Comprehensive Development of Reactive Sputtering Parameters for High-Performing Zirconium Nitride Thin Films for Earth-Abundant Plasmonics

Zirconium Nitride (ZrN) is a highly promising earth-abundant alternative for plasmonic materials due to its lower cost, high electrochemical stability, and greater mechanical durability than traditional metals like Au and Ag. While epitaxial films enable longer plasmonic propagation lengths than their polycrystalline counterparts, epitaxial ZrN has so far only been achieved with expensive, MgO substrates via unbalanced magnetron reactive sputtering. Therefore, research efforts have been focused on developing ZrN films on Si/SiO₂ substrates to achieve low-cost films with optical performance suitable for commercial plasmonic applications such as on-chip waveguiding and catalysis enhancement. However, a comprehensive understanding of the reactive sputtering deposition conditions necessary for high-performing ZrN thin films on Si/SiO₂ substrates is lacking; most research has relied on high-deposition temperatures and the tuning of N₂ flow rates to produce films of acceptable quality. Most notably, the influence of large substrate biases (&gt;100V) on ZrN film formation and its corresponding optical properties has been largely unexplored; given the strong impact of the increased kinetic energy that results from bias, this is a shortcoming of the existing studies.<br/><br/>Herein, for the first time, we explore the systematic tuning and effect of deposition temperature, nitrogen flow rate, and substrate bias, on the optoelectronic properties of ZrN thin films via XRD, XPS, and spectroscopic ellipsometry. First, to optimize the nitrogen concentration and generate stoichiometric ZrN, the ideal gas flow rate was found to be 2.0:30sccm N₂:Ar at a plasma power of 150W. Next, we demonstrate that a large, applied RF bias of 400V to the Si substrate enables optimum film formation conditions, likely arising from increased film densification, as evidenced by improved optical properties and higher surface plasmon polariton figures of merit (Q_SPP). Lastly, we show the effect of moderate, elevated temperatures of 400C, on improved film performance likely from decreased microstrain as evidenced similarly.<br/><br/>As a result, our highest performing ZrN films demonstrate a Q_SPP over 50 in the VIS-IR spectrum rivaling the best silicon-based ZrN films across literature. Without relying on expensive MgO substrates or extremely high deposition temperatures, we demonstrate the comprehensive effect of sputtering conditions on film formation and plasmonic performance. Given the significant performance achievements, this work can open the door for widely accessible, earth-abundant plasmonic materials suitable for electrochemical and optoelectronic systems and devices operating within the VIS-IR spectrum.