Electronically Driven Structure Changes of Si Captured by Femtosecond Electron Diffraction

Abstract

The excitation of a high density of carriers in semiconductors can induce an order-to-disorder phase transition due to changes in the potential-energy landscape of the lattice. We report the first direct resolution of the structural details of this phenomenon in freestanding films of polycrystalline and (001)-oriented crystalline Si, using 200-fs electron pulses. At excitation levels greater than approximately 6% of the valence electron density, the crystalline structure of the lattice is lost in <500 fs, a time scale indicative of an electronically driven phase transition. We find that the relaxation process along the modified potential is not inertial but rather involves multiple scattering towards the disordered state.

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

“…Nonthermal phase transitions have been observed in multiple semiconducting and dielectric materials: Si [1][2][3][4][5][6], Ge [7,8], GaAs [9], InSb [10], Ge 2 Sb 2 Te 5 [11], and C [12]. Band gap materials are particularly amenable to nonthermal processes because their band gaps inhibit electronic relaxation.…”

“…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.…”

Time-resolved structural dynamics of thin metal films heated with femtosecond optical pulses

“…Irradiation with femtosecond laser pulses induces lattice disorder (1), which could be (i) a purely electronic nonthermal order-disorder process by injecting carriers into bulk semiconductor crystals, such as Si (2,3) and GaAs (4, 5) within 1 ps; (ii) a purely thermal-disorder thermal process that depends on electron/phonon, phonon/phonon, and phonon/lattice interaction time in metal films, especially in noble metals, such as Au (6,7); (iii) a combination of thermal-and nonthermal-disorder with the ratio depending on the laser fluence (8). When the laser fluence is sufficient, the phase transition from solid to liquid will occur and the melting process is considered to involve both thermal and nonthermal processes, whereas in gold (9-11) and aluminum films, which is the dominant process is still under debate (12)(13)(14).…”

Photoinduced ultrafast local volume changes in intermediate-valence solids

“…10,25 It is well established that for fcc crystals the ͑111͒ Bragg peak is that of highest intensity,., Therefore we have computed the time dependence of the Bragg reflections during the lattice expansion from the structure factor,,…”