Time-resolved study of laser-induced disorder of Si surfaces

Abstract

Optical second-harmonic studies show that the electronic structure in the top 75-130 A of a crystalline Si surface loses cubic order only 150 fsec after the Si is excited by an intense 100-fsec optical pulse. This suggests that atomic disorder can be induced directly by electronic excitation, before the material becomes vibrationally excited. In contrast, the electronic properties of the equilibrium molten phase are not obtained for several hundreds of femtoseconds. PACS numbers: 78.47.+p, 61.80.Ba, 71.38.+i With use of ultrashort laser radiation pulses, it is possible to excite the electronic states of a solid before appreciable energy is transferred to the lattice vibrational states. Here, we explore the question posed several years ago by Van Vechten, Tsu, and Saris, 1 whether or not crystalline Si can be driven to disorder by electronic excitation without the lattice modes' equilibrating at a temperature above the melting temperature. This possibility has obvious practical importance in laser-assisted semiconductor processing and surface photochemistry in general. It is of fundamental interest because the dynamics of electron-phonon equilibration may directly effect the evolution of the phase transition. Indeed, a recent picosecond time-resolved study of graphite after pulsed laser excitation reported observation of an intermediate nonequilibrium phase. 2 The physical picture of how Si might be disordered by nonequilibrium electronic excitaton is appealing. Silicon is well described by a tight-binding model, and therefore electronic excitation might directly drive disorder by a process akin to photodissociation in molecules. In a more delocalized picture, the electronic excitation might weaken the interatomic bonding and thus lower the vibrational energy required to produce disorder. However, previous linear reflection 3,4 and second-harmonic generation 5 (SHG) experiments on laser-induced disorder of silicon can be interpreted in the normal picture of cascade electron-phonon relaxation and subsequent lattice melting in « 1 psec. Here, we report new time-resolved optical measurements. We find that the SH reflection changes abruptly in a way that is consistent with the top 75-130 A of the silicon surface's losing cubic symmetry in < 150 fsec. Because 150 fsec is less than two electron-phonon relaxation times, the result suggests that the laser induces disorder before the phonons equilibrate above the melting temperature. In addition, the data indicate that the electronic properties of the equilibrium molten phase are not obtained on this short time scale but rather with a > 300-fsec time constant. This suggests that considerable vibrational excitation is required to obtain the interatomic distance and/or correlations of the liquid.The experiments are very similar to those reported in Refs. 4 and 5. Laser pulses of 75 fsec duration (110 fsec FWHM autocorrelation) were obtained by dispersion compensation of the amplified output of a colliding-pulse mode-locked laser operating at 610 nm. To make the excitation...

Coherent phonons in condensed media

“…Most of them are based on time-resolved second harmonic generation (SHG), which vanishes when the crystal loses its symmetry. It has been reported, that this process occurs on time scales which are shorter than the typical energy transfer times from the electronic system to the lattice 103,104,105]. Theoretically it has been predicted that a non-thermal phase transition may occur when more than 9% of all valence electrons are excited into antibonding conduction band states 106, 1 0 7 ].…”

Ultrafast electronic disordering during femtosecond laser melting of GaAs

“…In a refinement of the second-harmonic generation experiment, Tom, Aumiller, and Brito-Cruz reported a loss of cubic order in crystalline Si only 150 fs after a 100-fs pulse [8]. [5].…”

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).…”