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Abstract
Randomly distributed plasmonic Ag nanoparticles (NPs) with various sizes were fabricated by a reflow process to an island-shaped Ag thin-film deposited on a Si photodiode. These NPs conformally enclosed by an antireflective (AR)-type SiNx/SiO2 bilayer reveal significantly diminished reflectance in a broad wavelength (500 nm - 1100 nm) as compared to the cases of Ag NPs or SiO2 layer enclosing Ag NPs on the Si substrate. Accordingly, the forward scattering and the total reflection along with wide-angle interference in between the dielectric bilayer incorporating the Ag NPs induce highly increased light absorption in the Si substrate. The fabricated Si photodiode adopting the plasmonic AR bilayer shows the responsivity peak value of 0.72 A/W at 835 nm wavelength and significant responsivity enhancement up to 40% relative to a bare Si photodiode in a wavelength range of 500 nm to 1000 nm.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Si photodiodes can detect optical signals in a wide wavelength from ultraviolet to infrared with high-speed response and low noise [1–3]. They also have advantages such as miniaturization and light-weight in a compact manner [4]. Recently, technologies for improving detection accuracy, miniaturization, high functionality, and low power consumption have been developed to overcome low output signal of the Si photodiodes [5–8]. As a practical application in bio-sensing field, wearable photoplethysmography (PPG) sensor for a healthcare device has been developed through multi-channelization by using a plurality of Si photodetectors to minimize dynamic noise characteristics and accurately measure the biometric signals [9,10]. The PPG sensor for measuring heart rate and oxygen saturation detects light reflected from blood vessels by illuminating LED light. Here, green-LED was applied to improve the performance of the heart rate detector with good absorption of Hb and HbO2, while red- and infrared-LEDs were adopted together for measuring blood oxygen saturation. The performance of light-sensing elements in these bio-sensors affects the crosstalk and signal to noise ratio, which significantly impact on the signal quality [11,12]. Thus, the photodetector, integrating the plural sensors as a single detector, with high responsivity in a broad wavelength of visible to near- infrared (NIR) is highly demanded in various applications including the bio-sensing field.
Recently developed optoelectronic devices including photodetectors, solar cells, and ultra-thin image sensors [13–15] require emergence of a high-performance antireflection (AR) layer with ultrathin thickness, insensitivity to the angle of incidence, and broadband feature. In this respect, a plasmonic structure/metasurface based AR coating is preferrable because it allows one to realize the improved forward-scattering toward the substrate and the enhanced electromagnetic confinement nearby the plasmonic structures. Thus, Ag-silica composite, Au-SiNx, and indium-TiO2 patterns [16–18] as a plasmonic AR layer were developed to improve the performance of the device. Since uniformly fabricated metal nanostructures induce the corresponding plasmonic resonance, the reflection can be diminished at a specific wavelength. Meanwhile, aperiodic metal nano-patterns or randomly dispersed nanoparticles (NPs) with different sizes can broaden the AR effect by overlapping the resonances. Thus, various aperiodic structures were developed such as an ultrathin AR coating for Si with Ag NPs encapsulated with a silica matrix, Au/Ag clusters, Au-TiO2 composite, and Ag-SiNx [19–22]. These nanocomposites were fabricated by utilizing the co-sputtering and thermal dewetting methods [23–25]. Here, the NPs produced by the dewetting process normally have a half spherical shape, and this process requires several hours to anneal the metal thin-film [24,25]. Meanwhile, a reflow process of a metal thin-film formed on a substrate takes only several minutes to fabricate randomly dispersed metal NPs with various shapes and sizes, resulting in broader resonance scattering and higher throughput than those of the dewetting method. Moreover, a dielectric film surrounding metal NPs placed on a high-index substrate, where nair < nfilm < nsubstrate, increase the fraction of light toward the substrate [26]. Thus, it is expected that the NPs fabricated by the reflow process combined with an optimized dielectric nanostructure in a compact manner can induce broadband AR function and plasmonic forward-scattering towards the active region to achieve significantly enhanced light absorption.
In this regard, we developed the plasmonic AR structure consisting of SiNx/SiO2 bilayer and randomly distributed Ag NPs fabricated by the reflow process of the sputtered Ag thin-film on a Si photodiode. The theoretical analyses reveal the highly diminished reflectance and improved plasmonic forward-scattering by the suggested plasmonic AR structure in a broad spectral range. Here, the dielectric bilayer encapsulating the Ag NPs induces light scattering to larger angles enhancing path length, as well as the broaden scattering resonance. Accordingly, the fabricated Si photodiode embedding the plasmonic AR structure shows significant improvement up to 40% of responsivity in a wavelength from 500 nm to 1000 nm with respect to a bare Si photodiode, which coincides well with the theoretical results. Therefore, the plasmonic AR layer including Ag NPs fabricated by the reflow process allows ones to realize broadband light absorption inside the absorbing materials with uncomplicated and high throughput procedure.
2. Experiment and simulation
2.1. Fabrication of randomly distributed Ag NPs on a Si lateral photodiode
The proposed process of the randomly distributed Ag NPs was applied to the lateral-type Si photodiode fabricated by using the 200 mm n-type (phosphorous doped), <100> oriented single-side polished Si wafer with a thickness of 725 μm and a resistivity of 1-3 ohm-cm as schematically shown in Fig. 1. The standard cleaning process was performed before the thermal oxidation with the thickness of 0.5 μm to block the dopants during ion implantation. The oxide layer was patterned for the detector windows (or to separate the dopant regions) and the 10 nm of oxide layer was grown to serve as a buffer layer for the ion implantation. The anode and cathode implant windows were defined by the i-line lithography process. Then, the implant process was carried out for the anode and cathode regions by using Boron with ion energy of 80 keV and does of 5×1014 cm-2 and Phosphorous with ion energy of 80 keV and does of 5×1015 cm-2, respectively. The processed wafer was annealed in the furnace at the temperature of 1100 ℃ for 3 hours to recrystallize the damaged layer during the implant process and diffuse the dopant with depth of 3 μm. The Boron with ion energy of 10 keV and does of 5×1014 cm-2 was also implanted in the detector windows, and the rapid thermal annealing (RTA) process at 1100℃ for 10 seconds was conducted to have high dopant concentration near the surface.
Fig. 1. Schematic cross-sectional view of the lateral-type Si photodiode with the suggested plasmonic AR structure for the broadband absorption.
A CMOS compatible reflow process of an island-growth thin-film on the Si substrate was developed to form randomly dispersed Ag NPs with various shapes and sizes instead of using the solution based dispersion process. Initially, the dilute hydrofluoric (DHF) cleaning process was performed to remove the buffer oxide layer and make the surface of the Si wafer hydrophobic. Then, Ag thin-film deposition was conducted by using the sputtering process under the condition with DC power of 2500 W, process pressure of 8 mTorr, and Ar gas of 100 sccm for 1.5 seconds. Here, the coral-like Ag nanostructures produced by the island-growth thin-film were annealed by the vacuum metal furnace at temperature of 900 ℃ and pressure of 10 mTorr for 10 minutes for the post-reflow process. Then, the PECVD processed SiO2 and SiNx double layers with thicknesses of about 17.7 nm and 33.8 nm, respectively, as an additional AR coating were sequentially deposited on the randomly dispersed Ag NPs on the Si surface. Here, the thickness of each dielectric layer was chosen to highly diminish the reflection at a target wavelength of 500 nm, which corresponds to the wavelength of detecting bio-signal such as heart rate. Then, the i-line lithography for the contact etch of anode and cathode electrodes was performed. The metallization was conducted by the typical lift-off process of Au thin-film. Here, the i-line lithography of negative photoresist, descum process, and BOE etch for several-seconds were adopted to remove the native oxide, and e-beam evaporation process of Ti/Au thin-film with a thickness of 30/300 nm was performed to form the ohmic contact with the Si substrate. After completing the metal lift-off process, the fabricated Si wafer was thinned to have a thickness of 150 μm.
2.2. FDTD simulation on the plasmonic AR layer on a Si substrate
The full-wave electromagnetic simulation using Lumerical’s finite-difference time-domain (FDTD) software (FDTD, Lumerical 2020 R2.4, Vancouver, BC, Canada) was performed to analyze the reflectance and the optical absorption of the Si substrate depending on the types of AR structures such as Ag NPs only, Ag NPs enclosed with SiO2, or Ag NPs enclosed with SiNx/SiO2 in order to compare their optical performance. The complex refractive indices (n, k) of Ag NPs, SiO2, SiNx, and Si were taken from the Ref. [27]. The plane wave source with a wavelength of 500 nm to 1100 nm was injected along the normal direction to the Si substrate. The periodic boundary conditions in both x- and y-axes and the perfectly matched layer in z-axis were implemented on the unit-cell structure. Then, the spectral performance of the developed plasmonic AR structure on the Si substrate was characterized in terms of reflection, absorption enhancement, and light absorption density.
2.3. Measurement of responsivity of the plasmonic Si photodiode
The experimental setup for measuring responsivity consists of a spectral light source combining a 600 W tungsten halogen lamp (with a wavelength of 400 nm) and a 200 W mercury xenon lamp (with a wavelength 400 nm) to a double lattice spectrometer (Bentham DTMc300), and a stage controlling the position of the detector. The parallel beam of ∼ 5 mm in diameter coming from the spectroscopic light source with radiation power varying from 100 pW to 10 nW was transmitted to the detector. To measure the sensitivity with respect to irradiance, a beam of the spectral light source was incident on a small integrating sphere with a diameter of 50 mm. Here, a spatially uniform radiation distribution was produced at the exit. After placing the detector at a predetermined position, we measured the photocurrent signal under the same conditions as the reference detector adhere to the integrating sphere calibrated the sensitivity in advance. Then, the sensitivity of the detector was estimated from the ratio of photocurrent of the plasmonic photodiode to that of the reference photodiode.
3. Results and discussion
3.1. Randomly distributed Ag NPs on a Si substrate
The Ag thin-film was deposited on the Si substrate under the condition of the high deposition rate (3.3 nm/s) for the sputtering process. The island growth of the Ag thin-film produces the coral-like nanostructures with the diameter ranging from 40 nm to 250 nm as shown in the top-view of field emission scanning electron microscopy (FESEM) image (Fig. 2(a)) and corresponding size distribution (Fig. 2(b)). Here, the Ag nanostructure has a filling factor of 30.1%, and the mean value of the size is about 120 nm. It is noted that the hydrophobic surface of the Si substrate by the DHF cleaning process increases the contact angle of the Ag nanostructure during the reflow process. Thus, the randomly dispersed Ag NPs with various shapes and diameters (20 nm to 160 nm) from the coral-like nanostructure on the substrate were fabricated by the post reflow process as shown in the top-view FESEM image (Fig. 2(c)) and corresponding size distribution (Fig. 2(d)). The fabricated Ag NPs show a reduced filling factor of 19.8% as compared to the coral-like nanostructures. The diameter of fabricated Ag NPs mostly ranges 40 nm to 120 nm with the mode of 80 nm in size, and the average inter-distance among the NPs is about 150 nm.
Fig. 2. (a) Top-view FESEM image of the coral-like Ag nanostructure deposited on a Si substrate and (b) corresponding size distribution. (c) Top-view FESEM image of the Ag NPs produced by the reflow process formed on a Si substrate and (d) corresponding size distribution.
Figure 3 shows the top-view of optical microscope images and the cross-sectional view of FESEM images of the AR-type SiNx/SiO2 layer with or without Ag NPs deposited on the Si photodiode. The measured thicknesses of SiNx and SiO2 are 33.8 nm and 17.7 nm, respectively, as shown in Fig. 3(a). As seen in the morphology of Fig. 3(b), the Ag NPs are conformally enclosed by the dielectric double layers consisting of SiO2 and SiNx. Here, the most important parameters for enhancing the efficiency of the localized surface plasmon resonance are the NP size and the inter-distance among the plasmonic NPs. Thus, the fabricated Ag NPs with various shapes and diameters are expected to overlap enhanced plasmonic resonances in a broad spectral range. Also, the reflection color of the AR-type bilayer without the Ag NPs on the Si substrate appears reddish-brown (Fig. 3(a)), while the reflection color of the AR bilayer with the Ag NPs is observed in blue (Fig. 3(b)) due to significant light absorption in the spectrum corresponding to green and red wavelength ranges. Moreover, it is expected that the conformally deposited bilayer with the Ag NPs on the Si substrate as shown in Fig. 3(b) increases the AR performance due to the gradually varying reflective index profile from air through the AR layer to the Si substrate.
Fig. 3. (a) Top-view of optical microscopic image and cross-sectional view of FESEM image for the Si photodiode deposited with the AR bilayer. (b) Top-view and cross-sectional view of the Si photodiode with the AR bilayer conformally enclosing the Ag NPs.
3.2. Reflectance and scattering property
To quantify the amount of reflectance and scattering property for the plasmonic AR structures, we performed FDTD simulation on the Ag NPs enclosed with the SiNx/SiO2 bilayer on the Si substrate in a broad wavelength range of 500 nm to 1100 nm, in comparison with the other plasmonic structures such as Ag NPs only and Ag NPs encapsulated with SiO2. Since the diameter distribution of the fabricated Ag NPs lies from 40 nm to 140 nm, we divided the diameter range by 20 nm intervals and selected the representative diameter for each interval. Also, we extracted the inter-particle distance of 150 nm from the mean value of the randomly distributed NPs. Then, the plasmonic NP was modelled as a spherical shape enclosed with the thin dielectric bilayer on top of the Si substrate.
The effect of the NPs on the light trapping can be estimated by the reflection property and corresponding light absorption inside the Si substrate before and after the deposition of Ag NPs with or without the dielectric layers. Figure 4 shows the simulated reflectance spectra of the Si substrate with the plasmonic AR structures in a wavelength range of 500 nm to 1100 nm. Here, the diameter of the Ag NPs was set to 80 nm corresponding to the mode of the size distribution for the randomly dispersed Ag NPs as shown in Fig. 2(d). It shows that the reflectance gradually decreases over the whole wavelengths except below 530 nm as the plasmonic structure changes from Ag NPs to Ag NPs enclosed with SiO2 and then Ag NPs enclosed with SiNx/SiO2. Since the extinction spectra related to the absorption and scattering properties of the Ag NPs shows the peak value at a wavelength of 440 nm for this Ag NP, the amount of reduced reflectance of the Ag NPs on the Si substrate with respect to a bare Si substrate decreases as the wavelength increases from 500 nm to 1100 nm. Thus, the decreased reflectivity by the Ag NPs on the Si substrate is attributed to the plasmonic absorption and forward scattering. For light scattering from metal NPs, dipolar plasmonic resonance is generally dominant across the visible to NIR range, while higher order excitation with different spatial distribution is significant at shorter wavelengths [28]. Moreover, the metal NPs in a multi-layered structure generally support the excitation of dipolar particles. Thus, the Ag NPs surrounded by the SiO2 film serve to increase the amount of dipole emitter toward the Si substrate by total reflection at the SiO2/air interface. It is noted that as the refractive index surrounding the NPs increases, the NPs’ extinction spectrum shifts to longer wavelengths. Moreover, since a material with a higher refractive index has a higher optical state density, forward scattering becomes strong toward the substrate with a high refractive index. In the case of Ag NPs surrounded by the SiNx/SiO2 bilayer, the effective refractive index of SiNx/SiO2 in the air/SiNx/SiO2 structure increases to 1.6, leading to increased optical cross-section that affects optical scattering. Accordingly, the amount of dipole emitters toward the Si substrate highly increases due to multiple reflections and total reflection at the Air/SiNx/SiO2/Si interface. It is noted that the fraction and angular properties of light scattered into the substrate are affected by reflective interference in the curved dielectric bilayer such as wide-angle interference between light emitted into the substrate and light reflected from the air/SiNx interface [26]. Such wide-angle interference appears predominantly when the refractive index of the dielectric layer on the substrate has a value between the refractive index of air and the substrate. Consequently, the AR structure consisting of SiNx/SiO2 bilayer enclosing the Ag NPs reaches up to 96.3% decrease in reflectance at a wavelength of 800 nm. The rates of decrease in the reflectance of the Si substrates with the three types of plasmonic AR structures with respect to the bare Si substrate are presented in Table 1 at the wavelengths of 500, 600, 700, 800, and 900 nm. It is also noted that the reflectance of the AR-type dielectric bilayer without the Ag NPs is still higher than that of the suggested plasmonic AR bilayer.
Fig. 4. Reflectance spectra of a bare Si substrate, SiNx/SiO2 bilayer on a Si substrate, and the Si substrates with the plasmonic AR structures such as Ag NPs, SiO2 layer enclosing Ag NPs, and SiNx/SiO2 bilayer enclosing Ag NPs. The diameter of Ag NPs is set to 80 nm.
Table 1. Rate of decrease in the reflectance of Ag NPs, SiO2 layer enclosing Ag NPs, and SiNx/SiO2 bilayer enclosing Ag NPs placed on the Si sub. with respect to a bare Si sub.
3.3. Absorption enhancement and absorption density
To analyze the degree of absorption enhancement according to the size of Ag NPs in the plasmonic AR bilayer structure formed on the Si substrate, we also performed the simulation on the optical absorption in a wavelength range of 500 nm to 1000 nm by considering the size distribution appeared in Fig. 2(d). Here, we assumed the Ag NPs are dispersed on the Si substrate with an average inter-particle distance of 150 nm. As observed in Fig. 5(a), the integrated absorption over the spectral range increases as the diameter of Ag NPs decreases from 140 nm to 40 nm since relatively large-sized Ag NPs can induce backward scattering. Also, the ratio of absorption cross section to scattering cross section decreases as the NP size increases. It is noted that smaller NPs result in higher parasitic absorption and induce a dominant peak at shorter wavelength with significant near-field enhancement, while larger NPs lead to stable scattering peaks in the broad range. Thus, the amounts of the forward-scattering and the optical cross section affect the absorption spectrum depending on the NP diameter and interspacing among them. Figure 5(b) shows the overall absorption enhancement by the Ag NPs distributed with diameters from 40 nm to 140 nm, which was obtained by summing the respective absorption enhancement depending on the size of the Ag NPs in Fig. 5(a) in consideration of the size distribution of Ag NPs in Fig. 2(d). Here, the absorption enhancement of around 40% is observed over the wavelength range from 625 nm to 800 nm. This improved performance is attributed to the enhanced forward scattering by the multireflection and total internal reflection along with wide-angle interference inside the dielectric bilayer enclosing the Ag NPs on the Si substrate.
Fig. 5. (a) Absorption enhancement in the Si substrate by the AR-type SiNx/SiO2 bilayer enclosing the Ag NPs with a diameter of 40, 60, 80, 100, 120, or 140 nm, and (b) overall absorption enhancement by the Ag NPs distributed with diameters from 40 nm to 140 nm.
To clarify the optical absorption in the Si photodiode, we present the profiles of the optical absorption per unit volume at representative wavelengths of 650 nm and 800 nm without and with the Ag NPs (diameter of 80 nm) enclosed with the dielectric layers as shown in Fig. 6. Here, we plot the logarithm of the absorption density using the same color scale to compare among the four different configurations (a bare Si sub., Ag NPs on a Si sub., SiO2 layer encapsulating Ag NP on a Si sub., SiNx/SiO2 layer encapsulating Ag NP on aSi sub.) at each wavelength. It is clearly observed that the Ag NPs increase the absorption in the Si substrate by enhancing the forward scattering resulting from the surface plasmon resonance. The SiO2 layer conformally enclosing the Ag NPs makes the incident light spread widely and deeply inside the Si substrate, increasing the amount of dipole emitters directed to the Si substrate than the Ag NPs on the substrate. Moreover, the curved dielectric bilayer incorporating the Ag NPs much more enhances light absorption inside the Si substrate as compared to the case of Ag NPs only or SiO2 enclosing Ag NPs. As observed in the absorption profile, the plasmonic SiNx/SiO2 layer enclosing Ag NPs makes the incoming light spread much wider and deeper inside the Si substrate than the case of Ag NPs only or Ag NPs enclosed with SiO2. Specifically, the rate of increase in the absorption densities for SiNx/SiO2/Ag NP at 650 nm (800 nm) relative to the Ag NPs on the Si substrate is estimated to be 25.7% (9.87%), which is higher than that of 17.2% (4.52%) for the case of SiO2/Ag NPs on Si substrate. Table 2 presents the maximum optical absorption per unit volume depending on the plasmonic structures at each wavelength. It shows that the maximum absorption density also gradually increases as the structure changes from Ag NPs to Ag NPs enclosed with SiO2, and then Ag NPs enclosed with SiNx/SiO2. Thus, the suggested AR structure consisting of the dielectric bilayer incorporating the Ag NPs shows the highest absorption density among the four different configurations. It is noted that the absorption of the Ag NPs is broadly enhanced due to the randomly dispersed property of the NPs produced by the reflow process.
Fig. 6. Absorption profiles of the bare Si substrate and the plasmonic Si substrates with Ag NPs, SiO2 layer enclosing Ag NPs, or SiNx/SiO2 bilayer enclosing Ag NPs at the wavelengths of (a) 650 nm and (b) 800 nm. The scale bar on the right of each figure represents the unit [Watt/μm3] of electric-field intensity in log-scale.
Table 2. Maximum absorption density [Watt/μm3] of Ag NPs, SiO2 layer enclosing Ag NPs, and SiNx/SiO2 bilayer enclosing Ag NPs placed on the Si substrate at the wavelengths of 650 nm and 800 nm
3.4. Responsivity of the plasmonic Si photodiode
We measured the responsivity of the Si photodiode with the Ag NPs enclosed by SiNx/SiO2 bilayer in the spectral range of 500 nm to 1000 nm as shown in Fig. 7, where the spectral response is broadly enhanced in comparison with the Si photodiode without the Ag NPs. Here, the cut-off wavelength of a Si substrate at room temperature is around 1100 nm, so the responsivity enhancement decreases as the wavelength increases above 1000 nm. Meanwhile, since the plasmonic wavelength of the fabricated Ag NPs is located in between the wavelength of 400 nm to 500 nm, the responsivity induced by the light absorption is suppressed below this wavelength corresponding to the surface plasmon resonance. Also, the measured responsivity enhancement is observed to be slightly lower than the amount of simulated absorption enhancement in Fig. 5 (b) because the responsivity of the fabricated Si photodiode is affected by the device structure, wavelength-dependent absorption coefficient of material, and external/ internal quantum efficiency. Nevertheless, the responsivity enhancement at the wavelengths of 500, 600, 700, 800, and 960 nm is 2.1, 20.4, 28.6, 31.1, and 40.6%, respectively, relative to the bare Si photodiode. It also shows a highly improved responsivity peak value of 0.72 A/W at the wavelength of 835 nm. This significant increase illustrates the strong forward scattering in a broad spectral range due to plasmonic AR effect as referred in the simulation analysis part. Since the peak of the spectral responsivity shifts to the longer wavelength with increasing the diameter of Ag NPs, the overlapped the resonance effect is observed by the dispersed Ag NPs with various shape and sizes as expected. Also, the dielectric double layers contribute the total reflectance along with wide-angle interference effect inside the AR-type bilayer enclosing the Ag NPs by the different refractive indices between the adjacent layers. Moreover, conformally structured dielectric bilayer incorporating the Ag NPs directs incoming beams omnidirectional inside the Si substrate effectively. Therefore, the plasmonic AR bilayer enclosing the Ag NP fabricated by the reflow process can be a promising nanostructure to enhance the photoresponse of the Si-based devices in a broadband as a single photodetector in various applications including bio-sensors.
Fig. 7. Measured responsivity of the plasmonic Si photodiode with the AR-type SiNx/SiO2 bilayer enclosing Ag NPs as a function of wavelength, in comparison with the case of a bare Si photodiode.
4. Conclusion
We developed the plasmonic AR structure by enclosing the Ag NPs with the favorable AR-type SiNx/SiO2 bilayer. Here, the post-reflow process to the Ag thin-film formed on the Si substrate was utilized to fabricate the randomly dispersed Ag NP with various shapes and sizes. The plasmonic forward scattering by the NPs and the total reflection with wide-angle interference effect in between the dielectric bilayer induce highly increased light absorption inside the Si photodiode. Thus, the fabricated Si photodiode with the suggested plasmonic AR bilayer shows the responsivity peak value of 0.72 A/W at 835 nm wavelength and reveals significant improvement up to 40% in responsivity in a wavelength range of 500 nm to 1000 nm with respect to the reference Si photodiode. Therefore, the suggested plasmonic AR bilayer covering the metal NPs produced by the reflow process can be a promising architecture for highly improving the performance of various photo-sensitive devices in a broad spectrum with cost-effective, high throughput, and compact features.
Funding
Institute of Information & Communications Technology Planning & Evaluation (2019-0-00720 (50%), 2019-0-01823 (30%)); Korea Evaluation Institute of Industrial Technology (20006476 (20%)).
Acknowledgments
The authors acknowledge the effort of the NNFC’s process engineers to fabricate the device.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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