Achieving Infrared Detection by All-Si Plasmonic Hot-Electron

Jul 3, 2019 - The success of all-Si based detection in NIR will surely start an era of .... In contrast, the absorption by the planar Si or its nanowi...
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Achieving Infrared Detection by All-Si Plasmonic Hot-Electron Detectors with High Detectivity Bo Feng,§ Jingyuan Zhu,§ Bingrui Lu,§ Feifei Liu,¶ Lei Zhou,¶ and Yifang Chen*,§

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§

Nanolithography and Application Research Group, State Key Lab of Asic and System, School of Information Science and Technology, Fudan University, Shanghai 200433, China ¶ Department of Physics, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: An improved architecture for all-Si based photoelectronic detectors has been developed, consisting of a specially designed metasurface as the antenna integrated into a Si nanowire array on the insulator by an electron beam lithography based self-alignment process. Simulation using the Finite Difference Time Domain (FDTD) method was carried out to ensure perfect absorption of light by the detector. Optic measurement shows a 90% absorption at 1.05 μm. Photoelectronic characterization demonstrates the responsivity and detectivity as high as 94.5 mA/W and 4.38 × 1011 cm Hz1/2/W, respectively, at 1.15 μm with the bandwidth of 480 nm, which is comparable to that of III−V/II−VI compound detectors. It is understood that the outstanding performances over other reported all-Si based detectors originate from the enhanced quantum efficiency in one-dimensional conduction channels with high density of states, which efficiently accommodate the emitted plasmonic hot electrons for high conduction in the Si nanowires, enabling the near-infrared detection by all-Si based detectors. KEYWORDS: all-Si photodetectors, plasmonics, hot electrons, near-infrared detection, perfect absorption barrier.7−12 The most important significance of such a brand of photoelectron detectors is that the photoelectron conversion occurs below the Si bandgap, allowing the detection in the NIR region (1−3 μm). Advantages of Si based detectors reside in many important aspects. Si is inexhaustible, inexpensive, and environmental friendly, being able to operate at room and/or even higher temperatures. The device process of Si is well established and compatible to CMOS technology. The success of all-Si based detection in NIR will surely start an era of remote sensing technology with revolutionary applications such as Si based FPAs in NIR with a 12 in. wafer scale, room temperature operation, miniaturized FPAs/circuits in one substrate, etc. So far, a big variety of device architectures have been proposed to enhance the external/internal quantum efficiency (EQE/IQE) of photon-to-electron conversion for achieving high responsivity to meet the requirement by practical applications.7−14 A perfect absorber of light,15,16 chiral metamaterials,17 Au-mirror structure,18 and channel-coupled

P

hotoelectron detection in near-infrared (NIR) wavelengths (1−3 μm) is of extreme importance and broad applications in optical communications,1 environmental protection,2 security monitoring,3 medicine/healthcare,4 etc. So far, semiconductors applied in 1−3 μm photoelectron detectors (PDs) are dominated by III−V compounds5 such as GaSb, InAs, and GaAs and II−VI ones such as HgCdTe1 because their bandgaps are responsive for the near-infrared (NIR) wavelengths. However, the drawbacks of using these semiconductors are the material toxicity, high cost, complexity in processing/growth, need for cooling, and incompatibility with Si based CMOS technology. Currently, focal plane arrays (FPA) based on photodetectors (PDs) are advancing toward a reduced sensing area, a large number of detectors, a large area in the wafer scale, multichannel detections, high temperature operation, and integration of detectors with read-out circuits in one substrate.6 To achieve these advances, a special brand of detectors needs to be developed. In recent years, Si based optoelectronic devices for NIR detection have come to the light, which are based on the generation of hot electrons in metallic antennas via the nonradiative plasmon decay,7 followed by internal photoemission (IPE) of the carriers into silicon channels through the metal/Si interface with a Schottky © XXXX American Chemical Society

Received: May 30, 2019 Accepted: July 3, 2019 Published: July 3, 2019 A

DOI: 10.1021/acsnano.9b04236 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Architecture design and absorption tunability of modes. (a, b) Schematic illustration of the proposed photodetectors with a metasurface integrated into a Si nanowire array on SOI substrate, which was a 500 nm thick oxide buried in silicon. The parallel Si nanowires with the height (H) of 100 nm and/or 150 nm, the width of 100 nm, and the pitch of 200 nm were replicated by nanofabrication on a lightly doped N-type silicon (1−20 Ω·cm). (c, d) The simulated optical absorption spectra of both TM-polarized (E-field at the x-axis direction) and TE-polarized (E-field at the y-axis direction) light, showing near perfect absorption for the TM mode. (e) The simulated absorption spectra of the three types of device configurations as illustrated in the insets. (f) The systematic simulations for the spatial distributions of the electric field (i), the Poynting vector (ii), the absorbed power (iii), the current density (iv), and the charge density (v) in the Au/Silicon interfaces at the peak absorption of 0.91 μm for TM mode. In the simulation, the antenna is a bilayer of 2 nm Ti/40 nm Au and its dimensions are W = 100 nm, P = 200 nm, and H = 150 nm.

nanogap structure19 have been tried. Other groups have invented hot hole based devices, in which the plasmongenerated hot holes are ∼2 eV more energetic than the hot electrons.20−23 Unfortunately, all these devices still remain a low-efficient injection of plasmonic hot carriers through the Schottky barrier in the metal/Si interface for photocurrents,24,25 resulting in extremely low quantum efficiency (QE) in the order of 0.01−0.2% only. For all-Si based detectors,7−9,12−15,17−19 the responsivity at the present is still struggling at the level lower than 10 mA/W, far below the average performance of its counterparts based on traditional III−V compounds.5 One way to enhance the QE is to lower Schottky barrier in the metal/Si interface, which is also beneficial to the detection in longer wavelengths. However, Au/Si interfaces with low Schottky barriers suffer from a large dark current unless the detectors are cooled down to cryogenic temperatures.26,27 Therefore, improved architectures must be innovated for the all-Si based detectors to meet the

performance competitive to the III−V compound devices for practical applications. This work reports our recent breakthrough in achieving high detectivity comparable to III−V/II−VI compound based detectors by developing a configuration in all-Si based PDs with perfect light absorption and enhanced quantum efficiency. The device architecture consists of a specially designed metasurface as the nanoscale antenna integrated into parallel nanowires in Si on insulators (SOI). The peak responsivity as high as 94.5 mA/W at 1.15 μm corresponding to the detectivity of 4.38 × 1011 cm Hz1/2/W and the external quantum efficiency of ∼12% at room temperature have been achieved. The whole process of photoelectron detection including the selective light absorption by the 3D cavities (the antennas) with surface plasmonic modes, hot-electron generation in Au through nonradiative decay of plasmons, and the effective transfer of the generated hot electrons from the metal to one-dimensional (1D) conduction channels in Si, known as the internal photoemission effect (IPE),7−9 was B

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Figure 2. Device fabrication, optical responsivity measurement, and further simulations. (a, b) Micrographs by a scanning electron microscope (SEM) of model ZEISS Sigma HD for the fabricated detectors on SOI. Between the side antennas and the Si nanowires, 10 nm gaps can be spotted. (c, d) The measured absorption spectra (1 − transmittance − reflectance) by the detectors for both the TM-polarized (c) and the TE-polarized light (d). Including the 10 nm gaps in the simulation model, the calculated absorption spectra are presented in (e, f), which are closer to the measured ones than the ones in Figure 1c,d, which neglect the 10 nm gaps. In the simulation, the antenna material is a bilayer of 2 nm Ti/40 nm Au and the other dimensions are W = 100 nm, P = 200 nm, and L = 200 nm. (g) The spatial distribution of the electric field at the peak absorption (point A) for H = 150 nm shows the effect of the 10 nm gaps on the absorption. Strong electric fields are observed inside the two gaps, which brings down the absorption intensity around the top antenna.

RESULTS AND DISCUSSION The schematic diagrams of the whole device built on SOI substrate are shown in Figure 1a,b. The conducting channels are the parallel Si nanowires (NWs) with the wire width of 100 nm and the height of 100−150 nm, respectively. Using electron beam lithography, nanoscale antennas as periodic gold blocks were integrated into the parallel conducting wires by a self-aligned method. Schottky barriers are formed in the interfaces between the antennas (Au) and the silicon nanowires. The source and the drain of the two-terminal

clearly elucidated. The close connection between the detection property and the device structure enables us to further develop all-Si based detectors with broader bandwidth covering the whole 1−3 μm in the future. The success of this work evidently demonstrates the feasibility of replacing the III−V/II−VI compounds by all-Si for NIR detectors, which will lead to an enormous impact on the sensing community as a whole with extensive applications in our life. C

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Figure 3. Configuration-dependent photoelectron performance. (a) The typical curves of photocurrent vs dc bias of the photodetector under the illumination of light at 1.15 μm with the intensity of 400 μW/cm2. The samples tested include planar Si, the Si nanowires, and the Si nanowires with the golden antennas at room temperature. Two heights of the nanowires are compared. The curves for the actual devices are labeled in the figure (b, c). The measured responsivities of the fabricated all-Si based detectors for the TM- and TE-polarized lights at VDS = 1.5 V (points). (d) The corresponding external quantum efficiencies measured (points). Solid lines are the fitting curves to the data of metasurface devices for the Schottky barrier height of 0.46 eV. Bias dependency of responsivity (e) and detectivity (f) under 1.15 μm wavelength light illumination with the intensity of 400 μW/cm2. (g) The measured detectivity and dark current density (inset) of metasurface detectors with the height of 100 and 150 nm, respectively. (h) The comparison of the detectivity achieved in this work with other reported ones based on III−V/II−V compounds: InGaAs,4,5 PV-InAs,28,35 PC-CQDs,33,34 PV-HgCdTe,1,35 and PC-PbS/PbSe.28,36 PV, Photovoltaic; PC, photoconductive.

detector were finally defined by the aluminum (Al) pads using registered photolithography. The geometry dimensions of the nanostructures in the PDs, such as the silicon layer thickness (H), the width (W), and the period (P) of the Si nanowires along the x-axis (Figure 1b), are the key parameters, deciding the surface plasmon modes in the metasurface. Such a design with disconnected Au blocks also helps us to eliminate the detection sensitivity to the polarization of light, which will be discussed later. The first function of the antennas in combination with silicon nanowires is expected to be a wavelength selector, determined by the plasmonic modes existing in the 3D cavity. Figure 1c shows the absorption spectrum of the TM-polarized (E-field at the x-axis direction) incident light, calculated by the Finite Difference Time Domain (FDTD) simulation method.

Near perfect absorption (99%) by the device is obtained, which occurs at the wavelength of 0.91 μm, corresponding to the device parameters (W = 100 nm, P = 200 nm, and H = 150 nm, where L is fixed to be 200 nm throughout the Article). When the height of the wire is reduced to 100 nm, the absorption peak is red-shifted to 1.0 μm and the absorption goes down to less than 90%. With the same height, when the W/P of the silicon wires is changed to 150 nm/400 nm, the absorption peaks are shifted from 0.91 to 1.25 μm, suggesting that the resonant frequency will be effectively determined by the antenna geometry. As for the TE-polarized light (E-field at the y-axis direction), the peak absorption is observed around the wavelength of 1.25 μm, as shown in Figure 1d. In contrast to the TM-polarized light, with the height increasing from 100 nm up to 150 nm, the peak absorption is reduced. It is also D

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ACS Nano necessary to mention that the absorption in 0.9−1.2 μm by the planar silicon and Si wires without the metallic metasurface is nearly zero, as shown by the black (for planar Si) and the red (for Si nanowries) curves in Figure 1c,d, showing the critical role of the antennas for the all-Si based detectors in NIR. In the unit cell of the metallic metasurface, three antennas surrounding one Si nanowire form a 3D cavity as schematically illustrated by the black inset in Figure 1e. To understand the responsibility of the specific antenna(s) for the resonant absorption, the whole cavity was broken down into two different configurations with the side antennas (in the red inset) and the top antenna (in the blue inset), respectively. The antenna on the top of the Si nanowire dominates the absorption at 0.9 μm for the TM-polarized light and that at 1.05 μm for the TE-polarized one, respectively. The side antennas, however, show an even stronger absorption than what the top antenna does at 1.2 μm (TM mode) and a weaker one around 0.85 μm (TE mode), respectively. The diverse resonances by different antennas lead to tunable absorption of the 3D cavity to the light in the wavelengths from 0.8 to 1.6 μm, when the geometry dimensions such as the wire width and the pitch of the antennas are altered, as shown in Figure 1c,d. To gain insight of where the absorbed energy is confined for the generation of hot electrons, the electric field distributions, the Poynting vector, and the absorption power profile were numerically simulated at the resonant wavelength of 0.91 μm, as shown in Figure 1f. The electric field (Figure 1f(i)) is highly concentrated around the antenna on the top of as well as the side of the Si nanowires. The Poynting vector (Figure 1f(ii)), representing the energy flow of the light, reveals the most important character of the designed cavity, that is the majority of the energy is accumulated in the lower part of the top antenna connecting to the Au/Si interfaces, where it is 3 × 103 times higher than that in the free space between two wires. Another profound feature to emphasize is that the Poynting vector between the side antennas and the silicon wire is negligible, suggesting that there is hardly contribution of photocurrent from the two antennas beside the nanowires. It is the plasmons condensed in the lower part of the top antennas that dissipate the energy by generating hot electrons in the metals through the nonradiative decay. The perfect absorption of light, as discussed above, helps to maximize the generation of hot electrons. Such a physical picture is supported by the simulation results for the absorbed power and the current density, which are located in the interfaces of Au/Si, as shown in Figure 1(iii),(iv), respectively. The existence of the charge density, as simulated in the Au/Si interfaces (Figure 1f(v)), also supports our understandings. Figure 2a,b presents the micrographs of the fabricated photodetectors in silicon. To clarify the functionality of 3D cavities, four different configurations of the device were prepared and tested. They are planar Si, Si nanowires, and the completed detectors with two different heights of the Si nanowires, 100 and 150 nm, respectively. The wire width (W), the pitch (P), and the antenna pitch (L) are kept unchanged as 100, 200, and 200 nm, respectively. The Si nanowire array occupies the overall area of 200 μm × 300 μm as the sensing region. The Au thickness for the antennas is fixed to be 40 nm, and a 2 nm thick Ti was used for both adhering the Au antennas to the silicon and determining the Schottky barrier height.7 Figure 2c,d shows the measured optical absorption spectra of the light with TM and TE polarizations, respectively, for the all-Si based photodetectors with the four configurations.

For the metasurface device with the wire height of 150 nm, the resonant absorption peak reaches as high as ∼90% with the full width at half-maximum (fwhm) of 300 nm to the TMpolarized light at 1.05 μm (Figure 2c). In contrast, the absorption by the planar Si or its nanowires is merely ∼10% in the same wavelengths, clearly proving the absorption enhancement by the antennas. As for the TE-polarized light, high absorption around 70% is also observed at the wavelength of 1.15 μm. When the absorption spectra are compared to those by the simulation (Figure 1c,d), it is found, however, that the measured absorption spectra do not agree with the simulation results, which should be caused by the structural deviation of the real device from the modeled one (Figure 1b). Careful inspection of the fabricated devices shows that there are 10 nm wide gaps between the bottom Au antennas and the Si nanowires, as marked in Figure 2b. When this was taken into account for the modified model of the device, the simulation was carried out and the results were plotted in Figure 2e (TM polarization) and Figure 2f (TE polarization), respectively. Apparently, the simulated absorption spectra agree with the measured ones closely. Also, the absorption peak (point A) at 1.0 μm in Figure 2e is reduced down to 88%. Although the spatial distribution of the electric field (|E|) by the simulation (Figure 2g) shows strong plasmonic modes in the gaps, the generated plasmonic hot electron in the metals fail to enter the Si nanowires via IPE because of the gaps, making no contribution to the photon-electronic current by the side antennas. Figure 3a presents the photocurrent (I)−voltage (V) curves measured from the devices at room temperature with four different configurations as described above under the illumination of light at 1.15 μm with the intensity of 400 μW/cm2. Typical I−V curves of photoelectron detectors are observed for the Si nanowires integrated with antennas, while those without the antennas show negligible photocurrent (Ilight − Idark) even at the dc bias of ±1.5 V. It reveals that the photocurrent is mainly caused by the plasmon decay-generated hot electrons. Responsivity (Rλ) was calculated according to Rλ = Iph/(PinS), where Iph is the photocurrent, Pin is the illumination intensity of the light, and S is the sensing area (200 μm × 300 μm). Both the experimental responsivity spectra (points) for the TM-polarized (Figure 3b) and the TEpolarized (Figure 3c) light, respectively, show a strong peak at 1.15 μm, which can be effectively governed by the height of the wires. The external quantum efficiency (EQE) of the photodetector is defined as4 EQE = hc/(qλ)·Rλ, where h is the Planck constant, c is the speed of light, q is the Coulomb charge, and λ is the wavelength. The maximum responsivity for the TM-polarized light, at the bias VDS = 1.5 V, reaches 94.5 mA/W with the bandwidth of 480 nm, corresponding to a quantum efficiency of 11.7% (Figure 3d), when the wire height is 150 nm. For TE-polarized light, a broadband responsivity with the peak of 70.1 mA/W and the quantum efficiency of 7.9% at 1.15 μm was obtained. In the wavelength up to 1.55 μm, the responsivity still remains at the high level of 18.2 mA/ W with the quantum efficiency of 1.5%. Considering the Schottky barrier is formed in the interface of antenna/Si, the responsivity (R) of an IPE based photodetector can be expressed as follows:8 R(v) = S(v) × ηi E

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Figure 4. All-Si based photodetector with the polarization sensitivity and the time-dependent properties. The polarization sensitivity is also related to the device geometry dimensions. (a, b) The photocurrent ratio of TM/TE modes for the nanowire heights of 100 and 150 nm, respectively. (c) Direct demonstration of the photoelectron currents under the modulation of incident lights at 1.15 μm wavelength with the light intensity of 400 μW/cm2.

1011 cm Hz1/2 W1− for the TM-polarized light and 3.25 × 1011 cm Hz1/2 W1− for the TE-polarized one, respectively. The curves in the inset show the dark current density of 9.08 × 10−8 A cm−2 (100 nm wire height) and 1.45 × 10−7 A cm−2 (150 nm wire height). By comparison, our metasurface covered all-Si photodetectors show the best performance than ever reported (see Table S1), which are even comparable to those by III−V/II−VI compounds in the same infrared range,1,4,5,28−36 as presented in Figure 3h. Furthermore, the metasurface architecture of all-Si based devices developed in this work also shows the detection sensitivity to the light polarization can be tuned by the wire height (H). For example, the discrimination ratio of TM over TE as 2.57 (Figure 4a) was measured at 1.32 μm for the wire height of 100 nm. When the height becomes 150 nm, it reaches unity at the same wavelength (Figure 4b). Direct optoelectronic effect is also demonstrated in Figure 4c. A rapid increase or decrease in the photocurrent under the modulated illumination of 400 μW/cm2 is observed at 1.15 μm. The curves also confirm the time dependence, yielding estimated rise and decay constants of 0.4 and 1.3 ms, respectively. Figure 5 schematically illustrates plasmonic hot-electron generation and transfer into the silicon. The energy band diagram in Au−Si interfaces with Schottky barriers along a 150 nm height (H) nanowire in SiO2 (buried oxide) is also plotted. The depletion width37 around the Si nanowire surface is

where S(v) is the plasmon absorption spectrum, which satisfies a Lorentzian line shape function11 (see the Supporting Information). ηi describes the number of hot electrons in the system with sufficient energy to overcome the barrier and can be approximated by the modified Fowler theory: 8 (hv − qϕ )2

ηi ≈ C F hv B , where CF is the device-specific Fowler emission coefficient, hv is the photon energy, and qϕB is the potential barrier. On the basis of this, the external quantum efficiency of the plasmonic hot-electron device can be evaluated by the extended Vickers formula:26,27

EQE = S(v) × P(d) × ηi ≈ S(v) × P(d) × CF

(hv − qϕB)2 hv

(2)

where P(d) is a scattering term and d is the metal thickness. By fitting the measured responsivity (points) and EQE (points) of the metasurface devices with eqs 1 and 2, respectively, as shown in Figure 3b−d (solid lines), a Schottky barrier height of ∼0.46 eV was extracted, which is in the same order of magnitude as reported.7,8 The excellent agreement between the fitting curves and the experimental data also proves that the photocurrent mainly comes from plasmonic hot electrons, even at the 0.9−1.1 μm region. On the basis of the measured responsivity, the detectivity of the photodetector can be expressed as4 D*(λ) = Rλ/(2q·Jd)1/2, where D* is measured in the units of cm Hz1/2 W1− (or Jones) and Jd is the dark current density (A/cm2). Figure 3e,f presents both the responsivity and the detectivity driven by the dc bias from the source to the drain bias, which are strongly associated with the device structure. Figure 3g presents the measured detectivity spectra. For the device with the wire height of 150 nm, under the dc bias of 1.5 V, the peak detectivity is 4.38 ×

estimated to be 30 nm using wd =

2εVbi qND

, where Vbi is the

built-in potential (0.2 eV for Ti−Si), ND is the doping concentration of Si (2.8 × 1015 cm−3), and ε is the permittivity, leaving the conducting space of about 120 nm (height) × 100 nm (width) inside the wire. The Fermi wavelengthλF = F

2π nsH

is estimated to be 123 nm for the electron DOI: 10.1021/acsnano.9b04236 ACS Nano XXXX, XXX, XXX−XXX

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be partially ascribed to the 1D transport of plasmonic hot electron in Si nanowires with high density of states for elevated photocurrents. With this special architecture developed in this work, further advance to expand the sensing wavelength to the whole NIR of 1−3 μm is highly prospective. The success of this work starts a special era by adding the all-silicon based detector to the family of sensing technology in the NIR with a number of advantages including room temperature operation, large sensing area up to 12 in. at least, mass production at low cost, and high compatibility with the CMOS technology, benefiting the advances of sensing technology with an enormous impact.

Figure 5. Schematic illustration of the band structure in the interface of the Si nanowire/Au antenna, showing the process of surface plasmon-induced hot-electron generation and transfer into the 1D conducting channels. At room temperature, the 1D subband in the Si nanowire is continuous, capable of collecting all the arrival electrons.

METHODS Device Fabrication. Nanofabrication for the Si based detectors with four different configurations was carried out using electron beam lithography based processes on a SOI substrate. The SOI structure was formed by a 500 nm thick buried oxide and a 150 or 100 nm lightly doped N-type silicon layer (1−20 Ω·cm). On the SOI, a 350 nm thick PMMA supplied by Micro-Chem Ltd. was first spin coated, followed by a soft-bake of the resist on a hot plate for 10 min at 180 °C. After the e-beam exposure by the beam writer of JEOL 6300FS (see Figure S3a), the exposed resist was developed in a MIBK/IPA (1:3) solution at 23 °C for 60 s, finished by a thorough rinse in IPA solution for 15 s. A wet etch in 2% buffered HF was applied to remove the native oxide on silicon just before transferring into a thermal evaporator for the deposition of a 25 nm Cr. A lift off process in warm acetone was then carried out to remove the unwanted metals. By this stage, Cr nanowires on Si were formed, occupying the overall area of 200 μm × 300 μm (see Figure S3b). Using the patterned Cr nanowires as etching masks, a reactive ion etch (RIE) in fluorine based plasma was carried out in a Samco etcher to form Si nanowires. The Cr mask was removed by a Chromium etchant from SigmaAldrich Ltd. (see Figure S3c). Using the same process flow, a metasurface architecture with a 2 nm Ti/40 nm Au bilayer was formed on the Si nanowire array by using a self-aligned technique (see Figure S3d−f). Annealing in nitrogen gas at 500 °C for 15 min was undertaken, aiming to reduce surface defects on the wires. Finally, large bond pads of Al were then laid down to connect the ends of the Si NWs, using a registration technique (see Figure S3g). Optical Measurement Techniques. Transmission and reflection measurements were carried out under ambient conditions at room temperature for samples on transparent quartz substrates. An in-house built setup including a monochromator, collimator, quarter-wave plate, linear polarizer, and the polarization detector in series was used in the measurement. The detection area had a spot diameter of 80 μm. For the reflection measurements in the infrared wavelengths, a Nicolet 6700 Fourier-transform infrared spectrometer equipped with a Continuum XL microscope (Olympus) was used. An infrared light source was focused onto the normal sample. The reflectance spectra were plotted as (Rsample/RAl) × 100%; Rsample was collected from an area with a sample present and RAl, from an adjacent Al pad as a total reflection mirror. Absorption was calculated as 1 − transmittance − reflectance. Electrical Characterizations. Photoelectronic properties of the fabricated detectors were systematically characterized using an inhouse built photoresponse characterization system. An optical radiation of a well-defined wavelength was selected from a supercontinuum laser source using acousto-optic tunable filters (Fianium). The light source was calibrated by a power meter supplied by OPM 35S Ltd. To investigate the time response of the detector, an additional shutter was added to the output as a square-wave function generator. The laser beam passed through a linear polarizer to form TM and TE modes and was then focused onto the whole detector’s sensing area. The measured current−voltage data at room temperature from the devices with different designs were extracted using a digital source meter (Keithley 2400).

density ns of 2.78 × 1015 cm−3, which is around the wire width of the Si channels. This indicates that the plasmonic hotelectron transport inside the Si nanowires in our detector is in a one-dimensional (1D) regime even at room temperature. The density of states (DOS) in the lowest energy is D(E) =

2 h

2mn* E − Ec

,

4πmn* h2

, and

4π (2mn*)3/2 h3

E − Ec for a 1D,

2D, and 3D system, respectively. They can be calculated as 1.6/eV, 0.67/eV, and 0.196/eV, respectively. So far, all the reported photoelectron detectors are based on 3D conducting silicon. The Si nanowires in this work takes the advantage of 1D DOS, which is one magnitude order higher than the existing works. The energetic plasmonic hot electrons in the antennas with energy higher than the Schottky barrier emit through the Au/Si interfaces and then drift under the dc bias VDS across the source-drain, leading to an expected photocurrent in the 1D Si nanowire system. The high density of states in the 1D wires is able to accommodate lots more entering electrons than in 2D/3D systems, resulting in the highest photocurrent ever reported.8,9 The detectivity achieved in this work by our PDs has reached the same level as those of the III−V/II−VI compound based detectors such as photovoltaic detectors in InAs28,35 and HgCdTe,1,35 photoconductive detectors in PbSe,28,36 PbS,28,36 and CQDs,33,34 etc. as presented in Figure 3f. It is also important to mention that the antennas in this work are formed by isolated Au blocks in the nanoscale instead of bulky metals, so that the localized plasmons find it is difficult to dissipate the power through heat conduction to its environment, the same as the generated hot electrons do. This is another non-negligible because of the achieved high responsivity in this work.

CONCLUSIONS An architecture for all-Si based photoelectronic detectors in the NIR has been designed and fabricated. The high responsivity and the detectivity as 94.5 mA/W and 4.38 × 1011 cm Hz1/2 W1− at 1.15 μm, respectively, have been successfully achieved. Such a detection performance reaches the same level as that of III−V/II−VI compound based detectors in the NIR region. The key to the success is mainly two-fold: one is the perfect absorption by the specially designed 3D cavity as the antennas; the other is the generation of hot electrons by the nonradiative decay of the plasmons that happens on the lower part of the top antennas connected to Si nanowires to ensure high injection efficiency. It is also important to emphasize that the high detection property may G

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ACS Nano Numerical Simulations. All the optical spectra, the spatial distributions of the electromagnetic field, and the Poyinting vector were calculated by solving the Maxwell equation, using the FDTD simulator, supplied by the Lumerical software package (3D-Finite Difference Time Domain (FDTD)). The refractive index or permittivity of the materials, including Ti, Au, silicon, and silicon oxide in NIR region, used in this work, are from the material database in the FDTD solution supplied by Lumerical Ltd. In all the simulations, the periodic boundary condition was adopted along both the x- and the y-axes and perfectly matched layers were used along the propagation directions. The light source is supposed to be a plane wave, propagating along the z-direction (Figure 1b) normal to the detector plane. Its polarization perpendicular to the Si nanowires (electric field at x-direction) is defined as the TM mode and that parallel to the nanowires (electric field at y-direction), as the TE mode. A reflection monitor was placed at the top of the simulation region, and a transmission monitor was placed beneath the Si substrate. Absorption was calculated as 1 − transmittance − reflectance. The current density and charge density in metals are obtained using the current charge density analysis groups (supplied by advanced analysis in the FDTD solution).

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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/acsnano.9b04236. Optical reflection, transmission spectra; comparison of the device performances; Lorentzian line shape function; fabrication processes of the detectors (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Y.C.). ORCID

Bingrui Lu: 0000-0001-7686-0734 Yifang Chen: 0000-0002-5746-2762 Author Contributions

B.F. and Y.C. planned the projects and designed the experiments. B.F. and J.Z. carried out the experiments. B.F., B.L., F.L., and L.Z. analyzed the data. B.F. wrote and edited the paper. Y.C. initiated the research topic and supervised all of the research. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Grant No. 61574043 and No. U1732104), the Fudan University-CIOMP Joint Fund (project number: FC2017-008), and the Basic Research Project of Shanghai Science and Technology Innovation Action (Grant No. 17JC1400300). REFERENCES (1) Lei, W.; Antoszewski, J.; Faraone, L. Progress, Challenges, and Opportunities for HgCdTe Infrared Materials and Detectors. Appl. Phys. Rev. 2015, 2, No. 041303. (2) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Biosensing with Plasmonic Nanosensors. Nat. Mater. 2008, 7, 442−53. (3) Willer, U.; Saraji, M.; Khorsandi, A.; Geiser, P.; Schade, W. Near- and Mid-Infrared Laser Monitoring of Industrial Processes, Environment and Security Applications. Opt. Laser. Eng. 2006, 44, 699−710. H

DOI: 10.1021/acsnano.9b04236 ACS Nano XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsnano.9b04236 ACS Nano XXXX, XXX, XXX−XXX