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Synergistic Effects of Plasmonics and Electrons Trapping in Graphene Short-Wave Infrared Photodetectors with Ultrahigh Responsivity Zefeng Chen, Xinming Li, Jiaqi Wang, Li Tao, Mingzhu Long, Shijun Liang, LAY KEE ANG, Chester C. T Shu, Hon Ki Tsang, and Jian-Bin Xu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b06172 • Publication Date (Web): 22 Dec 2016 Downloaded from http://pubs.acs.org on December 27, 2016
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Synergistic Effects of Plasmonics and Electrons 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
Keywords: Plasmonics; Short-wave infrared photodetector; Graphene; Ultrahigh responsivity: Fast photoresponse; 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 graphene-based SWIR photodetector with high responsivity and fast photoresponse. Particularly, a vertical built-in field is employed onto the graphene channel for trapping the photo-induced electrons and leaving holes in graphene, which results in the prolonging of photoinduced carrier lifetime. On the other hand, plasmonic effect 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
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of 83 A/W at the wavelength of 1.55 µm with a fast rising time less than 600 ns. This device design concept addresses key challenges for high-performance graphene SWIR photodetector and is promising for the development of mid/far infrared optoelectronic applications.
Short-wave infrared (SWIR) photodetectors, covering from 1.0 µm to 3.0 µm wavelength, are highly desired for various applications, e.g., biological imaging, remote control, and telecommunication.1,2 Over the past few years, the 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 environmental 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 Recent year, graphene’unique electronic and optical properties, such as the zero band gap and ultrahigh carriers mobility, have attracted people's attention. Broadband and ultra-fast photodetector based on graphene have been developed.8-16 However, the photoresponse of graphene photodetector is limited by its obvious intrinsic characteristics: low light absorption (~2.3%) of the atomic thickness of graphene17 and a ultrashort lifetime of photoinduced carriers.8,9 In order to generate a high photocurrent, the light absorption has to be enhanced and the separation of photoinduced carriers requires to occur on a sub-picosecond time scale.18 To overcome this two problems, people used other light-absorbing media (including quantum dots, carbon nanotube, nanoplates) to absorb infrared light for the generation of photo-induced carriers and used graphene as conduction channel.19-21 These SWIR photodetectors exhibit high
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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 the other absorption material 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 introducing two mechanisms to improve the photo-induced carriers generation and separation, representing a device design concept. The plasmonic effect is employed into the device to generate 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 photo-induced electrons out off graphene and leaving holes, which results in the prolonging of photoinduced carrier lifetime. This process is similar to the carrier trapping effect. Thanks to the synergy of above two mechanisms, experiment results show that the photocurrent can be enhanced by nearly 10 times with the responsivity up to 83 A/W at the wavelength of 1550 nm after introducing 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 Au nanoparticles (NPs) array, which is fabricated by
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nanosphere lithography.22-24 Figure 1a shows the schematic process of fabricating the plasmonic nanostructures coupled graphene SWIR photodetector. Firstly, polystyrene(PS) spheres with the diameter of 700 nm spontaneously formed a monolayer uniform film by using surface assembly method on water and could be transferred onto the SiO2/Si substrate. Figure 1b shows a wellordered close-packed structure of monolayer PS sphere with a triangle-like hole leaving between every three PS spheres. After that, 30 nm gold film was deposited onto the PS spheres. Then the PS spheres were removed using tape and Au NPs array was left on the SiO2/Si substrate. Atomic force microscopy (AFM) image shows a hexagonal arrangement of triangle-like Au NPs with a period of 700 nm (Figure 1c), which is corresponding to the size of monolayer close-packed PS sphere. The Au NPs arrays were then transferred to the re-prepared graphene photodetector with wetting transfer method. After transferring onto the graphene photodetector, the triangle-like Au NPs array structure is well kept with the same morphology (Figure 1d). Raman spectra of graphene (supporting information Figure S1) shown that the property of graphene is also well preserved after covering with gold nanoparticles array. Concept of the vertical built-in field in graphene SWIR photodetector Unlike other typical hybrid graphene SWIR phototransistors using the light-absorbing medium for the generation of photo-induced carriers,19-21 in our device photo-induced carriers are provided in graphene only and the plasmonic effect is introduced into for light trapping effect12,25,26 to enhance the generation of photo-induced carriers in graphene. On the other side, the silicon in this device is used for forming a built-in field at the interface through the graphenesilicon Schottky junction,27-30 rather than to provide photo-induced 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 Au NPs array on the electrical behavior of Schottky junction is
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tiny.31-35 Comparing 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 the SWIR light illumination (out of the absorption region of silicon, wavelength large than 1100 nm), the light with the wavelength matching with plasmonic resonance will be trapped by Au NPs and absorbed by graphene; then the photo-induced carriers generating 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 have to emphasize again that this design concept is totally different from other previous works. The key points are: 1) silicon provides built-in field but not photo-induced carriers; 2) graphene provides photo-induced carriers through plasmonic enhanced light absorption, which results in a photo-response at SWIR wavelength (e.g. 1550 nm); 3) the graphene device is working in conductor mode but not diode mode, which provides a high gain.
Figure 1. The fabrication process to integrate Au NPs array with graphene SWIR photodetector. (a)
Fabricate Au NPs array using monolayer polystyrene (PS) spheres as a mask and transfer the Au NPs 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 NPs array on SiO2 obtained by depositing gold using monolayer PS spheres as a mask. (d) The
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Au NPs array after transferring onto graphene. (e) Schematic diagram of the concept of graphene SWIR photodetector.
Plasmonic effect of triangle-like Au NPs It is well known that resonance wavelength of plasmonic effect is related to the Au NPs’size, shape, as well as the dielectric constant of the substrate.23,24 Here, through controlling the annealed time of monolayer PS spheres, the trilateral size of the triangle-like Au NPs can be adjusted, accompanied by the tunable resonance wavelength of plasmonic effect.Three monolayer PS spheres samples with annealed time of 30 s, 60 s and 120 s in 120 °C were fabricated, respectively. During the annealing, the PS spheres are softened and the hexagonal close-packed spheres began to bond together as the extension of annealed time, resulting in the shrinking of the interspace between every three spheres (supporting information Figure S3). Therefore, the trilateral size of triangle-like Au NPs decreases as the extension of the annealed time of PS spheres. Here, pattern 1, 2 and 3 of triangle-like Au NPs array correspond to the PS spheres annealing time of 30 s, 60 s and 120 s, respectively (Figure 2a). In theory, with the different trilateral size of triangle-like Au NPs, we can achieve plasmonic resonance at a different wavelength. To investigate the plasmonic resonance characteristics, spectral analysis is 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 NPs array samples after transferring to the silicon substrate. The plasmonic resonance peaks are found at the wavelength of about 1570 nm, 1750 nm and 1820 nm for pattern 3, 2 and 1, respectively. Meanwhile, the plasmonic resonance wavelengths for the three triangle-like Au NPs array on the glass substrate are blue-shifted to 800 nm, 900 nm and 950 nm (supporting information Figure S4).23 The characteristics of these
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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 supporting information Figure S4. Simulation of the plasmonic triangle-like Au NPs To confirm the plasmonic effect of this triangle-like Au NPs, the transmission and absorption spectrums of triangle-like Au NPs arrays on graphene-silicon substrate are 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 experiment results. Beyond the main resonance peak, there are same small resonance peaks in the visible light region, which is corresponding to higher order resonance mode or hybrid mode. The absorption spectrums show that near the plasmonic resonance wavelength (1570 nm, 1750 nm and 1820 nm), the light absorption of graphene is increased up to about 24%, which is enhanced about by 10 times higher than 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 local electric field, the transition probability for an electron from valence band to conductor band can be calculated based on Fermi’s gold rule,
ρT =
2π n ', k ' H1 ( Es 0 ) n, k h
2
f i (1 − f f )δ ( E f − Ei − hω ) , where ρT is the transition probability,
Es0 is the amplitude of electric field in the presence of plasmonic effect, h is Planck constant,
|n, k > is the wave function and fi is the Dirac fermi distribution. The absorption induced by the
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interband transition can be given by A=hωρT =
e2 Es 0 2h
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2
. If the electric amplitude of incident
2
light is E0, the incident energy flux is Wi =2 E0 /µ0 c . So absorption efficiency of graphene is
A/Wi =
Es 0 E0
2 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 get the enhancement factor, we firstly simulate electric field distribution near the triangle-like Au NPs at the 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 comparing with that without Au NPs. Therefore the absorption efficiency of graphene, in this 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 2 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. So 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 Thereby, only one resonance peak is observed in the transmission spectra.
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Figure 2. Characteristics 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, pattern 1, 2 and 3. (b) Experiment transmission spectra of different triangle-like Au NPs array corresponding to the morphology shown in (a). (c) Simulation results of triangle-like Au NPs array 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 is the light intensity distribution at the cross section. (e) The simulated near-field electric field intensity of triangle-like Au NPs for two indicate the polarization of excitation. Photoresponse of graphene SWIR photodetector To quantify the plasmonic effect of the triangle-like Au NPs 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 the photocurrent is increased with the increasing incident light
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power at the wavelength of 1550 nm. Besides, 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 (
