PNNL

Abstract

Soon after it was discovered that intense laser pulses of nanosecond duration from a ruby laser could anneal the lattice of silicon, it was established that this so-called pulsed laser annealing is a thermal process. Although the radition energy is transferred to the electrons, the electrons transfer their energy to the lattice on the time scale of excitation. The electrons and the lattice remain in equilibrium and the laser simply "heats" the solid to the melting temperature within the duration of the laser pulse. For ultrashort laser pulses in the femtosecond regime, however, thermal processes (which take several picoseconds) and equilibrium thermodynamics cannot account for the experimental data. Upon excitation with femtosecond laser pulses, the electrons and the lattice are driven far out of equilibrium and disordering of the lattice can occur because the inter-atomic forces are modified due to the excitation of a large (10% or more) fraction of the valence electrons to the conduction band. The present review focuses on the nature of the nonthermal transitions in semiconductors under femtosecond laser excitation.