서강대학교

초록/요약

Bismuth chalcogenides (Bi2Se3 and Bi2Te3) are 2-dimensional thermoelectric materials expected as the leading candidates of environmentally friendly energy generators by saving untapped waste heat from various sources. Moreover, their topological metallic and bulk insulating nature can be employed for advanced computing electronics. Later, the way was devised through which is manipulating its thermal conductivity by use of spatial phonon confinement in intrinsic quantum well structure along the 2D layer stacking direction. To understand mechanistic process of phonon quantum confinement, real-time measurement of lattice dynamics is required. Since coherent phonons and following lattice relaxation can be reproduced by laser excitation, laser pump x-ray probe technique can be exploited to study. In this thesis, we studied lattice relaxation dynamics of optically excited Bismuth Selenide (Bi2Se3) thin films. The femto-second laser illumination on the sample triggers the excitation of electrons and continuously generates optical and acoustic phonons. Ultrafast time resolved x-ray diffraction (trXRD) technique was employed to resolve picosecond time scale dynamics. We measured the (006) Bragg reflection of Bi2Se3 with thicknesses of 5 – 150 nm. Laser power was varied from 0.7 to 8.0mJ/cm2. We observed that a fast intensity decrease and recovery of the position of (006) Bragg reflection due to relaxation of electrons up to 200ps. An oscillatory behavior is appeared between 200ps and 1ns which is attributed to longitudinal acoustic phonons and beyond 1ns, due to a complex recovery behavior, lattice thermalization and incoherent phonon generation appeared.