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Plasmonics-Based Multifunctional Electrodes for Low-PowerConsumption Compact Color-Image Sensors Keng-Te Lin,† Hsuen-Li Chen,*,† Yu-Sheng Lai,*,‡ Yi-Min Chi,† and Ting-Wei Chu† †

Department of Materials Science and Engineering, National Taiwan University, 1, Sec. 4, Roosevelt Road, Taipei 10610, Taiwan National Nano Device Laboratories, National Applied Research Laboratories, 26, Prosperity Road I, Hsinchu 30076, Taiwan

‡

S Supporting Information *

ABSTRACT: High pixel density, efficient color splitting, a compact structure, superior quantum efficiency, and low power consumption are all important features for contemporary color-image sensors. In this study, we developed a surface plasmonics-based color-image sensor displaying a high photoelectric response, a microlens-free structure, and a zero-bias working voltage. Our compact sensor comprised only (i) a multifunctional electrode based on a single-layer structured aluminum (Al) film and (ii) an underlying silicon (Si) substrate. This approach significantly simplifies the device structure and fabrication processes; for example, the red, green, and blue color pixels can be prepared simultaneously in a single lithography step. Moreover, such Schottkybased plasmonic electrodes perform multiple functions, including color splitting, optical-to-electrical signal conversion, and photogenerated carrier collection for color-image detection. Our multifunctional, electrode-based device could also avoid the interference phenomenon that degrades the color-splitting spectra found in conventional color-image sensors. Furthermore, the device took advantage of the near-field surface plasmonic effect around the AlSi junction to enhance the optical absorption of Si, resulting in a significant photoelectric current output even under low-light surroundings and zero bias voltage. These plasmonic Schottky-based color-image devices could convert a photocurrent directly into a photovoltage and provided sufficient voltage output for color-image detection even under a light intensity of only several femtowatts per square micrometer. Unlike conventional color image devices, using voltage as the output signal decreases the area of the periphery read-out circuit because it does not require a current-to-voltage conversion capacitor or its related circuit. Therefore, this strategy has great potential for direct integration with complementary metal-oxidesemiconductor (CMOS)-compatible circuit design, increasing the pixel density of imaging sensors developed using mature Sibased technology. KEYWORDS: plasmonics, hole array, multifunctional electrode, color image sensors, low-power-consumption

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the size and thickness requirements of color filters face many challenges in the trend toward devices displaying high pixel density. Conventional image sensors feature organic dye−based filters for color splitting. The organic color filters absorb the unwanted portion of the spectrum to ensure red (R), green (G), and blue (B) transmitted colors.2 Nevertheless, organic dyes with low absorption coefficients require relatively thick color filters, thereby limiting the embodiment of high-density pixels in devices.4−6 In addition, the organic dye filters are susceptible to environmental factors;4−7 for example, the dyes may degrade under ultraviolet (UV) illumination, high temperature, and humid surroundings. Therefore, developing an alternative approach for color filtering is a challenging and potentially rewarding issue. The development of plasmonic structures may provide a new approach for color filtering.8−13 The phenomenon of surface

INTRODUCTION Efficient color splitting, high pixel density, high quantum efficiency, and low power consumption are all features of contemporary color-image sensors. In general, color-image sensors comprise microlenses, color filters, lightpipes, metal wire layers, photodetectors arrays, and accompanied electric circuit items. When the size of a single pixel in a color-image sensor is shrunk, each component will significantly affect the dimensions and performance of the image sensor.1 For example, when the pixel dimension is less than the scale of the wavelength of visible light, the microlens cannot focus the incident light properly, limiting the size of the pixel. Moreover, the shading effect of metal wires also decreases the amount of light received by each photodetector.2 Besides, the design of lightpipe layer concerns the capability of light focalization on the photodetector. Typically, the silicon nitride was used for the lightpipe layer1 because it has higher refractive index than the surrounding oxide layer; in addition, the length of lightpipe is generally several micrometers.1 Nevertheless, we found that the lightpipe layer will further affect the color spectra.3 In addition, © XXXX American Chemical Society

Received: November 25, 2015 Accepted: February 18, 2016

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DOI: 10.1021/acsami.5b11425 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

pixel density, low power consumption, efficient color splitting, and sufficient spectral responsivities under zero bias voltage. The key concept underpinning this device is the use of the multifunctional metal-semiconductor (Schottky) junction to directly extract the photoinduced carriers into Si upon receiving light from the metallic electrode-based plasmonic color filters. Even under low-light conditions of several femtowatts per square micrometer, our proposed Schottky-based color-image sensors could still provide sufficient voltage output for color image detection. Unlike conventional color-image sensors, the devices proposed herein provide sufficient color splitting capability even when the number of metallic holes is less than 50, as well as exploit the native plasmonic Schottky junction to collect photons in the near-field region and then directly transform them into the electrical output in the form of voltage. Therefore, this strategy has great potential for direct integration with circuit design to effectively decrease the circuit area and increase the number of pixels within a fixed device area.

plasma resonance (SPR)-induced extraordinary transmittance (EOT) has attracted attention14−17 because metal films featuring periodical subwavelength structures were found to display much higher transmittances than those predicted from classic diffraction theory. In addition, metal films possessing periodic hole arrays exhibit superior color splitting characteristics, allowing the development of filters with pass bands in the red, green, and blue regimes.5,6,15,18−21 Moreover, plasmonic filters based on metallic hole arrays have displayed transmissions in the range 40−50%5 when the filter area has been as small as 5 × 5 μm2. Plasmonic color filters have the attractive feature that only a single structured metal film could provide different color spectra merely through changing the period of the structure, suggesting their great potential for integration with commercial complementary metal-oxide-semiconductor (CMOS) color-image sensors.6 Although plasmonic color filters integrated with CMOS image sensors can provide efficient color splitting, the dimensions of the microlens, the length of the lightpipes, and the shading area of the metal wires would still hinder the development of high-pixel-density colorimage sensors. Therefore, there is a need to develop effective approaches to remove the microlenses and lightpipes from CMOS image sensors. Recently, nanostructures on semiconductor devices have been widely studied for their attractive photovoltaic, light trapping, optical detection, and sensing properties.4,22−30 For example, the core/shell p-i-n Si nanowire-based devices displayed high open-circuit voltages and short-circuit currents in spite of their small absorbing volumes.30 Seo et al. demonstrated that vertical silicon (Si) nanowires with distinct diameters exhibit a variety of colors covering the entire visible spectrum.22 Park et al. proposed a concept for developing a filter-free color image device based on an all-Si nanowire structure.4 They incorporated Si nanowires in p-i-n photodetectors such that the absorbed photons could be converted to a photocurrent directly; in addition, the spectral responsivities of the devices were controlled further by the size of the nanowire. Accordingly, this strategy abandoned the microlenses, color filters, and lightpipes used in conventional colorimage sensors and allowed shrinking of the size of each pixel. Nevertheless, the splitting ability of the colors would deteriorate gradually as the number of Si nanowires decreased; in addition, the required spacing of each Si nanowire (ca. 1 μm) limits the trend toward high pixel density. Moreover, low spectral responsivity (ca. 0.02 A W−1) also revealed that the quantum efficiency of Si nanowire-based devices was not high enough. The inefficient color splitting ability and low responsivities would restrict the application of this technology in practical color imaging detection. Generally, the photocurrent is the signal generated for light detection in a conventional image sensor. The current signals must, however, be further converted into voltage through extra capacitors that amplify the voltage signals in a peripheral readout circuit. Although capacitors and related circuits play important roles in signal conversion, they inevitably increase the chip area and decrease the response speed. Therefore, simplification of capacitor-based circuits is a major issue in read-out circuits. Herein, we propose a microlens-free, plasmonic color filterbased image sensor, constructed from a single-layer structured aluminum (Al) film in direct contact with a Si substrate, that exhibits enhanced device operation in terms of its photovoltage output. In addition, this device simultaneously exhibits high

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RESULTS AND DISCUSSION In this study, the plasmonic structures not only provided efficient color splitting, but also induced high electric fields within the near-fields to improve the conversion efficiency of the devices. The concept was readily achieved through constructing plasmonic metal hole array structures on n-type Si surfaces, where metal−Si Schottky barriers were formed. Figure 1a displays a schematic representation of the proposed AlSi Schottky-based plasmonic color-image sensors. The structured top-metal layer on the Si substrate performed three main functions: Schottky junction formation, as a plasmonic color filter, and as an electrode. Here, the color-selecting photoresponse in the Schottky-based plasmonic color-image sensors was realized by the photogenerated electron−hole pairs

Figure 1. A plasmonic Schottky-based color-image sensor featuring a metallic hole array structure for color-selective photodetection. (a) Schematic representation of the proposed Al−Si Schottky-based plasmonic photodetector. (b) Energy band diagram for an Al−Si Schottky-based device under illumination. (c) Top-view and crosssectional SEM images of Al hole array structures on Si substrates; the cross-sectional SEM image was recorded taken at a tilt angle of 52°. B


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positions of the color-matching function for the blue, green, and red spectra were located at 445, 555, and 595 nm, respectively.32 Therefore, we set out to determine the transmittance spectra of plasmonic structures that would match the peaks of the standard color response. Figure 2b displays the simulated transmittance spectra of Schottky-based devices comprising periodic hole arrays of Al films (thickness: 150 nm) on Si substrates. These periodic metallic hole-array structures displayed excellent color-splitting capability; besides, the light transmitted into Si layer is approximately 35−45% of the incident light at resonant wavelengths (see Figure S1 of the Supporting Information, SI). In addition, the peak positions in the transmittance spectra for blue, green, and red lines were very close to the peaks of each color-matching function. The spectral response and peak position could be fine-tuned easily by adjusting the dimensions of the holes and the period. Moreover, Figure 2c−e displays the electric field distributions obtained by simulating the incident light passing through the distinct metallic hole arrays designed for the colors blue, green, and red, respectively. Figure 2c displays the electric field intensities of light at wavelengths of 445, 555, and 595 nm, respectively, passing through the H022P044 structure. At a wavelength of 445 nm, near the SPR transmission peak of the H022044 structure, we observed an obvious transmission of electric field intensity passing through the metallic hole arrays. When we changed the wavelength of the incident light to 555 and 595 nm, the transmission intensity decreased significantly, because these wavelengths did not match the SPR of the H022044 structure. Similarly, Figure 2, parts d and e, displays the results of the simulations of the H028P056 and H0315P063 structures, which we had designed for the colors green and red, respectively. The simulations reveal obvious transmissions of electric field intensity passing through the metallic hole arrays at the resonance wavelengths of 555 and 595 nm, respectively. Therefore, we could exploit the metallic hole array structures having distinct SPR bands in the designed spectral regime to achieve the color-splitting function. Moreover, the EOT phenomenon would also simultaneously increase the light transmittance into the Si substrate and improve the photoresponse of the devices at the designed spectral band. Furthermore, we anticipated applying the metallic hole array structures as replacements for organic color filters in colorimage sensors. In general, conventional color-image sensors comprise color filters, lightpipes (silicon nitride layers), and Sibased devices (photodiodes). Here, we investigated the feasibility and applicability of integrating the metallic holearray structures into a commercial color-image device. To further study the optical behavior of metallic hole-array structures integrated with color-image sensors, we deposited the periodic metal hole arrays on a silicon nitride layer coated on the Si substrate. In general, the length (thickness) of the lightpipe (silicon nitride layer) is in the range from 1 to 2.5 μm. The intensity of light at distinct wavelengths propagating into the Si substrate would be affected by the thickness of the silicon nitride layer, due to interference. Figure S2 reveals that the interference effect degraded the color-splitting performance of the color-image sensors. A thicker silicon nitride layer resulted in more interference peaks and dips in the transmittance spectrum. Moreover, the interference phenomenon remained obvious even when the thickness of the silicon nitride layer was as low as 1 μm. Figure 3a displays the transmittance spectrum of a Si-based photodetector comprising periodical metallic hole arrays coated on a silicon nitride film having a thickness of 1.5

transferring over the Schottky barrier, in a process involving four steps. (i) The incident light of a specific color couples through the metallic hole structures to generate surface plasmons. (ii) The plasmons propagate along the metal surface and then transmit into the Si substrate to induce photoexcited carriers. (iii) The electron−hole pairs separate as a result of the built-in electric field formed at the metal−Si interface (Schottky junction), as displayed in Figure 1b. (iv) The separated carriers are collected effectively by the structured metal electrode. Accordingly, the plasmonic device, comprising only the structured single-layer metal film and the Si substrate, could perform efficient color splitting and transform to photoresponse directly into electrical signals. Figure 1c displays field-emission scanning electron microscopy (FESEM) images of metallic structures having distinct diameters and periods in a hexagonal arrangement on Si substrates. The hexagonal symmetry meant that, regardless of the polarization state of the incident light, the optical behaviors of this structure are almost the same. In Table 1, we label the Table 1. Structural Dimensions of Plasmonic Schottky-based Devices Featuring Al Hole Array Structures symbol

H022P044

H028P056

H03P06

diameter (nm) period (nm)

220 440

280 560

300 600

plasmonic color devices in terms of their distinct dimensions (hole diameter and period); for example, the device featuring a hole-array structure having a hole diameter of 220 nm and a period of 440 nm is named “H022P044”. The other metallic structures in the plasmonic color-image sensors are named H028P056 and H03P06. Furthermore, the interface between the metal and the Si substrate would significantly affect the photoresponse of plasmonic devices; accordingly, the etching conditions or deposition process for preparing the structured metal films were very critical. The cross-sectional images reveal that the Al layers provided good coverage on the surfaces of the Si substrates. Moreover, the etching process of the metal layer should be performed accurately on the Si surface such that the metal−Si interface would not be damaged and could result in good photoresponse in the form of electrical signals. To analyze the color splitting ability of the plasmonic structures, we used the three-dimensional finite-difference timedomain (3D-FDTD) method to investigate the performance of metallic hole-array structures having different structural parameters (e.g., hole diameter, period). Here, we set transverse-electric (TE)-polarized plane waves propagating from 1 μm above the metallic hole-array structures on Si substrates. To further study the optical behavior of the metallic hole-array structures, we set up detectors located 5 nm below the AlSi interface (inside the Si) for each plasmonic structure. The detector within the Si substrate could detect the degree of light transmittance through the metallic structures into the Si substrate and, thereby, determine the color splitting ability of each structure. In the simulation, we set the metal hole arrays to have a duty ratio (hole diameter-to-period ratio) of 0.5. All of the optical constants of the materials (Al, Si, silicon nitride) were obtained from the literature.31 Furthermore, we normalized all of the values in each color-splitting spectrum to the maximum value of transmitted light into the Si substrate. As displayed in Figure 2a, according to the ideal spectral response of the CIE 1964 10° color-matching function, the peak C


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Figure 2. (a) CIE 1964 XYZ color-matching functions. (b) Simulated transmittance spectra of the plasmonic Schottky-based devices; light was passed through distinct dimensions of the Al hole-array structures with infinite periodic holes and then transmitted into the Si substrates. (c−e) Calculated electric field intensity distributions of incident light of various wavelengths (445, 555, and 595 nm) passing through the Al hole-array structures of (c) H022P044, (d) H028P056, and (e) H0315P063, respectively.

μm. The transmission peak of each color has been shifted; moreover, the shapes of the spectra were also affected significantly by the interference phenomenon. Therefore, the interference effect must have a distorting effect on the colorsplitting spectra. For example, the peak positions of the red line in Figure 3a located at wavelengths of 405, 430, 455, 485, 535, 645, 715, and 770 nm correspond to the interference peaks of a silicon nitride film having a thickness of 1.5 μm coated on a Si substrate (see Figure S2b). Similarly, the color-splitting spectra of the Si substrates with silicon nitride-based lightpipes of distinct thicknesses would also reveal spectral distortion (see Figure S3). These fluctuations in the spectra of the different colors would largely degrade the color-performance of colorimage sensors. Next, we would like to investigate the colorsplitting behavior of the metallic hole-array structures with the goal of obtaining a high pixel density. We decreased the number of metallic holes to decrease the area of color filters and further increase the pixel density. We simulated the transmission spectra of metallic structures having 33 holes, set in a hexagonal arrangement; the corresponding areas for the blue, green, and red filters were 2.62 × 2.33, 3.28 × 2.91, and

3.67 × 3.24 μm2, respectively, comparable with the pixel sizes in a high-density CMOS image sensor. Figure 3, parts b and c, displays the transmittance spectra of light propagating into Sibased devices prepared without and with the silicon nitride lightpipe layer, respectively. Figure 3b reveals that the structured metallic film, even with only 33 holes, could still exhibit very good color-splitting characteristics to determine the spectra of RGB colors. In contrast, the transmittance spectra of the 33 metallic holes on a silicon nitride layer (1.5 μm) in Figure 3c were affected by the interference phenomenon, significantly decreasing the color-splitting capability. Therefore, plasmonic structures with finite numbers of holes would not be suitable for integration with conventional color-image sensors because the tendency is toward the development of a large number of pixels in a fixed device area. In contrast, our proposed single-layer plasmonic structures would be suitable for such systems because the light of distinct wavelengths propagating through the metallic holes would be transmitted directly into the Si substrate, thereby avoiding any interference from the lightpipe or any shading effect of the metal wires. Accordingly, we expected the Schottky-based plasmonic sensors D


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to have great potential for integrating in CMOS image sensors, simultaneously achieving a microlens-free structure, efficient color-splitting, and a high pixel density in a single chip. The plasmonic Schottky-based color-image sensor proposed in this study is a metal−semiconductor (MS) junction photodetector comprising Al hole arrays, an Al−Si Schottky junction on a lightly doped n-type Si substrate, and an ohmiccontact electrode on the backside of the Si substrate. The detailed fabrication processes and measurement setup are described in the Methods (please see the SI). We measured the current−voltage (I−V) performance of the plasmonic Schottkybased devices to confirm the rectification of the Schottky junction and their photoresponsive properties. As displayed in Figure 4a, we measured the I−V characteristics of the devices in the dark and under illumination of 100 mW cm−2 from a nonpolarized and broadband light source (Xe lamp). The measured dark current density of the Schottky-based device was approximately 1.2 nA cm−2 at zero bias, even though the device had undergone the dry etching process. Thus, the AlSi interface had not been destroyed during the etching process. Such a low dark current density could be conductive for a device capable of detection in low-light environments. In addition, the measured I−V characteristics under illumination revealed that the Schottky-based devices featuring metallic hole arrays were very sensitive to the incident light. Comparing the devices operated in the dark and under illumination, both the short-circuit current and the open-circuit voltage of the plasmonic Schottky-based device responded more significantly when illuminated. Therefore, we measured the spectral photoresponses of individual devices containing metallic hole arrays having diameters/periods of 220/440, 280/560, and 300/600 nm (H022P044, H028P056, and H03P06, respectively) in their “short-circuit current” and “open-circuit voltage” modesthe two modes used commonly in today’s integrated circuits. Initially, we measured the spectral response of the photocurrent of each device under zero bias, the so-called “shortcircuit current” mode and then calculated the spectral responsivities (units: A W−1) using the measured data (for details, please see the Methods in the SI). The inset in Figure 4(b) displays schematic representations outlining the operating mechanism of Schottky-based plasmonic color-image sensors in this mode. Upon illumination with incident light, the electron/ hole pairs generate in the SI. These carriers are swept from the Schottky junction by the built-in electric field of depletion region. Then, the holes and electrons move toward the top and backside electrode, respectively, and photocurrent could be produced. Figure 4b displays the measured responsivities of the plasmonic Schottky-based photodetectors. The peak positions of the photodetectors’ blue, green, and red spectral responsivities were located at 465, 560, and 600 nm, respectively, very close to the peak positions of the colormatching function for blue, green, and red spectra. Furthermore, the full widths at half-maximum (FWHMs) for the blue, green, and red detectors were approximately 100, 115, and 105 nm, respectively. The measured FWHMs of these spectral responses were slightly larger than the simulated values (Figure 2b), presumably because the shapes of the holes, the roughnesses of the holes’ sidewalls, and the hole diameters in the simulation setups differed slightly from those in the real devices. In addition, we measured a maximum responsivity of 0.14 A W−1 for the blue detector, 0.21 A W−1 for the green detector, and 0.25 A W−1 for the red detector at zero bias.

Figure 3. (a) Simulated transmission spectra of light propagating into Si-based photodetectors comprising of distinctly sized Al hole-array structures with infinite numbers of periodic holes, a silicon nitride− based lightpipe layer (thickness: 1.5 μm), and underlying Si substrate. (b, c) Simulated transmission spectra of the photodetectors featuring only 33 Al holes in a hexagonal arrangement in the (b) absence and (c) presence of a silicon nitride−based lightpipe layer (thickness: 1.5 μm) on the Si substrate. The thickness of the Al layer in each structure was 150 nm; structural details of the devices are depicted in the insets. E


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Figure 4. Electrical characteristics of Schottky-based devices featuring metallic hole array structures. (a) Current−voltage characteristics measured in the dark and under illumination of 100 mW cm−2 from a nonpolarized and broadband light source (Xe lamp). The measured dark current density of the Schottky-based devices was approximately 1.2 nA cm−2 at zero bias voltage. (b) Photoelectric characteristics of the plasmonic Schottky-based devices H022P044, H028P056, and H03P06, measured under zero bias voltage. The inset displays schematic representation of the operating mechanism of plasmonic Schottky-based color-image sensors in the “short-circuit current” mode. (c−e) Correlation between excess current and incident light power density for the plasmonic Schottky-based devices H022P044, H028P056, and H03P06 at wavelengths of 450, 550, and 600 nm, respectively. Within the incident power density range from several femtowatts per square micrometer to several picowatts per square micrometer, the excess currents of the plasmonic Schottky-based devices exhibited linear responses (R2 > 0.999).

film on a Si substrate, the responsivity of the Schottky-based device remained distinguishable when measured under zero bias voltage and low-light conditions. As displayed in the inset to Figure 4c−e, even when the incident light power density was less than 10 fW μm−2, the excess current from the devices remained greater than 1 nA. Figure 4c−e also display the correlations between the excess current and the incident light power density of the plasmonic Schottky-based devices at wavelengths of 450, 550, and 600 nm, respectively. The linear behavior of the excess current over the whole range of illumination powers is dominated by the conversion of incident photons into electron/hole pairs. The incident power density of the light from the Xe lamp ranged from several femtowatts per square micrometer to several picowatts per square micrometer; the excess current of the each device also exhibited a high degree of linearity (R2 > 0.999), regardless of the wavelength (450, 550, or 600 nm). Therefore, these plasmonic Schottkybased devices were very sensitive to the intensity of incident

Thus, even when these Schottky-based devices featuring metallic hole arrays are operated at zero bias, they can provide sufficient responsivities (Figure 4b) and external quantum efficiencies (see Figure S4) to distinguish light of distinct wavelengths. More importantly, the external bias voltage higher than 1 V is generally required to obtain sufficient photoresponsivities from conventional Si-based photodetectors.11,33 Operation at zero bias means that our proposed devices could achieve photodetection with ultralow power consumption. To further investigate the performance of the plasmonic Schottky-based devices, we characterized the photodetectors in terms of the excess current, defined as the difference between the photocurrent and the dark current, under illumination at distinct power densities. Figure 4c−e display the excess currents of the devices H022P044, H028P056, and H03P06 recorded under incident light at various power densities and wavelengths of 450, 550, and 600 nm, respectively. Even though each photodetector featured a 150 nm structured Al F


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Figure 5. (a) Schematic representation of the operating mechanism of plasmonic Schottky-based color-image sensors in the “open-circuit voltage” mode. (b) Measured responsivities (units: V W−1) of the plasmonic Schottky-based photodetectors H022P044, H028P056, and H03P06. (c−e) Correlation between excess voltage and incident light power density for the (c) H022P044 device at a wavelength of 450 nm, (d) H028P056 device at a wavelength of 550, and (e) H03P06 device at a wavelength of 600 nm. Within the incident power density range from several femtowatts per square micrometer to several picowatts per square micrometer, the excess voltages of the plasmonic devices exhibited high degrees of linearity (R2 > 0.999). Each plasmonic device provided a voltage output of greater than 1 mV under illumination at an incident light power density of 1 pW μm−2.

backside electrode can be used to determine the intensity of light, directly providing an electrical signal in the form of a voltage for subsequent operation of the integrated circuit [Figure 5a (iii)]. Accordingly, the strategy proposed herein can convert a photocurrent directly into a photovoltage, not only decreasing the circuit area without the need for an extra capacitor but also conveniently integrating the color-image sensor with the peripheral read-out circuit. Figure 5b displays the measured spectral responsivities (units V W−1) of the blue, green, and red colors of the plasmonic Schottky-based photodectors. In the “open-circuit voltage” measurement mode, the peak positions and FWHMs of the RGB detectors’ spectral responsivities were similar to those measured in the “short-circuit current” mode (cf. Figure 4b). The maximum responsivities were 168.1 V W−1 for the blue detector, 348.6 V W−1 for the green detector, and 367.1 V W−1 for the red detector. Accordingly, the Schottky-based plasmonic color-image sensors should provide sufficient voltage output for subsequent use by integrated circuits even when operated in surroundings under low-intensity light. To examine the performance of the Schottky-based devices featuring plasmonic

light, such that slight variations in light intensity could also be determined. Importantly, the devices could detect light at low intensity with ultralow power consumptionvery useful properties suggesting promising practical applications in daily life. Generally, color-image sensors provide a photocurrent to evaluate the light intensity at specific colors. To the best of our knowledge, there have been no previous literature reports of color-image sensors that transform the photocurrent directly into the form of a photovoltage. In this study, in addition to obtaining photocurrents, we also found that our plasmonic color-image sensors can generate a photovoltage output when operated in the “open-circuit voltage” mode. Figure 5a displays schematic representations outlining the operating mechanism of Schottky-based plasmonic color-image sensors in this mode. The open-circuit mode implied the absence of a current, as displayed in Figure 5a (i), upon illumination with incident light. The electron/hole pairs generated in the Si accumulate at the top electrode featuring the metallic hole arrays [Figure 5a (ii)] when the backside electrode is maintained grounded under illumination; as a result, the potential between the top and G



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hole arrays structures under low light, we characterized the photodetector in terms of the excess voltage (the difference between the photovoltage and the dark voltage) under illumination at distinct power densities. Figures 5c−e displays the excess voltages of the devices H022P044, H028P056, and H03P06 measured under the incident light at wavelengths of 450, 550, and 600 nm, respectively. Similar to the measurements in the “short-circuit current” mode, the excess voltage of each device exhibited a high degree of linearity (R2 > 0.999) under illumination over a very wide range of incident power densities. Each plasmonic color-image sensor could provide a voltage output of greater than 1 mV under illumination at an incident light power density of 1 pW μm−2. Importantly, the plasmonic Schottky-based devices could provide outputs of up to tens of microvolts even when the incident power densities were less than 10 fW μm−2 (insets to Figure 5c−e). Consequently, the plasmonic devices described herein exhibited color-image detection capability at ultralow levels of ambient visible light even when operated in the “open-circuit voltage” mode. Moreover, the sufficient signal output in the form of voltage (from several microvolts to millivolts) provided by the photodetectors should effectively minimize the need for capacitors and amplifiers in the fixed area of the circuits. Therefore, the concept behind our devices would benefit the design of subsequent processing circuitry and enlarge the effective area of color-image sensors to further increase the pixel number and density.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11425. Methods, simulated transmittance spectra of the plasmonic Schottky-based devices, interference phenomena of silicon nitride films with various thicknesses on Si substrates, color spectra of plasmonic Si photodetectors featuring a silicon nitride-based lightpipe layer with distinct thicknesses, and spectral external quantum efficiencies of plasmonic Schottky-based devices (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.-L.C.). *E-mail: [email protected] (Y.-S.L.). Present Addresses §

Department of Materials Science and Engineering, National Taiwan University, 1, Sec. 4, Roosevelt Road, Taipei, Taiwan. ∥ National Nano Device Laboratories, National Applied Research Laboratories, 26, Prosperity Road I, Hsinchu, Taiwan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology, Taiwan, for supporting this study under contracts MOST-103-2221-E-002041-MY3 and MOST-103-2221-E-002-092-MY3.

CONCLUSIONS In summary, we have developed surface plasmonics-based color-image sensors displaying superior photoelectric responses, microlens-free structures, and low power consumption. The sensors comprise only (i) a multifunctional electrode based on a structured single-layer Al film and (ii) an underlying Si substrate. The concept proposed herein significantly simplifies the device structure and fabrication processes; for example, the RGB color pixels of the device can be prepared simultaneously using a single lithography step. Moreover, the plasmonic electrodes perform multiple functions during colorimage detection, including color splitting, optical-to-electrical conversion, and collection of photogenerated carriers. The use of multifunctional electrodes also avoids the phenomenon of interference, which degrades the color-splitting spectra, in conventional color-image sensors, allowing the development toward photodetectors exhibiting ultrahigh pixel density. Our Schottky-based devices featuring metallic hole array structures take advantage of the near-field surface plasmonic effect around the AlSi junction to enhance the optical absorption of Si, resulting in excellent photoelectric performance, regardless of whether the output is in the form of a photocurrent or a photovoltage. The plasmonic Schottky-based color-image devices can convert a photocurrent directly into a photovoltage and provide sufficient voltage output for color-image detection even under low-light conditions (several femtowatts per square micrometer). Compared with conventional color-image sensors, the concept proposed herein not only provides sufficient color-splitting capability even when the number of metallic holes is less than 50 but also exploits the native plasmonic Schottky junction to effectively collect photons in the near-field region. Therefore, this strategy has great potential for direct integration with CMOS-compatible circuit designs, increasing the pixel density of imaging sensors while employing mature Si technology.

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