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Abstract
In this paper, we prove that indium tin oxide (ITO) can be a material of choice for wavelength-selective thermal emitters that enable operation in ambient air. The wavelength-selective thermal emission is achieved by plasmonic ITO perfect absorbers (IPAs) composed of ITO – Al2O3 – ITO tri-layers where the top ITO layer is a hexagonal array of ITO disks. Simulated optical spectra of the IPAs reveal nearly perfect absorptivities (i.e. 0.99) at obvious resonances, which can be easily tuned just by changing the size of the ITO disks. The fabricated IPAs that followed the pre-designed simulation also exhibit excellented resonant absorptivities (i.e. 0.92), as well as good resonance tunability, which indicate the appreciable performance of IPAs. Furthermore, we show that IPAs are also feasible to serve as highly-efficient thermal emitters that can operate in ambient air without the need for strict vacuum or inert gas conditions, providing another platform for IR plasmonic devices.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Engineering the thermal emission using wavelength-selective perfect absorbers [1] covers a wide range of applications in infrared (IR) photonics and thermal managements ranging from thermophotovoltaics [2–5] to non-dispersive IR (NDIR) gas sensors [6,7] and radiative coolers [8–11]. For the practical applications, plasmonic materials such as tungsten [12,13], molybdenum [14] or metal nitrides and metal carbides [15–19] have been proposed; however, they are not robust against oxidation if they are used at elevated temperature in air. Hence, the practical applications of thermal metallic emitters are limited to the strict vacuum or inert gas ambient conditions. Thus, serious demand exists for IR plasmonic materials that are cost-effective and robust at high temperature. In this regard, transparent conductive oxides (TCOs) are already in oxidized state and thus inherently robust in ambient air high-temperature operations. They exhibit screening plasma frequencies in the near-IR region owing to their lower carrier concentrations (1020–1022 cm-3) and thus leading to low-loss nature compared to the elemental metals (1023 cm-3), which render them as promising candidate of IR plasmonic materials.
In the past decades, commonly utilized TCOs including indium tin oxide (ITO) [20–24], fluorine doped tin oxide (FTO) [25,26] or doped zinc oxide (AZO and GZO) [27–29] have been found to be attractive in IR plasmonics [30]. Among them, ITO has shown significant advantages compared to other TCOs owing to its robustness in moist air, surface smoothness, mechanical stability and simple fabrication process [31]. Thus, ITO has been intensively applied in many studies subjected to localized plasmonic nanoparticles [32–34], optical metasurfaces [35,36], antenna arrays for surface-enhanced IR absorption spectroscopy [37–39], active tunable plasmonic devices [40–43]. Although ITO is a favourable IR plasmonic material for wavelength-selective perfect absorbers and thermal emitters with the chemical and electrical stability at elevated temperature [44], it has not been used for these applications so far.
In this paper, to our best knowledge, wavelength-selective thermal emitters made of IR plasmonic ITO operated in ambient air were demonstrated for the first time. The proposed ITO perfect absorbers (IPAs) were composed of tri-layered ITO – Al2O3 – ITO wherein the top ITO disk resonators array was separated to the bottom ITO film by the Al2O3 insulator. The IPA structures were numerically simulated to optimize the geometrical parameters and then realized using a scalable colloidal lithography process. The fabricated IPAs exhibited nearly perfect resonant absorptions (i.e. 0.92) with polarization independence and facile tunability. We demonstrated that the IPA thermal emitters can efficiently operate in ambient air with desirable wavelength selectivity, high emissivity and high stability against the temperature changes. The proposed IPAs presented in this work can be used for practical IR devices such as surface-enhanced IR absorption spectroscopy, NDIR sensors, as well as visible-transparent – IR wavelength-selective heaters for drying furnace or radiative cooling for smart window. Although this work demonstrates ITO thermal emitters in the IR region, the resonance can extend to the THz region, and the concept is also applicable to other TCOs.
The remainder of the paper is organized as follows. The second section describes the structural design and simulated optical properties of the proposed IPAs. The third section provides a detailed fabrication procedure to realize scalable IPAs. The thermal emission and emissivity spectra of IPAs are demonstrated in the fourth section. Finally, the paper is summarized in the fifth section.
2. Proposed IPA structure and optical properties
Prior to performing numerical simulation of the proposed IPAs, spectroscopic ellipsometry was conducted for the ITO film fabricated by sputtering to obtain the actual optical property of ITO for the accurate numerical simulations and further fabrications. Herein, a 200 nm ITO film was deposited on a Si (100) substrate (p-type, 10 Ω·cm) by RF sputtering (i-Miller CFS-4EP-LL, Shibaura) using an ITO target (In2O3/SnO2 90/10 wt %). The sputtered ITO film was then annealed at 400 °C in vacuum condition for 1 hour to improve the conductivity and optical property of ITO. The film was optically characterized by two spectroscopic ellipsometers (SENTECH, SE 850 DUV for UV – NIR region and SENDIRA for mid-IR region). The complex permittivity ($\varepsilon = {\varepsilon _1} + i{\varepsilon _2}$) over a wide wavelength range, from 0.19 μm to 25 μm, derived from the ellipsometry measurements of the ITO film is shown in Fig. 1(a) (red curves). As seen, the real part of the sputtered ITO’s permittivity approaches negative values beyond 1.8 μm in wavelength, which evidences the plasmonic behavior of ITO in the IR region. For a comparison, the complex permittivities of two typical metals that are commonly used in IR perfect absorbers and thermal emitters; a noble metal – Au (orange curves) and a refractory metal – W (blue curves) taken from literature [45] are also plotted in Fig. 1(a). We also plot the ratio (${{ - {\varepsilon _1}} \mathord{\left/ {\vphantom {{ - {\varepsilon_1}} {{\varepsilon_2}}}} \right.} {{\varepsilon _2}}}$) between the real part and imaginary part of permitivities of the three plasmonic metals (Fig. (1b)), which reflect their figure-of-merit (FoM) values to determine the field enhancement of localized surface plasmon resonance (LSPR). As seen in Fig. 1(b), although both real part and imaginary part of the sputtered ITO film are much smaller than those of Au and W, nevertheless, the FoM value of ITO is almost comparable to that of Au and W in the mid-IR region; it is slightly smaller than the FoM of Au but higher than the FoM of W. This indicates that ITO can be a good material for IR plasmonic devices utilizing LSPR, especially for IR plasmonic absorbers.
Fig. 1. (a) A comparison between measured permittivity of the sputtered ITO film (red curves) with permitivities of Au (orange curves) and W (blue curves). Negative real part of ITO’s permittivity clearly reveals plasmonic behavior of ITO in the IR region. The complex permitivities of Au and W taken from the literature [45] were divided by a factor of 10 for a better comparison. (b) Figure-of-merit ($- {{{\varepsilon _1}} \mathord{\left/ {\vphantom {{{\varepsilon_1}} {{\varepsilon_2}}}} \right.} {{\varepsilon _2}}}$, solid curves) and skin depths (dashed curves) of ITO (red), Au (orange) and W (blue).
Here, we applied IR plasmonic ITO for IR perfect absorbers by adopting a metal – insulator – metal configuration wherein the top layer made of a hexagonal array of ITO disks is isolated from the ITO bottom mirror via an Al2O3 film (Fig. 2(a)). Employing the rigorous coupled-wave analysis (RCWA) simulation (DiffractMOD, Synopsys’ RSoft) with the measured ITO permittivity, whereas the permitivities of Al2O3 and Si were taken from the literature [46], we demonstrated that with a proper design and optimal parameters, perfect absorptivities could be achieved. Figure 2(b) plots simulated spectra of an IPA array with a periodicity – p of 3 μm, diameter – d of 2.01 μm and insulator thickness – t of 0.4 μm. It is worth noting that because of a larger skin depth – $\delta$ ($\delta = {\lambda \mathord{\left/ {\vphantom {\lambda {({2\pi \kappa } )}}} \right.} {({2\pi \kappa } )}}$, with $\kappa$ is the extinction coefficient) compared to elemental metals (dashed curves in Fig. 1(b)), the thickness of ITO resonators and ITO film designed for absorbers are certainly thicker than those of elemental metals. In this work, the thicknesses of ITO disks and ITO films are optimized and kept unchanged at 0.15 μm and 0.4 μm, respectively. As seen in Fig. 2(b), IPA exhibits two main resonant bands at normal incidence; the first one locating at the shorter wavelength region (∼ 4.0 μm), namely M1, behaves like a dual-band resonance with high absorptivities (i.e. 0.93) whereas the second one at the longer wavelength region (8.4 μm), namely M2, reveals a nearly perfect absorptivity (i.e. 0.99). Interestingly, M2 can be easily tuned by changing the diameter – d of ITO disk [1,47,48]. As shown in Fig. 2(c), M2 readily tunes from 5 μm to 10 μm when the ITO diameter changes from 1.0 μm to 2.7 μm. In addition, the resonant peak of M2 is directly proportional to the disk diameter, indicating that M2 behaves like the dipole antenna resonance. In contrast, M1 acts as a hybrid resonance that splits into two peaks when the diameter of ITO disk increases.
Fig. 2. (a) Scheme of IPA configured by tri-layered ITO – Al2O3 – ITO with a hexagonal lattice ITO disk resonator array arranged on the top of Al2O3-ITO layered films. Dashed rectangle indicates the unit cell defined in the simulation. (b) Simulated optical spectra including absorptivity – A (red curve), transmittance – T (blue curve) and reflectance – R (black curve) of IPA with a periodicity – p of 3 μm, diameter – d of 2.01 μm and insulator thickness – t of 0.4 μm. (c) Resonance tunability of IPA by changing diameter – d of ITO disk resonator while keeping the other parameters unchanged (p = 3 μm, t = 0.4 μm). The thicknesses of ITO disk and ITO film are kept unchanged at 0.15 μm and 0.4 μm, respectively. In the simulation, the incident electromagnetic field propagated along the –z-axis, the electric field oscillated along the x-axis.
To further verify the optical properties of IPA, full wave simulations based on the finite-difference time-domain (FDTD) method (FullWAVE, Synopsys’ RSoft) were also performed to calculate distributions of electric fields (Ex and Ez) and magnetic field (Hy) in IPA excited at both M1 (Fig. 3(a)) and M2 (Fig. 3(b)). As seen, the induced electric fields Ez clearly indicate that the excited electric dipoles at the top and bottom metals oscillate in the inverse phases, resulting in significantly enhanced magnetic fields in the insulating layer between the top and bottom electric dipoles. The resonances are the so-called magnetic resonances; the first-order (fundamental) magnetic resonance at M2 and the third-order magnetic resonance at M1. In addition, we observe that the magnetic dipoles in each unit cell at M1 couple to the others along the incident plane, which suggests that M1 arises from the coupling between the third-order magnetic resonance with the photonic property of the hexagonal lattice. For a plasmonic hexagonal lattice, the angular-dependent dispersion relation of the surface plasmon polariton (SPP) resonance excited by an electromagnetic wave with a wavelength of $\lambda$ under an incidence of $\theta$ (see Fig. 2(a)) is given by
Fig. 3. Simulated electric fields (Ex and Ez) and magnetic field (Hy) distributions of the IPA (d = 2.01 μm, p = 3 μm, t = 0.4 μm) excited at two resonance modes: (a) at M1 (wavelength 4.0 μm) and (b) at M2 (8.4 μm). (c) Simulated angle-dependent absorptivity of the IPA with geometrical parameters of d = 2.01 μm, p = 3 μm, t = 0.4 μm. The dashed curves indicate SPPs at metal-dielectric interfaces of ITO disk hexagonal lattice (see Eq. (1)). (d) Simulated insulator thickness dependence of IPA’s absorptivity while keeping d and p unchanged at 2.01 μm and at 3 μm, respectively. In the simulation, the incident electromagnetic field propagated along the –z-axis, the electric field oscillated along the x-axis, and the incident amplitudes were normalized to 1.
3. Fabrication of IPAs
The proposed IPAs were fabricated following the geometrical parameters optimized by the pre-designed simulations. Here we used a colloidal lithography combined with the reactive-ion etching (RIE) process developed in our previous works to fabricate large-area IPAs with the size of 5×28 mm2 [48,49]. Figure 4(a) describes the fabrication process. Herein, tri-layered ITO (0.15 μm) – Al2O3 (0.4 μm) – ITO (0.4 μm) films followed the optimal parameters taken from the pre-designed simulations were deposited on 5×28 mm2 Si substrates by utilizing three RF sputtering steps. To fabricate the top ITO disk array, a monolayer of polystyrene (PS) spheres with diameter of 3.0 μm (Polybead Polystyrene Microspheres – Polysciences) was deposited on top of each tri-layered film as RIE mask. To reduce the size of the PS spheres, oxygen plasma etching (ULVAC CE-300I) with an etching rate of 2 nm/second was applied. The etching time on each sample was precisely controlled to achieve different PS sizes corresponding to the pre-designed ITO disks. Then, another RIE step was also employed to etch ITO with PS mask using the following recipe: mixture gases of BCl3 (10 sccm) and Cl2 (10 sccm); APC pressure of 0.3 Pa; RF antenna power of 200 W; RF bias power of 10 W; etching rate of 0.23 nm/second. IPAs were finally achieved by removing the PS mask in toluene and ethanol.
Fig. 4. (a) Schematic diagrams illustrate scalable fabrication process of IPAs using colloidal lithography combined two steps of RIE. (b) SEM image (top) and photo (bottom) of the fabricated IPA. Arrows indicate typical defects of periodic ITO disk array. (c) SEM images of two fabricated IPAs having diameters of 1.62 μm (S1) and 2.01 μm (S2) with the same other parameters (p = 3 μm, t = 0.4 μm). (d) Simulated absorptivities and (e) measured absorptivities of S1 (blue curves) and S2 (red curves).
Figure 4(b) represents a typical top-view scanning electron microscope (SEM) image (top) and a photo (bottom) of the fabricated IPAs. Despite some defects (white arrows in Fig. 4(b)) in the fabricated ITO disk lattice due to the non-uniformity and misalignments of PS array used as mask for RIE, the ITO disk hexagonal array was well-constructed as the pre-designed structure with grain sizes in the range of few tens to hundred microns scale. This result proves that the concise fabrication technique is applicable for realizing large-area IPAs. In this work, we fabricated two IPAs with the pre-designed geometrical parameters of p = 3 μm, t = 0.4 μm and different diameters obtained by simulations, namely S1 with d = 1.62 μm, and S2 with d = 2.01 μm. SEM images of the two fabricated absorbers are shown in Fig. 4(c). The transmittivity (T) and the reflectivity (R) spectra of the two fabricated IPAs were measured using a Fourier transform infrared (FTIR) spectrometer (Thermo Nicolet NEXUS 670, MCT detector, KBr beam splitter), then the absorptivity (A) spectra was achieved using the below relation
The simulated and measured absorptivity spectra of S1 (blue curves) and S2 (red curves) at normal incidence are plotted in Fig. 4(d) and Fig. 4(e), respectively. As shown, both S1 and S2 exhibit two main absorption bands; the SPP coupled third-order magnetic resonances at the shorter wavelength region are 3.5 μm (S1) and 4.0 μm (S2) and the fundamental magnetic resonances at the longer wavelength region are 7.5 μm (S1) and 8.4 μm (S2). The mode splitting at M1 resulted from the coupling between the third-order magnetic resonance and SPP (-1,0) mode of metallic hexagonal lattice is also clearly observed in both simulation and experiment, especially when the ITO disk diameter increases from 1.62 μm (S1) to 2.01 μm (S2), which is consistent with the above discussion (Fig. 2(c)). In particular, the measured absorptivity spectra agree well with the simulated absorptivity spectra, especially the resonant peaks and shapes, indicating the appropriate fabrication process used in this work. Furthermore, because of the high-rotational symmetry, the ITO periodic array absorber shows no dependence on the polarization as proven by both simulation (Fig. 5(a)) and measurement (Fig. 5(b)). This certainly makes the absorber more substantial in the practical applications such as highly efficient thermal emitters and other IR photonics devices with unpolarized light sources.Fig. 5. Polarization-independent absorptivity of IPA with geometrical parameters of d = 2.01 μm, p = 3 μm, t = 0.4 μm: (a) Simulation and (b) measurement.
4. Selective thermal emission of IPAs
A promising application of resonant perfect absorbers is spectrally selective thermal emitters. According to the Kirchhoff's law of thermal radiation, the thermal emissivity of an arbitrary body equals to its absorptivity at the thermodynamic equilibrium. Thus, a perfect absorber can inevitably serve as a highly-efficient emitter. Although most wavelength selective thermal emitters are made of metallic structures, only Au- and Pt-based emitters can operate in ambient air. Other metal-based emitters are not compatible with high-temperature ambient in the presence of oxygen gas or water vapor. In the following section, we demonstrate that ITO-based thermal emitters can work in air without any degradation in their performances and do not require any strict vacuum or inert gas ambient conditions. The fabricated S1 and S2 perfect absorbers were heated up using a heating plate and the emission spectra were characterized by a Fourier-transform infrared (FTIR) spectrometer (Thermo Nicolet NEXUS 670). The emitter can also be heated directly by applying an electrical current through the bottom ITO film. Figure 6(a) depicts the thermal emission measurement setup. As shown in this figure, the emission from IPAs are collected and guided to an FTIR spectrometer using a pair of concave metal coated mirrors. Emission spectra at normal angle of S2 IPA operated in ambient air at different temperatures varying from 178 °C to 307 °C measured at the surface of the emitter are described in Fig. 6(b). It is worth noting that in each measurement, the emitter was kept at a stabilized temperature and the emission spectrum was averaged for 100 spectra. Interestingly, the ITO emitter – S2 emits thermal radiation that has peaks at exactly the same resonant features of the IPA. In particular, the emission peaks almost did not change when the temperature increases from 178 °C to 307 °C, even kept at 307 °C for few hours, indicating the stability against the change of temperature and the robustness in ambient air of this wavelength-selective ITO emitter. Although the ITO emitter has shown good stability up to 307 °C after each time use, and the microstructure could retain the morphology at high temperatures; the device performance degraded gradually at higher temperatures above 400 °C (Appendix, Fig. 7). The ITO emitter can be another candidate besides gold emitter to work in air ambient; however, for the high temperature applications, refractory tungsten [13,50] and metal nitrides [15,16] are favorable materials since they have shown good thermal stability at very high temperatures.
Fig. 6. (a) Schematic diagrams of emissivity measurements. (b) Measured emission spectra of IPA – S2 at different temperatures ranging from 178 °C to 307 °C. Dashed black curve plots black body emission measured at 307 °C. (c) Simulated emissivity and (d) measured emissivity of S1 and S2.
To verify the emissivity of the IPAs, the emission spectrum from a black body paint (JSC-3, Japan Sensor Corp.) at 307 °C was also taken for the emissivity determination (dotted curve in Fig. 6(b)). The emissivity spectra of S1 and S2 were therefore calculated by dividing their emission spectra taken at 307 °C to the black body’s emission spectrum taken above. Figures 6(c)–(d) plot the simulated and measured emissivity spectra of S1 (blue curves) and S2 (red curves), respectively. It should be noted that the simulated emissivity is identical to the simulated absorptivity according to the Kirchhoff's law. Interestingly, the measured emissivity spectra of the fabricated IPAs agree well with the simulated spectra of the pre-designed devices with high emissivity (above 0.86), and they also match perfectly to their measured absorptivity spectra shown in Fig. 4(e). Thus, the proposed IPA and fabrication process are realizable and can be used for various practical applications. For examples, by designing the resonances of the IPA matching to the molecular vibrations, the IPA can be used as the IR source for NDIR gas sensors or wavelength-selective heaters for food and material drying. Because the IPA composed of ITO and Al2O3, which are transparency in the visible region, the IPA devices can be applied for visible-transparent – IR wavelength-selective emitters for transparent heaters or shelf-cooling smart windows by radiative cooling.
5. Summary
In summary, we have demonstrated that ITO is a promising plasmonic material of choice for IR wavelength-selective perfect absorbers and their application in thermal emitters. The proposed IPAs composed of an ITO disk hexagonal lattice isolated to the bottom ITO mirror via an Al2O3 insulator. The IPA structures were numerically simulated to verify their optical properties, and then successfully fabricated using a scalable colloidal lithography fabrication process. The fabrication recipe proposed in this work precisely embodied the pre-designed IPA devices. The fabricated IPAs exhibited desired performance as high-performance thermal emitters, such as polarization independence, near-perfect absorptivity, and resonance tunability. Furthermore, the fabricated IPA emitters stably work in ambient air up to 307 °C without any vacuum or inert gas ambient conditions, providing another platform that can reduce the cost and be merged into practical IR devices.
Appendix
Optical property of IPA was stable after operating at 307 °C but it was dramatically degraded after operating at 590 °C even though the microstructure could retain the morphology.
Fig. 7. (a) Absorptivity of IPA (S2) after operating at 307 °C (top) and 590 °C (bottom). (b) Top-view and (c) Cross-sectional view SEM images of IPA after operating at 590 °C. (d) – (g) EDX elemental maps of IPA corresponding to SEM image in (c).
Funding
Japan Society for the Promotion of Science (JSPS) (16F16315, 16H06364, 17K19045); Core Research for Evolutional Science and Technology (CREST) (JPMJCR13C3) from Japan Science and Technology Agency.
Acknowledgements
The authors would like to thank all the staffs at Namiki Foundry in NIMS for their kind supports, especially to Mr. Katsumi Ohno. Thang. D. Dao would like to thank the fellowship program (P16315) from JSPS.
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