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Tunable multispectral color sensor with plasmonic reflector Vladislav Jovanov, Helmut Stiebig, and Dietmar Knipp ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00402 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 28, 2017

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Tunable multispectral color sensor with plasmonic reflector Vladislav Jovanov,† Helmut Stiebig,‡,§ and Dietmar Knipp‡,#,* †

Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany

‡

Research Center Jülich, Institute of Photovoltaics, 52425 Jülich, Germany

KEYWORDS: plasmonics, optical sensors, color sensor,

ABSTRACT: Vertically integrated color sensors with plasmonic reflectors are realized. The complete color information is detected at each color pixel of the sensor array without using optical filters. The spectral responsivity of the sensor is tuned by the applied electric bias and the design of the plasmonic reflector. By introducing an interlayer between the lossy metal back reflector and the sensor, the reflectivity can be modified over a wide spectral range. The detection principle is demonstrated for a silicon thin film detector prepared on a textured silver back reflector. The sensor can be used for RGB color detection replacing conventional color sensors with optical filters. Combining detectors with different spectral reflectivity of the back reflector allows for the realization of multispectral color sensors covering the visible and the near infrared spectral range.

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Plasmonic nanostructures and plasmonic metasurfaces are promising elements to miniaturize optical components like lenses or improve the behavior of optoelectronic devices like optical detectors and solar cells.1,2 Plasmonic structures can be used to improve the absorption of optoelectronic devices, permitting a considerable reduction in the physical dimensions of detectors and solar cells. Major insights in the physics of surface plasmon polaritons and localized plasmon polaritons have been achieved by improved nanofabrication and optical modeling. However, high optical losses associated with plasmonic nanostructures have prevented the realization of plasmonic solar cells exhibiting record energy conversion efficiencies or low loss metal based metasurfaces that operate in the visible part of electromagnetic spectrum. The confinement of light at the nanoscale requires that plasmonic materials possess free carriers, which unavoidably results in absorption losses of the optoelectronic device. Optical losses can be reduced by using doped semiconductor like oxides and nitrides.3,4 However, most doped semiconductors exhibit plasmonic resonances in the infrared and not in the visible part of the optical spectrum.3-5 Rather than continuing to research on energy conversion devices like solar cells, where plasmonic losses ultimately limit the energy conversion efficiency, we focus on applications where optical losses enable new functionalities. By controlling and tuning the optical absorption new applications like colorimetric or thermal management devices can be realized.6,7 In this study we focus on the realization of red-green-blue (RGB) and multispectral color sensors fabricated on tunable plasmonic reflectors. By nanostructuring the metal reflector of the sensor the spectral reflectivity can be controlled from highly reflective surfaces to almost perfect absorbers. By integrating such structures in an optical sensor, the spectral responsivity can be tuned.

Imaging is usually performed by an array of sensor elements in combination with a color filter array (CFA). However, the quality and flexibility of conventional sensor arrays using color filter arrays 2 ACS Paragon Plus Environment

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are limited because three chromatic pixels for red, green and blue are required to generate a color pixel. To increase the performance, vertically integrated sensors have been developed. Here the color information is detected in the depth of the sensor. Various vertically integrated sensor structures were proposed and realized in previous years.8-15 Different materials, design concepts and contact configurations were used to realize vertically integrated color sensors. The wavelength dependent absorption of the applied semiconductor material leads to the absorption of photons at various depths, so that the color information can be detected in the depth of the device. The suggested sensors range from two terminal devices, which change their spectral sensitivity by varying the applied bias voltage to vertically stacked diodes.8,9,11-15 The most notable devices are the camera chips commercialized by Foveon based on a stacked arrangement of three silicon diodes.8,9 A vertical integrated thin film sensor was realized by Hewlett Packard.11 In order to realize color sensors with higher spectral resolution multispectral devices are required. The only vertically integrated multispectral sensors with up to 6 spectral channels have been developed by our group.16 However, the fabrication of these devices require the fabrication and patterning of a complex layer stack. In this paper we present a vertical color sensor with a plasmonic reflector. By engineering the plasmonic reflector, the reflectivity of the reflector and the spectral sensitivity of the color sensor can be adjusted. Schematic cross sections of the color sensors are shown in Fig. 1. To our knowledge for the first time a sensor is presented that combines a spatial color multiplexing scheme (side-by-side arrangement of the individual color channels) used by conventional color sensors with the time multiplexing scheme (sequential read-out of colors) of a vertically integrated sensor. The design of the plasmonic back reflector is investigated by optical simulations. The color detection principle of the sensor is explained and the spectral responsivity of the experimentally realized sensors is presented and discussed.


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“Figure 1. Schematic cross section of vertically integrated color sensor with a (a) highly absorbing back reflector and (b) a highly reflective back reflector.“

The plasmonic back reflector of the vertical color sensor plays a key role in determining the spectral responsivity of the color sensor. The main purpose of the back reflector is to reflect light back into the optical sensor. The reflectivity of the back reflector can be controlled in terms of the absolute reflection and wavelength dependence of the reflection. By controlling the design of the metal back reflector, the reflection can be tuned from high reflection to a low reflection. In previous studies, the influence of such plasmonic reflectors on the optics, quantum efficiency and energy conversion efficiency of silicon thin film solar cells was investigated.17-19 Metal reflectors covered with nano features exhibit large optical losses for long wavelengths. By introducing an interlayer with low refractive index, the quantum efficiency is enhanced and the optical losses of the back reflector are 4 ACS Paragon Plus Environment

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reduced.17-19 In this study, the nano features are introduced intentionally to control the spectral responsivity of optical detectors. 3D optical simulations are conducted to the study the effect of nano features on the optics of the thin film color sensors. The calculations are carried out using a Finite Difference Time Domain (FDTD) simulation approach. Silver is used as a back reflector material because flat silver films exhibit a high reflection in the visible and near infrared optical spectrum. The optical properties of the silver film are described by a Drude model.20 The aim of the simulation is to study the optics of the metal reflector covered with nano features. The overall design of the color sensor is described in a subsequent section of the manuscript. In order to separate the absorption of the silicon layer from the absorption of the metal reflector, the extinction coefficient of the crystalline silicon used for the simulations has been set to zero, while the refractive index has been set to 4. The morphology of the textured silver film is described by periodically arranged pyramids with a square shaped base. The height and period of the pyramids is 15 nm and 50 nm, respectively. The root-mean-square roughness of the metal reflector is 3.5 nm. Schematic cross sections of the different back reflectors designs are shown in Fig. 3(a-c). The calculated reflection of the different reflectors is shown in Fig. 2. The textured silver layer exhibits a high absorption in the red and infrared part of the optical spectrum, while the flat silver films exhibit a very high reflection. The spectral responsivity of the color sensor can be controlled by texturing the metal back reflector, since the absorption of the textured film can be increased to almost 100% by increasing the aspect ratio of the pyramids.


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“Figure 2. Absorption of silver back reflectors with of flat and pyramidal textured interfaces. A crystalline silicon film with and without zinc oxide interlayer is formed on the flat and textured silver reflectors. The refractive index of the silicon layer was set to 4, while the extinction coefficient of the silicon layer was set to 0. The height and period of the pyramidal texture is 15 nm and 50 nm.“ Sensors with flat and textured back reflectors could be realized together on the same substrate. However, the direct growth of a silicon layer on a silver films leads to a modification of the surface of the silver film. The plasma deposition of the microcrystalline n- and i-layer leads to a roughening of the silver film. In the case of the substrate with the nano features the plasma treatment has a minor effect, while in the case of the flat substrate the plasma treatment leads to an increased absorption of the silver film and a reduction of the reflectivity for longer wavelengths. To avoid the roughening of the silver layer an interlayer is introduced between the silver back reflector and the silicon diode. A lightly doped zinc oxide layer with an average refractive index of 2.2 is used as interlayer.


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Based on simulation results, flat and textured silver reflectors covered with a ZnO interlayer exhibit a high reflection for red and infrared light. The simulation of the flat silver reflector with a ZnO interlayer is given in the supplementary material (Fig. S.1). For short wavelengths ranging from 400 nm to 550 nm, the textured silver reflector with the ZnO interlayer exhibits high absorption, while the flat silver with the ZnO interlayer exhibits high reflection. However, the spectral responsivity of the color sensor is not affected, because light with a wavelengths of 400 nm to 550 nm does not reach the back reflector. This allows for the simplification of the color sensor design and fabrication process. Instead of fabricating color sensors with flat silver reflectors and a ZnO interlayer, we have fabricated a nanotextured silver layer combined with a ZnO interlayer. Hence, color sensors with and without ZnO interlayers on nanotextured silver are fabricated and compared. Plots of the electric field distribution for a flat and textured back reflector with and without a ZnO interlayer are shown in Fig. 3(d-i). The plots are shown for wavelengths of 455 nm and 730 nm. The flat back reflector exhibits low absorption for both wavelengths. For an incident wavelength of 730 nm the textured metal reflector without ZnO interlayer exhibits a maxima of the absorption. The corresponding power loss maps can be found in the Supplementary material (Fig. S.2). By inserting a ZnO layer, the absorption drops and the reflected electric field is increased. For an incident wavelength of 455 nm, the opposite behavior is observed. Without a ZnO interlayer almost all light is reflected. Inserting the ZnO layer leads to a shift of the plasmon resonsance, so that a high absorption is observed at 455 nm. The nano features are distinctly smaller than the wavelength of the incident light and a standing wave is formed on top of the metal back reflector. The textured metal films acts like an effective refractive index gradient. The fringes of the absorption spectra are caused by the periodic arrangement of the unit cell. In the case of the experimentally realized color sensors, a random arrangement of nano features is formed. Hence no interference fringes are observed, which is confirmed by the experimentally measured spectral responsivity. The measured spectral responsivities do not exhibit pronounced interference fringes. 7 ACS Paragon Plus Environment

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“Figure 3. Schematic cross section of back reflector of the color silicon sensors. (a) Flat interface, pyramidal textured interface with (c) and without (b) zinc oxide interlayer. (d-f) Electric field profile for an incident wavelength of 455 nm, (g-i) Electric field profile for an incident wavelength


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of 730 nm. Power loss maps for the 455 nm and 730 nm are provided in the Supplementary material.“

The vertical color detector consists of a p-i-i-i-n diode. The absorber of the sensor is divided in three regions. This is schematically illustrated in Fig. 4. The first region exhibits a large optical bandgap to absorb the blue part of the optical spectrum, while light with photon energies smaller than the bandgap is transmitted through the first region. A smaller bandgap is used to absorb the green part of the optical spectra in the 2nd region, while again light with photon energies smaller than the bandgap is transmitted. The bandgap of the 2nd region allows for absorbing blue and green light. However, the blue light is already absorbed by the first absorber region. Finally, red and infrared light is absorbed by the third region with the smallest bandgap. In order to optically design the color sensor 1D transfer matrix method simulations are used. Details on the 1D optical simulations of silicon thin film sensors are presented in literature.15 The optical bandgap and thickness of the individual layers are selected to match the color matching functions used to characterize color sensors.21 The photo generated charges are extracted depending on the applied bias voltage. To allow for the separation of photons absorbed in the different regions of the detector, a very thin lightly doped n-layer is introduced between absorber region I and II. The energy band diagram for different applied bias voltages is schematically shown in Fig. 4. The electric field under short circuit conditions is sufficiently large to extract the photo generated charges from region I. With increasing voltage applied to the color sensor charges are extracted from the other regions of the detector. For intermediate reverse voltages, the charge are extracted from region I and II, while the photo generated charges in region III still recombine. For high reverse bias all charges are extracted from the color detector.


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“Figure 4. Schematic band diagram of the color sensor for different applied reverse bias voltages.“

The color sensors are fabricated by plasma enhanced chemical vapor deposition (PECVD) at temperatures below 300 °C on the plasmonic reflectors. The devices are fabricated by the Research Center Jülich. Cross sections of the color detector with and without interlayer are shown in Fig. 5(b) and 5(c). The reflector consists of an electron beam evaporated aluminum layer on a glass substrate. The aluminum layer improved the adhesion of the subsequently prepared silver layer on the glass substrate and determines the roughness of the reflector. The roughness of the substrate can be controlled by the deposition conditions and thickness of the aluminum film. With increasing film 10 ACS Paragon Plus Environment

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thickness the roughness of the film increases. Afterwards, a thin silver film is thermally evaporated on the aluminum coated substrate. The roughness of the silver film originates from the roughness of the underlying aluminum film. A Scanning Electron Microscope image of the textured silver film is shown in Fig. 5(a). The film exhibit a root-means-square roughness of 3-5 nm. The film act as absorber for red and infrared light if the silicon detector is directly prepared on the substrate. Films with similar surface texture can be prepared by sputtering silver films at elevated substrate temperatures.22 By adding a lightly doped ZnO layer the absorption is shifted to shorter wavelengths. The doped ZnO layer is prepared by radio frequency magnetron sputtering. The film has a thickness of 100 nm. In the next step, the silicon detector is prepared. The detector consists of a 20 nm thick amorphous silicon n-layer followed by a 1250 nm thick microcrystalline silicon layer. Afterwards, a silicon layer with a bandgap of 1.8 eV is prepared on the substrate. The film has a thickness of 90 nm. The film is prepared at low deposition temperatures of only 180 °C using a silane to hydrogen dilution of 1/4. The third absorber consists of a 110 nm thick wide bandgap amorphous silicon carbon layer with a bandgap of 2.0 eV. Finally, a 10 nm thick wide bandgap player is prepared. The top contact of the sensor array are realized by radio frequency magnetron sputtered ZnO.


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“Figure 5. (a) Scanning electron microscope image of rough silver film used as back reflector of the optical detector. Cross section of detector with (c) and without (c) zinc oxide interlayer.“

The measured spectral responsivity of the sensors without a ZnO interlayer (Fig. 5(c)) is shown in Fig. 6(a). The spectral responsivity is plotted for applied voltages of 0V, -0.5V und -1.0V. The sensor exhibits peaks of the spectral responsivity at 450 nm, 480 nm and 535 nm, respectively.


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Longer wavelengths light reaching the back reflector is absorbed by the plasmonic reflector. Fig. 6(b) exhibits the spectral responsivity of a detector with a ZnO interlayer (fig. 5b). The reflector reflects most of the red and infrared light, so that the peaks of the spectral responsivity are shifted to

Spectral Responsivity [mA/W]

longer wavelengths. Peaks are measured for 450 nm, 505 nm and 550 nm.


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Wavelength [nm] “Figure 6. Measured spectral responsivity of color sensors prepared on nanotextured silver films with (b) and without (a) zinc oxide interlayer. The spectral responsivity is measured for applied voltages of 0V, -0.5V und -1.0V. “ 13 ACS Paragon Plus Environment

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In order to separate the color channels the following simple linear transformation was applied to the measured spectral responsibilities.

R  1 − 1 0  SR (V = −1.0 V ) G  = 0 1 − 1 × SR (V = −0.5 V )        B  0 0 1   SR (V = 0 V ) 

(1)

The color transformation can be applied under the assumption that the three color channels are linear independent. Previous investigations have shown that linear independent color channels can be extracted if the different regions of the sensor exhibit a different lifetime-mobility product.13 The resulting color channel are shown in Fig. 7 after normalization. Fig. 7(a) exhibits peaks of the spectral responsivity at 450 nm, 555 nm, and 600 nm and Full Width at Half Maxima (FWHM) of 180 nm, 165 nm and 175 nm of the sensor with a lossy back reflector. Most of the light reaching the back reflector is absorbed. The RGB channels exhibits a good agreement with commercial sensors using color filters and the CIE (International Commission on Illumination) color spaces.21,23 The color sensor with the ZnO interlayer (Fig. 7(b)) exhibits peaks of the spectral responsivity at 450 nm, 575 nm and 700 nm and FWHM of 180 nm, 215 nm and 335 nm. The high reflection of the back reflector leads to a shift of the peaks to longer wavelength. The green spectral responsivity moves from 555 nm to 575 nm, while the red spectral responsivity shifts from the red spectral region (600 nm) to the NIR spectral region (700 nm).


Normalized spectral responsivity [a.u.] Normalized spectral responsivity [a.u.]

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Normalized spectral responsivity [a.u.]

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Wavelength [nm] “Figure 7. Relative spectral responsivity of the individual color channels for sensors with (b) and without (a) zinc oxide interlayer. (c) Combination of the color sensors with and without zinc oxide interlayers.“ By inserting a low refractive index interlayer between the silver reflector and the silicon detector, the spectral responsibility can be controlled. The wavelength dependent absorption of the reflector


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is shifted to shorter wavelengths. Consequently, the spectral responsivity shifts to longer wavelengths. By combining sensors with different spectral responsivity, the spectral resolution can be increased and multispectral sensors can be realized. The combination of the color sensors with silicon and ZnO interlayers allows for the realization of a 5 channel color sensor as shown in Fig. 7(c). The sensor exhibits 4 color channels in the visible spectrum and one channel in the infrared spectrum. In particular the infrared channel is of interest to applications in surveillance, security, machine vision and biomedical imaging. Adding an additional visible sensor channel to the commonly used three color channels leads to a distinct improvement of color recognition and a reduction of the color rendering factor, which is determined by a colorimetric characterization of the sensor.21 The colorimetric characterization is based on the human vision system, which is represented by the color matching functions. The human vision system is very sensitive to variations of the green and red color matching functions.15 Hence, an additional sensor channel was added to the sensor structure distinctly improving the spectral characteristic of the sensor. In particular, the combination of the novel sensor with approaches from artificial intelligence and deep learning like deep convolutional neuronal networks should allow for the realization of novel cameras and sensor systems.24

A novel sensor is presented combining spatial and time color multiplexing schemes. The spatial multiplexing is achieved by using plasmonic reflectors, while the time multiplexing is required since a vertical sensor architecture employing a stack of absorbers with different bandgaps is used. The optical properties of the back reflector are controlled by using a lossy reflector in combination with different interlayers inserted between the reflector and the detector. The detection principle is demonstrated for a silicon thin film detector prepared on a textured silver back reflector using a zinc oxide interlayer. The spectral responsivities are comparable to conventional color sensors using


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optical filters. Combining detectors with differently tuned back reflector allows for the realization of multispectral sensor systems covering the visible and near infrared optical part of the spectrum.

AUTHOR INFORMATION Corresponding Author *

Email: [email protected] (D. K.)

Present Addresses §

H.S.: University of Bielefeld, Germany

#

D.K.: Geballe Laboratory for Advanced Materials, Department of Materials Science and

Engineering, Stanford University, Stanford, CA 94305, USA Author Contributions The experiment was designed by D.K & H.S. The devices were prepared and characterized by D.K. Optical simulations were carried out by V.J. All authors discussed the results and contributed to the manuscript preparation. All authors prepared the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors like to thank the Research Center Jülich for its support in preparing and characterizing the color sensors.

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Franken, R.H.; Stolk, R.L.; Li, H.; van der Werf, C.H.M.; Rath, J.K.; Schropp, R.E.I., Understanding light trapping by light scattering textured back electrodes in thin film n-i-ptype silicon solar cells, J. of Appl. Phys. 2007, 102, 014503.

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LeCun, Y.; Bengio, Y.; Hinton, G., Deep Learning, Nature 2015, 541, 436-444.


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Color ACS Paragon Plus Environment sensor

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1 2 + + 3 4 5 6 p-i-i-i-n 7 p-i-i-i-n diode 8 diode 9 10 11 12 13 14 low refractive 15 index interlayer 16 17 lossy reflector lossy reflector 18 19 ACS Paragon Plus Environment 20 21 22

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(a) 1 2 3 4 5 6 7 8 9 10 100 nm 11 12 13 14(b) (c) Incoming Incoming 15 Light 16 Light 17 18 ZnO:Al ZnO:Al 19 20 21 22 Color 23 Color sensor 24 sensor 25 ZnO: Al 26 interlayer 27 28 29 30 silver silver 31 Aluminium 32 Aluminium 33 ACS Paragon Plus Environment 34 Glass Glass 35 36


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Tunable Multispectral Color Sensor with Plasmonic ... - ACS Publications

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