Fabrication of Highly Metallic TiN Films by Pulsed Laser Deposition

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Article Cite This: ACS Photonics 2018, 5, 814−819

Fabrication of Highly Metallic TiN Films by Pulsed Laser Deposition Method for Plasmonic Applications Ramu Pasupathi Sugavaneshwar,*,† Satoshi Ishii,*,† Thang Duy Dao,† Akihiko Ohi,‡ Toshihide Nabatame,‡ and Tadaaki Nagao*,†,§ †

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Namiki Foundry, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan § Department of Condensed Matter Physics Graduate School of Science, Hokkaido University, Kita-10 Nishi-8 Kita-ku, Sapporo 060-0810, Japan S Supporting Information *

ABSTRACT: We report the fabrication of titanium nitride (TiN) films with the “best” plasmonic behavior reported so far by the pulsed laser deposition method. Even though the deposition is done at room temperature (∼25 °C) and grown on an amorphous native oxide of a silicon wafer, the plasmonic property of the TiN is comparable to that of gold, which is a conventional plasmonic material in the visible to near-infrared region. Because of the highly plasmonic nature of the TiN, the near field around the TiN nanostructure can be as high as that of a gold nanostructure. A room-temperature process without a strict requirement on the substrate allows depositing a TiN film even on a flexible polymer film without degrading its property. Our results pave the way for using TiN as a truly practical plasmonic material, replacing the use of noble metals. KEYWORDS: thin film, pulsed laser deposition, plasmonics, titanium nitride, flexible device

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exhibiting a high melting point.21−24 Over the years, research on plasmonic TiN has branched out to various fields, and it is currently being studied for various applications such as photothermal solar-heat converters,13 photothermally activated shape memory materials,25 heat-assisted magnetic recording (HAMR),26 and self-powered flexible photodetectors.27 In some of these applications there is an inevitable need for alternative plasmonic materials with significantly improved performance comparable to conventional plasmonic materials. For example, in applications for HAMR, which utilizes nearfield photothermal effects26 where currently Au is being used, there is a problem of shape distortions during the intense laser illumination. TiN is expected to substitute Au due to its comparable plasmonic properties at high temperatures.24−29 Although there are a number of investigations to improve and optimize the dielectric function of TiN, most of the works were done by heating the substrates and using lattice-matched template crystals such as sapphire or MgO.30−34 These two factors are serious drawbacks in advancing the plasmonic applications of TiN such as for applications in silicon microelectronics or flexible electronics. Herein, we report the fabrication of TiN films with “the best metallic behavior” reported to date grown at room temperature (∼25 °C) on a

esearch on plasmonics has been pursued widely by a tremendous number of researchers in various fields especially in the last two decades due to the exciting and promising results in several different application areas.1,2 Initially coinage elemental metals such as gold, silver, and copper were considered for the applications of plasmonics such as surface-enhanced Raman scattering3,4 and infrared absorption spectroscopy.2−6 But as the plasmonics field is progressing toward a broad research realm, the efforts toward finding optimum materials for targeted spectral windows become more and more important for successful applications.7−9 Major drawbacks in using elemental metals are inability to tune their dielectric properties and optical losses, and research on compound plasmonic materials is expected to overcome these issues.10 Currently, refractory conductive ceramics, or cermets, are attracting strong interest due to the recent progress of plasmonics in high-temperature applications,11,12 photothermal conversion,13 sensing,14 and active plasmonics.15 Titanium nitride (TiN) has been one of the most investigated compound plasmonic materials so far. While there are several earlier works on the plasmonic properties of TiN,16−18 some recent reports8,19,20 demonstrated that TiN offers good performance for many plasmonic applications. There are various advantages over conventional plasmonic materials, not only being cost-effective but also being CMOS compatible, being chemically and mechanically stable, and © 2017 American Chemical Society

Received: August 21, 2017 Published: December 21, 2017 814

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ACS Photonics SiO2/Si substrate by the pulsed laser deposition (PLD) method. We have successfully utilized this advantage to demonstrate the single-step fabrication of a plasmonic TiN film on a flexible polymer thin film for the first time. We have also estimated the absorption and scattering efficiency, electric field intensity, and reflectance spectra for TiN nanostructures and clarify that most of the optical characteristics of TiN are rather similar to those of Au. Figure 1 shows the dielectric functions (i.e., real (ε′) and imaginary (ε″) parts of the permittivity) of the four TiN films

Figure 2. Comparison of the measured permittivity data with previous reports on TiN against Au, W, and Mo. (a) Real part; (b) imaginary part. Permittivity data for TiN-R1 and TiN-R2 are extracted with permission from ref 19 (2012 OSA) and ref 21 (2015 OSA). Permittivity data for Au are from refs 37 and 38. Permittivity data for Mo and W are extracted from refs 39 and 40 for comparison.

permittivity of Au is more negative than all the TiN films. Nonetheless, in the visible and NIR region up to ∼1300 nm, our TiN film shows rather similar values to Au and better values than refractory metals such as Mo and W. Furthermore, the metallicity of the TiN films prepared by the PLD method are several times higher than that of the TiN films prepared by the other methods under similar low-temperature deposition conditions. Also our TiN shows better metallicity compared to the TiN film fabricated by sputtering with similar thicknesses (Figure S2 Supporting Information). Figure 3 represents the figure of merit (FOM) for different TiN films compared with Au. The FOM value of Au is higher

Figure 1. Complex permittivities of the TiN films deposited at different rates of deposition. (a) Real part and (b) imaginary part.

retrieved via spectroscopic ellipsometry (spectral range from 0.3 to 3 μm, angles of incidence varied from 50° to 70° in 10° steps). The samples were fabricated by the PLD method on SiO2/Si substrates using a TiN target. Each deposition was done at the identical condition except for the deposition rate; the rates for TiN-1, TiN-2, TiN-3, and TiN-4 were 0.1, 0.07, 0.05, and 0.03 Å/s, respectively, which were controlled by adjusting the deposition rate from 10 to 2 Hz. All the films exhibit excellent metallic behavior (negative permittivity, real part) in the visible to near-infrared spectral regions for wavelengths longer than ∼440 nm. The metallicity (negative real permittivity) is greater with films deposited with a higher rate of deposition. The accuracy of the extracted complex dielectric function of TiN films from ellipsometry fitting (Figure S1 Supporting Information) has been ensured by the agreement between the thicknesses extracted from ellipsometry and the thicknesses measured by a stylus profilometry, where the errors were within 5%. The Hall measurement was also performed to evaluate the carrier concentrations and showed good agreement with the highly metallic nature observed with the ellipsometry results. Generally nitrides have a carrier concentrations in the range of ∼1022 cm−3. In our case too we observed high carrier concentrations of around (7−8) × 1022, which is close to the range of elemental metals (Table S1 Supporting Information) in all the TiN films. Also the measured values were consistent over a period of time (4 months), which indicates the stability of the films. Here it should be noted that the carrier concentrations of the TiN films is calculated with the thickness values obtained from profilometry, which has around 5% variation. Furthermore, although the carrier concentration is an important factor for achieving metallicty, the metallicity of the TiN is also dominated by the carrier relaxation time. The low mobility values in the TiN films deposited with a lower rate reflects more defects in the films, which reduces the carrier relaxation time and thereby leads to low metallicity in these films.35,36 As a comparison, the permittivities of the TiN films deposited by the PLD method (TiN-1) and other literature values19,21 together with conventional metals37−40 are plotted in Figure 2. The figure clearly shows that the real part of the

Figure 3. Figure of merit (FOM) −ε′/ε″ of different TiN films compared with Au. Permittivity data for Au are extracted from refs 37 and 38 for comparison.

than that of TiN films. The FOM of TiN-1 is lower mainly due to the high loss as shown in Figure 2(b). It should be noted that in most SPP waveguide applications, the FOM is represented as shown in Figure S3 (Supporting Information). The FOM values of the TiN films here are smaller than those of Au, which means that the TiN films are not suitable for SPP waveguide applications. Our TiN has large negative ε′ values comparable to Au and larger ε″ values compared to Au. This means that it exhibits a large enough plasmonic response compared with Au and shows swifter conversion from light energy into heat/electrical energies than Au. Therefore, our TiN will be more suitable for energy conversion purposes such as for photothermal and photocatalytic applications. The current TiN-1 and TiN-2 show better FOM values compared to the other fabricated TiN films, which suggests they could have significant impact in practical applications.41 The surface profiles of the four TiN films obtained by atomic force microscopy (AFM) are shown in Figure 4. The rootmean-square (rms) roughness values of TiN-1, TiN-2, TiN-3, and TiN-4 films are 0.4, 0.5, 0.7, and 0.9 nm, respectively. Moreover the use of high laser pulse energy and high rate of 815

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at room temperature. Figures 5(a) and (b) show AFM images of poly(methyl methacrylate) (PMMA) of 90 nm thickness

Figure 4. AFM images of the (a) TiN-1 (b) TiN-2 (c) TiN-3, and (d) TiN-4 films on SiO2/Si.

deposition resulted in ablation of high enough TiN flux leading to a more two-dimensional growth (Figure S4 Supporting Information) with connected and closed morphology of the films.42,43 The θ−2θ X-ray diffraction (XRD) pattern (Figure S5 Supporting Information) can be indexed to TiN with the Fm3m space group, which confirms that our TiN films indeed formed a TiN phase. The pattern exhibits a polycrystalline nature, and from Halder−Wagner analysis, the grain sizes of the TiN films were determined to be 9.9, 11.5, 11.7, and 11.0 nm (Table S1 Supporting Information) for the TiN-1 TiN-2, TiN3, and TiN-4 films, respectively. Generally, the small grain size of a film will cause heavy carrier scattering and will increase the loss of the film. However, in spite of the small grain sizes of the current TiN films, the imaginary part of the permittivity values are only 4 times than that of Au.31 In our experiments we have adopted nitride targets in PLDgrown TiN films, although it has been viewed as a challenging method due to its high melting point and low vapor pressure.8 In our case we are successful in fabricating high-quality metallic TiN films, and furthermore, the metallicity can be varied by changing the rate of deposition, as we have shown above. Mainly for TiN films to be metallic it has been earlier reported that the stoichiometric ratio of Ti to N should be closer to 1.17 This has been demonstrated by results obtained under different nitrogen ambient, where low nitrogen partial pressure leads to higher metallicity (with higher loss), and TiN films with a more dielectric nature (with lower loss) resulted under high nitrogen partial pressure.19,20 We performed X-ray photoelectron spectroscopy (XPS) measurements to confirm the stoichiometry of the present TiN films (Figure S6 Supporting Information). The stoichiometric ratios of Ti and N in the films are closer to 1 (slightly N deficient), which is in agreement with the literature (Figure S6 Supporting Information).17 Obtaining precise stoichiometry for TiN-1 to TiN-4 was limited by the shortcomings of the XPS techniques because of the formation of surface oxide phases as the XPS measurements were done ex situ, but a major component of the TiN phase is suggested to be N-deficient TiN.44,45 We have also tried to deposit our TiN film on an organic thin film to demonstrate the advantage of high-quality film growth

Figure 5. (a) AFM image of PMMA-coated SiO2/Si substrate. (b) AFM image of the TiN film grown on the same PMMA substrate. (c, d) Complex permittivities of the TiN films deposited on different substrates. (c) Real part and (d) imaginary part. (e) Optical photograph of Au and TiN films on Si/SiO2 and PMMA. (f) Optical photograph of the TiN film on a PET substrate.

coated on a Si wafer and the TiN film deposited with a thickness of 180 nm at ∼0.1 Å/s rate of deposition on the PMMA. Roughness on the pristine PMMA template is around 2.0 nm (in rms value), and surface roughness of the TiN films grown on the PMMA film is around 6.0 nm. Figures 5(c) and (d) show the real and imaginary parts of the permittivity. Here, the metallicity is slightly lower than the value obtained on SiO2/Si with a similar rate of deposition; however, the color in the visible region is similar to that of the Au film as seen in Figure 5(e). We also did Halder−Wagner analysis in XRD (Figure S5 Supporting Information), and the grain size of the TiN films was determined to be 8.0 nm, which lead to loss comparable to that of films deposited on SiO2/Si. This value is slightly smaller than the values from the TiN on SiO2/Si substrate possibly due to the film growth with smaller diffusion length on the rough PMMA film. We believe that these results demonstrate the prospect of fabricating plasmonic TiN films on various types of nonstandard substrates including organic flexible substrates such as poly(ethylene terephthalate) (PET) (Figure S7 Supporting Information), as shown in the inset of Figure 5(f). This would play an important role in significantly widening the applications of this material in the emerging areas of research.11,23−25 To demonstrate the plasmonic properties of our TiN, we compare its plasmonic properties to those of gold in three representative configurations either analytically or numerically. Figures 6(a) and (b) show the analytically calculated scattering and absorption efficiencies of 50 nm radius spheres in water, respectively. The peak heights of the TiN nanosphere are in 816

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Figure 6. Analytically calculated (a) scattering and (b) absorption efficiencies of 50 nm radius spheres in water for the TiN-1 and gold. (c) Analytically calculated maximum near-field intensity around the nanospheres made of TiN-1 and gold in dipole approximation. (d) Angular-dependent reflectance of the TiN films on a glass substrate at 850 nm. The thicknesses of the TiN films are 20, 30, and 40 nm. Permittivity value of Au is obtained from ref 37. Schematic representations are provided in the inset for corresponding simulations. (e) Schematic representation of the TiN circular disks arranged in hexagonal arrays on a glass substrate used for the simulation (f). (f) Simulated transmittance spectra of the TiN-1 nanodisk hexagonal arrays with varying the diameter D (0.2−0.8 μm) of the disk and the periodicity P (P was chosen to be 2D) of the arrays while keeping the same TiN thickness of 50 nm.

In Figure 6(d), we also examined the reflectivities of our TiN films in the attenuated-total-reflection (ATR) geometry with Kretschmann configuration. There are sharp peaks that correspond to surface plasmon resonances (SPRs). The SPRs exhibit sharp enough features (e.g., at 42°) in the ATR spectra that are compatible with SPR sensing, indicating the suitability of our TiN film for sensing applications. Figures 6(e) and (f) represent the schematic and simulated transmittance spectra (calculated using the RCWA method, Synopsys’ RSoft) of the hexagonal TiN nanodisk arrays (50 nm thick) with different diameters (D, varied from 0.2 to 0.8 μm) and periodicities (P, chosen to be 2D) on a glass substrate. This result demonstrates the excellent tunability of plasmon resonance from the visible to the NIR region. Here it should be noted that the bandwidth of the TiN nanostructure is better in the NIR region compared with another report.21 Referring to refs 21 and 48−50, we believe our TiN can be also used in tunable mid-IR plasmonic nanostructures to design welldefined resonances. These results show that our fabricated TiN film could be a promising candidate for various plasmonic device applications including applications in high-temperature plasmonic devices.11,13 To summarize, we have investigated the optical properties of TiN films deposited by the PLD method on Si/SiO2 and organic substrates. Surprisingly, even at room temperature and without using lattice-matched substrate, we have succeeded in achieving comparable metallicity to Au from the visible to NIR region. We have estimated that the absorption/scattering efficiencies and local electric field intensity of the TiN nanospheres are almost comparable to those of Au nanospheres. Since TiN shows higher mechanical/thermal stability and has better cost effectiveness than Au, it is considered a practical plasmonic material that can replace noble metals. Our proposed fabrication method further widens the use of TiN by providing a simple fabrication route adoptable for Si microdevices and for flexible electronic/photonic devices as well. We believe that the knowledge provided by this work would lead to a broad avenue for realizing various types of TiN-based plasmonic nanostructures for futuristic applications.

similar regions to those of the gold nanosphere. It could also be observed that the TiN nanosphere offers better performance (scattering and absorption) in the NIR region compared to the Au nanoparticle. The bandwidth of the TiN nanosphere is comparable to the TiN nanostructures fabricated with the TiN that are fabricated at higher temperatures.29,30 Also, since the FOM of the spherical nanoparticle are closer to 1, which could be suitable for energy harvesting applications.44 This advantage could be utilized with a variety of perfect absorber designs for efficient performance in solar thermophovoltaics applications.11,24,39,46,47 Figure 6(c) represents the maximum field intensity at the nanosphere in the dipole approximation limit. Similar to the cases with scattering and absorption efficiencies that are in the far field, the near-field intensities of the TiN and gold nanospheres are comparable in the visible region. However, the TiN nanosphere shows better performance than Au in the NIR region (λ > 600 nm), which corresponds to the biological transparency window. This is a great advantage in medical applications to replace expensive Au with low-cost and nontoxic TiN. This feature is also useful for HAMR applications,28 where the use of complex nanoscale geometries is imminent.26

METHODS Fabrication. A KrF excimer laser (Compex 205, Coherent COMPexPro) operating at a repetition rate of 2−10 Hz with an energy density of 6 J/cm−2 was used. The target was hotpressed TiN (99.9%, purity), which was purchased from Kojundo Chemical Lab. Co., and the substrate was a Si wafer having a 100 nm thick SiO2 layer. The working distance between the target and substrate was 10 cm. The substrate temperature was monitored by a thermocouple attached to the substrate holder and was kept at room temperature (RT ≈ 25 °C) during all deposition processes. The background pressure throughout the deposition in the chamber was 5 × 10−6 Torr. Characterization. The thickness of the films prepared at various rates of deposition were measured using a step profiler (Dektak 150, Veeco Instruments). The surface profile measurement was performed by an atomic force microscope (Bruker). The crystal structure of the film was determined by using an Xray diffractometer (Smart Lab, Rigaku). The spectroscopic ellipsometry measurement was made from the ultraviolet (UV) to near-infrared (NIR) wavelength range (240 to 3000 nm) using a variable-angle spectroscopic ellipsometer (SE850DUV, SENTECH). The Drude−Lorentz model with a single Lorentz oscillator and the Drude term was used in all the fitting and

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ACS Photonics provided a good fit to the ellipsometric measurements with thickness values highly consistent with the surface profile measurements (within 5% deviation). Carrier concentration was determined by Hall measurement performed using a Resitest 8400 series (Toyo Corporation) in the ac field Hall measurement mode. The contacts were taken 1 mm from the edge of the TiN films,51 and the samples were of uniform thickness, as evidenced from the cross sectional SEM. The Hall measurement was performed with two different geometries using automated software, and the mobility and carrier concentration values obtained were within 5% deviation (measurement deviation arising from sample during Hall measurement) using two different geometries. The average values obtained from the measurement were reported in the article.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b00942. Ellipsometric fitting results of the TiN films on Si/SiO2 substrates by PLD; complex permittivities of the TiN films on Si/SiO2 substrates by PLD and sputtering, tabulated parameters of the TiN films on Si/SiO2 substrates by PLD containing Hall mobility, carrier concentration, particle size, and plasma frequency values; XRD patterns, XPS spectra, elemental percentages, FOM of different TiN films, cross sectional SEM image of the TiN films on Si/SiO2 substrates by PLD; XRD patterns and SEM image of the TiN films deposited on a PET substrate by PLD (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ramu Pasupathi Sugavaneshwar: 0000-0001-8485-852X Satoshi Ishii: 0000-0003-0731-8428 Thang Duy Dao: 0000-0001-5027-9079 Tadaaki Nagao: 0000-0002-6746-2686 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge Mr. Katsumi Ohno from Namiki Foundry (NIMS, Japan) for his help with the PLD equipment setup. This work is partially supported by JSPS KAKEHI (15K17447, 16H06364, 17H04801) and CREST “Phase Interface Science for Highly Efficient Energy Utilization” (JPMJCR13C3), Japan Science and Technology Agency, and the Japan Prize Foundation.

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REFERENCES

(1) Atwater, H. A. The promise of plasmonics. Sci. Am. 2007, 296, 56−62. (2) Lal, S.; Link, S.; Halas, N. J. Nano-optics from sensing to waveguiding. Nat. Photonics 2007, 1, 641−648. (3) McCall, S.; Platzman, P.; Wolff, P. Surface enhanced Raman scattering. Phys. Lett. A 1980, 77, 381−383. 818

DOI: 10.1021/acsphotonics.7b00942 ACS Photonics 2018, 5, 814−819

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DOI: 10.1021/acsphotonics.7b00942 ACS Photonics 2018, 5, 814−819