Multilayer ReS2 Photodetectors with Gate Tunability for High

Subscriber access provided by University of Sunderland

Surfaces, Interfaces, and Applications 2

Multilayer ReS Photodetectors with Gate Tunability for High Responsivity and High Speed Applications Kartikey Thakar, Bablu Mukherjee, Sameer Grover, Naveen Kaushik, Mandar Deshmukh, and Saurabh Lodha ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11248 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Multilayer ReS2 Photodetectors with Gate Tunability for High Responsivity and High Speed Applications Kartikey Thakar1, Bablu Mukherjee1,Ϯ, Sameer Grover2,ϮϮ, Naveen Kaushik1,ϮϮϮ, Mandar Deshmukh2, and Saurabh Lodha1,*

1

Department of Electrical Engineering, Indian Institute of Technology, Bombay. Mumbai 400076, IN.

2

Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai - 400005, IN.

K. Thakar, Dr. B. Mukherjee, N. Kaushik, Prof. S. Lodha* Department of Electrical Engineering Indian Institute of Technology, Bombay Mumbai 400076, India. * E-mail: [email protected] Dr. S. Grover, Prof. M. Deshmukh Department of Condensed Matter Physics and Materials Science Tata Institute of Fundamental Research Mumbai 400005, India. Keywords: ReS2, photodetectors, tunability, TMD, fast

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 37

ABSTRACT Rhenium disulfide (ReS2) is an attractive candidate for photodetection applications owing to its thickness independent direct bandgap. Despite various photodetection studies using twodimensional semiconductors, the trade-off between responsivity and response-time under varying measurement conditions has not been studied in detail. This report presents a comprehensive study of the architectural, laser power and gate-bias dependence of responsivity and speed in supported

and

suspended

ReS2

phototransistors.

Photocurrent

scans

show

uniform

photogeneration across the entire channel due to enhanced optical absorption and direct bandgap in multilayer ReS2. High responsivity of 4 A/W (at 50 ms response time) and low response time of 20 µs (at 4 mA/W responsivity) make this one of the fastest reported transition-metaldichalcogenide photodetectors. Occupancy of intrinsic (bulk ReS2) and extrinsic (ReS2/SiO2 interface) traps is modulated using gate-bias to demonstrate tunability of the response time (responsivity) over 4 orders (15×) of magnitude highlighting the versatility of these photodetectors. Differences in the trap distributions of suspended and supported channel architectures, and their occupancy under different gate-biases, enable switching the dominant operating mechanism between either photogating or photoconduction. Further, a new metric that captures intrinsic photodetector performance by including the trade-off between its responsivity and speed, besides normalizing for the applied bias and geometry, is proposed and benchmarked for this work.

Introduction Two-dimensional (2D) layered materials have seen significant research interest after the extraction of graphene from graphite using mechanical exfoliation.1 2D materials exhibit


2

Page 3 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


intrinsic metallic, semiconducting or insulating properties and a large number of them show transition from direct to indirect bandgap depending on their thickness. These properties open a door to various applications ranging from electronics and optoelectronics to photonics, valleytronics and much more.2-5 Transition metal dichalcogenide (TMD) materials are a set of layered 2D materials in the form of MX2, where M is a transition metal and X is a chalcogen. Rhenium disulfide (ReS2) is a TMD semiconductor material which has attracted a large number of researchers towards understanding its behavior and applications even before the aforementioned discovery of graphene.6-9 Unlike most TMDs, ReS2 shows little dependence of its direct bandgap on flake thickness, making it an attractive candidate for optoelectronic applications.10 It also shows intrinsic in-plane anisotropy arising from Re atom chains7-9,

11-13

along with air-stability unlike some of the elemental semiconducting 2D materials such as black phosphorus14, 15 and silicene.16 Beyond graphene and other 2D materials,17-21 this has led to a recent surge of interest in photodetectors based on ReS2.22-26 Table 1. Comparison of our work with previous photodetector reports. Materiala 1L MoS227 1L MoS228 1L MoS2b,29 1L MoS230 1L WSe231 Gr-Bulk MoS218 FL InSe32 FL ReS2b,24 CVD ReS226 FL ReS2b,23

VD [V] 1 8 5 >3 2 0.1 10 3 0.5 4

Measurement conditions VG Laser Intensity [V] λ [nm] [mW/cm2] 50 532 8×104 -70 561 0.24 100 532 5 nW 640 0.3 -60 650 0.38 -20 (pulse) 80 0 -50

Figure of merit Responsivity Response [A/W] time [s] -3 7.5×10 50×10-3 8.8×102 0.6 103 10 < 1×10-3 1.8×102 10

Study of architectural dependence No No No Yes No

Study of dependence of speed on VG No No No No No

635

10-8

5×108

4.3×104

No

No

633 532 500 532

350 0-3 mW 3.11 6 pW

1.57×102 103 6.04×102 8.86×104

4 2 2×10-3 102

No No No No

No No No No

ML ReS2b,25 5 30 405 5 pW 2.5×107 0.67 No No ML ReS2 0.5 -80-80 633 ~140 ~4 ~20×10-6 Yes Yes (This Work) -1 a) acronyms: 1L - monolayer, FL - few-layer, ML - multilayer, CVD - chemical vapor deposition. b)absolute laser power.


3


Page 4 of 37

Despite a vast and growing volume of work on 2D material-based photodetectors, there are significant lacunae in understanding fundamental device operation mechanisms and trade-offs, from a measurement29 as well as a device design perspective.30, 33 This has resulted in a large spread in the data set of reported performance parameters. Measurement conditions and key photodetector metrics of responsivity and response time (speed) are listed in Table 1 for a range of 2D materials. Large variability in measurement conditions along with a trade-off between responsivity and response time are evident, irrespective of the material used. More specifically, gate and drain bias, incident laser power, and trap distributions (through device architecture) can affect the responsivity and speed of the photodetector. For example, many reports have established the power-law dependence showing an increase in the responsivity with decrease in incident laser power.20, 21 However, a fundamental understanding of the impact of gate bias and traps on photodetector responsivity and speed metrics is critically needed to understand the variability in reported work as well as to guide future studies. This work reports comprehensive measurements of the responsivity, time dynamics and spatial uniformity of the photoresponse in multilayer (ML) ReS2 under variable incident power (633 nm, 5 - 125 µW), laser modulation frequency (1.67×10-3 - 2×104 Hz) and gate bias conditions (-60 V to 80 V) for supported and suspended channel ReS2 field-effect transistor (FET) architectures (varying trap distributions). Spatial and time evolution of the photocurrent analysed over these wide ranges of measurement conditions helps distinguish between large responsivity-slow speed and low responsivity-high speed operation. High responsivity value of 4 AW-1 is achieved for a 178 Hz modulated 100 µW, 633 nm laser beam. The same device can respond to a 20 kHz modulation of the laser with reliable photocurrent switching and a short response time of ~20 µs,


4

Page 5 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


making it the fastest ReS2 photodetector reported till date. These observations indicate the potential of ReS2 in reliable and high speed photodetection applications. The broad set of measurement data also helps delineate mechanisms governing the fundamental trade-off between responsivity and speed under different gate biasing and trap distribution conditions. Comparison of supported and suspended channel architectures allows to distinguish between the roles of intrinsic bulk traps in ReS2 vs extrinsic traps at the ReS2/SiO2 (gate-dielectric) interface during photodetector operation. Variation in gate bias (-60 V to 80 V) is shown to switch the operation from a photogating (PG) to a photoconductive (PC) regime (and vice-versa) by modulating trap occupancies leading to a device whose speed (responsivity) can be tuned from 367 s to 20 ms (8 to 0.53 mAW-1) and hence can cater to a wide range of performance requirements. We finally conclude by benchmarking our device against a wide range of reported photodetectors and also by proposing a novel, phenomenological photodetector metric that captures the intrinsic photodetector performance by including the trade-off between its responsivity and response time besides normalizing for the effects of applied bias and device geometry.

ReS2 Device Characterization Device Architecture ReS2 photodetectors with two different device architectures were fabricated in FET configuration. The words ‘FET’, ‘photodetector’ and ‘device’ are used interchangeably in this report. Supported ReS2 FET had the ReS2 channel in contact with bottom gate SiO2 (285 nm) dielectric whereas suspended34 ReS2 FET had a ~150 nm air gap between the ReS2 channel and


5


Page 6 of 37

Figure 1. Device structure (a) Schematic of supported ReS2 device. ReS2 is in contact with 285 nm thick SiO2 bottom gate dielectric. (b) False colour SEM image of the supported ReS2 device. Scale bar is 1 µm. (c) Schematic of suspended ReS2 device. ReS2 flake is embedded in the contacts as shown through partially transparent contacts. A ~150 nm air gap is present between ReS2 and SiO2. (d) False colour SEM image of the suspended ReS2 device. Curvature in the flake is due to stress from contacts. Scale bar is 500 nm. the bottom SiO2. Figure 1a (c) shows the schematic of a supported (suspended) ReS2 FET. ReS2 flakes were embedded in the source/drain contacts in suspended ReS2 FETs as shown in Figure 1c. Figure 1b (d) shows a false color scanning electron microscopy (SEM) image of the supported (suspended) ReS2 FET with a rectangular channel. Suspended ReS2 channel was slightly arched due to stress from the contacts as a result of the fabrication process. Channel length and width of the supported (suspended) ReS2 device were 2.6 µm and 0.6 µm (2.4 µm and


6

Page 7 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.6 µm), respectively. Detailed fabrication steps are discussed in the Methods section. ReS2 flakes were analyzed via Raman, photoluminescence (PL) and x-ray photoelectron spectroscopy (XPS) that suggest good quality of the multilayer flakes as shown in Figure 2. Two main Raman peaks at 162 cm-1 and 212 cm-1 were observed corresponding to E2g (in-plane vibration) and A1g (out-of-plane vibration) modes, respectively. PL peak obtained at ~1.5 eV indicates the multilayer nature of ReS2. XPS spectra also show sharp peaks corresponding to Rhenium (Re)


7


Page 8 of 37

Figure 2. Physical characterization (a) Raman spectrum of ReS2 flake. Typical E2g and A1g peaks are observed at 162 cm-1 and 212 cm-1, respectively, along with other labeled peaks. This confirms good crystalline quality of the flake. (b) Photoluminscence (PL) spectrum of the ReS2 flake showing an optical bandgap of ~1.5 eV. (c,d) X-ray photoelectron spectroscopy (XPS) spectra for Rhenium (Re) and Sulfur (S), respectively. Two XPS peaks at 162.1 eV and 163.4 eV are observed for sulfur corresponding to its 2p3/2 and 2p1/2 orbitals, respectively. and Sulfur (S) atoms at typically observed binding energies confirming good quality of the crystal. ReS2 flake thicknesses were measured to be ~15-30 nm using atomic force microscopy (AFM) (Figure S1, Supporting Information). Multilayer ReS2 flakes were chosen to enhance the light absorption as compared to mono- and few-layer flakes although techniques like plasmon coupling can also be incorporated to enhance the absorption in thinner flakes.35


8

Page 9 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Steady-state Electrical Characterization

Figure 3. DC measurements (a) Transfer characteristics of the supported ReS2 device for varying drain bias. The device shows good on-off current ratio of ~104. (b) Transfer characteristics of the suspended ReS2 device for varying drain bias. Poor off current results from reduced gate capacitance due to the 150 nm air gap. (c) Transfer characteristics of the supported ReS2 transistor under dark and illumination at VD = 1 V. Typical signatures of photoconductance (PC) - increase in current, and photogating (PG) - shift in threshold voltage, are observed. Inset shows the ReS2 transistor under focused beam illumination of a 633 nm, 100 µW laser. Additional measurements were taken at VG = 0 V and VG = 80 V. Corresponding points are shown in the figure. (d) Transfer characteristics of the suspended ReS2 transistor under dark and illumination at VD = 0.5 V. Complete off state is not seen due to poor gate control. Additional measurements were taken at VG = -60 V and VG = 60 V. Corresponding points are shown in the figure.


9


Page 10 of 37

Steady-state electrical measurements were performed for the supported and suspended channel devices. Figure 3a (b) shows the transfer characteristics of the supported (suspended) ReS2 FET without any illumination for a range of drain bias (VD). The supported channel FET shows good on-off current ratio of ~104 whereas the suspended channel device shows poor on-off ratio and subthreshold slope due to reduced gate control caused by introduction of the ~150 nm air gap. Channel width-to-length ratio (W/L) is indicated in the plots. The gate bias (VG) where transconductance (gm) reaches its maximum value was taken as the threshold voltage (VTh), and field effect carrier mobility (µ) was extracted at VTh according to the following relation. ߤ=

݀‫ܫ‬஽ ‫ܮ‬ , ܸ݀ீ ܹ‫ܸ ீܥ‬஽

where ID is drain current, L is channel length, W is channel width, VG is gate bias, VD is drain bias and CG is gate capacitance calculated as CG = εox/EOT. εox = 3.9ε0 and EOT is effective oxide thickness for SiO2. Mobility values of ~3 cm2V-1s-1 and ~8 cm2V-1s-1 were calculated for the supported and suspended ReS2 FETs, respectively. Higher mobility in the suspended ReS2 device as compared to the supported channel device is expected due to the absence of ReS2/SiO2 interface traps. Moreover, high value of subthreshold slope in both architectures indicates presence of bulk traps inside the ReS2 channel. We have listed the extracted electrical parameters for a range of supported and suspended ReS2 devices in Table S2 (Supporting Information) along with previously reported values. Devices used to characterize the photoresponse are representative of the set of fabricated devices and compare well with published reports. Figure 3c (d) shows the transfer curves for the supported (suspended) ReS2 photodetectors under dark and illumination conditions. The complete experiment setup for photocurrent detection is described in Figure S3 (Supporting Information). As shown in Figure 3c, a combination of PC and PG effects was observed in the illuminated supported ReS2 device. The


10

Page 11 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


effect of PC and PG on phototransistor transfer characteristics is explained elsewhere.20 PC is observed due to increase in mobile charge carrier density in the channel through photogeneration, whereas PG results from trapping of the charge carriers resulting in a shift in VTh. Inset in Figure 3c shows the supported ReS2 device under illumination with a focused laser beam of 633 nm. A horizontal shift in threshold was observed originating from the PG effect as compared to a vertical shift due to PC. Increase in VTh was observed after illumination suggesting that mainly electrons were being trapped. This assumption is justified for ReS2 since it has a large number of traps with energies near the conduction band edge (EC) as indicated in several reports.22,

23

The suspended ReS2 device showed only a vertical shift in the transfer

characteristics as the gate field-effect is reduced. However, charge trapping was not absent in the suspended ReS2 device as discussed later. The individual data points highlighted in Figure 3c,d were used to evaluate the dynamic (time-dependent) behavior of the photodetectors as discussed later.

Photocurrent Measurements Photocurrent Spatial Scans Photocurrent spatial scans under varying electrical bias and incident laser power

were

performed for the supported and suspended ReS2 photodetectors using the same setup as discussed in Figure S3 (Supporting Information). Figure 4a (d) shows a typical photocurrent spatial map for the supported (suspended) ReS2 device under 633 nm, 100 µW laser illumination. Both devices show uniform photocurrent generation across the channel area with peaks near the drain electrode. Figure 4b (e) shows extracted photocurrent linescans for different laser powers across the length of the channel for the supported (suspended) ReS2 photodetector. These


11


Page 12 of 37

Figure 4. Photocurrent map (a) Photocurrent map of the supported ReS2 device for VG = 0 V, VD = 1 V. Source, drain and channel are indicated by dashed lines. (b) Linescans of photocurrent along the channel for different illumination powers. Uniform photocurrent along the channel is observed as a direct consequence of the direct bandgap and multilayer nature of the ReS2. In comparison, similar linescan for a 1L MoS2 device at VG = 0 V, VD = 1 V and P = 5 mW shows very less photocurrent in the channel area due to insufficient light absorption. (c) Absolute and normalized responsivity values obtained from photocurrent linescan data for varying effective incident power. Power-law dependence of responsivity on the incident power with a negative exponent value indicates presence of traps in the device. (d) Photocurrent map of the suspended ReS2 device for VG = 0 V, VD = 1 V. Source, drain and channel are indicated by dashed lines. (e) Linescans of the photocurrent along the suspended ReS2 channel for different illumination powers. Higher photocurrent near the drain contact is due to the in-built electric field of the Schottky barrier. (f) Responsivity vs power graph as in (c), for the suspended ReS2 device. Normalized responsivity for the suspended ReS2 device is smaller as compared to the supported ReS2 device. linescans show uniform photocurrent generation across the entire channel resulting from the direct bandgap of ReS2. Enhanced photocurrent observed near the drain electrode is likely due to more efficient carrier separation resulting from the enhanced electric field near the Schottky barrier contact. Monotonic increase in photocurrent was observed with increasing incident power (5 - 125 µW) for both device architectures. Furthermore, Figure 4b shows the linescan for a supported monolayer (direct bandgap) MoS2 device under similar biasing (VG, VD) conditions. Negligible photocurrent in the channel in this case indicates a lack of photogenerated carriers due


12

Page 13 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to low photo-absorption and enhanced current near the drain electrode is likely due to photothermal effects. This further reinforces the importance of a direct bandgap in thicker, multilayer ReS2 to allow sufficient absorption of incident light. Figure 4c (f) shows the plot of responsivity (R) vs effective incident power (P) where R = Iph/P was calculated for the supported (suspended) ReS2 device from the integrated photocurrent (Iph) over the channel area. The calculation of effective laser power is described in detail in Figure S4 (Supporting Information). Both types of devices achieved a maximum responsivity value of ~4 AW-1 at low incident power. Power-law dependence of responsivity with incident power indicates the presence of traps.36 Exponent (η) values of 0.31 and 0.58 were obtained for the photocurrent dependence on incident power η ( I ph ∝ P ) for the supported and suspended ReS2 devices, respectively. A smaller exponent

indicates larger effective trap density and dominance of PG over PC. The suspended ReS2 device shows a larger exponent value as compared to the supported ReS2 device, as expected from reduced effective trap density. However, presence of trap states is also known to increase the photodetector gain (G) resulting in increased responsivity.23, 36 Photocurrent gain is given by G = Tl/Tt, where Tl is carrier lifetime in a trap state and Tt is carrier transit time through the channel given by Tt=L2/(µVD). For the supported ReS2 device with L ~ 2.6 µm, µ = 3 cm2V-1s-1 and VD = 1 V, we get Tt ~ 22 ns. Similarly, we get comparable Tt ~ 16 ns for the suspended channel device. As a result, with a lower Tl due to reduced trap density, we expect lower G, and hence R, in the suspended detector. However, despite smaller area and G, responsivity of the suspended ReS2 device is comparable to the supported one. The larger photocurrent in the suspended ReS2 device is likely due to reduced channel resistance (larger W/L) as well as higher electric field at the Schottky contacts due to shorter L. Hence we have calculated the normalized responsivity (RNorm) using the relation RNorm = R*(L/W) to consider the effect of channel resistance (via W/L ratio) on


13


Page 14 of 37

the photocurrent for a given bias and laser illumination. RNorm for the supported (suspended) ReS2 FET is shown using the right hand side y-axis in Figure 4c (f). As we expect, smaller RNorm values for the suspended ReS2 device suggest that it would exhibit smaller responsivity (R) for the same channel geometry as the supported ReS2 device. Dynamic Behaviour of the Photocurrent Dynamic Behaviour for sub-1 Hz Frequency

Figure 5. Temporal response Figures in each column are for the same device and bias conditions. Charge carriers in orange are due to photogeneration whereas those in green are due to the field-effect of applied VG. Two types of trap levels are shown- bulk traps inside the ReS2 channel and interface traps at the ReS2/SiO2 interface. The trap levels are assumed to be near the conduction band minimum of ReS2 as indicated in previous reports.19, 20 Interface traps are non-existent in the suspended ReS2 device. Blue arrows in the band diagrams show direction of flow of the charge carriers. (a-d) Band diagrams of the ReS2 transistors along the channel under different electrical bias conditions. (e-l) Photocurrent evolution with time for different time scales. Region in yellow indicates the time period during which the laser is on. (e-g,i) show the photogating effect (slow rise and decay in photocurrent) whereas (h,j-l) show that photoconduction (sharp increase or drop in photocurrent) dominates the photoresponse. Time constants τ1 and τ2 are extracted by fitting the photocurrent curves to exponential rise and decay functions, respectively.


14

Page 15 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


We further measured dynamic response of the ReS2 photodetectors to evaluate their speed of operation. Figure 5 shows the photocurrent measurements under 633 nm, 100 µW focused laser illumination for the supported as well as suspended ReS2 devices and explanation for the observed behaviour using a series of band diagrams. In order to measure only the photoresponse of the device, the current was manually set to zero using a dc current offset in the current to voltage converter before the starting point of each of the individual curves. Figure 5e to Figure 5l show photocurrent evolution with time under constant illumination (modulation frequency = 0 Hz) where region in yellow indicates the time when the laser is on. Each column in Figure 5 corresponds to a particular electrical bias with columns ordered from the slowest to the fastest photoresponse corresponding to effective trap densities from the largest to the smallest. Biasing conditions for photoresponse measurements were chosen such that the mobility is nearly independent of the gate voltage (charge density). Figure 5a to Figure 5d show band diagrams of the ReS2 devices across the length of the channel for the corresponding measurement condition. Photogenerated carriers are shown in orange and carriers induced by gate field-effect are shown in green. Two types of traps, namely bulk traps (NBT) in the ReS2 channel (dark crimson band) and interface traps (NIT) at the ReS2/SiO2 interface (dark grey band), are used to explain the dynamic photocurrent behavior. Each case going from left to right in Figure 5 is explained in detail next.

Supported ReS2 device in OFF condition The supported ReS2 device showed the slowest dynamic response when biased in OFF condition, i.e. VG < VTh. The bulk and interface traps in this condition are mostly empty owing to a lack of carriers in the channel. This allows a large number of photogenerated electrons (in orange) to get trapped near the conduction band while some of the gate induced electrons (in


15


Page 16 of 37

green), although small in number, can get trapped as well. This leads to a prominent PG effect (slow and large increase in current) visible in the photocurrent dynamic response. In Figure 5e, an increase in photocurrent up to ~600 nA (R ~ 8 mAW-1) with a rise time constant of ~367 s was observed during the laser on time of 600 s. After the laser was turned off at 600 s, a sharp drop in the photocurrent (~60 nA, ~20 ms) indicating PC effect was followed by a slow decay with a large time constant of ~2146 s indicating PG effect. All PG time constants were extracted by fitting explicit exponential functions whereas all PC time constants were measured between minimum and maximum current levels across the jump/drop in photocurrent. When laser on-off modulation time period was set to 30 s, a decrease in maximum photocurrent (~220 nA, R ~ 2.93 mAW-1) and the time constant (~4.2 s) was observed with little current relaxation during laser off period as shown in Figure 5i. This could be due to the inability of the slow bulk traps in the ReS2 channel to respond to relatively fast light modulation unlike the fast interface traps. A slow increase in the photocurrent level across many light modulation cycles suggests that the PG effect is dominant. This argument is further reinforced by the dynamic photoresponse in suspended channel photodetectors as discussed next.

Suspended ReS2 device under negative gate bias A bias of VG = -60 V will not be able to induce electrons in the suspended ReS2 channel device through the gate-field effect. Like the supported device, it was also dominated by the PG effect when operated under laser illumination for 600 s - shown in Figure 5f. However, owing to the fact that the interface traps no longer play a part in PG, and that the photogenerated carriers can be trapped only in the intrinsic bulk traps in ReS2, the maximum photocurrent reduced to ~220 nA and R ~ 2.2 mAW-1 (~600 nA for the supported device) and the time constant to ~287 s (~367 s for supported ReS2). The PC effect was observed as a sharp drop in the photocurrent


16

Page 17 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(~40 nA, 60 ms) after the laser was turned off which was followed by a slow decay. Similar to the previous sub-section, when the suspended ReS2 device was illuminated with a modulation time period of 30 s, it showed no sign of slow rise or decay (except across cumulative cycles) as shown in Figure 5j. On the contrary, the device showed sharp switching (~63 ms) with a photocurrent of ~50 nA (R ~ 0.5 mAW-1). This observation leads us to conclude that i) bulk traps in ReS2 are slow enough to not play a part in high speed switching; and ii) PC effect is more prominent in suspended architecture with a relatively clean channel surface.

Suspended ReS2 device under positive gate bias Biasing the suspended device at VG = 60 V induces electrons through the gate-field effect (in green) that get trapped in bulk traps in the ReS2 channel. A larger probability of gate induced electrons getting trapped in bulk ReS2 traps as compared to the previous case of VG = -60 V reduces trapping probability of the photogenerated carriers. As shown in Figure 5g, this resulted in further reduction in photocurrent to a maximum of ~170 nA (R ~ 1.7 mAW-1) and in the rise time constant to ~261 s. The effect of VG was seen for photocurrent decay also, the current level at t = 2400 s was significantly lower than in Figure 5f (VG = -60 V). Also, a sharp drop was observed in the photocurrent (~30 nA, 40 ms) after the laser was turned off. The same device when operated under a modulation time period of 30 s, showed the PC effect with fast switching (~35 nA, 45 ms) as shown in Figure 5k. Comparing Figure 5j and 5k, we observe a decrease in the current value before the start of each light modulation cycle, indicating reduced PG effect due to the filling of bulk traps with gate induced electrons.

Supported ReS2 device in ON condition


17


Page 18 of 37

Bulk and interface traps are nearly filled with gate induced electrons in the supported device in ON condition, i.e. VG > VTh, as shown in Figure 5d. Hence, the trapping probability of photogenerated carriers is very low. A strikingly fast response (~40 nA, 20 ms) for a measurement setup limit of ±20 ms was observed with the photocurrent being stable until the laser was turned off (~300 s) (Figure 5h). The same drop in current level (~40 nA, R ~ 0.53 mAW-1) was observed following the laser off event. A constant current of similar magnitude (~40 nA) as observed during the sharp drops in all previous cases reinforces PC as its source mechanism and provides a clear distinction between PC and PG effects. For 30 s modulation time period, Figure 5l shows PC dominated 45 nA (R ~ 0.6 mAW-1), 50 ms steps in the photocurrent with no evident increase in the base photocurrent level between consecutive levels (negligible PG). Localized heating due to the laser spot can also lead to thermal effects. Several studies have reported high Seebeck coefficient in TMDs like MoS237,

38

and WSe239 due to the

photothermoelectric (PTE) effect. However, PTE did not contribute significantly to Iph in this work due to the fact that (i) it amplifies mainly near the electrodes whereas the laser spot was incident at the midpoint of the channel far away from the contacts, (ii) the perturbation in local channel potential is expected to be much smaller (~mV) in the inversion regime (VG > VTh) as compared to applied VD (~V), and (iii) the currents generated from PTE are much smaller (~pA) when compared to the currents reported in this study (~nA).

Dynamic Behaviour for High Frequency (180 Hz - 20 kHz) High frequency (180 Hz - 20 kHz) photocurrent measurements were also performed for the supported and suspended ReS2 devices. Figure 6a (b) shows the photocurrent behaviour under laser modulation frequency of 10 kHz for the supported (suspended) channel device at VG = 0 V.


18

Page 19 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The duration during which the laser was on is shown in yellow. The rise times of 13 µs (supported) and 22 µs (suspended) were limited by the measurement setup (10-15 µs) and slower than the transit times of 22 ns (supported) and 16 ns (suspended). This suggests that a faster measurement setup could result in sub-µs rise times. Moreover, the effect of laser power and flake thickness on high frequency performance of the supported ReS2 devices has also been


19


Page 20 of 37

explored (Figure S5, Supporting Information). As expected, Iph shows linear dependence on the incident laser power. Stable photodetection was observed with detectable photocurrents up to 20 kHz modulation frequency. Measured response time of 100 s) which reduces with applied gate bias and/or a change in the device architecture (supported vs suspended). Reduction of NIT in total effective trap density (top arrow in Figure 7a) resulting from the suspended channel architecture resulted in a small change in rise time suggesting that interface traps are not the bottleneck for photodetector speed. Reduction by NBT in effective trap density (middle arrow in

Figure 7. Performance comparison (a) Extracted response time vs estimated effective trap density for the supported and suspended ReS2 photodetectors. Photoconductive (PC) and photogating (PG) dominated regions are shown. Responsivity values (in mAW-1) corresponding to the points are also shown. Devices presented in this work demonstrate gate bias tunability over large responsivity (15×) and response time (4 orders of magnitude) windows. (b) A new, empirical internal dynamic photoresponse (IDP) that represents the intrinsic performance of a photodetector by capturing the trade-off in its dynamic behaviour, i.e. responsivity vs speed, besides normalizing for the effects of applied bias and geometry, is shown for recent reports and this work. (c) Responsivity vs response time for a large number of reported photodetector studies. ReS2 photodetectors in this work are amongst the fastest reported TMD-based photodetectors (‘high speed’ point). 22 ACS Paragon Plus Environment

Page 22 of 37

Page 23 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7a) due to higher laser modulation frequency improved the response time. This was more pronounced in the suspended ReS2 device since only bulk traps contribute to PG. Lastly, application of gate bias in supported ReS2 device achieved the maximum reduction in response time (bottom arrow in Figure 7a) along with a stable photoresponse. Corresponding responsivity values ranging from ~8 to 0.5 mAW-1 are also shown for the corresponding data points in Figure 7a. Gate bias is able to tune the photodetector performance over 4 orders of magnitude for response time (~367 s to 20 ms) for a corresponding 15× (~8 to 0.5 mAW-1) modulation in responsivity. Traps do not play a role in high frequency switching and response time in the range of ~µs could be achieved easily. The NBT and NIT values calculated above are only a quantitative approximation that help to identify the underlying physical phenomena - which is a key goal of this manuscript. A recent report on broadband photoresponse in suspended bulk MoS2 transistors also discusses similar dominance of PC in suspended channel device architecture with decay time in the range of ~1-3 ms.41

A New Photodetector Metric The results and discussion so far indicate a fundamental trade-off between responsivity and response time. We propose an empirical internal dynamic photoresponse (IDP) metric that captures this trade-off in the dynamic behaviour of a photodetector, besides the effects of applied bias and geometry, ‫= ܲܦܫ‬

‫ܫ‬௉௛ (‫ܮ‬/ܹ) , [‫ି ܸ ∙ ܣ‬ଵ ‫ି ݏ‬ଵ ] ܸ஽ ‫ݐ‬௥௘௦௣௢௡௦௘

where Iph is the absolute maximum photocurrent under given illumination, L and W are the channel length and width, respectively, VD is the bias applied across the photodetector and tresponse is the response time of the device. We choose absolute photocurrent over responsivity


23


Page 24 of 37

because it is possible to achieve high responsivity at very low incident power with relatively low photocurrent. In other words, appreciable photocurrent level can be achieved with prolonged exposure to low incident power intensity to ensure high responsivity at the expense of speed. Here L/(WVD) factor normalizes the photocurrent to the applied bias and the channel resistance due to geometry. IDP is analogous to the widely accepted metric of the gain-bandwidth product. While IDP allows us to compare the performance of various 2D material-based photodetectors reported so far, a proper gain-bandwidth analysis is needed to establish the suitability and to compare the performance of these devices with competing non-2D technologies for photodetection applications. Future reports on 2D material-based photodetectors should include the gain-bandwidth analysis of their devices. IDP is plotted for some recent reports of high performance TMD based photodetectors in Figure 7b. The first two data points correspond to the supported device in this work operating in the PG and PC regimes. Although the values of responsivity and speed are significantly different for the two operating regimes, the IDP value in both cases is the same indicating the intrinsic performance of the device irrespective of operating conditions. Alternatively, higher value of IDP indicates a fundamental improvement in that the photodetector is able to give higher responsivity at given speed or faster response at given responsivity. Thus, IDP provides a single unifying benchmark to evaluate intrinsic photodetector performance without having to worry about varying operating conditions that can be chosen so as to highlight either high responsivity at the expense of speed or fast response at the expense of responsivity.

Benchmarking Against Reported Values Figure 7c shows a responsivity vs response time scatter plot for various photodetector reports employing a wide range of 2D materials and their heterostructures. Photodetectors based on


24

Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


individual TMDs span a large range of responsivity/speed with only a few reports of fast response times in the range of ~µs. Other elemental 2D materials such as graphene (Gr)42 and black phosphorus (BP),43-45 and hybrid structures such as Gr-MoS2-Si,19 Si-BP46 and carbon nanotube (CNT)-MoS247 can perform faster, close to commercially available high performance Si and InGaAs photodetectors;20 while other devices based on TMDs cover a large area of the plot.27, 28, 31, 32, 41, 48-61 Alloyed 2D materials62, 63 and topological insulators like Bi2Te3, alloyed with Si64 and as contacts to SnSe65 and SnS66 channels have also been employed for photodetection. Two points (red squares in Figure 7c) are marked for this work on ReS2corresponding to high responsivity (~4 AW-1: at low incident power) and high speed (~20 µs: at high modulation frequency). No explicit distinction is made between supported and suspended channel devices as they perform similarly at the device level as far as photodetection metrics (responsivity and speed) are concerned.

Conclusions In summary, the dependence of ReS2 photodetector performance on incident power, device architecture, and gate bias has been demonstrated. Supported and suspended channel devices were fabricated to study architectural dependence. The presence of PG and PC effects was observed in transfer characteristics of the ReS2 photodetector under illumination. Photocurrent scans established uniformity in photodetection over the entire channel area as well as a high responsivity value for the photodetector. Furthermore, evolution of photocurrent with time was studied under different gate bias demonstrating high speed operation using gate bias to enhance PC over PG. A model based on bulk traps in ReS2 and interface traps at the ReS2/SiO2 interface was used to explain the gate bias dependence of the speed and responsivity. High responsivity of


25


Page 26 of 37

~4 AW-1 and fast and stable photodetection with response time less than 20 µs for a detectable photocurrent of ~40 nA on the same device demonstrated a large window of operation to suit varied applications. The fast response time was limited by the measurement setup (~10-15 µs) and does not indicate the intrinsic limit of the photodetectors. Also, tunability over 4 orders of magnitude (15×) in the photodetector response time (responsivity) from ~367 s to ~20 ms (~8 to ~0.53 mAW-1) was demonstrated via modulation of only the gate bias. The benchmarking of this work with other reports, against standard metrics of responsivity and speed as well as an improved metric (IDP) that captures the intrinsic performance (responsivity vs speed trade-off), puts this work amongst the fastest TMD photodetectors reported till date and better intrinsic performance than recent reports.

Methods Device fabrication and characterization: Both the supported and suspended ReS2 devices were fabricated on a SiO2 (285 nm)/p++-Si substrate acting as the global back gate. ReS2 flakes were mechanically exfoliated from a crystal (purchased from hqGraphene) on the substrate using a scotch tape. The samples for supported ReS2 devices were resist-coated with EL9/PMMA-A4 bilayer at 3200/2000 RPM. To make suspended channel devices, ReS2 was exfoliated after coating EL9 copolymer, followed by PMMA coating. Typical electron-beam lithography (EBL) technique was used to pattern the source and drain contacts followed by metal deposition of Cr/Au using a 7-target sputtering tool from AJA International, USA. Cr/Au electrode thicknesses for supported and suspended devices were 5 nm/80 nm and 10 nm/250 nm, respectively. The metal lift-off was done by keeping the samples in acetone for ~8-12 h at room temperature. Raman and PL measurements were carried out on exfoliated ReS2 flakes in LabRAM HR800, Horiba Scientific with a 1 µm 532 nm laser spot. XPS results were obtained from larger (~20 ×


26

Page 27 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 µm) ReS2 flakes located using secondary electron (SXI) imaging using Phi 5000 VersaProbe II, Ulvac-Phi Inc., XPS system. AFM measurements were performed in MFP-3D, Asylum Research Inc., USA. Agilent B1500A fast measurement SMU was used for electrical measurements under ambient conditions. Photocurrent measurements: For optical measurements, the devices were loaded in a custommade vacuum chamber designed to hold the pressure up to ~1 mbar and mounted under the Mitutoyo 100X (NA = 0.55) long working distance objective lens in a WiTec Alpha300R system. Photocurrent scans were performed using WiTec Alpha300R. Electrical signals were applied and sensed using a National Instruments (NI) data acquisition (DAQ) system (USB 6216-BNC) through LabView software using a current to voltage converter (DL Instruments 1211) and a lock-in amplifier (SR830, Stanford Research Systems). Laser source of 633 nm (part of Witec Alpha300R) with adjustable output power was used and modulated using a function generator (DS360, Stanford Research Systems) and an optical modulator (MT200-R18-FIO, AA Optoelectronics) to characterize the optical behaviour of the ReS2 devices. Low frequency (sub-1 Hz) dynamic photocurrent measurements were done using NI DAQ (USB 6216-BNC) and Lecroy DSO (WaveAce 2024) was used to acquire high frequency photocurrent response.

ASSOCIATED CONTENT Supporting Information. It contains details on AFM image, extracted electrical parameters for the supported and suspended ReS2 devices, photocurrent scan measurement setup, calculation of effective laser power, effect of laser power and flake thickness on high frequency photoresponse and calculation of trap densities. AUTHOR INFORMATION


27


Page 28 of 37

Corresponding Author *Email: [email protected] Present Addresses Ϯ International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan. ϮϮ Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot- 7610001, Israel. ϮϮϮ Device Technology Solutions, Micron Technology, Bangalore 560008, India. Author Contributions K.T., B.M., M.D. and S.L. conceived the idea and created the experiment plan. K.T., N.K. and B.M. fabricated the ReS2 photodetectors. N.K. and B.M. measured Raman, PL and XPS spectra for ReS2. K.T. measured electrical response. K.T., B.M. and S.G. did photocurrent scan measurements. K.T. and S.G. did the photocurrent dynamic measurements. K.T. analysed the experimental data and benchmarked against the previous reports. All authors contributed in writing the manuscript. Funding Sources K.T. acknowledges Visvesvaraya PhD Scheme from Ministry of Electronics and Information Technology (MeitY), Govt. of India and S.L. acknowledges the Department of Science and Technology (DST), Govt. of India for funding support. ACKNOWLEDGMENT


28

Page 29 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Authors acknowledge Indian Institute of Technology, Bombay Nanofabrication Facility (IITBNF) for usage of its facilities for device fabrication and characterization.

REFERENCES [1]

Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.;

Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science, 2004, 306, 666. [2]

Zhang, X.; Hou, L.; Ciesielski, A.; Samorì, P. 2d Materials Beyond Graphene for High‐

Performance Energy Storage Applications. Adv. Energ. Mater., 2016, 6. [3]

Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and

Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol., 2012, 7. [4]

Chhowalla, M.; Jena, D.; Zhang, H. Two-Dimensional Semiconductors for Transistors.

Nat. Rev. Mater., 2016, 1, 16052. [5]

Schaibley, J. R.; Yu, H.; Clark, G.; Rivera, P.; Ross, J. S.; Seyler, K. L.; Yao, W.; Xu, X.

Valleytronics in 2d Materials. Nat. Rev. Mater., 2016, 1, 16055. [6]

Murray, H.; Kelty, S.; Chianelli, R.; Day, C. Structure of Rhenium Disulfide. Inorg.

Chem., 1994, 33, 4418. [7]

Ho, C. H.; Huang, C. E. Optical Property of the near Band-Edge Transitions in Rhenium

Disulfide and Diselenide. Journ. Alloys and Comp., 2004, 383, 74. [8]

Ho, C. H.; Huang, Y. S.; Tiong, K. K. In-Plane Anisotropy of the Optical and Electrical

Properties of ReS2 and ReSe2 Layered Crystals. Journ. Alloys and Comp., 2001, 317, 222.


29


[9]

Page 30 of 37

Tiong, K. K.; Ho, C. H.; Huang, Y. S. The Electrical Transport Properties of ReS2 and

ReSe2 Layered Crystals. Solid State Commun., 1999, 111, 635. [10] Tongay, S.; Sahin, H.; Ko, C.; Luce, A.; fan, W.; Liu, K.; Zhou, J.; Huang, Y. S.; Ho, C. H.; Yan, J; Ogletree, D. F.; Aloni, S; Ji, J; Li, S; Li, J; Peeters, F. M.; Wu, J. Monolayer Behaviour in Bulk ReS2 Due to Electronic and Vibrational Decoupling. Nat. Commun., 2014, 5, 3252. [11] Ho, C. H. Dichroic Electro-Optical Behavior of Rhenium Sulfide Layered Crystal. Cryst. Struct. Theor. Appl., 2016, 2, 65. [12] Chenet, D. A.; Aslan, O. B.; Huang, P. Y.; Fan, C.; Zande, A. M. v. d.; Heinz, T. F.; Hone, J. C. In-Plane Anisotropy in Mono- and Few-Layer ReS2 Probed by Raman Spectroscopy and Scanning Transmission Electron Microscopy. Nano Lett., 2015, 15, 5667. [13] Lee, D.; Sim, S.; Cho, S.; Shim, W.; Choi, H. Ultrafast Anisotropic Dynamics of NonDegenerated Excitons in in Atomically Thin ReS2. Conference on Lasers and Electro Optics, San Jose, CA (USA), 2017. [14] Abate, Y.; Akinwande, D.; Gamage, S.; Wang, H.; Snure, M.; Poudel, N.; Cronin, S. B. Recent Progress on Stability and Passivation of Black Phosphorus. Adv.Mater., 2018, 1704749. [15] Goyal, N.; Kaushik, N.; Jawa, H.; Lodha, S. Enhanced Stability and Performance of FewLayer Black Phosphorus Transistors by Electron Beam Irradiation. Nanoscale, 2018, 10, 11616. [16] Tao, L.; Cinquanta, E.; Chiappe, D.; Grazianetti, C.; Fanciulli, M.; Dubey, M.; Molle, A.; Akinwande, D. Silicene Field-Effect Transistors Operating at Room Temperature. Nat. Nanotechnol., 2015, 10, 227. [17] Zhang, Y.; Liu, T.; Meng, B.; Li, X.; Liang, G.; Hu, X.; Wang, Q. J. Broadband High Photoresponse from Pure Monolayer Graphene Photodetector. Nat. Commun., 2013, 4, 1811.


30

Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[18] Roy, K.; Padmanabhan, M.; Goswami, S.; Sai, T. P.; Ramalingam, G.; Raghavan, S.; Ghosh, A. Graphene-MoS2 Hybrid Structures for Multifunctional Photoresponsive Memory Devices. Nat. Nanotechnol., 2013, 8, 826. [19] Tao, L.; Chen, Z.; Li, X.; Yan, K.; Xu, J. B. Hybrid Graphene Tunneling Photoconductor with Interface Engineering Towards Fast Photoresponse and High Responsivity. 2D Mater. Appl., 2017, 1, 19. [20] Buscema, M.; Island, J. O.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; Zant, H. S. J. v. d.; Gomez, A. C. Photocurrent Generation with Two-Dimensional Van Der Waals Semiconductors. Chem. Soc. Rev., 2015, 44, 3691. [21] Xie, C.; Mak, C.; Tao, X.; Yan, F. Photodetectors Based on Two‐Dimensional Layered Materials Beyond Graphene. Adv. Funct. Mater., 2017, 27, 1603886. [22] Rahman, M.; Davey, K.; Qiao, S. Z. Advent of Rhenium Disulfide (ReS2): Fundamentals to Applications. Adv. Funct. Mater., 2017, 27, 1606129. [23] Liu, E.; Long, M.; Zeng, J.; Luo, W.; Wang, Y.; Pan, Y.; Zhou, W.; Wang, B.; Hu, W.; Ni, Z.; You, Y.; Zhang, X.; Qin, S.; Shi, Y.; Watanabe, K.; Taniguchi, T.; Yuan, H.; Hwang, H. Y.; Cui, Y.; Miao, F.; Xing, D. High Responsivity Phototransistors Based on Few-Layer ReS2 for Weak Signal Detection. Adv. Funct. Mater., 2016, 26, 1938. [24] Liu, F.; Zheng, S.; He, X.; Chaturvedi, A.; He, J.; Chow, W. L.; Mion, T. R.; Wang, X.; Zhou, J.; Fu, Q.; Fan, H. J.; Tay, B. K.; Song, L.; He, R. H.; Kloc, C.; Ajayan, P. M.; Liu, Z. Highly Sensitive Detection of Polarized Light Using Anisotropic 2d ReS2. Adv. Funct. Mater., 2016, 26, 1169. [25] Shim, J.; Oh, A.; Kang, D. H.; Oh, S.; Jang, S. K.; Jeon, J.; Jeon, M. H.; Kim, M.; Choi, C.; Lee, J.; Lee, S.; Yeom, G. Y.; Song, Y. J.; Park, J. H. High-Performance 2d Rhenium


31


Page 32 of 37

Disulfide (ReS2) Transistors and Photodetectors by Oxygen Plasma Treatment. Adv.Mater., 2016, 28, 6985. [26] Hafeez, M.; Gan, L.; Li, H.; Ma, Y.; Zhai, T. Large-Area Bilayer ReS2 Film/Multilayer ReS2 Flakes Synthesized by Chemical Vapor Deposition for High Performance Photodetectors. Adv. Funct. Mater., 2016, 26, 4551. [27] Yin, Z.; Li, H.; Li, H.; Jiang, L.; Shi, Y.; Sun, Y.; Lu, G.; Zhang, Q.; Chen, X.; Zhang, H. Single-Layer MoS2 Phototransistors. ACS Nano, 2011, 6, 74. [28] Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2. Nat. Nanotechnol., 2013, 8, 497. [29] Furchi, M. M.; Polyushkin, D. K.; Pospischil, A.; Mueller, T. Mechanisms of Photoconductivity in Atomically Thin MoS2. Nano Lett., 2014, 14, 6165. [30] Klots, A. R.; Nawaz, A. K. M.; Wang, B.; Prasai, D.; Krzyzanowska, H.; Caudel, D.; Ghimire, N. J.; Yan, J.; Ivanov, B. L.; Velizhanin, K. A.; Burger, A.; Mandrus, D. G.; Tolk, N. H.; Pantelides, S. T.; Bolotin, K. I. Probing Excitonic States in Ultraclean Suspended TwoDimensional Semiconductors by Photocurrent Spectroscopy. Sci. Rep., 2014, 4, 6608. [31] Zhang, W.; Chiu, M. H.; Chen, C. H.; Chen, W.; Li, L. J.; Wee, A. T. S. Role of Metal Contacts in High-Performance Phototransistors Based on WSe2 Monolayers. ACS Nano, 2014, 8, 8653. [32] Tamalampudi, S. R.; Lu, Y. Y.; Kumar U, R.; Sankar, R.; Liao, C. D.; Moorthy B, K.; Cheng, C. H.; Chou, F. C.; Chen, Y. T. High Performance and Bendable Few-Layered Inse Photodetectors with Broad Spectral Response. Nano Lett., 2014, 14, 2800. [33] Mukherjee, B.; Thakar, K.; Kaushik, N.; Lodha, S. Suspended ReS2 FET for Improved Photocurrent-Time Response. Device Research Conference, South Bend, IN (USA), 2017.


32

Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[34] Kaushik, N.; Ghosh, S.; Lodha, S. Low-Frequency Noise in Supported and Suspended MoS2 Transistors. IEEE Tans. Elect. Dev., 2018, 1. [35] Mukherjee, B.; Kaushik, N.; Tripathi, R. P.; Joseph, A.; Mohapatra, P.; Dhar, S.; Singh, B.; Kumar, G. P.; Simsek, E.; Lodha, S. Exciton Emission Intensity Modulation of Monolayer MoS2 Via Au Plasmon Coupling. Sci. Rep., 2017, 7, 41175. [36] Fang, H.; Hu, W. Photogating in Low Dimensional Photodetectors. Adv. Sci., 2017, 4, 1700323. [37] Buscema, M.; Barkelid, M.; Zwiller, V.; van der Zant, H. S.; Steele, G. A.; CastellanosGomez, A. Large and Tunable Photothermoelectric Effect in Single-Layer MoS2. Nano Lett., 2013, 13. [38] Zhang, Y.; Li, H.; Wang, L.; Wang, H.; Xie, X.; Zhang, S.-L.; Liu, R.; Qiu, Z.-J. Photothermoelectric and Photovoltaic Effects Both Present in MoS2. Sci. Rep., 2015, 5. [39] Groenendijk, D. J.; Buscema, M.; Steele, G. A.; Michaelis de Vasconcellos, S.; Bratschitsch,

R.;

van

der

Zant,

H.

S.;

Castellanos-Gomez,

A.

Photovoltaic

and

Photothermoelectric Effect in a Double-Gated WSe2 Device. Nano Lett., 2014, 14. [40] Kaushik, N.; Mackenzie, D. M.; Thakar, K.; Goyal, N.; Mukherjee, B.; Boggild, P.; Petersen, D. H.; Lodha, S. Reversible Hysteresis Inversion in MoS2 Field Effect Transistors. npj 2D Materials and Applications, 2017, 1, 34. [41] Saenz, G. A.; Karapetrov, G.; Curtis, J.; Kaul, A. B. Ultra-High Photoresponsivity in Suspended Metal-Semiconductor-Metal Mesoscopic Multilayer MoS2 Broadband Detector from UV-to-IR with Low Schottky Barrier Contacts. Sci. Rep., 2018, 8, 1276. [42] Xia, F.; Mueller, T.; Lin, Y. M.; Valdes-Garcia, A.; Avouris, P. Ultrafast Graphene Photodetector. Nat. Nanotechnol., 2009, 4, 839.


33


Page 34 of 37

[43] Buscema, M.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; Van Der Zant, H. S.; Castellanos-Gomez, A. Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors. Nano Lett., 2014, 14, 3347. [44] Buscema, M.; Groenendijk, D. J.; Steele, G. A.; Zant, H. S. J. v. d.; Castellanos-Gomez, A. Photovoltaic Effect in Few-Layer Black Phosphorus PN Junctions Defined by Local Electrostatic Gating. Nat. Commun., 2014, 5, 4651. [45] Youngblood, N.; Chen, C.; Koester, S. J.; Li, M. Waveguide-Integrated Black Phosphorus Photodetector with High Responsivity and Low Dark Current. Nat. Photonics, 2015, 9, 247. [46] Chen, C.; Youngblood, N.; Peng, R.; Yoo, D.; Mohr, D. A.; Johnson, T. W.; Oh, S. H.; Li, M. Three-Dimensional Integration of Black Phosphorus Photodetector with Silicon Photonics and Nanoplasmonics. Nano Lett., 2017, 17, 985. [47] Jariwala, D.; Sangwan, V. K.; Wu, C. C.; Prabhumirashi, P. L.; Geier, M. L.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C. Gate-Tunable Carbon Nanotube–MoS2 Heterojunction PN Diode. Proc. Natl. Acad. Sci., 2013, 110, 18076. [48] Chen, J.; Liu, B.; Liu, Y.; Tang, W.; Nai, C. T.; Li, L.; Zheng, J.; Gao, L.; Zheng, Y.; Shin, H. S. Chemical Vapor Deposition of Large‐Sized Hexagonal WSe2 Crystals on Dielectric Substrates. Adv.Mater., 2015, 27, 6722. [49] Lei, S.; Ge, L.; Najmaei, S.; George, A.; Kappera, R.; Lou, J.; Chhowalla, M.; Yamaguchi, H.; Gupta, G.; Vajtai, R. Evolution of the Electronic Band Structure and Efficient Photo-Detection in Atomic Layers of InSe. ACS Nano, 2014, 8, 1263.


34

Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[50] Jacobs-Gedrim, R. B.; Shanmugam, M.; Jain, N.; Durcan, C. A.; Murphy, M. T.; Murray, T. M.; Matyi, R. J.; Moore, R. L.; Yu, B. Extraordinary Photoresponse in Two-Dimensional In2Se3 Nanosheets. ACS Nano, 2013, 8, 514. [51] Tsai, D. S.; Liu, K. K.; Lien, D. H.; Tsai, M. L.; Kang, C. F.; Lin, C. A.; Li, L. J.; He, J. H. Few-Layer MoS2 with High Broadband Photogain and Fast Optical Switching for Use in Harsh Environments. ACS Nano, 2013, 7, 3905. [52] Kwon, J.; Hong, Y. K.; Han, G.; Omkaram, I.; Choi, W.; Kim, S.; Yoon, Y. Giant Photoamplification in Indirect‐Bandgap Multilayer MoS2 Phototransistors with Local Bottom‐ Gate Structures. Adv.Mater., 2015, 27, 2224. [53] Li, X.; Zhu, M.; Du, M.; Lv, Z.; Zhang, L.; Li, Y.; Yang, Y.; Yang, T.; Li, X.; Wang, K. High Detectivity Graphene‐Silicon Heterojunction Photodetector. Small, 2016, 12, 595. [54] Pradhan, N. R.; Ludwig, J.; Lu, Z.; Rhodes, D.; Bishop, M. M.; Thirunavukkuarasu, K.; McGill, S. A.; Smirnov, D.; Balicas, L. High Photoresponsivity and Short Photoresponse Times in Few-Layered WSe2 Transistors. ACS Appl. Mat. Inter., 2015, 7, 12080. [55] Abderrahmane, A.; Ko, P.; Thu, T.; Ishizawa, S.; Takamura, T.; Sandhu, A. High Photosensitivity

Few-Layered

MoSe2

Back-Gated

Field-Effect

Phototransistors.

Nanotechnology, 2014, 25, 365202. [56] Zhang, W.; Huang, J. K.; Chen, C. H.; Chang, Y. H.; Cheng, Y. J.; Li, L. J. High‐Gain Phototransistors Based on a CVD MoS2 Monolayer. Adv.Mater., 2013, 25, 3456. [57] Kufer, D.; Konstantatos, G. Highly Sensitive, Encapsulated MoS2 Photodetector with Gate Controllable Gain and Speed. Nano Lett., 2015, 15, 7307. [58] Lan, C.; Li, C.; Yin, Y.; Liu, Y. Large-Area Synthesis of Monolayer WS2 and Its Ambient-Sensitive Photo-Detecting Performance. Nanoscale, 2015, 7, 5974.


35


Page 36 of 37

[59] Yang, S.; Tongay, S.; Li, Y.; Yue, Q.; Xia, J. B.; Li, S. S.; Li, J.; Wei, S. H. LayerDependent Electrical and Optoelectronic Responses of ReSe2 Nanosheet Transistors. Nanoscale, 2014, 6, 7226. [60] Huo, N.; Yang, S.; Wei, Z.; Li, S. S.; Xia, J. B.; Li, J. Photoresponsive and Gas Sensing Field-Effect Transistors Based on Multilayer WS2 Nanoflakes. Sci. Rep., 2014, 4, 5209. [61] Zhang, E.; Jin, Y.; Yuan, X.; Wang, W.; Zhang, C.; Tang, L.; Liu, S.; Zhou, P.; Hu, W.; Xiu, F. ReS2‐Based Field‐Effect Transistors and Photodetectors. Adv. Funct. Mater., 2015, 25, 4076. [62] Zheng, Z.; Yao, J.; Yang, G. Centimeter-Scale Deposition of Mo0.5W0.5Se2 Alloy Film for High-Performance Photodetectors on Versatile Substrates. ACS Appl. Mat. Inter., 2017, 9, 14920. [63] Yao, J.; Zheng, Z.; Yang, G. Promoting the Performance of Layered-Material Photodetectors by Alloy Engineering. ACS Appl. Mat. Inter., 2016, 8, 12915. [64] Yao, J.; Shao, J.; Wang, Y.; Zhao, Z.; Yang, G. Ultra-Broadband and High Response of the Bi2Te3–Si Heterojunction and Its Application as a Photodetector at Room Temperature in Harsh Working Environments. Nanoscale, 2015, 7, 12535. [65] Yao, J.; Zheng, Z.; Yang, G. All‐Layered 2D Optoelectronics: A High‐Performance UV– Vis–NIR Broadband SnSe Photodetector with Bi2Te3 Topological Insulator Electrodes. Adv. Funct. Mater., 2017, 27, 1701823. [66] Yao, J.; Yang, G. Flexible and High‐Performance All‐2d Photodetector for Wearable Devices. Small, 2018, 14, 1704524.


36

Page 37 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC FIGURE:


37

Multilayer ReS2 Photodetectors with Gate Tunability for High

Recommend Documents