Modeling of Thermal, Electronic, Hydrodynamic, and Dynamic Deposition Processes for Pulsed-Laser Deposition of Thin Film
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epitaxial and low temperature growth of homogeneous and heterogeneous films by utilizing
energetic species, stoichiometric ablation of constituent species of the target, and growth of metastable phases layer-by-layer under nonequilibrium ablation conditions. However, there are still several issues, such as inclusion of particulates in the growing films and spatial nonuniformities of the films over large areas, that must be resolved before PLD can be widely used for industrial applications. While experimentalists are trying to find optimal conditions for thin film growth by PLD, a systematic effort in modeling of various physical processes during deposition is needed. In this paper, we present preliminary results from theoretical modeling of laser-ablation phenomena using a variety of computational techniques. For computational modeling of the complicated processes such as occur during PLD, one faces the challenge not only to better understand the fundamentals of the processes, but also to utilize the most appropriate computational techniques in the modeling. II. Pulsed laser melting and evaporation A variety of phenomena and issues, such as melting, solidification, overheating and undercooling, nucleation, coexisting multiple phases, rapidly moving multiple phase boundaries, interfacial kinetics, temperature/phase-dependent optical and thermal properties of the materials, etc., have to be dealt with in order to properly model laser-solid interactions. In the LASER8 programs of 1-D [1,2] and 2-D [3,4] models, finite difference (FD) methods were used to solve heat flow equations in terms of enthalpy and temperature. A concept of state arrays was developed to monitor phase status and other parameters in each finite 675 Mat. Res. Soc. Symp. Proc. Vol. 354 01995 Materials Research Society
difference cell. These models allow nonequilibrium melting and solidification to occur at temperatures other than the thermodynamic phase transformation temperatures and thus have tremendous flexibility in treating the many complicated phenomena that occur during pulsed laser annealing and ablation. A previously developed 2-D LASER8 model has been applied recently to study several problems during annealing and ablation of solid materials by pulsed laser beams [3,4], including: (1) the dynamics of "explosively" propagating buried liquid layers during pulsed laser annealing of a-Si layer on c-Si substrate, (2) the dynamics of Si flake nucleation and growth and the final structures obtained with different laser energy densities, (3) ablation due to absorption by localized but extended defects such as in the case of MgO, (4) particulate ejection during laser ablation and matrix assisted laser desorption ionization (MALDI). Fig. 1 shows one example from our modeling and simulations of explosive crystallization. Quantitative and qualitative agreement achieved between experiment and theory demonstrates that the approach employed in the simulations can be generally applied to a variety of problems and for many other materials. The 2-D progr
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