The results of a molecular dynamics study of phase change in a system of condensed Lennard-Jones atoms is presented. These molecular dynamics results are compared to the rates of phase change predicted by two analytical phase change models. Both analytical models are supported by the molecular dynamics results.

A new non-equilibrium Molecular Dynamics (NEMD) computer simulation method has been developed to study ultra-rapid melting and resolidification processes, e.g. laser annealing, ion implantation, etc. An atomic-level description of the material is combined with a new simulation technique to produce thermodynamic, structural and kinetic information as a function of time. Experimentally realistic values of the energy fluence, pulse duration and substrate temperature are used as input to the simulation. Rapid heat transfer simulating the action of the energy input is then set up allowing a complete prediction of the undercooling and associated kinetic properties. As such this new method offers the most realistic simulation model for rapid thermal processing to date.

The structure of ion-implanted a-Si has been studied using Raman spectroscopy. It is shown that the average bond-angle distortion and the strain energy stored in the random network vary drastically upon annealing. Relaxation is enhanced in the case of heating by laser irradiation. The results imply that the apparent melting temperature of a-Si may vary from one experiment to another, depending on the heating rate and -method.

Heat transfer processes and phase transitions at the Si-water interface during nanosecond pulsed laser irradiation were studied in real time using transient optical reflectance measurements. Measurements at two probe laser polarizations suggest that during irradiation a steam phase is formed near the Si surface. Relaxation of this phase occurs several hundred nanoseconds after solidification of the Si is completed. Acousto-optical measurements indicate that a shock wave, propagating with the sound velocity in water, is initiated near the Si-water interface.

Time resolved reflectivity (TRR) measurements were carried out during pulsed laser irradiation of silicon immersed in water. It was found that the TRR in water was similar to that in air though the signal deteriorated after about 100 ns from the starting point of the laser for incident energy densities above 1.4 J/cm2 (unlike what is observed in air). The total melt duration in water was about 2.8 to 1.6 times less than that in air at the same absorbed energy density. It was estimated that 20 % of the absorbed energy was taken away by the water layer. For the same energy coupled into the solid the melt-in/regrowth kinetics was speeded up by the presence of the water layer at the surface by about a factor 2 consistant with the results of Polman et. a15.

We have investigated the formation of thin layers of carbides and nitrides by irradiating silicon (100) single crystal immersed in clear organic solvents or liquid ammonia. The liquids were transparent to the excimer laser (λ=308 nm, τ45 ns) used, at energy densities from 0.5 to 3.0 Jcm−2. Most of the laser energy was absorbed by the silicon specimens above a certain threshold to cause melting of the surface to a depth of approximately 250 nm. The pool of liquid silicon reacts with the solvents or ammonia in a saturated high pressure vapor phase for a duration of approximately 200 ns per pulse. The specimens were irradiated with a number of pulses ranging from 1 to 50. The films were then analyzed for structure and composition using TEM, AES, and IR spectroscopy. We report here calculations of laser-solid interactions, microstructure, and properties of the resulting thin films.

The use of thermodynamic analysis of phase relationships to identify metastable conditions for generating undercooled liquids by pulsed laser melting, and to interpret the resulting microstructures is illustrated from studies of Mn, FeV and TiAl. Phases with simple, disordered structures are found to nucleate and grow within tens of nanoseconds in liquids undercooled by 50–100 K. An extended transformation depth is found in which the stable high-temperature a phase replaces the metastable a phase in FeV due to the heat release from the rapidly forming a phase. The formation of the disordered bcc structure and of a metallic glass in the melt of the ordered compound γ-TiAl indicate that regrowth of this phase is sufficiently slow to generate undercoolings of – 1000 K.

Four Cu-Zr alloys, Cu56Zr44, Cu50Zr50, Cu47Zr53, and Cu33Zr67, were surface melted with electron and pulsed laser beams to compare their kinetics of nucleation, growth and glass formation. It was observed that the ease of glass formation increased in the order: Cu33Zr67, Cu47Zr53, Cu56Zr44, and Cu50Zr50. The nucleation and regrowth produced different metastable phases. At the equiatomic composition, the preferred phase is a CsCl-type (B2) BCC structure. As the composition deviates from this, the preferred phase is either orthorhombic or tetragonal with a much larger unit cell not previously reported in the literature. The maximum growth velocity of these metastable phases was found to be about 0.025 m/s. The slow kinetics are responsible for the ease of glass formation in these systems.

A line-source electron-beam system has been used to heat thin surface layers of metastable phases at a rate which precludes solid-state transformations to stable phases, thus permitting the observation of melting transitions normally missed with slow heating. A detailed example of a new approach to this method is shown for metastable icosahedral Al-Re and. crystalline Al6Re.

A requirement of any theory for the kinetics of interface motion during a phase transformation is that the force law which relates fluxes to thermodynamic forces satisfies Onsager's reciprocity relations. In this paper we show how to apply Onsager's relations to a phase transformation mediated by a moving interface in a two-component system. We use the result to evaluate two proposed models for interface motion during alloy solidification.

Transient conductance measurements reveal the metastability of amorphous Gallium (a-Ga): Gallium can be amorphized for a certain “lifetime” at temperatures as high as 60 K, i.e. well above the crystallization temperature of 16 K.

For the first time the lower part of the C-shaped transition curve in a TTT-diagram can be measured for a pure metal. The results are in good agreement with undercooling behavior of liquid Ga-droplets.

NiSi and Ni2Si layers on silicon substrates as well as high fluence Si(As) ion implanted layers,have been rapidly melted by 30 ns Nd laser pulse irradiation.The energy density ranged between 0.4 and 1.2 J/cm2. Bilayer structures have been observed when the energy density has been chosen properly.

Buried epitaxial layers together with an amorphous or a policrystalline layer on top,have been detected by RBS and TEM measurements.

We have used excimer laser radiation to mix single layers of Ti directly on annealed substrates of 304 stainless steel. This process produces much thicker surface alloy layers and is much faster than ion mixing techniques. Liquid state diffusion results in mixed layers of the order of 1.0 μm thick from 0.5 μm Ti overlayers. In single pulse experiments, fluences of up to about 4 times melt threshold could be used before ablation occurred. The fluence in these experiments determines the melt duration and therefore the amount of mixing. In order to achieve greater mixing without ablation of the surface, multiple pulses at lower fluence were used. Profiles of laser mixed surface layers and microstructural characterization of the layers are presented.

The minimum energy in a 248 nm, 25 ns long excimer laser pulse required to remove thin Au and Cr films from optical quartz has been measured. Heating of the films by the laser has been modelled with a finite element calculation. Assuming that at threshold, all of the laser energy contributes to film removal, the calculations show that the gold films are removed when the heated gold surface reaches the atmospheric boiling point, and that temperatures well in excess of the atmospheric boiling point are required to remove the Cr films, with the required temperatures increasing with film thickness.

High resolution electron microscope (HREM) studies provide the ability to study desorption and sputtering from the perspective of the analysis of the resultant materials, their structure, composition and atomic registry (orientation with respect to the original,material and the irradiation). This is a neglected facet of surface irradiation effects research, yet it is the most important from the technological point of view. In the current study, surface electron irradiation processes in oxides were studied in-situ in a Hitachi H-9000 HREM operated at incident electron energies of 100–300 keV. It was found that a wide range of processes occur in the HREM which are dependent on the energy and flux of the incident electrons and on the material properties. Both ballistic and electronic irradiation damage was observed and the material responses included surface sputtering, amorphisation, chemical disordering, desorption of O and metal surface layer creation, surface roughening and bulk defect creation.

Polycrystalline CuO samples are irradiated in UHV by excimer laser pulses at 308nm. Mass spectrometry combined with time-of-flight measurements is used to determine the composition of the desorbing particles and the desorption mechanisms. Additional information is provided by Scanning-Electron-Microscopy of the etched surfaces. At fluences below 1.8 J cm−2 the products Cu, O and O2 are desorbed by a purely thermal mechanism at temperatures up to 6000 K and a copper rich surface mask is formed. At higher power densities an additional ablative contribution is monitored which increases with power density, and much less surface area is covered by a Cu-rich mask.

Time-of-flight (TOF) experiments were performed to study the desorption of trans-l,2-dichlorocyclohexane molecules from molecular films condensed on a quartz-crystal microbalance at 77 K. The wavelength dependence of the desorption signal was investigated by resonant excitation of the internal vibrational mode of the molecule in the 10 μm region with a pulsed line-tunable TEA CO2 laser. The TOF temperatures showed the same spectral dependence as the optical absorption coefficient. The TOF distributions were narrower than Maxwellian distributions for multilayer desorption yields and approached Maxwellian distributions for submonolayer desorption yields. A saturation behavior was found at high laser intensities, indicating that the kinetic energy of the desorbing molecules is limited to a maximum value.

Pulsed laser mixing has been used as surface modification technique for the improvement in the mechanical properties of ceramics. Thin metallic layers of nickel were deposited on structural silicon nitride and were irradiated with Xenon Chloride (XeCl) laser pulses. The laser parameters were optimized to lead to the formation of mixed layers. The mixed interfacial layers were analyzed using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Rutherford Backscattering (RBS) techniques. Detailed heat flow calculations were performed to simulate the effects of intense laser irradiation on metal coated ceramic structures. The melt lifetimes and the interfacial temperatures obtained using these calculations, were applied to understand the laser mixing phenomena occuring in these layered structures. Thermodynamics of chemical reactions between the metal overlayers and the substrate were done to predict the formation of mixed interfacial layers during laser irradiation.

Single shot pulses from a Nd:YAG laser were used to generate different sized holes and patterns in tape-cast ceramic sheets for electronic packaging applications. The ceramic sheets were based on formulations in the BaTiO3 and MgO-Al2O3-SiO2 systems. The single shot laser pulses were targeted using a computer controlled mirror deflection system and a programmed (cavitation) pattern which could readily be modified. Morphology and structure of the drilled holes before and after sintering were examined by SEM. The surface radial temperature profile resulting from the laser pulse was directly mapped using a lacquer thermocoat. Effects of the laser pulse energy and beam spot size on laser/ceramic interactions and cavitation process were also explored.

In earlier work we reported the consequences of simultaneous exposure of polymers to mechanical stress and electron bombardment. In this study we examine the response of highly stressed polyimide films to excimer laser radiation (20 ns pulses @ 248 nm wavelength) in air. The exposed surfaces show evidence of surface and near surface damage, crack initiation, and eventually crack growth over a wide range of applied stress and laser fluence. These results show that the morphology of the stressed material has a significant influence on the resulting damage and suggest that the regions of highest damage are those experiencing the highest local stress. A growth of nodules on the polyimide surface are also observed which are missing when exposed in vacuum.