We have fabricated two-dimensional periodic arrays of titanium nitride (TiN) nanoparticles from epitaxial thin films. The thin films of TiN, deposited on sapphire and single crystalline magnesium oxide substrates by a pulsed laser deposition, are metallic and show reasonably small optical loss in the visible and near infrared regions. The thin films prepared were structured to the arrays of nanoparticles with the pitch of 400 nm by the combination of nanoimprint lithography and reactive ion etching. Optical transmission indicates that the arrays support the collective plasmonic modes, where the localized surface plasmon polaritons in TiN nanoparticles are radiatively coupled through diffraction. Numerical simulation visualizes the intense fields accumulated both in the nanoparticles and in between the particles, confirming that the collective mode originates from the simultaneous excitation of localized surface plasmon polaritons and diffraction. This study experimentally verified that the processing of TiN thin films with the nanoimprint lithography and reactive ion etching is a powerful and versatile way of preparing plasmonic nanostructures.
© 2016 Optical Society of America
22 January 2016: Corrections were made to the body text and acknowledgments.
Metallic nanostructures can manipulate light at the nanoscale through the excitation of surface plasmon polariton(SPP), which is the collective oscillation of conduction electrons coupled to lightwaves. The science on SPPs, i.e., plasmonics, has been extensively explored to open a broad range of applications [1, 2]. With the advance in nanofabrication techniques, various metallic nanostructures, such as bow-ties [3,4], Yagi-Udas [5,6], and periodic particle arrays [7–11 ] to name a few, have been produced to achieve the designed plasmonic responses.
As the expansion of fundamental and application research in plasmonics, there is an increasing demand on better materials [12, 13]. The demand for lowering optical loss is in particular strong, since it directly improves the performance of plasmonic devices. The search for materials complementary to gold and silver has been launched to find several important groups of materials, such as nitrides working in the visible region [14, 15] and transparent conducting oxides in the infrared [16–18 ]. In parallel to the quest for new materials, an effort for improving the crystal quality has been devoted to minimize the unwanted scattering of the conduction electrons at the defects and grain boundaries. Better plasmonic responses for nanostructures made of high-quality crystals has been demonstrated for noble metals [19–21 ] and some complementary materials [15,22].
Another important and required property for plasmonic materials is the processability. Although gold and silver show excellent plasmonic responses, they cannot be nanostructured with selective dry etching techniques. This limitation makes the fabrication of nanostructures of gold and silver complex and tricky. Titanium nitride (TiN) has been proven to be a promising material having the compatibility with nanofabrication techniques. TiN, which is composed of abundant elements of titanium and nitrogen, has a gold-like color together with a high thermal stability and a mechanical hardness. The thin-film fabrication techniques have been established for TiN because of its technological and industrial importance [23–26 ]. For plasmonic applications, TiN/(Al,Sc)N alternating epitaxial layers have been fabricated by sputtering so as to explore their properties as the hyperbolic metamaterial . Lithographical fabrication of TiN nanoparticles has also been studied, where the array of TiN nanoparticles has been prepared by a lift-off process using a chromium (Cr) mask to examine the local heating effect . In electronics, TiN is incorporated in silicon-based complementary metal oxide semiconductor (CMOS) devices as a diffusion barrier layer, so that it can be a material that merges plasmonics with electronics. Fabrication of plasmonic strip waveguide for 1.55 μm light was demonstrated for the future implementation of plasmonic interconnects in electronic devices .
In the present study, we have fabricated periodic arrays of TiN nanoparticles by taking advantage of its compatibility with nanofabrication technique. Epitaxial thin films of TiN were firstly prepared by using a pulsed laser deposition (PLD) method. The thin films prepared were then processed with nanoimprint lithography and reactive ion etching (RIE) to make periodic arrays of TiN nanoparticles. The choice of periodic array comes from its ability to support collective plasmonic mode; thanks to the periodicity on the order of the light wavelengths, the localized SPPs excited on each nanoparticles are coupled through diffraction [8, 9, 30–33 ]. The collective mode accompanies the intense field spatially extended in the plane of the array, so that it is advantageous for many optical applications including fluorescence enhancement [34, 35], surface-enhanced Raman scattering , and solar cells. It is noted that the collective mode was not observed in the previous work on TiN nanoparticle array  because of the mismatch between the wavelength of localized SPP resonances and that of the diffraction. We have prepared two different arrays having the same pitch of the periodicity but different particle sizes to tune the position of the localized SPPs. Numerical simulation(COMSOL) showed that the collective plasmonic modes are associated with the accumulation of electromagnetic field both on the particles and in between the particles.
2.1. Fabrication of thin films
TiN thin films were fabricated by using a PLD technique on MgO and Al2O3 (sapphire) substrates. Synthetic MgO (100) single-crystal substrates were heated at 1300 °C in O2 for 10 h, and sapphire (0001) substrates were heated at 1000 °C in air for 3 h, and then further heated at 750 °C in air for 3 h to obtain an atomically flat surface. A dense TiN ceramics (Toshima, Japan) was utilized as the PLD target. Background pressure of the deposition chamber was 1 × 10− 3 Pa, and the nitrogen gas pressure was kept at 1 Pa during the deposition. A KrF excimer laser beam (wavelength, 248 nm; repetition frequency, 5 Hz) was focused onto the ceramic target with a photon power density of about 1.8 J/cm2/pulse. The substrate, facing the target, was rotated mechanically and maintained at 780 °C during the deposition.
2.2. Characterization of thin films
Crystalline orientation was analyzed by x-ray diffraction (XRD) (ATX-G, Rigaku Co.) using the Cu Kα radiation. The surface morphology of the thin films was observed by atomic force microscopy (AFM). Temperature dependence of resistivity of the thin films was measured by using a standard four point probe in a Quantum Design physical properties measurement system (PPMS). The dielectric function of the thin films was examined by spectroscopic ellipsometry (FE-5000, Otsuka Electronics Co.) for the wavelengths between 300 to 800 nm. The thickness of the thin films was evaluated to be about 130 nm by a surface profiler.
2.3. Fabrication of nanoparticle arrays
For the fabrication of TiN nanoparticle arrays, we have used 30 nm thick TiN thin films on sapphire substrate. The process flow is schematically shown in Fig. 1. First, the thin films were deposited with a resist(TU2-170, thickness = 200 nm) and pre-baked for 5 min at 95°C. Separately, a silicon mold consisting of the periodic square array of cylinders (diamater 150 nm, height 200 nm, array pitch 400 × 400 nm),was prepared by electron beam lithography(F7000s-KYT01, Advantest) and silicon deep etching(RIE-800iPB-KU, Samco). Then, the surface structure of silicon mold was duplicated on the resist by nanoimprint lithography (EntreTM3, Obducat). The sample was then structured by RIE(RIE-101iPH, Samco). As the etching gas, the mixture of Ar, BCl3, and Cl2 was used with the flow rates of 8, 5, and 15 sccm, respectively . The pressure was set to 2 Pa, the rf power 700 W, the bias DC power 100 W, and the typical etching time 40 sec. The residual resists were removed by O2 ashing (RIE-10NR, Samco).
2.4. Characterization of nanoparticle arrays
The surface structure of the array was examined by using a scanning electron microscope(SEM)(SU8000, Hitachi). The zeroth order transmission was measured as a function of the angle of incidence. The samples were mounted on a computer controlled rotation stage, and rotated to change the angle of incidence on the array. As an incident light, a linearly collimated beam(beam diameter of 1 mm) from a halogen lamp was used. The absolute zeroth-order transmission was obtained by normalizing the transmission of the incident light through the samples to that of the substrate.
3. Results and discussion
3.1. Structural and electrical characterization of thin films
The thin films prepared show a metallic and gold-like appearance (see the left inset in Fig. 2(a)). Figures 2(a) and 2(b) show the out-of-plane XRD patterns of the TiN thin films grown on sapphire (0001) and MgO (100) substrates, respectively. In Fig. 2(a), diffraction peaks of TiN 111 and 222 are observed together with the sapphire 0006 peak, indicating the oriented deposition of TiN in 〈111〉 direction. The 〈111〉 -oriented growth was also observed for sputter -made TiN thin films on sapphire (0001) . In-plane XRD pattern (data not shown) indicates the thin film is grown epitaxially, with the in-plane orientation of sapphire 〈10 0〉 // TiN 〈110〉. The rocking curve of the main TiN diffraction peak (see the right inset) shows the full width at half maximum of 0.038 °, showing a good crystallinity with a low mosaicity. For the thin film on MgO (100)(Fig. 2(b)), an intense peak due to 200 reflection from the thin film is found at 2θ = 42.59 °, along with that from the substrate at 42.91°. The growth orientation is different from that on sapphire because of the very small lattice mismatch between TiN (100) and MgO (100). The out-of-plane lattice constant calculated by using the Bragg equation is a = 0.4245 and 0.4215 nm for TiN thin film and MgO substrate, respectively.
Figure 2(c) shows a high-resolution reciprocal space map of XRD for the TiN thin film on MgO, measured around the reflection from TiN (113) plane. The TiN 113 reflection was aligned vertically to the reflection of MgO 113. The calculated position for the bulk TiN 113 with the lattice constants of a = b = c = 0.4242 nm  was also indicated in the figure. Compared to the bulk TiN, the lattice of the thin film shrunk along the  direction by 1 % to fit the MgO lattice. In contrast, the lattice constant along the out-of-plane direction is similar to its bulk value, indicating no strain is applied in this direction. The width of the TiN spot in  direction (along x axis in the figure) is narrower than that of the MgO substrate, demonstrating the excellent quality of the epitaxial thin film with no relaxation and no mosaic disorder in lateral direction.
The good quality of the thin film is also manifested by the surface morphology. The AFM image in Fig. 3(a) shows that the thin film on MgO has a very regular step and terrace structure. The cross sectional profile (right panel of Fig. 3(a)) shows a step height of approximately 1.17 nm, corresponding to the three times of the (100) plane spacing of TiN. The average roughness (Ra) of a single terrace is < 0.1 nm. The AFM measurement clarifies that the thin film on sapphire is not atomically flat (data not shown), but the surface is also smooth (Ra = 0.23 nm with a scan size of 5 × 5 μm).
In order to examine the stoichiometry of the thin film, the temperature dependence of resistivity was measured for the TiN thin film on MgO (Fig. 3(b)). With the decrease in temperature, resistivity becomes lower, and drops abruptly at 2.4 K, showing a transition to superconducting phase. TiN is a superconductor with T c = 5 K, and the transition temperature varies with stoichiometry, i.e., T c varies with x of TiN1− x . Comparison with literature suggests the composition of the present film is TiN0.97, i.e., the amount of defects is small and the film is close to stoichiometry.
3.2. Dielectric function of thin films
The dielectric function of the TiN thin films was examined by using a spectroscopic ellipsometry. In the experiment, we measured the reflectance ratio for p- and s-polarized light, r p/r s = tan(Ψ)exp(iΔ), where tan(Ψ) and Δ are the amplitude ratio and the phase shift, respectively. The data were analysed with a three layer model comprising a substrate(sapphire and MgO), the TiN layer and air on the top. The dielectric function of the substrate was determined by a Cauchy fit, and that of TiN was modelled by one Drude and two Lorentz terms(eq.(1)), where the Drude term presents the contribution from the conduction electrons, while the Lorentz terms correspond to the interband transitions :Figure 4 shows the experimental data and the fitting results for the TiN thin films on sapphire and MgO. The agreement between the ellipsometry data and the fitting function was reasonably good. The fitting parameters used are summarized in Table. 1. We can fit the experimental data with reasonable values, which were comparable to those reported in literature .
The dielectric functions calculated from the fit are shown in Figs. 5(a) and 5(b), together with that of one of the best-quality epitaxial thin films in literature by Naik et al. . In the real part, ε′ of our thin films becomes negative, i.e., they behave as metals, for the wavelengths longer than 475 nm for the thin film on MgO and 465 nm for that on sapphire. The real part in dielectric constant reflects the polarizability of the material, and negative large ε′ means a strong screening effect toward the incident light. The magnitude in ε′ of our thin films in metallic region is larger than that of the thin film by Neik et al., indicating the higher screening effect of our thin film. The imaginary part, ε″, shows a local minimum at around 420 nm for our thin films. The rise in ε″ at shorter wavelengths is due to interband transitions , and that with increasing wavelength is due to the oscillation loss of conduction electrons. Compared to the thin film by Naik et al., our thin films have a larger ε″ or a larger optical loss especially at longer wavelengths, which is partly the cost of the larger screening effect. It should be noted that because of their high crystal quality, ε″ of our thin films is still smaller than those of many other TiN thin films in literature [14,15,40].
In Figs. 5(c) and 5(d), we compare the quality factors for SPP propagation, defined as QSPP = ε′ 2/ε″, and localized SPP resonance, QLSPR = −ε′/ε″ . For the performance of plasmonic material, both ε′ and ε″ are important because ε′ dominates the field distribution and the loss depends on ε″. Our thin films show larger QSPP and QLSPR for shorter wavelength region of the plot, indicating a better performance of our thin films as supporting media for SPPs in this wavelength range.
3.3. Plasmonic properties of periodic array
For the fabrication of nanoparticle array, we have prepared 30 nm thick TiN thin films on sapphire substrate. We selected the thin films on sapphire because of its higher quality factors. Figure 6 shows the SEM top-view image of the nanoparticle arrays. Both the arrays A and B were nanostructured by using the same silicon mold, and the size of the nanoparticles is controlled by the etching time; the arrays A and B consist of the square array (400 nm pitch) of cylinders with the diameter of around 180 and 260 nm, respectively. We defined x and y directions on the sample as in Fig. 6 for the optical transmission experiment.
Figure 7 shows the optical transmission spectra for p-polarized light as a function of incident angle. The incident angle was varied so as to provide momentum only in x direction. For array A (Fig. 7(a)), a dip in transmission appears at 1050 nm at normal incidence, which corresponds to the excitation of localized SPP resonance at each nanoparticles. For array B, a deeper dip appears at a longer wavelength around 1150 nm, reflecting its larger nanoparticle size compared to that in array A. In the plots, the dotted line indicates the Rayleigh anomaly (RA) of the array, or the onset of diffraction, i.e., the wavelength λ and the angle of incidence θ in at which a diffracted wave is grazing to the plane of the array. When the incident wave vector where n is the refractive index in the plane of the array) does not have a component in the y direction, the RAs satisfy the relation8,9,30] known as surface lattice resonance(SLR) [31–33 ,35].
We simulated the transmission spectra by using finite element method (COMSOL). The model consisted of the sapphire substrate(n = 1.79), a TiN cylinder(diameter 180 or 260 nm, height 30 nm, dispersive complex refractive indices from the ellipsometry)and air(n = 1.00) on the top. The unit size of the cell is 400 nm × 400 nm × 2000 nm, and the periodic boundary condition was applied to x and y directions. A plane wave was incident from the top of the model, and the perfectly matched layer was set on the bottom. The simulated transmissions are exhibited in Figs. 7(c) and 7(d), showing a good agreement with the experiment. In order to visualize the field distribution in the sample, we plot in Figs. 7(e) and 7(f) the field distribution in the xy plane 20 nm above the substrate surface for the model with nanoparticle diameter of 180 nm, for λ = 1000 nm at two different incident angles of θ in = 0 and 46 °(indicated by the stars in Fig. 7(c)). At θ in = 0 °(Fig. 7(e)), the field is accumulated at the surface of nanoparticles, indicating the excitation of localized SPPs. In contrast, when θ in = 46 °(Fig. 7(f)), not only in the nanoparticles, but also the field is accumulated in between the particles. This indicates both the localized SPPs and diffraction are excited simultaneously on this condition.
In this study, we have fabricated plasmonic arrays of nanoparticles from TiN thin films. Atomically flat surface was achieved for the thin film deposited on MgO substrate. Dielectric function was deduced from ellipsometry to find that our thin films have reasonably good quality factors as SPP-supporting media in the visible and near infrared. By using nanoimprint lithography and RIE, the thin films of TiN were nanostructured to periodic arrays of nanoparticles. The TiN array has many potential applications due to its ability to support collective plasmonic mode. We believe that the present work shows the possibility of exploring plasmonics with TiN, a thermally stable and mechanically strong material which comprises abundant elements of Ti and N. It is also noted that the simple strategy to make plasmonic nanostructure in this work is robust and applicable not only to periodic arrays but also to various complex structures.
We thank Prof. I. Kakeya(Kyoto U.) for transport measurements. A part of this work was supported by Kyoto University Nano Technology Hub and NIMS Nanofabrication Platform in ”Nanotechnology Platform Project” sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Financial support from the Asahi Glass Foundation, the Yazaki Memorial Foundation for Science and Technology, and from Grant-in-Aids for Scientific Research (C, No. 24560824) and Challenging Exploratory Research (No. 26620192) from MEXT were gratefully acknowledged.
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