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Abstract
Due to the strongly concentrated electromagnetic field and the ability to detect the below-bandgap photon energies, surface-plasmon-based photodetections have attracted considerable attention. However, the manipulation of plasmonic resonance is complicated with a high cost in fabrication; moreover, the performance of hot-electron photodetectors is generally unsatisfactorily low. Here, we demonstrated that a tunable absorption can be realized by using the nanohole patterned metal-spacer-metal (MSM) structure, which can be wafer-scale fabricated by the nanosphere lithography technology. The angle- and polarization-insensitive absorption is realized under the excitation of the gap-mode plasmons, which can be facilely manipulated in the near-infrared band by varying the thicknesses and material of the spacer as well as the diameter and period of the nanohole arrays. An asymmetrically bended electrical system is proposed to efficiently convert the highly absorbed photon energies into the photocurrent. Results show that the responsivity of the prepared MSM structure can be up to ∼2.82 mA/W at the wavelength of 1150 nm.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metallic nanostructures have received extensive attention due to the strong electric field localization in nanoscale volumes by exciting surface plasmons (SPs) [1,2], which find a broad range of applications to enhance the optical and optoelectronic performance, e.g., perfect absorbers, biosensing, nonlinear optics, photovoltaics and photodetectors [3–5]. Recently, interests are also paid on the hot electrons generated by the nonradiative decay of SPs in various metallic nanostructures including nanowires, nanoparticles, gratings, and waveguides [6–8]. Especially, the photodetection based on plasmonic hot electrons in a metal-semiconductor configuration has been proposed due to the superiority in detecting photons with energy well below the semiconductor bandgap. Besides, the hot-electron photodetectors (HE PDs) have advantages in room-temperature & self-driven operation, ultrafast response, and the high tunabilities on the working wavelength and polarization dependence. However, on one hand, the related fabrication (especially for the large-area device) is relatively difficult with a high cost; on the other hand, the quantum yield is still too low to promote the practical applications due to a number of optical and electrical losses (e.g., imperfect optical absorption, severe hot-electron thermalization loss and low emission efficiency) [9–11].
To improve the device performance, SPs from various well-designed structures have been widely employed to improve the hot-electron generation and transport efficiencies. For example, by incorporating Au or Ag nanoparticles into nanoporous TiO2 [12,13], incident photon-to-electron conversion efficiency (IPCE) can be boosted at the resonant wavelength of SPs. An Au-nanodisk-antenna system leads to the hybrid mode of localized SPs and Fabry-Pérot (FP) cavity modes and contributes to 2−10 fold enhancement of the photoresponsivity [14]. In addition, a broadband super absorber for the efficient hot-electron photoconversion over the entire visible band is realized in the conformal metallic nanorod array system. The average absorption is 16 times compared to the planar reference and the electric field is strongly bounded to the Ag/TiO2 interface, which contributes to a significantly enhanced hot-electron generation and transport efficiency [15]. Currently, metallic film coated on the nanocone or nanopillar structured substrates is demonstrated to be a satisfactory optical absorber, and several types of broadband or multiband HE PDs are proposed [16–18]. However, most of these proposed devices are fabricated by the high-cost and time-consuming nanotechnology and equipment. For practical photodetection applications, the large-area and low-cost fabrication with tunable optical absorption is necessary.
In this study, we experimentally demonstrate a high optical absorption in the near infrared band by exciting gap-mode plasmons from metal-spacer-metal (MSM) configuration with ordered nanohole arrays formed in the upper metal layer. By manipulating the nanohole diameter & period or the spacer thickness & material, the excited gap-mode plasmons are verified to be tunable and insensitive to the polarization and incident angle of the light. Furthermore, we theoretically prove that a high-performance HE PD can be constructed by using the Au-TiO2-Al configuration, where the upper Au nanohole arrays are used as the light absorber and electric anode, the middle TiO2 is used as the spacer to form the FP cavity and the hot-electron transport layer, and the lower Al is used as the optical reflector and electric cathode. Being different from the conventional metal-insulator-metal (MIM) HE PDs [19–21], the absorption of the lower metal in this proposed device is very low (optical benefit) and we electrically design the device junctions by asymmetrically bending the energy bands to minimize the reverse photocurrent (electrical benefit); therefore, the resultant photocurrent is unidirectionally high, leading to a very high responsivity.
2. Device and method
Optical simulation is carried out by using finite difference time domain (FDTD) method with a commercial software (Lumerical, FDTD solutions). The refractive indices of Al2O3, ZnO, TiO2 and Al are from Palik [22], which are further fitted in the optical simulation. The refractive indices of Au are from Lorentz-Drude model [23]. We use finite-element method (COMSOL Multiphysics software) to calculate the electrodynamic of the hot electrons. The detailed procedure is given in our previous reports [24,25].
The MSM structure is prepared with the help of polystyrene (PS) nanosphere lithography, and the main steps are schematically shown in Fig. 1. First, Au (or Al) film and TiO2 (Al2O3 or ZnO) film are subsequently coated on the SiO2 substrate by electron beam deposition and atomic layer deposition (ALD). Then, closely packed monolayer PS spheres are obtained on water-air interfaces by self-assembly with micro-propulsive injection method, and then transferred to the substrate [26]. The final diameters of these PS spheres are decreased by inductively coupled plasma reactive ion etching (ICP-RIE). The upper Au film is then coated on the PS arrays. After removing these PS spheres, the nanohole-patterned MSM structure is obtained. Practically, the deposition rate of these films in-situ affects the planeness, so we employ the as slow as possible growth rates (i.e., 0.2−0.5 Å/s). Typical digital photographs of the as-prepared wafer-scale MSM structures with nanohole arrays are inserted in Fig. 1.
Fig. 1. Fabrication progress of the large-area MSM structure with nanohole arrays patterned in upper metal. All the scale bars are 2000nm.
3. Results and discussion
Via changing the original and final diameters of the PS nanospheres, the center-to-center distance (i.e., period, P) of the neighbor nanoholes and the nanohole diameter (D) in the as-prepared MSM structure can be tuned. The schematic of the proposed MSM structure and the typical scanning electronic microscope (SEM) images are shown in Fig. 2. The thicknesses of the upper metal, middle spacer and lower metal are d1, d2 and d3, respectively. Cross-section SEM reveals the boundaries of two different materials are distinct and flat. These SEM analyses suggest that the desired MSM structures with homogeneous nanoholes and highly ordered arrangement are achieved.
Fig. 2. (a) Three-dimensional and (b) side view of the proposed MSM structure. D is the nanohole diameter, P is the period, d1, d2, d3 are the thicknesses of the upper metal, middle spacer and lower metal, respectively. (c) Top-view (P = 1000 nm and D = 750 nm) and (d) cross-sectional (P = 1000 nm and D = 650 nm) SEM images of the as-prepared MSM structures. In (c) and (d), d1 = 40 nm, d2 = 70 nm, and d3 = 100 nm. The dotted ellipse in (d) indicates an individual nanohole.
Figure 3(a) shows the relative reflectance spectra of the nanohole patterned MSM structures with varying D and P = 1000 nm. In this figure, to focus on the resonance shifts under various system configurations, the reflectance spectra have been horizontally shifted. Two obvious reflectance dips can be observed for each sample in the wavelength range of interest. While for the unpatterned counterpart (i.e., the upper Au film without nanoholes), around 99% reflectivity is measured in wavelength range of 700–2200 nm, as shown in plane curve [Fig. 3(a)]. The distinct difference implies the existence of a special optical resonance due to the introduction of these ordered nanoholes. Figure 3(b) represents the relative reflectance spectra of the other group of diameter-varying MSM structures, where the period is 600 nm and the middle layer is ALD-grown TiO2. Similar to the first group with P = 1000 nm, there are two characteristic reflectance dips for each sample. As D decreases, the dip on the left is barely shifted, while the one on the right is substantially red-shifted.
Fig. 3. (a) and (b) Measured reflectance spectra of the MSM structures with varying nanohole diameters. The thicknesses of the upper Au, middle spacer, and lower Au are 40 nm, 50 nm, and 100 nm, respectively. The spacer materials in (a) and (b) are ALD-grown Al2O3 and TiO2, respectively. (c) Simulated reflectance of these MSM structures with the same sizes and materials as that of (a). (d) The spatial distributions of normalized magnetic field at λ = 896 nm and λ = 1732nm for the MSM structure with D = 750 nm in (c).
To uncover the physical nature of these optical resonances, the electromagnetic simulation is carried out by FDTD method. The simulated unit is shown in Figs. 2(a) and 2(b) with the geometric dimensions from the experiments. Unless being specifically indicated, the incident angle (θ) is 0° and the electric-field polarization is along the y axis (i.e., φ = 0°). Figure 3(c) shows the simulated spectra of these MSM structures corresponding to Fig. 3(a). One can see that the overall optical characteristics of the simulated reflectance spectra are consistent with the measurements. For the simulation, the reflectance dips are lower, and some subordinate valleys are present, which can be ascribed to that the patterned MSM structure in the simulation is perfect in the size uniformity, nanohole arrangement and interfacial flatness. Figure 3(d) shows the normalized magnetic field distributions in the outermost y-z plane of the simulated unit. For λ = 896 nm, the magnetic field is strongly localized on the top surfaces of the upper Au nanostructure and the lower Au film. These features imply that the hybridization of surface plasmon polariton (SPP) at the Au-air and the upper Au-TiO2 interfaces is excited, and the resonance wavelength is mainly determined by the period. For λ = 1732nm, the magnetic field is intensively concentrated in the gap of the upper Au nanostructure and the lower Au film, which can be attributed to the lower Au-TiO2 interface enhanced by the coupled metal mirror (i.e., the hybridization of SPP and the F-P cavity modes). The substantial optical resonances from metal nanostructures coupled with a metal film are usually named as gap-mode plasmons [27–29]. Therefore, we can conclude that the reflectance dip on the short-wavelength side is from the SPPs at the Au-air and upper Au-TiO2 interfaces, and that on the long-wavelength side is from the gap-mode plasmons. Considering the convenience of optical tunability and the hot-electron collection for the HE PD application, only the gap-mode plasmons are discussed in the following.
Compared to varying the periods and the diameters of the nanoholes, controlling the thickness and material of the spacer is more feasible. So we try to manipulate the gap-mode plasmons by varying the spacer thickness and material [30]. As shown in Fig. 4(a), as the spacer thickness increases from 20 nm to 100 nm, the characteristic reflectance dip becomes more and more remarkable, and the resonant shift is getting slower. When the spacer material is changed from TiO2 to ZnO and then to Al2O3, substantial blue shift of the gap-mode resonance is observed [Fig. 4(b)], which can be explained by the obvious reduction in the refractive index of the spacer. The same tendency in the variation of the characteristic reflectance dips with the varying spacer is also obtained by our calculation [Fig. 4(c) and 4(d)]. Note that the slight difference in the resonant wavelength between the experimental and simulated results is from the distinguished refractive indices of the materials. These results suggest that the strong gap-mode plasmonic resonances require an appropriate thickness of the spacer, e.g., 50−100 nm.
Fig. 4. Measured reflectance spectra for the MSM structures with varying spacer thicknesses (a) and varying spacer materials (b). In (a), P = 1000 nm, D = 750 nm, and spacer materials is Al2O3. In (b), P = 1000 nm and D = 700 nm. (c) and (d) Simulated reflectance spectra corresponding to these MSM structures in (a) and (b).
The effects of the polarization and incident angle on the reflectance spectra are discussed in Fig. 5. Theoretically, the simulated results show that the gap-mode plasmons are independent of the polarization and incident angles, due to the highly ordered and symmetric arrangement of nanoholes. In principle, our measurement agrees with the above expectation; however, there is a slight shift of the resonance under varying incident angle since the prepared nanohole arrays are not as perfect as designed.
Fig. 5. (a) and (b) Measured reflectance under different polarization and incidence angles (φ is the polarization angle, θ is the incidence angle). (c) and (d) Simulated reflectance at different polarization and incident angles. The employed structure is Au-Al2O3-Au configuration with P = 1000 nm, D = 750 nm and d2 is 100 nm.
Finally, we demonstrate that the high-responsivity HE PDs in the near-infrared band can be achieved based on the as-prepared MSM structure with the ordered nanohole arrays. To maximize the photocurrent, we propose the asymmetric energy-band bending for the MSM structure by using two different metals. The spacer material is chosen as n-type TiO2 (working as hot electron collection and transport layer), then the upper (lower) metal is chosen as high work-function Au (low work-function Al). Considering the difference in the work function, the Schottky contact between Au and TiO2 can be formed, and the TiO2/Al contact can be treated as an Ohmic contact. Under the assumption of the desired absorptivity (i.e., the absorptivity of the upper Au nanostructures is as close as possible to the unity, and the absorptivity of the lower Al layer is as small as possible), these photogenerated hot electrons in the upper Au with energy higher than the Schottky barrier (ΦΒ) and within the escaping cone can be injected into the middle TiO2, and then transported and extracted by the lower Al electrode, as shown in Fig. 6(a). While these hot electrons, generated in the lower metal from the small portion of absorption of the incident photons, can hardly be extracted out by the upper electrode due to the upward bending of the energy band. So the proposed MSM structure with asymmetric energy-band bending shows electrical benefit compared to the conventional MIM structure with symmetric energy-band bending.
Fig. 6. (a) Band diagram and hot-electron flow direction for the Au-TiO2-Al structure under the desired optical absorption. (b) Dark I-V behavior of the prepared Au-TiO2 contact. (c) Calculated absorptivity of the upper Au (solid lines) and the lower Al (dashed lines) for the nanohole patterned Au-TiO2-Al structures, where P = 600 nm d1 = 40 nm, d2 = 50 nm and d3 = 100 nm. (d) Calculated responsivity spectra of the Au-TiO2-Al structures with the optical absorption spectra corresponding to (c).
The contact barrier for our prepared Au and TiO2 films is estimated to be 0.89 eV by fitting the dark current-voltage (I-V) behaviors using thermionic emission equation [Fig. 6(b)] [31]. The nanoholed Au-TiO2-Al structures with P = 600 nm, d1 = 40 nm, d2 = 50 nm, d3 = 100 nm and varying nanohole diameters are employed for the investigation of the hot-electron detection. The calculated absorption spectra of the upper Au nanostructures and the lower Al film are shown in Fig. 6(c). One can see that the lower Al film absorbs only 15%–25% of the incident light at the peak, and the upper Au nanostructures absorb 60%–70%. For the case with D = 500, the absorptivity peaks of for the lower and upper metals are separately higher than those for the case with D = 550 nm. It implies that the excitation of the gap-mode plasmons will improve the absorptivities of both top and bottom metals. Fortunately, the photocurrent from lower Al to upper Au can be suppressed by the asymmetric band diagram of the Au-TiO2-Al structure. The photo-responsivity is obtained and shown in Fig. 6(d). One responsivity peak is present (i.e., ∼2.80 mA/W at λ = 1150 nm for the case with D = 550 nm) from the optical absorption of the upper Au. It can be further confirmed that the wavelength of the responsivity peak is consistent with that of the absorptivity spectra. When D is decreased to 500 nm, the responsivity peak is red-shifted, which can be explained by the wavelength shift of gap-mode plasmons. Moreover, the two responsivity peaks are obviously asymmetrical, and the case with D = 500 nm shows an obviously smaller responsivity at the peak [as indicated in Fig. 6(d)]. Nevertheless, the optical absorptivity spectra for the two cases with D = 550 nm and 500 nm are distributed symmetrically on the short- and long-wavelength sides, and the case with D = 500 nm shows a slightly higher absorptivity. The disagreements of the optical and electrical responses are ascribed to the lower energy of these photons at longer wavelengths, which leads to that the flux of the hot electrons injected into the TiO2 from optical absorption at longer wavelength is relative smaller. Note that the optical absorptivity in the upper Au nanostructures in Fig. 6(c) is not optimized, and can be further improved by optimizing the nanohole periods and diameters, as well as the Au thicknesses. Higher optical absorption in the upper Au will guarantee the higher responsivity of the hot-electron detector.
4. Conclusions
In summary, we have demonstrated that a tunable characteristic reflectance dip and narrowband absorption in the near-infrared band can be obtained via exciting the gap-mode plasmons. The gap-mode system was composed by the MSM structure with ordered nanoholes in the upper metallic layer fabricated through the large-area and low-cost polystyrene nanosphere lithography. The gap-mode plasmons was shown to be facilely tuned by varying the thicknesses and materials of the spacer, or by the nanohole diameters and periods. The special resonance was found to be insensitive to the incident angle and the light polarization. We further designed the asymmetrical metal-semiconductor contacts to achieve the unidirectional photocurrent. It was found that the hot-electron photodetector based on the proposed MSM structure without an optical optimization exhibits a narrowband and high photoresponse (i.e., with responsivity ∼ 2.82 mA/W at λ = 1150 nm). This work provides a low-cost and efficient route to exploit the plasmonic applications for new-type photodetectors, biochemical sensors, analytical spectroscopy, and many related fields.
Funding
National Natural Science Foundation of China (61504088, 61675142, 61875143, 61905170); Natural Science Foundation of Jiangsu Province (BK20180042, BK20181169, BK20190816); China Postdoctoral Science Foundation (2017M611898, 2018T110549); Major Basic Research Project of the Natural Science Foundation of the Jiangsu Higher Education Institutions (17KJA480004); Priority Academic Program Development of Jiangsu Higher Education Institutions.
Disclosures
The authors declare no conflicts of interest.
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