Recently, a collaborated team led by Prof. Luo Jun-Wei from the Institute of Semiconductors of the Chinese Academy of Sciences (CAS), has investigated the process of structural nonthermal melting in laser-induced silicon crystal annealing. This work was published in Science Advances on July 06, 2022.

The method of ultrafast laser annealing was proposed in the 1970s, but its physical mechanism has not been clearly understood. With the advent of a new generation of light sources, the observation of ultrafast dynamics within atomic scale can be achieved, so this controversy of ultrafast laser annealing has recently returned to the forefront.

In this work, the researchers have used the real-time time-dependent density functional theory (rt-TDDFT) to study the laser-induced structural nonthermal melting during the laser annealing process in silicon. They reveal that the photoexcited electrons from the bonding states of the valence band to the antibonding states of the conduction band generate the atomic driving force that lengthens the Si-Si bonds. After undergoing the electron-phonon self-amplificated process, the excited carrier distribution and the atomic driving force localize at some certain atoms to cause the locally atomic motion, forming the nucleation seeks (Figure). The seeks rapidly expand into another region to result in the overall nonthermal melting within a sub-picosecond timescale. It is significantly different from the picture of simultaneously broken all atomic bonds as considered by classical theoretical models of inertial models, phonon instability, or Coulomb repulsion.

Surprisingly, the carrier localization driven by the electron-phonon self-amplificated process is effective when excited electrons only occupy the conduction band edges. Therefore, in a short-wavelength photoexcitation, a sufficient amount of photoexcited hot carriers occupying high energy levels is required to cool down to the band edge, leading to a longer structural melting time than a long-wavelength laser. Finally, they propose that the nonthermal melting process can be effectively manipulated by laser wavelength.