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Color-Selective and CMOS-Compatible Photodetection Based on Aluminum Plasmonics Bob Y. Zheng, Yumin Wang, Peter Nordlander, and Naomi J. Halas* Over one billion silicon-based image sensors were produced worldwide in the last year alone.[1] Typically, imaging sensors use p–n or p–i–n junctions to separate and collect photoexcited electron–hole pairs. Silicon-based image sensors absorb over a broad spectral range, requiring color filters for spectral selectivity. In contemporary image sensors, color selection is performed with dielectric or organic dye filters, which impose practical limitations on scalability or durability.[2] With reduction in pixel size, the integration of dielectric filters with image sensors becomes challenging due to the need for complex and time-consuming alignment procedures. The use of on-chip organic dyes is limited by their durability; currently available dyes degrade over time under exposure to ultraviolet light.[3] For low-light illumination conditions and high-density pixel geometries, a single photodiode produces a very small amount of photocurrent, which is limited by the quantum efficiency of light absorption per unit volume and requires extremely lownoise electronics to amplify the signal. A photodetector that could achieve color sensitivity without incorporating multiple materials and, in addition, which could enhance detection sensitivity, would be a substantial improvement over current technology. Plasmonic gratings offer an attractive route towards fully integrated, spectrally sensitive detectors and have been used extensively as tunable optical band-pass filters.[4–7] Plasmonic gratings utilize the interference effects of surface plasmons, the coherent oscillations of conduction-band electrons, in a periodic structure to tune the center wavelength and line shape of the transmission bandwidth. Recent work on plasmonic imaging devices has focused on silver filters,[8–12] due to their low loss in the visible region of the spectrum. However, Ag is not compatible with complementary metal oxide semiconductor (CMOS) fabrication processes, which severely limits its ultimate manufacturability. Aluminum is a promising substitute for Ag, possessing outstanding optical properties in the
B. Y. Zheng, Prof. P. Nordlander, Prof. N. J. Halas Department of Electrical and Computer Engineering Rice University Houston, Texas 77005, USA E-mail: [email protected] Prof. P. Nordlander, Prof. N. J. Halas, Dr. Y. Wang Laboratory for Nanophotonics Rice University Houston, Texas 77005, USA Dr. Y. Wang, Prof. P. Nordlander Department of Physics and Astronomy Rice University Houston, Texas 77005, USA
DOI: 10.1002/adma.201401168
6318
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visible and near-UV regions of the spectrum and compatibility with CMOS processes.[13–16] Al color filters show full color tunability, but thus far their use has focused exclusively on their farfield properties, where the filter is placed hundreds of nanometers or microns, above the photosensitive element.[17–22] Limited knowledge of the influence of oxygen as a bulk impurity on the plasmonic properties of Al has led to a large variability in the color selectivity of Al filters.[14] To further improve upon a color-selective photosensitive element, it would also be desirable to incorporate an intrinsic gain mechanism. The most common methods for producing photocurrent gain involve charge trapping or impact ionization.[23] Impact ionization, or avalanching, requires large DC power consumption to generate large electric fields, which limits the usefulness of avalanche photodetection as a strategy in imaging applications. Charge trapping involves trapping either photogenerated holes or electrons in a semiconductor. By trapping carriers, it is possible to decrease the transit time so that the overall generation rate is greater than the transit rate. Consequently, charges are generated at a faster rate than they are collected, which results in photocurrent gain.[24,25] Here we report a new strategy that takes advantage of recent advances in Al plasmonics to achieve a fully integrated, colorselective Al–Si plasmonic photodetector with the additional advantage of intrinsic gain for improved photodetection sensitivity. The design is based on a band-gap engineering approach to charge trapping that utilizes two back-to-back metal-semiconductor heterojunctions to form a potential energy well (Figure 1a). This energy well forces charges to accumulate, resulting in a build-up of charges at the Schottky junction (inset). This build-up of carriers generates image charges, which results in a high electric field at the metal–semiconductor junction and a reduced Schottky barrier height.[26–31] This modification to the Schottky barrier height exponentially increases the current through the photodetector, giving the photodetector an intrinsic mechanism to amplify its photocurrent signal. This basic metal–semiconductor–metal (MSM) geometry has been utilized extensively for ultrafast photodetectors (with speeds > 300 GHz),[32–35] implemented in GaAs or InGaAs at telecommunications wavelengths near 1550 nm for industry applications.[25] At high speeds, MSM photodetectors typically exhibit low responsivities and no photocurrent gain. However, at lower speeds (
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Color-Selective and CMOS-Compatible Photodetection Based on Aluminum Plasmonics Bob Y. Zheng, Yumin Wang, Peter Nordlander, and Naomi J. Halas* Over one billion silicon-based image sensors were produced worldwide in the last year alone.[1] Typically, imaging sensors use p–n or p–i–n junctions to separate and collect photoexcited electron–hole pairs. Silicon-based image sensors absorb over a broad spectral range, requiring color filters for spectral selectivity. In contemporary image sensors, color selection is performed with dielectric or organic dye filters, which impose practical limitations on scalability or durability.[2] With reduction in pixel size, the integration of dielectric filters with image sensors becomes challenging due to the need for complex and time-consuming alignment procedures. The use of on-chip organic dyes is limited by their durability; currently available dyes degrade over time under exposure to ultraviolet light.[3] For low-light illumination conditions and high-density pixel geometries, a single photodiode produces a very small amount of photocurrent, which is limited by the quantum efficiency of light absorption per unit volume and requires extremely lownoise electronics to amplify the signal. A photodetector that could achieve color sensitivity without incorporating multiple materials and, in addition, which could enhance detection sensitivity, would be a substantial improvement over current technology. Plasmonic gratings offer an attractive route towards fully integrated, spectrally sensitive detectors and have been used extensively as tunable optical band-pass filters.[4–7] Plasmonic gratings utilize the interference effects of surface plasmons, the coherent oscillations of conduction-band electrons, in a periodic structure to tune the center wavelength and line shape of the transmission bandwidth. Recent work on plasmonic imaging devices has focused on silver filters,[8–12] due to their low loss in the visible region of the spectrum. However, Ag is not compatible with complementary metal oxide semiconductor (CMOS) fabrication processes, which severely limits its ultimate manufacturability. Aluminum is a promising substitute for Ag, possessing outstanding optical properties in the
B. Y. Zheng, Prof. P. Nordlander, Prof. N. J. Halas Department of Electrical and Computer Engineering Rice University Houston, Texas 77005, USA E-mail: [email protected] Prof. P. Nordlander, Prof. N. J. Halas, Dr. Y. Wang Laboratory for Nanophotonics Rice University Houston, Texas 77005, USA Dr. Y. Wang, Prof. P. Nordlander Department of Physics and Astronomy Rice University Houston, Texas 77005, USA
DOI: 10.1002/adma.201401168
6318
wileyonlinelibrary.com
visible and near-UV regions of the spectrum and compatibility with CMOS processes.[13–16] Al color filters show full color tunability, but thus far their use has focused exclusively on their farfield properties, where the filter is placed hundreds of nanometers or microns, above the photosensitive element.[17–22] Limited knowledge of the influence of oxygen as a bulk impurity on the plasmonic properties of Al has led to a large variability in the color selectivity of Al filters.[14] To further improve upon a color-selective photosensitive element, it would also be desirable to incorporate an intrinsic gain mechanism. The most common methods for producing photocurrent gain involve charge trapping or impact ionization.[23] Impact ionization, or avalanching, requires large DC power consumption to generate large electric fields, which limits the usefulness of avalanche photodetection as a strategy in imaging applications. Charge trapping involves trapping either photogenerated holes or electrons in a semiconductor. By trapping carriers, it is possible to decrease the transit time so that the overall generation rate is greater than the transit rate. Consequently, charges are generated at a faster rate than they are collected, which results in photocurrent gain.[24,25] Here we report a new strategy that takes advantage of recent advances in Al plasmonics to achieve a fully integrated, colorselective Al–Si plasmonic photodetector with the additional advantage of intrinsic gain for improved photodetection sensitivity. The design is based on a band-gap engineering approach to charge trapping that utilizes two back-to-back metal-semiconductor heterojunctions to form a potential energy well (Figure 1a). This energy well forces charges to accumulate, resulting in a build-up of charges at the Schottky junction (inset). This build-up of carriers generates image charges, which results in a high electric field at the metal–semiconductor junction and a reduced Schottky barrier height.[26–31] This modification to the Schottky barrier height exponentially increases the current through the photodetector, giving the photodetector an intrinsic mechanism to amplify its photocurrent signal. This basic metal–semiconductor–metal (MSM) geometry has been utilized extensively for ultrafast photodetectors (with speeds > 300 GHz),[32–35] implemented in GaAs or InGaAs at telecommunications wavelengths near 1550 nm for industry applications.[25] At high speeds, MSM photodetectors typically exhibit low responsivities and no photocurrent gain. However, at lower speeds (
