Hybrid Fabrication of High Quality Titanium Nitride Nanostructures for Plasmonic Applications

Nanomaterials for plasmonic devices have attracted enormous and multidisciplinary interest and efforts over the last decade. However there are few examples of plasmonic devices mainly due to materials issues, since gold and silver, the “traditional” plasmonic materials, show limited tunability of their plasmonic response across the spectra and low melting point, which makes them incompatible with CMOS technology.<br/>Transition metals nitrides (TMN) emerge as alternative plasmonic nanomaterials suitable for a wide range of applications from microelectronics to photonics and medicine. The TMN are conductive ceramics with exceptional properties such as substantial electronic conductivity, high melting point (&gt;3000 K), tunable work function, while the TMNs are particularly stable in hostile chemical environments, high temperature, and strong electric fields, such as in lasers. Among them, titanium nitride (TiN) is recently emerging as significant candidate material for plasmonic applications (biosensors, catalysis and photochemistry, solar energy harvesting, photo-detection, and optical storage of information).<br/>In this work, we focus on the hybrid fabrication of TiN nanostructures with controlled spacing and tunable dimensions (thickness and lateral dimensions) using a combination of Nanosphere Lithography (NSL) and reactive magnetron sputtering (MS). Several MS techniques such as DC, Closed-Field Unbalanced MS and Highly Power Impulse MS were used with the target to succeed high quality materials. NSL appears as a very promising approach, due to its rapid implementation and compatibility with wafer-scale processes, combines the advantages of both top-down and bottom-up approaches and includes: (a) preparation of the nanospheres colloidal mask, (b) deposition of the desired material in the empty space between the nanospheres and (c) removal/lift-off of the nanosphere colloidal mask to “reveal” the deposited material that keeps the ordered patterning of the mask interstices. Specifically, a suspension of monodisperse polystyrene nanospheres (diameter, d=552 nm or d=175 nm) was spin coated on a substrate (several substrates were used such as Si (001), glass, and flexible PET to target applications in flexible printed photovoltaics) to form the colloidal mask. A UV ozone process was used to confine the triple-junction vias of the polystyrene mask. Subsequently, the selective growth of TiN was made by the above mentioned MS in Ar/N atmosphere by varying the TiN thickness from 10 to 30 nm, while the MS process parameters were also fine-tuned to increase the directionality of deposited species, TiN crystal quality (low concentration of point defects). In particular, the substrate-to-target distance was maximized to improve geometrical directionality, and a negative bias voltage was used to “guide” the ionic species deep into the vias between the nanospheres. The arrays of ordered TiN nanostructures appear after the lift-off of the mask. The proposed process produces well-defined TiN triangular nanoislands with low concentration of point defects, similar structure with the continuous TiN films of high electrical conductivity and plasmonic performance, and durability at least up to 400^o C. Finally it should be noted that TiN is used as a case study as very good representative of other refractory TMNs, such as ZrN, HfN, NbN, and TaN.<br/>Refs: P. Patsalas, N. Kalfagiannis, S. Kassavetis, G. Abadias, D.V. Bellas, Ch. Lekka, E. Lidorikis, Materials Science and Engineering R 123 (2018) 1–55.<br/>Panos, S., Tselekidou, D., Kassavetis, S., Fekas, I., Arvanitidis, J., Christofilos, D., Karfaridis, D., Dellis, S., Logothetidis, S. and Patsalas, P., Phys. Status Solidi B, 258 (2021) 2000573