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Abstract
Hot electrons from the nonradiative decay of surface plasmons have drawn extensive attention due to the outstanding performance in realizing below-bandgap photodetection. However, the widely employed metallic nanostructures are normally complex and delicate with a great challenge in large-area fabrication, and there is a great limitation to achieve substantial photoresponse at relatively long wavelengths (e.g., 2000nm) with polarization- and incident-angle independence. In this study, we theoretically and experimentally demonstrate a broadband, omnidirectional, and polarization-insensitive absorber based on wafer-scale silicon honeycomb nanobowls with 20-nm-thick gold overlayer. The average absorption across the long wave near infrared band (LW-NIR, i.e., 1100−2500 nm) is higher than 82%, which is contributed from the random nature and multimode localized plasmonic resonances excited on the side walls of nanobowls. Benefitted from the well-connected thin Au film and relatively low Schottky barrier, the generated hot electrons have a high transport probability to reach Schottky interface and participate in the interfacial charge transfer process. As a result, the hot-electron photodetector under no bias realizes a broadband photodetection up to 2000nm wavelength with a responsivity of 0.145 mA/W, and its cutoff wavelength is predicted up to 3300 nm by fitting the experimental result with Fowler theory. Our proposed Au/Si nanobowls photodetector could open a pathway to further extend the detection wavelength of Si-based photodetectors with a large-area and low-cost fabrication process, which promotes practical hot-electron applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Direct utilization of hot-electrons arising from the nonradiative decay of surface plasmons (SPs) has received extensive attention due to its broad applications in photodetection [1–10], surface imaging [11–13], photocatalysis [14], and solar energy harvesting [15–17]. Normally, it is challenging for conventional Si-based photodetectors to realize infrared photodetection beyond 1100 nm due to the limitation of Si bandgap. Alternately, many efforts focus on approaches of germanium (Ge) or III-V semiconductor epitaxial growth or wafer bonding on silicon (Si) [18,19], two-photon absorption and bulk/surface defect-mediated absorption [20,21]. However, these ways usually involve complicated and costly fabrication processes, thermal mismatch, silicon-based integration difficulties and poor reliability, etc. Specifically, through the regulation of Schottky contact at the metal-semiconductor interface, the hot-electron photodetector has the capability of detecting low-energy photons below the semiconductor bandgap, operating under room-temperature without electrical bias and controllable working wavelength with polarization dependence [22]. However, the responsivity of the hot-electron photodetector is extremely low due to the low optical absorption and poor hot-electron injection [23]. Therefore, various metallic nanostructures including gratings [2,3,24], nanorods [8], nanowires [25], pyramids [26–28], nanoantennas [1,4,29], and waveguides [30] have been extensively employed to boost the hot-electron generation and extraction efficiency. For example, perfect absorption is realized in the metamaterial absorber with a gold (Au) film of 15 nm [5], which ensures an efficient hot-electron generation and transport relative to that in the thick film of low absorption. The performance can be improved in metallic gratings arising from the absorption enhancement and combination of free-carrier absorption in highly doped Si [31]. By the excitation of surface plasmon polaritons (SPPs) in Si channel-separated interdigitated Au gratings, the spectrally selective photodetection is realized [32,33]. Nevertheless, almost all of these metallic nanostructures require the complicated/costly fabrication technology with a low throughput, support a relatively narrowband plasmonic resonance with their optical and electrical responses sensitive to the polarization and incident angles, and have difficulty maintaining satisfactory performance at longer wavelengths. To realize the practical photodetection application, a large-area, broadband, omnidirectional and polarization-insensitive hot-electron photodetector with a low-cost fabrication process and observable photoresponse at longer wavelengths is highly desired.
In this study, we propose and experimentally demonstrate a broadband hot-electron photodetector based on random Au/Si honeycomb nanobowls (Au/Si-NBs), which can be large-area fabricated by evaporation coating, thermal treatment and chemical etching. Relative to the steep side walls in the nanostructures of nanoholes and nanosquares, our prepared nanobowls have a larger opening and smoothly circular arc shape, leading to that the post-deposited thin metallic overlayer can be compact and electrically connected, as well as possessing the strong localization of multimode electric field. The Au/Si-NBs device exhibits a polarization-insensitive absorption of higher than 82% across the long wave near infrared band (LW-NIR), and the absolute absorption declines less than 10% as the incident angle increases from 8° to 68°. We systematically characterize the optical and electrical responses of the various hot-electron photodetectors with varying morphologies including the nanoholes, nanospikes and deep/shallow nanobowls. By taking advantages of the efficient hot-electron generation and transport from the strong optical absorption in the well-connected thin metal overlayer assisted by shallow nanobowl structures, a large-area, broadband, omnidirectional and polarization-insensitive Si-based photodetector is realized. Specifically, the Au/Si-NBs photodetector could maintain an unbiased responsivity of 0.145 mA/W at the wavelength of 2000nm, which is an order of magnitude higher than the ever reported values in Si-based photodetectors at this wavelength to our knowledge, and its cutoff wavelength is predicted up to 3300 nm by fitting the experimental data with Fowler theory. The nanobowl-based strategy could open a pathway for the efficient and low-cost hot-electron photodetection, photocatalysis and photovoltaic systems over a broader spectrum.
2. Device fabrication and characterization
The fabrication process of the proposed Au/Si-NBs structure is schematically shown in Fig. 1(a). First, a silver (Ag) film of ∼ 20 nm was deposited by thermal evaporation on the cleaned n-type Si substrate (2−4 Ω·cm) pretreated by ultrasonic cleaning, and then treated by rapid thermal process (RTP) for 4 minutes under the protection of nitrogen at 800 °C. After the RTP treatment, the sample turned to a uniform navy blue color and the continuous Ag film changed to randomly distributed Ag nanoparticles with variable sizes, as shown in Fig. 1(b). The size of Ag nanoparticles exhibited a Gaussian distribution with a large population in the diameter range of 120−240 nm (average value ∼ 167 nm). Here, the nanostructured Ag layer was used as the metal catalyst for the following metal-assisted chemical etching (MACE) of Si [34]. The etching solution was composed of HF (40 wt.%), H2O2 (30 wt.%) and deionized water at a volume ratio of 20:1:79, the etching process lasted for 1 minute at 40 °C. After MACE, the Si directly underneath the Ag nanoparticles was etched into disordered nanoholes (NHs) with a mean depth h = 700 nm and diameter d = 167 nm, as shown in Fig. 1(c). Then the Ag catalyst residues were completely removed by the concentrated solution of HNO3. Subsequently, the as-etched Si was drily oxidized annealed in a muffle furnace in air condition for 1 h at 1000 °C. During the oxidation process, silica layer in-situ grew up on the surface of NHs and would reach a saturation status when the Si under the silica layer would not get oxidized further. After the oxidation treatment, the formed silica layer was removed by immersing in the 2.5 wt.% HF solution. The deep Si-NBs were obtained after one circle of dry oxidation and silica-etching processes, and had a smaller depth (larger top diameter) of 450 nm (245 nm) in average, as shown in Fig. 1(d). The circle of dry oxidation and silica-etching processes was further repeated one and two more times and the corresponding structures were named as shallow NBs and nanospikes, as illustrated in Figs. 1(e) and 1(f), respectively. The shallow Si-NBs had a mean depth (top diameter) of 365 nm (310 nm) with deep dark blue color. Next, overlayers of Ti/Au (3 nm/20 nm) were deposited by electronic beam evaporation on the above-mentioned varying Si nanostructures. The smaller depth, larger top-diameter and slowly-changing arc-shape of the shallow NBs enabled the Ti/Au thin film to be uniformly coated on the side walls of the nanostructured Si substrate, as shown in Fig. 1(g). Finally, Ohmic contact on the back of the Si substrate was formed by indium soldering.
Fig. 1. (a) Schematic diagram of the fabrication process of the random Au/Si hybrid nanostructures. (b) Top-view SEM image of Ag nanoparticles formed on the Si substrate after RTP and the corresponding digital photograph (as the insert). (c–f) Cross-sectional SEM images and the corresponding digital photographs of the samples after MACE with various circles of dry oxidation and silica-etching processes [(c): NHs after etching; (d): deep NBs for one circle of oxidation and silica-etching; (e): shallow NBs for two circles of oxidation and silica-etching; (f): nanospikes for three circles of oxidation and silica-etching]. (g) Cross-sectional SEM image and the corresponding digital photograph of the Au/Si shallow NBs device (∼ 3 nm Ti is pre-deposited for improving the adhesive strength between Au and Si).
The microscopic morphologies were characterized by a field emission scanning electron microscope (SEM) (ZEISS, Sigma 300). Incident angle dependent optical properties were test by a spectrophotometer without an integrating sphere (PekinElmer, Lambda 750 UV/vis/NIR). Other optical properties were all measured by another spectrophotometer with a 150 mm InGaAs integrating sphere (PekinElmer, Lambda 1050 S+ UV/vis/NIR). For the electrical measurement, the device was illuminated by a NKT Photonics supercontinuum laser with an acousto-optical tunable filter to select the wavelength. The photocurrent was recorded with a semiconductor parameter analyzer (Agilent, B1500A). The optical power was measured by optical power meter (Thorlabs, S148C).
3. Optical and electrical responses
The broadband, omnidirectional and polarization-insensitive characteristics of the Au/Si-NBs hot electron photodetector are demonstrated from the optical and electrical perspectives in detail. The optical absorption (A) was acquired by measuring the reflection (R) and transmission (T) with A = 1 − R− T. Figure 2(a) shows the optical absorption spectra of the Au/Si hybrid structures with various Si morphologies (i.e., NHs, deep/shallow NBs, nanospikes and planar device). It is found that: (1) Au/Si shallow NBs device has a highest absorption beyond 82% across the entire LW-NIR (mainly absorbed by the metallic overlayer [4,29]), and the absorption is 8 times higher than the planar reference; (2) the absorptions in the Au/Si NHs and deep NBs are comparable and around 75% across the LW-NIR, a bit lower than that in the shallow NBs due to the relatively weak light confinement in the less continuous Au film; (3) more circles of oxidation and silica-etching processes cause the absorption of nanospikes significantly reduced because of the destruction of nanobowl structures, following the similar trend as the planar reference. It is clear from Fig. 2(b) that the efficient absorption in the Au/Si shallow NBs can be maintained over a broad wavelength band and a large incident angle even up to 68° (with the reflection only about 10% in absolute value higher than that under the incident angle of 8°). Figure 2(c) shows the effect of polarization angles on the optical absorption. The optical responses are insensitive to light polarizations due to the random size, spatial distribution and symmetric arc-shaped structures of NBs, similar as truncated and cone shaped Si structures [35]. The outstanding optical performance is in contrast to previously reported metallic gratings [3,31–33] and nanowires [36,37], where the SPs are excited under specific light polarization/incident angles, resulting in an angle/polarization sensitive optical response. Furthermore, compared to the metamaterial and metasurface with deep-subwavelength pitches [5] and embedded trench-like antennas [29], the random nanostructures of NBs can avoid costly lithography manufacturing processes and show greater potential for large-area and high-throughput applications.
Fig. 2. (a) Measured absorption spectra of the five kinds of Au/Si hot-electron devices based on various surface morphologies. (b) Measured reflection spectra of the Au/Si shallow NBs device under various incidence angles. (c) Measured absorption spectra of the Au/Si shallow NBs device under light polarization angles of 0° and 90°. The inset shows the absorption polar chart of the Au/Si shallow NBs device at various polarization angles for λ = 1550 nm. Noted that the spectra in (b) were measured by a spectrophotometer without an integrating sphere, while the spectra in (a and c) were obtained by another spectrophotometer with a 150 mm InGaAs integrating sphere. (d) Calculated absorption spectra of the single and super (2×2) shallow NB arrays. (e and f) Cross-sectional distributions of the normalized electric field in the single and super shallow NB arrays at their peak wavelengths, respectively.
In order to reveal the underlying physics for the broadband absorption, the optical responses of the Au/Si shallow NBs with different typical sizes and spatial distributions from SEM images are numerically studied with three-dimensional finite element method (FEM) simulation, using periodic boundary conditions and perfect matched layer (PML). Considering the typical single NB (d = 260 nm, h = 365 nm) arrays and super NB arrays consisting of four different NBs (d1 = 260 nm, d2 = 300 nm, d3 = 290 nm, d4 = 280 nm, h = 365 nm), the simulated and measured absorption spectra are shown in Fig. 2(d). Refractive indices of Au and Si materials in the simulation are from Palik [38], and the light is vertically incident from the Au side. It is found that the single and super NB arrays have a peak absorption at λ = 1345 nm and λ = 1520 nm with the full width at half maximum (FWHM) of around 400 nm and 600 nm, respectively. Besides, the peak absorption values are close to that from experiment. Thus, the measured broadband absorption arises from the synthetic effects of Au/Si shallow NBs with random sizes and spatial distributions [39]. It is noted that the oxidation and silica-etching processes make the as-prepared nanobowls surface rough, so the measured absorption is inevitably higher than the calculated value. Further, the cross-sectional distributions of the electric field at the peak wavelengths in the single (λ = 1345 nm) and super (λ = 1520 nm) NB arrays are explored in Figs. 2(e) and 2(f), respectively. It is obvious that multimode optical resonances are excited in the different cross-sections of NBs, contributing to the broadband absorption [8]. Besides, the electric field strength is enhanced to be over 7 times relative to that of incidence (E0) and strongly localized in the metallic interface, indicating the excitation of SPs [4,5].
The side-view spatial distributions of the normalized electric field (|E|/E0) and the hot-electron generation rate (G) in the single and super NB arrays at λ = 1345 nm and 1520 nm are plotted in Figs. 3(a) and 3(b), respectively. It is clear that the electric field is strongly bounded to the metallic interface with many high-intensity nodes on the side walls of NBs, further verifying the excitation of localized plasmonic resonances. Inherent from the strong electric field, hot electrons are efficiently generated in the thin Au film with the generation rate obtained by [8]
where ${\eta _{\textrm{eh}}}$ is the efficiency of plasmons decay into hot electrons, ${\varepsilon _i}$ the imaginary part of the material permittivity, $\hbar $ the reduced Planck constant. Further, based on the assumption of an isotropic momentum distribution and using the exponential attenuation model, the spatially dependent hot-electron transport probability to reach Schottky interface before thermalization is calculated by [7] where L is the distance from the generation position to the Schottky interface, and MFP is the energy-dependent hot-electron mean free path [40]. Benefitting from the 20 nm thin Au film, the generated hot electrons have a high probability to reach Schottky interface and participate in the interfacial charge transfer process [2], especially in the side walls of NBs [see Fig. 3(c)].Fig. 3. Side-view spatial distributions of the (a) electric field enhancement, (b) hot-electron generation rate, and (c) hot-electron transport probability in the single and super shallow NB arrays at their peak wavelengths, respectively.
Next, we show the detail performance of the Au/Si-NBs broadband absorber in infrared photodetection. Figure 4(a) illustrates the working principle for the hot-electron photodetection. Through the optical absorption in path (i), hot electrons are generated in Au, and will transport to the Au/Si interface within the mean free path before thermalization [41]. Those hot electrons with correct momentum and enough energy at the Au/Si interface can overcome the Schottky barrier to be injected into the conduction band of Si, and then extracted by the back electrode (i.e. In solder), resulting in the photocurrent through the external circuit [1]. The detection process of the photons absorbed by Si same as the conventional Si photodetector is also depicted in path (ii). As shown in Fig. 4(b), all of the hot-electron devices exhibit a clear rectifying behavior in the dark current-voltage (I-V) characteristics, indicating the formation of Schottky junction between the coated Au and the employed Si substrate. Schottky barrier heights (${\phi _\textrm{B}}$) of the hot-electron devices can be extracted from the measured I-V curves via the thermionic emission equation [42]:
Fig. 4. (a) Energy diagram and working principle of the hot-electron photodetector (hν1, hν2 represent the incident photons respectively absorbed by Au and Si materials). (b) I-V curves of the five kinds of the hot-electron photodetectors under dark condition. (c) Photoresponses and (d) statistical distributions of the photocurrents at the wavelength of 1550 nm in the five kinds of Au/Si hot-electron devices. (e) Photocurrent polar chart of the Au/Si shallow NBs device at the wavelength of 1550 nm under various polarization angles. (f) Power dependence of the photocurrent at the wavelength of 1550 nm of the Au/Si shallow NBs device.
To further demonstrate the superiority of the shallow NBs structure, Fig. 4(c) compares the photocurrents of the five types of hot-electron devices under light illumination of λ = 1550 nm at zero bias. It is clear that all of the presented devices show an on-off switching of the illumination and the photocurrent is dramatically increased from 0.054 µA in the planar device to 0.59 µA in the NHs device by modification into plasmonic structures. Interestingly, the photocurrent can be further increased to 1.12 µA and 1.52 µA in the deep and shallow NBs devices, respectively. Further modification of the plasmonic Au/Si-NBs structure into nanospikes structure leads to a lower photocurrent of 0.46 µA due to the significantly decreased optical absorption and hot-electron generation rate. Since the spikes in the NBs and nanospike nanostructures relax the momentum requirements during the hot-electron internal emission process [44–46], the responsivity of the NHs device is much lower and even close to that of the nanospikes device although the absorption is much higher. Next, the replicability and stability of the proposed Au/Si-NBs device is examined by statistically analyzing the photoresponses of the vast hot-electron devices with the five types of different surface morphologies at λ = 1550 nm, as shown in Fig. 4(d). It is found that the Au/Si shallow NBs device has a good replicability with the highest responsivity among the five types of devices. Inherent from the polarization-insensitive optical absorption, the photoresponse of the Au/Si shallow NBs device is predicted to be independent to light polarizations, as shown in Fig. 4(e). It is clear from Fig. 4(f) that the observed photocurrent linearly increases with the incident light power, which suggests a single photon-electron conversion process [47], and we can still get obvious on-off current of our device at 0.165 mW of excitation light source.
Finally, we investigate the key performance parameters of the hot-electron photodetector. Obvious photoresponse of the Au/Si shallow NBs device can be seen from the I-V curves under illumination with different wavelengths [ Fig. 5(a)]. Figure 5(b) also presents the photoresponses of the Au/Si shallow NBs device under alternately laser-on and laser-off illumination of various wavelengths. It is noted that the obvious on-off switching of the illumination can be observed even for the wavelength up to 2000 nm (note that further longer wavelength is not used in the experiment due to the limit of our employed light source and the optical power value at 2000nm wavelength is about 2.8 mW). As shown in Fig. 5(c), the Au/Si shallow NBs device has the highest responsivity among the five types of hot-electron devices across the infrared band. The measured responsivity at wavelength as long as 2000nm is 0.145 mA/W, an order of magnitude higher than the current reported highest responsivity of 0.085 mA/W of Al/Si nanopyramids structure at this wavelength [48], and the reverse-biased Au/Si shallow NBs device [Fig. 5(d)] shows almost 1.5 times higher responsivity at the reverse bias of 0.1 V than that of zero bias.
Fig. 5. (a) I-V curves of the Au/Si shallow NBs device under wavelength-varying light illumination. (b) Photoresponses of the Au/Si shallow NBs device under wavelength-varying light irradiation. (c) Responsivity spectra of the five kinds of hot-electron photodetectors. (d) Reverse bias dependence of the responsivity of the Au/Si shallow NBs device. (e) Fowler fitting to the measured EQE curves. (f) Detectivity spectra of the five kinds of hot-electron photodetectors.
To further elaborate the superiority of Au/Si shallow NBs device in broadband photodetection, we obtain the external quantum efficiency (EQE) spectra based on the measured responsivity and fit them to the modified Fowler equation [49]:
where $h\nu $ is the photon energy, and ${C_F}$ is the Fowler constant. As demonstrated in Fig. 5(e), the fitted Fowler curves are in very good approximation to all the measured values. The intersection of the Fowler curve with the x axis indicates the cutoff detection wavelength, where the photon energy is equal to the Schottky barrier height. Thus, the predicted cutoff wavelength of the Au/Si shallow NBs device is estimated to be ∼ 3300 nm, which is around 2 times of that in the device based on disordered nanocomposites structure (1700nm) [50]. There is a small gap between the barrier heights get from the fitted Fowler curve and extracted from the measured I-V curve because the reverse saturation current read from the I-V curve is less than its actual value, which is limited by the interval of the applied reverse bias and the amplitude range set in the semiconductor parameter analyzer. A comparison of the cutoff wavelengths of the state-of-the-art broadband Si-based photodetectors is shown in Table 1. One could find that it is very difficult to realize observable photoresponse over 2000nm, and extension of the detectable wavelength requires the development of novel materials and corresponding complicated fabrication processes [48,51,52]. Recently, a molecular optomechanical platform was proposed to up-convert photos from the far- and mid-infrared into the VIS-NIR domain to realize single-photon detection at wavelengths beyond 2000nm [53]. Compared with these works, our nanobowl-assisted strategy obviously provides a more direct and easier way to extend the detection wavelength to mid-IR, and has higher responsivity at λ = 2000nm under no bias. This promotes the use of plasmonic enhanced Si-based photodetectors for a broad range of applications including mid-IR circuitry and biochemical sensing [48].
Table 1. A comparison of the cutoff wavelengths of the state-of-the-art broadband Si-based photodetectors with different device structures.
Figure 5(f) shows the detectivity spectra, which describe the ability of photodetectors to detect weak light signal. It is found that the Au/Si-NHs device has the highest detectivity due to the relatively low dark current. However, benefit from the high photoresponse, the detectivity in the nanobowl-based device is on the same order of magnitude as that in the Au/Si-NHs, Au nanoislands [54], Au grating [55], and Au stripes [56] devices. The absorption and responsivity [ Figs. 6(a) and 6(b)] of the shallow NBs device also show obvious advantages over the planar device at the wavelengths smaller than 1100 nm with the measured responsivity of ∼ 0.226 A/W at 750 nm wavelength.
Fig. 6. (a) Absorption spectra and (b) responsivity spectra of the Au/Si planar and shallow NBs hot electron photodetectors in the wavelength range of 750−1100 nm.
4. Conclusions
In summary, we have experimentally demonstrated a broadband, omnidirectional, and polarization-insensitive hot-electron photodetector with the detection wavelength up to 2000nm, and the device structure is consisted merely of low-cost and large-area Si honeycomb nanobowls and the post-coated 20 nm Au film. The synergistic effects of multiple scattering and localized surface plasmon resonances contribute to a polarization-insensitive and omnidirectional absorption over 82% across the LW-NIR. The broadband absorption in the thin plasmonic structures, along with the well-connected Au/Si-NBs contact allow an efficient hot-electron generation and extraction, which result in the broadband photodetection with an unbiased responsivity of 0.145 mA/W at the wavelength of 2000nm, and the cutoff wavelength is predicted up to 3300 nm by fitting the experimental result with Fowler theory. Our proposed device can be lithography-freely fabricated merely by a combination of evaporation coating, thermal treatment and chemical etching, and have a good repeatability in the performance. This work opens up an alternate path to extend the detection wavelength of Si-based photodetectors, promoting the practical application of hot-electron devices.
Funding
National Natural Science Foundation of China (61675142, 61875143, 61905170, 62075146); Natural Science Foundation of Jiangsu Province (BK20180042, BK20180208, BK20181169, BK20190816); Natural Science Research of Jiangsu Higher Education Institutions of China (18KJD416001, 20KJA510003); Priority Academic Program Development of Jiangsu Higher Education Institutions; Qinglan Project of Jiangsu Province of China.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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