Ultrasensitive Near-Infrared Photodetectors Based on a Graphene

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Ultrasensitive near-infrared photodetectors based on grapheneMoTe-graphene vertical van der Waals heterostructure 2

Kun Zhang, Xin Fang, Yilun Wang, Yi Wan, Qingjun Song, Wenhao Zhai, Yanping Li, Guangzhao Ran, Yu Ye, and Lun Dai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14483 • Publication Date (Web): 23 Jan 2017 Downloaded from http://pubs.acs.org on January 24, 2017

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ACS Applied Materials & Interfaces

Ultrasensitive near-infrared photodetectors based on graphene-MoTe2-graphene vertical van der Waals heterostructure Kun Zhang,† Xin Fang,† Yilun Wang,† Yi Wan,† Qingjun Song,† Wenhao Zhai,† Yanping Li,† Guangzhao Ran,† Yu Ye,*, †, ‡ and Lun Dai*, †, ‡ †

State Key Lab for Mesoscopic Physics and School of Physics, Peking University, Beijing

100871, China ‡

Collaborative Innovation Center of Quantum Matter, Beijing 100871, China

*

Correspondence to: [email protected], [email protected]

ACS Paragon Plus Environment

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ABSTRACT Graphene and other layered materials, such as transition metal dichalcogenides (TMDs), have rapidly established themselves as exceptional building blocks for optoelectronic applications, due to their unique properties and atomically thin nature. The ability to stack them into van der Waals (vdWs) heterostructures with new functionality has opened a new platform for fundamental research and device applications. Nevertheless, near-infrared (NIR) photodetectors based on layered semiconductors are rarely realized. In this work, we fabricate graphene-MoTe2-graphene vertical vdWs heterostructure on SiO2/p+-Si substrate by a facile and reliable site controllable transfer method, and apply it for photodetection from visible to the NIR wavelength range. Compared to the layered semiconductors photodetectors reported thus far, the graphene-MoTe2-graphene

photodetector

has

superior

performance,

including

high

photoresponsivity (~110 mA W−1 at 1064 nm and 205 mA W−1 at 473 nm), high external quantum efficiency (EQE, ~12.9% at 1064 nm and ~53.8% at 473 nm), rapid response and recovery processes (rise time of 24 µs, fall time of 46 µs under 1064 nm illumination), and free from an external source-drain power supply. We have employed scanning photocurrent microscopy to investigate the photocurrent generation in this heterostructure under various back gate voltages and found that the two Schottky barriers between the graphenes and MoTe2 play an important role in the photocurrent generation. In addition, the vdWs heterostructure has a uniform photoresponsive area. The photoresponsivity and EQE of the photodetector can be modulated by the back gate (p+-Si) voltage. We compared the responsivities of thin and thick flakes and found that the responsivity had a strong dependence on the thickness. The heterostructure has promising applications in future novel optoelectronic devices, enabling nextgeneration high responsivity, high speed, flexible, and transparent NIR devices.


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KEYWORDS transition metal dichalcogenides, vdWs heterostructure, MoTe2, near-infrared photodetector, photoresponsivity Introduction Graphene and other layered materials, such as transition metal dichalcogenides (TMDs), have rapidly established themselves as exceptional molecular building blocks for optoelectronic applications,1-3 due to their unique properties, such as strong light-matter interactions, flexibility, van der Waals (vdWs) assembly, etc.4-5 Owing to its unique electronic properties, graphene has led to the discovery of several new phenomena, such as the half-integer quantum Hall effect and Klein tunneling.6-7 However, the weak light absorption and zero bandgap nature of graphene limit its applications in optoelectronic devices. Other layered semiconductors, such as MoS2, SnS2, InSe, GaTe and WS2, have been demonstrated plenty of unique and complementary optoelectronic properties.8-12 Near-infrared (NIR) photodetectors have attracted extensive interest due to their various applications in telecommunication, biological imaging and remote sensing.13-14 There are two main types of NIR photodetectors, either based on photodiodes or photoconductors.14 A photoconductor can achieve high photoresponsivity but with low speed and requiring an external power supply. A photodiode usually has low photoresponsivity but with high speed and can work without external power, crucial in the current global energy crisis. So far, most of the photodetectors based on layered semiconductors operate in the visible wavelength range, due to their large bandgaps. The reported NIR photodetectors based on layered semiconductors show either a low photoresponsivity (< 10 mA W−1)15-16 or low speed (~1 ms)15,

17

, requiring an

additional external source-drain power supply as well. A self-powered NIR photodetector with


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higher speed and photoresponsivity is still under tight demand for next-generation optoelectronic applications. As a member in the TMDs family, MoTe2, with an indirect bandgap of 1.0 eV in its bulk form,18 is a good candidate for NIR photodetector applications.19 The recent progress in the precise transfer of layered materials while maintaining their qualities enables the combination of different material physical properties in one device by assembling them into vdWs heterostructures. These structures provide a new platform for both the fundamental study and device applications. In this work, we fabricated NIR photodetectors based on graphene-MoTe2graphene vertical vdWs heterostructure via a site-controllable layer by layer transfer method. We studied the photoresponse properties of the as-fabricated photodetectors in detail. The results show that the vdWs heterostructure photodetector has uniform photoresponsive area and can operate in a wavelength range from visible to NIR under self-powered conditions. Upon 1064 nm laser illumination, the photodetector shows high photoresponsivity (110 mA W−1), high external quantum efficiency (EQE, 12.9%), and high speed (rise time of 24 µs, fall time of 46 µs). We also employed the scanning photocurrent microscopy to investigate the photoresponse mechanism of the vdWs heterostructure. We find that the Schottky barriers between graphenes and MoTe2 play a key role in the photocurrent generation. By modulating the barrier height of the bottom Schottky junction via the back gate voltage, one can optimize the device performance. Furthermore, we compared the responsivities of thin and thick flakes and found that the responsivity had a strong dependence on the thickness. Our findings suggest the graphene-MoTe2-graphene vdWs vertical heterostructure to be a promising candidate for future NIR flexible and transparent optoelectronics. Results and Discussion


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The photodetector was made of a MoTe2 flake, sandwiched by top and bottom graphenes (Fig. 1a,b). To fabricate such a device, the bottom graphene (GB) was first transferred to 300 nm SiO2/p+-Si substrate and patterned to be 10 µm wide. Then, the MoTe2 flake and top graphene (GT) were transferred sequentially via a site-controllable transfer method20 to make the grapheneMoTe2-graphene vdWs heterostructure (see Methods). Using atomic force microscopy (AFM), the thickness of the MoTe2 flake was determined to be about 140 nm. In addition to this thick device, another device with a 10 nm MoTe2 flake was also fabricated and successfully studied (Supporting Information Section S1). The size of the MoTe2 was chosen to be larger than 10×10 µm to prevent the short circuit between the top and bottom graphenes. The GB and GT served as the source and drain contact electrodes of the photodetector, respectively. The source-drain current-voltage (Ids-Vds) curve of the heterostructure shows nonlinear behavior in dark, indicating the Schottky contact nature between the graphenes and MoTe2 (Fig. 1c). Under focused continuous-wave 1064 nm laser illumination (Fig. 1c), the device shows a clear photovoltaic behavior with a short-circuit current (Isc) of about 1.5 µA and an open-circuit voltage (Voc) of about 82 mV, indicating the clear NIR photoresponse of the device.

Figure 1. Structure and photoresponse of the vertical graphene-MoTe2-graphene vdWs heterostructure. (a) Schematic illustration of the heterostructure. The MoTe2 flake is sandwiched by the top and bottom graphenes. (b) Optical image of the heterostructure with 140 nm MoTe2. The black and red dashed lines denote the bottom and top graphenes, respectively. The MoTe2 flake and top graphene were stacked by a site-controllable transfer method. (c) The Ids-Vds curve


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recorded in dark (blue) and under illumination (red) by a focused laser (1064 nm, 65 µW, 2 µm in diameter). The heterostructure exhibits a clear photoresponse with a Isc of about 1.5 µA and Voc of about 82 mV. Inset: the red spot denotes the laser beam spot. The scale bars in (b) and inset of (c) are 10 µm.

To study the origin of the photoresponse, we measured the gate-modulated electrical transport characteristics of the device under various back gate voltages (Vg) (Fig. 2a). The p+-silicon substrate was taken as the back gate. During the measurement, the bottom graphene (i.e., GB) was grounded. We can see that the Ids-Vds curves generally show obvious asymmetric transport behaviors, except for Vg of about −20 V, suggesting the different Schottky barrier heights between the n-type MoTe2 and two p-type graphenes (see Supporting Information, Section S2). The asymmetric transport behavior becomes more obvious as the positive back gate voltage increases. This is easily understood, considering the relative Schottky barrier heights between MoTe2 and the graphenes. The work function of MoTe2 was reported to be 4.1-4.3 eV.21-23 To determine the work function of the transferred graphene, we performed ultraviolet photoelectron spectroscopy to extract the work function to be 4.92 eV (Supporting Information, Section S3). It indicates that our graphene is p-type, consistent with our graphene FETs characteristics (Supporting Information Fig. S2b). Based on the fact that the work function of graphene is much larger than that of MoTe2 as well as the n-type nature of MoTe2 (Supporting Information Fig. S2a), the Fermi level of MoTe2 should be higher than that of graphenes. Due to ambient water vapor and oxygen p-doping the top graphene,24 the Fermi level of GT should be lower than that of GB, so the Schottky barrier height of GT/MoTe2 is higher than that of GB/MoTe2 (Supporting Information, Fig. S3b,c). A positive back gate voltage will increase the Fermi level of the GB (i.e. decrease the work function of it), and therefore, the GB/MoTe2 Schottky barrier height is lowered. Since the Schottky barrier height of GT/MoTe2 is less sensitive to the back gate, due to


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the screening effect arising from the bottom graphene and the thick (140 nm) MoTe2, the GT/MoTe2 Schottky barrier height does not appreciably change. Hence, the difference between the two barrier heights becomes larger. At higher positive back gate voltage, GT/MoTe2 dominated transport behavior appears (Fig. 2a). In contrast, negative back gate voltage can increase the work function of GB, and accordingly increase the barrier height of GB/MoTe2, which leads to equal barrier heights of GB/MoTe2 and GT/MoTe2, and correspondingly a symmetric Ids-Vds curve, at Vg of about −20 V. Notably, the Ids (Vds=0.5 V) increases by one order when the Vg changes from −30 V to 30 V (Fig. 2b), confirming again that the device structure is changed from back-to-back Schottky barriers to one Schottky barrier (GT/MoTe2).

Figure 2. Gate-modulated electrical transport and photoresponse characteristics of the heterostructure. (a) Ids-Vds curves under dark condition. The Vg varies from −30 V to 30 V by a step of 5 V. (b) Corresponding transfer curve of the heterostructure, the Vds is set at 0.5 V. The Ids increases by about one order with Vg varying from −30 V to 30 V, which is ascribed to the reduction of Schottky barrier height of GB/MoTe2. (c) The Ids-Vds curves of the heterostructure under 1064 nm laser illumination, with Vg varies from −30 V to 30 V. (d) The dependence of Isc and Voc on Vg. The black and red lines represent Isc and Voc, respectively.


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We also investigated the photoresponse behavior of the device under various back gate voltages using the 1064 nm laser with an incident power of 9.6 µW. Both the Isc and Voc increase with the back gate voltage (Fig. 2c). When the back gate voltage changes from −30 V to 30 V, the Isc increases monotonically from 0.12 µA to 0.89 µA, and Voc increases monotonically from 26 mV to 111 mV (Fig. 2d), indicative of the strong photoresponse dependence on the back gate voltage. Scanning photocurrent microscopy is a powerful tool to investigate the optoelectronic properties of nanostructures and can be used to visualize the spatial-resolved photocurrent generation. Herein, we use the scanning photocurrent microscopy to study the origin of the photocurrent of the vdWs heterojunction photodetector. The scanning photocurrent microscopy images with different back gate voltages (−20 V, 0 V, 20 V, 30V) are shown in Fig. 3a-3d. Unlike the lateral structures that usually have a small photoresponse area near the junction,25 the vertical vdWs heterostructure has a uniformly large photoresponse area for photocurrent generation. At back gate voltage of −20 V (Fig. 3a), negative photocurrent (corresponding to Isc) was observed at regions where GT overlaps with MoTe2, while positive photocurrent was observed at regions where MoTe2 flake overlaps with GB, indicating the photocurrent is mainly from the contact region. The positive photocurrent vanishes gradually, while the negative photocurrent increases with increasing back gate voltage from −20 to 30 V (Fig. 3b-d). The observed phenomenon can be explained by plotting the band diagrams (Fig. 3e-h). As discussed before, the vertical vdWs heterostructure is comprised of back-to-back Schottky barriers of graphene and MoTe2. When the back gate voltage is −20 V, the back-to-back Schottky barriers are symmetric (Fig. 3e). Under laser illumination, the built-in field in the Schottky junction can separate the photogenerated electrons and holes, and generate photocurrent. However, in this


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case, the photocurrent is small for both the positive and negative ones. In addition, according to Beer’s law, due to the absorption of MoTe2, the laser intensity reaching at the GB/MoTe2 interface is much less than that at the GT/MoTe2 interface, thus the amplitude of generated photocurrent at GB/MoTe2 junction is smaller than that at GT/MoTe2. Therefore, the negative photocurrent is observed at the GB/MoTe2/GT overlap region. When the back gate voltage increases from −20 to 30 V, the barrier height of GB/MoTe2 (Fig. 3f-3h) reduces, and the GT/MoTe2 Schottky junction gradually dominates the transport behavior of the photodetector. Consequently, the negative photocurrent increases, and the positive photocurrent fades out (Fig. 3b-d).

Figure 3. Gate-modulated photocurrent in the graphene-MoTe2-graphene vdWs heterostructure. (a-d) Scanning photocurrent microscopy images (focused 633 nm laser with 2 µm diameter) under different back gate voltages (−20 V, 0 V, 20 V, 30V), respectively. The dashed lines indicate the contours of the graphenes and MoTe2 flake. From (a) to (d), the negative photocurrent increases gradually and the positive photocurrent decreases gradually. (e-h) Schematic band diagrams of the heterostructure under different back gate voltages (−20 V, 0 V, 20 V, 30V). There are two Schottky barriers (GB/MoTe2, GT/MoTe2) in the heterostructure, which offer built-in electrical fields to separate the photogenerated carriers. The barrier height of GB/MoTe2 is tuned via the electric field applied from the back gate. Therefore, the photocurrent behavior of the heterostructure is modulated.


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To characterize the performance of the vdWs heterostructure NIR photodetector, photoresponsivity (R) and EQE are two significant performance parameters. R is defined as the photocurrent generated per unit power of incident light on the device, R=Isc/P, where Isc is the short-circuit current, P is the incident light power. EQE is defined as the ratio of the number of carriers collected by the electrodes to the number of incident photons: EQE=(hc/e)×(Isc/Pλ), where h is the Planck’s constant, c is the light speed in vacuum, e is the electron charge, λ is the wavelength of the incident light.26 For a photovoltaic photodetector, the EQE is always less than one. Furchi et al. have reported a WSe2/MoS2 vdWs heterostructure with a EQE of 1.5% at 590 nm laser excitation.26 Yu et al. have reported a graphene-MoS2-graphene vdWs heterostructure with an EQE of 55% at 488 nm laser excitation. 27 The NIR photoresponse performance of the graphene-MoTe2-graphene vdWs heterostructure photodetector is shown in Fig. 4a,b. For a fixed incident light power, the EQE increases as the back gate voltage changes from −30 V to 30V. At Vg=30 V, the photoresponsivity and EQE remain constant at first, and then decrease monotonically with incident light power (Fig. 4b). It is worth noting that when the incident 1064 nm laser power is less than 5 µW, the EQE of the photodetector can be as high as ~12.9%, corresponding to R of ~110 mA W−1. These values are higher than their counterparts of other layered semiconductor based photodetectors in NIR range.15-16 Notably, the EQE of the photodetector is even higher in visible wavelength range (~53.8% at 473 nm, corresponding to R of ~205 mA W−1, see Supporting Information, Section S4). We ascribe the higher performance in visible range to the higher absorption coefficient of MoTe2 in visible range (5.6 × 105 cm−1 at 473 nm).25 The decreasing of photoresponsivity and EQE at higher incident light power was also observed in other nanostructure based photodetectors.28-29 It is possibly due to the reduction of the number of carriers that could be


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collected under high photon flux.15 In addition, the photodetector with thinner MoTe2 flake (10 nm) shows similar photoresponse behaviors (Supporting Information, Section S1). However, the responsivity (3.4 mA W−1) and EQE (0.4%) are lower than those of the thick one. The decrease in the responsivity and EQE is ascribed to the decrease in the light absorption of MoTe2 due to the decreased thickness. Moreover, the reported characteristics suggest that thin MoTe2 suffer from considerable degradation with respect to its bulk counterparts30, will result in deteriorated device performance.

Figure 4. NIR photoresponse performance of the graphene-MoTe2-graphene vertical vdWs heterostructure photodetectors. (a) The dependence of its EQE on the back gate voltage and incident laser power. (b) The dependence of its photoresponsivity and EQE on the incident light power, the back gate voltage is set at 30 V. The photoresponsivity and EQE reach 110 mA W−1 and 12.9%, respectively, while the incident power is less than 5 µW.

Photoresponse speed is crucial for a photodetector. The reported response times for most layered semiconductor based photodetectors are in the range from several milliseconds to a few seconds,8, 11-12, 15, 17 which cannot meet the demand for high-speed photodetection. To measure the

temporal

photoresponse

of

the

graphene-MoTe2-graphene

vdWs

heterostructure

photodetector, a mechanical chopper was used to modulate the incident 1064 nm laser and the photocurrent was recorded using a digital oscilloscope. The photodetector exhibits a repeatable and stable photoresponse to a 3930 Hz incident laser (Fig. 5a). From a single normalized


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modulation cycle, the rise (fall) time, defined as the time required for photocurrent to increase (decrease) from 10% to 90% (90% to 10%) of the maximum photocurrent,15 can be extracted to be about 24 µs (46 µs) (Fig. 5b). It is worth noting that considering the intrinsic response time of the mechanical chopping process (~10 µs, Supporting Information, Section S5), the real response and the recovery times of the device should be even shorter.

Figure 5. Temporal photoresponse of the graphene-MoTe2-graphene vertical vdWs heterostructure photodetectors. (a) Normalized photocurrent response under 1064 nm laser illumination with a 3930 Hz light switching frequency. The photocurrent shows a repeatable and stable response to the chopped incident laser. (b) A single normalized modulation cycle measured at 3930 Hz. The red lines represent the levels of 10% and 90% of the maximum photocurrent.

In a photodiode based detector, like the Schottky junction based detector in this work, the photo-generated carriers are extracted by the built-in electric field under zero bias. The vertical graphene-MoTe2-graphene heterostructure provides a short transmit distance between source and drain for photo-generated carriers, enabling a combination of high speed and high responsivity (110 mA W−1, EQE of 12.9%, rise time of 24 µs, fall time of 46 µs at 1064 nm). Moreover, it is self-powered, i.e., it works at zero source-drain bias voltage, which is beneficial for low energy consumption.


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Table 1. Comparison of several important parameters of the graphene-MoTe2-graphene vertical vdWs heterostructure photodetectors in this work and those of other layered semiconductor based photodetectors Materials

Vds (V)

Responsivity (mA W−1)

Response time (ms)

Wavelength

Reference

MoTe2

0

110

0.024

1064 nm

this work

b-P

0.2

Ultrasensitive Near-Infrared Photodetectors Based on a Graphene

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