Transient Optical Properties of Titanium Aluminum Nitride (Ti_1-xAl_xN) Epilayers

Titanium nitride is a refractory material with excellent thermal and mechanical stabilities as well as optical and plasmonic properties in the visible and near-infrared (NIR) regions. Alloying different concentrations of aluminum element in TiN can not only change the dielectric properties from metallic to dielectric but also tune the epsilon-near-zero wavelength (λ_ENZ) over a wide spectral range. Understanding the role of Al in the transient optical responses of Ti_1-xAl_xN under femtosecond excitation is crucial for optoelectronic, photovoltaic, and photothermal applications. Recently, the electron-phonon (e-ph) coupling rate and time of TiN have been a controversial issue, and moreover, little is known about the transient optical properties of Ti_1-xAl_xN. In this work, the broadband transient reflectance of highly crystalline Ti_1-xAl_xN epitaxial films with various Al concentrations (0 ≤ x ≤ 0.67) is investigated by an ultrafast pump-probe experiment. With increasing Al concentration, the optical absorption in the visible to near-infrared region is drastically increased in the Ti_1-xAl_xN films, showing great potential to serve as an efficient absorbing layer for photovoltaic cells. From the carrier dynamics studies, we found that TiN undergoes wavelength-dependent e-ph coupling processes with distinctly different lifetimes: sub-picosecond (≤0.2 ps) in a narrow spectral region near λ_ENZ and a few tens of picoseconds in the metallic region, followed by a very long heat dissipation process on the nanosecond timescale. As for Ti_1-xAl_xN, the spectral region where the ultrafast e-ph coupling occurs is extended to the whole visible range. While ultrafast and strong e-ph coupling is advantageous in hot carrier engineering applications, prolonged preservation of heat in the lattice for a nanosecond makes TiN and TiAlN emerging photothermal materials with high conversion efficiency.