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Abstract
Transparent conductive oxide (TCO) films showing epsilon near zero (ENZ) properties have attracted great research interest due to their unique property of electrically tunable permittivity. In this work, we report the effect of oxygen stoichiometry on the structure, optical and ENZ properties of indium tin oxide (ITO) films fabricated under different oxygen partial pressures. By using spectroscopic ellipsometry (SE) with fast data acquisition capabilities, we observed modulation of the material index and ENZ wavelength under electrostatic gating. Using a two-layer model based on Thomas-Fermi screening model and the Drude model, the optical constants and Drude parameters of the ITO thin films are determined during the gating process. The maximum carrier modulation amplitude ΔN of the accumulation layer is found to vary significantly depending on the oxygen stoichiometry. Under an electric field gate bias of 2.5 MV/cm, the largest ENZ wavelength modulation up to 27.9 nm at around 1550 nm is observed in ITO thin films deposited with oxygen partial pressure of ${P_{{O_2}}}$=10 Pa. Our work provides insights to the optical properties of ITO during electrostatic gating process for electro-optic modulators (EOMs) applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Transparent conductive oxides (TCOs) such as ITO, Al-doped ZnO (AZO) and Ga-doped ZnO (GZO) etc. have attracted great research interest recently [1–3]. These materials show metal like optical properties and electric-field tunable permittivity in the visible to infrared wavelength range, which is promising for a variety of applications in silicon waveguide EOMs and plasmonic waveguide modulators [1,4–17]. As a member of the TCOs materials, ITO shows epsilon-near-zero (ENZ) property in the near infrared [2,3,10,15,16,18–25]. In a silicon optical waveguide, this property allows significant modulation of the modal profile and propagation loss in ITO under applied electrostatic gating, which can be applied for waveguide electro-absorption modulator (EAM) applications [8,10,26]. On the other hand, as a promising alternative plasmonic material, ITO also shows many advantages over traditional noble metals (such as gold and silver) [27–31]. For example, ITO thin films show much lower loss compared to gold and silvers in the visible and near-infrared wavelengths owing to its much lower free carrier concentrations [32]. Its carrier concentrations can also be tuned under an applied electric field, leading to a variation of the optical properties. These properties allow the development of active plasmonic devices such as plasmonic waveguide modulators and optical phase arrays [27,30,33–39].
Characterization and understanding the optical properties of ITO thin films, especially under electrostatic gating is crucial for the above-mentioned applications. In early studies, Feigenbaum et al. [40] characterized the modulation of ITO optical constants in an Au/ITO/SiO2/Au multilayer stack. Under an applied electric field of 0.5 MV/cm, they observed unity order index modulation of ITO in the visible. In a later work by Sorger et al. [5], the authors characterized a similar material stack on a silicon waveguide, and demonstrated waveguide based EAM devices under an applied electric field up to 1.5 MV/cm. Optical constant modulation by electrostatic gating is observed by Melikyan et al.. In a Ag/ITO/SiO2/Ag structure, using the Thomas-Fermi screening model, the authors observed index modulation from n = 0.096, κ=4.28 to n = 0.095, κ=4.37 at 1550 nm wavelength under 3.3 MV/cm applied electric field. The difference from previous reports may be originated from different starting material properties of the ITO thin films. According to the Drude model, the electrical and optical property of ITO is strongly related to its free carrier concentrations. Oxygen stoichiometry in such materials play an important role of tuning the initial carrier concentrations, because oxygen vacancies are electron donors [1,41]. Therefore, a study on how ITO material properties, especially oxygen stoichiometry influences the material’s optical property under electrostatic gating is desired. Resolving this question will provide useful insight to guide the material and device fabrication for efficient and active nanophotonic device applications.
In this work, we report a study on the structure, optical and ENZ properties of ITO thin films fabricated under different oxygen partial pressures by pulsed-laser deposition (PLD). Using spectroscopic ellipsometry, we collected the ellipsometry data from 210 nm to 1690 nm wavelength within 8 seconds at each electric field gate bias, therefore eliminating long time gating induced ionic migration effects. Using a two-layer model based on the Thomas-Fermi screening model, we extract all the Drude parameters in the ITO thin films during electric field gating, demonstrating a strong influence of the material optical property by oxygen stoichiometry. An optimum oxygen partial pressure during fabrication is observed for the largest index modulation amplitude in ITO thin films, which leads to an index tuning range from n = 0.631, κ=0.574 to n = 0.570, κ=0.653 at 1550 nm wavelength under 2.5 MV/cm bias, together with an ENZ wavelength tuning from 1558.2 nm to 1530.3 nm.
2. Experiment and methods
ITO/SiO2 thin film stacks were deposited on Si (100) substrates. 20 nm thick SiO2 thin films were firstly deposited by radio frequency (RF) sputtering at room temperature using a SiO2 target in a magnetron sputtering (MS) system. The chamber was pumped to a base pressure of 8×10−4 Pa before deposition. During sputtering, the RF power was kept at 80 W. A pure Ar gas ambient was kept at 0.5 Pa. ITO thin films were subsequently deposited by PLD on the SiO2 thin films using a 248 nm KrF excimer laser (Compex Pro 205). The ITO target (Alfa Aesa, purity 99.99%) was a ceramic pellet with the composition of In2O3:SnO2 = 93%:7% by weight. Before deposition, the background pressure of the chamber was evacuated to be below 5×10−4 Pa. During deposition, the target-substrate distance was kept at 5.5 cm. The substrate temperature was fixed at 300°C and the ambient oxygen partial pressures of the chamber were kept at 0.1 Pa, 1 Pa, 10 Pa, 30 Pa, respectively. A laser fluence of 2 J/cm2 and 10 Hz repetition rate yielded a thin film deposition speed of 45.6 nm/min. After deposition, the substrate was naturally cooled to room temperature without any post-annealing. For electric field gating experiments, some ITO thin films were also deposited at the same time through a shadow mask, forming a 2 mm diameter disk top electrode.
Phase characterization of the ITO thin films was carried out by X-ray diffraction (XRD) on a Shimadzu XRD-7000 X-ray diffractometer with Cu K${\alpha }$ radiation (${\lambda }$=0.1542 nm). The surface roughness and morphology was characterized by atomic force microscopy (AFM). Hall measurement (Ecopia HMS-5000) was performed to obtain the carrier concentration and Hall mobility of the ITO films. Spectroscopic ellipsometry (RC2, J. A. Woollam Co., Inc.) was applied to measure the optical constants of ITO thin films. For electric field gating characterizations, a DC voltage ranging from 0 to 5 V was applied between the ITO thin film and Si across the SiO2 layer by using coaxial probes and a semiconductor device analyzer (Keysight Technologies B1500A). Ellipsometry data was then collected within 8 s in the wavelength range of 210 nm–1690 nm. The software of Complete EASE was used to fit the measured data to obtain the optical constants of the ITO thin films.
3. Results and discussion
Figure 1(a) shows the XRD pattern of ITO thin films fabricated under different oxygen partial pressures. From the diffraction patterns, we could see that the crystal structure of the ITO films is highly dependent on the oxygen partial pressures during deposition. The diffraction peaks of (211), (222), (400), (440), (622) indicated a polycrystalline structure for all deposition conditions. The diffraction peaks were consistent with the cubic crystal structure of In2O3 (JCPDS card no.06-0416) [42–44]. As increasing the oxygen partial pressures from 0.1 Pa to 30 Pa, the lattice constant decreased from 10.396 Å to 10.314 Å [41,45], indicating lower oxygen vacancy concentrations for higher deposition oxygen partial pressures. The average grain size for thin films deposited at 0.1 Pa, 1 Pa, 10 Pa and 30 Pa were 20.7 nm, 35.7 nm, 46.2 nm and 53.5 nm respectively. Figure 1(b) shows the surface morphology of ITO thin films fabricated under an oxygen partial pressure of 10 Pa measured by AFM. A root-mean-square (RMS) surface roughness of 1.46 nm is observed in this film. The RMS surface roughness of ITO films deposited at 0.1 Pa, 1 Pa and 30 Pa is 0.77 nm, 1.23 nm and 2.02 nm, respectively. The RMS surface roughness of the films increases with the increasing of oxygen partial pressures, which is consistent with previous reports [46].
Fig. 1. (a) XRD pattern of ITO thin films deposited under different oxygen partial pressures. Within the figure, black, red, green and blue curves represent thin films deposited under oxygen partial pressures of 0.1 Pa, 1 Pa, 10 Pa and 30 Pa, respectively. (b) AFM image ($1 \times 1\;\mu {m^2}$) of ITO thin films deposited at 10 Pa.
Figure 2 shows the refractive index and extinction coefficient of ITO films fabricated under different oxygen pressures and measured by spectral ellipsometry (SE). The refractive indices are fitted using the Drude-Lorentz model [7,47,48]:
Fig. 2. Refractive index n (solid line) and extinction coefficient ${\kappa }$ (dotted line) as a function of wavelength for ITO thin films fabricated under different oxygen partial pressures.
The mean-square-error (MSE) obtained by fitting the SE data was below 5 for all ITO films. In this fitting, we used three oscillators: the Drude oscillator describing the absorption of free carriers, the Lorentz oscillator describing the edge absorption, and the Guassian oscillator describing the near-infrared phonon absorption [49]. It can be seen from Fig. 2 that as the oxygen pressure increases, the refractive index of the ITO film increases, and the extinction coefficient decreases in the visible to near infrared. An increase in oxygen concentration results in a decrease in oxygen vacancies and a consequent decrease in carrier concentration, leading to lower optical loss of the ITO thin films. This result is also consistent with previous reports [1,2,44].
To verify the measured optical constants, we also carried out the Hall effect measurements on the ITO thin films. The carrier concentrations and Hall mobility characterized by both methods are shown in Table 1. The results obtained by both methods are consistent with each other, justifying the fitting results of the Drude-Lorentz model. The quantitative differences may be originated from the characterization sample size. For the Hall effect measurement, the transport data is collected on a large area (1×1cm2) of ITO films, while for the SE characterization, the data is collected from a micrometer size area (light spot diameter of 200 µm). From the table, we can see that the carrier concentration of the ITO thin films increases from 1.1×1020 cm−3 to 7.3×1020 cm−3 when the oxygen pressures is decreased from 30 Pa to 1 Pa due to more donor type oxygen vacancy formation at lower oxygen partial pressures. The carrier mobility also decreases, which may be attributed to scattering of free carriers resulted from excess oxygen vacancies, as can be observed in Fig. 1(a).
Table 1. Comparison of Hall effect measurements and Drude-Lorentz model fitting parameters for ITO thin films fabricated under different oxygen partial pressures
To characterize the optical constants the ITO thin films under applied electric fields, we fabricated ITO/SiO2/Si thin film stack structures as shown in the Fig. 3(a). The 40 nm thick ITO thin films deposited through a shadow mask formed a disk with a diameter of about 1 mm. In this stack, the ITO thin films act as the top electrode, while the p-type silicon substrate (0.005 Ω•cm) acts as the bottom electrode. The gate SiO2 oxide thickness is around 20 nm. A positive DC bias from 0 V to 5 V is applied on the silicon substrate, leading to electron accumulation in the ITO thin films at the interface, as shown in Fig. 3(b). The optical microscope image of this structure is shown in Fig. 3(c), where the gray disc is the ITO film and the light region is the silicon dioxide film. A DC bias is applied via the tungsten probe.
Fig. 3. (a) Schematic of the ITO/SiO2/Si thin films stack structure. A DC bias of 0 V to 5 V was applied a cross the SiO2 layer during the ellipsometry characterizations. (b) Cross-sectional view of the two-layer model under applied bias. (c) Optical micrograph pattern under the DC bias of the ITO films. The green circle indicates the incident light spot location of the ellipsometer.
We use a two-layer model based on Thomas-Fermi screening model to fit the optical constants of the ITO thin films under applied bias, where ${t_{TF}},\;\;{t_{bulk}}$ represent the thicknesses of the accumulation layer and the bulk layer of the ITO thin films, respectively. We fixed the optical constants of the bulk layer of the ITO thin films as the measured optical constants at 0 V and fitted the optical constants of the accumulation layer using the Drude-Lorentz model, as indicated in Eq. (1). The Kramers-Kronig relation is maintained during the fitting process [50]. According to the Thomas-Fermi screening model, the thickness of the accumulation layer ${t_{TF}}$ is given by [51,52]
Figure 4 shows the optical constants of the interface layer measured under different applied voltages for ITO thin films deposited at different oxygen partial pressures. We discarded the data of ITO films deposited at 0.1 Pa because the modulation amplitude was too small. Figure 4(a), (c), (e) show the refractive indices and extinction coefficients of the accumulation layer in the wavelength range from 210 nm to 1690 nm, with the applied voltage varied from 0 V to 5 V. A reversible change in the refractive index and extinction coefficients is observed in all films. Figure 4 (b), (d) and (f) show the zoom-in spectra in the near infrared wavelength range. A monotonic decrease of the refractive index and increase of the extinction coefficient is observed with increasing the applied voltage. This observation is consistent with the Drude model for the case of increasing electron concentrations in the accumulation layer. We find the maximum change of the optical constants takes place in the ITO thin films deposited at 10 Pa. When the applied voltage is 5 V(corresponding to an applied field of 2.5 MV/cm), for ITO thin films deposited at 1 Pa, 10 Pa and 30 Pa, the changes of refractive indices Δn are 0.008, 0.061, 0.01, and the change in the extinction coefficient Δκ are 0.023, 0.079, 0.006 at 1550 nm wavelength. The largest index modulation is observed in ITO thin films deposited at 10 Pa. This is because for low deposition pressures such as 0.1 Pa or 1 Pa, the initial electron concentration is very high. The electric field induced charge carrier accumulation is relatively too low to modulate the optical constants. Whereas when the deposition pressure is too high such as 30 Pa, the accumulation layer thickness is large according to Eq. (3), leading to a small change in the carrier concentration which is normalized by the accumulation layer volume under applied bias. Therefore, an optimum deposition pressure exists for the most efficient modulation of the optical constants of the interface layer.
Fig. 4. Refractive index (left axis) and extinction coefficient (right axis) of ITO films deposited under ${P_{{O_2}}}$ of (a) 1 Pa. (c) 10 Pa. (e) 30 Pa. and with different applied bias in a wavelength range from 210 to 1690 nm. Also shown are the zoom-in view of corresponding refractive index and extinction coefficient for ITO deposited at ${P_{{O_2}}}$ of (b) 1 Pa. (d) 10 Pa and (f) 30 Pa, respectively.
The above arguments can be further understood by calculating the carrier concentrations in the accumulation layer under electrostatic gating. Table 2 shows the variation of the Drude model parameters under different applied bias conditions. The high frequency permittivity ${\varepsilon _\infty }$, the carrier concentration n and the damping rate of charge carriers Γ are presented. From the table, we can see that ${\varepsilon _\infty }$ remains at around 4.10 with different applied biases, whereas the carrier concentration and the damping rate change significantly. At 5 V, the carrier concentrations increase by 7.1×1019 cm−3, 6.7×1019 cm−3 and 4.8×1019 cm−3 for ITO thin films deposited at 1 Pa, 10 Pa and 30 Pa, respectively. Meanwhile, the damping rate increases for all samples. This trend agrees with the scenario of electron accumulation under positively applied electric fields and free carrier induced scattering [32,54]. To quantitatively validate the carrier concentration modulation amplitude, we calculated the electron concentrations ${n_{acc}}$ by investigating the charge accumulation in the ITO/SiO2/Si capacitor structure under applied bias [8,27]:
Where Q is the charge accumulation of the capacitor under a DC bias, A is the unit area of the capacitor. We know that $Q = C \times {V_g} = {V_g} \times {\varepsilon _0} \times {\varepsilon _i} \times A/{t_{Si{O_2}}}$, where C is the capacitor, ${V_g}$ is the applied voltage, ${\varepsilon _i} = 3.9$ is the static relative permittivity of SiO2 and ${t_{Si{O_\textrm{2}}}}$ is the thickness of SiO2 films, which are 20.08 nm, 20.46 nm, 20.97 nm fitted from ellipsometry data, for the films deposited under oxygen partial pressures of 1 Pa, 10 Pa and 30 Pa, respectively. Assuming a uniform distribution of charge carriers in the accumulation layer, we substitute Q into Eq. (4), therefore obtaining ${n_{acc}} = \frac{{Q/e}}{{A \times {t_{TF}}}}{\ =\ }\frac{{{\varepsilon _0}{\varepsilon _i}{V_g}}}{{{t_{Si{O_\textrm{2}}}} \times {t_{TF}} \times e}}$. Under an applied voltage of ${V_g}$=5 V, the change of carrier concentrations in the accumulation layer is 8.0×1019 cm−3, 7.7×1019 cm−3 and 5.9×1019 cm−3 for ITO thin films deposited under ${P_{{O_2}}}$ of 1 Pa, 10 Pa and 30 Pa, respectively. These numbers agree very well with the results presented in Table 2, therefore verified the accuracy of our models.Table 2. The variation of the parameters obtained by the Drude model fitting under different oxygen pressures with an applied gate voltage
Finally, we show electric field tuning of the ENZ properties in Fig. 5. Electric field modulation of the carrier concentration in the interface layer lead to a modulation of the ENZ wavelength of the ITO thin films. Under 0 applied bias, the ENZ wavelength for ITO thin films deposited at 1 Pa, 10 Pa and 30 Pa are at 1392.8 nm, 1558.2 nm and 1564.7 nm, respectively. Under an applied bias, the ENZ wavelength blue-shifted to shorter wavelengths for all samples, consistent with the charge accumulation process. The maximum ENZ wavelength modulation is observed in ITO thin films deposited at 10 Pa, which shows an ENZ wavelength ranging from 1558.2 nm to 1530.3 nm for 0 V and 5 V applied DC bias. This is accompanied by a permittivity modulation amplitude of 0.15 at 1550 nm wavelength. These results show that ITO thin films with SiO2 gate oxides can be efficiently tuned around the telecommunication wavelength under optimum deposition oxygen partial pressures, which is important for electro-optic modulator and phase shifter applications [52,55].
Fig. 5. Tuning the ENZ wavelength of ITO thin films under different applied bias for ITO thin films deposited at: (a) 1 Pa. (b) 10 Pa and (c) 30 Pa.
4. Conclusions
In summary, we report the effect of oxygen stoichiometry on the structure, optical and ENZ properties of ITO thin films deposited by PLD. By using a two-layer model based on the Thomas-Fermi screening model, we observed a systematic tuning of the optical constants of ITO thin films by applied DC bias. Under an accumulation gate bias, a monotonic increase of the carrier concentration and damping rate is observed in all ITO thin films fabricated under different oxygen partial pressures. An optimum oxygen stoichiometry and deposition partial pressure is observed for achieving the largest modulation amplitude in ITO thin films. For applied DC field of 2.5 MV/cm, a maximum ENZ wavelength modulation range from 1530.3 nm to 1558.2 nm is observed in ITO thin films deposited at ${P_{{O_2}}}$=10 Pa, accompanied with a refractive index modulation of Δn = 0.061 and Δκ=0.079 at 1550 nm. Our results provide insights for the design and fabrication of electro-optic modulators, phase shifters based on ITO thin films.
Funding
National Natural Science Foundation of China (51522204, 6147503); Ministry of Science and Technology of the People's Republic of China (2016YFA0300802, 2018YFE0109200).
References
1. Z. Ma, Z. Li, K. Liu, C. Ye, and V. J. Sorger, “Indium-Tin-Oxide for High-performance Electro-Optic Modulation,” Nanophotonics 4(1), 198–213 (2015). [CrossRef]
2. M. G. Wood, S. Campione, S. Parameswaran, T. S. Luk, J. R. Wendt, D. K. Serkland, and G. A. Keeler, “Gigahertz speed operation of epsilon-near-zero silicon photonic modulators,” Optica 5(3), 233–236 (2018). [CrossRef]
3. N. Kinsey, C. DeVault, J. Kim, M. Ferrera, V. M. Shalaev, and A. Boltasseva, “Epsilon-near-zero Al-doped ZnO for ultrafast switching at telecom wavelengths,” Optica 2(7), 616–622 (2015). [CrossRef]
4. A. Melikyan, N. Lindenmann, S. Walheim, P. M. Leufke, S. Ulrich, J. Ye, P. Vincze, H. Hahn, T. Schimmel, C. Koos, W. Freude, and J. Leuthold, “Surface plasmon polariton absorption modulator,” Opt. Lett. 19(9), 8855–8869 (2011). [CrossRef]
5. V. J. Sorger, N. D. Lanzillotti-Kimura, R. M. Ma, and X. Zhang, “Ultra-compact silicon nanophotonic modulator with broadband response,” Nanophotonics 1(1), 17–22 (2012). [CrossRef]
6. H. W. Lee, G. Papadakis, S. P. Burgos, K. Chander, A. Kriesch, R. Pala, U. Peschel, and H. A. Atwater, “Nanoscale conducting oxide PlasMOStor,” Nano Lett. 14(11), 6463–6468 (2014). [CrossRef]
7. K. Shi, R. R. Haque, B. Zhao, R. Zhao, and Z. Lu, “Broadband electro-optical modulator based on transparent conducting oxide,” Opt. Lett. 39(17), 4978–4981 (2014). [CrossRef]
8. S. Zhu, G. Q. Lo, and D. L. Kwong, “Design of an ultra-compact electro-absorption modulator comprised of a deposited TiN/HfO2/ITO/Cu stack for CMOS backend integration,” Opt. Express 22(15), 17930–17947 (2014). [CrossRef]
9. H. Zhao, Y. Wang, A. Capretti, L. D. Negro, and J. Klamkin, “Broadband Electroabsorption Modulators Design Based on Epsilon-Near-Zero Indium Tin Oxide,” IEEE J. Sel. Top. Quantum Electron. 21(4), 192–198 (2015). [CrossRef]
10. L. Jin, Q. Chen, W. Liu, and S. Song, “Electro-absorption Modulator with Dual Carrier Accumulation Layers Based on Epsilon-Near-Zero ITO,” Plasmonics 11(4), 1087–1092 (2016). [CrossRef]
11. J. Yoon, M. Zhou, M. A. Badsha, T. Y. Kim, Y. C. Jun, and C. K. Hwangbo, “Broadband Epsilon-Near-Zero Perfect Absorption in the Near-Infrared,” Sci. Rep. 5(1), 12788 (2015). [CrossRef]
12. S. G. C. Carrillo, G. R. Nash, H. Hayat, M. J. Cryan, M. Klemm, H. Bhaskaran, and C. D. Wright, “Design of practicable phase change metadevices for near infrared absorber and modulator applications,” Opt. Lett. 24(12), 13563–13573 (2016). [CrossRef]
13. J. S. Kim and J. T. Kim, “Silicon electro-optic modulator based on an ITO-integrated tunable directional coupler,” J. Phys. D: Appl. Phys. 49(7), 075101 (2016). [CrossRef]
14. A. O. Zaki, K. Kirah, and M. A. Swillam, “Hybrid plasmonic electro-optical modulator,” Appl. Phys. A 122(4), 473 (2016). [CrossRef]
15. A. Anopchenko, L. Tao, C. Arndt, and H. W. H. Lee, “Field-Effect Tunable and Broadband Epsilon-Near-Zero Perfect Absorbers with Deep Subwavelength Thickness,” ACS Photonics 5(7), 2631–2637 (2018). [CrossRef]
16. Q. Gao, E. Li, and A. X. Wang, “Ultra-compact and broadband electro-absorption modulator using an epsilon-near-zero conductive oxide,” Photonics Res. 6(4), 277–281 (2018). [CrossRef]
17. L. Tao, A. Anopchenko, S. Gurung, J. Zhang, and H. W. H. Lee, “Gate-Tunable Plasmon-Induced Transparency Modulator Based on Stub-Resonator Waveguide with Epsilon-Near-Zero Materials,” Sci. Rep. 9(1), 2789 (2019). [CrossRef]
18. X. Liu, J.-H. Kang, H. Yuan, J. Park, Y. Cui, H. Y. Hwang, and M. L. Brongersma, “Tuning of Plasmons in Transparent Conductive Oxides by Carrier Accumulation,” ACS Photonics 5(4), 1493–1498 (2018). [CrossRef]
19. S. Vassant, A. Archambault, F. Marquier, F. Pardo, U. Gennser, A. Cavanna, J. L. Pelouard, and J. J. Greffet, “Epsilon-near-zero mode for active optoelectronic devices,” Phys. Rev. Lett. 109(23), 237401 (2012). [CrossRef]
20. N. Engheta, “Pursing Near-Zero Response,” Science 340(6130), 286–287 (2013). [CrossRef]
21. Y. C. Jun, J. Reno, T. Ribaudo, E. Shaner, J. J. Greffet, S. Vassant, F. Marquier, M. Sinclair, and I. Brener, “Epsilon-near-zero strong coupling in metamaterial-semiconductor hybrid structures,” Nano Lett. 13(11), 5391–5396 (2013). [CrossRef]
22. P. Moitra, Y. Yang, Z. Anderson, I. I. Kravchenko, D. P. Briggs, and J. Valentine, “Realization of an all-dielectric zero-index optical metamaterial,” Nat. Photonics 7(10), 791–795 (2013). [CrossRef]
23. S. Campione, I. Brener, and F. Marquier, “Theory of epsilon-near-zero modes in ultrathin films,” Phys. Rev. B 91(12), 121408 (2015). [CrossRef]
24. M. Z. Alam, I. D. Leon, and R. W. Boyd, “Large optical nonlinearity of indium tin oxide in its epsilon-near-zero region,” Science 352(6287), 795–797 (2016). [CrossRef]
25. L. Caspani, R. P. Kaipurath, M. Clerici, M. Ferrera, T. Roger, J. Kim, N. Kinsey, M. Pietrzyk, A. Di Falco, V. M. Shalaev, A. Boltasseva, and D. Faccio, “Enhanced Nonlinear Refractive Index in epsilon-Near-Zero Materials,” Phys. Rev. Lett. 116(23), 233901 (2016). [CrossRef]
26. A. Ciattoni, A. Marini, and C. Rizza, “All-optical modulation in wavelength-sized epsilon-near-zero media,” Opt. Lett. 41(13), 3102–3105 (2016). [CrossRef]
27. F. Yi, E. Shim, A. Y. Zhu, H. Zhu, J. C. Reed, and E. Cubukcu, “Voltage tuning of plasmonic absorbers by indium tin oxide,” Appl. Phys. Lett. 102(22), 221102 (2013). [CrossRef]
28. Y. Wang, A. Capretti, and L. Dal Negro, “Wide tuning of the optical and structural properties of alternative plasmonic materials,” Opt. Mater. Express 5(11), 2415 (2015). [CrossRef]
29. Y. Wang, A. C. Overvig, S. Shrestha, R. Zhang, R. Wang, N. Yu, and L. Dal Negro, “Tunability of indium tin oxide materials for mid-infrared plasmonics applications,” Opt. Mater. Express 7(8), 2727–2739 (2017). [CrossRef]
30. Y. Li and C. Argyropoulos, “Exceptional points and spectral singularities in active epsilon-near-zero plasmonic waveguides,” Phys. Rev. B 99(7), 075413 (2019). [CrossRef]
31. Y. Wang, H. Zhao, D. Huo, H. Su, C. Wang, and J. Zhang, “Accumulation-layer hybridized surface plasmon polaritions at an ITO/LiNbO3 interface,” Opt. Lett. 44(4), 947–950 (2019). [CrossRef]
32. G. V. Naik, V. M. Shalaev, and A. Boltasseva, “Alternative plasmonic materials: beyond gold and silver,” Adv. Mater. 25(24), 3264–3294 (2013). [CrossRef]
33. P. R. West, S. Ishii, G. V. Naik, N. K. Emani, V. M. Shalaev, and A. Boltasseva, “Searching for better plasmonic materials,” Laser Photonics Rev. 4(6), 795–808 (2010). [CrossRef]
34. B. Dastmalchi, P. Tassin, T. Koschny, and C. M. Soukoulis, “A New Perspective on Plasmonics: Confinement and Propagation Length of Surface Plasmons for Different Materials and Geometries,” Adv. Opt. Mater. 4(1), 177–184 (2016). [CrossRef]
35. P. Guo, R. D. Schaller, J. B. Ketterson, and R. P. H. Chang, “Ultrafast switching of tunable infrared plasmons in indium tin oxide nanorod arrays with large absolute amplitude,” Nat. Photonics 10(4), 267–273 (2016). [CrossRef]
36. A. Forouzmand and H. Mosallaei, “Real-Time Controllable and Multifunctional Metasurfaces Ultilizing Indium Tin Oxide Materials: Phase Array Perspective,” IEEE Trans. Nanotechnol. 16(2), 296–306 (2017). [CrossRef]
37. A. Forouzmand, M. M. Salary, S. Inampudi, and H. Mosallaei, “A Tunable Multigate Indium-Tin-Oxide-Assisted All-Dielectric Metasurface,” Adv. Opt. Mater. 6(7), 1701275 (2018). [CrossRef]
38. Y. Li and C. Argyropoulos, “Tunable nonlinear coherent perfect absorption with epsilon-near-zero plasmonic waveguides,” Opt. Lett. 43(8), 1806–1809 (2018). [CrossRef]
39. T. Cui, B. Bai, and H.-B. Sun, “Tunable Metasurfaces Based on Active Materials,” Adv. Funct. Mater. 29(10), 1806692 (2019). [CrossRef]
40. E. Feigenbaum, K. Diest, and H. A. Atwater, “Unity-order index change in transparent conducting oxides at visible frequencies,” Nano Lett. 10(6), 2111–2116 (2010). [CrossRef]
41. S. Peng, X. Cao, J. Pan, X. Wang, X. Tan, A. E. Delahoy, and K. K. Chin, “X-ray Photoelectron Spectroscopy Study of Indium Tin Oxide Films Deposited at Various Oxygen Partial Pressures,” J. Electron. Mater. 46(2), 1405–1412 (2017). [CrossRef]
42. S.-I. Jun, T. E. McKnight, M. L. Simpson, and P. D. Rack, “A statistical parameter study of indium tin oxide thin films deposited by radio-frequency sputtering,” Thin Solid Films 476(1), 59–64 (2005). [CrossRef]
43. X. Fang, C. L. Mak, S. Zhang, Z. Wang, W. Yuan, and H. Ye, “Pulsed laser deposited indium tin oxides as alternatives to noble metals in the near-infrared region,” J. Phys.: Condens. Matter 28(22), 224009 (2016). [CrossRef]
44. H. Kim, C. M. Gilmore, A. Piqué, J. S. Horwitz, H. Mattoussi, H. Murata, Z. H. Kafafi, and D. B. Chrisey, “Electrical, optical, and structural properties of indium–tin–oxide thin films for organic light-emitting devices,” J. Appl. Phys. 86(11), 6451–6461 (1999). [CrossRef]
45. L. Hao, X. Diao, H. Xu, B. Gu, and T. Wang, “Thickness dependence of structural, electrical and optical properties of indium tin oxide (ITO) films deposited on PET substrates,” Appl. Surf. Sci. 254(11), 3504–3508 (2008). [CrossRef]
46. M. M. Munir, F. Iskandar, K. M. Yun, K. Okuyama, and M. Abdullah, “Optical and electrical properties of indium tin oxide nanofibers prepared by electrospinning,” Nanotechnology 19(14), 145603 (2008). [CrossRef]
47. J. Gwamuri, M. Marikkannan, J. Mayandi, P. K. Bowen, and J. M. Pearce, “Influence of Oxygen Concentration on the Performance of Ultra-Thin RF Magnetron Sputter Deposited Indium Tin Oxide Films as a Top Electrode for Photovoltaic Devices,” Materials 9(1), 63 (2016). [CrossRef]
48. D. B. Tice, S. Q. Li, M. Tagliazucchi, D. B. Buchholz, E. A. Weiss, and R. P. Chang, “Ultrafast modulation of the plasma frequency of vertically aligned indium tin oxide rods,” Nano Lett. 14(3), 1120–1126 (2014). [CrossRef]
49. P. Guo, R. P. H. Chang, and R. D. Schaller, “Transient Negative Optical Nonlinearity of Indium Oxide Nanorod Arrays in the Full-Visible Range,” ACS Photonics 4(6), 1494–1500 (2017). [CrossRef]
50. P. Uprety, M. M. Junda, K. Ghimire, D. Adhikari, C. R. Grice, and N. J. Podraza, “Spectroscopic ellipsometry determination of optical and electrical properties of aluminum doped zinc oxide,” Appl. Surf. Sci. 421, 852–858 (2017). [CrossRef]
51. K. E. Peiponen and E. M. Vartiainen, “Kramers-Kronig relations in optical data inversion,” Phys. Rev. B 44(15), 8301–8303 (1991). [CrossRef]
52. A. V. Krasavin and A. V. Zayats, “Photonic signal processing on electronic scales: electro-optical field-effect nanoplasmonic modulator,” Phys. Rev. Lett. 109(5), 053901 (2012). [CrossRef]
53. J. Baek, J. B. You, and K. Yu, “Free-carrier electro-refraction modulation based on a silicon slot waveguide with ITO,” Opt. Express 23(12), 15863–15876 (2015). [CrossRef]
54. F. Neumann, Y. A. Genenko, C. Melzer, S. V. Yampolskii, and H. von Seggern, “Self-consistent analytical solution of a problem of charge-carrier injection at a conductor/insulator interface,” Phys. Rev. B 75(20), 205322 (2007). [CrossRef]
55. Y. W. Huang, H. W. Lee, R. Sokhoyan, R. A. Pala, K. Thyagarajan, S. Han, D. P. Tsai, and H. A. Atwater, “Gate-Tunable Conducting Oxide Metasurfaces,” Nano Lett. 16(9), 5319–5325 (2016). [CrossRef]
