Time-resolved x-ray diffraction study of longitudinal optical phonons in zincblende semiconductors

Sammanfattning: Anharmonic decay of longitudinal optical (LO) phonons in zincblende semiconductors is not at present well characterized due to the polar and many-body nature of the problem. Femtosecond THz radiation offers a novel route to this study, as the interaction of this radiation is directly with the phonon lattice and these THz excited phonons do not have enough energy to excite electrons over the band gap. Therefore, their decay is entirely due to anharmonic interactions with other phonon modes in the lattice. In this thesis, it has been hypothesized that by the correct choice of THz polarization and for achievable THz field strengths, the atomic motion in the THz excited LO phonon modes and their decay could be directly observed as an intensity modulation of the nearly forbidden reflections of the zincblende structure with time-resolved x-ray diffraction (TXRD). In the framework of an ultrafast THz pump, TXRD probe experiment, this thesis consists of investigating this hypothesis using a theoretical model. Modelling the LO phonons as harmonic oscillators driven by the electric fields of the THz radiation and where the material parameters have been simulated using density functional perturbation theory for InSb. The results show that for a 200 fs Gaussian THz pulse centered at the LO phonon frequency, 20% increase in the TXRD intensity can already be observed for a THz field strength of 0.3 MV/cm. Due to the anharmonic decay of the LO phonon, significant intensity modulation can be observed even 10 ps after the THz pulse. These results on InSb suggest that if THz radiation can be generated close to the LO phonon frequency, significant TXRD intensity modulation can be observed in any zincblende semiconductor for achievable THz field strengths. Therefore, the present thesis paves a way for a new ultrafast time-resolved technique to measure the anharmonic lifetimes of LO phonons in zincblende semiconductors, for which the data is at present lacking, and which are needed for the advancement of hot carrier solar cells.