Ab initioMolecular Dynamics Simulation of Laser Melting of Silicon

Abstract

The method of ab initio molecular dynamics, based on finite temperature density functional theory, is used to simulate laser heating of crystal silicon. We have found that a high concentration of excited electrons dramatically weakens the covalent bond. As a result, the system undergoes a melting transition to a metallic state. In contrast to ordinary liquid silicon, the new liquid is characterized by a high coordination number and a strong reduction of covalent bonding effects. [S0031-9007(96)01372-5] PACS numbers: 61.20. Ja, 64.70.Dv, 65.50.+m The phenomenon of laser-induced annealing in semiconductors has attracted, over the years, considerable attention and contradictory explanations [1,2]. One of the first attempts to explain the observed phenomenology has been the "plasma annealing" (PA) model [1]. In this model it is suggested that a tetrahedral semiconducting material, subject to intense laser irradiation, can be driven directly into a disordered state, long before the system has time to become vibrationally excited. In fact, laser irradiation induces electronic transitions from the bonding to the antibonding states, thus depleting the bond charges. A high level of electronic excitation could severely weaken the interatomic bonds so that room temperature atomic motions can lead to the disordering of the lattice. The alternative explanation [2] was that direct energy transfer between the excited electrons and the ions leads to ordinary thermal melting (TM).For laser pulses lasting 20 ps and longer, TM appears to be [2,3] the dominant mechanism. However, for very short irradiation times ͑ϳ100 fs͒, recent time-resolved experiments have strongly strengthened the PA model. In fact several groups [4] have observed laser-induced melting of a GaAs crystal under high laser irradiation. Tom, Aumiller, and Brito-Cruz [5] have reported a loss of cubic order in crystalline Si only 150 fs after an intense 100 fs optical pulse. Shank, Yen, and Hirlimann [6] have observed melting of silicon on a time scale of less than 1 ps after a 90 fs pump pulse, as evidenced by reflectivity and second-harmonic generation. In spite of this strong experimental evidence, a clear understanding of the processes that take place is still missing.Since reasonable estimates [7] suggest that the relaxation time for the electrons, t ee ϳ10 214 sec, is much shorter than the electron-ion relaxation time, t eI ϳ10 212 sec, we can treat the subsystems of electrons and ions in a different way. We assume that, after irradiation and for times smaller than t eI , the electron subsystem remains in internal equilibrium at the initial laser-induced temperature, which is different from that of the ions. The ions instead are allowed to evolve freely. In order to describe this situation we used the ab initio molecular dynamics (MD) simulation technique introduced by Alavi et al. [8]. This method is based on finite temperature density functional theory (DFT), and incorporates self-consistently the effects of thermal electronic excitations and fractiona...

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

“…Most ab initio studies of nonthermal melting in silicon have been built on two assumptions: thermalized electrons and the BornOppenheimer approximation, which excludes nonadiabatic effects such as electron-phonon coupling, e.g., Refs. [16][17][18][19][20][21][22]. These studies do predict bond softening at high excitations but they predict melting to occur at higher excitations than expected based on experiment.…”

“…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.…”

Structural, dynamic, and electronic properties of liquid tin: An ab initio molecular dynamics study

“…We have also evaluated the electrical conductivity , which as in other previous ab initio calculations, 56 has been computed by the Kubo-Greenwood formula. 57 It is obtained by extrapolating to zero frequency, using Drude's model formula, the optical conductivity, ϵ ͑0͒ = lim →0 ͑ ͒, where ͑ ͒ is computed by a configurational average of…”

Atomic/Molecular-Level Simulations of Laser–Materials Interactions

“…The description of the effect of the electronic excitation on the material properties is typically performed by means of computationally expensive electronic structure calculations, e.g., [23][24][25][26][27][28][29][30][31][32][33][34][35][36]. Simulations based on electronic structure calculations provide information on the changes in the interatomic bonding and the ultrafast atomic dynamics induced by the electronic excitation.…”