Progress has been made In the study of photoinduced phase transition mechanism of IN line on silicon where semiconductors are located

Since the early 20th century, quantum theory has made significant contributions to technological development. Despite the success of quantum theory, its applications are mainly limited to equilibrium systems due to the lack of a framework for non-equilibrium quantum systems. The production of ultrashort laser pulses and free electron accelerator X-rays has promoted the development of the whole field of non-equilibrium ultrafast dynamics. Ultrafast phenomena, such as photoinduced phase transitions, photoinduced demagnetization, high-energy ion collisions and molecular chemical reactions, have attracted much attention in physics, chemistry and biology. The field of non-equilibrium ultrafast has become a hot topic due to its abundant experimental results. However, the atomic/molecular shift at atomic scale cannot be obtained experimentally, so the understanding of excited state dynamics is controversial. In order to study ultrafast dynamics, theoretical simulation is very important. In order to promote the development of the ultrafast field and uncover many mysteries in ultrafast dynamics, Luo Junwei's team and Wang Linwang's team from the Institute of Semiconductors of the Chinese Academy of Sciences have developed a series of time-varying evolutionary algorithms and applied these algorithms to different fields.

Recently, researchers have applied this algorithm to the phase transition of Si (111) surface In line, which has solved many controversies in the experiment. A single indium atomic layer is adsorbed on the surface of Si (111), and a quantum wire structure composed of Si (111) - (4×1) -In two parallel zigzag In chains is formed at room temperature (Figure 1b), which has metallic properties. When the temperature drops below 125 K, the In atoms rearrange into a twisted hexagon with (8×2) reconstruction (FIG. 1a), resulting in a one-dimensional charge-density wave (CDW) with periodic lattice distortion, and opening the band gap to become an insulator phase in condensed matter physics (narrow-gap semiconductor) (FIG. 1c). Laser pulse irradiation can realize the ultra-fast transformation of In line on silicon between semiconductor phase and metal phase. However, the In line on silicon irradiated by laser pulse rapidly attenuates its coherent phonon oscillation after the semiconductor phase transition, and the oscillation between two phases which is common in other quantum phase change materials does not appear.

In order to investigate the microscopic mechanism of rapid attenuation of coherent phonon oscillations on silicon In line after phototransformation. This work uses time-dependent density functional theory (rt-TDDFT) method to simulate the dynamic process of In line (In/Si (111)) on silicon under laser pulse irradiation, and theoretically reproduces the ultrafast process of semiconductor phase conversion to metal phase observed in the experiment (FIG. 1g) (FIG. 1, 2). It is found that the laser pulse excites the valence electrons In silicon to the surface states S1 and S2 conduction bands In the In line, and since the S1 and S2 energy bands come from the bonding states In dimer on the single In sawtooth chain, the optical excitation forms an atomic force that makes the In dimer longer and drives the In atom to move towards the semiconductor phase. The integrated motion of In atoms in the lattice period forms the CDW coherent phonon pattern, leading to the structural phase transition (FIG. 3, 4). It has been shown that after switching to a semiconductor phase, the S1 and S2 bands switch to atoms across the two sawteeth In chains. This switching of band components causes the direction of the atomic driving force to rotate about π / 6, preventing the collective motion of the In atoms in the CDW phonon mode. This study explains from the local atomic driving force, provides a simpler physical image of the photoinduced phase transition process, and provides an intuitive theoretical guidance for experimental regulation of structural phase transition. All the above simulations can be realized in PWmat software.

Relevant research results are based on the Origin of Immediate Damping of Coherent Oscillations in Photoinduced Charge-Density Wave Transition. 4 in the journal Physical Review Letters. The research work is supported by the National Natural Science Foundation of China for Outstanding Young Scholars, the Key Research Program of Frontier Sciences of Chinese Academy of Sciences, and the Strategic Leading Science and Technology Project of Chinese Academy of Sciences.

Figure 1. Dynamic simulation and experimental comparison of photoinduced semiconductor phase (CDW) to metal Phase transition

Figure 2. Evolution of atomic structure, atomic forces and photoexcited electron distribution over time

Source: Institute of Microelectronics, Chinese Academy of Sciences