

In this work we are utilizing a Pulsed Cold Cathode Electron Beam (PCCEB) for steel surface treatment that presents distinct properties as compared to other beam sources for rapid heating of metal surfaces. The possibility of producing different surface transformations like, melting and solid–solid transformations by adjusting the gun control parameters is demonstrated. It is shown that for the same base material, different pulse conditions give rise to distinct structures, a roughened or smooth martensitic surface or a smooth pearlite layer several microns thick. For the liquid–solid transformation, it is shown for the first time that with an adequate impurity content the instability onset threshold (giving rise to a rough topography) is different from the melting threshold. Hence, by controlling the discharge parameters, it is possible to obtain different topographies with similar crystallographic structure and hardness, allowing an adjustment of the final roughness to the particular application requirements.

Computational study of the generation of crystal defects in a bcc metal target irradiated by short laser pulses

The generation of crystal defects in a Cr target irradiated by a short, 200 fs, laser pulse is investigated in computer simulations performed with a computational model that combines the classical molecular dynamics method with a continuum description of the laser excitation of conduction band electrons, electron-phonon coupling, and electron heat conduction. Interatomic interactions are described by the embedded atom method (EAM) potential with a parametrization designed for Cr. The potential is tested by comparing the properties of the EAM Cr material with experimental data and predictions of density functional theory calculations. The simulations are performed at laser fluences close to the threshold for surface melting. Fast temperature variation and strong thermoelastic stresses produced by the laser pulse are causing surface melting and epitaxial resolidification, transient appearance of a high density of stacking faults along the {110} planes, and generation of a large number of point defects (vacancies and self-interstitials). The stacking faults appear as a result of internal shifts in the crystal undergoing a rapid uniaxial expansion in the direction normal to the irradiated surface. The stacking faults are unstable and disappear shortly after the laser-induced tensile stress wave leaves the surface region of the target. Thermally activated generation of vacancy-interstitial pairs during the initial temperature spike and quick escape of highly mobile self-interstitials to the melting front or the free surface of the target, along with the formation of vacancies at the solid-liquid interface during the fast resolidification process, result in a high density of vacancies, on the order of ${10}^{\ensuremath{-}3}$ per lattice site, created in the surface region of the target. The strong supersaturation of vacancies can be related to the incubation effect in multipulse laser ablation/damage and should play an important role in mixing/alloying of multicomponent or composite targets.

Pulsed laser dewetting of patterned thin metal films: A means of directed assembly

Thin nickel films were patterned into various shapes and treated with a series of laser pulses. The edges and vertices of the patterned shapes act as programable instabilities, which enable directed assembly via dewetting when the laser energy density is above the melting threshold. The pattern formations were monitored as a function of laser pulse and the retraction process was attributed liquid dewetting and a subsequent resolidification. The calculated retraction velocity (83m∕s) and liquid lifetime (12.3ns) were consistent with the measured nickel retraction distances. The vertices of the shapes had an initially larger retraction velocity which was attributed to an additional in-plane curvature.

Laser-induced formation of conical bumps on the surface of superrefractory metals

The formation of conical bumps on W, Ta, and Mo surfaces as a result of irradiating them with a laser pulse in an atmosphere of air, nitrogen, or helium and in vacuum have been experimentally studied at an irradiation-energy density above the melting threshold but below the blowout threshold of the melt from the irradiation zone by vapor pressure. A strong dependence of the bump height on the composition of the gaseous atmosphere is detected. The minimum radius of curvature of the tip of 0.5μm and a tip angle of the conical bump of 60° were observed on molybdenum for a bump height of 19μm above the original surface level. It is shown that the regularities of formation of the conical bumps on the metals are associated with the final contact angle of wetting of the solid phase by its melt and with the dependence of the contact angle on the composition and chemical activity of the atmosphere. A model has been constructed of the formation of a conical bump during the crystallization of a limited bath of the melt.

Insight into electronic mechanisms of nanosecond-laser ablation of silicon

We present experimental and theoretical studies of nanosecond ArF excimer laser desorption and ablation of silicon with insight into material removal mechanisms. The experimental studies involve a comprehensive analysis of the laser-induced plume dynamics and measurements of the charge gained by the target during irradiation time. At low laser fluences, well below the melting threshold, high-energy ions with a narrow energy distribution are observed. When the fluence is increased, a thermal component of the plume is formed superimposing on the nonthermal ions, which are still abundant. The origin of these ions is discussed on the basis of two modeling approaches, thermal and electronic, and we analyze the dynamics of silicon target excitation, heating, melting, and ablation. An electronic model is developed that provides insight into the charge-carrier transport in the target. We demonstrate that, contrary to a commonly accepted opinion, a complete thermalization between the electron and lattice subsystems is not reached during the nanosecond-laser pulse action. Moreover, the charging effects can retard the melting process and have an effect on the overall target behavior and laser-induced plume dynamics.

Constitutive relationships and physical basis of fault strength due to flash heating

We develop a model of fault strength loss resulting from phase change at asperity contacts due to flash heating that considers a distribution of contact sizes and nonsteady state evolution of fault strength with displacement. Laboratory faulting experiments conducted at high sliding velocities, which show dramatic strength reduction below the threshold for bulk melting, are well fit by the model. The predicted slip speed for the onset of weakening is in the range of 0.05 to 2 m/s, qualitatively consistent with the limited published observations. For this model, earthquake stress drops and effective shear fracture energy should be linearly pressure‐dependent, whereas the onset speed may be pressure‐independent or weakly pressure‐dependent. On the basis of the theory, flash weakening is expected to produce large dynamic stress drops, small effective shear fracture energy, and undershoot. Estimates of the threshold slip speed, stress drop, and fracture energy are uncertain due to poor knowledge of the average contact dimension, shear zone thickness and gouge particle size at seismogenic depths.