Transient Optical Properties of Titanium Aluminum Nitride (Ti_1-x Al _x N) Epilayers

Titaniumnitride is a refractory material with excellent thermaland mechanical stabilities as well as optical and plasmonic propertiesin the visible and near-infrared (NIR) regions. Alloying differentconcentrations of aluminum element in TiN can not only change thedielectric properties from metallic to dielectric but also tune theepsilon-near-zero wavelength (& lambda;(ENZ)) over a wide spectralrange. Understanding the role of Al in the transient optical responsesof Ti1-x Al (x) N 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 controversialissue, and moreover, little is known about the transient optical propertiesof Ti1-x Al (x) N. In this work, the broadband transient reflectance of highly crystallineTi(1-x )Al (x) N epitaxial films with various Al concentrations (0 & LE; x & LE; 0.67) is investigated by an ultrafast pump-probeexperiment. With increasing Al concentration, the optical absorptionin the visible to near-infrared region is drastically increased inthe Ti1-x Al (x) N films, showing great potential to serve as an efficientabsorbing layer for photovoltaic cells. From the carrier dynamicsstudies, we found that TiN undergoes wavelength-dependent e-phcoupling processes with distinctly different lifetimes: sub-picosecond(& LE;0.2 ps) in a narrow spectral region near & lambda;(ENZ) and a few tens of picoseconds in the metallic region, followed bya very long heat dissipation process on the nanosecond timescale.As for Ti1-x Al (x) N, the spectral region where the ultrafast e-ph couplingoccurs is extended to the whole visible range. While ultrafast andstrong e-ph coupling is advantageous in hot carrier engineeringapplications, prolonged preservation of heat in the lattice for ananosecond makes TiN and TiAlN emerging photothermal materials withhigh conversion efficiency.