Atomistic-continuum modeling of short laser pulse melting of Si targets

Abstract

We present an atomistic-continuum model to simulate ultrashort laser-induced melting processes in semiconductor solids on the example of silicon. The kinetics of transient non-equilibrium phase transition mechanisms is addressed with a Molecular Dynamics method at atomic level, whereas the laser light absorption, strong generated electron-phonon non-equilibrium, fast diffusion and heat conduction due to photo-excited free carriers are accounted for in the continuum. We give a detailed description of the model, which is then applied to study the mechanism of short laser pulse melting of free standing Si films. The effect of laser-induced pressure and temperature of the lattice on the melting kinetics is investigated. Two competing melting mechanisms, heterogeneous and homogeneous, were identified. Apart of classical heterogeneous melting mechanism, the nucleation of the liquid phase homogeneously inside the material significantly contributes to the melting process. The simulations showed, that due to the open diamond structure of the crystal, the laser-generated internal compressive stresses reduce the crystal stability against the homogeneous melting. Consequently, the latter can take a massive character within several picoseconds upon the laser heating. Due to negative volume of melting of modeled Si material, -7.5%, the material contracts upon the phase transition, relaxes the compressive stresses and the subsequent melting proceeds heterogeneously until the excess of thermal energy is consumed. The threshold fluence value, at which homogeneous nucleation of liquid starts contributing to the classical heterogeneous propagation of the solid-liquid interface, is found from the series of simulations at different laser input fluences. On the example of Si, the laser melting kinetics of semiconductors was found to be noticeably different from that of metals with fcc crystal structure.Comment: 38 pages, 27 figure

Soft x-ray induced femtosecond solid-to-solid phase transition

“…This timescale is much shorter than any known timescale of electron-phonon coupling and thermal phase transition, which are usually expected to occur on a picosecond to nanosecond timescale [7,21,32]. Moreover, it is even faster than any non-thermal melting (solid-to-liquid) transitions which require at least some 300-500 fs, for example, in silicon, gallium arsenide, and other covalently bonded semiconductors [1,3,33].…”

Simulating electronically driven structural changes in silicon with two-temperature molecular dynamics

“…The degree of order in the atomic structure over this ( ,t)-space can be quantified with the centrosymmetry order parameter [26] as shown in Fig. 3(a).…”

“…This is justified by the following observations: The experimental probe observes a smaller spot size than the pulse, and on the picosecond timescale, there will be negligible lateral thermal diffusion over the ∼10 2 μm length scale. Regarding the deposition as a function of depth, previous modeling [26] of silicon nanofilm irradiation that accounts for the temperature-dependence of the optical properties indicates a near-uniform excitation within the first 50 nm. In our model, the energy is delivered to the electrons via the heat source Q in the electronic heat Eq.…”

“…(4) may be numerically integrated more efficiently with use of Suzuki-Trotter decomposition [51]. Some previous efforts [26,27,41,52] to model silicon with a two-temperature model have been based on the framework of van Driel [23], which parametrizes the system with the carrier density, in addition to the usual electronic and nuclear temperatures. The carrier density evolves according to a separate differential equation that captures carrier generation, diffusion, and recombination.…”

“…Silicon is a widely studied material in the context of strongly driven phase transitions, both experimentally [1][2][3][4][5][6] and theoretically [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29], where it is found to melt on a subpicosecond timescale at high excitations. Most theoretical studies of nonthermal melting in silicon have employed ab initio simulation methods.…”