Letter pubs.acs.org/NanoLett
Metal−Insulator−Semiconductor Diode Consisting of TwoDimensional Nanomaterials Hyun Jeong,†,‡ Hye Min Oh,†,§ Seungho Bang,†,§ Hyeon Jun Jeong,†,§ Sung-Jin An,†,§ Gang Hee Han,† Hyun Kim,†,§ Seok Joon Yun,†,§ Ki Kang Kim,∥ Jin Cheol Park,†,§ Young Hee Lee,†,§ Gilles Lerondel,‡,§ and Mun Seok Jeong*,†,§ †
Center for Integrated Nanostructure Physics, Institute for Basic Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea ‡ Laboratoire de Nanotechnologie et d’Instrumentation Optique, Institut Charles Delaunay, CNRS-UMR 6281, Université de Technologie de Troyes, BP 2060, 10010 Troyes, France § Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea ∥ Department of Energy and Materials Engineering, Dongguk University, Seoul 100-715, Republic of Korea S Supporting Information *
ABSTRACT: We present a novel metal−insulator−semiconductor (MIS) diode consisting of graphene, hexagonal BN, and monolayer MoS2 for application in ultrathin nanoelectronics. The MIS heterojunction structure was fabricated by vertically stacking layered materials using a simple wet chemical transfer method. The stacking of each layer was confirmed by confocal scanning Raman spectroscopy and device performance was evaluated using current versus voltage (I−V) and photocurrent measurements. We clearly observed better current rectification and much higher current flow in the MIS diode than in the p−n junction and the metal−semiconductor diodes made of layered materials. The I−V characteristic curve of the MIS diode indicates that current flows mainly across interfaces as a result of carrier tunneling. Moreover, we observed considerably high photocurrent from the MIS diode under visible light illumination. KEYWORDS: Graphene, h-BN, monolayer MoS2, metal−insulator−semiconductor diode, carrier tunneling
R
p−n junction devices consisting of layered materials have been of particular interest recently.29,30 However, metal−insulator− semiconductor (MIS) diodes made using only atomically thinlayered materials have not been reported to date. Although p−n junction diodes have lower current levels than MIS diodes, the p−n junction diodes are more popular for most applications because of their easily controlled doping process.31 However, MIS diodes have greater potential to be used in high power and high speed devices than p−n junction diodes because MIS diodes are based on metal−semiconductor (MS) structures.32 Additionally, the advantages that MIS diodes have over MS diodes include lower reverse saturation currents, caused by interfacial layers with higher barrier heights, and significantly smaller effective Richardson constants.33 Herein, we propose a novel MIS diode consisting of graphene, h-BN, and 1-L MoS2. As graphene is mechanically stable when in contact with h-BN, because of their almost identical lattice constants, h-BN was adopted as an insulating
ecently, layered materials with strong in-plane chemical bonds and weak out-of-plane bonds have attracted much attention for nanoelectronic applications, because they can be fabricated by conventional methods using existing siliconbased-device production facilities.1−4 Moreover, these layered materials are suitable for use in flexible devices because of their atomic-scale thicknesses.5−7 The most extensively studied metallic layered material is graphene.8−10 As semiconducting layered materials, two-dimensional (2D) transition metal dichalcogenides (TMD) have emerged as a promising materials.11−17 In particular, monolayer (1-L) MoS2, having a direct optical transition band gap of 1.8 eV, has been extensively researched in recent years because of its stable and strong optical properties.18−21 Hexagonal boron nitride (hBN), which has very high thermal and dielectric stabilities, has a band gap of ∼6.0 eV and is representative of insulating layered materials.22−24 Currently, vertically stacked heterostructures of these layered materials are of great interest because their excellent optoelectronic properties and flexibilities allow for their use in high-performance nanodevices, such as multifunctional photoresponsive memory devices,25 light-emitting devices,26 and field-effect transistors.27,28 Investigations into © 2016 American Chemical Society
Received: December 3, 2015 Revised: February 5, 2016 Published: February 17, 2016 1858
DOI: 10.1021/acs.nanolett.5b04936 Nano Lett. 2016, 16, 1858−1862
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Figure 1. Fabrication and optical properties of MIS heterojunction structure. Schematics of the stacking process for (a) graphene, (b) h-BN, and (c) 1-L MoS2. Graphene, h-BN, and 1-L MoS2 were sequentially stacked on the SiO2/Si and acted as metal, insulator, and semiconductor, respectively. (d) Perspective and (f) cross-sectional schematics of the final MIS structure. The footprint of the top layer was kept smaller than that of the lower layer to prevent leakage currents. Optical microscope images taken at (e) high and (g) low magnifications. (h) Local PL spectrum of MoS2, the top layer of the MIS structure. The narrow, sharp peaks in the PL spectrum are the result of Raman scattering from overlapping layers.
layer. The 1-L MoS2 grown by chemical vapor deposition (CVD) was used for the semiconducting layer of the MIS diode because of its large scale and direct band gap characteristics. The MIS device studied in this work was transparent, ultrathin, flexible, and transferrable with large scale structure, owing to the characteristics of CVD grown layered materials. Results and Discussion. Figure 1a−c illustrates the fabrication process used to produce the MIS diode consisting of graphene, h-BN, and 1-L MoS2. Graphene was grown on copper foil by atmospheric pressure chemical vapor deposition and transferred to the SiO2/Si substrate using the wet chemical etching method.34 A 300 nm thick SiO2 layer was used to electrically isolate the MIS structure from the Si substrate. The graphene layer transferred to the SiO2/Si substrate was 2 × 2 cm in size. Multilayer h-BN samples were grown on metal foil by CVD, using borazine as the precursor.35 The as-grown h-BN multilayers were then transferred to the graphene/SiO2/Si template. The size and thickness of the transferred h-BN were 1 × 1 cm and 20 nm, respectively. Because the crystal quality of the insulating layer is the factor that most significantly influences carrier tunneling efficiency in MIS structures, h-BN was thermally annealed in a furnace. The 1-L MoS2 flakes were grown on SiO2/Si substrates by CVD under a static N2 flow of 500 sccm.36 The 1-L MoS2 flakes were then chemically transferred to a prepared h-BN/graphene/SiO2/Si template. To remove the PMMA residue on the transferred 2D materials, we have conducted chemical cleaning with acetone and thermal annealing. The transferred 1-L MoS2 was approximately 500 × 500 μm in size. Finally, we obtained a vertically stacked MIS heterostructure where graphene, h-BN, and 1-L MoS 2
corresponded to metal, insulator, and semiconductor layers, respectively, as depicted in Figure 1d. Figure 1e presents an optical microscope image of the top surface of the MIS structure, clearly showing 1-L MoS2 flakes on h-BN/graphene. The average dimensions of the linked-triangle-shaped MoS2 flakes were approximately 200 × 200 μm (Supporting Information Figure S1 and S2). The white dotted line in Figure 1e indicates the border between a 1-L MoS2 flake and the h-BN. Figure 1f shows a cross-sectional schematic of the stacked MIS structures. To prevent leakage currents between layers, the upper layer was designed to be smaller than the lower layer. To distinguish each layer more clearly, a magnified optical microscope image of the surface of the MIS structure is presented in Figure 1g. The white dotted lines in Figure 1g have been included to highlight the different regions, which are graphene, h-BN/graphene, and MoS2/h-BN/graphene. Figure 1h shows the confocal PL spectrum of the stacked 1-L MoS2. The exciting light source was a 532 nm diode pumped solidstate laser. The inset of Figure 1h shows a magnified view of the PL spectrum in a wavelength range of 536−555 nm. The peaks at 543.1 and 547.0 nm, indicated with black arrows, correspond to the Raman peaks of 1-L MoS2 and Si, respectively.37 The main PL peaks of MoS2 are present at 624.1 and 674.2 nm and correspond to B and A exciton transitions, respectively.38 The small peaks at 560.3, 580.6, and 620.4 nm, indicated with black arrows in the PL spectrum, are Raman peaks associated with the Si substrate, h-BN layer, and graphene layer, respectively.39 This indicates that the 1-L MoS2, h-BN, and graphene layers were vertically stacked on the SiO2/Si, as expected. Moreover, two-dimensional Raman scattering images of the MIS structure 1859
DOI: 10.1021/acs.nanolett.5b04936 Nano Lett. 2016, 16, 1858−1862
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Figure 2. Device performance of the MIS diode. Three-dimensional schematics of (a) the MIS and (b) the p−n diodes for electrical measurement. (c) I−V curves for the MIS and the p−n diodes. The inset is I−V characteristic in log scale. Current rectification with significantly high current flow was clearly observed in the MIS diode. Energy band diagrams of the MIS diode (d) at equilibrium and under (e) forward and (f) reverse biases. The observed current rectification of the MIS diode can be explained by carrier tunneling.
higher current for the MIS diode compared with that of the p− n diode confirms that operating mechanism of MIS diode is based on carrier tunneling. To verify the current rectification the MIS diode, we measured I−V curve of MS diode that consists of graphene and 1-L MoS2 (Supporting Information Figure S5). Although current flow of MS diode shows higher than MIS diode, we observed better current rectification in the MIS diode. This confirms that Schottky barrier between Cr and 1-L MoS2 does not significantly affect the current rectification of the MIS diode. Moreover, we observed neither negligible leakage currents between the graphene and h-BN layers nor between the h-BN and 1-L MoS2 (Supporting Information Figure S6). This supports that charge transport across the MIS diode was mainly due to carrier tunneling. To interpret the operating mechanism of the MIS diode, energy band diagrams depicting the diode at equilibrium and under forward and reverse biases are shown in Figure 2d−f, respectively. The work function values of graphene and MoS2 are 4.6 and 4.6−4.9 eV, respectively.43,44 The electron affinity values of h-BN and MoS2 are 2−2.3 and 4.2 eV, respectively.45,46 Consequently, based on the energy band parameters, we predicted an energy band diagram for the equilibrium state, which is depicted in Figure 2d. While below the threshold voltage, charge carriers can accumulate at the interface between graphene and h-BN as the forward bias is increased. Above the threshold voltage, however, the current is expected to increase exponentially because of tunneling charge carriers, which accumulate at the graphene/hBN interface, as described in Figure 2e. Conversely, charge carriers cannot tunnel through the insulating layer under reverse bias, as shown in Figure 2f. This model clearly explains the current rectification of the MIS diode, as revealed in the I− V curve. Because 1-L MoS2 is a semiconductor with a direct optical transition of 1.8 eV, it is expected that MIS diodes based on 1-L
support that the layered materials were well vertically stacked (Supporting Information Figure S4). Note that if a transparent and flexible template was employed as a substrate, a transparent MIS diode with good flexibility could be achieved using this method because all of the layers of the MIS structure were deposited by chemical transfer. To estimate device performance, current versus voltage (I− V) curves were measured for the MIS diode consisting of graphene, h-BN, and 1-L MoS2. As a reference, p−n junction structure consisting of 1-L MoS2 and 1-L WSe2 were fabricated on SiO2/Si substrate. Figure 2a,b presents three-dimensional (3D) schematics of the MIS and the p−n diodes for I−V measurements. To fabricate the robust metal pad, we created square patterns by photolithography. The Cr/Au was deposited on the patterned sample using e-beam evaporator. Figure 2c shows I−V curves for the MIS and the p−n diodes. Red square and black circle indicate I−V data points for the MIS diode and the p−n diode, respectively. Voltages were applied in a range of −10 ∼ +10 V. For the MIS diode, the graphene layer acted as a cathode while the MoS2 acted as an anode. The current at 10 V for the MIS and the p−n diodes are 2.7 × 10−7 A and 1.2 × 10−8 A, respectively. The inset is I−V characteristics of MIS and p−n diodes plotted by log scale of current. As represented in I−V curve, current rectification was clearly observed from the MIS diode. This current rectification is attributed to less carrier concentration of 1-L MoS2 than that of graphene. A typical saturation current near 0 A was observed in the reverse bias region and an exponential current increase was observed above the threshold voltage in the forward bias region. The threshold voltage and series resistance of the MIS diode were 6.5 V and 1.5 × 107 Ω, respectively. The high threshold voltage of 6.5 V is quite typical value of MIS structures.40 Because MIS structures contain insulating layers, carriers can tunnel into semiconductor layer at high voltages.41,42 Moreover, approximately 10 times 1860
DOI: 10.1021/acs.nanolett.5b04936 Nano Lett. 2016, 16, 1858−1862
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deposition (CVD). To transfer graphene, poly(methyl methacrylate) (PMMA) was used as the supporting material. Graphene was floated along with PMMA on the copper etchant. The floating graphene and PMMA were transferred to the SiO2 substrate. Then, the PMMA on the graphene was removed with an acetone solution. Finally, the graphene was dried and heated in the oven to eliminate the residual PMMA on the graphene. Fabrication of h-BN. h-BN was grown from a borazine precursor on a metal foil with a thickness of 0.1 mm by CVD. PMMA was used as a supporting material and coated onto the as-grown h-BN to separate it from the metal foil. The metal foil was chemically etched. Next, the back side of the h-BN was washed with a diluted solution of HCl used to remove residual chemical etchant. The h-BN with PMMA was then transferred to a previously prepared graphene-on-SiO2 template. Acetone was sprayed onto the transferred h-BN to remove the PMMA. Finally, h-BN was annealed at 500 °C for 3 h to improve crystallinity. Fabrication of Monolayer MoS2. Monolayer (1-L) MoS2 was grown on a SiO2 substrate by CVD. PMMA was coated onto the as-grown 1-L MoS2 to act as a support for the 1-L MoS2. SiO2 was etched by dipping it into a dilute HF solution. The floating 1-L MoS2 and PMMA were transferred to a previously prepared graphene and h-BN template. The PMMA was removed with an acetone solution. Finally, the sample was dried at 80 °C for 1 h to eliminate any residual PMMA. Characterization of the MIS Diode. Photoluminescence spectra were obtained using a confocal PL spectrometer equipped with an objective lens with high numerical aperture of 0.7 and a diode-pumped solid-state laser (532 nm). Confocal Raman spectroscopy was conducted using a commercial multifunctional microscope (NTEGRA, NT-MDT). I−V curves were measured using a conventional probe station system equipped with an accurate current detector (Sourcemeter 2400, Keithley).
MoS2 have photoresponses to visible light. To investigate the photoresponse of the MIS diode, photocurrent measurement was carried out by using conventional probe station with a visible light illumination (Supporting Information Figure S7). The spectral range of the lamp is from ∼500 to ∼700 nm. Figure 3 shows the photocurrent of the MIS diode with
Figure 3. Photocurrent dynamics of the MIS diode induced by visible light illumination. The Ilight/Idark ratio of photocurrent is around 6.6 at an incident light intensity of 7 mW and a photoresponsivity of 0.3 mA/W.
illustration of measurement configuration. A static voltage of 10 V was applied to the MIS diode to measure the time-dependent photocurrent, as shown in Figure 3. The current was increased when lamp was turned on and photocurrent decayed when the light source was turned off. The rise and decay times of the photocurrent are 10 and 11 s, respectively. The Ilight/Idark ratio of photocurrent is around 6.6 at an incident light intensity of 7 mW, and a photoresponsivity of 0.3 mA/W was calculated. In consideration of power density of the light source (2.8 W/m2), this value is approximately 10 times higher than previous reported, which would be the graphene−MoS2 heterostructure.47 This indicates that the MIS diode could be available to apply to the photoresponsive devices such as photodiode and photovoltaic cell. Conclusions. We have demonstrated a novel MIS diode structure consisting of graphene, h-BN, and 1-L MoS2 that act as metal, insulator, and semiconductor, respectively. Raman and PL measurements confirmed that the layers were vertically stacked well on the SiO2/Si substrate using a chemical transfer method. The diode’s conventional current versus voltage curve of the MIS diode clearly showed current rectification with considerably higher current flow compared to that of p−n junction diode. Moreover, significantly high photocurrent was observed in the MIS diode under the visible light illumination. Consequently, we suggest that this MIS diode, consisting of layered materials, has immense potential for applications in extremely thin optoelectronics and integrated circuit with large photoresponsivities and good flexibilities. Experimental Methods. Fabrication of Graphene. Graphene was grown on copper foil by chemical vapor
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b04936. Optical microscopy image of MIS diode with low magnification (S1), AFM image of 1-L MoS2 (S2), AFM images of graphene (S3), Optical characterization of MIS heterojunction structure (S4), I−V curve of MS contact (S5), I−V curves for graphene−h-BN and 1-L MoS2−h-BN (S6), and the light emission spectrum in the visible light range for the photocurrent (S7). (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by IBS-R011-D1 of Korea and the Human Resources Development program (No. 20124010203270) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), funded by the Ministry of Trade, Industry, and Energy of the Korean government, and 1861
DOI: 10.1021/acs.nanolett.5b04936 Nano Lett. 2016, 16, 1858−1862
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partially supported by the FUI MULTISS project (F1305008 M).
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DOI: 10.1021/acs.nanolett.5b04936 Nano Lett. 2016, 16, 1858−1862
