Multimodal Photodiode and Phototransistor Device Based on Two

Nov 2, 2016 - Multimodal Photodiode and Phototransistor Device Based on Two-Dimensional Materials. Seon Namgung, Jonah Shaver, Sang-Hyun Oh, and Steve...
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Multimodal Photodiode and Phototransistor Device Based on Two-Dimensional Materials Seon Namgung, Jonah Shaver, Sang-Hyun Oh, and Steven J. Koester* Department of Electrical and Computer Engineering, University of Minnesota, 200 Union Street SE, Minneapolis, Minnesota 55455, United States S Supporting Information *

ABSTRACT: With strong light−matter interaction in their atomically thin layered structures, two-dimensional (2D) materials have been widely investigated for optoelectronic applications such as photodetectors and photovoltaic devices. Depending on the aim of optoelectronic applications, different device structures have been employed. Lateral phototransistor structures have been employed for high optical gain, while vertical photodiode structures have been employed for fast response and low power operation. Herein, we demonstrate a multimodal photodetector platform based on 2D materials, combining both a phototransistor and a photodiode and taking the corresponding desirable characteristics from each structure within a single device. In this platform, a multilayered transition-metal dichalcogenide flake is transferred on top of metal electrodes, and a transparent gate electrode is employed. The channel region of the flake between electrodes operates as a phototransistor providing a high gain mode, while the electrode region in the same flake operates as a vertical Schottky photodiode providing a fast response mode. These modes can be dynamically selected by controlling the drain voltage and gate voltage. KEYWORDS: multimodal, photodiode, phototransistor, 2D materials, Schottky barrier 5.5 ps.21 The fast response of the vertical structure relies on a strong built-in electric field across the 2D material in the vertical direction. However, the quantum efficiency of these vertical structures is limited to less than unity due to the lack of a gain mechanism. Thus, a promising way to achieve both high gain and fast photoresponse in a device is to develop a hybrid structure composed of both a phototransistor and a photodiode. Recent studies have demonstrated devices showing a gate-tunable optical gain and photoresponse time,22−24 but those devices still exclusively rely upon the control of the trap sites in the phototransistor structure, potentially limiting the speed, and also require large drain voltages for operation. Here, we demonstrate a multimodal photodetector platform which adopts both a lateral field-effect transistor (FET) structure that produces high optical gain and a vertical photodiode structure, which allows large-area detection and produces fast optical response. In this platform, multilayered TMD material is assembled directly on top of prefabricated electrodes, and transparent ionic liquid is used as a top-gate electrode. The channel region between the electrodes operates as a phototransistor with lateral transport, while the electrode region operates as a vertical Schottky photodiode device. We

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espite their atomic-scale thickness, two-dimensional (2D) materials can exhibit strong interactions with light and also display unique and prominent electrical properties.1−4 These unique optoelectronic properties allow us to utilize 2D materials for photodetectors and photovoltaic devices.5−7 For instance, graphene-based photodetectors exhibit fast photoresponse, sufficient for optical communications, and ultrabroadband photoresponse due to the massless Dirac cone band structure.8 However, graphene photodetectors suffer from excessive dark current, making them unsuitable for most applications. On the other hand, TMDs, a class of layered 2D materials with a finite band gap (typically between 1 and 2 eV), exhibit high light absorption particularly at short wavelengths.9−14 TMD-based phototransistors have demonstrated high responsivity in the range of 102−107 A/W.15−17 Notably, the high gain in the TMD-based phototransistors is associated with trapping of photogenerated charges, which can act to amplify the current flowing between the device electrodes. This trap-based mechanism, however, results in slow photoresponse. On the other hand, 2D material-based vertical devices have shown fast photoresponse, with reasonable (but still less than unity) quantum efficiency. For instance, photodiodes fabricated using TMDs vertically stacked with graphene sheets have shown high quantum efficiency even with a zero bias condition. 18−20 In a WSe 2 -based vertical heterostructure, photoresponse time is measured as low as © 2016 American Chemical Society

Received: September 25, 2016 Accepted: November 2, 2016 Published: November 2, 2016 10500

DOI: 10.1021/acsnano.6b06468 ACS Nano 2016, 10, 10500−10506

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Figure 1. Schematic diagrams of 2D material-based multimodal photodetector platform including both a vertical photodiode and a lateral phototransistor structure and experimental setup.. (a) Device structure of a multimodal photodetector where a TMD flake is assembled on top of metal electrodes and a transparent gate electrode is used. In the region between the electrodes, the device operates as a phototransistor with high gain achieved by trapping photogenerated charges (photogating mechanism). In the region above the electrodes, the device operates as a photodiode with fast photoresponse achieved by a strong electric field based on Schottky barrier (photovoltaic mechanism). (b) Experimental SPCM setup to reveal the different mechanisms of photocurrent generation on the channel and the electrode regions. In this setup, the current through the device is measured as the stage position is rastered, and a focused laser beam is incident upon the device through stationary optics.

Figure 2. PV mode of operation of multimodal Schottky photodetector. (a) Optical image of MoS2 flakes transferred on top of Au electrodes (top left) and spatial maps of photocurrents at VD = 0 and different values of VG. (b) Band diagrams depicting photovoltaic mechanism for photocurrent generation by means of vertical charge collection which is modulated by VG. (c) Optical image of a WSe2 flake assembled on Ti and Pd electrodes (left) and spatial photocurrent maps at VD = 0 for different values of VG. (d) Semilog plots of absolute values of photocurrents vs VG on both Ti and Pd electrodes at VD = 0. The relative Schottky barrier height difference can be determined by the shift of forward bias turn-on which is indicated by the dashed lines.

quantum efficiency as high as 48.5% under zero bias conditions for a TMD photodetector operating in the PV mode, and we also show responsivity as high as 1270 A/W for the devices when operated in PG mode.

use scanning photocurrent microscopy (SPCM) measurements to confirm the device operation, where we show that photocurrent generation in the channel region and the electrode region are governed by a photogating (PG) and a photovoltaic (PV) mechanism, respectively. We further confirm the formation of a Schottky photodiode on the electrode region by gate voltage (VG)-dependent photocurrent measurements which can determine the relative Schottky barrier height of different metal electrode materials. In accordance with the corresponding mechanisms, we demonstrate a high gain operation mode in the channel region as well as a fast response operation mode in the large electrode region. The characteristics of each mode are dynamically tuned by a VG and a drain voltage (VD). In the fast response mode, we report external

RESULTS AND DISCUSSION Figure 1a shows a schematic diagram depicting our device structure operating in both the PG mode in the region between the device electrodes and the vertical-transport PV mode above the large electrode contact regions. In our platform, a TMD flake is assembled directly on top of metal electrodes to build a large-area Schottky photodiode between the metal electrode and the TMD. It should be noted that a Schottky barrier on the electrodes creates an electric field in the vertical (i.e., out of 10501

DOI: 10.1021/acsnano.6b06468 ACS Nano 2016, 10, 10500−10506

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MoS2 and hot electron generation in the metal electrodes are negligible. Since all of these measurements were performed with zero VD condition, the photocurrent generation in our devices is ascribed to the PV effect due to the vertical built-in Schottky barrier slope in the 2D materials on top of metal electrodes. When a 2D material flake is laid over metal electrodes, the Schottky barrier produces an electric field along the out-ofplane direction due to the difference in the work functions of metal and the 2D material. This strong intrinsic electric field effectively separates the photoexcited charge carriers. When we apply a VG via the ionic liquid, the effective work function of the 2D material is changed,26 thus changing the potential profile as shown in Figure 2b. For positive VG, the increased field separates the photogenerated carriers more efficiently, which increases the magnitude of photocurrent. For negative VG, the electric field changes sign, and the opposite polarity current is induced. Notably, due to our inverted contact geometry, the Schottky contact region is formed over the entirety of the electrode region, which is larger than that in a conventional device geometry where the electrodes are on top and the high-field region only occurs at the contact edges.27−29 To further confirm that the photocurrent generation on the electrodes relies on the vertical Schottky barrier, we measured photocurrent using a bimetallic electrode structure, as shown in Figure 2c. Here, WSe2 was used as the 2D material, and the electrodes were composed of Ti and Pd. Previous reports have shown that Ti (Pd) with low (high) work function creates smaller Schottky barrier for electrons (holes), creating n-type (p-type) contacts, respectively.16,30 We performed a SPCM experiment on the device with a 532 nm laser with 12 nW optical power and zero VD. We observed the transition of the photosensitive area from the Ti electrode (source) to Pd electrode (drain) as the VG increased, while maintaining negative polarity of photocurrent. As shown in Figure 2c, on the Ti electrode, negative photocurrent is observed at negative VG and becomes negligible as the VG increases. On the other hand, on the Pd electrode, we only observe photocurrent when VG > 0, and the photocurrent is always negative. Finally, in the channel region between the electrodes, we observe smaller photocurrent and only in the intermediate VG range. The external quantum efficiency was found to be as high as 48.5%, which is larger than the values obtained in the other vertical structures with a MoS220 or WSe221 flake with a larger thickness than ours (15 nm). The high external quantum efficiency of the PV mode in our device is mainly attributed to strong band bending along the vertical direction of the WSe2, which results from the extremely high capacitance of the ionic liquid gate. In this case, nearly all of the applied voltage appears across the WSe2, whereas in devices with solid gate dielectrics, a large portion of the applied voltage drops across the dielectric layer. In addition, in our device, reflectance from the metal electrode beneath the 2D material further enhances the external quantum efficiency. The transition of photosensitive area and consistent polarity can be attributed to the built-in asymmetric Schottky barrier in the bimetallic device. For more detailed analysis, we plot the absolute values of the photocurrents measured on both electrodes on a semilog plot (Figure 2d). The original photocurrents are shown in Figure S2 in the Supporting Information. As shown by the schematic band diagrams, the results show that above the Ti electrode, the Fermi level lies in the gap, nearer to the conduction band of WSe2. At negative VG, the conduction band is raised near the surface, resulting in

plane) direction which can be used for effective charge collection. Since the flake is thin ( 0, while lower, constant responsivity indicative of a PV mechanism is observed in VD = 0. (e, f) Photocurrent vs time for the WSe2 device in the PG and PV dominant condition, respectively. The plots correspond to the dashed gray regions in (a) with the same colors.

photocurrent measured at VD = 0.1 V. It is clear that the gatevoltage dependence of these two parameters are well matched over a wide bias range. This strong correlation can be used to extract the equivalent threshold voltage shift (ΔVth) in the FET induced by the charge trapping mechanism that underlies the PG effect. To do this, we note that the PG photocurrent should be proportional to the transconductance as shown in the following equation:24

To further characterize the multimodality of these devices, we measured the optical power dependency of the responsivity with different combinations of VG and VD (Figure 4a). With a condition of VD = 0 V and VG = 1 V (shown in red), the responsivity of the MoS2 device was nearly the same regardless of optical power, which is characteristic of the PV mechanism. However, with a condition of VD = 0.5 V and VG = 0.5 V (shown in black), the responsivity decreased as the optical power increased, which is characteristic of the PG mechanism. These results support that the PG effect becomes dominant with a finite VD and positive VG, as shown in Figure 3d. The responsivity measured in the PG regime is lower than the previously reported values. A likely explanation is that transferred charges from ionic liquid partially fill up the trap sites,35 which decreases overall responsivity in PG effect. However, the decreasing responsivity with optical power indicates that PG effect is still in effect, albeit with relatively low responsivity. For practical applications, this low responsivity can be improved by using a different type of transparent gate electrode such as ITO or graphene. We also characterized the photoresponse speed under PV- and PG-dominant conditions (Figure 4b,c). In the PG dominant condition, we observe an initial fast increase (decrease) of the drain current, followed by the slow increase (decrease) of the drain current with turning on (off) the excitation light (Figure 4b). This combined fastthen-slow response is attributed to a mixed response due to the initial collection of PV-generated carriers, followed by slow trapping and then the flow of PG-induced current. These results support that the total photocurrent is a sum of photocurrents from PV and PG effects as shown in Figure 3e,f. The time constants of slow rise and falling are measured as 2 and 8.1 s, respectively. On the other hand, in the PV dominant condition, we observe the fast photoresponse (Figure 4c), which confirms the PV effect governs the photodetection in this regime. Thus, the operation mode of our device can be dynamically controlled by tuning VD and VG.

IPG = I(VG + ΔVth) − I(VG) ≈ (dI /dVG)ΔVth = gmΔVth (1)

where ΔVth is the constant of proportionality between IPG and gm. For the data in Figure 3f, and using eq 1, we extract a value of ΔVth = 9 mV. This result indicates that the trapped charge has a positive sign. We do note that a slight discrepancy exists at negative VG, where positive photocurrent is observed indicating a negative PG effect. This behavior could be due to a change in sign of the trapped charge, which could lead to negative PG-induced gain. However, a more likely explanation is that this effect is due to the fact that the PV-induced current could have a slight VD dependence, which can induce errors when the PG current is small. We also note that Figure S4 in the Supporting Information shows SPCM results on the WSe2 device with bimetallic contacts. These results confirm that PGinduced photocurrent only occurs for illumination in the channel region and does not occur when light is incident in the large-area electrode contact regions. The results in Figures 2 and 3 indicate that our devices can operate both as vertical Schottky photodiodes and lateral fieldeffect phototransistors with different photocurrent generation mechanisms in each mode of operation. The electrode region operates based on PV mechanism, while the channel region operates mainly based on PG mechanism. These different mechanisms are activated with different VG and VD. Thus, our devices can be utilized as a multimodal photodetector, where the mode is dynamically selectable by VG and VD. 10504

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ASSOCIATED CONTENT

We also demonstrated similar multimodality of the WSe2 bimetallic device. The responsivity of the device is measured as a function of optical power in different VD conditions (Figure 4d). At VD = 0, the responsivity of the device was nearly independent of optical power, which is characteristic of the PV mechanism. On the other hand, at VD = 0.5 V, the responsivity decreased with increasing optical power, characteristic of the PG mechanism. We also observed that the response time of photodetection was different depending on the applied bias conditions (Figure 4e,f). With VD = 0.5 V, we observed a slow photoresponse, which further suggests the PG mechanism (Figure 4e). With VD = 0, we observed the fast response of the drain current to turning on and off the excitation light, which points toward the PV mechanism (Figure 4f). The response time at VD = 0 is less than the measurement limit (28 ms) of our experimental setup. Therefore, these results demonstrate that our device operates in different modes cooperatively with the operation condition tunable with a VD and a VG.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06468. Photocurrent maps measured with different wavelengths, photocurrent measured on different regions of the WSe2 bimetallic device, Schottky barrier height determination with multielectrodes, and photocurrent maps of the WSe2 bimetallic structure with different drain voltages (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported primarily by the Air Force Office of Scientific Research under award no. FA9550-14-1-0277 (S.N., S.J.K.) This work was also supported in part by the National Science Foundation (NSF) through the University of Minnesota MRSEC under award no. DMR-1420013 (J.S., S.‑H.O., S.J.K.). Device fabrication was performed at the Minnesota Nanofabrication Center at the University of Minnesota, which receives partial support from the NSF through the National Nanotechnology Coordinated Infrastructure (NNCI). Portions of this work were also carried out in the University of Minnesota Characterization Facility, which received capital equipment funding from the NSF MRSEC.

CONCLUSION In conclusion, we demonstrated a hybrid device platform adopting both a lateral phototransistor structure and a vertical Schottky photodiode structure, which can be utilized as a multimodal photodetector providing a fast response mode as well as a high gain mode. The device is fabricated simply by transferring a multilayered TMD flake on the prebuilt electrodes, and transparent ionic liquid is employed as a gate electrode. We confirmed the formation of vertical Schottky photodiode on electrode regions by SPCM method. We further confirmed the vertical Schottky barrier formation by the different VG dependence of the photocurrent on different kinds of metal electrodes, and we also showed that this method can be utilized as a means to determine the relative Schottky barrier height at the metal/2D materials interface. In the vertical photodiode on electrodes, PV mechanism based on the Schottky barrier slope provides fast photoresponse and high charge collection efficiency over a large area. In the lateral phototransistor channel region, PG mechanism provides a responsivity as high as 1270 A/W with a small VD of 0.5 V. These operation modes based on each structure are dynamically selected by controlling the bias conditions in the device. Our multimodal photodetector platform based on 2D materials will promote the development of high performance multifunctional optoelectronic devices.

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METHODS The device fabrication began with electrode formation on a 300 nm SiO2/Si wafer by photolithography and metal deposition through electron beam evaporation. A 10 nm Ti adhesion layer was used for Au, Pd, or Ni (50 nm) deposition. Mechanically exfoliated MoS2 or WSe2 flakes were transferred on the electrode regions using a microscope-based alignment system. The electrodes outside the active device region were passivated with photoresist to block unwanted leakage current to an ionic liquid gate. Electrical measurement was performed using Keithley 2450 source meters at room temperature. 1Butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) was used for ionic liquid gating, and a Pt electrode was used to make electrical connection to the liquid gate. Lasers with wavelengths of 532, 785, and 1550 nm were used as the light sources, and they were focused on the devices using microscope system with a 50× objective (0.55 N.A.). For the SPCM experiments, a three-axis piezo-stage (Mad City Labs, Inc.) was mounted on the microscope system. Samples were mounted on the piezo-stage, and the stage was controlled to move in a raster pattern. 10505

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