Silicon-Based Embedded Trenches of Active Antennas for High

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Silicon-Based Embedded Trenches of Active Antennas for HighResponsivity Omnidirectional Photodetection at Telecommunication Wavelengths Keng-Te Lin,† Chih-Jie Chan,† Yu-Sheng Lai,*,‡ Lung-Tai Shiu,† Ching-Che Lin,† and Hsuen-Li Chen*,†,§ Department of Materials Science and Engineering and §Center of Atomic Initiative for New Materials (AI-MAT), National Taiwan University, 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan ‡ National Nano Device Laboratories, National Applied Research Laboratories, 26, Prosperity Road I, Hsinchu 30078, Taiwan ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/10/19. For personal use only.

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S Supporting Information *

ABSTRACT: Although the use of plasmonic nanostructures for photodetection below the band gap energy of the semiconductor has been intensively investigated recently, efficiencies of such hot electron-based devices have, unfortunately, remained low because of the inevitable energy loss of the hot electrons as they move and transfer in active antennas based on metallic nanostructures. In this work, we demonstrate the concept of high-refractive-index material-embedded trench-like (ETL) active antennas that could be used to achieve almost 100% absorbance within the ultrashallow region (approximately 10 nm) beneath the metal−semiconductor interface, which is a much smaller distance compared with the hot electrons’ mean free path in the noble metal layer. Taking advantage of these ETL-based active antennas, we obtained photoresponsivities under zero bias at wavelengths of 1310 and 1550 nm of 5854 and 693 nA mW−1, respectivelyvalues higher than most those previously reported for active antenna-based silicon (Si) photodetectors that operate at optical telecommunication wavelengths. Furthermore, the ETL antenna strategy allowed us to preserve an omnidirectional and broadband photoresponse, with a superior degree of detection linearity of R2 = 0.98889 under the light of low power density (down to 11.1 μW cm−2). The photoresponses of the ETL antenna-based device varied by less than 10% upon changing the incident angle from normal incidence to 60°. Because these ETL-based devices provide high responsivity and omnidirectional detection over a broad bandwidth, they show promising potentials for use in hot electronbased optoelectronics for many applications (e.g., Si photonics, energy harvesting, photocatalysis, and sensing devices). KEYWORDS: plasmon, hot electron, high-refractive-index material, embedded, Schottky

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INTRODUCTION The outstanding optoelectronic, photochemical, and photothermal properties of hot electron-based devices have led to a wide variety of applications in many fields, including solar energy harvesting, 1−3 photodetection, 3−13 photocatalysis,2,14−16 sensing,17,18 and imaging technologies.19,20 In terms of photodetection, plasmonic nanostructures in contact with semiconductor materials can convert incident electromagnetic waves directly into electrical signals through the generation of plasmon-induced hot electrons. Such hot electron-based photodetectors can break through the native limitations of the semiconductor to allow photodetection at photon energy well below the band edge of the semiconductor.4−13 Many types of optically active antennas, including nanorods,4 nanowires,5 nanogratings,6 plasmonic metamaterials,9,10 and deep-trench/thin metal (DTTM) structures,8 on silicon (Si) substrates have been demonstrated for photodetection at telecommunication wavelengths. Knight et al. first demonstrated a nanorod antenna contacted with a Si substrate for photodetection in the infrared (IR) regime4 in © XXXX American Chemical Society

2011. This approach opened up the possibility of designing Sibased devices. Nevertheless, photoresponses of nanorod antenna-based devices have remained poor because such planar nanorod antennas on Si substrates have only one plasmonic decay path. Therefore, Knight et al. further proposed a method5 of embedding the nanowire antenna into the Si substrate to increase the number of paths of plasmonic decay and improve the photoresponse of antennabased devices.5 Efficient absorption is another very important factor for improving photoresponses of antenna-based devices. Sobhani et al. demonstrated a nanograting antenna for narrowband photodetection in the IR regime.6 This periodic nanograting antenna could couple the electromagnetic wave to the surface plasmons propagating through structures, resulting in high absorbance at resonant wavelengths. Almost all absorption Received: September 13, 2018 Accepted: December 27, 2018

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DOI: 10.1021/acsami.8b15914 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic representation of the energy band diagram and the transfer process of hot electrons over the metal−Si interface. Steps 1−3 reveal the generation, movement toward the metal−Si interface, and injection into the conduction band of the Si of the hot electrons. (b,c) Schematic representation of the optoelectronic behavior of hot electron-based detectors (b) reported in previous studies4−6,8−10 and (c) proposed herein. (d) Schematic representation of the different structures of TL and ETL antennas; the TL antenna is a 3D structure composed of a singlelayer DTTM film on the structured Si substrate and patterned with hexagonal symmetry; in contrast, the ETL antenna is the inverse TL structure covered with the HRI material (i.e., Si).

metamaterial-based device for photodetection in the IR regime.9 Their concept was to integrate the functions of a metamaterial structure and a Schottky diode into a so-called metamaterial perfect absorber (MPA)-based device. Such MPA structures can achieve high absorbances at designed wavelengths.9 Furthermore, the use of MPAs means that the optical absorption of such structures will not be sensitive to the incident angle or polarization state of the light,9 which are both important characteristics for a photodetector. Such promising characteristics would provide the optoelectronic devices featuring omnidirectional and polarization-insensitive photodetection capabilities for use in various practical applications, such as wearable electronics, functional clothes, and epidermal electronics.27,28 In general, however, exploiting the nanoscale features of MPA-based devices requires complicated manufacturing processes. In a previous study,8 we found that DTTM-based broadband devices exhibit many promising capabilities, such as superior photoelectronic conversion efficiency, high degree of

occurred near the interface of the nanograting antenna and Si. Accordingly, the nanograting approach improved the photoresponsivity of antenna-based devices up to approximately 600 nA mW−1 at zero bias voltage in the IR regime.6 Nevertheless, the optical properties of nanowire and nanograting antennas result in optoelectronic detectors featuring the mentioned structures exhibiting only narrowband photodetection and limited photodetection capability of specific polarization states. Thus, there are still many challenges in developing practical active antenna-based devices, particularly those providing high absorbance, broadband photodetection, and almost the same photoresponse toward distinct polarization states. Metamaterials are commonly used structures for manipulating the behavior of electromagnetic waves.9,10 Because most metamaterials allow the efficient absorbance and tailorable collection of electromagnetic waves, they have been applied in, for example, solar energy harvesting,21,22 photodetection,9,10 emissivity control,23,24 and thermal imaging technologies.25,26 In terms of photodetection, Li and Valentine demonstrated a B


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states (trap centers) at the Schottky interface, the hot electrons would be depleted and would not lead to the electrical signal. Accordingly, the behavior of hot electrons in each step will significantly affect the final number of hot electrons existed in the conduction band of the semiconductor and thereby directly influence the photoresponsivity of the hot electronbased device. Figure 1b provides a schematic of the optoelectronic behavior of hot electron-based detectors.4−6,8−10 In this case, the light was transmitted above the air−plasmonic nanostructure interface to the semiconductor substrate. As displayed in the inset of Figure 1b, the light with specific wavelengths couples through plasmonic nanostructures to generate surface plasmon polaritons (SPPs); subsequently, the hot electrons generated by plasmon decay then move in the structured metal layer toward the metal−semiconductor junctions. The movement of hot electrons in plasmonic nanostructures means that they will lose some of their energy obtained from the plasmon decay. Therefore, in previous studies,8,9 the structured metal films in hot electron-based devices have generally been designed to be thinner than the hot electrons’ mean free path in the metal, thereby minimizing the energy loss of the hot electrons in the metal. Nevertheless, some of the energy of the hot electrons will still be lost in the metal before they arrive at the metal−semiconductor interfaces. Accordingly, decreasing the distance moved by hot electrons toward the Schottky interfaces should improve the photoresponse of a hot electron-based device. Herein, we propose such a concept using an HRI material-embedded active antenna. Figure 1c displays the schematic of the optoelectronic behavior of an ETL-based device. In this strategy, the light was propagated from above the surface of the HRI material to its interface with the trench-like (TL) structure. Here, the HRI material should be a material that is transparent at the designed wavelength regime. For example, Si is an HRI material featuring a high refractive index of approximately 3.4830 at telecommunication wavelengths; furthermore, Si features a band gap energy of 1.12 eV, meaning that it is transparent in the broadband IR regime. Therefore, when such an ETL-based device is illuminated with the IR light, the light will transmit through the Si and interact directly with the metallic inverse-TL structuresthat is, the incident light would arrive at the Si−gold (Au) interfaces without passing through the metal layer on the structures. As shown in the inset of Figure 1c, when the incident light propagates into the inverse-TL structures, the SPPs are excited and then they decay to generate hot electrons, which are positioned very close to the Si−Au interfaces. Therefore, this strategy can effectively decrease the distance moved by hot electrons toward the Schottky interfaces. Furthermore, the inelastic collisions of hot electrons inside the metal film can be decreased significantly. The hot electrons could keep most of the energy obtained from plasmon decay as they pass across the Schottky interface, therefore, effectively contributing to the electrical signal. Accordingly, the performance of hot electronbased devices is improved when using this strategy. Figure 1d illustrates schematically the difference in structure of a TL antenna and an ETL antenna. The TL antenna (left-hand image in Figure 1d) is a 3D structure composed of only a single-layer DTTM film and patterned with hexagonal symmetry on the structured Si substrate, as we reported in a previous study.8 In this case, the TL antenna exploits the 3D cavity effect, the SPR phenomenon, and the large-area Schottky interfaces to efficiently gather hot electrons

detection linearity, and polarization-insensitive photodetection in the IR regime. The DTTM structures were, however, sensitive to the incident angle of light, caused by the phenomenon of surface plasmon resonance (SPR). The absorption peaks of the plasmonic DTTM structure would shift to longer wavelengths upon increasing the incident angle. In 2018, Wen et al. successfully demonstrated the hot electron harvesting in the plasmonic/photonic disordered nanocomposites.29 They not only proposed a simple method to fabricate hot electron-based devices featuring high conversion efficiencies but also clearly distinguished the photothermal heat relaxation behaviors and the photoelectric ejection phenomena in the metallic nanocomposites. Furthermore, they expected that the disordered nanocomposites would be insensitive to both the polarization states and angles of incidence. However, they did not discuss these parts in depth. Accordingly, a highly efficient hot electron-based device would have to possess the following properties: efficient absorbance over a broad bandwidth; large-area metal−semiconductor junctions; shortdistance movement of hot electrons in the structured metallic layer; and omnidirectional photoresponse. To the best of our knowledge, no hot-electron-based Si photodetectors operating at telecommunication wavelengths, and possessing all of the features above, have been proposed previously. In this article, the active antennas embedded in highrefractive-index (HRI) materials have been developed and demonstrated to efficiently improve photoresponsivities of hotelectron-based devices. The embedded trench-like (ETL) structures proposed herein exploits an HRI material (i.e., Si) to shorten the effective wavelength of the electromagnetic wave and uses the three-dimensional (3D) cavity and SPR effects to achieve efficient and broadband absorption. Furthermore, this approach significantly decreases the distance travelled by hot electrons inside the metal layer and maintains large-area Schottky interfaces to efficiently collect the generated hot electrons. With these hot electron-based Si detectors, taking advantage of the superior properties of ETL active antennas, we have obtained devices providing high photoresponsivities at telecommunication wavelengths while also displaying broadband and omnidirectional photoresponses.

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RESULTS AND DISCUSSION In general, the photoresponse of a hot electron-based detector can be understood as a three-step process9 involving light absorption, hot electron generation, and the transfer of hot electrons over the Schottky interface, as illustrated in Figure 1a. In step 1, the light would be absorbed by the structured metal film to generate plasmon-induced hot electrons. Thus, the optical absorption of plasmonic nanostructures initially determines the total amount of energetic hot electrons. In step 2, a portion of the energetic hot electrons makes movements in the metal layer and is transported to the Schottky interface prior to thermalization. In this step, the hot electrons may lose energy through inelastic collisions in the metal layer before they transport to the Schottky interface; thereby, the hot electrons that lose energy cannot convert into the electrical signals. In step 3, the hot electrons transfer across the Schottky barrier and then propagate into the conduction band of Si to generate an electrical signal. According to the modified Fowler theory,4 the hot electrons reaching the Schottky interface have a certain probability of being injected into the semiconductor. In addition, if there are some defect C


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At first, we studied the optical behaviors of ETL antennas having various structural parameters and optimized the maximum absorbance at the wavelength of 1550 nm. The spectral absorbance could be tuned readily by adjusting the period and width of hole; furthermore, the EH012P05 antenna displayed near-unity absorbance at 1550 nm and broadband absorbance (>80%) from 1250 to 1800 nm (for details, see Supporting Information Figure S1). Next, we investigated the optical behaviors of the TL and ETL antennas. Figure 2a presents the calculated spectral absorbances of the TL and ETL antennas. The H069P15 TL structure, having a trench depth of 1.2 μm and an Au film thickness of 30 nm, exploited the SPR phenomenon and the 3D cavity effect to sufficiently absorb the incident light at resonant wavelengths; thereby, it featured an absorption peak (approximately 85%) at 1540 nm. Except for both the SPR- and cavity-induced absorption effects, the TL structures can also confine and then absorb the light in the cavities by exploiting only the cavity effects at the nonresonant wavelengths. Therefore, compared with other active antennas, 4,6 this TL antenna exhibited higher absorbance over a broadband regime. Furthermore, in this study, the ETL antenna combined the functions of a TL antenna and an HRI material to efficiently absorb the incident light at the designed wavelengths. As displayed in Figure 2a, the absorptions of the EH012P05 ETL antenna, having a trench depth of 1 μm, were much higher than those of the H069P15 TL structure over a broadband spectral regime. Here, with the same structural geometries and dimensions, but with different depths of the inverse-TL metallic cavities, the maximum absorbance of the ETL structure, having a trench depth of 0.5 μm, was only slightly lower than that of the ETL structure having a trench depth of 1 μm. The absorptions of the ETL structure, having a trench depth of 0.5 μm, were, however, higher than those of the H069P15 TL structure over a broad bandwidth. We attribute the broadband and efficient absorption phenomenon to the ETL antenna, confining and absorbing incident light of distinct wavelengths within the inverse TL cavities. Here, the Si features a high refractive index (ca. 3.48)30 and is a transparent material at telecommunication wavelengths. Accordingly, the effective wavelength of incident light could be significantly shortened as the light propagates into Si. For example, the effective wavelengths of light in the telecommunication regime (wavelengths from 1250 to 1650 nm in air) are in the range from 360 to 475 nm when propagating in Si. Furthermore, the optical path length in the HRI material is much larger than that in air as the light propagates over the same physical length in both media.31 Thus, the ETL antenna maintained its good absorption ability even when the physical depth of the inverse trenches of the ETL antenna was lowered down to 0.5 μm. To study the absorption mechanism, the EH012P05 ETL structure, having a depth of inverse metallic cavities of 1 μm, was selected for simulation of its electric field distributions within the near-field regime at various wavelengths. Figure 2b shows distributions of electric field intensity as the incident light having wavelengths of 1310, 1550, and 1800 nm transmitting through the EH012P05 ETL antenna. Most of the light at each wavelength could propagate into the ETL structure and interact with it. Furthermore, the EH012P05 ETL antenna structure provided high-intensity electric fields within the near-field regime around the Si−Au interfaces at each wavelength. We attribute these phenomena to smaller differences in effective wavelength among the three wave-

originating from the plasmon decay; therefore, it provides superior optoelectronic properties. Nevertheless, most of the hot electrons should move a certain distance, passing through the metal layer, before arriving at the metal−semiconductor interfaces. In this study, the ETL antenna was an inverse TL antenna (right-hand image in Figure 1d), which possesses the inverse TL structure covered with the HRI material (i.e., Si). In general, the structural parameters might affect the optical behaviors of an ETL antenna, including the refractive index of the covered materials, the width of hole (H), period (P), the depth of the trenches (D), and the metal layer thickness (t). Accordingly, the absorption properties of the ETL-based devices can be manipulated by adjusting the structural parameters of the ETL antenna. Furthermore, this strategy means that the incident light would arrive at the Si−Au interfaces without passing through the metal layer on the inverse TL structures. Therefore, the ETL not only possesses all of the advantages of a TL antenna but also sufficiently shortens the distance moved by hot electrons inside the metal layer. Accordingly, we expected that the photoresponses of the ETL antenna-based devices would be much better than those of corresponding TL antenna-based devices. To analyze the absorption properties of the ETL antenna, we used the 3D finite-difference time-domain (FDTD) method to simulate the optical behavior of ETL antennas with various structural parameters. As displayed in Table 1, we Table 1. TL- and ETL-Based Devices Defined in Terms of Their Structural Dimensions TL-based devices

ETL-based devices

symbol

H065P13

H069P15

EH0098P045

EH012P05

hole width (μm) period (μm)

0.65 1.3

0.69 1.5

0.098 0.45

0.12 0.5

labeled the TL- and ETL-based devices in terms of their different sizes (period and width of hole); for instance, the device with a TL antenna is named “H065P13” to denote that it has the hole width of 0.65 μm and the period of 1.3 μm; in addition, the device featuring an ETL antenna with the hole width of 0.098 μm and the period of 0.45 μm is denoted by “EH0098P045.” The other active antennas in the TL- and ETL-based devices are named H069P15 and EH012P05, respectively. In simulations, we fix the plane waves with transverse electric (TE) polarization transmitting from 800 nm above the inverse-TL structures in the Si. We also set two detectors for each antenna: the first one is 0.1 μm below the Au−air interface (in air) and the second one is 0.2 μm above the incidence (inside the Si). These two detectors measured the transmittance (T) and reflectance (R) of ETL antennas, respectively; hence, the absorbance (A) could be calculated by using the equation A = 1 − R − T. In addition, we set up plane waves with TE polarization transmitting from 800 nm above the TL antenna structure on the Si substrate and two detectors for the TL antenna structure: one 0.1 μm below the Au−Si interface (inside the Si) and the other 0.2 μm above the incidence (in air). These two detectors detected the values of T and R of the TL antenna structure, respectively, allowing the calculation of the value of A of this structure. In the simulation, we fixed the Au film thickness of ETL and TL structures at 50 and 30 nm, respectively; the optical constants for Au and Si are obtained from the literature.30 D


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Figure 2. Optical behavior of TL- and ETL-antenna structures. (a) Simulated spectral absorbance of the TL- and ETL-antennas: the TL structure (black solid line), having a width of hole (H) of 0.69 μm, a period (P) of 1.5 μm, an Au thickness (t) of 30 nm, and a depth (D) of 1200 nm; the ETL structures having distinct dimensions D [1000 nm (red line); 500 nm (blue dot line)], with other structural parameters kept constant (H = 0.12 μm; P = 0.5 μm; t = 50 nm). (b) Simulated distributions of the electric field intensity as the light at wavelengths of 1310, 1550, and 1800 nm transmitting through the EH012P05 ETL structure, having a depth of inverse metallic cavities at 1 μm. (c−e) Calculated absorption maps of incident light at (c) 1310, (d) 1550, and (e) 1800 nm passing through the EH012P05 ETL structure, having a depth of inverse metallic cavities at 1 μm. Almost all of the absorption occurs close to the Au−Si interfaces. (f) Squares of the electric field amplitude (|E|2) at distinct positions in the metallic sidewalls, at a depth of 100 nm below the top surface, of the EH012P05 ETL antenna, at 1550 nm. All of the values of |E|2 generated in the Au layer and at surfaces have been normalized to their strongest value, which is generated at the surface of the ETL antenna. The values of |E|2 within the metallic sidewalls of the ETL structure decreased dramatically upon increasing the distance from the metal−semiconductor interfaces.

lengths in the Si. The dimensions of the ETL antenna could match the resonant conditions of the SPR and cavity effects to ensure a large absorption of incident light. As displayed in Figure 2b, the EH012P05 ETL structure displayed very low reflectance, near-zero transmittance, and near-unity absorbance (R/T/A = 0.58/0.01/99.41%) at a wavelength of 1550 nm. When we adjusted the wavelength to 1310 and 1800 nm, most of the incident light can still transmit into the inverse cavity structures and then be absorbed by the ETL structure, with little incident light reflected back (R/T/A = 19.16/0.13/ 80.69% at 1310 nm; R/T/A = 13.76/0.03/86.21% at 1800 nm). Accordingly, the ETL antenna could exploit the SPR phenomenon, the cavity effects, and the small differences in

effective wavelengths in the HRI material to achieve broad and efficient absorption at telecommunication wavelengths. Figure 2c−e displays the absorption maps obtained when simulating incident wavelengths of 1310, 1550, and 1800 nm, respectively, propagating through the EH012P05 ETL antenna, having a depth of inverse metallic cavities of 1 μm. At shorter wavelengths (1310 and 1550 nm), the light can transmit into the inverse metallic cavities, and then gradually be depleted by the structured Au film (Figure 2c,d). Therefore, the high absorbance region extended along the sidewalls of the cavities to the bottom. When we adjusted the wavelength to 1800 nm, the effective wavelength was approximately 520 nm, close to the half dimension of the hole width. The incident light would resonate strongly when it propagated into metallic E


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Figure 3. Omnidirectional photodetection capability of an ETL-based device. (a) Schematic representation of the IR light traveling from air into the ETL antenna structure. The relationship between the incident angles (in air) and refraction (inside the Si) were described by Snell’s law. (b−d) Angle-dependent absorption spectra of the EH012P05 ETL antenna under illumination of IR light with (b) TE-, (c) TM-, and (d) random polarization states. (e,f) Absorbances at wavelengths of (e) 1310 and (f) 1550 nm by the EH012P05 ETL antenna under illumination of the IR light with TE-, TM-, and random polarization states at various incident angles.

interfaces. Moreover, the distances moved by the energetic hot electrons were much shorter than the mean free path of hot electrons in Au (ca. 35 nm).32,33 Therefore, we expected that the ETL antenna-based strategy would efficiently enhance the performance of hot electron-based devices. In addition, we used 3D-FDTD simulations to investigate the omnidirectional photodetection capability of the ETLbased devices. In general, light will change its direction of propagation in response to a change in its transmission medium. Furthermore, Snell’s law explains the relationship between the incident angles and refraction when light propagates through an interface between two different media34

cavities. As displayed in Figure 2e, the high absorbance region was close to the top surface of the ETL structures. Moreover, the calculated absorption maps at these wavelengths reveal that almost all of the absorption occurred very close to the Au−Si interfaces. Next, we investigated the absorption distributions within the structured metal layer. Figure 2f displays the simulated squares of the electric field amplitudes (|E|2) at different positions within the ETL structure at a wavelength of 1550 nm. As depicted, several detectors are fixed within the metallic sidewalls of the ETL structure at a depth of 100 nm below the top surface to record the values of |E|2. We normalized all of the values of |E|2 at these different positions relative to the highest value of |E|2 generated at the surface of the ETL structure. The value of |E|2 on the surfaces of the metallic sidewalls of the ETL structure was larger than that inside the metal layer. Furthermore, the values of |E|2 within the sidewalls of the ETL antenna decreased to 9% at only 10 nm away from the Au−Si interfaces. According to previous studies, the values of |E|2 should be proportional to the input energy flux density. The values of |E|2 decreased dramatically within 10 nm of the metal layer, implying that not only did almost all of the absorption occur within the shallow region (ca. 10 nm) beneath the metal−semiconductor interfaces but also that less of the light energy could propagate through the deep region beneath the Au interface. Therefore, most of the hot electron generation would occur at the Schottky (Au−Si)

n1 sin θ1 = n2 sin θ2

(1)

where n1 and n2 are the refractive indices of the respective materials, and θ1 and θ2 are the angles measured from the normal of the interface at each material. Here, we exploited materials featuring a high refractive index to change the direction of light propagation prior to the light interacting with metallic structures. Figure 3a provides a schematic representation of IR light traveling from air into the ETL antenna structure. According to Snell’s law, the angles of incidence (θ1) at which IR light travels through air (n1 = 1) in the range from 0 to 89° correspond to angles of refraction (θ2) at which IR light travels into the Si [n2 = 3.48] in the range only from 0 to 16.69°. Thus, a small variation in the direction of light F


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Figure 4. Electrical properties of ETL-based devices. (a) Schematic of the ETL-based plasmonic device. (b) Top-view and cross-sectional FESEM images of ETL antennas having distinct hole widths and periods. (c) Current−voltage properties of an ETL-based device measured in the darkness; the dark current density of the device was around 13 nA cm−2 at zero bias. (d) Responsivity spectra of the proposed ETL-based device and of the DTTM-based device described in our previous study.8 (e) Correlation of the excess current with light power for the EH012P05 ETL-based device at 1550 nm. In the power density range of 10−50 μW cm−2, the excess currents of the ETL-based devices displayed a high linear response of R2 = 0.98889. (f) Angle-dependent photoresponses of ETL-based devices at 1310 and 1550 nm.

propagation occurs in Si even when the angles of incidence in air are large. On the basis of this concept, the ETL antenna structures are not sensitive to the angles of incidence and thereby readily maintain the absorption properties in the IR regime. Figure 3b−d displays the maps of absorbance spectra simulated at various incident angles for the EH012P05 ETL structure under TE-, transverse magnetic (TM)-, and randomly polarized light, respectively. Herein, we directly averaged the absorbance of the TE- and TM-polarized light to calculate the absorbance of the randomly polarized light. On increasing the incident angle, the absorption properties of the ETL structure for TE-polarized light were maintained well (Figure 3b). As displayed in Figure 3e,f, the absorbances at wavelengths of 1310 and 1550 nm were maintained at above 80 and 90%, respectively, for angles of incidence of up to 80°

under TE-polarized light. Furthermore, the absorption behaviors of the EH012P05 ETL antenna varied gradually for the TM-polarized light on increasing the incident angle (Figure 3c). This behavior is attributed to the electric field perpendicular to vertical metallic structures diminishing the light of TM polarization states under the condition of oblique incidence; thus, the interaction between the light and the ETL antenna structure decreased and, accordingly, the absorption also slightly decreased. Nevertheless, the absorbances at wavelengths of 1310 (Figure 3e) and 1550 (Figure 3f) nm were maintained at greater than 70% for incident angles of 80 and 70°, respectively, under the TM-polarized light. Moreover, for the light-collection capability under the light of random polarization states, the most practical situation in the applications of hot electron-based devices, the spectral G


Research Article

ACS Applied Materials & Interfaces absorbance of the ETL antenna structure varied slightly on increasing the incident angle (Figure 3d). Absorbances at wavelengths of 1310 (Figure 3e) and 1550 (Figure 3f) nm remained nearly constant and decreased only slightly, respectively, for angles of incidence of up to 80° under the light of random polarization states. Therefore, although the absorption properties of the ETL antenna are angle-dependent for the light of TM polarization, the ETL antenna exhibited efficient and omnidirectional absorption under the light of random polarization states over a broad wavelength regime. Accordingly, we expected that ETL-based devices would exhibit high photoresponsivities while also preserving omnidirectional and broadband responses at telecommunication wavelengths. The proposed plasmonic ETL-based device in this work is a Schottky junction photodetector consisting of a single-layer/ inverse TL metallic structure, an Au−Si Schottky junction on a Si substrate, a single-layer antireflection coating (ARC), and an electrode ohmic contact on the back side of the Si substrate (Figure 4a). The ARC layer positioned on the back side of the Si substrate could effectively decrease the reflection of Si at telecommunication wavelengths. Therefore, most of the light could propagate into the Si and then interact with the ETL antenna structures. Details of the fabrication process and the measurement setup are shown in the Methods section (more details, please see the Supporting Information). Figure 4b displays the top-view and cross-sectional field-emission scanning electron microscopy (FESEM) images of ETL antennas, featuring different hole widths and periods with hexagonal symmetry. As displayed in top-view FESEM images, the designed TL structures with various hole widths and periods were etched into the Si surface. In addition, the crosssectional FESEM images demonstrate that the surface of the TL structures is well-covered by the Au layer, and the thickness of the coated Au films are approximately 50 nm. The current− voltage (I−V) properties of the ETL-based devices were measured to demonstrate the rectification characteristics of the metal−semiconductor junction. Figure 4c presents the I−V properties of the ETL-based devices measured in the darkness. Although the ETL-based device had undergone the dry etching process, the dark current density of the device was around 13 nA cm−2 at zero bias. Herein, based on the measured I−V characteristics, further characterization of the barrier height and the quality of the fabricated metal−semiconductor interface of the ETL antenna-based device was carried out. In general, the thermionic I−V properties of a Schottky diode at a bias voltage could be described by eq 235 I = Is[e(qV / nkT ) − 1]

Therefore, we conclude that the Au−Si interfaces were not destroyed during fabrication processes. The low dark-current density and near-unity ideality factor mean that only a few defects were present at the Schottky interfaces. The energetic hot electrons could transfer across the metal−semiconductor interface, instead of depleting at the trap centers and then contributing to the photocurrent. Next, the photocurrents of the EH012P05 ETL-based device were measured under illumination with the IR light (the wavelength regime from 1250 to 1650 nm) and zero bias to calculate the spectral photoresponsivities with a unit of nA mW−1 from the measured data (for details, please see the Methods section in the Supporting Information). In Figure 4d, we compared the data for the H065P13 DTTM-based device (reported in our previous paper8) with those of the ETL-based device obtained in this study. The responsivities of the ETL-based device under zero bias were approximately two to five times higher than those of the DTTM-based device at telecommunication wavelengths. We attribute this behavior to both the broadband absorption enhancement in the IR regime and effective shortening of the distance moved by the hot electrons inside the metal layer. Figure 4e displays the excess currents of the EH012P05 ETL-based device under illumination with IR light of a wavelength of 1550 nm at distinct power densities (tungsten lamp; 10 to 50 μW cm−2). The degree of linearity between the excess current and the incident power density of light is as high as R2 = 0.98889. Furthermore, the linear property of the excess current could be maintained even after decreasing the incident power density of light down to 11.1 μW cm−2. Therefore, the plasmonic ETL-based devices were extremely sensitive to the incident power of the IR light. To further study the characteristics of the ETL-based devices, we measured their excess currents under illumination with IR light of random polarization states at 1310 and 1550 nm and at various incident angles. As shown in Figure 4f, the excess currents of the ETL-based device varied only slightly upon varying the incident angle from the normal incidence to 60° under the randomly polarized light at both wavelengths (1310 and 1550 nm). The tendency of measured currents of fabricated ETL-based active antenna under oblique incidence was slightly different to that of simulated results (Figure 3e,f), presumably because the shapes of the TL structures in the real ETL-based devices (with taper shape of structures) fabricated by lithography and etching processes were different from those in the simulation setups. Nevertheless, such small variations in photoresponse at different angles of incidence could be conducive to developing a device capable of omnidirectional photodetection. Thus, the devices featuring the ETL-based antenna structure displayed high photoresponsivities while maintaining omnidirectional and broadband responses at telecommunication wavelengths. Table 2 compares the performances and structural types of previously reported hot electron-based photodetectors with those of our new ETL-based devices4,6,8−10,29,38 operating in the IR regime. Recently reported hot electron-based devices have exhibited responsivities in the range from 8 to 5100 nA mW−1 at 1310 nm and from 1.5 to 1400 nA mW−1 at 1550 nm. These Si-based devices featuring plasmonic nanostructures, including nanorods, nanogratings, metamaterials, and DTTM antennas, use the generated hot electrons for photodetection below the band gap of Si. Nevertheless, the great majority of hot electrons generated in the plasmonic nanostructures of these devices requires hot electrons to move approximately the

(2)

where q denotes the absolute value of the electronic charge, n denotes the ideality factor, k denotes the Boltzmann constant, T denotes the absolute temperature, and Is denotes the reverse saturation current and is given by eq 335 Is = AA*T 2e−(qφB / kT )

(3)

where A is the area of diode, A* denotes the Richardson constant and the value36,37 is 120 A cm−2 K−2 for n-type Si, and φB denotes the barrier height of the Schottky diode. The calculated barrier height and ideality factor of the ETL antenna-based device are approximately 0.73 eV and 1.06, respectively. The ideality factor of Schottky diode near unity indicates the good quality of metal−semiconductor interface.35 H



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Table 2. Properties (Structural Types and Photoresponsivities) of Recently Reported Hot ElectronBased Si Detectors Operated at Optical Telecommunication Wavelengths hot electron-based device

responsivity (nA/mW)

structure type

λ@1310 nm λ@1550 nm

nanorod antenna nanograting antenna MPA chiral metamaterial disordered nanocomposite random gold nanoparticles absorber DTTM antenna ETL antenna

ca. ca. ca. ca. ca. ca.

8 75 2800 2100 5100 1500

2688 5854

ca. 1.5 ca. 475

1400 ca. 140 167 693

8 this work

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b15914. Methods and simulated spectral absorbance of ETL antennas having distinct dimensions of structural parameters (PDF)

refs 4 6 9 10 29 38

Research Article

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AUTHOR INFORMATION

Corresponding Authors

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

Hsuen-Li Chen: 0000-0002-7569-572X Author Contributions

K.T.L., H.L.C., and Y.S.L. contrived the idea and designed the experiments. K.T.L. conducted the optical simulations; K.T.L., L.T.S., C.J.C., and C.C.L. prepared the samples and measured optoelectronic characteristics of devices. K.T.L. and H.L.C. wrote the paper. All authors contributed to data analysis and scientific discussion.

distance of the metal’s thickness before arriving at the metal− semiconductor interfaces. In contrast, the ETL-based devices we developed in this study exhibit a superior capability for generating and collecting the hot electrons. In this strategy, the energetic hot electrons are generated close to the metal− semiconductor interfaces and thereby move relatively short distances to arrive at the Schottky interfaces; they then transfer across the metal−semiconductor interface and contribute directly to the electrical signals. In this study, photoresponsivities of the ETL-based device were 5854 and 693 nA mW−1 at 1310 and 1550 nm, respectively; these values are higher than most of those described in previous reports. Because it provides high responsivities and omnidirectional and broadband responses, the strategy demonstrated in this work should be readily useful for the applications in energy harvesting, photocatalysis, and sensing technologies.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Ministry of Science and Technology, Taiwan, under contracts MOST 106-2221-E-002158-MY3, MOST 106-2221-E-002-105-MY3, 107WFA0410904, and NTU-107L9008.

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CONCLUSIONS In conclusion, the first HRI material-embedded active antennas for photodetection, well below the band gap of Si, at optical telecommunication wavelengths, have been developed and experimentally demonstrated. Realizing near-unity absorption within the ultrashallow region (approximately 10 nm) beneath the metal−semiconductor interfaces allows the energy loss of hot electrons to be decreased significantly in the active antenna structures. We have measured high photoresponsivities under zero bias at wavelengths of 1310 and 1550 nm: 5854 and 693 nA mW−1, respectively; these values are higher than those of the most published active antenna-based photodetectors. Moreover, the ETL antenna strategy allowed us to preserve an omnidirectional and broadband photoresponse, with a superior degree of detection linearity (R2 = 0.98889) under light of low power density (down to 11.1 μW cm−2). Photoresponses of the ETL antenna-based devices varied by less than 10% when the incident angle varied from normal incidence to 60° under randomly polarized light. Therefore, such compact ETL-based devices have a great potential for integration into onchip optoelectronics in a variety of photovoltaic, photocatalysis, and sensing systems.

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REFERENCES

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The designed ETL-based devices were fabricated by the conventional semiconductor manufacturing processes (for details, please see the Methods section in the Supporting Information). I


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Silicon-Based Embedded Trenches of Active Antennas for High

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