Growth of Wafer-Scale Standing Layers of WS2 for Self-Biased High

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Growth of Wafer-Scale Standing Layers of WS2 for SelfBiased High-Speed UV-Visible-NIR Optoelectronic Devices Hong-Sik Kim, Malkeshkumar Patel, Joondong Kim, and Mun Seok Jeong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16397 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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ACS Applied Materials & Interfaces

Growth of Wafer-Scale Standing Layers of WS2 for Self-Biased High-Speed UV-Visible-NIR Optoelectronic Devices Hong-Sik Kim,†, ^, # Malkeshkumar Patel, †,^ Joondong Kim*, †,^ and Mun Seok Jeong # †

Photoelectric and Energy Device Application Lab (PEDAL), Multidisciplinary Core Institute

for Future Energies (MCIFE), Incheon National University, 119 Academy Rd. Yeonsu, Incheon, 22012, Republic of Korea ^

Department of Electrical Engineering, Incheon National University, 119 Academy Rd.

Yeonsu, Incheon, 22012, Republic of Korea #

Department of Energy Science, Sungkyunkwan University, Suwon 440746, Republic of

Korea KEYWORDS. Wafer-scale; 2D material; WS2; vertically aligned growth; RF sputtering; selfbiased, broadband photodetector;

ACS Paragon Plus Environment

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ABSTRACT.

This work describes the wafer-scale standing growth of (002)-plane-oriented layers of WS2 and their suitability for use in self-biased broadband high-speed photodetection. The WS2 layers are grown using large-scale sputtering, and the effects of the processing parameters such as the deposition temperature, time, and sputtering power are studied. The structural, physical, chemical, optical, and electrical properties of the WS2 samples are also investigated. Based on the broadband light absorption and high-speed in-plane carrier transport characteristics of the WS2 layers, a self-biased broadband high-speed photodetector is fabricated by forming a type-II heterojunction. This WS2/Si heterojunction is sensitive to ultraviolet, visible, and near-infrared photons and shows an ultrafast photoresponse (1.1 µs) along with excellent responsivity (3 mA/W) and specific detectivity (1010 Jones). A comprehensive Mott-Schottky analysis is performed to evaluate the device parameters of the device, such as the frequency-dependent flatband potential and carrier concentration. Further, the photodetection parameters of the device, such as its linear dynamic range, rising time, and falling time, are evaluated in order to elucidate its spectral and transient characteristics. The device exhibits remarkably improved transient and spectral photodetection performances as compared to those of photodetectors based on atomically thin WS2 and two-dimensional materials. These results suggest that the proposed method is feasible for the manipulation of vertically standing WS2 layers that exhibit high inplane carrier mobility and allow for high-performance broadband photodetection.

INTRODUCTION Photonic and optoelectronic devices based on two-dimensional (2D) semiconducting materials demand the production of high-quality materials at the wafer scale. The use of


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transition metal dichalcogenides (TMDCs), also known as non-graphene materials, is on the rise, as they have the potential to further the applicability of 2D materials. Bulk crystals of TMDCs consisting of Van der Waals-bonded layers can be separated into structures with atomic-scale thickness, which exhibit desirable stable mechanical, electronic, and optical properties, in contrast to their bulk counterparts. TMDCs have the common formula MX2, where M represents Mo, W, Nb, Ta, or Re, and X represents Se, S, or Te. Of the various TMDCs being explored, WS2 and MoS2 in particular show material properties that make them suitable for use in optoelectronics and energy applications.1–6 Specifically, WS2 exhibits indirect bandgaps of 1.4 eV and 2.1 eV in the bulk and monolayer states, respectively.7–11 WS2 is thus used in the layer form in devices such as phototransistors,2,12,13 photodetectors,2,14–17 Zener phototunnel diodes,18 gas sensors,13,19,20 photoelectrodes and catalysts for water splitting,21–25 and solar cells.26–29 Recently, various 2D materials have been reported by HfS2,30 MoS2,31 SnS2,32 SnS,33 MoSe2, WSe2,34 and ReS2,35 which also inspired us to grew vertically oriented WS2 layers. A top-down approach relies on the chemical/mechanical exfoliation of bulk crystals while a bottom-up approach relies on fabrication methods such as chemical vapor deposition (CVD),11,12,36–38 atomic layer deposition (ALD),19 chemical vapor transport (CVT),39 and pulsed laser deposition (PLD),40–43 and molecular beam epitaxy.44–47 The large-area wafer-scale growth of WS2 would significantly increase its commercial applicability. However, there are no reports on the wafer-scale synthesis of this material. In this context, the sputtering method would certainly be advantageous as compared to the other aforementioned processes, owing to its simplicity, choice of substrates, high deposition rate, high reproducibility, reactant-free nature, and thickness control.


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In this work, we attempted to grow wafer-scale vertically aligned WS2 layers by magnetron sputtering and evaluated their suitability for use as self-biased, high-speed, broadband—namely, ultraviolet (UV), visible, and near-infrared (NIR)—photodetectors. The effects of the processing parameters such as the deposition temperature, time, and sputtering power on the vertically oriented WS2 layers were analyzed. The highly crystalline WS2 layers were successfully grown and the preferential orientation was investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) analyses. The oriented WS2 layers were used to construct a heterojunction with p-Si; the resulting device was fully characterized with respect to its suitability for broadband photodetection applications. Significantly, the WS2/Si photodetector exhibited photovoltaic-activity-enabled self-biased, broadband photodetection. This self-driven large-area (1 cm2) WS2/Si photodetector showed outstanding performance, including an ultrafast photoresponse time (1.1 µs), high dynamic range (90 dB), high responsivity (4 mA/W), and high specific detectivity (~ 2×1010 Jones). These values can be attributed to the excellent diode properties of the WS2/Si interface, including its low dark current ( 20000 at +2 V), as determined by MottSchottky analysis. The photodetection performance of the WS2/p-Si device, which was significantly higher than that of other WS2-based devices (Table 1), as well as conventional devices and devices based on other 2D materials, highlights the suitability of the proposed method for fabricating high-performing self-biased broadband photonic devices. RESULTS AND DISCUSSION Figure 1 illustrates the wafer-scale production of WS2 films. To fabricate the films, a 2-inch (diameter) WS2 target was RF-sputtered in flowing Ar (50 sccm) at a deposition pressure of 10 mTorr. The WS2 growth was evaluated for various deposition temperatures (Td), sputtering


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powers (W), and deposition times (t). The wafer-scale WS2 film formed on a 4-inch quartz wafer is shown in Figure 1b.

Figure 1. (a) Wafer-scale fabrication of WS2 film. Effects of deposition temperature, time, and sputtering power were studied. (b) Wafer-scale sample of vertically oriented WS2 layers. Table 1. Characteristics of photodetector based on vertically oriented WS2 layers/Si heterojunction and their comparison with those of other similar previously reported devices. Here A, λ, τr, R, D, LDR are the active area of the device, wavelength of light, rising time, responsivity, detectivity, and linear dynamic range, respectively. Device structure

WS2 fabrication

Condition A (mm2)

Bias, λ

τr

R

D (Jones)

Vertical oriented WS2

RF sputtering 100 (wafer scale)

0V, 520 nm

Year/Ref. (dB) 52

0V, 365 nm Al/WS2/p-Si/Al

LDR

1.1 µs

4 mA/W

1.5×1010

0V, 850 nm

71

This work

90

W sulfurization

4

-5V, (340-1100 0.67 ms nm)

5.5 A/W

-

-

2017 / 18

WO3+sulfure @ 950oC

~0.01

2V, 405 nm

91 s

950 A/W

-

34

2017 / 16

0.2

5V, 365 nm

-

53.3 A/W 1.2×1011

-

2016 / 48

(on quartz)

Sputtering, postannealing

Cr/Au/Gra./WS2/Gra. /Au/Cr

CVD, transferred

10-6

5V, 532 nm

-

3.5 A/W

9.9×1010

58

2016 / 37

Mechanical exfoliation

0.0003

1V, 633 nm

＜20 ms

5.7 A/W

-

54

201413

CVD

0.05

5V, 514 nm

5.3 ms

15 µA/W

-

60

201317

Ag/WS2/p-Si/Ag WS2/Gra. (on SiO2/Si) Au/Ti/WS2/Ti/Au

Au/WS2/Au (on SiO2/Si) Au/Ti/WS2/Ti/Au (on quartz)


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FESEM images (topographical and cross-sectional) of the WS2 samples grown on Si substrates at room temperature, 200 °C, and 400 °C are shown in Figure 2a, b, and c, respectively. It can be seen that Td affects the morphology of the grown samples. The topographical and cross-sectional images of the WS2 films grown at 200 °C and 400 °C confirm the formation of standing layers at the nanoscale. The FESEM images shown were taken at a magnification of 100k for convenience. The images suggest that the number density of the vertical layers is significantly higher at 400 °C. However, the thicknesses of the two samples are almost similar. Further, owing to the increased number density of the vertical layers, the degree of interconnection between the vertical nanolayers is also higher; this is desirable for fast carrier transport, particularly in the in-plane direction. The XRD patterns of these samples, which are shown in Figure 2d, indicate that the WS2 layers grown at 400 °C are highly crystalline, as a single diffraction peak is present in their pattern, at 2θ of 14.16°; this peak corresponds to the (002) plane of hexagonal WS2 (crystallography open database, COD- 9009145). Thus, it was confirmed that highly crystalline and layered WS2 was grown preferentially on a large scale. Further, the diffraction intensity was greater than 15k for the fast scan (total scan time of 30 min). Moreover, the observed highintensity XRD peak suggests that the WS2 layers grown at 400 °C not only had a higher number density but were also more crystallized than the ones grown at 200 °C, even though the thickness of the former was lower (Figure 2c, 43.1 nm). Thus, the results of the XRD analysis suggest that a Td of 400 °C is preferable for growing high-quality vertically oriented WS2 layers with the lattice dimensions a = 3.14 Å and b = 12.5 Å. The atomic structure of WS2 is shown in Figure 2e; the (002) plane can be seen in the figure. Thus, vertically oriented WS2 layers, which are highly desirable for ensuring rapid in-plane carrier transport, were synthesized successfully using


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the proposed method.23,29,49,50 The vertical, layered growth of WS2 can probably be attributed to the compression and extrusion of the fabricated WS2 plates, as is also the case for MoS2.24,51,52 Figure 2f shows the thickness dependent Raman spectra of the WS2 samples (grown at 400 oC on Si substrates) for wavenumbers of 150–800 cm-1 as measured at room temperature. Raman spectroscopy is particularly useful for studying layered materials.17 The Raman spectra of these samples contain 1st- and 2nd-order characteristic peaks.36,38 The 1st-order peaks were the following: LA(M) at 175 cm-1, E12g at 356 cm-1, and A1g at 417 cm-1. Further, the 2nd-order peaks were as follows: A1g(M)-LA(M) at 230 cm-1, 2LA(M)-3E12g(M) at 265 cm-1, 2LA(M)-2E22g(M) at 296 cm-1, 2LA(M) at 250 cm-1, A1g(M)+LA(M) at 585 cm-1, and 4LA(M) at 705 cm-1. These peaks are like those observed in the case of monolayer WS2, suggesting that the vertically standing samples were either monolayer or multiple-layer but not bulk. A strong Raman peak at 520 cm-1 is corresponding to the Si substrate. The lattice resonance of WS2 monolayers and the ratio of the intensities of the A1g and 2LA(M) Raman peaks are related to the number of layers.53,54 The differences in the wavenumbers of the A1g and 2LA(M) Raman peaks corresponding to mono-, bi-, and trilayer WS2 are 65.8 cm-1, 67.1 cm-1, and 68.1 cm-1, respectively.54 The Raman characteristic peaks corresponding to these lattice resonances, as shown in Figure 2g, had a wavenumber difference of 69.43 cm-1, which indicated that the vertically standing WS2 sample probably consisted of more than three layers.53


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Figure 2. Topographical and cross-sectional FESEM images of WS2 samples prepared at (a) room temperature, (b) 200 °C, and (c) 400 °C. Sputtering deposition time and power were 30 min and 25 W, respectively. (d) XRD spectra (background corrected) of WS2 samples. (e) Atomic structure of hexagonal WS2, (002) planes shown confirm vertical orientation of WS2 layers. Gray and orange spheres represent W and S atoms, respectively. (f) Thickness dependent Raman spectra of WS2 films prepared at 400 oC, and (g) E12g and A1g vibrational modes of WS2 layer. The phase purities and chemical states of the grown WS2 samples were confirmed by Xray photoelectron spectroscopy (XPS). Figure 3a and b show the high-resolution W 4f and S 2p


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spectra, respectively, of the WS2 samples prepared at RT, 200 °C, and 400 °C. The W 4f7/2, W 4f5/2, and W 5p3/2 peaks, which appear at approximately 33.0 eV, 35.5 eV, and 38.1 eV, respectively, were indicative of the oxidation states of W. Further, the S 2p1/2 and 2p3/2 peaks at 162.4 eV and 163.5 eV, respectively, suggested that S was present as S2-. These peaks are identical to those seen in the case of high-purity WS2.24,55 Moreover, the 4f7/2 peak of W 4f and the 2p1/2 peak of S 2p remained unresolved in the case of the RT sample. However, the XPS peaks of the WS2 samples prepared at 200 °C and 400 °C suggested that the chemical states of W and S in the samples were similar. Figure 3c shows the transmittance (T) and absorbance (A) spectra of high-quality vertically standing WS2. This sample was prepared on a quartz wafer. It can be seen that the ultrathin WS2 film exhibited strong light absorption. The exciton-induced absorption of photons by WS2 results in bands at wavelengths of 628 nm, 525 nm, and 454 nm; these are the A, B, and C exciton bands of WS2. However, these absorptions peaks could not be resolved in the case of the vertically oriented WS2 layers.53,56 The optical bandgap (Eg) was calculated using the Tauc relation (αhν )1/γ = β(hν-Eg); here, hν is the incident photon energy, β is a constant, and γ is the exponent.57 The Tauc relation for the indirect allowed optical transition, as shown in Figure 3d, suggested that the Eg value was 1.38 eV, which is similar to that reported previously for WS2.54,58,59 Next, we explored the effects of the sputtering power and deposition time on the growth of vertically standing WS2 multilayers. For this, the sputtering power was varied to 25 W, 50 W, and 100 W, while the deposition time was varied to 30 min and 60 min. All the samples were prepared at 400 °C. FESEM images of these samples showing their surface and cross-sectional morphologies, captured at a magnification of 100k, are shown in Figure 4.


9


4f7/2 4f5/2

(a)

(b)

RT RT oC 200 oC 200 oC 400 oC 400

5f3/2

2p2/3

S 2p

25

30 35 40 Binding energy (eV)

45

155

RT oC 200 200 oC oC 400 400 oC

2p1/2

Intensity

Intensity

W 4f

160 165 170 Binding energy (eV)

175

(d)

(c) 100 80

C B

60

0.4 A

40

0.2 20

WS2 @ 0

400

400oC,

600

25W

800

0.5 -1 0.5 (α α hv) (cm eV)

0.6

Absorbance

Transmittance (%)

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

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3x105 2x105 1x105

0.0 1000 1200 1400

Wavelength (nm)

Eg = 1.38 eV Indirect

1

2 3 hv (eV)

4

Figure 3. XPS spectra of WS2 samples prepared at various temperatures. (a) W 2f, and (b) S 2p spectra. (c) Transmittance and absorbance spectra of WS2 film prepared at 400 °C on glass substrate. (d) Tauc plot of indirect allowed optical bandgap at 1.38 eV. The topographical images indicate that the lateral dimension of the vertically grown WS2 layers increased with increases in the sputtering power and deposition time. As a result, the layer width could be increased from 60 nm (RF power of 25 W, deposition time of 30 min) to 600 nm (RF power of 100 W, deposition time of 60 min) by controlling the sputtering power and deposition time. The layer thickness of the vertically grown sample corresponding to 100 W and 60 min was estimated to be approximately 5 nm (see brighter lines in topographical image). As per the lattice dimensions, this meant that the sample consisted of 8 layers. Further, the layer thickness of the vertically grown sample corresponding to 25 W and 30 min was estimated to be 2–2.8 nm, suggesting that the sample consisted of 4–5 vertically grown layers of WS2. These results are in keeping with the wavenumber difference of 69.4 cm-1 in the characteristic Raman


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peaks (E12g and A1g). Thus, this analysis confirmed that changing the sputtering power and deposition time not only modulates the width and height of the WS2 layers grown but also their number density.

Figure 4. Effects of RF sputtering power and time on growth of vertically oriented layers of WS2. Deposition temperature was 400 °C. Cross-sectional images show topography of corresponding samples. All FESEM images were obtained at magnification of 100k. The cross-sectional images of these samples confirmed that elongated and highly oriented WS2 layers were grown along the vertical direction. Theses layers seem very compact and tall. The growth rate as a function of the sputtering power for the two cases, that is, for, t = 30 min and 60 min, is shown in Figure S1. The growth of the sample corresponding to t = 60 min could be represented by the expression tWS2 = 4.85W + 35.5 while that for the sample corresponding to t =


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30 min could be expressed as tWS2 = 2.68W – 17; here tWS2 is the thickness of the WS2 film in question. While there may have been discrepancies from errors in thickness estimation. These expressions indicate that the growth rate of WS2 in the initial stage is not high. The vertical WS2 layers can be tuned from 6 nm to 13.5 nm, according to the RF power and deposition time. Figure 5a shows a schematic of the device fabricated to study the heterojunction formed by the vertical layers of WS2 and the Si wafer. This device had the structure Al/p-Si/n-WS2(40 nm)/Al and an area of 1 cm2. The WS2 layers absorbed the incoming photons to generate photogenerated carriers, which were transported rapidly across the junction without external bias. The dark current-voltage characteristics of the WS2/Si device, shown in Figure 5b, confirm the formation of a heterojunction. The estimated dark saturation current was less than 0.1 µA and the diode ideality factor, n, was 1.43. The value of n was estimated using the relation =

, where q, K, and T are the elementary charge, Boltzmann’s constant, and absolute

temperature, respectively. The segment used for calculating n is shown in Figure 5b. Moreover, the device exhibited outstanding rectifying characteristics (Figure S2) when subjected to a large bias of up to ±2 V. The dark current remained as low as 5 µA at -2 V and was greater than 100 mA at +2 V. Thus, the device exhibited a rectifying ratio 20000 or higher at room temperature without breakdown. This performance of the WS2/Si heterojunction revealed large-currentpassing capabilities and can be attributed to the vertically aligned nanolayer network, which branched out the current at the nanoscale and collectively exhibited outstanding diode properties and a low turn-on voltage (0.38 V). The diode properties were analyzed further by measuring the I-V characteristics at a low temperature (-40 °C, Figure S3). The results showed an extremely low dark saturation current (< 100 pA) as well as a low reverse bias current and a mark-free forward current. The diode characteristics are attributed to the interface between of p-Si and n-


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WS2, which can be demonstrated by the Ohmic contact profiles of Al/p-Si/Al and Al-WS2-Al (Figure S4). Further, the WS2/Si heterojunction exhibited excellent photoresponsive current-voltage characteristics, as shown in Figure 5b. The heterojunction had an open-circuit voltage (VOC) of 210 mV and a zero-bias photoresponse, with its signal-to-noise ratio (Ihv/Id) being higher than 9000 for NIR radiation (850 nm). Here, Ihv and Id are the photocurrent and the dark current, respectively. The photoresponsive attributes of the WS2/Si device were examined further, in order to evaluate its spectral characteristics and photodetection performance. For this, its currentvoltage characteristics were measured under a monochromatic light source that emitted radiation in the UV, visible, and NIR regions, as shown in Figure 5c. The intensity of the light source was 15 mW/cm2, unless stated otherwise. The heterojunction responded well to the broadband radiation. These results suggest that the fabricated vertically aligned WS2 layers should find use in zero-bias UV,60 visible,61 and NIR33 photodetection. The device performance obtained from the sole p-Si device (Al/p-Si/Al) exhibited no photoresponses under pulsed light illumination condition (Figure S5). This is due to the lack of the junction formation to show no current changes by light illumination and clearly demonstrates the function of WS2/p-Si junction for the origin of the photoresponses. To quantify the spectral photodetection performance of the WS2/Si heterojunction, we estimated its responsivity (R) and specific detectivity (D). R can be calculated using the relation

λ =

, where Phv is the intensity of the monochromatic light source and Ihv is the

photocurrent (Ihv = Ihv(measured) – Id(dark)). This relation suggests that the device current is dependent on the light intensity. The R value as a function of the light wavelength is shown in Figure 5d. The broad-ranging R values of the WS2 device can be attributed to the light-


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absorption capabilities of the vertically aligned layers. Moreover, the device exhibited peak R values at 420 nm, 680 nm, and 800–1000 nm. These results suggest that the WS2 film probably exhibited excitonic absorption.53,56 Interestingly, the WS2/Si heterojunction showed enhanced R values (~ 6 mA/W), particularly in the UV and NIR regions. Further, the D value can be calculated using the relation λ = λ

=

, where A is the device area, and Jd is

the dark current density. Therefore, we used the measured value of the current to determine D, which depends on R and Id. Of these, Id is the key parameter for improving the D performance of photodetectors.62,63 The D value as a function of the photon wavelength is shown in Figure 5b, which indicates that the device exhibited consistent photodetection performance over the entire wavelength range. The D value of the device was as high as 2.4 × 1010 Jones. It should be noted that we estimated the R and D values at relatively high light intensities and are likely to be higher at lower light intensities. The linear dynamic range (LDR) of a photodetector determines its image quality.64–67 For example, an LDR of 48 dB results in a depth of 8 bits, which corresponds to a 256-grayscale image. This is the case for conventional photodetectors. On the other hand, LDR values of 70 and 90 dB result in depths of 12 and 14 bits, which correspond to 4096- and 16384-grayscale images, respectively.67 The LDR value can be estimated from the relation LRDλ = %&'

20!"# $

%()

*, where Pmax and Pmin are light intensities of the photocurrent exhibiting a linear

trend. We estimated the LDR values corresponding to λ values of 365 nm, 520 nm, and 850 nm, as shown in Figure 5e. A photocurrent was consistently observed at these wavelengths as a function of the light intensity (up to 10 mW/cm2). The LDR value was estimated from the linear relation of Ihv versus Phv. The determined LDR values corresponding to wavelengths of 365 nm,


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Figure 5. (a) Schematic and photograph of WS2/p-Si heterojunction device (active area of 1 cm2). (b) I-V characteristics (dark and 850 nm light), diode ideality factor (n), rectification ratio at 0.7 V, open-circuit voltage (VOC), and signal-to-noise ratio (Ihv/Id) are shown. (c) Spectral I-V characteristics. (d) Responsivity and specific detectivity as functions of incident photon wavelength (zero-bias operation). (e) Photocurrent vs. light intensity plot for estimating LDR. (f) Spectral transient photocurrent analysis of rising time (τr) and falling time (τf); inset shows transient photocurrent response for λ = 385 nm and 850 nm and values of τr and τf. τr extends from 10% to 90% of signal along rising edge (lower to higher), and τr extends from 90% to 10% of signal along falling edge (higher to lower). 520 nm, and 850 nm were 52 dB, 71 dB, and 90 dB, respectively. These values further highlight the potential of vertically oriented WS2 layers for use in high-dynamic-range imaging photodetectors, particularly those for the NIR region. The LDR value corresponding to 365 nm


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(UV) was restricted to 52 dB owing to the deviation from linearity at higher intensities. Note that the LDR values were determined for the zero-bias condition. After analyzing the steady-state current-voltage characteristics and photoresponse of the WS2/Si heterojunction, its transient photocurrent was evaluated. We measured the zero-bias transient photocurrent under a pulsed light to quantify the photoresponse speed and the spectral attributes of the device. For this, the rising (τr) and falling times (τf) were estimated from the transient photoresponse. The estimated values of τr and τr as functions of λ are shown in Figure 5f; the transient photoresponse corresponding to wavelengths of 385 nm and 850 nm is shown in the inset. The photoresponse remained constant over the entire wavelength region, including the UV region, with the device exhibiting a high sensing speed. It can be seen that the τr and τf values are equal in magnitude at approximately 40 µs; this is owing to the high data acquisition rate used for the chronoamperometry method. The high photodetection rate of the device can be attributed to the high in-plane mobility of the photogenerated charge carriers in the WS2 layers and the formation of a high-quality heterojunction with the Si wafer, owing to which the device showed broadband photodetection (Table 1). In addition, the further investigation is required to find the optimum thickness of the WS2 film for the enhanced photoresponses (Figure S6). Junction properties such as the built-in potential and acceptor carrier concentration as well as the frequency attributes of the junction can be studied by Mott-Schottky analysis. Hence, we measured the capacitance of the device as a function of the applied bias under dark conditions. Figure 6a shows the frequency-dependent Mott-Schottky characteristics, that is, the (1/Csc)2 versus V characteristics, of the WS2/Si device. Here, Csc is the measured space-charge capacitance of the device. The negative slope of the curve for the Mott-Schottky analysis data under reverse bias is due to the p-type character of the


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Si wafer as well as the formation of an abrupt junction between the WS2 film and the Si wafer. Therefore, we can consider the fabricated WS2 sample a highly doped semiconductor. From the Mott-Schottky characteristics, one can estimate the flat-band potential (VFB), which is the point where these curves intersect the potential axis, and the free-carrier concentration (accepter (p-Si), NA), which can be calculated from the slope value. These values are shown as functions of the applied frequency, which ranged from 1 MHz to 1 kHz, in Figure 6b. The frequency dependence on these parameters can be seen clearly, with the curves showing two distinct regions. The VFB and NA values increases progressively on the left side, that is, for low frequencies, and remain constant in the higher-frequency region. Thus, we considered the values corresponding to higher frequencies (i.e., those for 40 kHz to 600 kHz). The VFB value was estimated to be 0.4 V while the NA value was ~1014 cm-3. It is worth mentioning that a VFB value of 0.4 V of the WS2/Si heterojunction allowed for self-biased photodetection. Further, the estimated NA value is in keeping with the specifications for the Si wafers used in this study. VFB can be determined from the energy band edges of WS2 and Si, which are shown in Figure 6c. It can be seen that there is a work function difference (Øbi) between the Fermi levels (Ef) of the two materials, which results in the formation of a barrier at the interface of WS2 and Si. The experimentally measured VFB value was comparable to that shown in the figure. Ideally, Øbi can be increased by increasing the carrier concentration (conductivity) of the WS2 layers. Figure 6d shows the energy bands of Si/WS2 interface and confirms that a type-II heterojunction was formed. The WS2 film absorbed the UV- and visible-wavelength photons while the NIR photons were absorbed at the Si/WS2 interface, owing to which the device exhibited excellent photodetection performance.


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1x1017

8x1016

3x10

(b)

1 MHz 500 kHz 250 kHz 100 kHz 50 kHz 10 kHz 5 kHz 1 kHz

5x1016

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2x1014

VFB

0.4

NA 1x1014

0.2

frequency > 40 kHz

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0 -0.6

-0.4

-0.2

0.0

0.2

0.4

NA (cm-3)

(a)

VFB (V)

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

(A/CSC)2 (cm2 F-1)2


8x1013

0.0

0.6

1k

Voltage (V)

10k

100k

1M

Frequency (Hz)

WS2 (c)

(d)

4.15 eV EC

Si EF

5.25 eV

φbi 0.41 eV

EV

NIR photons

4.5 eV

EC - terminal

EF

WS2

Visible photons

+ terminal

Si 5.88 eV Electron Hole

EV

UV photons

Heterojunction (Type – II)

Figure 6. (a) Mott-Schottky characteristics measured at various applied frequencies. (b) Estimated values of flat-band potential (VFB) and acceptor carrier concentration (NA) for given frequency at room temperature. (c) Energy band edges of p-Si and n-type WS2. (d) Energy band diagram of WS2/p-Si interface, which confirms formation of type-II heterojunction. In order to elucidate the advantages of rapid in-plane charge transport in the WS2 layers, high-speed photovoltage measurements were performed on the device. This was done by exposing the device to a pulsed light source (850 nm) and performing measurements using a high-speed digital oscilloscope. We also examined the stability of the device by performing these measurements in air for several days. The observed stable photodetection operation of the WS2/Si heterojunction was attributable to the WS2 overlayer. The photovoltages obtained for a pulsed light (square wave) at 50 Hz and 10 kHz are shown in Figure 7a and b, respectively. In both cases, the photovoltage was 210 mV, with the value remaining constant. The pulsing frequency of the light only affected the background of the transient photovoltage and not the peak value. The frequency-dependent transient photovoltage was analyzed for the relative balance, τr, and τr. The relative balance was defined using the expression (Vmax-Vmin)/Vmax,


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where Vmax and Vmin are the maximum and background values of the photovoltage corresponding to a particular pulsing frequency. The relative balance as a function of the frequency and the measurement setup used are shown in Figure 7c. These results demonstrate that the WS2/Si device exhibited high-speed operation in response to light pulses with a frequency of 10 kHz. To further highlight the high-speed photodetection performance of the WS2/Si device, τr and τf are shown as functions of the pulsing frequency in Figure 7d. It can be seen that the photodetection speed was as high as 1.1 µs for the 10 kHz signal. The remarkably fast photodetection performance of the WS2/Si heterojunction can be ascribed to the high in-plane carrier mobility of the device, which was much higher than that of conventional Si broadband and wide-bandgap UV photodetectors.60,61,68 According to the SCAPS simulation (Figure S7 and Table S1),69,70 the Si/WS2 heterostructure device may provide the power conversion efficiency (η) of 12.06% with an open circuit voltage (VOC) of 0.454 V, short circuit current density (JSC) of 40.26 mA/cm2 and fill factor (FF) of 66%. Thus, the broadband photo-responsiveness and high charge-carrier transport properties of the vertically oriented layers of WS2 grown at the wafer scale should pave the way for their use in a wide range of applications and devices, including phototransistors,2,12,13 photodetectors,2,14–17 Zener phototunnel diodes,18 gas sensors,13,19,20 photoelectrodes and photocatalysts for water splitting,21,23–25 and solar cells.26–29


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Figure 7. Transient photovoltage of WS2/Si heterojunction photodetector under pulsed 850 nm light signal: (a) 50 Hz, and (b) 10 kHz. (c) Relative balance (Vmax-Vmin)/Vmax as function of pulsed frequency; inset shows schematic of measurement setup consisting of oscilloscope and signal generator used for modulating pulsing frequency of incident light. (d) Rising time and falling time as functions of light pulse frequency as estimated from transient photovoltage signal. CONCLUSION In conclusion, the results of this study suggest that vertically oriented layers of WS2 can serve as broadband light-sensitive structures that exhibit intrinsic optoelectronic characteristics at the wafer scale. A deposition temperature of 400 °C is preferable for growing vertically (002)plane-oriented hexagonal layers of WS2, whose thickness, width, and height can be tailored based on the deposition process parameters such as the sputtering power and deposition time. We demonstrated the usefulness of the fabricated WS2 films by forming a type-II heterojunction with a WS2/Si interface. This interface showed excellent diode properties, including a high


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rectification ratio (>20000), low dark saturation current (

Growth of Wafer-Scale Standing Layers of WS2 for Self-Biased High

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