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Abstract
Designing an efficient emitter design is an important step for achieving a highly efficient TPV conversion process. Wavelength-selective emissivity, spectra match between the emitter and TPV cells, and high thermal stability are three main characteristics that must be considered before implementing the emitter. In this work, an indium tin oxide (ITO)/sapphire emitter structure is investigated for TPV application over the temperature range from 200°C to 1000°C. A 1-µm-thick ITO layer is deposited on a 650-µm-thick sapphire substrate. In addition, 50-nm-thick SiO2 is deposited on top of the ITO to enhance the performance of emitter at high temperatures. High-temperature emissivity and absorptivity measurement of the emitter samples are obtained using FTIR and a Hitachi U-4100 spectrophotometer, respectively. The resultant SiO2/ITO/sapphire/stainless-steel planar emitter structure has selective emission with high emissivity of ∼0.8 in the 1–1.6 µm wavelength regime at 1000°C. This emission range lies at the bandgap edge of silicon TPV cells and thus can be used to harness the true potential for making a low-cost thermophotovoltaic system.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Thermophotovoltaic (TPV) system, first introduced in 1956 [1], is an efficient energy conversion system that operates at a high-temperature and converts thermal radiative energy to electrical energy [1–4]. TPVs became the extensive research area because of its ability to exceed the efficiency of the conventional Shockley-Queisser limit for a photovoltaic cell [5,6]. In the TPV system, photovoltaic (PV) cells are irradiated with high thermal emission from a local heat source, instead of solar energy, to generate electrical power. The basic components of the modern TPV system include (i) a combustor or a heat source, (ii) spectrally selective emitter (SE), and (iii) an array of a low bandgap PV cell. For an efficient TPV system, ideally, an emitter should have these characteristics: high emissivity, spectrally tailored long and short wavelength emission, and high thermal stability.
High operating temperature is beneficial for the TPV system to procure a high radiative power, as stated by the Stefan-Boltzmann law (radiated power ∝ T4). An emitter is heated (>800°C) by using any heat source such as solar energy [7], combustible fossil fuel [8], radiative isotopes [9], waste heat [10], etc. The heated emitter at elevated temperatures radiates photons having a spectrum over a broadband range. These radiated thermal photons will be collected by the PV cell to generate charge carriers to produce electric power. Most of the radiated photons have long-wavelength having energies less than the PV cell’s bandgap is transparent to the PV cell. These photons will eventually be absorbed by the surrounding, e.g., packaging, and will increase the operating temperature of the PV cell and thus decrease its external quantum efficiency. These photons that may rather be unusable are suppressed by using a spectrally selective emitter [11]. Also, selective emitter must be able to tailor its emissivity peak slightly above the wavelength (λEg) corresponding to the bandgap energy (Eg) of the PV cell to ensure the maximum absorption since at bandgap energy the density of states is zero. Finally, emission of the photons with energies much higher than the bandgap of the PV cell also needs to be suppressed to minimize the thermalization loss [12] and hence enhances the efficiency of the TPV system [13]. The total power emitted by the emitter should be equal to the ideal blackbody spectrum multiplied by the emitter’s selective emissivity. On the other hand, the TPVs overall efficiency is an amalgamation of the various component’s individual efficiency, such as combustion, spectral, radiant, filter, view-factor, PV cell, and inverter efficiency [14]. Therefore, the overall thermophotovoltaic system’s efficiency can also be summarized as an integration of the radiant, spectral, and photovoltaic cell efficiencies [15].
Designing high-performance selective emitters for practical application has great challenges, and various efforts have been made in the past for the experimental realization of the TPV emitters [16–24]. Selective emitters need to maintain the vital functional properties under the extremely high-temperatures with a strong thermally induced stress in conjunction with an oxidizing surrounding. Thus, designing an effective absorber/emitter structure for tuning of the spectral response is the crucial step for the overall optimization of an efficient TPV system [25–27]. For practical applications, selective emitters must be able to spectral tailor the emission wavelengths and sustain at the high-temperatures. It should have low emissivity for low energy photons (usually lower than the bandgap of PV cell) and high emissivity for photons having energies just above the bandgap of the PV cell [28–30]. Several materials have been used earlier for fabricating efficient emitter, including rare-earth materials such as erbium-doped aluminum garnet (ErAG) emitter material based double side filtering structure [31], Er2O3/Yb2O3 [32], photonic crystals (PhCs) [27,33,34], plasmonic metamaterials [30,35], and frequency selective surfaces [36]. Although these efforts are remarkable, further improvements can be made in the aspects of alleviating surface deformation at elevated temperatures, reducing layer thickness and layer number, and eliminating the requirement of lithography and deep plasma etching. The improvements can further enhance the stability, cost-effectiveness, and feasibility of TPV technology.
Here, we present a planar indium tin oxide (ITO) on a sapphire transparent emitter structure for the TPV application, as shown in Fig. 1. ITO, a transparent conducting oxide (TCO), is a wide-bandgap (Eg>4.1 eV) semiconductor that possesses high transmittance (>85%) in the visible spectrum with relatively low electrical resistivity (10−4 Ω-cm) [37,38]. It is mostly used for semiconductor devices such as TPV cells, gas sensors, flat panel displays, anti-reflection coating, etc. [39]. Some previous research works have been published specifying the thermal stability of ITO thin films in the temperature range from 350°C-1250°C [40–43]. ITO deposition on high thermal conductivity substrate, such as sapphire, has improved optical lifetime performance and damage thresholds [44]. Also, the sapphire substrate has decent interface stability with ITO [45] at high temperatures. Furthermore, to avoid major mechanisms that degrade the emitter structure, i.e., oxidation [46], surface diffusion, and nanostructure deformation [47,48] of TPV emitters at high-temperatures, ITO/sapphire structure can be beneficial. Additionally, SiO2/ITO/Sapphire/Stainless-steel planar emitter structure has a selective emission with high emissivity in the 1–1.6 µm wavelength regime at 1000°C. This selective emission is ideal for Si (Eg = 1.1 eV) and GaSb (Eg = 0.72 eV) based TPV systems. Moreover, having a planar design for the device eliminates the use of lithography and etching processes [49]. In this work, SiO2 is deposited on top of the ITO to enhance the performance at high temperatures [50]. Stainless steel (type 304, one side polished) is used as a metallic backplate to avoid transmittance from the transparent device at a short wavelength. It will be shown experimentally that ITO/Sapphire transparent structure can provide high emittance at near-infrared while maintaining a visible spectrum without any backplate. Some of the initial results can be found in our previous papers [24,49] and student theses [51–53].
Fig. 1. (a) The emitter structure in this study with tSiO2 = 50 nm, tITO = 1 µm, tSapphire = 650 µm, tsteel = 0.8 mm, (b) emitter sample without steel before high temperature measurements, and (c) emitter sample without steel after high temperature measurements.
ITO and other TCOs have been widely used for near-field TPV [54–59] using fluctuation electrodynamic due to their near-infrared surface plasmonic properties. However, here the ITO/sapphire emitter structure is more relevant to the far-field TPV system. Near-field TPV does not follow the ideal Planck law for blackbody radiation [57] and can emit at near-infrared radiation at a lower temperature. These theoretical results have been very encouraging, yet, the experimental realization of these near-field TPV systems is quite difficult because of the ∼µm TPV cell-emitter spacing that hampers the TPV cell operation. Thus, a vacuum environment is inevitable to prevent heat conduction and convection from the emitter to the TPV cells [60].
Moreover, ITO/sapphire structure is transparent at the visible spectrum, which can be incorporated with the micro combustors and impart the advantage of monitoring the combustor interior structure stability and flame quenching effect at elevated temperatures [61]. Also, an ITO on the sapphire structure is more chemically stable at high-temperature than solely ITO due to interfacial Al-In bonds [45]. Usually, operating temperature for a typical TPV lies in the range of 800-1200°C, and to date, only a few studies have actually provided the emissivity/absorption measurement [59,62–68]. In this regard, our planarized emitter structure possesses high stable thermal radiation property due to the superior interfacial property of the planar ITO-sapphire interface.
2. Methods
2.1. Sample fabrication
A 650 µm thick double-sided polished sapphire is bought from the Summit-tech, a Taiwan based company. First, a 1 µm ITO layer is deposited on one side of the sapphire substrate via DC sputtering process. For the entire process, argon and oxygen, precursor gases are maintained at 200 °C. A flow rate of oxygen is maintained at 0.5 sccm, and the vapor partial pressure is kept at ∼60%. The deposition is done at 300 V with a DC power and current of 0.4 KW and 1.28 A, respectively. The vacuum pressure is kept at ∼1×10−5 torr.
Then, 50 nm thick SiO2 is deposited on top of the ITO by using PECVD method. The flow rate of the precursor gases SiH4, N2O, and N2 is maintained at 8.5 sccm, 710 sccm, and 161.5 sccm, respectively. The RF power, temperature, and pressure for the whole deposition period are maintained at 20 W, 300 °C, and 1000 mtorr, respectively.
2.2. Sample measurement
2.2.1. Room-temperature (RT) measurement
Hitachi U-4100 Spectrophotometer and Bruker’s FTIR machine is used to measure Reflectance (R), Transmittance (T), and Absorption (A) at room temperature. Hitachi U-4100 spectrophotometer with a built-in integration sphere is used for measuring the reflectance (R) and transmission (T) in the ultra-violet, visible, and near-infrared wavelength regime (UV-VIS-NIR) ranging from 300 nm to 2600 nm. Absorption (A) can be calculated by using the formula:
The detailed information of the measurement setup is mentioned in [69]. Room-temperature (RT) Fourier transform infrared spectrophotometer (FTIR) has been used to measure the R, T, and A of the ITO/sapphire structure. The wavelength range of the RT FTIR is from 1µm–20 µm. At room temperature, Bruker FTIR uses a silicon carbide (SiC) glow bar as a light source. For detection, a pyroelectric detector using deuterated lanthanum α alanine doped triglycine sulfate (DLATGS) with KBr is used. For the RT FTIR measurements, Bruker IFS66 V/S is used. Wavenumber ranges from 7500 to 160 cm−1 with IFS66 V/S stepper scanner PAD 28a card. The mid-IR time-resolved spectra resolution is 5 ns, and the spectral resolution is 5cm−1. The aperture size is 5 mm. Cold Field Emission Scanning Electron Microscope (Hitachi SU8010) and Energy Dispersive Spectrometer are used for obtaining SEM images and EDS analysis of the elements. The electron source is a cathode electron gun, and the voltage is 0.1kV∼30 kV. The resolution is 10 Å (at 15 kV) or 13 Å (at 1 kV). The elemental range that can be detected is from Be4 to U92.
2.2.2. High-temperature (HT) measurement
According to Kirchhoff’s law of thermal radiation, under thermal equilibrium, the absorptivity and emissivity of a surface are equal at a specific temperature and wavelength. Thermo Fisher’s FTIR machine is used to measure the emissivity (ε) at high temperatures. The high-temperature emissivity measurement system contains a sample heater, mirror, blackbody oven, and FTIR. The sample and the blackbody are heated to the same temperature. With the help of a mirror and an external port, radiation intensities from the sample and reference blackbody are directed inside the FTIR machine for measurements. The detector is DTGS/KBr, and the wavelength range is from 1µm to 18µm. The model of HT FTIR is Thermo Fisher Scientific Inc. Nicolet iS50 FTIR. For the rotation setup and resolution, the mirror is controlled by a step motor, and the resolution of the rotation angle is 0.12°, OMNIC software is used to control the FTIR. The setting of FTIR in the wavelength band (400cm−1 ∼ 10420 cm−1), as shown in Table 1, does not require segmental adjustment. When measuring, each spectrum is averaged after 32 scans by FTIR, and the resolution of the measurement is 0.482 cm−1.
Table 1. Settings of high-temperature FTIR.
In the sample heater, a heating coil is used to heat the ceramic surface, and the ceramic surface then transfers the heat to the sample. The ceramic surface is behind the sample holder made of stainless steel, and the radiation of the sample comes out from the radiation hole of ZnSe glass. The ceramic surface also emits radiation, but the direction is opposite to the radiation hole, and thus it does not affect the measurement. The sample heater is connected with a PID (proportional-integral-derivative) temperature controller (TAIE, FY900). A thermocouple (Highlight Tech Corp.) is used to estimate the temperature of the sample while operating. The whole emissivity measurement from 200°C to 1000°C takes ∼6 hours (5 hours for heating and 1 hour for measurement). The whole system, including the sample holder, is in a vacuum having a pressure of 1 torr during the measurement. The complete setup of the FTIR machine and the heater are shown in Fig. 2. The emissivity is defined as the sample radiation intensity divided by the blackbody radiation intensity, and its value is between 0 and 1. The formula is
Fig. 2. System setup (a) Diagram of the high-temperature emissivity measurement system, (b) 3D design of the sample heater.
3. Results and discussion
The cross-sectional SEM images of the emitter sample before and after emissivity measurement are shown in Fig. 3. The 1µm thick ITO layer is sandwiched between a top protective layer of SiO2 and a bottom thick sapphire layer of thickness 50 nm and 650µm, respectively. Simulation results of ITO-based emitters can be found in our previous work [49]. Figure 3(a) is the SEM image taken before 1000°C vacuum measurement, and Fig. 3(c) is taken after high-temperature emissivity measurement at 1000°C in vacuum.
Fig. 3. (a) Cross-sectional SEM image of emitter sample before annealing along with EDS analysis of (b) oxygen, aluminum, indium, and tin elements, measured at room temperature before heating, (c) Cross-sectional SEM image of emitter sample after annealing along with EDS analysis of (d) oxygen, aluminum, indium, and tin elements, measured at room temperature after emissivity measurement.
3.1. Room-temperature results
EDS analysis of elements distribution in emitter sample before heating is shown in Fig. 3(b). The whole section of ITO and sapphire is selected for the line scan. Indium (blue), tin (purple), and oxygen (red) elements are confirmed in the ITO layer as expected. EDS analysis also confirmed the diffusion of indium and tin atoms into the sapphire layer.
The prerequisite for an emitter to have high emissivity is to have high optical absorption, as stated by the Kirchhoff’s law. Room temperature R, T, and A measurement of the sample measured by RT FTIR for the wavelength range of 1.2-10 µm is shown in Fig. 4(a) and (b) [49]. It is evident from the measurement that the emitter structure without stainless steel has very low transmittance (∼60%) at a wavelength near 1 µm, and transmittance of the sample with stainless steel is zero. Transmittance decreases to zero for both of the samples at a wavelength of ∼2 µm, and this corresponds to the application where the see-through-emitter is not desired. For both emitter structure, high absorption is observed at λ∼1.7 µm and after that absorptivity decrease sharply for λ∼2-4 µm. High absorption of ∼93% is observed for ITO/sapphire with the inclusion of the steel as compared to the ∼88% of the sample without stainless steel. This peak in NIR region in as-deposited 1 µm thick ITO is because of the free charge carrier absorption phenomenon. The carrier concentration (1020−1021cm−3) in as-deposited ITO is lower than metals. Therefore, ITO exhibits plasma frequency in the NIR region [59] below the fundamental absorption edge at the bandgap, where the semiconductor should normally be transparent. This free charge carrier absorption can be explained by using Drude’s free electron model [70–72]:
Fig. 4. Reflectance (R), Transmittance (T), and Absorption (A) measurement of the emitter sample at room temperature using (a) FTIR measurement of emitter sample without stainless steel, and (b) with stainless steel, (c) UV-VIS-NIR spectrophotometer measurement of emitter sample without stainless steel, and (d) with stainless steel.
Hitachi Spectrophotometer R, T, A measurement at room temperature of the sample for the UV-VIS-NIR wavelength ranging from 300 nm to 2600 nm is shown in Fig. 4(c) and (d) [49]. High transmission at the visible range can be observed for the sample without stainless steel (Fig. 4(c)) because of the transparency of the ITO and sapphire in the visible spectrum. The high absorption curve is observed at the range of λ∼1.6-2 µm for both samples. Likewise, RT FTIR measurement, spectrophotometer measurement also indicates the peak absorption intensity for both of the emitter samples, w/ or w/o stainless steel can be observed at λ∼1.7 µm. Figure 4(b) and (d) shows the enhancement in peak absorption intensity due to the inclusion of the steel backplate. In addition, the absorptivity decreases sharply after λ∼2 µm. Hence, the long wavelength emissivity suppression capability of the sample is verified, which is beneficial for the TPV application. Also, long-wavelength absorption/emission suppression of the sample is evident from both measurements at room temperature.
3.2. High-temperature results
EDS analysis of elements distribution in the emitter sample after heating the sample in vacuum at 1000°C is shown in Fig. 3(d). Even after high-temperature vacuum measurement, the element distribution is quite consistent and shows no excess diffusion of elements in the sample.
The normal working temperature of a typical TPV emitter is at around 800°C -1200°C, and therefore, high temperatures emissivity measurement of ITO/sapphire emitter sample will verify its ability to radiate at high emissivity with long-wavelength emission suppression. Emissivity measurement of stainless steel for different temperatures at the normal emission angle is shown in Fig. 5. The temperature is gradually varied from 200°C to 1000°C, as mentioned above. For this measurement, the emissivity value increases slightly with increased temperature. For the normal emission angle measurement, the emissivity of steel gradually decreases in the wavelength range from 1–6µm and maintains consistency from 6–10µm. We show the emissivity data from low (200°C) to high temperature (1000°C) to see the gradually changed emissivity to gain confidence on the high temperature data. Low-temperature emssivity is, in general, easier to measure. If we do not see gradual transition from the low to high temperature, high-temperature data cannot be confirmed.
Fig. 5. Detailed emissivity measurement of stainless steel at normal emission angle at different temperatures. The shaded zone indicates a noisy portion where data is not usable.
Similarly, the emissivity measurement of the ITO/sapphire emitter structure without and with stainless steel at the normal angle is shown in Fig. 6. The signal of the thermal emission has temperature and wavelength dependence according to blackbody radiation and Wien’s displacement law. The short wavelength (λ=1-3µm) portion of the emissivity is thus quite noisy at low temperature. Fortunately, the TPV operating temperature is at 800°C -1200°C, so we are mainly interested at high temperature emission spectrum. We use a method to determine the wavelength limitation for emissivity measurement at different temperatures, which depends on the ratio of the background and the sample signals:
Fig. 6. Detailed emissivity measurement of ITO/sapphire emitter sample (a) without stainless steel at normal emission angle, (b) with stainless steel at normal emission angle, at different temperatures. The shaded zone indicates a noisy portion where data is not usable.
In Fig. 6, we observe the high emissivity zone in short wavelength at 800-1000°C. In addition, the suppressed emissivity at long wavelength (>2.5µm) is also observed. The successfully demonstrated selective emission using ITO/sapphire emitter ensure the application in high efficiency TPV systems. It is also observed that the emissivity/absorptivity peak at 1000°C shifts toward shorter wavelength, compared to the room-temperature absorption peak at λ=1.7µm. It should be emphasized that the n-k at high temperature is, in general, deviated from the n-k at room temperature. This is common in most of the materials, especially when temperature goes up to 1000°C [67]. Thus while n-k is already changed at 1000°C, it is not very surprising that the plasmon absorption peak shifts. From Drude model in Eq. (4) and Eq. (5), it can be inferred that the carrier concentration increases when the plasmonic absorption peak has blue shift. The free carrier concentration N, according to Eq. (5), can be rearranged in terms of plasma wavelength λp :
The equation shows the inverse relation between N and square of plasma wavelength λp, which clearly depicts that the plasmonic emission/absorption peak will shift to shorter wavelength with an increased carrier concentration. Therefore, we can observe from Fig. 6 that the emissivity peak gradually shifts from room-temperature absorption peak at λ=1.7µm to 1000°C emissivity peak at λ=1µm. The corresponding bandgap matching to TPV cell changes from GaSb (Eg = 0.72 eV) to Si (Eg = 1.1 eV). This strengthens our emitter structure because Si-based photovoltaics are cheap and mature in technology as compared to GaSb based TPV system. As for the physical reasons for increased carrier concentration at elevated temperature, it can be explained by dopant ionization phenomenon, i.e., incomplete dopant ionization at room temperature. In addition, the increased intrinsic carrier concentration at high temperature due to Fermi Dirac distribution and bandgap narrowing can also lead to increased carrier concentration. In general, it is a common phenomenon that carrier concentration is increased with increased temperature in semiconductors. Since there is very few literature on the n-k values, material properties, or the bandstructure calculations of ITO at 1000°C, it can be difficult to judge at this point which physical mechanisms are the dominant one leading to the increased carrier concentration at 1000°C, and more research on ITO high-temperature material property is required. From room temperature measurement in Fig. 4 it can be seen that samples w/ or w/o stainless steel backplate all lead to selective emission with long-wavelength suppression. It is also observed in room temperature data that there is residual transmittance at short wavelengths (<2µm). While this residual transmittance is beneficial for visible, transparent TPV emitters, it can be somehow problematic in high-temperature emissivity measurement. This is because in the high-temperature emissivity measurement, the apparatus is different from room temperature FTIR absorptivity measurement. In emissivity measurement, photons are emitted from the sample, and there is no external light source. Without a polished stainless steel backplate, the sample is placed on a rough, unpolished stainless steel sample stage. This can lead to lower emissivity since the random light scattering by the unpolished sample stage leads to radiation loss. As a result, we use a polished stainless steel backplate to eliminate the effect of the sample stage. The polished stainless steel backplate enhance the thermal emission, as can be seen in Fig. 6.
In order to have a sustainable emitter structure, the reproducibility of the emissivity measurement is conducted to ensure the feasibility of the SiO2/ITO/sapphire structure. To achieve this, the new emitter sample is fabricated and placed in an air oven and heated up to 1000°C for 6 hours in ambience under atmospheric pressure, and then the temperature is lowered to the room temperature. This procedure is repeated three times, and then the sample is placed back to the vacuum chamber and heated again to 1000°C to perform the high-temperature emissivity measurement. The resultant emissivity of the new emitter sample is shown in Fig. 7(a). It is evident from Fig. 7(a) and (b) that 1000°C heating of the emitter sample in the ambient environment has minimal effect on the spectral emissivity of the ITO/sapphire emitter, and the measured emissivity at 1000°C is stable and the will not change with further heating.
Fig. 7. Detailed emissivity measurement of ITO/sapphire emitter sample with stainless steel at normal emission angle at different temperatures. (a) The measurement taken after heating the emitter sample in the ambience for 6 hours and cooling down to room temperature, this step is repeated 3 times. (b) is the same as Fig. 6(b). The shaded zone indicates a noisy portion where data is not usable.
As the final note, it worth pointing out that in our vacuum sample furnace measurement, the effect of vacuum (∼1 torr) annealing and air oven annealing is almost the same. This is because the pressure of our vacuum sample furnace is ∼1 torr, which is low vacuum, and thus there is significant oxygen residual inside it. The reason to pump down the sample furnace to 1 torr is to reduce noise, especially at 1000°C measurement, but we do not need ultra-high vacuum for this purpose.
Conclusively, it can also be observed in our sample that the high-temperature emissivity measurement data is quite consistent with room temperature FTIR absorption measurement, validating the Kirchhoff’s law of thermal radiation. A slight change in enhanced peak emissivity can be observed due to the difference between material properties, i.e., dielectric responses, at high temperatures and room temperature. Therefore, our emitter structure is appropriate and satisfies all of the critical properties that are expected from an ideal TPV emitter. It is worth to mention that because of the planar ITO structure in this work, there are no surface plasmonic effects and has an ideal spectral emission at far-field because of the bulk dielectric response of the ITO. Finally, having a planar design for device eliminates the use of lithography and etching process, this infuses the advantage of large emitter area scaling, lower cost, and the prevention of nanostructure deformation at high temperature.
4. Conclusion
In this work, the experimental work demonstrated the spectral characteristics of the planar ITO/sapphire emitter sample using mainly the bulk dielectric response of ITO. High emissivity (>80%) can be observed in the wavelength range of 1-1.6µm at temperature 800-1000°C. This emission range resembles well with the bandgap of Si or GaSb TPV cells. Besides cut-off, highly suppressed emissivity at long wavelengths can also be observed in this work due to the innate bulk dielectric response of the ITO layer at long wavelengths. This, in turn, enables less energy wastage by mid-IR heat and thus enhances the TPV system performance. In addition, SiO2 is deposited on top of the ITO to prevent oxidation at high temperatures [50]. Additionally, ITO deposition on high thermal conductivity substrate, such as sapphire, proved to have improved optical lifetime performance and damage thresholds [44]. The sapphire substrate has decent interface stability with ITO at high temperatures, and thus at ITO/sapphire structure is beneficial for high-temperature TPV applications. Due to the above-mentioned properties, our planar ITO-sapphire selective emitter with the top SiO2 thin layer eradicates the fast oxidation of TPV emitters and the deformation of the photonic nanostructures at high temperatures. This facilitates the use of ITO-sapphire emitter in ambient conditions instead of in vacuum environments.
Funding
Ministry of Science and Technology, Taiwan (MOST 106-2628-E-007-006-MY3, MOST 106-2628-E-009 -010 -MY3, MOST 108 2218 E 007 009).
Acknowledgements
We acknowledge the assistance from Yen-liang Tu, Guan-wen Lai, and Tzu-hsiang Huang in experiments.
Disclosures
The authors declare no conflicts of interest.
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