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Jun 13, 2014 - Graphene is a highly promising material for high speed, broadband, and multicolor photodetection. Because of its lack of bandgap, indiv...
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Large-Area, Transparent, and Flexible Infrared Photodetector Fabricated Using P‑N Junctions Formed by N‑Doping Chemical Vapor Deposition Grown Graphene Nan Liu,† He Tian,‡,§ Gregor Schwartz,† Jeffrey B.-H. Tok,† Tian-Ling Ren,*,‡,§ and Zhenan Bao*,† †

Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States Institute of Microelectronics and §Tsinghua National Laboratory for Information Science and Technology (TNList), Tsinghua University, Beijing 100084, China

‡

S Supporting Information *

ABSTRACT: Graphene is a highly promising material for high speed, broadband, and multicolor photodetection. Because of its lack of bandgap, individually gated P- and Nregions are needed to fabricate photodetectors. Here we report a technique for making a large-area photodetector on the basis of controllable fabrication of graphene P-N junctions. Our selectively doped chemical vapor deposition (CVD) graphene photodetector showed a ∼5% modulation of conductance under global IR irradiation. By comparing devices of various geometries, we identify that both the homogeneous and the PN junction regions contribute competitively to the photoresponse. Furthermore, we demonstrate that our two-terminal graphene photodetector can be fabricated on both transparent and flexible substrates without the need for complex fabrication processes used in electrically gated three-terminal devices. This represents the first demonstration of a fully transparent and flexible graphene-based IR photodetector that exhibits both good photoresponsivity and high bending capability. This simple approach should facilitate the development of next generation high-performance IR photodetectors. KEYWORDS: Graphene, flexible and transparent, IR photodetector, chemical doping, P-N junctions from 300 nm to 6 μm.11 In addition, ultrafast metal− graphene−metal photodetector has also been demonstrated12,13 with an observed constant photoresponse under optical modulations up to 40 GHz. They predicted that the intrinsic bandwidth of graphene photodetectors may exceed 500 GHz.12 Subsequently, many research efforts have been focused on enhancing the device’s parameters, such as photoelectric gain, quantum efficiency, and responsivity. Among the numerous reported strategies, introducing a small bandgap or incorporating dielectric materials can help to effectively inject or separate charges and suppress the dark current, thus leading to higher photoelectric gains. Specifically, reported methods include using bilayer graphene biased under transverse electric fields, nanoribbons,14,15 or formation of a hybrid interface with PdS, TiO2 and ZnO.16−20 The use of plasmonic nanostructures and microcavities has previously been reported to convert incident light more efficiently, thus increasing the quantum efficiency.21−23 In addition, controlling the morphology of metallic plasmonic nanostructures can selectively amplify the photoresponse of graphene at different wavelengths, enabling detection of multicolors.24 Applying split gates to form

T

here is a growing number of diverse applications of infrared (IR) photodetector for both military and civilian use, such as health care, security monitors, rail safety, gas leak detection, scanning imaging for thermal night vision goggles, and space observations.1 Traditional IR photodetectors are made from group IV or III−V semiconductors,2 which absorb photons with energy matching their bandgaps and change the current output based on the intensity of absorbed light.1 Nextgeneration IR photodetector systems require new materials that have broadband absorption in combination with fast response and easy integration with silicon to address the need for wideband detection.3 Recently, flexible and transparent photodetectors built on lightweight and bendable plastic substrates have received increasing attention as compared to conventional rigid photodetectors. This is due to their numerous potential applications, such as flexible cell phones, curved digital cameras, large-area foldable displays, and other flexible electronic systems. Graphene is a promising material for both electronics4,5 and photonics applications.6−8 It has a strong and broadband absorption per unit mass, covering a wide wavelength range from visible to IR.9 Together with its extremely high mobility, it can be used in ultrafast broadband photodetector.10−12 The IBM group reported the first graphene-based photodetector10−12 and observed that their operation wavelength ranges © XXXX American Chemical Society

Received: October 23, 2013 Revised: June 12, 2014

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Figure 1. Schematic showing the process of fabricating chemical doping CVD grown graphene into P-N junctions. Channel length (L) is 50 μm and width (W) is ∼ 1 cm. The area of N-doped region varies.

Figure 2. (a) Optical images of the graphene P-N junctions, highlighting both the P- and N- regions. Within the channel, the top region represents the graphene doped by o-MeO-DMBI-I−, whereas the bottom region represents the graphene covered by a photoresist layer to protect the P-type region. (b) Raman spectra of undoped (P-type, blue) and o-MeO-DMBI-I− (6 nm)-doped (red) and photoresist-covered (black) CVD-graphene. (c) Maps of G and 2D peak position in the junction region. (d) Transfer curves of graphene P-N junctions with different area ratios of P- and N- region. (e) Transfer curves of graphene P-only and N-only devices with the same doping conditions as used in (d) (N-region: 1.5 nm o-MeO-DMBI-I− film). (f) Transfer curves of graphene P-N junctions with various thickness of o-MeO-DMBI-I− films (AN/AP = 1.7:1). (g) Separation of energy (black) and two Dirac Points (blue) as a function of the thickness of o-MeO-DMBI-I− film. All of the device’s channel width is ∼1 cm and the channel length is ∼50 μm.

photodetectors with graphene that are scalable, flexible, and transparent. With the recent development in the growth of graphene via chemical vapor deposition (CVD),32,33 large area, transparent, and flexible graphene-based photodetector implementation should be possible, a feat that cannot be achieved using traditional IV or III−V semiconductor-based technologies. Moreover, the electronic property of graphene can be easily controlled by tuning its chemical potential, leading to an

electrically induced P-N junction is another effective approach to enhance the photoresponsivity and detectivity.25,26 The photoresponse mechanism of a graphene-based device has been mainly attributed to three mechanisms, namely the photovoltaic, photothermoelectric, and bolometric effects.26−30 Apart from the broadband absorption and fast response, ultrathin and highly visible-transparent characteristics are two other desired parameters for IR photodetectors.31 However, there have yet any reports to date in making such IR B

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graphene should shift to a larger wavenumber, while the 2D peak intensity of doped-graphene will decrease to a lower intensity compared to pristine exfoliated graphene.39 Using a shadow mask, we defined as-prepared graphene (P-type) and N-doped region in a single CVD graphene film for Raman study. Again, compared to the Raman spectra of pristine exfoliated graphene, the G peak of both undoped (P-type) and N-doped CVD graphene upshifted.39 To obtain the average level of doping density in each case, we performed a 5 μm × 15 μm Raman mapping in the adjacent P-type and N-doped regions, using 0.2 μm as a step increment (Figure 2c). For the N-doped (6 nm o-MeO-DMBI-I) CVD graphene, the average G and 2D peak positions from 500 data points were measured to be 1595.07 ± 2.65 and 2682.20 ± 1.70 cm−1, respectively. This corresponds to an electron concentration of ∼1.5 × 1013 cm−2 in air.40 For the transferred CVD graphene (P-type, as shown in Figure 2d), the G and 2D peak positions are at 1593.22 ± 1.86 and 2683.68 ± 1.44 cm−1, respectively. This corresponds to a hole concentration as high as ∼1.0 × 1013 cm−2.40 As a comparison, the pristine exfoliated graphene typically have G and 2D peaks at ∼1582 and ∼2682 cm−1, respectively. This indicates that heavily P- and N-doped regions can be fabricated in micrometer scale by patterning of the oMeO-DMBI-I− film, therefore leading to P-N junctions. Next, the presence of graphene P-N junctions fabricated by spatially selective N-doping was further confirmed by electrical measurements. Several devices with different P- and N- area ratios are shown in Figure 2a. Their transport behaviors were characterized in a N2-filled glovebox. Resistance measured as a function of back gate voltages is presented in Figure 2e. The measured P-N junctions all exhibited characteristic double charge neutrality points (CNPs),41 which are fixed at VCNP,1 = ∼10 V and VCNP,2 = ∼120 V regardless of the P:N area ratio. On the other hand, the N-type and undoped (P-type) CVD graphene only exhibit one Dirac point, at the same location as the observed CNPs in the P-N junctions (Figure 2d). However, the ratio of the peak resistance at the two CNPs (RCNP,1/ RCNP,2) is lower for a lower N/P area ratio. Additionally, the amount of o-MeO-DMBI-I− applied can be controlled to tune the relative doping density of the P-N junction. Figure 2f compares the transfer curves with different relative thicknesses of o-MeO-DMBI-I− films, measured by the quartz crystal monitor during deposition, with a fixed area ratio of the N/P regions (AN/AP = 1.7:1). The dopants aggregate on the graphene surface. Therefore, the change in thickness results in different surface coverage of dopants (see Supporting Information, Figure S3). The CNPs of N-doped region shifts positively with increasing o-MeO-DMBI-I− film thickness, while the CNPs of P-doped region did not show any observable changes. Thus, this leads to an increase in the separation energy (|VCNP,2 − VCNP,1|) between the two CNPs (Figure 2g). This can be described as Fermi energy difference between the P- and N- doped graphene region and estimated by the relation shown in eq 1,42 where α ≈ 7.0 × 1010 cm−2 V−1 is the gate voltage to carrier density conversion factor. A larger |VCNP,2 − VCNP,1| corresponds to a larger Fermi energy difference in the formed P-N junctions.

enhanced IR photoresponse. This latter approach is usually achieved by electrical doping.34,35 Here, we incorporate a new efficient N-type dopant to the CVD-grown graphene to enable large area, flexible, and transparent IR photodetectors. We demonstrate that charge transfer doping of CVD-grown graphene can be achieved in selective regions to produce a large number of P-N junctions. Formation of the P-N junction is found to be crucial in determining the polarity and amplitude of the photoresponse in our devices. Furthermore, because no gate voltage is needed to tune the charge carrier density, the charge transfer doped P-N junctions can thus be fabricated onto any substrate, leading to a fully transparent and flexible photodetector. In addition, after subjecting our photodetector to numerous cycles of bending experiments, its performance remains unaffected. This is the first demonstration of a transparent and flexible graphene IR photodetector, which represents a major step toward the future of graphene-based light and portable optoelectronics. The process used for patterning the P-N junctions is shown in Figure 1. For large area thin-film devices, we choose CVDgrown graphene on Cu foil. Because of the low carbon solubility in Cu, predominantly monolayer graphene on Cu foil are obtained,32 as confirmed by microscopic observations and atomic microscope imaging. The CVD-grown graphene sheets were then transferred onto a Si substrate, which consists of a 300 nm SiO2 dielectric layer, capped with a 20 nm thermally cross-linked divinyltetramethyldisiloxane bis(benzocyclobutene) (BCB) as a surface passivation layer for electron and hole traps.36 A layer of photoresist was subsequently spin-coated on top of the graphene film and Au electrodes were patterned by photolithography. The compound 2-(2-methoxyphenyl)-1,3-dimethyl-2,3-dihydro-1H-benzoimidazole (o-MeO-DMBI) is a strong n-type dopant for CVDgrown graphene,37 which can efficiently tune the graphene electronic property from P-type to ambipolar and, finally, to Ntype. Its doping can be easily performed either by vapor deposition or inkjet printing and provides features of large area and uniform doping effect without affecting its visible transparency (see Supporting Information, Figure S1). oMeO-DMBI-I− films were then patterned using an additional photolithography step so that certain regions of the channel are covered with o-MeO-DMBI-I− films, while the unexposed regions contained the undoped CVD graphene. CVD graphene was already heavily P-doped after growth and transfer process in air. The unexposed parts (protected by the photoresist) are still P-doped, confirmed by electrical measurements (see Supporting Information, Figure S2). Therefore, upon completion of the fabrication steps our obtained channel is composed of both N- and P-type regions. The above process allowed us to selectively control desired doping areas. For example, we can selectively control the area ratio of P- to N-regions depending on the photolithographically defined feature sizes. As seen in Figure 2a, our graphene channel is partly covered with o-MeO-DMBI-I− films and photoresist. Raman spectroscopy was used to confirm whether the photoresist layer indeed protects the P-type region of graphene against the N-type dopants.38 No Raman peak was observed in the photoresist-covered graphene region (Figure 2b). Together with electrical measurement (see Supporting Information, Figure S2), we confirmed that our applied photoresist film is thick and dense enough to fully insulate both the graphene and o-MeO-DMBI-I− molecules. It was previously reported that the G peak position of doped-

ΔE = ℏυF(πα|VCNP,2 − VCNP,1|)(1/2)

(1)

This indicates that by changing the thickness of o-MeO-DMBII− film we can thus tune the Fermi energy difference of P-N junctions. When the thickness of o-MeO-DMBI-I− film is 9 nm, C

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Figure 3. (a) Schematic drawing (left) and real (right) photoresponse test setup. The channel is composed of P-type (red, left) and N-type (blue, right) graphene with corresponding Dirac cones. (b,c) Representative photoresponse of P-N junction (b, ∼50 μm in length, ∼1 cm in width; c, ∼3 μm in length, ∼160 μm in width) under IR illumination (>780 nm). Tests were performed in a N2-filled glovebox.

the Fermi energy separation can be as high as ∼340 meV and saturated at around this value as the thickness is further increased. It is also noted that such graphene P-N junctions do not resemble the conventional semiconductor diodes in that they do not show any obvious “rectification effect” due to the lack of bandgap and pseudospin conservation of graphene.43 Thus, such a structure cannot be used for photodiodes, but only as photodetectors. We also note that these large-area graphene devices likely incorporate defects such as cracks and pinholes introduced by the transfer process and thus have nominally low field-effect mobilities (780 nm) was used to illuminate the whole device, including the P- and N- doped graphene channels and their corresponding electrodes. The photoresponse of long- and short-channel devices show several distinctive features. Most prominently, the long-channel graphene photodetectors showed negative photoresponse, that is, the photocurrent decreased upon IR irradiation (Figure 3b), while the shortchannel devices exhibited positive photoresponse (Figure 3c)

characteristic of conventional semiconductor photodetectors.44 Second, the long-channel device showed a much longer response time both in the rising and falling edges of the photocurrent versus time plot than the short-channel devices. We fitted the rising/falling edges from the long-channel device to exponential rise/decay, respectively, that is, I = I0 − A exp(−[(t − t1)/τ1]) for the rising edge (light-on to light-off) and I = I0 + A exp(−[(t − t2)/τ2]) for the falling edge (light-off to light-on).45 The fittings yield characteristic photoresponse time, τ1 and τ2, of 20.1 ± 2.3 and 23.8 ± 1.6 s, respectively. On the other hand, the response times of the short-channel devices are less than 1 s on both the rising and falling edges and are limited by the sampling rate of the electrical measurement instrument. Lastly, the photocurrent to dark-current ratio of the short-channel device (5.15 ± 0.24%) is ∼10 times higher than that of the long-channel device (0.53 ± 0.03%), allowing more robust operation with good signal-to-noise ratio. The photoresponse amplitude of our device is on a par with those recorded for devices using graphene as both the photoabsorber and the conduction channel (see Supporting Information, Table S1).20,29,45 We note that extremely high photosensitivity has been reported for graphene quantum-dot (QD) hybrid structures where the QDs served as a highefficiency photoabsorber.16 The spectral range of photodetection of these devices,46 however, is limited by the band gap of the QD materials. We also note that another work reported that the photoresponse was substantially enhanced by introducing electrotrapping centers, but unfortunately their response speed became slower (on the order of 100 s) at the same time.20 Overall, our work used selectively chemical doping to enhance the photoresponse without sacrificing the photoresponse speed and the fabricated photodetector is responsive D

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N-regions and that from the P-N junction. The resistances of the P-, N- and junction regions are represented as RP, RN, and RPN respectively. RP and RN increase while RPN decreases upon IR irradiation. In the long-channel devices, the increase in RP and RN is much more significant than the decrease in RPN, resulting in a net effect of negative photoresponse. On the other hand, in the short-channel device the contributions from RP and RN are significantly reduced while that from RPN is unchanged, resulting in a net positive photoresponse. We note that the graphene−metal contacts may also contribute to the photoresponse due to local band bending. It is important, however, to emphasize that only the P-N junction devices gave positive photoresponse while the homogeneous devices gave negative response, which highlights the critical role of the chemically derived junction in our photodetector devices. The exact mechanisms of the photoresponses in our devices deserve future investigation. We note that in electrostatically doped graphene junction devices, both photovoltaic and photothermoelectric effects were found to give rise to positive photoresponse at the junction regions.25,27 On the other hand, the rise of lattice temperature can lead to increased resistivity in homogeneous graphene.29 The latter effect can result from irradiation induced heating of the Si/SiO2 substrate or Joule heating of the graphene channel. Indeed, we observed a rise of the surface temperature in our long-channel devices upon IR irradiation (Supporting Information, Figure S6). Because the chemical doping-generated P-N junction does not require either the gate or dielectric layers, device fabrication can easily be accomplished to prepare ultrathin all-transparent flexible photodetector.25 The transparent photodetector is fabricated on a flexible polyethylene terephthalate (PET) substrate with ITO as electrodes (Figure 5a,b). The device shows a transmittance >∼90% over a wavelength range of 400 to 2000 nm (Supporting Information, Figure S7). This ITObased transparent photodetector showed the same IR response trend as the rigid nontransparent device discussed earlier (Figure 5d, Supporting Information, Figure S8). To the best of our knowledge, this is the first report of a graphene-based alltransparent and flexible IR photodetector. In addition to its transparency, the photodetector as fabricated on PET substrate allows for effective photodetection under various bending angles. Using metal (e.g., Au or Pd) as the electrodes (Figure 5c), we observed better contacts between the graphene and its electrodes. Figure 5e showed the comparison of photocurrent characteristics of metal-based flexible photodetector upon exposure to IR irradiation at different bending angles. As expected, photodetectors with different channel lengths (L∼ 50 μm and ∼3 μm) showed completely inversed photoresponse. Our device also showed good reproducibility and stability when the PET substrate was repeatedly bent back and forth numerous times (>20) to various angles, for example, 33, 60, 70, and 80°, relative to the horizontal level. As our photodetector is able to function even when bent to 80°, this suggests its applicability toward the development of flexible graphene-based optoelectronics. In conclusion, we have developed a technique to fabricate large-area, flexible and transparent graphene photodetectors. This is enabled via controlled fabrication of P-N junctions on CVD-grown graphene. Contrary to most other graphene-based IR photodetectors, the device described herein is derived through a selected-area chemical doping process. Together with the broadband adsorption, our chemically doped CVD-grown graphene photodetector can be potentially fabricated on a large

over broad IR range. The photoresponse can be further enhanced by increasing the light absorption efficiency by, for example, integrating waveguide with the graphene photodetector.47 Table 1 summarizes the photocurrent density, photocurrent to dark-current ratio, and measurement conditions for the long- and short-channel devices. Table 1. IR Photoresponse Parameters of P-N Junction (NDopant Thickness: 6 nm Graphene Devices) structure PN (L ∼ 50 μm) PN (L ∼ 3 μm) a

Vds (V) 0.1 5

photocurrent density (10−4A/cm2)a −(10.16 ± 0.55) (5.50 ± 0.47) × 104

photocurrent versus dark current (%)a −(0.53 ± 0.03%) 5.15 ± 0.24%

Set dark current direction as positive.

The depletion length in our chemically doped graphene P-N junction should be independent of the channel length. The fact that the long- and short-channel devices have opposite photoresponse thus suggests that the homogeneous p- and nregions contribute to the photoreponse oppositely to the junction region. To further shed light on the different roles of the homogeneous and P-N junction regions, we performed several experiments. First, we measured the photoresponse of a pure P-type graphene channel with 5 μm length under the same IR illumination (Supporting Information, Figure S5). This device showed a negative photoresponse. Second, we compared the photoresponse of 50-μm-channel P-N junction devices with various ratios between the area of n- and p-regions (Figure 4).

Figure 4. Comparison of the IR photoresponse of graphene P-N junction upon (i) depositing different thicknesses of o-MeO-DMBI-I− film, (ii) different area ratios of P- and N- regions, and (iii) different device channel lengths.

Notably, the polarity and amplitude of the photoresponse show no significant change in the N/P area ratio ranging between 0.6 and 2.9. This observation indicates that the homogeneous Nand P-regions contribute similarly to the photoresponse. Lastly, we fabricated and measured 3 μm channel P-N junction devices with different N-dopant thickness (Figure 4), which, as shown above, is effective to tune the Fermi energy difference at the PN junction. Indeed, the photoresponse shows >30% enhancement when the thickness of the N-dopant is increased from 3 to 9 nm. On the basis of the above observations, we interpret the geometry-dependent photoresponse of our devices as a competition of contributions from the homogeneous P- and E

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Figure 5. (a,b) Photos showing the transparency and flexibility of graphene photodetector built on PET substrates. (c) Bending testing and the definition of bending angles (top). Optical image of the long channel device with arrows showing the bending direction (bottom). (d) Photoresponse of the ITO-based all transparent photodetector. The inset schematic shows the structure of the device. (e) Photoresponses of the metal-based photodetectors (top, channel length ∼3 μm; bottom, channel length ∼50 μm) on PET substrate at different bending angles (33, 60, 70, and 80°). Left and right columns are corresponding with pre- and postbending photoresponse, respectively. The slightly decrease in the observed photoresponsivity after bending may be due to both the decrease in illuminated area and the incurred damage on graphene or electrode while bending.

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scale. Next, we deduced that the homogeneous and the junction regions contribute competitively to the photoresponse with tunable dominating effects via device geometry design to achieve optimized sensitivity and speed. Because our described chemical doping process does not require complex fabrication process for metallic gate electrodes, it makes all-transparent and flexible IR photodetector possible. Taken together, this work represents the first demonstration of a fully transparent and flexible photodetector that shows good photoresponsivity and high bending capability. Our study shows that efficient chemical doping of CVD grown graphene will offer a promising path to enable flexible and portable graphene-based optoelectronics, allowing a pathway for the next generation high-performance IR photodetectors.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (T.-L.R.) [email protected]. *E-mail: (Z.B.) [email protected]. Author Contributions

N.L., H.T. and Z.B. conceived and designed the experiment. N.L. and H.T. performed the experiment. G.S. provided technical guidance. N.L., H.T., G.S., T.L.R., and Z.B. analyzed the data and discussed the results. N.L. and Z.B. cowrote the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the AFOSR (Grant FA9950-09-1-0256), the Global Climate and Energy Project (GCEP) and National Natural Science Foundation of China (61025021) for the support of this work.

ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental process and figures of UV-vis-IR transmittance spectra, transfer curves of undoped transferred CVD graphene, AFM image of transferred CVD graphene, photoresponse of short-channel p-type graphene photodetector, representative photoresponse of long-channel P-N junction graphene photodetector under IR illumination, schematic showing the process of fabricating chemically doping CVD-grown graphene on PET substrates into P-N junctions, and table of comparison of photoresponse of our device with literature. This material is available free of charge via the Internet at http://pubs.acs.org.

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