Article Cite This: ACS Photonics 2018, 5, 2617−2623
Plasmonic Hot-Carriers in Channel-Coupled Nanogap Structure for Metal−Semiconductor Barrier Modulation and Spectral-Selective Plasmonic Monitoring Ya-Lun Ho,†,⊥ Yi-Hsin Tai,‡,⊥ J. Kenji Clark,† Zhiyu Wang,† Pei-Kuen Wei,‡,§,∥ and Jean-Jacques Delaunay*,† †
Department of Mechanical Engineering, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan Research Center for Applied Sciences, Academia Sinica, Taipei 115-29, Taiwan § Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan ∥ Institute of Biophotonics, National Yang-Ming University, Taipei 11221, Taiwan
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‡
S Supporting Information *
ABSTRACT: Plasmonic hot-carriers, which are induced by plasmons at metal surfaces, can be used to convert photon energy into excited carriers over a subwavelength region and provide a new means to realize photodetection within the subband-gap region of semiconductor materials. However, the barrier height of the metal−semiconductor junction affects the behavior of the plasmon-induced hot-carriers and limits the electrical response of photodetection. High electrical responsivity, achieved by manipulating the barrier height using plasmon-induced hot electrons, is desired to broaden the possible applications. Here we report a plasmonic channelcoupled nanogap structure, where the barrier height of the metal−semiconductor junction is altered upon the excitation of plasmon-induced hot-carriers. The structure consists of semiconductor channels and metal slabs forming nanogaps, which sustain coupled plasmons and confine light to the semiconductor−metal interfaces. In contrast to conventional Schottky barriers and ohmic contacts, in which plasmon-induced hot-carriers and the generation of electron−hole pairs by photoabsorption cause an increase in the photocurrent, the generation of plasmon-induced hot-carriers at the resonant wavelength results in an increase in the junction barrier height and a decrease in the photocurrent induced by photoabsorption. By modifying the barrier height, the plasmon resonance can be monitored from the electrical response with a high spectral resolution and a large modulation. KEYWORDS: hot-carriers, plasmonics, spectral selectivity, nanogaps, plasmonic coupling
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In nanostructures sustaining plasmon-induced hot-carriers, the hot-carriers either transfer over a Schottky barrier,12,28−31 formed by the bending of the semiconductor energy band at a metal−semiconductor junction, or in the case of an ohmic contact,32,33 where there is no energy band bending, are injected directly into the semiconductor and generate a photocurrent. However, the generation of plasmonic carriers is still relatively inefficient in comparison to the generation of electron−hole pairs by photoabsorption (quantum efficiency is about 1 when light energies are larger than the band gap34), and the photocurrent variation during photodetection is limited.34−36 A large contrast in the electrical response between TM- and TE-polarized light induced by the generation of plasmon-induced hot electrons is desired in order to broaden the potential applications of hot-carrier-based devices.
lasmonic nanostructures, with the ability to locally confine light at the interfaces of metal and semiconductor/ dielectric by surface plasmon resonances (SPRs), have been used to assist in the conversion of photon energy into usable forms in nanostructures with subwavelength dimensions, showing their potential for miniaturizing and improving efficiency in solar energy harvesting,1−9 photocatalysis,1,4,10,11 photodetection,12−21 and lasing22−25 devices. The mechanism by which surface plasmons enhance the photon energy conversion has been widely investigated. Due to the strong focusing at the plasmonic hot spots, the generation of electron−hole pairs results from photoabsorption in a small region in the semiconductor. Surface plasmons can also directly induce hot-carriers1−3,26,27 in the metal, which are then injected into the semiconductor. Unlike the generation of electron−hole pairs by photoabsorption, where the efficiency depends on the band gap of the semiconductor and varies based on the extinction coefficient, the generation of plasmon-induced hotcarriers can occur at energies below the band-gap energy. © 2018 American Chemical Society
Special Issue: Recent Developments and Applications of Plasmonics Received: November 1, 2017 Published: February 27, 2018 2617
DOI: 10.1021/acsphotonics.7b01307 ACS Photonics 2018, 5, 2617−2623
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Figure 1. Light confinement in the plasmonic channel-coupled nanogap structure. (a) Illustration of the channel-coupled nanogap structure. The semiconducting channels with adjacent metallic slabs form nanogaps. Inset: Schematic cross-section of the nanochannel structure with light behavior. (b) Simulated absorptance spectra of the structures on the basis of Si nanochannels for TM-polarized normal incident light. (c) Simulated timeaveraged Poynting vector fields at point A defined in (b). Electric field density distribution at (d) points A and (e) B defined in (b).
In this work, we present a plasmonic channel-coupled nanogap structure in which excited plasmons modify the barrier height of the metal−semiconductor junction and effect a change in the electrical properties of the structure that opposes the change caused by electron−hole pairs generated by photoabsorption. The plasmon resonance thus can be monitored by an electrical response with a large modulation and narrow bandwidth. The structure consists of a semiconducting nanochannel with adjacent metallic slabs forming a nanogap (Figure 1a). The coupling of the plasmons at the nanogap on both sides of the nanochannel focuses and confines light to the corners of the gap edges, which perfectly corresponds to the current density distribution when the structure is connected to an external bias in order to measure the electrical response. By selecting a semiconductor (lightly doped n-Si) and a metal (Ti) with similar work functions, a small barrier height of the metal−semiconductor junction is formed. At the plasmonic resonance wavelength, irradiation by TE-polarized light results in a larger decrease in the barrier height than irradiation by TM-polarized light. This causes a large contrast between the electrical response of the device under TE irradiation and TM irradiation. Figure 1a illustrates the plasmonic channel-coupled nanogap structure consisting of semiconducting (Si) channels with adjacent metallic (Au, with/without a Ti adhesion layer) nanoslabs on a bottom substrate forming nanogaps. The illustration also shows the period of the structure p, the width of the Si channels w, the height of the channels h, and the thickness of the Au nanoslabs t. The structure is surrounded by air (refractive index n = 1), and the light is TM-polarized and
normally incident. The optical properties of the channelcoupled nanogap structure are simulated with a rigorous coupled wave analysis, and the absorptance spectrum (Figure 1b) shows a sharp peak (point A) at a wavelength λ of 1244 nm with a narrow bandwidth (fwhm = 41 nm) and a modulation of 0.82 (relative to the incident wave) in the sub-band-gap region of Si. The origin of the strong and narrow-band absorption at point A can be clarified by examining the Poynting vector field (Figure 1c) and the electric field density distribution (Figure 1d), computed using the finite-difference time-domain technique. The Poynting vector field represents light flow and reveals the ability of the structure to guide and focus light to the corners of the nanogaps. Light is strongly confined to the interfaces of the nanochannel and the gaps. The electric field density distribution at the resonance condition (λ = 1244 nm, point A in Figure 1b) indicates a strong surface plasmon resonance at the semiconductor−metal slab interfaces. By comparison, the distribution at the nonresonance condition (λ = 1140 nm, point B in Figure 1b), as shown in Figure 1e, does not show clear evidence of a resonant electric field enhancement. To clarify the plasmonic behavior, the electric field magnitude and phase distributions in the z- and x-direction are provided in Figure 2a−d. Surface plasmon resonances are observed on the top and bottom corners of nanoslab edges as hot spots with strong electric field enhancement. Between two adjacent slabs, the phase distribution in the z-direction shows an antisymmetric behavior, whereas in the x-direction it is symmetric and has an electric field magnitude distribution that reveals the coupling of the surface plasmons on opposite sides of the nanogaps. 2618
DOI: 10.1021/acsphotonics.7b01307 ACS Photonics 2018, 5, 2617−2623
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Figure 3. Fabricated channel-coupled nanogap structure. (a) Measured reflectance spectrum of the nanochannel structures for ppolarized normal incident light. Inset: Scanning electron microscopy cross-section and top-view images of nanochannel−gap structures. (b) Simulated reflectance, transmittance, and absorptance spectra based on the fabricated structure. The resonance of the plasmonic coupled mode in the channel-coupled nanogap structure has a narrow bandwidth and is shown by a strong absorption and low reflectance.
Figure 2. Electric field magnitude and phase distribution in the channel-coupled nanogap structure. Electric field magnitude distribution in the (a) z- and (b) x-direction and electric field phase distributions in the (c) z- and (d) x-direction at point A defined in Figure 1b.
spectrometer VIR-300, JASCO, Tokyo, Japan) and compared with the simulated reflectance/absorptance spectra based on the parameters of the fabricated structure in Figure 3a (also see Figure S3 in the Supporting Information). The simulated spectra, as shown in Figure 3b, reproduce the measured spectrum of the fabricated structure in terms of position, bandwidth, and modulation of the resonance dips. The difference between the simulated and measured spectra is a result of the refractive index used in simulation varying slightly from the actual material refractive index. The measured spectrum has a sharp reflectance dip at a wavelength λ of 1286 nm with a narrow bandwidth (fwhm = 54 nm) and low reflectance of 0.025 (relative to the incident wave). At the resonance with the narrow bandwidth and low reflectance in the sub-band-gap region of the Si, light is strongly focused and confined at the edges of the nanoslabs through the coupling of the nanochannels and gaps as discussed above. A broad dip appears at λ = 1143 nm with a relatively small reflectance modulation as a result of the transmission of light through the nanochannel. Due to the strong coupled plasmons with narrow resonance bandwidths, the channel-coupled nanogap structure, which exhibits strong light confinement and electric field enhancement especially at the interface between the semiconducting nanochannel and the metal gaps, offers great potential for spectrally selective and light-sensitive optoelectronic devices. First, a discussion of the electrical behavior of the channelcoupled nanogap structure under dark conditions with different metal−semiconductor junctions is provided below. Figure 4a−c show the energy band profile of the nanochannel with the n-Si/ Au metal−semiconductor−metal junction, the nanochannel with the highly doped n-Si/Ti metal−semiconductor−metal junction, and the nanochannel with the lightly doped n-Si/Ti metal−semiconductor−metal junction, respectively. At equilibrium, the Fermi levels in the n-Si and Au should coincide. Due to the difference between the work function of Au and n-Si, upward band bending is generated and a Schottky barrier forms on both sides of the channel, as shown in Figure 4a. The barrier height is calculated and shown in Figure S5b. Upon connecting the metal−semiconductor−metal junction to an external bias, the Au slab on the reverse bias side of the structure becomes more negatively charged and repels electrons in the semi-
The simulated absorptance spectra of different structures with the same Si nanochannels but different Au layer shapes are shown in Figure 1b. The incident light is TM-polarized and normally incident. The structure fully covered by an Au layer has a low absorptance peak, indicating weak light absorption by the structure (spectrum in purple). The structure with Au in the space between nanochannels has a strong and broad absorptance peak corresponding to the cavity modes of the nanochannel (spectrum in green). Another strong peak with a narrower bandwidth is shown in the absorptance spectrum of the structure with the side and bottom covered by the Au layer (spectrum in blue). This peak corresponds to the coupling of the cavity mode and the horizontal SPRs on the bottom layer. However, the sharp absorptance peak obtained in this structure has a strong dependence of the wavelength on the angle of the incident light as a result of the coupling to the horizontal SPRs,37 hampering its practical use. The absorptance spectrum of the nanochannel with only the bottom surface covered by Au, the investigated channel-coupled nanogap structure, has a strong and narrowband peak corresponding to the nanochannel−-gap coupled mode, which has more flexibility in light incident angle compared to the coupled mode of horizontal SPR and cavity mode. The variation of the light incident angle and wavelength in the nanochannel-coupled slab structure is shown in Figure S1 (see the Supporting Information). The measured resonance wavelength of the coupled mode shifts by 3 nm as the incident angle is varied from 0 to 3 degrees (see Figure S2 in the Supporting Information), whereas the SPR mode has a 35 nm shift of the resonance wavelength over the same angle range, corresponding to a 6.0 times larger shift than the nanochannel−gap coupled mode when normalized by the resonance wavelength. Insets of Figure 3a show electron microscopy images of the fabricated channel-coupled nanogap structures in a top view and cross-section view. A detailed description of the fabrication process is reported in the Methods section. The reflectance spectrum of the fabricated structure is characterized (FT-IR 2619
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Figure 4. Metal−semiconductor−metal junctions of channel-coupled nanogap structures. Equilibrium energy band diagrams, with and without an applied external bias, of three different metal−semiconductor−metal contacts: (a) Au and n-Si, (b) Ti and highly doped n-Si, and (c) Ti and lightly doped n-Si. (d) Current−voltage characteristics of the three types of metal−semiconductor−metal contact with a forward bias. The current is shown on a linear scale. Inset: The characteristics of the lightly doped n-Si/Ti junction structure with both a forward and reverse bias shown with a logarithmic y-scale.
conductor, resulting in an increase in the built-in-voltage barrier. On the forward bias side of the structure, electrons flow toward the Au junction, decreasing the built-in-voltage barrier. When the Ti is added between Au and n-Si as an interlayer, as shown in Figure 4b, the Schottky barrier is lower than that of the n-Si/Au contact. This is because there is a smaller difference between the work functions of n-Si and Ti than n-Si and Au. This is illustrated in Figure 4c, where the Schottky barrier height of the n-Si/Ti junction is shown to be smaller than the Schottky barrier height of the junctions in Figure 4a and b. The current−voltage characteristics under dark conditions for the channel-coupled nanogap structure with the three different types of metal−semiconductor−metal junctions in Figure 4a−c are shown in Figure 4d. The current−voltage curve (I−V curve) was obtained with an ammeter operating with a dc source swept from −2 to 2 V in amplitude. A logarithmic-scale I−V curve of the structure with a lightly doped n-Si/Ti junction is shown as the inset of Figure 4d and shows a symmetric behavior, which can be expected based on the symmetric structure. Based on the symmetric behavior of the curve, only the forward biases of the other curves are shown for discussion. In Figure 4d, the I−V curve of the structure with a lightly doped n-Si/Ti junction shows a linear relation in the region of interest, which is an ohmic contact-like behavior as shown in Figure 4c. In the cases of the n-Si/Au and highly doped n-Si/Ti junctions, the I−V curves show several regions with different slopes. For a low external bias, the small number of carriers in the metal with an energy larger than the Schottky barrier height (EC − EF) are injected into the semiconductor and sustain a current. This current is limited by the thermal equilibrium distribution of carriers in the metal, and hence saturation with only a small current increase is seen for larger biases. The current−voltage characteristics of the structure with the lightly doped n-Si/Ti junctions are shown in Figure 5a for the cases where the structure is illuminated by TE-polarized laser light, illuminated by TM-polarized laser light, and not illuminated at all (dark condition). Illumination was done at the resonance wavelength of the structure (1280 nm) and a laser intensity of 0.5 W/cm2, using a femtosecond laser equipped with a high-power collinear optical parametric
Figure 5. Variation in the junction behavior of the channel-coupled nanogap structure induced by plasmons. (a) Current−voltage characteristics of the structure with lightly doped n-Si/Ti junction under TE- and TM-polarized light irradiation and under dark conditions. Inset: Current density distribution of the structure induced by the external bias. (b) Function H(I) variation versus the current under TE and TM polarized light irradiation, and under dark conditions. The lines are the linear fits to the experimental data, from which the corresponding barrier heights are calculated. (c) Energy band diagrams under TE- and TM-polarized light irradiation and under dark conditions. (d) Current variations versus incident light wavelength for TM-polarized light with different bias voltages. Inset: Conductance variations versus incident light wavelength.
amplifier (ORPHEUS-HP, Light Conversion, Vilnius, Lithuania). When the structure is illuminated with TE-polarized light, the slope of the current−voltage curve increases. When the structure is illuminated with TM-polarized light at the resonance wavelength, however, excited surface plasmons result in strong light confinement and electric field enhancement at the interface of the Si and the Ti, and the slope of the curve decreases to a value between that of the TE and that under dark conditions. The current−voltage characteristics are further analyzed using the thermionic emission model, which is the dominant mechanism for carrier transport across Schottky junctions.38 The thermionic emission current is given by ⎤ ⎛ qφ ⎞⎡ ⎛ qV ⎞ I = AA*T 2 exp⎜ − B ⎟⎢exp⎜ ⎟ − 1⎥ ⎥⎦ ⎝ kBT ⎠⎢⎣ ⎝ kBT ⎠
(1)
where A is the area of the contact junction, A* is the Richardson constant, T is the temperature, φB is the barrier height, kB is the Boltzmann constant, q is the electronic charge, and V is the voltage drop across a junction. Since the nanogap structure is a combination of junctions and resistors, the structure is treated as an equivalent junction with a barrier height φB* in series with a resistance R, through which a current I flows.39 The voltage V can be expressed in terms of the bias 2620
DOI: 10.1021/acsphotonics.7b01307 ACS Photonics 2018, 5, 2617−2623
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ACS Photonics voltage V′ across the structure, as V = V′ − IR. For V > 3kBT/q, the bias voltage can be expressed by V ′ = IR + φB* +
kBT ⎛ I ⎞ ln ⎜ ⎟ q ⎝ AA*T 2 ⎠
the modulation at the desired operating frequency. The impedance variation of the structure with a lightly doped nSi/Ti junction versus the wavelength of incident light is shown in Figure 6a,b, together with the reflectance spectrum of the
(2)
To calculate φ*B , H(I) is introduced and defined as H (I ) ≡ V ′ −
kBT ⎛ I ⎞ ln ⎜ ⎟ q ⎝ AA*T 2 ⎠
(3)
φ*B can be obtained by combining eqs 2 and 3 to obtain H(I) = IR + φ*B . Figure 5b shows the variation of H(I) values with the current for the lightly doped n-Si/Ti structure under dark conditions, under TE illumination, and under TM illumination (IR light source with 1200 nm long-pass filter). The junction barrier heights of the structure under dark conditions, TE illumination, and TM illumination were 0.173, 0.127, and 0.143 eV, respectively. The decrease in the junction barrier height of the structure under illumination by TE-polarized light is a result of the generation of electron−hole pairs by photoabsorption in the semiconducting channel. The holes, which are separated from electron−hole pairs by the band bending, move toward the metal under an external bias. They are then trapped at the interface between the semiconductor and the metal and lower the barrier height, as shown in Figure 5c. When the illuminating light changes from a TE to a TM polarization, the generation of plasmon-induced hot-carriers increases, and the generation of electron−hole pairs by photoabsorption decreases. With fewer electron−hole pairs generated, the barrier-lowering effect is not as pronounced, and relative to the barrier height under TE illumination, the barrier height increases. To determine the overlap between the current density distribution induced by the external bias and the electric field distribution at the resonance condition of the nanochannel− gap coupled mode, a simulated current density distribution of the structure with an external dc bias is shown as the inset of Figure 5a (also see Figure S4 in the Supporting Information). The large current density at the corners of the nanoslab edge matches closely the enhancement of the electric field at the resonance wavelength in Figure 2a,b. As a result of the good overlap between the current density distribution and electric field enhancement of the structure upon illumination, the effect of the plasmonic resonance, which appears as a narrowbandwidth dip in the reflectance spectrum, can be monitored as an electrical signal variation. The variation of the current versus TM-polarized incident light wavelength for various bias voltages is shown in Figure 5d. The overall decrease in the current as the wavelength of the incident light increases is a result of the decrease in the extinction coefficient of Si and the corresponding decrease in the generation of the electron−hole pairs by photoabsorption. However, a minimum in the current is observed at the resonance, showing the spectral selectivity of the current variation, and evidence of the plasmonic effect. The inset of Figure 5d shows the conductance variations versus incident light wavelength. When the bias voltage increases, the spectrally selective behavior becomes less pronounced as compared to smaller bias voltages. Since each nanogap in the channel-coupled nanogap structure is symmetric (as shown in Figure 1a), each nanogap can be modeled as an equivalent RC nanocircuit, and it is possible to examine the spectral selectivity of the device with an ac bias by either resistance or reactance variation and maximize
Figure 6. Plasmon-induced impedance modulation of the channelcoupled nanogap structure with a lightly doped n-Si/Ti junction. (a) Resistance and (b) reactance variations versus TM-polarized incident light wavelength for different operating frequencies. (c) Resistance and (d) reactance ratio between the light irradiation with TM and TE polarization versus incident light wavelength with different operating frequencies. The reflectance spectrum is given for comparison.
structure for comparison. The impedance measurement was obtained with an LCR meter (E4980A, Agilent Technologies, Santa Clara, CA, USA) operating with an ac source of 1 V in amplitude and a frequency swept from 20 Hz to 2 MHz. The measured resistance and reactance variations show a local maximum and local minimum around 1285 nm (sweep increment Δλ = 10 and 5 nm), respectively, which is close to the resonance wavelength of the measured narrow-bandwidth reflectance dip, indicating the light confinement and the surface plasmons at the edge of the metal slab are responsible for this phenomenon. The overall variation peak/dip in the impedance measurement as wavelength increases agrees with the reflectance spectrum with regard to originating from the changing extinction coefficient of Si.40 The narrowband and spectrally selective electrical variation12,19,28,32 can be obtained for a wide range of operating frequencies through the exact resistance and reactance change. In the resistance measurement, the largest local variation, 4.9 kΩ between 1285 and 1320 nm, occurs for the lowest two operating frequencies of 20 and 200 Hz, whereas the smallest variation, 2.6 kΩ, occurs at the high operating frequency of 2 MHz. In the reactance measurement, the local variation, 3.2 kΩ, at the highest operating frequency of 2 MHz is much larger than the variation seen at lower operating frequencies. To further isolate the effect of the plasmonic resonance on the impedance variation of the structure, the ratio of the resistance under TM and TE light illumination at the resonance wavelength and a nonresonance wavelength is provided in Figure 6c. At the nonresonance wavelength, the resistance ratio is about 1 in both the short- and longwavelength region, demonstrating the independence of the measurement on the light polarization and the fact that surface plasmons do not contribute to the behavior in these regions. At 2621
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layer boundary conditions are applied in the z-direction at the top and bottom of the simulated domain, and periodic boundary conditions are applied in the x-direction. All intensities in figures are given relative to that of the incident light. The current density distribution was computed using the finite-difference time-domain technique (COMSOL Multiphysics, COMSOL, Inc., Burlington, VT, USA).
the resonance wavelength, the ratio increases to 147%, 147%, 144%, and 136% at 20 Hz, 200 Hz, 2 kHz, and 20 kHz, respectively. It was noted that the resistance ratio between the measurements with TM and TE incident light is more symmetric than the absolute resistance measurement and better matches the reflectance spectrum. This is because the general resistance increase caused by the decrease in the Si extinction coefficient for longer wavelengths is present for both the TM and TE polarizations, and normalization by the TE curve removes this trend from the TM curve. For high operating frequencies as shown in Figure 6d, the reactance ratios have the same behavior as the resistance ratio, increasing to 160% at 20 kHz and 133% at 2 MHz for the resonance wavelength. The channel-coupled nanogap structure demonstrated in this work focuses and confines TM light at the structure’s resonant wavelength to a region at the metal and semiconductor interfaces, which matches the region with a high current density in the structure. This confinement leads to the generation of surface plasmons and plasmon-induced hot-carriers and less photoabsorption than is observed for off-resonance wavelengths or TE light. As photoabsorption results in a decrease in the Schottky barrier height and an increase in the photocurrent, the reduced photoabsorption at the resonant wavelength results in a reduced photoconductivity and a strong impedance change. The effect of the plasmonic resonance can be visualized by a spectrally selective electrical response variation without requiring far-field optical detection, and the channel-coupled nanogap structure can be used as a plasmonic monitor for nano/microdevices. With the ability to control the barrier height of the metal−semiconductor junction by selective excitation of plasmon-induced hot-carriers, the channel-coupled nanogap structure is promising for use in switching, filtering, and other optoelectrical devices at the nanoscale and in the subband-gap regime of materials.
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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/acsphotonics.7b01307. Simulation of absorptance variation with the light wavelength and the incident angle in the nanogap structure, measurement of the reflectance spectra of the nanogap structure at different incident angles, and simulated current density distribution of the nanogap structure connected to the external bias (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ya-Lun Ho: 0000-0001-8274-5978 Pei-Kuen Wei: 0000-0002-3002-0526 Jean-Jacques Delaunay: 0000-0003-2175-0620 Author Contributions ⊥
Y.-L. Ho and Y.-H. Tai contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported through Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Numbers (17K18867, 17H03229) and JSPS Core-to-Core Program (Advanced Research Networks type A), Japan. A part of this work was conducted in Center for Nano Lithography & Analysis, The University of Tokyo, supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We would like to extend our grateful appreciation to Professor Makoto Kuwata-Gonokami, Professor Kuniaki Konishi, and Dr. Eric Lebrasseur from The University of Tokyo for important technical support.
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METHODS Fabrication Process. The channel-coupled nanogap structure was fabricated as follows. Si substrates, 525 μm thick, with different resistivity (1−20, 1−100, 100, 1−1000, and >1000 Ω·cm) were cleaned, and an electron beam resist (ZEP520A, Zeon Corporation, Tokyo, Japan) was spin coated on the substrates. A lithography process was performed with an electron beam lithography system (F7000S-VD02, Advantest, Tokyo, Japan) to form a line-and-space resist pattern. Then, the Si was etched by a reactive ion etching system (Plasmalab 80 Plus, Oxford Instruments, Abingdon-on-Thames, UK). After the etching process, Au layers with and without a Ti adhesion layer were sputtered on the resist pattern by electron-beam deposition (Peva-400E, Advanced System Technology Co., Tokyo, Japan). Finally, the resist together with its Au top layer is removed by lift-off in an ultrasonic bath. Simulation Details. The simulated spectra of reflectance, transmittance, and absorptance and the dispersion diagrams were computed using the rigorous coupled-wave analysis (DiffractMOD, Rsoft Design Group, Ossining, NY, USA). The time-averaged Poynting vector fields, electric energy density distributions, and electric field magnitude and phase distributions were computed using the finite-difference timedomain technique (FullWAVE, Rsoft Design Group, Ossining, NY, USA). The refractive index of Si is modeled according to literature values.40 The complex permittivity of Au is described by the Lorentz−Drude dispersion model.41 Perfectly matched
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REFERENCES
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DOI: 10.1021/acsphotonics.7b01307 ACS Photonics 2018, 5, 2617−2623
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DOI: 10.1021/acsphotonics.7b01307 ACS Photonics 2018, 5, 2617−2623
