Electron-Phonon Coupling with the Soft Phonon Mode and Slow Electronic Dynamics in the Ferroelectric Semiconductor SbSI

The study of electron-phonon interactions in semiconductors involving strongly anharmonic phonon modes has recently of interest with regards to understanding the outstanding optoelectronic properties of novel perovskite materials. Ferroelectric semiconductors provide an excellent model system in which to study electron-anharmonic phonon coupling owing to the presence of a strongly anharmonic “soft” phonon mode that drives the structural transition from the low symmetry ferroelectric phase to the higher symmetry paraelectric phase. While this “soft mode” or so-called “lattice dynamical” theory of structural phase transitions in ferroelectrics has been known since the 1960’s, direct, mode-specific experimental studies of electron-phonon coupling involving the anharmonic soft phonon mode in ferroelectric semiconductors have been rare. Here, using coherent phonon spectroscopy, we directly observe electron-phonon coupling in the time domain involving the soft TO-phonon mode for above-gap electronic excitations in a ferroelectric semiconductor SbSI. Furthermore, we observe that the above-gap electronic excitation coupled to the ferroelectric soft mode has an exceptionally long recombination lifetime that we estimate to be on the order of ~µs, with transient spectral features that suggest an electron-phonon coupling mechanism involving charge localization at ferroelectric domain boundaries. Our results give direct experimental evidence to suggest the importance of understanding electron-anharmonic phonon interactions towards their potential role in influencing charge carrier dynamics in polar semiconductors with broken lattice inversion symmetry.