Synergistic Effects of Plasmonics and Electron Trapping in Graphene

Dec 22, 2016 - In particular, a vertical built-in field is employed in the graphene channel for trapping the photoinduced electrons and leaving holes ...
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Synergistic Effects of Plasmonics and Electron Trapping in Graphene Short-Wave Infrared Photodetectors with Ultrahigh Responsivity Zefeng Chen,† Xinming Li,*,† Jiaqi Wang,† Li Tao,† Mingzhu Long,† Shi-Jun Liang,‡ Lay Kee Ang,‡ Chester Shu,† Hon Ki Tsang,† and Jian-Bin Xu*,† †

Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong SAR, China SUTD-MIT International Design Centre (IDC), Engineering Product Development, Singapore University of Technology and Design, 8 Somapah Road, Singapore 487372

‡

S Supporting Information *

ABSTRACT: Graphene’s unique electronic and optical properties have made it an attractive material for developing ultrafast short-wave infrared (SWIR) photodetectors. However, the performance of graphene SWIR photodetectors has been limited by the low optical absorption of graphene as well as the ultrashort lifetime of photoinduced carriers. Here, we present two mechanisms to overcome these two shortages and demonstrate a graphene-based SWIR photodetector with high responsivity and fast photoresponse. In particular, a vertical built-in field is employed in the graphene channel for trapping the photoinduced electrons and leaving holes in graphene, which results in prolonged photoinduced carrier lifetime. On the other hand, plasmonic effects were employed to realize photon trapping and enhance the light absorption of graphene. Thanks to the above two mechanisms, the responsivity of this proposed SWIR photodetector is up to a record of 83 A/W at a wavelength of 1.55 μm with a fast rising time of less than 600 ns. This device design concept addresses key challenges for highperformance graphene SWIR photodetectors and is promising for the development of mid/far-infrared optoelectronic applications. KEYWORDS: plasmonics, short-wave infrared photodetector, graphene, ultrahigh responsivity, fast photoresponse

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enhanced, and the separation of photoinduced carriers requires the reaction to occur on a subpicosecond time scale.18 To overcome these two problems, other light-absorbing media (including quantum dots, carbon nanotubes, and nanoplates) have been used to absorb infrared light for the generation of photoinduced carriers, and graphene has been used as a conduction channel.19−21 These SWIR photodetectors exhibit high photoconductivity gain because the lifetime of photoinduced carriers is largely prolonged by the strong photogating effect and the carriers’ transit time in graphene channel is very short. However, the response time and spectral range of these graphene SWIR photodetectors are restricted because the light absorption relies on other absorption materials instead of graphene. The optimal strategy to achieve high responsivity with fast photoresponse is to improve the generation rate of photoinduced carriers in graphene while maintaining the appropriate carrier lifetime in SWIR photodetector. In this work, we demonstrate a graphene SWIR photodetector with high responsivity and fast photoresponse through

hort-wave infrared (SWIR) photodetectors, covering wavelengths from 1.0 to 3.0 μm, are highly desired for various applications, e.g., biological imaging, remote control, and telecommunication.1,2 Over the past few years, detection of SWIR photons generally relies on narrow-bandgap semiconductor compounds such as InxGa1−xAs, InSb, and HgTe since silicon has an upper limited detection of approximately 1.1 μm wavelength.3,4 However, these semiconductors are not only high-cost but also environmentally unfriendly (e.g., arsenic and mercury). Beyond the photoinduced carriers in a semiconductor, a continuous flow of hot electrons generated on a gold thin film by photon absorption (or internal photoemission) can also be used for photodetection and can be amplified by localized surface plasmon resonance.5−7 Recently, graphene’s unique electronic and optical properties, such as zero band gap and ultrahigh carrier mobility, have attracted attention. Broadband and ultrafast photodetectors based on graphene have been developed.8−16 However, the photoresponse of graphene photodetectors is limited by its obvious intrinsic characteristics: low light absorption (∼2.3%) of the atomic thickness of graphene17 and an ultrashort lifetime of photoinduced carriers.8,9 In order to generate a high photocurrent, the light absorption has to be © 2016 American Chemical Society

Received: September 12, 2016 Accepted: December 22, 2016 Published: December 22, 2016 430

DOI: 10.1021/acsnano.6b06172 ACS Nano 2017, 11, 430−437

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Figure 1. Fabrication process to integrate Au NP array with graphene SWIR photodetector. (a) Fabricate Au NP array using monolayer polystyrene (PS) spheres as a mask and transfer the Au NP array to graphene SWIR photodetector. (b) Scanning electron microscopy (SEM) image of the monolayer PS spheres on SiO2 substrate obtained by self-assembly after annealing. (c) Au NP array on SiO2 obtained by depositing gold using monolayer PS spheres as a mask. (d) Au NP array after transfer onto graphene. (e) Schematic diagram of the concept of graphene SWIR photodetector.

Concept of the Vertical Built-in Field in Graphene SWIR Photodetector. Unlike other typical hybrid graphene SWIR phototransistors using a light-absorbing medium for the generation of photoinduced carriers,19−21 in our device, photoinduced carriers are provided in graphene only and the plasmonic effect is introduced for a light-trapping effect12,25,26 to enhance the generation of photoinduced carriers in graphene. On the other hand, the silicon in this device is used for forming a built-in field at the interface through the graphene−silicon Schottky junction27−30 rather than to provide photoinduced carriers. The barrier height and the ideality factor of the Schottky junction are about 0.5 eV and 1.4 (Figure S2), respectively, and the effect of the Au NPs array on the electrical behavior of Schottky junction is tiny.31−35 Compared with the traditional graphene photodetector,11 the built-in field can trap electrons (holes) into silicon and reduce the recombination of electrons and holes. Under SWIR light illumination (out of the absorption region of silicon, wavelength large than 1100 nm), the light with wavelength matching with plasmonic resonance will be trapped by Au NPs and absorbed by graphene, the photoinduced carriers generated in graphene will be separated by the built-in field, and one kind of carriers (electron or hole) will be swept in silicon, leading to the net increase of carrier concentration. The schematic diagram is shown in Figure 1e. Here, we emphasize again that this design concept is totally different from previous works. The key points are that (1) silicon provides built-in field but not photoinduced carriers, (2) graphene provides photoinduced carriers through plasmonic enhanced light absorption, which results in a photoresponse at SWIR wavelength (e.g., 1550 nm), and (3) the graphene device works in conductor mode but not diode mode, which provides a high gain. Plasmonic Effect of Triangle-like Au NPs. It is wellknown that the resonance wavelength of the plasmonic effect is related to the Au NPs’ size and shape as well as the dielectric constant of the substrate.23,24 Here, by controlling the annealed time of monolayer PS spheres, the trilateral size of the trianglelike Au NPs can be adjusted, accompanied by the tunable resonance wavelength of the plasmonic effect.Three monolayer PS sphere samples with annealed times of 30, 60 and 120 s in 120 °C were fabricated, respectively. During the annealing, the PS spheres are softened, and the hexagonal close-packed spheres begin to bond together as an extension of annealed time, resulting in shrinking of the interspace between every

introduction of two mechanisms to improve the photoinduced carriers’ generation and separation, representing a device design concept. The plasmonic effect is employed in the device to generate a light-trapping effect and enhance the light absorption of graphene. On the other hand, a vertical built-in field is employed onto the graphene channel to sweep the photoinduced electrons out of graphene, leaving holes that result in a prolonged photoinduced carrier lifetime. This process is similar to the carrier-trapping effect. Thanks to the synergy of the above two mechanisms, experimental results show that the photocurrent can be enhanced by nearly 10 times with responsivity up to 83 A/W at a wavelength of 1550 nm after introduction of the plasmonic effect, which is the highest reported responsivity among the graphene-based SWIR photodetector at this wavelength thus far. Furthermore, this graphene SWIR photodetector enables a fast rising time less than 600 ns, which is also faster than other graphene hybrid SWIR photodetectors reported.

RESULTS AND DISCUSSION Graphene SWIR Photodetector Fabrication and Characterization. The plasmonic effect is introduced by the Au nanoparticle (NP) array, which is fabricated by nanosphere lithography.22−24 Figure 1a shows the schematic process of fabricating the plasmonic nanostructure coupled graphene SWIR photodetector. First, polystyrene (PS) spheres with a diameter of 700 nm spontaneously formed a monolayer uniform film by using a surface assembly method on water and could be transferred onto the SiO2/Si substrate. Figure 1b shows a well-ordered close-packed structure of monolayer PS sphere with a triangle-like hole between every three PS spheres. Then 30 nm gold film was deposited onto the PS spheres. The PS spheres were removed using tape, and the Au NP array was left on the SiO2/Si substrate. The atomic force microscopy (AFM) image shows a hexagonal arrangement of triangle-like Au NPs with a period of 700 nm (Figure 1c), which corresponds to the size of a monolayer close-packed PS sphere. The Au NP arrays were then transferred to the reprepared graphene photodetector with a wetting transfer method. After transfer onto the graphene photodetector, the triangle-like Au NPs array structure is well kept with the same morphology (Figure 1d). Raman spectra of graphene (Figure S1) show that the property of graphene is also well preserved after being covered with a gold nanoparticle array. 431

DOI: 10.1021/acsnano.6b06172 ACS Nano 2017, 11, 430−437

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Figure 2. Characteristics of the plasmonic effect of triangle-like Au NPs array. (a) Morphology of triangle-like Au NPs with a different annealing time of monolayer PS spheres, from left to right, patterns 1, 2, and 3. (b) Experimental transmission spectra of different triangle-like Au NP arrays corresponding to the morphology shown in (a). (c) Simulation results of triangle-like Au NP arrays on graphene−silicon heterojunction. The solid lines and the dashed lines are the transmission and absorption spectra, respectively. (d) Light intensity at the surface of silicon versus the incident light’s wavelength. Inset: light intensity distribution at the cross section. (e) Simulated near-field electric field intensity of triangle-like Au NPs for two indicate the polarization of excitation.

about 24%, which is enhanced about by 10 times compared to the 2.3% absorption of pure monolayer graphene. This absorption enhancement can be attributed to the light-trapping effect of the plasmonic Au NPs. Once the surface plasmons around Au NPs are excited, the local electric field around the plasmonic nanostructure will be dramatically modified. As a result, the local electric field is enhanced compared to the incident electromagnetic wave. Under the action of the local electric field, the transition probability for an electron from valence band to conductor band can be calculated based on Fermi’s gold rule,

three spheres (Figure S3). Therefore, the trilateral size of triangle-like Au NPs decreases as an extension of the annealed time of PS spheres. Here, patterns 1−3 of the triangle-like Au NPs array correspond to the PS spheres’ annealing time of 30, 60 and 120 s, respectively (Figure 2a). In theory, with the different trilateral sizes of triangle-like Au NPs, we can achieve plasmonic resonance at different wavelengths. To investigate the plasmonic resonance characteristics, spectral analysis was used to study the scattering characteristics of these triangle-like Au NPs array on silicon substrate and glass substrate, respectively. Figure 2b shows the transmission spectra of the three corresponding triangle-like Au NP array samples after transfer to the silicon substrate. The plasmonic resonance peaks are found at wavelengths of about 1570, 1750, and 1820 nm for patterns 3, 2, and 1, respectively. Meanwhile, the plasmonic resonance wavelengths for the three triangle-like Au NP arrays on the glass substrate are blue-shifted to 800, 900, and 950 nm (Figure S4).23 The characteristics of these spectra can be understood by employing an effective medium model in which an effective dielectric function of a system of isolated nanostructures is given by a sum of Lorentzians. 36 More discussion about the transmission characteristics can be found in Figure S4. Simulation of the Plasmonic Triangle-like Au NPs. To confirm the plasmonic effect of these triangle-like Au NPs, the transmission and absorption spectra of triangle-like Au NP arrays on graphene−silicon substrates were simulated using commercial software Comsol as shown in Figure 2c. The main plasmonic resonance peaks are found in the SWIR light region, which is consistent with the experimental results. Beyond the main resonance peak, there are some small resonance peaks in the visible light region, which corresponds to a higher order resonance mode or hybrid mode. The absorption spectra show that near the plasmonic resonance wavelength (1570, 1750, and 1820 nm) the light absorption of graphene is increased up to

ρT =

2π ℏ

⟨n′, k′|H1(Es0)|n , k⟩|2 fi (1 − f f )δ(Ef − Ei − ℏω),

where ρT is the transition probability, Es0 is the amplitude of electric field in the presence of plasmonic effect, ℏ is Planck’s constant, |n,k⟩ is the wave function, and f i is the Dirac Fermi distribution. The absorption induced by the interband e 2 |E |2

transition can be given by A = ℏωρT = 2ℏs0 . If the electric amplitude of incident light is E0, the incident energy flux is Wi = 2|E0|2/μ0c. Thus, the absorption efficiency of graphene is A /Wi =

|Es0|2 |E0|2

πα = β × 2.3% where α is the fine structure

constant and β is the enhancement factor of electric field induced by the plasmonic effect. To obtain the enhancement factor, we first simulate electric field distribution near the triangle-like Au NPs at a wavelength of 1550 nm (the inset in Figure 2d). The distribution indicates that light intensity is enhanced near the triangle-like Au NPs array. We further integrated field intensity at the surface of silicon to achieve average light intensity versus the incident light’s wavelength, shown in Figure 2d. Around the resonance peak, the intensity of electric field with the assistance of plasmonic effect is enhanced by about 10 times compared with that without Au NPs. Therefore, the absorption efficiency of graphene, in this 432

DOI: 10.1021/acsnano.6b06172 ACS Nano 2017, 11, 430−437

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ACS Nano

Figure 3. Photocurrent and photoresponsivity measurement of the plasmon resonance enhanced graphene SWIR photodetectors. (a) Photocurrent versus bias voltage of graphene SWIR photodetector under different incident light powers at a wavelength of 1550 nm. (b) Photocurrent as a function of incident light power and electric bias at for a graphene SWIR photodetector. (c) Photoresponse of the devices versus illumination power. The solid circle and open circle represent the graphene SWIR photodetector with and without triangle-like Au NPs array, respectively. (d) Photoresponse spectrum of the graphene SWIR photodetector with the different trilateral size of triangle-like Au NPs array at the infrared region.

graphene channel and metal−graphene interaction region at the electrodes.30 At the applied voltage of 10 V, the photocurrent increases from 0.025 to 3.8 mA when the incident power varies from 0.3 μW to 7.2 mW, as shown in Figure 3c. In particular, this photocurrent is high enough for direct measurement without any amplifier. Compared to the graphene SWIR photodetector without a triangle-like Au NPs array (Figure 3c), both the photocurrent and responsivity are enhanced by nearly 10× after introduction of the plasmonic effect of the trianglelike Au NPs array. The calculated responsivity of graphene SWIR photodetector for an illumination wavelength of 1550 nm is up to 83 A/W, which is the highest reported responsivity at this wavelength among the graphene SWIR photodetector. In addition, the noise−equivalent−power (NEP) of the graphene SWIR photodetector can be achieved to about 10−10 W/Hz1/2 at a modulation frequency of 1 Hz, and the corresponding detectivity is about ∼108 cm Hz1/2 W−1 (Figure S5),40 which are comparable with photodetector based on the same working mode (photoconductive effect), such as PbSe (∼109 cm Hz1/2 W−1 at 1550 nm) and can be further optimized by reducing the dark current through further device design. The photoconduction gain also shows an improvement after introduction of the plasmonic effect, shown in Figure S6. Furthermore, the spectral-dependent photoresponse of the SWIR photodetector with the varying trilateral sizes of the triangle-like Au NPs is shown in Figure 3d. The spectraldependent photocurrent exhibits photoresponse peaks at wavelengths around the corresponding plasmonic resonance peaks of the triangle-like Au NPs array (around 1570, 1750, and 1850 nm). This result strongly suggests that the photoresponse enhancement can be attributed to the plasmon resonance of triangle-like Au NP arrays in the graphene SWIR photodetector. With this approach, spectral selective photodetection

case, can be up to 24%, which agrees with the absorption spectra. Normally, the plasmonic eigenmodes of the triangle-like Au NPs can be approximately reduced to the charges at the vertex of the structure because the induced charges upon plasmon excitation are concentrated at the vertex, while the induced charge density in the center of each structure is always very small. For a triangle-like Au NP, only two independent variables Qi (charges density at the ith vertex) exist due to the conservation of charge required that ΣQi = 0. A set of variables {Q1, Q2} gives an eigenvector of the plasmonic mode, and these eigenvectors can be chosen to be orthogonal. Thus, there exist two plasmon eigenmodes for a triangle-like Au NP. The two orthogonal modes are shown in Figure 2e. However, these two modes degenerate because the triangle-like Au NPs belongs the C3v symmetry group, similar to the previous results.37−39 Thus, only one resonance peak is observed in the transmission spectra. Photoresponse of Graphene SWIR Photodetector. To quantify the plasmonic effect of the triangle-like Au NP array enhancement on the performance of the graphene SWIR photodetector, we measured the photoresponse of the SWIR photodetector as a function of incident light power and applied voltage, plotted in Figure 3a,b. Here, the photocurrent is defined as the current subtracting the dark current from the light current (Ilight − Idark). Figure 3a shows that the photocurrent is increased with increasing incident light power at a wavelength of 1550 nm. In addition, the photocurrent is almost linearly dependent on the applied voltage, which shows a tunability of responsivity, and the higher response can be readily achieved by applying a higher bias voltage, shown in Figure 3b. the nonlinear behavior in the low voltage region (