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Laser-solid interaction and dynamics of the laser-ablated materials

Description: Rapid transformations through the liquid and vapor phases induced by laser-solid interactions are described by the authors` thermal model with the Clausius-Clapeyron equation to determine the vaporization temperature under different surface pressure condition. Hydrodynamic behavior of the vapor during and after ablation is described by gas dynamic equations. These two models are coupled. Modeling results show that lower background pressure results lower laser energy density threshold for vaporization. The ablation rate and the amount of materials removed are proportional to the laser energy density above its threshold. The authors also demonstrate a dynamic source effect that accelerates the unsteady expansion of laser-ablated material in the direction perpendicular to the solid. A dynamic partial ionization effect is studied as well. A self-similar theory shows that the maximum expansion velocity is proportional to c{sub s}{alpha}, where 1 {minus} {alpha} is the slope of the velocity profile. Numerical hydrodynamic modeling is in good agreement with the theory. With these effects, {alpha} is reduced. Therefore, the expansion front velocity is significantly higher than that from conventional models. The results are consistent with experiments. They further study how the plume propagates in high background gas condition. Under appropriate conditions, the plume is slowed down, separates with the background, is backward moving, and hits the solid surface. Then, it splits into two parts when it rebounds from the surface. The results from the modeling will be compared with experimental observations where possible.
Date: July 1, 1995
Creator: Chen, K. R.; Leboeuf, J. N.; Geohegan, D. B.; Wood, R. F.; Donato, J. M.; Liu, C. L. et al.
Partner: UNT Libraries Government Documents Department

Mechanisms affecting kinetic energies of laser-ablated materials

Description: Laser materials processing techniques are expected to have a dramatic impact on materials science and engineering in the near future and beyond. One of the main laser materials processing techniques is Pulsed Laser Deposition (PLD) for thin film growth. While experimentalists search for optimal approaches for thin film growth with pulsed laser deposition (PLD), a systematic effort in theory and modeling of various processes during PLD is needed. The quality of film deposited depends critically on the range and profile of the kinetic energy and density of the ablated plume. While it is to the advantage of pulsed laser deposition to have high kinetic energy, plumes that are too energetic causes film damage. A dynamic source effect was found to accelerate the plume expansion velocity much higher than that from a conventional free expansion model. A self-similar theory and a hydrodynamic model are developed to study this effect, which may help to explain experimentally observed high front expansion velocity. Background gas can also affect the kinetic energies. High background gas may cause the ablated materials to go backward. Experimentally observed plume splitting is also discussed.
Date: December 1995
Creator: Chen, K. R.; Leboeuf, J. N.; Wood, R. F.; Geohegan, D. B.; Donato, J. M.; Liu, C. L. et al.
Partner: UNT Libraries Government Documents Department

Laser-solid interaction and dynamics of laser-ablated materials

Description: An annealing model is extended to treat the vaporization process, and a hydrodynamic model describes the ablated material. We find that dynamic source and ionization effects accelerate the expansion front of the ablated plume with thermal vaporization temperature. The vaporization process and plume propagation in high background gas pressure are studied.
Date: September 1995
Creator: Chen, K. R.; Neboeuf, J. N.; Wood, R. F.; Geohegan, D. B.; Donato, J. M.; Liu, C. L. et al.
Partner: UNT Libraries Government Documents Department

Diffuse x-ray scattering from short-period W/C multilayers at in-plane momentum transfers 0.10-0.17 {angstrom}{sup -1}.

Description: X-ray scattering measurements at 10 keV from multilayers having a period of 24.8 {angstrom} and consisting of 100 W/C bilayers are reported. Specular scans revealed first-order reflectivities in the range 73.5% to 78.0% with bandpasses in the range of 1.5% to 1.7%. Total roughness (or interface grading) values deduced from fitting to the specular data only were in the range 2.5 to 3.0 {angstrom} for the last-to-grow surface of the W layers. Diffuse scattering measurements were made in a geometry that permitted investigation of in-plane momentum transfers up to 0.17 {angstrom}{sup {minus}1}. This is roughly an order of magnitude larger than is possible in conventional rocking scans. Reasonable fitting results were obtained for an in-plane correlation function that has a Fourier transform proportional to exp(-vq{sub y}{sup 2}{vert_bar}z{sub i}-z{sub j}{vert_bar}), where z{sub i}-z{sub j} is the average separation between the i{sup th} and j{sup th} interfaces and q{sub y} is the in-plane momentum transfer.
Date: April 20, 1999
Creator: Headrick, R. L.; Liu, C. L. & Macrander, A. T.
Partner: UNT Libraries Government Documents Department

Dynamical modeling of laser ablation processes

Description: Several physics and computational approaches have been developed to globally characterize phenomena important for film growth by pulsed laser deposition of materials. These include thermal models of laser-solid target interactions that initiate the vapor plume; plume ionization and heating through laser absorption beyond local thermodynamic equilibrium mechanisms; gas dynamic, hydrodynamic, and collisional descriptions of plume transport; and molecular dynamics models of the interaction of plume particles with the deposition substrate. The complexity of the phenomena involved in the laser ablation process is matched by the diversity of the modeling task, which combines materials science, atomic physics, and plasma physics.
Date: September 1, 1995
Creator: Leboeuf, J. N.; Chen, K. R.; Donato, J. M.; Geohegan, D. B.; Liu, C. L.; Puretzky, A. A. et al.
Partner: UNT Libraries Government Documents Department

Modeling of dynamical processes in laser ablation

Description: Various physics and computational approaches have been developed to globally characterize phenomena important for film growth by pulsed-laser deposition of materials. These include thermal models of laser-solid target interactions that initiate the vapor plume, plume ionization and heating through laser absorption beyond local thermodynamic equilibrium mechanisms, hydrodynamic and collisional descriptions of plume transport, and molecular dynamics models of the interaction of plume particles with the deposition substrate.
Date: December 31, 1995
Creator: Leboeuf, J.N.; Chen, K.R.; Donato, J.M.; Geohegan, D.B.; Liu, C.L.; Puretzky, A.A. et al.
Partner: UNT Libraries Government Documents Department

Modeling of plume dynamics in laser ablation processes for thin film deposition of materials

Description: The transport dynamics of laser-ablated neutral/plasma plumes are of significant interest for film growth by pulsed-laser deposition of materials since the magnitude and kinetic energy of the species arriving at the deposition substrate are key processing parameters. Dynamical calculations of plume propagation in vacuum and in background gas have been performed using particle-in-cell hydrodynamics, continuum gas dynamics, and scattering models. Results from these calculations are presented and compared with experimental observations.
Date: December 31, 1995
Creator: Leboeuf, J.N.; Chen, K.R.; Donato, J.M.; Geohegan, D.B.; Liu, C.L.; Puretzky, A.A. et al.
Partner: UNT Libraries Government Documents Department

Modeling and simulation of pulsed laser annealing and ablation of solid materials

Description: A previously developed one-dimensional (ID) computational model for heat flow and nonequilibrium phase change phenomena induced by pulsed-laser irradiation has been extended to two-dimensions. The 2D modeling focuses attention on the heat flow from localized sources embedded in an otherwise planar matrix. For example, nucleation events occurring in undercooled liquids such as molten Si formed by pulsed-laser melting of amorphous Si (a-Si) and inhomogeneous absorption due to randomly occurring defects in targets used for pulsed-laser ablation can be treated. Concepts introduced in the ID modeling, such as the state diagram and the state array are extended to 2D and refined. As an example of the calculations that are now possible, the laser-induced formation and propagation of buried liquid layers are followed in two dimensions for the case of a-Si on a crystalline silicon substrate. It is demonstrated how solid phase growth from individual nucleation sites gives rise to a nearly planar liquid layer propagating through the a-Si. Another example briefly addresses questions related to the early stages of the laser ablation of insulators such as MgO, where it is believed that the absorption of the laser radiation occurs at localized but extended regions of high concentrations of defects. The 2-D program has been rewritten for massively parallel machines such as the Intel Paragons in ORNL`s Center for Computational Sciences by one of us (CLL), thus allowing larger and more accurate calculations for complex systems to be carried out in reasonable times.
Date: April 1995
Creator: Liu, C. L. & Wood, R. F.
Partner: UNT Libraries Government Documents Department

Modeling of thermal, electronic, hydrodynamic, and dynamic deposition processes for pulsed-laser deposition of thin films

Description: Various physical processes during laser ablation of solids for pulsed-laser deposition (PLD) are studied using a variety of computational techniques. In the course of the authors combined theoretical and experimental effort, they have been trying to work on as many aspects of PLD processes as possible, but with special focus on the following areas: (a) the effects of collisional interactions between the particles in the plume and in the background on the evolving flow field and on thin film growth, (b) interactions between the energetic particles and the growing thin films and their effects on film quality, (c) rapid phase transformations through the liquid and vapor phases under possibly nonequilibrium thermodynamic conditions induced by laser-solid interactions, (d) breakdown of the vapor into a plasma in the early stages of ablation through both electronic and photoionization processes, (c) hydrodynamic behavior of the vapor/plasma during and after ablation. The computational techniques used include finite difference (FD) methods, particle-in-cell model, and atomistic simulations using molecular dynamics (MD) techniques.
Date: November 1, 1994
Creator: Liu, C.L.; LeBoeuf, J.N.; Wood, R.F.; Geohegan, D.B.; Donato, J.M.; Chen, K.R. et al.
Partner: UNT Libraries Government Documents Department

Vapor breakdown during ablation by nanosecond laser pulses

Description: Plasma generation through vapor breakdown during ablation of a Si target by nanosecond KrF laser pulses is modeled using 0-dimensional rate equations. Although there is some previous work on vapor breakdown by microsecond laser pulses, there have been no attempts made on vapor breakdown by nanosecond laser pulses. This work intends to fill the gap. A kinetic model is developed considering following factors: (1) two temperatures of both electrons and heavy-body particles (ions, neutrals, and excited states of neutrals), (2) absorption mechanisms of laser energy include inverse bremstrahlung (IB) processes and photoionization of excited states, (3) ionization acceleration mechanisms included are electron-impact excitation of ground state neutrals, electron-impact ionization of exited states of neutrals, photoionization of excited states of neutrals, and all necessary reverse processes. The rates of various processes considered are calculated according to the formula given by Zel`dovich and Raizer. The authors use a second order predictor-corrector numerical scheme for iterations of the rate equations. The rate equations are solved for five quantities, namely, densities of electrons, neutrals, and excited states of neutrals, and the temperatures of electrons and heavy-body particles. The total breakdown times (sum of evaporation time and vapor breakdown time) at different energy fluences are then calculated. The results are compared with experimental observations of Si target ablation using a KrF laser. A more detailed description of the model and the results will be published later.
Date: April 1, 1995
Creator: Liu, C.L.; Leboeuf, J.N.; Wood, R.F.; Geohegan, D.B.; Donato, J.M.; Chen, K.R. et al.
Partner: UNT Libraries Government Documents Department

Sequence Compaction to Preserve Transition Frequencies

Description: Simulation-based power estimation is commonly used for its high accuracy despite excessive computation times. Techniques have been proposed to speed it up by compacting an input sequence while preserving its power-consumption characteristics. We propose a novel method to compact a sequence that preserves transition frequencies. We prove the problem is NP-Complete, and propose a graph model to reduce it to that of finding a heaviest weighted trail on a directed graph, along with a heuristic utilizing this model. We also propose using multiple sequences for better accuracy with even shorter sequences. Experiments showed that power dissipation can be estimated with an error of only 2.3 percent, while simulation times are reduced by 10. Proposed methods effectively preserve transition frequencies and generated solutions that are very close to an optimal. Experiments also showed that multiple sequences granted more accurate results with even shorter sequences.
Date: December 12, 2002
Creator: Pinar, Ali & Liu, C.L.
Partner: UNT Libraries Government Documents Department