Robust plasmonic properties of epitaxial TiN films on highly lattice-mismatched complex oxides

High-quality titanium nitride (TiN) film is a prominent plasmonic material in the fields of plasmonics and nanophotonics, which is usually synthesized on several limited substrates (such as MgO, silicon, and sapphire). With the rapid development of hybrid optoelectronic devices composed of plasmonic materials and functional oxides, it is promising to integrate high-quality TiN films with a variety of functional oxides, which requires to accommodate TiN films to a broad spectrum of lattice mismatch. In this work, we synthesized high-quality single-crystalline TiN films on various oxide substrates $[\mathrm{MgO}, {\mathrm{MgAl}}_2{\mathrm{O}}_4,$ $({\mathrm{LaAlO}}_3{)}_{0.30}\text{−}({\mathrm{SrAl}}_{1/2}{\mathrm{Ta}}_{1/2}{\mathrm{O}}_3{)}_{0.70}, \mathrm{and} {\mathrm{YAlO}}_3]$ by high-pressure magnetron sputtering. It is surprising that the epitaxial TiN films can accommodate extremely large lattice mismatch up to $-15.39\%$. The crystal and electronic structures of the TiN films were characterized by high-resolution x-ray diffraction, atomic force microscopy, and x-ray photoemission spectroscopy measurements. The optical and electrical properties of the TiN films on various oxide substrates were investigated by spectroscopic ellipsometry and Hall effect measurements. It is revealed that the remarkable optical properties of TiN films are robust even on highly lattice-mismatched oxide substrates. Our work paves a way to integrate plasmonic TiN films with a variety of functional oxides for high-performance hybrid optoelectronic devices.

Figure 1

(a,b) Schematic of TiN crystal structure and TiN films on oxide substrates. (c) In-plane lattice parameters and lattice mismatch values of TiN films on various oxide substrates MgO, MAO* (half of its bulk lattice parameter), LSAT, and YAO (pseudocubic structure). (d)–(g) Surface morphology and roughness of TiN films on various substrates measured by AFM with tapping mode. The scanning area is $2\times 2$ $\mu {\mathrm{m}}^2$.

Figure 2

(a) $2\theta \text{−}\omega$ scans of TiN films on various oxide substrates. The black triangles indicate the (002) diffraction peaks of TiN films which are around $42.{415}^{\circ }$, $42.{555}^{\circ }$, $42.{565}^{\circ }$, and $42.{575}^{\circ }$ for MgO, MAO, LSAT, and YAO, respectively. All curves were vertically offset for clarity. (b) Phi scans around the (202) diffraction of TiN film and the YAO substrate (labeled as pseudocubic structure).

Figure 3

RSM patterns of TiN films around the (113) Bragg diffractions on (a) MgO, (b) MAO, (c) LSAT, and (d) YAO.

Figure 4

XPS Ti $2p$ spectra of TiN films on MgO, MAO, LSAT, and YAO substrates at room temperature.

Figure 5

(a) The real part ${\varepsilon }_1$ and (b) imaginary part ${\varepsilon }_2$ of TiN films on MgO, MAO, LSAT, and YAO substrates.

Figure 6

(a) Carrier density and plasmon energy [estimated from Fig. 5] of TiN films on various oxide substrates at room temperature. (b) Carrier mobility of TiN films at room temperature.

Figure 7

The plasmonic performance of TiN films on MgO, MAO, LSAT, and YAO substrates evaluated by the quality factor for (a) SPP and (b) LSPR.