Measurements of electron and phonon transport and scattering in ferroelectric thin films using ultrafast and infrared spectroscopy

Ferroelectric materials feature a spontaneous electrical polarization that originates from their unit-cell structure, with a spatial orientation that can be switched between stable states when subjected to an external electric field. This characteristic sets them apart as a unique class of materials with enhanced functionality among piezoelectrics. The push toward the miniaturization of piezoelectric sensors and actuators, particularly in microelectromechanical systems (MEMS), along with the integration of ferroelectric properties into integrated circuit (IC) technology and the rise of new applications relying on polarization control, has sparked significant scientific and commercial interest in ferroelectric thin films. Many key ferroelectric materials are oxide perovskites, which often suffer from limitations such as low paraelectric transition temperatures, non-linear displacements, and poor compatibility with technologies like complementary metal-oxide-semiconductor (CMOS) or III-nitride systems. These challenges currently hinder the widespread adoption of ferroelectric functionality in microtechnology.

In this study, we focus on ferroelectric wurtzite nitrides, which exhibit unique electronic and phononic properties that enable dynamic tunability and control over light, charge, spin, and heat. We present characterizing the phonon transport, sound speed, and phononic lifetimes of Al_1-xB_xN ferroelectric thin films using ultrafast spectroscopy. Additionally, when combined with infrared variable angle spectroscopy ellipsometry (IR-VASE), these methods allow for precise monitoring of scattering rates. Temperature-dependent measurements provide further insights into the changes in optical phonon energies. Understanding these changes will enhance our knowledge of phonon dynamics and could lead to improved ferroelectric materials for next-generation microtechnology applications.