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Multi-Functional Silicon Optoelectronics Integrated with Plasmonic Scattering Color Long Wen, Qin Chen, Xin Hu, Huacun Wang, Lin Jin, and Qiang Su ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b05960 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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Multi-Functional Silicon Optoelectronics Integrated with Plasmonic Scattering Color Long Wen, Qin Chen*, Xin Hu, Huacun Wang, Lin Jin and Qiang Su Key Lab of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (CAS), Suzhou 215123, People’s Republic of China *[email protected]

ABSTRACT: Plasmonic scattering from metallic nanoparticles have been used for centuries to create the colorful appearance in stained glass. Beside the usage as passive spectral filtering components, multi-functional optoelectronic applications can be expected by integrating the nanoscatters with semiconductors that generate electricity using the complementary spectral components of plasmonic colors. To suppress the usual degradation of both efficiency and gamut of plasmonic scattering coloration in highly asymmetric index configurations like a silicon host, Aluminum (Al) nanodiscs on Indium tin oxide (ITO) coated silicon were experimentally studied and demonstrated color sorting in full visible range along with photocurrent generation. Interestingly, although the power loss for coloration the photocurrents were found to be comparable to the reference devices with only antireflection coatings. Detailed investigation shows that ITO serves as both the impedance matching layer for promoting the backward scattering and schottky contact with silicon, and moreover plasmonic nanoscatters efficiently harvest the complement spectrum components for charge generation. The present approach combines the capacities of nanoscale color sorting and photoelectric converting at a negligible cost of efficiency, thus provides a broad flexibility of being utilized in various optoelectronic applications including self-powered display, filter-free imaging and colorful photovoltaics.

KEYWORDS: plasmonic, scattering, Silicon, color filtering, light trapping

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Utilizing surface plasmon excitation in metal nanostructures for efficient light manipulation and concentration at nanoscale is a rapidly emerging field of nanophotonics that has demonstrated great potential for sensing, color filtering, imaging, anti-counterfeiting, and light harvesting.1-7 Particularly, the application of plasmonic nanoparticles in structural coloration can be traced back for centuries, famously exemplified by the Lycurgus Cup. It is well-established that the optical properties of metal nanoparticles are governed by collective oscillations of free electrons, which give rise to a localized surface plasmon resonance (LSPR) at characteristic frequencies in the visible.8 By simply adjusting the shape and the size of those isolated nanostructures, LSPR can be spectrally tuned throughout the entire visible range with more attractive features,9-13 rendering them as color filters,14,15 holograms16 and polarimetry.17 Moreover, an ultrahigh spatial resolution of 100,000 dpi was demonstrated using plasmonic coloration.18 As compared with the pigment-based color schemes, the incorporation of the single layered sub-wavelength plasmonic elements not only allows the colors to be tailored in more efficient and compact forms but also gains the advantages of low cost, ease of fabrication, and long-term stability. In previous studies, the plasmonic filtering elements have been attached or fabricated on CMOS image sensors and demonstrated their full functionality as efficient passive optical components.19,20 More recently, integration of the structural colors with the photovoltaic (PV) effects represents an attractive direction for self-power display, filter-free imaging and aesthetically pleasing solar cells.21-25 Among these cutting-edge researches, Si-based or Si-compatible strategies are in particular interested for the PV-integrated filters used in energy-harvesting reflective displays, or integrated electro-optical devices which feature in a compact form factor by merging of the color filter and sensor into a single one.21,22 The striking light trapping characteristics arising from the plasmonic scattering,26 near-field localization27,28 or light guiding29,30 can substantially enhance the PV conversion. Thus, bridging structural coloration and light trapping in optoelectronic device with plasmonic elements renders aforementioned multifunctional applications holding high quantum efficiency. 1


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The structural colors relying on the scattering characteristics of nanostructures are highly dependent on the dielectric environment. In previous approaches, plasmonic nanoscatters were fabricated on low-index substrate for the purpose of obtaining the scattered additive reflection colors or subtractive transmission colors.14,15,17,31-34 However, for the case of plasmonic nanoscatters sitting on high index optoelectronic semiconductors like silicon, the reflective color extraction efficiency can be significantly diminished because of the narrowed escape cone in the optically dense medium according to the Snell’s law.35 In addition, the resonance peak becomes red shifted and broadened, primarily owing to the increased retardation effect,36 resulting in the poor chromaticity control. The above limitations impede the progress of the plasmonic scattering coloration for broader application in optoelectronics. In this paper, both theoretical and experimental studies on integrated plasmonic coloration and light harvesting towards the silicon optoelectronic platform were performed by incorporating of the metallic nanodiscs on top of ITO coated silicon. The proposed devices display distinct additive colors across the visible regime and harvest the complement spectrum component for efficient photocurrent extraction. Further, at the non-resonant wavelengths the better light coupling and angular spread of the forward scattered waves in the substrates allow the colored devices sustaining comparable power conversion efficiency against the anti-reflective reference devices. Meanwhile, the proposed approach is also capable of polarization-switchable coloration by utilizing elliptical shaped nanodiscs.

RESULTS AND DISCUSSION Theoretical analysis of the scattering behavior of the isolated metallic nanoscatters was performed focusing on the impacts of the underlying substrate. To achieve the spectral control across the whole visible region, plasmonic nanodiscs made from Aluminum (Al) were employed herein because of its higher plasma frequency as compared with noble plasmonic metals and meanwhile the attractive features including low-cost and the self-limiting oxidation induced stability.15,32 Figure 1a,b shows the calculated forward/backward scattering cross-sections (Qscat) of the isolated 2


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Al nanodiscs free-standing in air and on substrates with varied refractive indices (The simulation methods and parameters are given in Supporting Information 1). In general, there exists a dominant peak in the scattering spectra for all dielectric configurations, corresponding to the LSPR of the Al nanodiscs. For the case of free-standing Al nanodiscs, the incident wave is scattered strongly and symmetrically to the backward and forward directions. This symmetric scattering behavior is broken while an underlying substrate appears beneath the Al nanodiscs. With the increasing refractive index of the substrate, the scattering peaks red-shift and broaden in line-width because of the retardation effects. Moreover, it is observed that the intensity difference between the backward and the forward scattering becomes significant especially for the high index substrate. The backward scattering cross-sections are greatly diminished for the cases that the refraction index of substrate nsub is over 2.0. In contrast, the forward scattering on resonance maintains relatively high values and becomes even more considerably broader. The prominent and broad features of the forward scattering are highly preferred to the light trapping purpose in photovoltaic cells.26,35 However, to obtain the reflective coloration, the efficiency of backward scattering should be improved. The decrease in backward scattering on high index substrate is mainly ascribed to the narrowed escape cone for light in the optically dense medium. Considering the refractive indices of Silicon (n≈4), the critical angle for the light escape cone is about 15°. As a result, only a small fraction of the scattering light can be radiated back into air. To this end, the impedance matching layer with moderate index was inserted between the nanodiscs and the substrate, aiming at directional shaping of the scattered light. As shown in Figure 1a and 1b, the backward scattering efficiency from the impedance matched structure is significantly improved as compared to the case of the nanodiscs directly sitting on Silicon. Meanwhile, it is observed that the impedance matched structure exhibits a back scattering peak located at the same resonant wavelength but at least two-fold enhanced over the case of that homogeneous substrate with the same refractive index as the impedance matching layer. Further insight into the scattering behavior for the dielectric medium separated plasmonic nanoscatters can be obtained 3


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by plotting the electromagnetic field distributions as depicted in Figure 1c. In consistence with the calculated Qscat, the near-field patterns around the free-standing nanodiscs clearly show the symmetrically scattering features in which the incident wave is radiated efficiently for both forward and backward directions; while for the high index substrate supported nanodiscs it prefers to guide the scattering wave into the forward direction as witnessed by the dramatically altered strength and angular distribution of the field patterns in air. There are two main consequences of the addition impedance matching layer on scattering properties can be addressed from above analysis. The first is spectrally reshaping the resonance to match the situation of low index substrate and enhancing the backward scattering extraction by breaking the limitations arising from the small escape cone of Silicon. The second is the further increased back-scattering over the homogeneous substrate with low index due to the interference effect of the layered plasmonic-dielectric systems. To confirm the physical origin of these two facts, quantitative studies relying on the modified long wavelength approximation (MLWA) spheroid theory were carried out.37-39 (see Supporting Information 2 for details). As shown in Figure 2c, the backward scattering cross-section obtained from the MLWA (blue hollow circle) is found to be consistent with the numerical calculations (red solid circle) very well in both the resonant peak position and the line-width for the case of homogeneous substrate (nSub=2) supported nanodiscs. When plasmonic nanodiscs are separated from a substrate (Silicon) with high refractive index by a dielectric film with low refractive index, the local driving field at the position of nanoscatters for LSPR is determined by the interference between the incident and reflected field, which results in strong modified amplitude of the scattering efficiency.40,41 Figure 2a shows the calculated driving field (Ed2=|E/Ei|2, normalized electric field intensity at the interface of air and dielectric film) of the semi-infinite dielectric substrate and the layered dielectric/Silicon substrate configurations in the absence of nanodiscs. As compared with the featureless and flat spectra of the semi-infinite dielectric structure, the layered structures with varied thickness of the dielectric films show obvious spectral peaks and valleys due to the constructive or 4


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destructive interference of the incident and reflected fields. In Figure 2b, the backward scattering cross-sections of the plasmonic nanodiscs were calculated numerically by mounting the nanoscatters on the corresponding substrate configurations. There is a clear spectral dependence of the scattering behaviors of the nanodiscs on the local driving field as depicted in Figure 2a and 2b. When the constructive or destructive driving fields occur at a specific spectral position which coincides with the LSPR, the resulting Qscat can be significantly enhanced or diminished, as confirmed by the cases of the separated dielectric films with thickness of 70 and 130 nm. The contour plots in Figure 2c show the impedance matched structure has high electric field strengths inside the dielectric spacer and clear amplified hot-spots around the bottom corners of the nanodiscs, illustrating the enhanced driving field and thus the resonant scattering over the homogenous substrate. On account of the driving field effect, it is also possible to verify this speculation qualitatively. By multiplying the enhancement ratio of driving field intensity for the impedance matched structures over the homogenous substrate on the Qscat, one can see that the modified MLWA scattering efficiency of the impedance matched plasmonic nanoscatters shown in Figure 2c is well consistent with the numerical results, except the slightly blue-shifted resonance. It should be noted that the total reflection from the structures comprises not only the scattered portion but also the specular reflection. For the sake of ensuring high color saturation and wide gamut, it is crucial to suppress the specular reflection across the whole visible regime. Here, we chose the ITO film as the impedance matching layer and its thickness is fixed at 64 nm for the best trade-off between the suppression of the blue and red-end background colors. Thin layer of ITO has excellent conductivity and possesses high transparency throughout the visible and near-infrared regimes. Meanwhile, it typically forms a schottky-like contact with n-type Silicon, thus can be considered as an efficient electrode herein for the collection of the photogenerated carriers in Silicon. The proposed plasmonic nanoscatters integrated optoelectronic devices combing the capacities of color sorting and photocurrent generation are schematically 5


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illustrated in Figure 3a. By tailoring the geometric parameters including the diameter and packing density of the nanodisc arrays, the devices fulfill their coloration capability by output distinct reflective colors with desired luminosity. Unlike the previous filtering schemes,1,3,13,14,16,42 where the coloration is always accompanied by energy loss, as significant amount of light is rejected or parasitically absorbed by the filters, our approach attempts to harvest the complementary portions of light for the usage of photocurrent output. For the purpose of ensuring high purity coloration and PV harvesting, the thickness of nanodiscs is fixed at 30 nm, as it is found to be optimal for narrowing the resonant scattering and concomitantly suppressing parasitic metallic absorption. Figure 3b shows a contour plot of the calculated reflection spectra as a function of the nanodisc diameter. The well-defined narrow band light reflection arising from the backward scattering of the nanodiscs shifts its spectral positions continuously (from 400 to 700 nm) with the increasing diameter, suggesting an efficient visible color sorting capability of the proposed devices. To provide a quantitative measure of the saturation of the resulting reflective colors, the chromaticity coordinates in accordance with the spectral reflection were calculated and superimposed in the CIE 1931 xy color space diagram, as depicted in Figure 3c. The black solid circle represents the predicted color for the bare reference devices in absence of Al nanodiscs which tends to be a pinkish red. The incorporation of Al nanodiscs atop the layered substrates will significantly alter the color perceptions of the devices. It is found that the proposed devices permit the coloration to encompass all the primary colors by simply adjusting the diameter of the nanodiscs. Experimental realization of the proposed plasmonic nanoscatters integrated optoelectronic devices are conducted combing the functionalities of color sorting and photocurrent generation. The starting substrate is n-type (1-10Ω‧cm) and doublepolished Si with a thickness of 300 µm. The high conductive and transparent ITO layer was deposited on the RCA cleaned Si substrate by the magnetron sputtering and subsequently annealed at 300 °C for 1 h. As a critical parameter for suppression of the background specular reflection, the thicknesses of the prepared ITO films were carefully examined by spectroscopic ellipsometer. For the rear reflective electrode, 6


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300 nm thick Al was deposited on Si and then thermally treated below the Al-Si eutectic temperature. Plasmonic nanodisc arrays were fabricated on top of the ITO layer by means of electron beam lithography (EBL), Al evaporation, and liftoff. A scanning electron microscopy (SEM) image of one nanodisc array is shown in Figure 4a. The fabricated nanodiscs are round shaped with a uniform size distribution for the 20×20µm2 plasmonic pixels. Bright-field optical microscope images of the fabricated color pixels are presented in Figure 4b, demonstrating a high degree of color homogeneity and saturation for the five primary colors. The optical spectral responses of the fabricated plasmonic pixels are characterized in the visible range with a microscope-based fiber spectrometer measurement setup. As shown in Figure 4c, the bare ITO/Si devices in absence of plasmonic nanoscatters show a typical anti-reflection feature of the one-quarter coating, in which a reflection minima appears at λ=480 nm. For the integrated devices containing array of Al nanodiscs with small diameters and ultra-low surface coverage (e.g. in the cases of D=80, 95 nm, the surface coverage of nanodiscs is only around 7-8%), the resonant backward scattering of nanoscatters results in a dominate reflection peak at short wavelengths. The reflection peaks of these sparsely patterned nanodisc arrays have small peak amplitudes but provide the dominant contribution for color perception (refers to Figure 4b) due to the greatly suppressed red-end portion of reflection. With an increase of the diameter and surface coverage of the Al nanodiscs, the resonant reflection peaks tend to red-shift and possess higher peak amplitudes, thus leading to varied scattering colors shown in Figure 4b. The measured reflection spectra are verified with numerical simulations using the FDTD method as shown in Figure 4d. By adopting the actual experimental geometry of the nanodiscs, the numerical results show good agreement with the experimental data. Because of their localized resonance origins, the scattering colors are mainly sensitive to the profile and the size of Al nanodiscs. For moderate packing densities, i.e. the nanodiscs interact weakly with each other, increasing the packing densities can improve the luminosity of the plasmonic pixels without significantly altering their color perceptions. The microscope images along with the measured reflection spectra 7


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of the fabricated plasmonic color pixels with different diameters and filling ratios are plotted in Figure 5a. It is obviously that the amplitudes of the scattering intensities and thus the luminosities of the pixels increase with the increasing of filling ratio. Besides, for a given diameter, slight color variations are also observed in the chromaticity space (Figure 5b) with increasing period as a result of the blue-shifted localized resonance that is attributed to the radiative dipolar coupling between the nanodiscs in pixel array.43 Figure 5c shows the predicted RGB colors of the plasmonic pixels with varying diameter and filling ratio, by using the experimental reflection spectra plotted in Figure 5a. The predicted RGB colors provide direct visual perception of each color pixels, illustrating the full visible coloration capacity of the plasmonic pixels under the standard CIE-D65 white light illumination. We anticipate that by properly tailoring the sizes and packing densities of the Al nanodiscs, plasmonic pixels with desired primary colors and luminosities can be produced. In above sections, the proposed devices containing plasmonic nanodiscs with circular shapes are polarization independent under normal incidence, owing to their four-fold rotational symmetry and square lattice arrangement. However, in some specific applications such as polarization-resolved imaging techniques for remote sensing and surveillance, the coloration along with the polarization-dependent character is also highly preferred.44 This attempt can be straightforwardly achieved based upon elliptical shaped nanodiscs, by means of tailoring their long and short axes. The polarization-switchable coloration were successfully demonstrated in Figure 6a and 6b, where two selected plasmonic pixels containing arrays of asymmetric plasmonic nanoscatters were characterized in terms of the spectral response and color perceptions at different polarization angles θ. The polarization angles were controlled by inserting an optical polarizer in the optical path of microscope. In both cases, the reflectance varied almost linearly with respect to the continuous rotation of the polarization angle. Under illumination with a polarized angle of θ=0°, these two color pixels have different resonant reflection peaks and thus display distinctive colors due to their different sizes in the long axis of the elliptical nanodiscs. For θ=90° the polarization parallels with the short axis of the elliptical nanodiscs, resulting in similar 8


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spectral reflection and color perception as shown in Figure 6a and 6b. Based upon the polarization-switchable coloration features of the elliptical nanodiscs, two sets of optical information including the shapes and colors can be stored in one picture with pre-designed color pixels. As shown in the first plot of Figure 6c, the plasmonic pattern consists of four color pixels (marked by the numbers), where different Al nanodiscs are arranged in periodic lattices. The pixel 1 containing symmetric configuration of nanodiscs represents the violet-blue colored background of the cartoons, which is irrelevant to the polarization states. In the remaining three pixels, different elliptical nanodiscs are arranged in a rectangle lattice. The main axis of the elliptical nanodiscs in these pixels are orientated in x or y directions. To achieve favorable image shift between two orthogonal polarization states, simulations were performed to calibrate the anisotropic coloration response in terms of the reflectance amplitudes and profiles (see Supporting Information 4 for details). Under non-polarized illumination, these four color pixels can be easily recognized from the microscope plasmonic pattern (the left plot), but their colors are the results of mixing two primary scattering colors associated with the short- and long-axis polarization components. In contrast, with an illumination polarized linearly along x or y direction, some color pixels are immersed in the background and thus only one of the cat cartoons with specific designed color can be clearly resolved as demonstrated in the middle and right plots of Figure 6c. Since our plasmonic nanoscatters were designed to output specific on-resonance reflective colors in visible, substantial portions of incident light can transmit into the underlying silicon and contribute to the photocurrents. Meanwhile, several gain mechanisms for light trapping can take place at the off-resonance regimes of the nanoscatters, as confirmed in various literatures.7,26 Therefore, from a perspective of photon harvesting, we calculate the enhancement of absorption in the plasmonic nanoscatters integrated devices over the bare reference. As shown in Figure 7a, obvious dips located at the wavelengths near LSPRs are observed for all the plasmonic devices, owning to the construction of the reflective coloration. However, absorption enhancements are observed at rest wavelengths for the plasmonic devices. 9


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The sharp enhancement peaks appearing at short wavelengths are ascribed to the excitations of the surface Plasmon polaritons (SPPs) modes around the interface of Al and air. This can be easily confirmed by checking their phase-matching conditions. At long wavelengths, plasmonic devices show spectrally broad enhancement over the reference structure, implying an efficient non-resonant coupling of the incidence. Apart from above plasmonic in-coupling mechanisms, the angular distribution of the propagation waves in Si lengthens the optical path and thus promotes the extraction efficiency of the photogenerated carriers. As depicted in Figure 7b, the field distributions at the on-resonance and off-resonance wavelengths show typical periodic diffraction patterns. Due to the presence of the nanodiscs atop the silicon, the transmitted or scattered waves are coupled into various diffraction modes, allowing high absorption within relative shallow depths. The white lines superposed in the right plot are deduced from the diffraction equation by taking account the first order diffraction. Considering the finite diffusion length of minority carriers in n-type Silicon used herein, the angular distribution of diffracted beam in Si can substantially increase the internal quantum efficiency (IQE) of the devices. Figure 7c show the typical dark current-voltage curves of the reference devices. The rectifying performance of ITO/Si junction is crucial for achieving efficient photocurrent conversion. According to our experiments, we found that the thermally annealed ITO layer forms a more reliable schottky-like contact with Si as compared with the as-deposited films. Further, incorporation of an ultrathin oxide (2-4 nm) layer by chemical oxidation process between ITO and Si greatly improves the barrier height and suppresses reverse saturation current effectively. The light characteristics of the reference cells with such an optimized MIS configuration were experimentally evaluated under the simulated AM1.5 G illumination. As shown in the inset, the short-circuit current is 29.7 mA/cm2, open-circuit voltage around 0.39 V and the conversion efficiency is found to be 8.3%. The conversion efficiency of the devices can be significantly improved by establishing the commonly employed P-N junction at the front surface and the back surface field (BSF) scheme at the rear of the Si, but this substantially raises the device's cost and complexity. 10


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As an important device metric for optoelectronics, the external quantum efficiency (EQE) of the plasmonic devices should be evaluated. The result of external quantum efficiency (EQE) measurement on reference cell is shown in Figure 7d (black diamond). The corresponding internal quantum efficiency (IQE) can be extracted by using the simple relationship: IQE=EQE/(1-R), where R represents the experimental reflectance of the reference cell (refers to Supporting Information 5). Assuming that the plasmonic devices have the same internal carrier extraction efficiency, the EQE of the plasmonic devices therefore can be calculated by weighting the above IQE data with the numerically determined optical absorption of Si substrates (ASi=1-R-Ap, where Ap is the sum of parasitic absorption in ITO and nanodiscs layer, as shown in Figure 7d). Since the diffraction enhancements on IQE are neglected, this approach is regarded to give conservative but reasonably accurate predictions of EQE for plasmonic devices. To better mimic the realistic optical response, the simulations of the complete plasmonic devices including 300 µm thick Si substrates were carried out. The validation of the numerical methodological is confirmed by comparing the reflectance with the measured one. As shown in Figure 7d, the EQE curves of the plasmonic colored devices are comparable or higher than the reference, except for the resonance regime where plasmonic coloration and parasitic absorption take place. Considering white light illuminations (e.g. AM1.5G), these three selected plasmonic devices can provide photocurrents that are around 96-99% of the anti-reflective reference device (see Supporting Information 6), i.e. achieving coloration at a negligible cost of the power conversion efficiency. Spectral filtering, polarizing and light harvesting via plasmonic nanostructures are flourishing fields that hold

great importance

in their respective

device

applications.2-5,7,14 The present work aims to find an elegant combination of these merits for construction of the plasmonic optoelectronics relying on Si platform with multi-functional capacities. The integrated devices selectively scatter light only over desired ranges of wavelengths to access the s-RGB colors, while efficiently convert their complementary spectrum portions for electricity generation. Thus, we foresee that such an energy-efficient strategy can be employed in the reflective displays 11


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without resorting to the use of pigments filters, meanwhile generates power to charge the devices under bright ambient light illumination. In contrast, color reflective display based on interferometric absorption reported recently relies on the electrostatic actuation but has no power generation.44 Similarly, the proposed devices can be considered as the compact, CMOS-compatible filter integrated photodetector, therefore applicable in the imaging sensors. The advantages over the image sensors19,20 with plasmonic filters fabricated atop include, 1) less spatial optical crosstalk, 2) light harvesting for improved photoresponsitivity, 3) improved integrity, and 4) simplified fabrication eliminating alignment. Besides the possible interests from above two functionalities, the demonstrated polarization controlled coloration is also attractive. For instance, this feature can be used in polarization-resolved imaging for remote-sensing or surveillance.45 It should also be noted that some existing limitations need to be addressed for practical applications in the above mentioned fields. 1) Low chromaticity of plasmonic colors towards the red end. As shown in Figure 4 and 5, the linewidth of plasmonic scattering color of large nanodiscs becomes broad in the long wavelength region. It results in reduced chromaticities hindering applications in full visible range. Linewidth narrowing strategies should be explored on the presented scattering framework for future studies. For example, improved chromaticities can be observed in Figure 8c, where a thin low-index capping layer is coated on the pixels as shown in Figure 8a. It not only provides protection for the plasmonic nanoscatters but also improves both the efficiency and the gamut of the scattering coloration. More vivid colors in red can be observed as shown in Figure 8b compared to those in Figure 5a. 2) Angular dependence of the scattering colorations. Both theoretical and experimental studies on the impact of incident angle to the spectral reflectance of the plasmonic pixels were performed and the results are shown in Figure 9. The theoretical results are calculated by FDTD method and averaged by S- and P-polarized reflectance for difference incident angles. The reflection measurements on the fabricated color pixel (Al nanodiscs: D=110 nm, P=240 nm; pixel size is 20×20 µm2) were carried out by Microscopic Angle Resolved Photonic Spectroscopy (MARPS, Ideaoptics, Inc.). In 12


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general, increasing the oblique angle of the incidence, the amplitudes of the scattering peaks decrease and the spectral positions of the peaks slightly red-shift. The locations of the reflection peaks shift less than 10 nm when the incident angle varies from 10° to 30°. Generally, the f-number of the common camera ranges from 1.8 to 16, the corresponding incident angle is less than 17 degree.20 Therefore, the proposed pixels can sustain their color chromaticity with an oblique incident angle up to 30° owing to their localized resonance features, which is reasonably good for imaging. However, the decreasing in amplitude is more obvious and further optimization is required for display applications. For example, non-periodic or quasi-periodic patterning of the nanoscatters can enable excellent angle-insensitive (up to 60°) performance of the scattering colorations.46 CONCLUSIONS In conclusion, we have proposed and successfully demonstrated multiple functional optoelectronics on silicon platform by integrating plasmonic nanoscatters onto a schottky diode. Reflective coloration across the full visible spectrum was achieved together with a power conversion efficiency of 8.3% that is 96%-99% of the reference device without nanoscatters, i.e. realizing coloration at a negligible cost of the power conversion efficiency. The integration offers both color sorting and light harvesting capabilities with superiorities such as reduced spatial optical crosstalk, improved photoresponsitivity, self-charging, high integrity and so on in potential applications including advanced display, imaging and photovoltaics. Modified long wavelength approximation (MLWA) spheroid theory and numerical simulation reveal the underlying physics of the proposed structures, where the incorporation of an impedance matching layer provides spectral and spatial controls on the scattering behaviors of the plasmonic nanodiscs and enlarges the local driving force that increases the scattering efficiency. Further, based upon the asymmetric nanodiscs, we show two sets of color image information can be stored or shifted by controlling the polarization state. The presented concepts are generalizable to other optoelectronic material platforms, thus hold great potentials for development of the integrated displaying and sensing optoelectronics. 13


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Acknowledgment. We are thankful for the technical support from Nano-X of SINANO, CAS. This work was supported by the grants from the National Natural Science Foundation of China (No. 11274344, 11604367 and 61574158), the National Key Research and Development Program (No. 2016YFB0402501), the Cutting-edge Key Research Program of Chinese Academy of Sciences (No. QYZDB-SSW-JSC014), the Natural Science Foundation of Jiangsu Province (NO.BK20150369) and the Suzhou Science and Technology Development Program Foundation (No. SYG201529). Supporting Information Available: Additional information is noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES: 1.

2. 3. 4. 5. 6.

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Figure 1. The calculated (a) backward and (b) forward scattering cross-sections of the Al nanodiscs (diameter D=120nm, thickness t=30 nm) in various configurations including free-standing in air, sitting directly on the substrates (nsub=1.5, 2.0, 2.5 and 3.0) and the impedance matching layer (n=2.0, thickness is 70nm) coated Silicon. The inset in (a) illustrates the numerical model used in the total-field scattered-field (TFSF) source method. (c) The normalized magnetic field distributions in various configurations. 170x140mm (300 x 300 DPI)


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Figure 2. (a) Normalized driving field intensity of the homogeneous substrate with index of 2 and the dielectric (nd=2, td=20, 70 and 130 nm) coated Silicon. (b) Backward Qscat of the same substrate configurations as (a) but including the Al nanodiscs (D=120 nm). (c) (top) The typical electric field distributions on resonance illustrating the driving field enhancement of the layered substrate over the homogenous structure. (bottom) MLWA and numerical calculated backward Qscat for the cases of the homogeneous substrate and the dielectric (td=70 nm) separated Silicon. 170x119mm (300 x 300 DPI)


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Figure 3. (a) Schematic drawings of the proposed plasmonic Si optoelectronics. The Al nanodiscs with fixed thickness of 30 nm are arranged in a square lattice and characterized by their diameter D, period P. The supporting layered structures consist of a 64 nm thick ITO transparent anode, n-type Si substrate and high reflective Al cathode. (b) Contour plot of the reflectance as a function of the diameter of nanodiscs. D varies from 0 to 200 nm (where D=0 corresponding to the bare ITO/Si in absence of nanodiscs). The fill factor F defined by D/P is fixed to 0.5. (c) CIE 1931xy Chromaticity coordinates represent for the predicted colors of the multi-colored devices with reflectance corresponding to the spectra calculated in (b). (see Supporting Information 3). 170x150mm (300 x 300 DPI)


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Figure 4. (a) SEM image of the Al nanodisc array fabricated by EBL. The D and P are estimated to be 165 and 392 nm, respectively. The scale bar is 2µm. (b) Microscope image of the fabricated plasmonic color pixels (20×20µm2). The illumination source of the microscope is tungsten halogen lamp. From left to right, the pixels contain arrays of nanodiscs with diameters of 80, 95, 115, 135 and 165nm and periods of 264, 296, 328, 360 and 392nm, respectively. (c) The calculated and (d) measured reflection spectra corresponding to the pixels shown in (b). 170x140mm (300 x 300 DPI)


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Figure 5. (a) (left) Microscope images for plasmonic pixels with varying diameters (D=80~160 nm) and fill factor (F=0.33~0.5) of the nanodiscs. (right) Measured reflection spectra of four selected pixel rows. (b) CIE 1931xy chromaticity coordinates of the plasmonic pixels with the experimental reflectance shown in (a). The impact of the increasing period on the color chromaticities is indicted by arrows for each diameter. (c) Predicted RGB colors of the plasmonic pixels under standard D65 source illumination, by using the experimental spectra shown in (a). 170x99mm (300 x 300 DPI)


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Figure 6. (a-b) The spectral reflection spectra and microscopy images of two plasmonic color pixels under varied polarization angles. The color pixels (20 µm ×20 µm) consist of elliptical nanodisc arrays with different geometry parameters: (left) short axis DS=70 nm, long axis DL=130 nm; (right) DS=70 nm, long axis DL=170 nm. The nanodiscs were assembled in a rectangle lattice which has a same period of PS=240 nm in the direction parallel to short axis of the nanodiscs, but different periods (PL=360 and 400nm for the left and right pixels) in the perpendicular direction. (c) Microscopic plasmonic patterns of a cat cartoondisplay different image information at different polarization states. The numbers marked in the left image represent four color pixels containing different arrays of elliptical nanodiscs. The size of the pattern is 60 µm×60 µm. 170x129mm (300 x 300 DPI)


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Figure 7. (a) The absorption enhancement of three selected plasmonic nanoscatters integrated Si optoelectronic devices over the bare reference cell. (b) The normalized magnetic field distributions of the colored device at the on-resonance and off-resonance wavelengths. The white lines in the right plot were deduced by diffraction equations. (c) The typical current-voltage curves for the ITO coated Si devices with different experimental treatments. The insert plot is the experimental results of the current- and efficiencyvoltage curves measured under the one-sun illumination. (d) The predicted EQE and parasitic absorption in ITO and nanodiscs of the plasmonic devices. The experimental EQE of the reference device is also plotted. 170x129mm (300 x 300 DPI)


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Figure 8. (a) SEM image (45◦ titled view, scale bar is 1µm) shows the plasmonic nanodiscs embedded in a 40nm SiO2 capping layer. (b) The measured reflectance and microscope images of the embedded plasmonic nanodiscs configurations. The black line represents the reflection spectra of the bare SiO2/ITO/Si. The results for five nanodiscs integrated pixels with varying diameters and periods (D=80,100,120,140 and 160 nm, P=240, 260, 290, 320 and 360 nm) are plotted in the colored lines (red, blue, dark cyan, magenta and dark yellow). (c) CIE 1931xy chromaticity coordinates represents for the predicted colors of the pixels with the experimental spectra corresponding to (b). 170x55mm (300 x 300 DPI)


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Figure 9. Simulated (a) and measured (b) reflection spectra of the plasmonic color pixel at different incident angles. 170x70mm (300 x 300 DPI)


Multifunctional Silicon Optoelectronics Integrated ... - ACS Publications

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