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Precipitation of metal nitride nanoparticles from amorphous (M,Si)-(N,O) thin films (M =Nb, Zr)
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ScienceDirect Materials Today: Proceedings 16 (2019) 173–179
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ISAC-6_2018
Precipitation of metal nitride nanoparticles from amorphous (M,Si)-(N,O) thin films (M =Nb, Zr) Yuji Masubuchi*, Yuko Miyamoto, Shinichi Kikkawa Faculty of Engineering, Hokkaido University, N13 W8, Kita-ku, Sapporo, 060-8628, Japan
Abstract Crystallization of metal nitride nanoparticles from amorphous matrix upon thermal annealing was investigated by studying the plasmonic absorption behavior of the nanoparticles. Amorphous silicon oxynitride thin films containing various amounts of M (M = Zr, Nb) were deposited by RF magnetron sputtering from a M/Si composite target using nitrogen as a reactive sputtering gas. The optical transmittance at around 700 nm of the annealed thin film decreased due to plasmonic absorption by the metallic NbN nanoparticles. The nanoparticles precipitated homogeneously in the Si(N,O) matrix upon annealing the Nb1-xSix(N,O) thin film with x = 0.7 at 700ºC. In contrast, for Zr1-ySiy(N,O) thin films with y = 0.4 and 0.5, ZrN nanoparticles did not precipitate even after thermal annealing due to the stability of zirconium oxide in the Si(N,O) matrix, resulting in no significant change in the interference color of the thin films. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 6th International Symposium on Advanced Ceramics. Keywords: metal nitride; precipitation; optical property; X-ray absorption
1. Introduction Nanoparticles of noble metals such as gold and silver are an important class of plasmonic materials that are applied in optical and electromagnetic devices as well as biosensors [1-4]. Transition metal nitrides have recently attracted significant attention as another group of promising candidates for plasmonic materials [5, 6]. Zirconium nitride has optical resonance similar to that of gold in visible light region. Resonance at near-infrared wavelength has also been enhanced in titanium nitride nanoparticles with respect to that for gold nanoparticles [6].
* Corresponding author. Tel.: +81-11-706-6742; fax: +81-11-706-6740. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 6th International Symposium on Advanced Ceramics.
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Nanoparticles of transition metal nitrides have been fabricated by a wide variety of methods, including the direct nitridation of metal particles, the ammonolysis of oxide particles, chemical vapor deposition, and laser ablation methods [7-10]. TiN nanoparticles have been reported to crystallize in an amorphous silicon oxynitride matrix upon thermal annealing of an amorphous Ti-Si-N-O thin film deposited by RF sputtering [11]. The optical and electronic properties of the resultant film were controlled by changing the Ti/Si ratio and the annealing conditions. A wide color variation appeared in the annealed thin films, from yellow for the as-deposited thin film, to green and blue for thin films successively annealed at 800 and 900°C. Plasmonic absorption occurred at the interface between the precipitated 2.8-nm TiN nanoparticles and the dielectric silicon oxynitride matrix. The homogeneous formation of TiN nanoparticles in the amorphous Si-N-O matrix was confirmed by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray absorption spectroscopy. Figure 1 shows that the energy of formation is similar for TiO2 and SiO2, while TiN is more stable than Si3N4 [12,13]. Therefore, the TiN nanoparticles are preferentially crystallized in the residual SiO(N) matrix during thermal annealing of quaternary Ti-Si-O-N. Metals, oxides, and nitrides have been prepared as nanoparticles by the thermal annealing of metastable parent amorphous thin films [14-16]. The particle size of the precipitates can be controlled by the annealing process because the amorphous matrix acts as a diffusion barrier for nanoparticle growth. There are several different kinds of transition metal nitrides that exhibit metallic electrical conductivity similarly to that of TiN. Metallic ZrN nanoparticles have been expected to be applied as a plasmonic material in the visible light region because they have a surface plasmon resonance similar to that of gold nanoparticles [6]. NbN is also a metallic conductor that exhibits superconductivity at low temperature [17,18]. NbN nanoparticles are also promising candidates as alternative plasmonic materials to Au and Ag nanoparticles. The energies of formation for both Zr oxide and nitride are much lower than those of Si oxide and nitride, as shown in Fig. 1. The formation of NbN is more favorable than that of Si3N4, while the free energy of SiO2 formation is lower than that of Nb2O5. A more stable compound could thus precipitate by thermal annealing of amorphous silicon oxynitride containing Zr or Nb, similar to the case of TiN nanoparticles [11].
Fig. 1. Standard free energies for the formation of selected (a) oxides and (b) nitrides [12,13].
Here we report the crystallization behavior of nitride nanoparticles formed by the thermal annealing of amorphous Zr and Nb oxynitride thin films prepared by RF-magnetron sputtering. The optical properties of the annealed thin films were investigated with respect to the precipitated crystalline phase of the annealed thin film. In addition, the formation of metal nitrides is discussed in relation to their free energy of formation.
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2. Experimental Magnetron sputtering (JEC-SP360M; JEOL, Japan) was used to deposit thin films on SiO2 glass substrates (10×20×0.5 mm3) under an RF power of 100 W for 2 h. A number of Si chips (5×5 mm2, 99.97%, Furuuchi Chemical Co., Japan) were distributed on Nb (⌀76.2 mm, 99.95%, Furuuchi Chemical Co., Japan) or Zr (⌀76.2 mm, 99.2%, Kojundo Chemical Laboratory Co., Japan) targets to change the Si/(M+Si) ratio. The N2 gas pressures for sputtering Nb1-xSix(N,O) and Zr1-ySiy(N,O) thin films were 4 and 2 Pa, respectively. The as-deposited thin films were placed in an evacuated SiO2 glass tube below 10-1 Pa and then annealed at temperatures ranging from 600 to 900°C for 5 h. XRD patterns were collected using a diffractometer with monochromatized Cu Kα radiation (Ultima IV, Rigaku, Japan) in the θ-2θ scan mode. The film texture, thickness, and Si content, x in Nb1-xSix(N,O) and y in Zr1-ySiy(N,O), were analyzed using scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEMEDX; JSM-6390LV, JEOL, Japan). Unintended contamination of oxygen into the deposited thin films was evaluated by the EDX measurements. The microstructure of the annealed thin films was observed using TEM (JEM2010, JEOL, Japan) with cross-sectional thin slice samples finished by Ar ion beam thinning. Transmittance spectra of the films on their SiO2 glass substrates were measured using a UV-vis spectrometer (V-550, JASCO, Japan). Xray absorption near-edge spectroscopy (XANES) was applied to obtain Zr and Zb K-edge spectra, which were obtained in fluorescence mode using the BL-9A and NW10A beamlines at the Photon Factory/KEK, Tsukuba, Japan. The electrical conductivity at room temperature was measured using the van der Pauw method with a resistivity measurement apparatus (ResiTest 8300, Toyo Co., Japan). 3. Results and discussion 3.1. Nb1-xSix(N,O) thin films Broad diffraction peaks for rock-salt type Nb(N,O) were observed in the XRD pattern of the as-deposited Nb1-xSix(N,O) thin film with x = 0 as shown in Fig. 2(a). The thin film was black in color with a metallic luster, which indicated the electrical conductive nature of this product. The crystallinity of the film was significantly improved by post-annealing at 800°C, as shown in Fig. 2(b). However, the as-deposited Nb1-xSix(N,O) thin films with x = 0.48-0.72 were amorphous by XRD analysis. The rock-salt type structure was only slightly evident for the
Fig. 2. XRD patterns of (a) the as-deposited Nb1-xSix(N,O) thin films and (b) those annealed at 800°C. Filled circles indicate the diffraction positions for the rock-salt type Nb(N,O) lattice.
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Fig. 3. Change in color transmission for the as-deposited and annealed Nb1-xSix(N,O) thin films.
film with x = 0.48 after annealing at 800°C in an evacuated SiO2 tube. The color of the as-deposited thin film varied from orange at x = 0.48 to canary yellow at x = 0.72, as shown in Fig. 3. The color became dark brownish by annealing at elevated temperatures. A metallic luster also appeared in the thin films by annealing at 700°C for x = 0.48 and above 800°C for the entire range of x examined. XRD measurements indicated that crystalline phase did not appear in the annealed thin films with higher Si contents (x = 0.54-0.72), although both metallic luster and electrical conductivity were evident. The formation of a rock-salt-like local structure around Nb was investigated by measuring XANES spectrum of the Nb1-xSix(N,O) thin film with x = 0.72. The pre-edge peak on the Nb2O5 reference was assigned to a 1s to 4d transition induced by p-d hybridization due to a distorted coordination around Nb [19]. This feature was observed for the as-deposited thin film, and its intensity decreased with an increase in annealing temperature. The local structure around Nb was similar to that of Nb2O5 and gradually changed to that of NbN after annealing. The pre-edge peak disappeared in thethin film annealed at 900°C, as shown in Fig. 4(a). With an increase in the annealing temperature, the absorption edge shifted to lower energy, which indicated a decrease in the formal charge of Nb from Nb5+ in Nb2O5 to Nb3+ in NbN. The distinct peak labeled A in Fig. 4(a) can be used as a fingerprint for the formation of NbN [20]. The peak
Fig. 4. (a) Nb K-edge XANES spectra for Nb1-xSix(N,O) thin films with x = 0.72 annealed at various temperatures. Spectra for Nb2O5 (monoclinic) and NbN (rock-salt) are also shown as references. (b) Transmittance spectra for the as-deposited and annealed Nb1-xSix(N,O) thin films with x = 0.72. (c) Cross-sectional TEM image of the thin film with x = 0.72 annealed at 700°C. The inset ED pattern indicates the formation of rock-salt type NbN nanocrystals. The white semicircles represent the diffraction positions of the rock-salt lattice.
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intensity increased with the annealing temperature, which also supports the fact that the local structure around Nb changed to the rock-salt type NbN from the Nb2O5-like distorted coordination. Figure 4(b) shows that the UV-vis transmittance spectra of both the as-deposited thin film (x = 0.72) and the film annealed at 600°C exhibited interference. The optical transmittance of the film annealed at 700°C was significantly lower at all measured wavelengths due to plasmonic absorption by the precipitated metallic NbN nanoparticles but exhibited a slight monotonic increase with wavelength above 450 nm. The optical transmittance of the thin films annealed at 800°C and above was less than 5% for all wavelengths. These films exhibited an additional broad plasmon absorption at around 700 nm. Figure 4(c) shows a cross-sectional TEM image and electron diffraction (ED) pattern for the thin film with x = 0.72 annealed at 700°C. Diffraction rings shown in the inset are corresponding to 111, 200 and 220 planes in the NbN rock-salt lattice. The film contains rock-salt type NbN and the dark grains in the TEM image are small precipitates of NbN with sizes of approximately 2 nm. The NbN nanoparticles are dispersed in the insulating Si(N,O) matrix. The electrical resistivities of the as-deposited thin films increased from 2.0×107 μΩ cm for x = 0.48 to 1.9×109 μΩ cm for x = 0.60 with an increase in the amount of Si(N,O) matrix, while the as-deposited thin film with x = 0.72 was an electrical insulator. After annealing above 700°C, the electrical insulating properties of the thin film with x = 0.72 were maintained, even though metallic NbN nanoparticles precipitated in the Si(N,O) matrix. This was because the NbN nanoparticles were well separated by the insulating Si(N,O) matrix. The interface between the metallic nanoparticles and the dielectric host matrix exhibits a plasmonic resonance at a specific wavelength that depends on the size, electrical conductivity and dispersity of the metallic nanoparticle [6]. The thin film with x = 0.72 annealed at 700°C was light brown in color, as depicted in Fig. 3. Plasmonic resonance appeared at the interface between the nanosized metallic NbN precipitates and the amorphous Si(N,O) matrix as a new broad absorption around 700 nm, similar to our previous observation of TiN nanoparticles in a Si(N,O) matrix [11]. 3.2. Zr1-ySiy(N,O) thin films XRD measurements shown in Fig. 5(a) indicated amorphous thin films were obtained for as-deposited Zr1-ySiy(N,O) thin films at y = 0.41 and 0.53 (not shown in Fig. 5(a)). Zirconium nitride was not crystallized in the thin films by post-annealing in an evacuated silica tube at 900°C. Rock-salt type ZrN appeared only at y = 0. The film color changed from dark green in the as-deposited thin film to greyish blue in the annealed thin film at y = 0.41. Similar colors were observed for both as-deposited and post-annealed thin films, although the colors were slightly
Fig. 5. (a) XRD patterns of the as-deposited Zr1-ySiy(N,O) thin films with y = 0 and 0.41, and those annealed at 900°C. Filled circles and diamonds indicate the diffraction positions for the ZrN and Zr4O5N2, respectively. (b) Transmittance spectra for as-deposited and post-annealed Zr1-ySiy(N,O) thin films with y = 0.41 (red lines) and 0.53 (blue lines). (c) Zr K-edge XANES spectra for Zr1-ySiy(N,O) thin films with y = 0 and 0.41. Spectra for ZrO2 (baddeleyite) and ZrN (rock-salt) are also shown as references.
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brighter with y = 0.53 than with y = 0.41. Interference pattern as depicted in Fig. 5(b) appeared on their UV-vis transmittance spectra. XANES Zr K-edge spectra were measured to investigate the local structure around Zr in the Zr1-ySiy(N,O) thin films. The as-deposited Zr(N,O) (y = 0) thin film exhibited a pre-edge peak at 17986 eV as shown in Fig. 5(c), similar to that for the monoclinic ZrO2 reference, which had been assigned as a Zr 1s to 4d transition [21]. After annealing at 900°C, the pre-edge peak almost disappeared, and the spectrum became similar to that of the rock-salt type ZrN reference, as with the XRD pattern. However, the XANES spectrum for the thin film with y = 0.41 did not change upon annealing, since the crystallization of ZrN was suppressed in Si(N,O) matrix. The higher stability of ZrO2 than SiO2 and ZrN means that ZrN nanoparticles are not precipitated from amorphous Zr1-ySiy(N,O). 3.3. Precipitation behavior NbN nanoparticles were precipitated in a Si(N,O) matrix by the thermal annealing of amorphous Nb1-xSix(N,O) thin films. Metallic ZrN did not precipitate from the amorphous parent thin films, and the zirconium oxide-like local structure was maintained even after annealing of the thin films. The formation of metal oxide and nitride from M/Si oxynitride can be understood with respect to the thermodynamic stability. ZrO2 is more stable than SiO2. The formation of ZrN is not favorable in a Si(N,O) matrix. On the other hand, Nb2O5 is less stable than SiO2, and the Nb in the amorphous Nb-Si-N-O system forms NbN instead of Nb2O5. The crystallization of NbN nanoparticles dispersed in the Si(N,O) matrix can be achieved by thermal annealing of the oxynitride thin film containing both Nb and Si. The thin film with NbN nanoparticles exhibited plasmonic absorption at the interface between the NbN nanoparticles and the Si(N,O) matrix. 4. Conclusion Nb1-xSix(N,O) thin film was deposited by RF-magnetron sputtering of Nb-Si composite targets in a nitrogen atmosphere. Nanoparticles of metallic NbN dispersed in a Si(N,O) amorphous matrix were obtained by successive thermal annealing of the films above 700°C. Thermodynamically stable NbN was crystallized from Nb-Si oxynitride thin films instead of Si3N4 and Nb2O5. ZrN was not precipitated from similar Zr-Si oxynitride thin films due to the presence of the stable ZrO2 phase. With annealing, plasmonic absorption occurred at the interface between the NbN nanoparticles and the dielectric Si(N,O) matrix and the transmittance decreased at around 700 nm. Acknowledgements XANES measurements were conducted under the approval of the Photon Factory Advisory Committee (Proposal No. 2015G602). References [1] J.A. Dionne, H.A. Atwater, MRS Bull. 37 (2012) 717-724. [2] S.K. Ghosh, T. Pal, Chem. Rev. 107 (2007) 4797-4862. [3] P.K. Jain, X. Huang, I.H. El-Sayed, M.A. El-Sayed, Plasmonics 2 (2007) 107-118. [4] U. Guler, V.M. Shalaev, A. Boltasseva, Mater. Today 18 (2015) 227-237. [5] A. Boltasseva, H.A. Atwater, Science 331 (2011) 290-291. [6] U. Gular, G.V. Naik, A. Boltasseva, V.M. Shalaev, A.V. Kildishev, Appl. Phys. B 107 (2012) 285-291. [7] X. Yang, C. Li, L. Yang, Y. Yan, Y. Qian, J. Am. Ceram. Soc. 86 (2003) 206-208. [8] R. Marchand, F. Tessier, F.J. DiSalvo, J. Mater. Chem. 9 (1999) 297-304. [9] D. Li, C.J. Choi, B.K. Kim, Z.D. Zhang, J. Magn. Magn. Mater. 277 (2004) 64-70. [10] A. Reinholdt, R. Pecenka, A. Pinchuk, S. Runte, A.L. Stepanov, T.E. Weirich, U. Kreibig, Eur. Phys. J. D 31 (2004) 69-76. [11] A. Sawada, Y. Masubuchi, T. Motohashi, S. Kikkawa, J. Am. Ceram Soc. 97 (2014) 1356-1358. [12] O. Knacke, O. Kubaschewski, K. Hesselmann, Thermochemical Properties of Inorganic Substances; 2nd ed.; Springer-Verlag: Berlin, 1991. [13] I. Barin, Thermochemical Data of Pure Substances; 3rd ed.; VCH Verlagsgesellschaft: Weinheim, 1995. [14] Y. Kawaai, A. Yamada, T. Takeda, S. Kikkawa, Jpn. J. Appl. Phys. 43 (2004) 5671-5672. [15] Y. Masubuchi, Y. Sato, A. Sawada, T. Motohashi, H. Kiyono, S. Kikkawa, J. Eur. Ceram. Soc. 31 (2011) 2459-2462. [16] A. Sawada, Y. Masubuchi, T. Motohashi, S. Kikkawa, Mater. Lett. 115 (2014) 198-200.
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[17] N. Pessall, R.E. Gold, H.A. Johansen, J. Phys. Chem. Solids 29 (1968) 19-38. [18] X. Liu, G. Zhou, S.W. Or, S. Ran, Y. Sun, Ceramic Int. 41 (2015) 849-852. [19] M.R. Antonio, I. Song, H. Yamada, J. Solid State Chem. 93 (1991) 183-192. [20] Y. Wakai, T. Hara, K.K. Bando, N. Ichikuni, S. Shimazu, Top. Catal. 52 (2009) 1517-1524. [21] G. Mountjoy, D.M. Pickup, R. Anderson, G.W. Wallidge, M.A. Holland, R.J. Newport, M.E. Smith, Phys. Chem. Chem. Phys. 2 (2000) 2455-2460.
ScienceDirect Materials Today: Proceedings 16 (2019) 173–179
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ISAC-6_2018
Precipitation of metal nitride nanoparticles from amorphous (M,Si)-(N,O) thin films (M =Nb, Zr) Yuji Masubuchi*, Yuko Miyamoto, Shinichi Kikkawa Faculty of Engineering, Hokkaido University, N13 W8, Kita-ku, Sapporo, 060-8628, Japan
Abstract Crystallization of metal nitride nanoparticles from amorphous matrix upon thermal annealing was investigated by studying the plasmonic absorption behavior of the nanoparticles. Amorphous silicon oxynitride thin films containing various amounts of M (M = Zr, Nb) were deposited by RF magnetron sputtering from a M/Si composite target using nitrogen as a reactive sputtering gas. The optical transmittance at around 700 nm of the annealed thin film decreased due to plasmonic absorption by the metallic NbN nanoparticles. The nanoparticles precipitated homogeneously in the Si(N,O) matrix upon annealing the Nb1-xSix(N,O) thin film with x = 0.7 at 700ºC. In contrast, for Zr1-ySiy(N,O) thin films with y = 0.4 and 0.5, ZrN nanoparticles did not precipitate even after thermal annealing due to the stability of zirconium oxide in the Si(N,O) matrix, resulting in no significant change in the interference color of the thin films. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 6th International Symposium on Advanced Ceramics. Keywords: metal nitride; precipitation; optical property; X-ray absorption
1. Introduction Nanoparticles of noble metals such as gold and silver are an important class of plasmonic materials that are applied in optical and electromagnetic devices as well as biosensors [1-4]. Transition metal nitrides have recently attracted significant attention as another group of promising candidates for plasmonic materials [5, 6]. Zirconium nitride has optical resonance similar to that of gold in visible light region. Resonance at near-infrared wavelength has also been enhanced in titanium nitride nanoparticles with respect to that for gold nanoparticles [6].
* Corresponding author. Tel.: +81-11-706-6742; fax: +81-11-706-6740. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 6th International Symposium on Advanced Ceramics.
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Nanoparticles of transition metal nitrides have been fabricated by a wide variety of methods, including the direct nitridation of metal particles, the ammonolysis of oxide particles, chemical vapor deposition, and laser ablation methods [7-10]. TiN nanoparticles have been reported to crystallize in an amorphous silicon oxynitride matrix upon thermal annealing of an amorphous Ti-Si-N-O thin film deposited by RF sputtering [11]. The optical and electronic properties of the resultant film were controlled by changing the Ti/Si ratio and the annealing conditions. A wide color variation appeared in the annealed thin films, from yellow for the as-deposited thin film, to green and blue for thin films successively annealed at 800 and 900°C. Plasmonic absorption occurred at the interface between the precipitated 2.8-nm TiN nanoparticles and the dielectric silicon oxynitride matrix. The homogeneous formation of TiN nanoparticles in the amorphous Si-N-O matrix was confirmed by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray absorption spectroscopy. Figure 1 shows that the energy of formation is similar for TiO2 and SiO2, while TiN is more stable than Si3N4 [12,13]. Therefore, the TiN nanoparticles are preferentially crystallized in the residual SiO(N) matrix during thermal annealing of quaternary Ti-Si-O-N. Metals, oxides, and nitrides have been prepared as nanoparticles by the thermal annealing of metastable parent amorphous thin films [14-16]. The particle size of the precipitates can be controlled by the annealing process because the amorphous matrix acts as a diffusion barrier for nanoparticle growth. There are several different kinds of transition metal nitrides that exhibit metallic electrical conductivity similarly to that of TiN. Metallic ZrN nanoparticles have been expected to be applied as a plasmonic material in the visible light region because they have a surface plasmon resonance similar to that of gold nanoparticles [6]. NbN is also a metallic conductor that exhibits superconductivity at low temperature [17,18]. NbN nanoparticles are also promising candidates as alternative plasmonic materials to Au and Ag nanoparticles. The energies of formation for both Zr oxide and nitride are much lower than those of Si oxide and nitride, as shown in Fig. 1. The formation of NbN is more favorable than that of Si3N4, while the free energy of SiO2 formation is lower than that of Nb2O5. A more stable compound could thus precipitate by thermal annealing of amorphous silicon oxynitride containing Zr or Nb, similar to the case of TiN nanoparticles [11].
Fig. 1. Standard free energies for the formation of selected (a) oxides and (b) nitrides [12,13].
Here we report the crystallization behavior of nitride nanoparticles formed by the thermal annealing of amorphous Zr and Nb oxynitride thin films prepared by RF-magnetron sputtering. The optical properties of the annealed thin films were investigated with respect to the precipitated crystalline phase of the annealed thin film. In addition, the formation of metal nitrides is discussed in relation to their free energy of formation.
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2. Experimental Magnetron sputtering (JEC-SP360M; JEOL, Japan) was used to deposit thin films on SiO2 glass substrates (10×20×0.5 mm3) under an RF power of 100 W for 2 h. A number of Si chips (5×5 mm2, 99.97%, Furuuchi Chemical Co., Japan) were distributed on Nb (⌀76.2 mm, 99.95%, Furuuchi Chemical Co., Japan) or Zr (⌀76.2 mm, 99.2%, Kojundo Chemical Laboratory Co., Japan) targets to change the Si/(M+Si) ratio. The N2 gas pressures for sputtering Nb1-xSix(N,O) and Zr1-ySiy(N,O) thin films were 4 and 2 Pa, respectively. The as-deposited thin films were placed in an evacuated SiO2 glass tube below 10-1 Pa and then annealed at temperatures ranging from 600 to 900°C for 5 h. XRD patterns were collected using a diffractometer with monochromatized Cu Kα radiation (Ultima IV, Rigaku, Japan) in the θ-2θ scan mode. The film texture, thickness, and Si content, x in Nb1-xSix(N,O) and y in Zr1-ySiy(N,O), were analyzed using scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEMEDX; JSM-6390LV, JEOL, Japan). Unintended contamination of oxygen into the deposited thin films was evaluated by the EDX measurements. The microstructure of the annealed thin films was observed using TEM (JEM2010, JEOL, Japan) with cross-sectional thin slice samples finished by Ar ion beam thinning. Transmittance spectra of the films on their SiO2 glass substrates were measured using a UV-vis spectrometer (V-550, JASCO, Japan). Xray absorption near-edge spectroscopy (XANES) was applied to obtain Zr and Zb K-edge spectra, which were obtained in fluorescence mode using the BL-9A and NW10A beamlines at the Photon Factory/KEK, Tsukuba, Japan. The electrical conductivity at room temperature was measured using the van der Pauw method with a resistivity measurement apparatus (ResiTest 8300, Toyo Co., Japan). 3. Results and discussion 3.1. Nb1-xSix(N,O) thin films Broad diffraction peaks for rock-salt type Nb(N,O) were observed in the XRD pattern of the as-deposited Nb1-xSix(N,O) thin film with x = 0 as shown in Fig. 2(a). The thin film was black in color with a metallic luster, which indicated the electrical conductive nature of this product. The crystallinity of the film was significantly improved by post-annealing at 800°C, as shown in Fig. 2(b). However, the as-deposited Nb1-xSix(N,O) thin films with x = 0.48-0.72 were amorphous by XRD analysis. The rock-salt type structure was only slightly evident for the
Fig. 2. XRD patterns of (a) the as-deposited Nb1-xSix(N,O) thin films and (b) those annealed at 800°C. Filled circles indicate the diffraction positions for the rock-salt type Nb(N,O) lattice.
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Fig. 3. Change in color transmission for the as-deposited and annealed Nb1-xSix(N,O) thin films.
film with x = 0.48 after annealing at 800°C in an evacuated SiO2 tube. The color of the as-deposited thin film varied from orange at x = 0.48 to canary yellow at x = 0.72, as shown in Fig. 3. The color became dark brownish by annealing at elevated temperatures. A metallic luster also appeared in the thin films by annealing at 700°C for x = 0.48 and above 800°C for the entire range of x examined. XRD measurements indicated that crystalline phase did not appear in the annealed thin films with higher Si contents (x = 0.54-0.72), although both metallic luster and electrical conductivity were evident. The formation of a rock-salt-like local structure around Nb was investigated by measuring XANES spectrum of the Nb1-xSix(N,O) thin film with x = 0.72. The pre-edge peak on the Nb2O5 reference was assigned to a 1s to 4d transition induced by p-d hybridization due to a distorted coordination around Nb [19]. This feature was observed for the as-deposited thin film, and its intensity decreased with an increase in annealing temperature. The local structure around Nb was similar to that of Nb2O5 and gradually changed to that of NbN after annealing. The pre-edge peak disappeared in thethin film annealed at 900°C, as shown in Fig. 4(a). With an increase in the annealing temperature, the absorption edge shifted to lower energy, which indicated a decrease in the formal charge of Nb from Nb5+ in Nb2O5 to Nb3+ in NbN. The distinct peak labeled A in Fig. 4(a) can be used as a fingerprint for the formation of NbN [20]. The peak
Fig. 4. (a) Nb K-edge XANES spectra for Nb1-xSix(N,O) thin films with x = 0.72 annealed at various temperatures. Spectra for Nb2O5 (monoclinic) and NbN (rock-salt) are also shown as references. (b) Transmittance spectra for the as-deposited and annealed Nb1-xSix(N,O) thin films with x = 0.72. (c) Cross-sectional TEM image of the thin film with x = 0.72 annealed at 700°C. The inset ED pattern indicates the formation of rock-salt type NbN nanocrystals. The white semicircles represent the diffraction positions of the rock-salt lattice.
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intensity increased with the annealing temperature, which also supports the fact that the local structure around Nb changed to the rock-salt type NbN from the Nb2O5-like distorted coordination. Figure 4(b) shows that the UV-vis transmittance spectra of both the as-deposited thin film (x = 0.72) and the film annealed at 600°C exhibited interference. The optical transmittance of the film annealed at 700°C was significantly lower at all measured wavelengths due to plasmonic absorption by the precipitated metallic NbN nanoparticles but exhibited a slight monotonic increase with wavelength above 450 nm. The optical transmittance of the thin films annealed at 800°C and above was less than 5% for all wavelengths. These films exhibited an additional broad plasmon absorption at around 700 nm. Figure 4(c) shows a cross-sectional TEM image and electron diffraction (ED) pattern for the thin film with x = 0.72 annealed at 700°C. Diffraction rings shown in the inset are corresponding to 111, 200 and 220 planes in the NbN rock-salt lattice. The film contains rock-salt type NbN and the dark grains in the TEM image are small precipitates of NbN with sizes of approximately 2 nm. The NbN nanoparticles are dispersed in the insulating Si(N,O) matrix. The electrical resistivities of the as-deposited thin films increased from 2.0×107 μΩ cm for x = 0.48 to 1.9×109 μΩ cm for x = 0.60 with an increase in the amount of Si(N,O) matrix, while the as-deposited thin film with x = 0.72 was an electrical insulator. After annealing above 700°C, the electrical insulating properties of the thin film with x = 0.72 were maintained, even though metallic NbN nanoparticles precipitated in the Si(N,O) matrix. This was because the NbN nanoparticles were well separated by the insulating Si(N,O) matrix. The interface between the metallic nanoparticles and the dielectric host matrix exhibits a plasmonic resonance at a specific wavelength that depends on the size, electrical conductivity and dispersity of the metallic nanoparticle [6]. The thin film with x = 0.72 annealed at 700°C was light brown in color, as depicted in Fig. 3. Plasmonic resonance appeared at the interface between the nanosized metallic NbN precipitates and the amorphous Si(N,O) matrix as a new broad absorption around 700 nm, similar to our previous observation of TiN nanoparticles in a Si(N,O) matrix [11]. 3.2. Zr1-ySiy(N,O) thin films XRD measurements shown in Fig. 5(a) indicated amorphous thin films were obtained for as-deposited Zr1-ySiy(N,O) thin films at y = 0.41 and 0.53 (not shown in Fig. 5(a)). Zirconium nitride was not crystallized in the thin films by post-annealing in an evacuated silica tube at 900°C. Rock-salt type ZrN appeared only at y = 0. The film color changed from dark green in the as-deposited thin film to greyish blue in the annealed thin film at y = 0.41. Similar colors were observed for both as-deposited and post-annealed thin films, although the colors were slightly
Fig. 5. (a) XRD patterns of the as-deposited Zr1-ySiy(N,O) thin films with y = 0 and 0.41, and those annealed at 900°C. Filled circles and diamonds indicate the diffraction positions for the ZrN and Zr4O5N2, respectively. (b) Transmittance spectra for as-deposited and post-annealed Zr1-ySiy(N,O) thin films with y = 0.41 (red lines) and 0.53 (blue lines). (c) Zr K-edge XANES spectra for Zr1-ySiy(N,O) thin films with y = 0 and 0.41. Spectra for ZrO2 (baddeleyite) and ZrN (rock-salt) are also shown as references.
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brighter with y = 0.53 than with y = 0.41. Interference pattern as depicted in Fig. 5(b) appeared on their UV-vis transmittance spectra. XANES Zr K-edge spectra were measured to investigate the local structure around Zr in the Zr1-ySiy(N,O) thin films. The as-deposited Zr(N,O) (y = 0) thin film exhibited a pre-edge peak at 17986 eV as shown in Fig. 5(c), similar to that for the monoclinic ZrO2 reference, which had been assigned as a Zr 1s to 4d transition [21]. After annealing at 900°C, the pre-edge peak almost disappeared, and the spectrum became similar to that of the rock-salt type ZrN reference, as with the XRD pattern. However, the XANES spectrum for the thin film with y = 0.41 did not change upon annealing, since the crystallization of ZrN was suppressed in Si(N,O) matrix. The higher stability of ZrO2 than SiO2 and ZrN means that ZrN nanoparticles are not precipitated from amorphous Zr1-ySiy(N,O). 3.3. Precipitation behavior NbN nanoparticles were precipitated in a Si(N,O) matrix by the thermal annealing of amorphous Nb1-xSix(N,O) thin films. Metallic ZrN did not precipitate from the amorphous parent thin films, and the zirconium oxide-like local structure was maintained even after annealing of the thin films. The formation of metal oxide and nitride from M/Si oxynitride can be understood with respect to the thermodynamic stability. ZrO2 is more stable than SiO2. The formation of ZrN is not favorable in a Si(N,O) matrix. On the other hand, Nb2O5 is less stable than SiO2, and the Nb in the amorphous Nb-Si-N-O system forms NbN instead of Nb2O5. The crystallization of NbN nanoparticles dispersed in the Si(N,O) matrix can be achieved by thermal annealing of the oxynitride thin film containing both Nb and Si. The thin film with NbN nanoparticles exhibited plasmonic absorption at the interface between the NbN nanoparticles and the Si(N,O) matrix. 4. Conclusion Nb1-xSix(N,O) thin film was deposited by RF-magnetron sputtering of Nb-Si composite targets in a nitrogen atmosphere. Nanoparticles of metallic NbN dispersed in a Si(N,O) amorphous matrix were obtained by successive thermal annealing of the films above 700°C. Thermodynamically stable NbN was crystallized from Nb-Si oxynitride thin films instead of Si3N4 and Nb2O5. ZrN was not precipitated from similar Zr-Si oxynitride thin films due to the presence of the stable ZrO2 phase. With annealing, plasmonic absorption occurred at the interface between the NbN nanoparticles and the dielectric Si(N,O) matrix and the transmittance decreased at around 700 nm. Acknowledgements XANES measurements were conducted under the approval of the Photon Factory Advisory Committee (Proposal No. 2015G602). References [1] J.A. Dionne, H.A. Atwater, MRS Bull. 37 (2012) 717-724. [2] S.K. Ghosh, T. Pal, Chem. Rev. 107 (2007) 4797-4862. [3] P.K. Jain, X. Huang, I.H. El-Sayed, M.A. El-Sayed, Plasmonics 2 (2007) 107-118. [4] U. Guler, V.M. Shalaev, A. Boltasseva, Mater. Today 18 (2015) 227-237. [5] A. Boltasseva, H.A. Atwater, Science 331 (2011) 290-291. [6] U. Gular, G.V. Naik, A. Boltasseva, V.M. Shalaev, A.V. Kildishev, Appl. Phys. B 107 (2012) 285-291. [7] X. Yang, C. Li, L. Yang, Y. Yan, Y. Qian, J. Am. Ceram. Soc. 86 (2003) 206-208. [8] R. Marchand, F. Tessier, F.J. DiSalvo, J. Mater. Chem. 9 (1999) 297-304. [9] D. Li, C.J. Choi, B.K. Kim, Z.D. Zhang, J. Magn. Magn. Mater. 277 (2004) 64-70. [10] A. Reinholdt, R. Pecenka, A. Pinchuk, S. Runte, A.L. Stepanov, T.E. Weirich, U. Kreibig, Eur. Phys. J. D 31 (2004) 69-76. [11] A. Sawada, Y. Masubuchi, T. Motohashi, S. Kikkawa, J. Am. Ceram Soc. 97 (2014) 1356-1358. [12] O. Knacke, O. Kubaschewski, K. Hesselmann, Thermochemical Properties of Inorganic Substances; 2nd ed.; Springer-Verlag: Berlin, 1991. [13] I. Barin, Thermochemical Data of Pure Substances; 3rd ed.; VCH Verlagsgesellschaft: Weinheim, 1995. [14] Y. Kawaai, A. Yamada, T. Takeda, S. Kikkawa, Jpn. J. Appl. Phys. 43 (2004) 5671-5672. [15] Y. Masubuchi, Y. Sato, A. Sawada, T. Motohashi, H. Kiyono, S. Kikkawa, J. Eur. Ceram. Soc. 31 (2011) 2459-2462. [16] A. Sawada, Y. Masubuchi, T. Motohashi, S. Kikkawa, Mater. Lett. 115 (2014) 198-200.
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