Titanium nitride (TiN) is a plasmonic material, which efficiently absorbs the solar spectrum and is useful for light-to-heat conversion. Although TiN nanostructures have been developed for plasmonic applications including heat generators, TiN oxidizes to titanium dioxide (TiO2) at relatively low temperatures, as low as 350 °C in air, which limits the application of TiN. In this study, we protect TiN nanocylinder arrays with conformal coatings of insulator layers fabricated by atomic layer deposition. The coating increases the oxidation temperature of the TiN array in air up to 750 °C through suppression of oxygen diffusion by the insulator layer. We also demonstrate the benefit of the thermal oxidation of TiN as a route for nanofabrication of TiO2, a transparent high-refractive-index material. The TiN nanocylinders are converted to TiO2 nanocylinders without disturbing the periodic arrangement. The resultant TiO2 nanocylinder arrays act as two-dimensional (2D) photonic crystals.
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Noble metals such as Au and Ag have been investigated as plasmonic materials owing to their optical properties suitable for the excitation of surface plasmon polaritons in the visible spectral region. In addition to these conventional materials, titanium nitride (TiN) has been exploited as an alternative material supporting surface plasmon polaritons in the visible and near-infrared regions . Many TiN nanostructures have been studied, ranging from nanoparticles [2,3] to array of nanocylinders [4–12]. Although TiN is similar to Au in appearance, its optical loss or absorption is larger than that of Au in the visible range. Recently, due to the larger absorption, TiN has attracted significant attention from the viewpoint of the development of a light-to-heat converter. The spectrally broadband absorption of TiN accompanied by the localized surface plasmon resonance (LSPR) is beneficial to efficient absorption of the solar spectrum and conversion into heat .
The thermal stability of TiN has been investigated for its application to light-to-heat converters, for which the oxidation should be suppressed because titanium dioxide, TiO2, a compound eventually formed by the oxidation of TiN, is an insulator and does not absorb the visible light. TiN films retain their optical properties up to approximately 1200 °C during heating in vacuum [14,15], whereas they are oxidized to TiO2 over 400 °C and their optical properties change irreversibly during heating in air . The thermal oxidation of TiN was studied earlier in a different context as a coating material for reinforcement of strength, fracture toughness and wear resistance, and the diffusion of oxide ions was found to cause the oxidation [17,18].
Aside from the light-to-heat conversion, TiO2 has been intensively studied as an optically functional material having one of the highest refractive indices (n) in the visible region without absorption, which makes it a very useful photonic material in the visible range. Nanostructured TiO2 including one, two- and three-dimensional photonic crystals has been fabricated using the physical vapor oblique angle deposition , interference lithography , colloidal [21–23] and lithographically-made [24–31] templating and nanoimprint [32,33] in combination with sol–gel technology, evaporation, sputtering or atomic layer deposition (ALD). However, the design freedom and precise control of the structures are much less and poor than those of the Si photonics technology, which enables a large design freedom and precise nanofabrication owing to the selective dry etching technology originally developed for Si. Top–down fabrication through selective etching has not been yet established for TiO2 owing to the absence of gas that can selectively etch TiO2, while TiN is compatible with the selective dry etching. Hence, oxidation of TiN nanostructures could be a smart route to fabricate TiO2 nanostructures.
In this study, we fabricated TiN nanocylinder arrays covered with insulator layers by ALD in order to inhibit the diffusion of atmospheric oxygen and improve the thermal stability of TiN in air. The ALD achieves conformal coating with a high controllability of the thickness. As coating insulators, we chose aluminum oxide (Al2O3), which is a representative ALD material, and silicon nitride (Si3N4), which is frequently used as a surface protection layer . Preceding works on the micro-composites of Al2O3/TiN and Si3N4/TiN show that the oxidation of TiN occurs at a lower temperature than Si3N4, and thus the conformal coating of TiN nanocylinder with these materials should suppress the oxidation of TiN [17,18]. Reasonably-high thermal conductivity of Al2O3 (∼30 W m−1 K−1) and Si3N4 (∼30 W m−1 K−1) compared to TiN (∼12 W m−1 K−1) [35–37] would not prevent heat generation from TiN nanostructures for light-heat conversion application. In addition to the fabrication of the TiN nanocylinder array stable at high temperatures owing to the coating, we prepared a TiO2 nanocylinder array by intentionally oxidizing the TiN nanocylinder array. We analyzed the optical properties of the TiO2 nanocylinder array as a 2D photonic crystal.
2. Results and discussion
2.1 Change of optical properties with heat treatment
Figure 1(a) shows an SEM image of the TiN array before the coating. The array consists of cylinders with diameters of 230 nm, which are arranged in a square lattice with a periodicity of 350 nm. This design was chosen to provide light diffraction in the visible range for the demonstration of the photonic crystal behavior of the TiO2 nanocylinder array, as described sec. 2.1. Upon the deposition of the 50-nm-thick layer of Al2O3, the cylinders become larger (Fig. 1(b)). The coating is conformal; no apparent pits and cracks are observed in the SEM images.
Figure 2(a) shows optical transmittance spectra of the samples heat-treated at different temperatures. The bare TiN array before the heat treatment exhibits a broad dip around λ = 820 nm, attributed to the excitation of LSPR. The transmittance gradually increases in a wavelength range longer than about λ = 500 nm as the heat treatment temperature is raised over 350 °C, suggesting that the oxidation of TiN starts to occur around this temperature. After the heat treatment at 450 °C (or higher temperatures), the dip at λ = 800 to 900 nm disappears and a new dip emerges at λ = 500 nm. The new dip corresponds to the in-plane diffraction, which occurs at λ = n·(period) at normal incidence, as denoted by the vertical dotted line, obtained using the refractive index of the substrate (n = 1.46). The oxidation of the TiN nanocylinder at 450 °C is consistent with the behavior of the TiN thin film examined in the present study, as shown in Appendix Fig. 7, and of the TiN thin film reported previously . The sample heat-treated at higher temperatures is transparent in the longer-wavelength region, typically for λ > 600 nm, indicating that TiN is fully oxidized to TiO2. The X-ray diffraction patterns and Raman shifts (Figs. 7 and 8 in Appendix) indicate that the TiO2 obtained through oxidation of TiN is a rutile phase.
The transmittance of the array with the 50-nm-thick Al2O3 coating exhibits an LSPR at λ = 900 to 1000 nm, which is red-shifted with respect to that of the bare TiN array by $\varDelta$λ = 80 nm owing to the increase in n in the surrounding of the nanocylinders. A gradual increase in the transmittance in a wavelength range longer than about λ = 500 nm starts over 600 °C, indicating that the Al2O3 coating improves the thermal resistivity of the TiN array. After the heating at a higher temperature, the in-plane dip is observed around λ = 500 nm and the sample is transparent at the spectral range of λ > 600 nm, indicating that TiN is oxidized to TiO2. The transmittance of the array with the 10-nm-thick Si3N4 coating reveals that the resistivity toward oxidation further increases; the change in the wavelength dependence of the transmittance becomes observable after the heat treatment over 750 °C. Compared with that of the bare TiN array, as shown in Appendix Fig. 9, the spectral position of the LSPR is shifted by $\varDelta$λ = 50 nm. The red-shift is smaller than that for the Al2O3-coated sample, despite the higher refractive index of Si3N4 (n = 2.0) than that of Al2O3 (n = 1.8), owing to the thinner coating.
The change in transmittance is summarized in Fig. 2(b), where the transmittance at the wavelength, corresponding to the spectral dip position due to the LSPR observed before the heating, is plotted as a function of the heat treatment temperature. For the bare TiN array, the transmittance rapidly increases around the heat treatment temperature of 400 °C, and then is saturated to be unity. For the Al2O3- and Si3N4-coated arrays, the transmittances are constant up to 650 and 750 °C, respectively. These results indicate that the 10-nm-thick Si3N4 coating is more effective than the 50-nm-thick Al2O3 coating for the protection of the TiN nanocylinders from oxidation via reduction of oxygen diffusion at high temperatures . The effect of heating duration at 400 °C on the oxidation is examined in Appendix Fig. 10, where the bare TiN array is fully oxidized in 8 h.
Figure 3 shows SEM images after the heat treatment. For the bare TiN array, the cylinder shape is retained even after the heat treatment at 900 °C, at which TiN is oxidized to TiO2, as indicated by the transmittance measurements. Considering the molar mass and density of TiN (61. 9 g/mol and 5.4 g/cm3) and TiO2 (rutile, 79.9 g/mol and 4.23 g/cm3), the conversion from TiN to TiO2 is supposed to involve a volume increase of 59%. This corresponds to an isotropic expansion of 17%. The size change accompanying the oxidation is examined for the bare TiN array via close inspection of SEM images in Appendix Fig. 11, showing the isotropic increase of the volume of nanocylinders by 73%, which is similar to the expectation. For the array with the 50-nm-thick Al2O3 coating, the cylinder shape and the square arrangement are retained after the heat treatment at 900 °C. In contrast, the array with the 10-nm-thick Si3N4 coating exhibits a secondary phase after the heat treatment at 900 °C. We have not yet identified the composition of the secondary phase, although a preceding work on oxidation of Si3N4 reported the precipitation of cristobalite upon heat treatment higher than 1000 °C . X-ray photoemission spectra after the heat treatment at 900 °C (Appendix Fig. 12) indicates that Ti species migrate to the surface for the array with the 10-nm-thick Si3N4 coating, while the migration of Ti to the surface is largely suppressed for the array with the 50-nm-thick Al2O3 coating. The impurity phase is transparent in the range of the measurement considering the optical transmittance (Fig. 2) and has a minor impact on the far-field optical properties.
To reproduce the optical change with different heating temperatures for the bare array, we simulate the transmission using a surface-oxidized TiN nanocylinder (Fig. 4(a)). In this model, the air-side surface of the nanocylinder is changed to TiO2 with a volume fraction x, assuming that the oxidation starts from the air side. With increasing the volume of surface TiO2 layer, the dip in the simulated optical transmission becomes weak and slightly redshifts (Fig. 4(b)). This is in agreement with the experimental observation in Fig. 2(a). Instead of modelling a core-shell type cylinder, we also simulate the transmission with the modified dielectric function of TiN; the plasma frequency of the Drude term in the Drude-Lorentz function is changed from 6.45 to 5.45 eV to emulate the decrease of carrier density in the oxidation (see sec. 4.2. for the Drude-Lorentz model used to extract the dielectric functions). The result, shown by the dashed curve in Fig. 4(b), redshifts more than the experimental observation. The comparison in two models strongly suggests that the oxidation starts from the interface with air by the diffusion of oxygens. We also evaluate the effect of expansion of the nanocylinders after oxidation by increasing the size of cylinder in the simulation model. The resonance dip becomes deeper while the spectral position and the width do not depend on the size severely by the volume expansion up to 60 %, which is the experimentally observed value in Appendix Fig. 11.
We also simulate the spatial light energy distribution under the illumination at the resonance conditions (Fig. 5). The accumulation of light energy at the edge of the nanocylinder is observed for both the bare array and the coated arrays, and the intensities of the accumulated light are comparable among them. The accumulated light is fully localized inside the coating layer and the substrate for the array with the 50-nm-thick Al2O3 coating, while it is extended to the air for the array with the 10-nm-thick Si3N4 coating. This field distribution indicates that for applications that utilize the local field enhancement such as sensing, the 10-nm-thick Si3N4 is more beneficial.
2.3 Oxidation as a route for two-dimensional TiO2 nanostructures
The preservation of the shape and the arrangement even after the heat treatment at higher temperatures implies that a TiO2 nanocylinder array can be synthesized through the oxidation of a TiN array. We measured the optical transmittance of the oxidized array as a function of the angle of incidence (Fig. 6). The homogeneous refractive index around the array is critical to induce the photonic band structures via in-plane diffraction (see Fig. 6(a)). When the nanocylinders are surrounded by air, the extinction at normal incidence shows a broad peak around λ = 500 nm due to the Mie resonance in each TiO2 nanocylinder. In contrast, the immersion of the array with oil (Cargille) and the coverage with a silica glass plate leads to a sharp extinction resonance at λ = 535 nm due to the diffractive coupling of Mie resonances. The narrow peak means a strong light confinement caused by this mode. The quality factor of this mode at λ = 535 nm is ∼ 45, which is comparable to the highest value reported for TiO2 nanostrutures . The dispersion relation is better visualized by plotting the extinction as a function of in-plane wavevector (see Figs. 6(c) and (d)). The homogeneous refractive index around the array narrows the photonic bands, which follow the condition of in-plane diffraction, i.e., Rayleigh anomaly. A photonic bandgap appears at 2.6 eV and k|| = 9.3 mrad nm−1 as indicated in Fig. 6(d) by dotted circle. The clear observation of photonic bands indicates that the TiO2 nanocylinder arrays are useful as high-refractive-index metasurfaces with low optical losses.
In summary, we investigated the oxidation behaviors of the TiN nanocylinder arrays under heat treatments in air. The ALD coating helped the suppression of oxidation. The temperature at which the oxidation starts to occur is increased to 600 and 750 °C for the 50-nm-thick Al2O3 and the 10-nm-thick Si3N4 coatings, respectively, although a precipitate was observed at higher temperatures for the 10-nm-thick Si3N4 coating. For applications that utilize the local field enhancement such as sensing, the 10-nm-thick Si3N4 is more beneficial, while for light-to-heat conversion, both Si3N4 and Al2O3 would not hinder the heat generation of the TiN nanostructures thanks to their high thermal conductivities. We also demonstrated that the oxidation of the TiN nanocylinder arrays fabricated by the selective dry etching could provide TiO2 nanocylinder arrays without severe pattern destruction. The TiO2 array obtained works as a 2D photonic crystal and clearly shows a photonic bandgap.
4.1. Array fabrication and optical characterization
A TiN nanocylinder array was fabricated from a TiN thin film (thickness = 100 nm, purchased from Geomatic) on a silica glass substrate by nanoimprint and reactive-ion etching technologies . The array before coating with the insulator layers is referred to as bare array. On top of the bare array, Al2O3 (50 nm) or Si3N4 (10 nm) was deposited by ALD (FlexAL, Oxford). The arrays were heated in air from room temperature (∼20 °C) at a rate of 2 °C/min, kept at the preset temperature for 2 h, and cooled to room temperature in the furnace. The morphologies of the arrays were characterized by scanning electron microscopy (SEM, SU8000, Hitachi). The zeroth-order optical transmittance at normal incidence (θin = 0 °) was measured at room temperature (∼20 °C) by an ultraviolet-visible-near-infrared spectrophotometer (V770, JASCO) using an unpolarized light source with a silica glass as a reference. A varied-incident angle optical transmission analysis was performed using a home-made setup with a halogen lamp as a light source [38,39].
4.2. Dielectric function of TiN
The dielectric function of the TiN thin film was examined by using the spectroscopic ellispometry. We used a three-layer model to fit the reflection spectra obtained comprising a SiO2 substrate, the TiN layer and air on the top. As the dielectric constant of TiN, we used a function with one Drude term and two Lorentz terms, where the Drude term presents the contribution from conduction band, while the Lorentz terms correspond to the interband transition [5,40]
The optical characteristics of the arrays were simulated using the finite-element method (COMSOL Multiphysics). Three-dimensional models were used with periodic boundary conditions on the lateral coordinates to model the square lattice with a periodicity of 350 nm. The simulated structures consisted of a SiO2 glass substrate and TiN nanocylinder. The shape and the size of nanocylinder were selected to obtain the best-fit to the experimental transmission. To simulate the variation of optical transmission with heat treatment temperatures, the air-side surface of the TiN nanocylinder was replaced by TiO2. For the coated samples, the conformal layer of 50 nm Al2O3 or 10 nm Si3N4 were set on the structure. The refractive indices (n) and extinction coefficients (k) were deduced from the fits to the spectroscopic ellipsometry data for TiN and TiO2; the n of the SiO2 glass substrate was set to 1.46. A plane wave with an electric field oscillating in the x-direction was incident from the top boundary at normal incidence to investigate the optical response of the model.
Ministry of Education, Culture, Sports, Science and Technology (19H02434, Nanotech Cupal), Asahi Glass Foundation.
We thank Dr. Osamu Tomita (Kyoto University) for the X-ray photoemission spectra measurements. This study was partly supported by the Nanotechnology Hub (Kyoto University), Nanofabrication Platform (National Institute for Material Science), and Nano-Processing Facility (National Institute of Advanced Industrial Science and Technology) with the Nanotechnology Platform Project sponsored by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.
The authors declare no conflicts of interest.
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