Broad-Band Photocurrent Enhancement in MoS2 Layers Directly

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Broad-band Photocurrent Enhancement in MoS2 Layers Directly Grown on Light Trapping Si Nanocone Arrays Yunae Cho, Byungjin Cho, Yonghun Kim, Jihye Lee, Eunah Kim, Trang Thi Thu Nguyen, Ju Hyun Lee, Seokhyun Yoon, Dong-Ho Kim, Jun-Hyuk Choi, and Dong-Wook Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15418 • Publication Date (Web): 30 Jan 2017 Downloaded from http://pubs.acs.org on January 31, 2017

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ACS Applied Materials & Interfaces

Broad-band Photocurrent Enhancement in MoS2 Layers Directly Grown on Light Trapping Si Nanocone Arrays Yunae Cho,† Byungjin Cho,‡ Yonghun Kim,‡ Jihye Lee,§ Eunah Kim,† Trang Thi Thu Nguyen,† Ju Hyun Lee,† Seokhyun Yoon,† Dong-Ho Kim,‡ Jun-hyuk Choi,§ and Dong-Wook Kim*,†

†

Department of Physics, Ewha Womans University, Seoul 03760, Korea

‡

Department of Advanced Functional Thin Films Department, Korea Institute of Materials

Science (KIMS), Changwon 51508, Korea §

Department of Nanomanufacturing Research, Korea Institute of Machinery & Materials (KIMM),

Daejeon 34103, Korea

KEYWORDS: MoS2, photoresponse, chemical vapor deposition, Si, nanocone

ABSTRACT. There has been a growing research interest in realizing optoelectronic devices based on the 2D atomically thin semiconductor MoS2 owing to its distinct physical properties that set it

1 ACS Paragon Plus Environment


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apart from conventional semiconductors. However, there is little optical absorption in these extremely thin MoS2 layers, which presents an obstacle toward applying them for use in highefficiency light absorbing devices. We synthesized tri-layers of MoS2 directly on SiO2/Si nanocone (NC) arrays using chemical vapor deposition and investigated their photodetection characteristics. The photoresponsivity of the MoS2/NC structure was much higher than that of the flat counterpart across the whole visible wavelength range (for example, it was almost an order of magnitude higher at  = 532 nm). Strongly concentrated light near the surface that originated from a FabryPerot interference in the SiO2 thin layers and a Mie-like resonance caused by the Si NCs boosted the optical absorption in MoS2. Our work demonstrates that MoS2/NC structures could provide a useful means to realize high performance optoelectronic devices.

INTRODUCTION MoS2, as a typical atomically thin semiconductor, has a sizable bandgap in the energy region corresponding to visible light, which has stimulated research activities toward the development of 2D optoelectronic devices.1 2D semiconductors exhibit distinct physical properties owing to the quantum confinement effect along their thickness direction compared with their conventional 3D counterparts. While multilayer MoS2 has an indirect bandgap, monolayer MoS2 has a direct bandgap.2 The bandgap energy can be tuned by controlling either the layer thickness2 or the strain state.3 All these intriguing characteristics provide opportunities to realize devices with new functionalities and high performances. The material’s extremely small thickness, however, limits the optical absorption in MoS2 despite its very high absorption coefficient.4 This hinders the implementation of MoS2-based light absorbing devices, such as photo-detectors5 and solar cells.6


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There have been intensive efforts to improve the optical spectral response of MoS2. Many research groups have placed metal nanostructures on or under the MoS2 layers to increase absorption via surface plasmon (SP) excitation.7,8 Jeong et al. studied the optical properties of MoS2 layers deposited on dielectric spacers and metal thin films.9 They reported that the thickness and refractive index of the dielectric layer as well as the surface roughness of the metal thin film affects the light absorption in the MoS2 layers. Bahauddin et al. enhanced the optical absorption in MoS2 using plasmonic metal nanoparticles as antennae and underlying dielectric layers with back reflectors.10 The dielectric underlying layers functioned as Fabry-Perot resonators and increased the absorption through multiple internal reflections in MoS2 near the resonant wavelengths.11 When the metal nanoantennae were used, SP effects caused parasitic Ohmic loss as well as limiting the bandwidth tunability.12 In addition, charge transfer at the metal/MoS2 interface may alter the physical properties of MoS2.13 As an alternative approach, integrated structures, consisting of dielectric photonic nanostructures and MoS2 layers, can be used to tune and enhance the optical spectral response of MoS2. Wang et al. demonstrated the use of Fano-resonant photonic crystals, composed of TiO2 cubes, an Al2O3 spacer layer, and a Ag back reflector, to significantly boost absorption in MoS2.14 The absorption in MoS2 was as high as 90% at the resonance wavelength. This result is remarkable, but the absorption enhancement occurred only in a narrow wavelength range. Recently, Huang et al. demonstrated both narrow- and broad-band strong light absorption in MoS2 films prepared on GaN nanostructure arrays.15 Both the theoretical understanding of and fabrication techniques for such semiconductor-based photonic nanostructures are well established. Broad-band light trapping strategies are well-known for Si owing to its solar cell applications.16 Most importantly, Si is the dominant semiconductor material used in logic and memory circuits. Thus, integration of MoS2-



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based optoelectronic devices on Si is an important step toward realizing high-density optoelectronic integrated circuits, including both photon- and charge-manipulation elements. Photodetectors can have simple structures and are an important type of device that can be fabricated using MoS2. In particular, MoS2 metal-semiconductor-metal (MSM) structures are easy to fabricate and have shown promising device performances. In many early studies, MoS2 layers grown using chemical vapor deposition (CVD) were used, since CVD can produce high quality, large-scale MoS2 layers with a high throughput.17−20 Tsai et al. reported that their MoS2-based MSM photodetectors were capable of broad-band detection from the visible into the UV at working temperatures of up to 200 C.18 Lim et al. proposed a new low-temperature (450 C) synthesis method and successfully fabricated MoS2 photodetectors; they were even able to fabricate the detectors on flexible polymer substrates.19 Thus, all these studies have successfully demonstrated the suitability of CVD-grown MoS2 layers for practical applications. Here, we proposed a new 2D photodetector architecture comprising CVD-grown MoS2 layers on light trapping nanocone (NC) arrays. We investigated the structural and optical properties of the tri-layer MoS2 thin films. The NC arrays, consisting of nano-patterned Si wafers with 50-nmthick thermally grown SiO2 layers, significantly reduced optical reflection and concentrated light near the surface at visible wavelengths. Thus, the NC arrays enhanced light absorption in the MoS2 layers and increased the photoresponsivity of the MoS2 MSM devices. The design and fabrication techniques of Si-based light trapping nanostructures are well established, and hence our approach based on the integration of 2D MoS2 layers on 3D Si nanostructures provides a new pathway toward realizing high performance optoelectronic devices.


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EXPERIMENTAL SECTION Fabrication of NC arrays The NC arrays were fabricated using photolithography and subsequent etching on 8-in-diameter Si wafers. Photoresist was spin-coated on the wafers, then exposed using a KrF stepper (NSRS203B, Nikon, Japan) and a dark-field phase shift mask. The resist patterns were used as etching masks to form the NCs. An inductively coupled plasma reactive ion etching was performed using a Cl2/HBr plasma (TCP-9400DFM, Lam Research, USA). 50-nm-thick SiO2 layers were grown on the NC sample via thermal oxidation. The pattern consisted of hexagonal arrays of SiO2/Si NCs with a bottom diameter, spacing between nearest neighbors, and height of 225, 300, and 415 nm, respectively. The structural properties of the fabricated SiO2/Si NC array were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Figure 1a and b). We fabricated Si NC arrays without SiO2 layers, whose geometrical configuration was almost identical to that in this work, using nanoimprint technique and investigated their light trapping capabilities.21 The Mie-resonance-mediated strong light confinement in the NCs enabled broadband antireflection (AR) effects and also significantly enhanced photoluminescence intensity of the Si NC array.

Figure 1. (a) Side- and (b) cross-sectional view SEM images of the NC arrays onto which 50nm-thick SiO2 layers were deposited via thermal evaporation. The light and dark gray areas above the NCs in (b) are regions containing epoxy with diffused Pt and epoxy used for the specimen preparation, respectively.



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Fabrication of MoS2 devices MoS2 thin films were synthesized via CVD as reported in a previous study [20]. MoO3 films were deposited onto the SiO2/Si NC wafers using a thermal evaporator and sulfurized in a furnace by sublimating sulfur powder. Interdigitated Au/Cr (50/3 nm) electrodes (width of 400 µm and an inter-electrode gap of 100 µm) were deposited on the MoS2 films using a thermal evaporator and a shadow mask. In order to investigate the electrical properties of our CVD-grown tri-layer MoS2 films, we fabricated back-gated MoS2 field effect transistor (FET) devices (see Figure S1). The on/off ratio of our FET was as high as ~106 and field effect mobility of the MoS2 layer was estimated to be ~1 cm2/Vs. All these results showed that we could prepare high quality MoS2 thin films using our CVD technique. Optical simulations and characterizations Reflection spectra and electric-field intensity distributions for the MoS2 thin films on flat and NC substrates were obtained using finite-difference time-domain (FDTD) simulations with a normally incident broadband plane-wave source. The refractive index of SiO2 was set to n = 1.46.22 The thickness of the tri-layer MoS2 was set to 2 nm and the dielectric function of MoS2 was taken from Li et al.'s experimental data.23 Optical reflection measurements were performed for wavelengths between 400 and 800 nm using an ultraviolet/visible/near-infrared spectrophotometer (Lambda 750, PerkinElmer, USA). Room temperature Raman scattering spectra were measured using a spectrometer (207, McPherson, USA) equipped with a nitrogen cooled charge-coupled device array detector. The samples were excited with 2 mW of a 488 nm laser, focused to ~1 μm diameter spots using a microscope objective lens (×100). Low powers were used to ensure that the samples did not decompose by localized laser heating. Optimal results were obtained with 2 mW laser power, 10 second integration for flat substrates, and 30 second integration for NC substrates. The


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electrical properties of the MoS2 devices were measured by a semiconductor characterization system (4200-SCS, Keithley, USA). For the photocurrent measurements, laser diodes ( = 405, 532, and 650 nm) with various powers were used and the spot size of the laser beam was 0.4 cm2. RESULTS AND DISCUSSION Figure 2a and b show in-plane and cross-sectional high-resolution TEM images of MoS2 thin films grown on flat SiO2/Si substrates. The FFT analysis clearly revealed a hexagonal lattice structure with a lattice spacing of 0.27 and 0.16 nm, which were assigned to the (100) and (110) planes of MoS2 (Figure 2c).20 The FFT pattern also indicates the formation of polycrystalline layers.

Figure 2. (a) In-plane and (b) cross-sectional TEM images of a MoS2 thin film grown on flat SiO2/Si substrates. (c) shows the FFT pattern of the in-plane TEM image in (a).

Figure 3a-c show a cross-sectional TEM image and energy-dispersive X-ray spectroscopy maps of a MoS2 layer grown on a 50-nm-SiO2/Si NC sample. Tri-layer MoS2 thin films were grown on top of the NCs and on the flat region between the NCs; these were similar to the thin films grown on the flat substrates. Conversely, isolated island-like MoS2 layers, rather than a continuous layer, were formed on the inclined sides of the NCs. Based on the TEM analyses, the MoS2/NC array structures can be schematically illustrated in Figure 3d. Since the thickness of the electrode was



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only 50 nm, the electrodes were electrically connected to the MoS2 layers grown on the flat region between the NCs but not the MoS2 layers on the NC tops. In our CVD process, the MoO3 precursor layers were grown on substrates by thermal evaporation, and then sulfurized to form MoS2 layers. Thus, the conformal coating of MoO3 thin films is critical to prepare uniform layers of MoS2 on the substrates. The TEM analyses in Figure 3a-c suggest that the MoO3 layers cannot be deposited well on the steep sides of our high aspectratio NC arrays. Such thin precursor layers might agglomerate and form islands during the sulfurization process. As an alternative method, sputtering or sol-gel techniques could be

Figure 3. (a) TEM images of tri-layer MoS2 grown on SiO2/Si NC. Energy-dispersive X-ray spectroscopy maps of (b) S and (c) Mo. (d) Schematic diagrams of the MoS2/NC array structures.


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considered to grow the precursor layers. In the sputtering process, a very strong electric field might appear around the NP top due to the lightning rod effect during plasma generation. As a result, it may be possible to grow thicker layers on top of the NPs compared with on the flat region between the NPs and the sides of the NPs. If the sol-gel process were used, solution-based source materials should fill the flat region between the NPs while barely coating the NC surface. Thus, either of these two methods is not likely to form uniform MoO3 precursor layers on NC substrates. If all the source materials can be supplied in gas form (without using precursor thin films) and reacted to form MoS2 layers, then conformal growth of MoS2 layers on the high aspect-ratio structures might be possible. The MoS2 layers on the flat and NC substrates were studied using Raman spectroscopy, as shown in Figure 4. The spectra of both samples have peaks at 383 and 408 cm−1, which can be assigned to the in-plane vibrational mode of the Mo and S atoms (E12g) and the out-of-plane vibrational mode of the S atoms (A1g), respectively.24 The difference in the peak positions between E12g and A1g was approximately 23 cm−1, indicating the formation of tri-layer MoS2 for both

Intensity (arb. units)

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A1g

1

E2g

NC(x8) Flat 360

380

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420 -1

Raman shift (cm )

Figure 4. Raman spectra of the MoS2 thin films grown on flat (square) and NC (circle) substrates.



samples.25−27 The Raman intensity of the NC sample was lower than that of the flat sample, which we attribute to light scattering by the nanostructure array.

40

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Bare

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Figure 5. Calculated (lines without symbols) and experimental (lines with symbols) optical reflection (R) spectra of flat and NC samples with and without MoS2 thin films. Open and filled symbols indicate the bare and MoS2-coated samples, respectively.

Figure 5 shows the simulated and experimental optical reflection (R) spectra of the flat and NC substrates with and without the MoS2 layers. The experimental data agree well with the FDTD calculation results. In spite of the MoS2 layer being atomically thin (2 nm), the R of the MoS2 thin films on the flat substrate was drastically reduced compared with that of the bare substrate. FabryPerot (FP) interference occurs in the thermally grown SiO2 layers, and the light-MoS2 interaction can be significantly enhanced, resulting in a notable reduction of R.11 The reflection spectra of the MoS2 thin films on the flat substrate have two narrow dips at  = 630 and 680 nm, which correspond to absorption caused by direct transitions at the K point of the Brillouin zone, which are associated with the generation of B and A excitons, respectively.23 The NC samples displayed a very low level of optical reflection across a broad wavelength range owing to the AR effects of


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the surface nanostructure array, which originate from its graded refractive index, its multiple scattering property, and the Mie-like geometrical resonance.16,21 Coating of the MoS2 layers on the NC samples did not significantly affect the R spectra.

Figure 6. (a) A cross-sectional schematic of the MoS2/NC sample under illumination. The FDTD-calculated electric field intensity (|E/E0|2) distributions in the sample at  = (b) 400 nm, (c) 530 nm, and (d) 650 nm. The incident light was linearly polarized, as illustrated in (a). E0 indicates the magnitude of the electric field of the incident light.

Figure 6 shows the cross-sectional distributions of the FDTD-simulated light intensity (|E/E0|2, where E0 indicates the magnitude of the electric field of the incident light) in the MoS2/NC sample under blue ( = 400 nm), green ( = 530 nm), and red ( = 650 nm) light. The light intensity in the flat substrate decreases exponentially with increasing depth into the sample. In contrast, strong field confinement exists near the surface in the NC sample, which we attribute to the Mie-like resonance caused by the NCs.21,28 Geometrical optical resonance can increase scattering and absorption cross-sections for gradually varying conical shapes across a broad wavelength range.16 This effect explains the broad-band surface field enhancement (Figure 6b–d) as well as the notable AR effects at visible wavelengths (Figure 5). We propose that the multiple reflections caused by the SiO2 layers further boost the optical absorption in the MoS2 layers. Taghinejad et al. conducted



a systematic study on how the FP interference, which occurs in the SiO2 layer, enhances optical absorption in MoS2 layers by varying both the wavelength of the incident light and the SiO2 layer thickness.11 According to their work, a 50-nm-thick SiO2 layer can significantly increase the optical absorption in the MoS2 layers in the visible wavelength range. All these results clearly indicate that our SiO2/Si NC arrays concentrate incoming light in the MoS2 layers via the FP interference and Mie-like resonance, enabling the photo-generation of numerous carriers in the MoS2 layers.

NC

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60 40 20 0

Flat NC Flat NC Flat NC R G B

Figure 7. (a) Iph (lines with filled symbols) and R (lines with open symbols) of the MoS2 layers on flat (square) and NC (circle) substrates. (b) R for red (R), green (G), and blue (B) light ( = 650, 532, and 405 nm) for the flat and NC samples.

Figure 7a shows the photocurrent (Iph) and photoresponsivity (R) of the flat and NC samples under illumination of a green laser ( = 532 nm) with various powers. Iph was estimated by subtracting the dark current (Idark) from the current under illumination (Ilight) (i.e., Iph = Ilight − Idark). R is defined as, Rλ = Iph/(PS)


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where P is the incident light power and S is the active illuminated area. R is an important parameter for evaluating the performance of an illuminated photodetector. Remarkably, the R of the MoS2/NC sample was much higher than that of the flat sample—approximately 10 times higher for the green light at ~ 4 mW. Iph and R of the flat and NC samples were also measured under illumination with red ( = 650 nm) and blue ( = 405 nm) lasers (see Figure S2). In the case of the NC sample, the flat region between the NCs determines S, since continuous MoS2 layers were formed only on the flat region between NCs (Figure 3a-d) and the 50-nm-thick electrodes could not collect the current from the isolated island-like MoS2 layers near the NC tops (see Figure S3). Thus, the active illuminated area of the NC device is smaller than that of the flat device. Also, it should be noted that the 50-nm-thick thermally grown SiO2 layer blocked any current from the underlying Si substrates in both of the flat and NC samples. The estimated Schottky barrier height of our device was only 2 meV,20 and hence the bulk resistance of the MoS2 thin film, rather than the contact resistance, dominantly determined the photoresponse of our MSM photodetector. Figure 7b shows the R of the flat and NC samples under the red (R), green (G), and blue (B) light illumination with ~ 4 mW. This clearly shows that the NC sample exhibits enhanced photoresponsivity in the whole visible range, compared with the flat counterpart. A nearly identical response for multiple cycles was observed with a train of on–oﬀ illumination (see Figure S4). It should also be noted that our device was operated with a very small voltage. The R values in Figure 7b were measured with an applied voltage of 1 V, and the electric field in the MoS2 layer was only 210−3 V/m. As discussed above, we attribute the large R to the surface-concentrated light caused by the NC array.



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There have been many reports regarding the spectral response of the photocurrent in MSM photodetectors.17,18 The optical absorption of the incoming light in the active semiconductor material determines the amount of the photo-generated electron-hole pairs, some of which can contribute to the photocurrent. However, some of the electron-hole pairs undergo recombination prior to reaching the electrodes. Sometimes, the Schottky contacts can be used to raise the carrier collection efficiency, since the potential gradient near the contacts can separate electron-hole pairs and suppress the recombination.29 Illumination of the light can produce current via photothermoelectric5 and/or plasmonic30 effects at the contacts as well as the photo-excited carrier generation in the semiconductors. Therefore, the physical properties of both electrode and semiconductor materials should be carefully considered to understand and control the wavelengthdependence of the carrier generation and collection efficiency of the photodetectors. Our work suggested that the integration of the 2D semiconductor and the 3D light trapping nanostructures could modify the optical absorption spectra of the 2D layers and resulting photoresponse of the photodetectors. More systematic attempts to optimize the electrode materials and nanostructure geometry enable us to improve the performance of the MoS2 photodetectors.

CONCLUSION We fabricated integrated nanostructures with CVD-grown MoS2 thin films on NC arrays. The photoresponsivity of the MoS2/NC photodetector was much larger than that of the flat counterpart in the visible wavelength range (for example, almost an order of magnitude higher at  = 532 nm), although the photocurrent came from the MoS2 layers grown on the flat region between NCs only. The enhancement was primarily attributed to the strongly concentrated light near the surface caused by the FP resonance (via the thin SiO2 layers) and the Mie-like resonance (via the NC


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array). Our 2D MoS2-3D NC integrated structures thus provide a means to tune and improve the optical spectral response of MoS2 thin layers, which is a very attractive property for a variety of optoelectronic device applications.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. The transfer characteristics of a MoS2 field effect transistor; The photocurrent and responsivity of the flat and NC samples, under illumination of blue and red light; The active illuminated area of the flat and NC samples; The photocurrent data measured during multiple on-off illumination cycles.

AUTHOR INFORMATION Corresponding Author * Dong-Wook Kim. E-mail: [email protected]

Author Contributions Y.C., B.C., D.-H.K., J.-H.C. and D.-W.K. conceived and designed the research study. Y.C., B.C., Y.K., J.L., E.K., T.T.T.N., and J.H.L performed the experiments and analyzed the data. Y.C., B.C., S.Y., D.-H.K., J.-H.C. and D.-W.K. contributed to preparing the manuscript and all the authors have given approval to the submitted manuscript.



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Acknowledgements This study was supported by the National Research Foundation of Korea (2013R1A1A2007951, 2014M3C1A3052569,

2014R1A1A1036139,

2016R1D1A1B01009032,

and

2016R1D1A1A09917491). B.C., Y.K., and D.H.K. were also supported by the Fundamental Research Program (PNK5290) of the Korean Institute of Materials Science (KIMS).

Notes The authors declare no competing financial interest.


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TOC FIGURE


Broad-Band Photocurrent Enhancement in MoS2 Layers Directly

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