This award support research and education in optical interactions of lasers and semiconductor materials. This award is jointly supported by Chemical, Bioengineering, Environmental, and Transport Systems in the Division of Engineering and the Division of Materials Research. Research investigates how materials interact with lasers under conditions relevant to laser processing of covalent semiconductors. The work develops methods for predicting how processing conditions affect the resulting material structure. The work includes the development and use of novel computer simulation methods to elucidate the fundamental physical processes relevant to laser processing of covalent semiconductors. The general approach applies to intense femto-second pulses interacting with silicon.
The researchers address the fundamental technical challenges relevant to the development of a multiscale model of heat and mass transport appropriate for far-from-equilibrium conditions. Electronic heat transport is treated at the continuum level, while the lattice dynamics are treated using classical molecular-dynamics. A crucial component of the proposed work is that the interatomic interactions will depend on the local electronic temperature TE. Parameters in the interactions will be based on a new modification of the popular Tersoff potential, with the dependence of the parameters on TE established by fitting to a large database of energies from finite-temperature ab initio calculations. This novel approach will capture nonthermal effects known to be important for sub-picosecond melting. Heat transport by excited charge carriers is addressed using ab initio simulations of excited liquids and the Kubo-Greenwood method. The coupling between the continuum description of the electrons and the lattice will be driven using Langevin dynamics with the damping parameter fit to experiment. The focus of the proposed work develops a model for silicon as a test case. The model will be tested in its treatment of the fundamental physics of laser ablation of crystalline silicon and laser annealing of amorphous silicon. Comparison to experiment is used to validate the results of the model, and produce new insight into the role of bond-weakening and ultrafast non-thermal processes to melting and ablation.
Carrying out this project requires researchers to address the fundamental technical challenges relevant to the development of a multiscale model of heat and mass transport appropriate for far-from-equilibrium conditions.
The effort includes both scientific and educational elements. The theoretical and computer simulation methods will expand researchers' ability to model the fundamental physics of laser ablation and will be applied to the technologically relevant processes for crystalline silicon and laser annealing of amorphous silicon. The work as educational value in developing student skills, particularly the graduates and undergraduates who are directly involved in the research and the activities aid in recruiting new students for graduate study in materials simulation. The researchers and students engage in workshop activities that introduce students to materials simulation, including molecular-dynamics simulation and visualization which is coordinated with the Florida Society for Materials Simulation. The work integrates education and research through the development of course in materials simulation.
NONTECHNICAL SUMMARY: This award support research and education in optical interactions of lasers and semiconductor materials. Research investigates how materials interact with intense lasers beams under conditions relevant to laser processing of semiconductors. The work develops methods for predicting how processing conditions affect the resulting material structure. The work includes the development and use of novel computer simulation methods to elucidate the fundamental physical processes relevant to laser processing of covalent semiconductors. The general approach applies to intense ultrafast laser pulses interacting with silicon. In carrying out this project, researchers will address the fundamental technical challenges relevant to the development of a multiscale model of heat and mass transport appropriate for far-from-equilibrium conditions.
The effort includes both scientific and educational elements. The theoretical and computer simulation methods will expand researchers' ability to model the fundamental physics of laser ablation and will be applied to the technologically relevant processes for laser etching of silicon. The work as educational value in developing student skills, particularly the graduates and undergraduates who are directly involved in the research and the activities aid in recruiting new students for graduate study in materials simulation. The researchers and students engage in workshop activities that introduce students to materials simulation, including computer simulation and visualization which is coordinated with the Florida Society for Materials Simulation. The work integrates education and research through the development of course in materials simulation.
The interaction of intense, pulsed lasers is an important technology for processing materials. One important advantage of laser processing techniques can dramatically alter materials by depositing a large amount of energy in a small volume. As a result, processing can be done with great precision and with lower heat generation overall. Lower heat generation can be important, for example, in processing applications where materials with different melting points. To better understand this problem, the PI Schelling and graduate student Lalit Shokeen developed computational models to study laser-solid interactions. The principle challenge of this project was to include in a consistent framework the dynamics of the electrons, which directly absorb the radiation, alongside the atomic dynamics. The behavior of semiconductor materials interacting with lasers is quite complex, since the system is driven into extremes where the temperature of the electrons can be vastly greater than the temperature of the lattice. As a result, melting and ablation of materials can occur in ways that are distinct from ordinary melting. In particular, strong electronic excitation leads to destabilization of the atomic structure by abruptly breaking the chemical bonds, leading to melting and ablation in timescales of less than 1 picosecond. In this project, we developed a model for silicon interacting with laser pulses. The model was used to study the thermodynamics and kinetics of silicon in conditions of strong laser excitation. Several important aspects were determined. For example, the behavior of ordinary pathways of thermal melting were elucidated, where the behavior of the liquid state is surprisingly sluggish and the transition temperature sharply suppressed. The properties of liquid silicon with highly-excited electrons was also elucidated. We have also explored the conditions, mechanisms, and timescales by which silicon undergoes dramatic changes in its material state. We expect that the general approach we have developed in this project will become of use in other areas, including radiation damage where highly-excited electrons are important, but difficult to treat theoretically. There were several material outcomes of the project. The project resulted in two publications, with another article to be submitted shortly after the end of the project in late 2011. The work was presented as an invited talk to the Spring 2011 meeting of the Materials Research Society. Two important computer codes were produced for developing models of this kind and performing large-scale simulations. The graduate student Shokeen is expected to defend his PhD thesis in materials science in Fall of 2012. The graduate student and PI interacted with several undergraduate students through another funded NSF-REU project. These interactions included exposure to this research project. Finally, two regional meetings were hosted at the University of Central Florida where undergraduate students were able to learn about this project and about aspects of materials simulation.
