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Alloyed 2D metal-semiconductor atomic layer junctions Ah Ra Kim, Yong Hun Kim, Jaewook Nam, Hee-Suk Chung, Dong Jae Kim, Jung-Dae Kwon, Sang Won Park, Jucheol Park, Sun Young Choi, Byoung Hun Lee, Ji Hyeon Park, Kyu-Hwan Lee, Dong-Ho Kim, Sung Mook Choi, Pulickel M Ajayan, Myung Gwan Hahm, and Byungjin Cho Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b05036 • Publication Date (Web): 03 Feb 2016 Downloaded from http://pubs.acs.org on February 4, 2016
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Alloyed 2D metal-semiconductor atomic layer junctions Ah Ra Kim1†, Yonghun Kim1,2†, Jaewook Nam3†, Hee-Suk Chung4, Dong Jae Kim3, Jung-Dae Kwon1, Sang Won Park1, Jucheol Park5, Sun Young Choi1, Byoung Hun Lee2, Ji Hyeon Park6, Kyu Hwan Lee7, Dong-Ho Kim1, Sung Mook Choi7, Pulickel M. Ajayan8*, Myung Gwan Hahm9†* and Byungjin Cho1* 1
Department of advanced Functional Thin Films, Surface Technology Division, Korea Institute
of Materials Science, 797 Changwondaero, Sungsan-Gu, Changwon, Gyeongnam 51508, Korea, 2
School of Materials Science and Engineering, Gwanju Institute of Science and Technology
(GIST), 261 Cheomdan-gwangiro, Buk-Gu, Gwangju 61005, Republic of Korea, 3School of Chemical Engineering, Sungkyunkwan University, 300 Cheongcheon-dong, Suwon, Gyeonggido 16419, Republic of Korea, 4Jeonju Center, Korea Basic Science Institute, Jeonju, Jeollabukdo 54907, Republic of,
5
Structure Analysis Group, Gyeongbuk Science & Technology
Promotion Center, Future Strategy Research Institute, 17 Cheomdangieop 1-ro, Sangdongmyeon, Gumi, Gyeongbuk 39171, Republic of Korea, 6Department of Electricity and Electronic Engineering, University of Ulsan, 93 Daehak-ro, Nam-gu, Ulsan 44610, Republic of Korea, 7
Electrochemistry Department, Surface Technology Division, Korea Institute of Materials
Science (KIMS), 797 Changwondaero, Sungsan-gu, Changwon, Gyeongnam 51508, Republic of Korea, 8Department of Materials Science and NanoEngineering, Rice University, 6100 Main
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Street, Houston, Texas 77005, USA, 9School of Materials Science and Engineering, Inha University, 100 Inharo, Nam-Gu, Incheon 22212, Korea †These authors contributed equally to this work.
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ABSTRACT. Heterostructures of compositionally and electronically variant two-dimensional (2D) atomic layers are viable building blocks for ultrathin optoelectronic devices. We show that the composition of interfacial transition region between semiconducting WSe2 atomic layer channels and metallic NbSe2 contact layers can be engineered through interfacial doping with Nb atoms. WxNb1-xSe2 interfacial regions considerably lower the potential barrier height of the junction, significantly improving the performance of the corresponding WSe2-based field-effect transistor devices. The creation of such alloyed 2D junctions between dissimilar atomic layer domains could be the most important factor in controlling the electronic properties of 2D junctions and the design and fabrication of 2D atomic layer devices. KEYWORDS. Transition metal dichalcogenide, WSe2, NbSe2, Heterostructure, Atomic-layered FET.
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Two-dimensional atomic layer building blocks such as graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDs) have recently gained significant attention as unique components for next-generation miniaturized optoelectronic devices.1–7 In particular, atomically thin TMDs are excellent 2D semiconductors due to their finite bandgaps and various other interesting electronic properties.3,4,7 In addition to drawing interest due to their potential use in individual-atomic-layer devices, combinations of atomic layers with different compositions into van der Waals (vdW) heterostructures have gained interest because of the possibility of generating a large number of electronically variant systems.5,6,8–11 In these multipleatomic-layer devices, the Schottky barriers originating from Fermi level misalignment present a critical issue that must be addressed.12–15 In this study, we were the first to synthesize atomiclayered metallic NbSe2 as an electrical contact via chemical vapor deposition (CVD). Moreover, herein we propose a new means of realizing a 2D semiconductor–metal system with a low Schottky barrier height for use in TMD-based electronic devices, by creating a precisely controllable mixed-composition transition region between the 2D semiconductor and metallic regions of the device. This system is based entirely on 2D units, with the WSe2 semiconducting layer in contact with the NbSe2 metallic electrode layer via an interfacial transition region with mixed composition, facilitating charge transport through the contact region.12,14 To demonstrate the proposed approach, we first designed a simple bottom-gate fieldeffect transistor (FET) with a WSe2 channel and NbSe2 electrodes. Figure 1a presents the optical image and schematic drawing of the designed FET. The semiconducting WSe2 channel (the bottom layer) and metallic NbSe2 (the top layer) electrical contacts were synthesized on Si wafers with thermal oxide using thermal CVD. Tungsten trioxide (WO3) and niobium pentoxide (Nb2O5) thin films were directly selenized onto each substrate at 1000 °C by CVD, forming
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WSe2 and NbSe2 (see detailed methods, Figure S1, S2 in Supporting Information (SI) for more details). Figures 1b and 1d show high-resolution transmission electron microscopy (HR-TEM) images of the as-synthesized WSe2 and NbSe2, respectively. The HR-TEM images clearly indicate that the as-synthesized WSe2 and NbSe2 films have quasi-hexagonal honeycomb atomic structures. Figures 1c and 1e show the Raman spectra recorded from the surfaces of the WSe2 and NbSe2 films, respectively. The spectra exhibit two distinguishable phonon modes: A1g (corresponding to an out-of-plane mode) and E12g (corresponding to an in-plane mode).16,17 The X-ray photoelectron spectra (XPS) show the elemental compositions of the as-grown atomiclayered WSe2 and NbSe2 films (Figure S3). Figures 1f and 1h show the results of TEM-electron energy loss spectroscopy (EELS) analysis. Again they substantiate the chemical compositions of the as-synthesized WSe2 and NbSe2, respectively, with nanoscale spatial resolution. As shown in Figure 1g, the excitonic absorption peaks A and B of the optical absorbance spectrum recorded from the as-synthesized WSe2 correspond to direct gap transitions at the K point of the Brillouin zone.18 In addition, the overlap of the Se p orbitals with the W d orbitals yields additional features, A' and B', in the WSe2 absorbance spectrum.19 These peaks correspond to the splitting of the ground and excited states of the A and B transitions due to inter- and intra-layer perturbations of the d electron band by the Se p orbitals.20 The Hall measurement results show that the as-synthesized NbSe2 film has metallic characteristics and that the majority of carriers are holes (Figure 1i). The most interesting aspect of this work is the introduction of an interfacial transition region (WxNb1-xSe2) between semiconducting WSe2 and metallic NbSe2 to reduce the Schottky barrier height. The transition interfacial layers were designed systematically via a combination of thermal evaporation of WO3, plasma-enhanced atomic layer deposition (PEALD) of Nb2O5, and
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a sequential selenization process to form Nb-doped WSe2. The PEALD process was adopted to control precisely the Nb doping in the WxNb1-xSe2. The use of 1, 3, and 5 PEALD cycles for the Nb2O5 deposition led to thicknesses of 0.8 nm, 1.2 nm, and 1.9 nm, respectively (Figures S4 and S5 and Table S1). To elucidate the atomic structures and compositions corresponding to the different amounts of Nb doping in WSe2, atomic-resolution annular dark-field (ADF) scanning TEM (STEM) was conducted on the three WxNb1-xSe2 transition layers (top left of Figures 2a–c and Figure S6). The high-resolution ADF-STEM top-view images of the WxNb1-xSe2 transition layers indicate that Nb atoms are substituted for W sites in the atomic structure. Thus, thermally diffusing Nb atoms relocated to the W sites to form the complex W–Nb–Se atomic structures. Figures 2a–c (top right) show the intensity profiles along the normal to the interface passing through the W/Nb and Se sub-lattices, which are indicated by the green boxes in the raw ADFSTEM images (top left). These images demonstrate that the three different atoms can be distinguished by the pixel intensities, because they are approximately proportional to the square of the corresponding atomic numbers. Furthermore, the color-rendered images (bottom left of Figures 2a–c) show clear brightness differences between the W and Nb (red circles: Nb sites, green circles: W sites), suggesting that the intensity of W is stronger than that of Nb due to the relatively high atomic number of W. To investigate the crystal structure in depth, gray-scale ADF-STEM images were analyzed (Figure S10). As discussed before, the pixel intensities are used to classify atoms: the intensities of the W sites are greater than those of the Se and the Nb sites in the ADF-STEM images, as shown in Figures 2a–c (top left). Therefore, the pixel intensities can be used to classify atoms. However, regular image segmentation methods cannot be used, because the boundary between the atoms and the background cannot be determined precisely on the
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molecular scale, due to the uncertainty principle.21 Instead, we exploited the nearly periodic structure of the TMD layer to identify the regions associated with the atoms. The strictly periodic pattern was obtained by considering the six intensity peaks in the Fourier domain (Figure S10).22 The size-invariant circle detection method was used to determine the approximate positions of the atoms, which could then be corrected by the method described in Figures S10 and S11. The averaged intensities inside the detected circles were classified as W atoms or collections of Se and Nb atoms by Otsu’s method for three classes.23 The classification results for the three different cycles are shown in Figures 2a–c (bottom right). The XPS spectra (Figures S7) and EELS results (Figure 2d) reveal the chemical compositions (W, Nb, and Se) of the three mixed transition layers. While maintaining a consistent amount of Se, the atomic ratio of Nb to W was adjusted by varying the number of PEALD cycles used to introduce the Nb dopants, as shown in Figure S7 and Figure 2d. It is evident that the amount of Nb doping in WxNb1-xSe2 increases as number of cycles increases from 1 to 5 (Figures S7, S9, and 2d). On the other hand, the atomic ratio of W decreases since Nb atoms occupy W sites (Figure S7a). The Raman spectra also substantiate the existence of Nb–Se vibrational modes for the WxNb1-xSe2 transition layers. As shown in Figure 2e, in the spectra recorded from the transition layers, the vibrational modes of Nb–Se (A1g and E12g: dashed arrows 1 and 2, respectively) emerge with increasing Nb concentration, resulting in total band broadening. On the contrary, the vibrational modes of W–Se (E12g and A1g: dashed arrows 3 and 4, respectively) decrease and disappear as the composition changes from pure WSe2 to NbSe2.16,17 To elucidate the electrical behaviors of the WxNb1-xSe2 transition layers, Hall measurements were conducted (Figure S8). Figure 2f shows the variations of the Hall parameters (sheet carrier density, mobility, and conductivity) with increasing Nb content in the WxNb1-xSe2
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transition layer. The sheet carrier density is proportional to the amount of Nb doping, while the mobility shows the maximum near 6% Nb content. The conductivity increases as the PEALD cycle increases and eventually reaches to a level close to bare NbSe2. Thus, doping of the Nb atoms donate holes, which increases the hole carrier density. Next, we turned our attention to the roles of transition layers between the semiconducting and metallic TMDs in FET devices with global back gates. (Figure S12: (i) metal (Pd)–semiconducting WSe2 (MS) junction, (ii) NbSe2–WSe2 vdW junction, and (iii) mixed layer containing NbSe2–NbxW1-xSe2–WSe2 junction, more specifically denoted as m-vdW: 1 cycle, 3 cycles, and 5 cycles). The detailed fabrication methods and atomic force microscopy and optical images of each junction devices are shown in Figures S12-16. In the drain-source current vs. drain voltage (IDS–VDS) curves for each junction (Figure S17) were compared, indicating the m-vdW: 1 cycle junction devices exhibit relatively symmetric and linear IDS–VDS behaviors. Introduction of metallic NbSe2 and WxNb1-xSe2 atomic layers with NbSe2 increased the drain current, enhancing charge transport across the contacts. The statistical WSe2-based FET device data (10 devices each with MS, vdW, or m-vdW junctions, where the m-vdW junctions had 1, 3, or 5 cycle junctions) show the p-type behavior of WSe2(Figure 3a). Channel-length-dependent FET properties for each junction are also shown in Figure S18. The most remarkable observation is that the transfer curves of IDS vs. VBG (with VDS = -5 V) of the FET devices with the mixed transition layers (m-vdW: 1 cycle) exhibit the largest ON currents and transconductances. The superior contact property of the mixed transition layer of the m-vdW: 1 cycle device also improves the output characteristics (IDS–VDS modulated by VBG of 20 V to -60 V with step size of -20 V) (Figure S19). Specifically, the contact resistance values of the MS, vdW, and m-vdW: 1cycle devices, determined by transfer length method, were measured to be ~8.2, 3.1, and 1.6
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MΩ·µm under Vg = 0 V and 71.8, 37.2, and 14.7 kΩ·µm under Vg = –60 V (Figure S20 and Table S2). Thus, it is believed that the improvement of FET performance on the m-vdW: 1cycle device was ascribed to the reduced contact resistance. Improvement in subthreshold swing and mobility was also observed in the m-vdW: 1 cycle device (Figure S21). The average mobility values of MS, vdW, and m-vdW: 1cycle devices were calculated to be ~0.08, 0.11, and 0.38 cm2V-1s-1. On the other hand, the MS junction devices exhibit relatively low ON currents with significant fluctuations. Furthermore, the statistical data reveal that all of the 2D TMD junction devices (vdW and m-vdW: 1, 3, and 5 cycles) show relatively small variations of the ON and OFF states, indicating their atomically clean and dangling-bonding-free 2D heterojunction interfaces (Figure 3b). The electrical contact properties of all FET devices with the 2D TMD junction also remained stable over time (Figure S22). The histograms depicting the data for the ON (VBG = -60 V) and OFF states (VBG = 30 V) show that the employment of NbSe2 electrodes instead of Pd metal increases the charge injection efficiency and, moreover, that the insertion of an additional transition layer with a low Nb doping level (i.e., WxNb1-xSe2: 1 cycle film) most significantly improves the charge transport across the metal–semiconductor TMD interface. The number of hole carriers increases as the Nb ratio in WxNb1-xSe2 increases, which corresponds to the Hall measurement results (Figure S23). The high carrier concentration at the mixed transition layer interface reduces the effective Schottky barrier height, as in the case of charge transport enhancement via degenerate doping in conventional bulk Si metal contacts.24 As discussed above, three mixed transition layers were formed by controlled doping of Nb into WSe2. Obviously, their electrical characteristics were closely related to the atomic ratio between Nb and W, as shown in the Hall measurement results (Figure S23). Interestingly, the performance of the m-vdW: 1 cycle FET device was the best among all of the devices, including
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those with MS and vdW FET junctions; however, overdose of Nb atoms in the transition region did not improve the electrical performance and even degrade it as in the cases of m-vdW: 3 and 5 cycles (Figure 3b). There are considerable differences between the temperature-dependent mobility obtained by Hall measurement of the WxNb1-xSe2: 1 cycle device and those of the other TMDs (WxNb1-xSe2: 3 and 5 cycles and NbSe2) (Figure S23). The WxNb1-xSe2: 1 cycle film only shows a positive dµ/dT, while the other films exhibit negative slopes (i.e., dµ/dT < 0) or negligible temperature dependences. The sheet carrier densities of all of the films were found to be almost temperature-independent, showing high hole carrier densities of 1014–1015 cm-2 (Figure S23). These results imply that the conduction mechanism switches from semiconducting behavior (the case of the WxNb1-xSe2: 1 cycle film) to metallic behaviors (the cases of the WxNb1-xSe2: 3 and 5 cycles and NbSe2 film) with increasing Nb doping in WSe2 (Figure S23).25 Thus, the best transition layer should maintain its semiconducting behavior. Furthermore, the crystal structures of the mixed transition layers are distorted by substitution of Nb atoms onto W sites of WSe2 due to the different binding lengths of W–Se and Nb–Se. This non-negligible structural distortion adversely affects the charge transport throughout the vertical heterojuctions.26,27 As the amount of Nb atoms increases for the transition WxNb1-xSe2 layers, the degree of disorder changes more rapidly, which eventually might disturb the charge transport due to the irregular lattice structure. This is the reason why the transition layers with relatively large amount of Nb dosage did not improve the FET performance anymore and even degrade it (as the cases of m-vdW: 3 and 5 cycles). Thus, the PEALD-based light-doping approach to 2D TMD materials is a unique method of systematically manipulating their physical properties. Since even the transition layers have high conductivities of 102–104 Scm-1, it is important to elucidate the exact role of the metallic NbSe2 placed on the transition layers. Thus,
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we fabricated 10 devices without and 10 with final NbSe2 top electrodes on the transition channel layers of WxNb1-xSe2/WSe2 and then compared the statistical VBG–IDS data of the two types of devices. The charge collection efficiency clearly increased by introducing the NbSe2 electrode, as shown in Figure S24. The high contact resistances of the devices without NbSe2 induced low ON currents, indicating that the use of a transition layer alone is not sufficient to collect the hole charges that accumulate throughout channel/SiO2 interfaces. To demonstrate the electrical uniformity of an 8 × 10 array of integrated m-vdW: 1 cycle FET devices with channel lengths varying from 10 µm to 50 µm, we measured all 80 devices and then mapped their OFF (IDS @ Vg = 30 V) and ON (IDS @ Vg = -60 V) states (Figures 3c and S25). The yield of devices with ON/OFF ratios of over 103 is ~85% (68 functioning devices out of 80 FET), indicating that the p-type transistors are spatially uniform. To elucidate the electrical contact properties of the different junctions, we evaluated the drain currents at different temperatures and extracted the barrier heights using the 2D thermionic emission model (Figure SS26 and S27, and Table S3). The slopes of the Arrhenius plots of ln(IDS/T3/2) vs. 1000/T at a fixed VDS of 0.5 V for the MS, vdW, and m-vdW: 1 cycle devices are -3.6, -2.3, and -2.0, respectively (Figure S27). The negative slopes indicate the existence of Schottky barriers in all of the tested FET devices. The Schottky barrier heights (@ VDS = 0 V) for the MS, vdW, and m-vdW: 1cycle devices, estimated from the extrapolation curves of the slopes vs. VDS, are 326, 199, and 176 meV, respectively (Figure 3d). Compared with the vdW and m-vdW: 1cycle, relatively large barriers for m-vdW: 3 and 5 cycles are responsible for the reduced transistor performance (Figure S28). The m-vdW junction was most effective at lowering the electronic contact barrier height. The energy band diagrams of the WSe2 channel between the source and drain illustrate the interface-induced lowering of the Schottky barrier height; for the m-vdW: 1 cycle junction, the tunnelling effect
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can be also considered (Figure 3e). In all of the considered junctions (MS, vdW, and m-vdW: 1 cycle junctions), Schottky barriers exist for the holes, as experimentally observed and shown in Figure 3d. It is very likely that interfacial traps in metal-semiconductor junctions cause Fermilevel pinning, suppressing barrier height modulation by metal electrode materials with varying work functions. This behavior explains why the MS junction shows an unexpectedly large Schottky barrier height corresponding to p-type WSe2 (Figure 3e(i)). The metallic NbSe2 abruptly lowers the Schottky barrier height (Figure 3e(ii)), because the work function of NbSe2 aligns more closely with the electron affinity of WSe2. The dangling-bonding-free and atomically sharp interface in the vdW junction results in the low contact barrier.12 The barrier height and thickness are significantly reduced for the m-vdW: 1 cycle junction, and a fraction of the holes would quantum-mechanically tunnel through the barrier (Figure 3e(iii)). Thus, using a mixed-composition m-vdW interface between 2D heterolayers is likely to become a new strategy for reducing the barrier height and increasing the charge injection efficiency across metal– semiconductor interfaces. Therefore, the strategy of creating novel 2D vdW structures by stacking could be carefully and precisely controlled via the PEALD-CVD hybridization technique suggested in this study, ultimately enabling atomic-scale lego devices with the necessary functionality. In summary, we demonstrated a unique and effective vdW stacking approach that can result in significant 2D TMD FET performance improvement by using mixed-composition atomic layers between semiconducting WSe2 and metallic NbSe2. The electrical characteristics of the WxNb1-xSe2 mixed transition layers strongly depend on the Nb dosage, which can be precisely controlled via PEALD-CVD. The presence of a WxNb1-xSe2 mixed-transition atomic layer in between the semiconducting WSe2 and metallic NbSe2 layers considerably reduces the
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Schottky barrier height, leading to a high ON current for the WSe2 FET. The Schottky barrier lowering and tunnelling are considered to contribute simultaneously to the enhanced charge transport throughout such novel mixed 2D atomic layer heterojunctions. The described composition-engineered junctions in 2D atomic layer heterostructures promise to open new avenues in research on the fabrication of atomic-layer-based electronic devices.
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FIGURES.
Figure 1. (a) Optical image and schematic drawing of WSe2-based bottom-gate FET with NbSe2 electrode. Scale bar is 10 µm. (b) HR-TEM images of as-grown WSe2 film. Scale bar is 5 nm. (c) Raman spectrum recorded from as-synthesized WSe2 film. Spectrum shows two major phonon modes: out-of-plane mode (A1g) and in-plane mode (E12g). (d) Top-view HR-TEM image of asgrown NbSe2, showing quasi-hexagonal honeycomb atomic structure. Scale bar is 5 nm. (e) Raman spectrum recorded from as-synthesized NbSe2 film, revealing two major vibrational modes (A1g and E12g). (f) EELS analysis substantiates chemical composition of as-synthesized WSe2. (g) Excitonic absorption peaks A and B of optical absorbance spectrum recorded from assynthesized WSe2. (h) EELS analysis substantiates chemical composition of as-grown NbSe2. (i) Hall measurement results, showing metallic characteristics of as-synthesized NbSe2 film. 14 Environment ACS Paragon Plus
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Figure 2. (a), (b), and (c) Sets of high-resolution atomic-resolution ADF-STEM images and image analysis of WxNb1-xSe2 transition layers with Nb2O5 1, 3, and 5 PEALD cycles, respectively. Magnified STEM and color-rendered images, illustrating W and Nb sites (green: W sites and red: Nb sites). Intensity profile along normal to interface passing through W/Nb and Se sub-lattices corresponding to green boxes in raw ADF-STEM images. Results of atomic classifications (bottom right of a, b, and c) for W (gray), Nb (hunter green), and Se (yellow) atoms. (d) EELS analysis substantiates increased Nb doping on transition layers. (e) Raman spectra also validate existence of Nb–Se bonding on WxNb1-xSe2 transition layers. Vibrational modes of Nb–Se (A1g and E12g) emerge and increase as amount of Nb doping increases. (f) Hall measurements show that carrier densities and conductivities increase with increasing Nb doping. The thicknesses of approximately 4, 4.5, and 7 nm for WxNb1-xSe2: 1, 3, and 5 cycle transition layers were considered for the Hall measurement.
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Figure 3. (a) Transfer characteristics (IDS–VBG) of MS, vdW, and m-vdW: 1, 3, and 5 cycle junction devices. Data were fitted by averages and standard deviations of 10 devices with each junction type. IDS was measured at VDS of -5 V in devices with channel length of 50 µm and width of 100 µm. (b) Histograms of OFF (IDS @ VBG = 30 V and VDS = -5 V) and ON (IDS @ VBG = -60 and VDS = -5 V) states in MS, vdW, and m-vdW junction devices. OFF and ON parameters were extracted from VBG–IDS curves of Figure 3a. (c) Top: optical image of 8 × 10 array of integrated m-vdW: 1 cycle FET devices on SiO2/Si wafer. Top inset: Zoomed-in image of devices with channel lengths varying from 10 µm to 50 µm depending on row line. Bottom 16 Environment ACS Paragon Plus
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images: mapping images of OFF (IDS @ VBG = 30 V and VDS of -5 V) and ON (IDS @ VBG = -60 and VDS of -5 V) states obtained from 80 devices. Device yield (defined as devices with ON/OFF ratio of over 103) is ~85%, indicating high structural and electrical uniformity of stacked 2D layers. (d) Schottky barrier heights of MS, vdW, and m-vdW junction devices. In each case, Schottky barrier height was calculated from Y-intercept of extrapolated slope vs. VDS curve. (e) Energy band diagrams for (i) MS, (ii) vdW, and (iii) m-vdW junctions. qΦMS, qΦvdW, and qΦmvdW
indicate Schottky barrier heights of MS, vdW, and m-vdW junctions, respectively. Dashed
green curves of junctions represent thermionic emission, while dashed red line for m-vdW junction indicates tunneling.
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ASSOCIATED CONTENT Supporting Information. Detailed methods for synthesis and fabrication, detailed process of image analysis, supporting figures S1 –S28, and supporting table S1 – S2. This material is available free of charge via the Internet at http://pubs.acs.org
AUTHOR INFORMATION Corresponding Author *To whom correspondence should be addressed: e-mail: [email protected] (M.G.H.); [email protected] (P.M.A.); [email protected] (B.C.)
ACKNOWLEDGMENT This study was supported by the Fundamental Research Program (PNK4580 & PNK4890) of the Korean Institute of Materials Science (KIMS). M. G. H. and B. C. are grateful for the support from the Basic Science Research Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2014R1A1A1006214 and NRF-2014R1A1A1036139). J. N. was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013R1A1A1004986, NRF-2015M1A8A1053681). H.-S. Chung acknowledges support of Korea Basic Science Institute (KBSI-C35928).
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Communication
Alloyed 2D metal-semiconductor atomic layer junctions Ah Ra Kim, Yong Hun Kim, Jaewook Nam, Hee-Suk Chung, Dong Jae Kim, Jung-Dae Kwon, Sang Won Park, Jucheol Park, Sun Young Choi, Byoung Hun Lee, Ji Hyeon Park, Kyu-Hwan Lee, Dong-Ho Kim, Sung Mook Choi, Pulickel M Ajayan, Myung Gwan Hahm, and Byungjin Cho Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b05036 • Publication Date (Web): 03 Feb 2016 Downloaded from http://pubs.acs.org on February 4, 2016
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Alloyed 2D metal-semiconductor atomic layer junctions Ah Ra Kim1†, Yonghun Kim1,2†, Jaewook Nam3†, Hee-Suk Chung4, Dong Jae Kim3, Jung-Dae Kwon1, Sang Won Park1, Jucheol Park5, Sun Young Choi1, Byoung Hun Lee2, Ji Hyeon Park6, Kyu Hwan Lee7, Dong-Ho Kim1, Sung Mook Choi7, Pulickel M. Ajayan8*, Myung Gwan Hahm9†* and Byungjin Cho1* 1
Department of advanced Functional Thin Films, Surface Technology Division, Korea Institute
of Materials Science, 797 Changwondaero, Sungsan-Gu, Changwon, Gyeongnam 51508, Korea, 2
School of Materials Science and Engineering, Gwanju Institute of Science and Technology
(GIST), 261 Cheomdan-gwangiro, Buk-Gu, Gwangju 61005, Republic of Korea, 3School of Chemical Engineering, Sungkyunkwan University, 300 Cheongcheon-dong, Suwon, Gyeonggido 16419, Republic of Korea, 4Jeonju Center, Korea Basic Science Institute, Jeonju, Jeollabukdo 54907, Republic of,
5
Structure Analysis Group, Gyeongbuk Science & Technology
Promotion Center, Future Strategy Research Institute, 17 Cheomdangieop 1-ro, Sangdongmyeon, Gumi, Gyeongbuk 39171, Republic of Korea, 6Department of Electricity and Electronic Engineering, University of Ulsan, 93 Daehak-ro, Nam-gu, Ulsan 44610, Republic of Korea, 7
Electrochemistry Department, Surface Technology Division, Korea Institute of Materials
Science (KIMS), 797 Changwondaero, Sungsan-gu, Changwon, Gyeongnam 51508, Republic of Korea, 8Department of Materials Science and NanoEngineering, Rice University, 6100 Main
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Street, Houston, Texas 77005, USA, 9School of Materials Science and Engineering, Inha University, 100 Inharo, Nam-Gu, Incheon 22212, Korea †These authors contributed equally to this work.
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ABSTRACT. Heterostructures of compositionally and electronically variant two-dimensional (2D) atomic layers are viable building blocks for ultrathin optoelectronic devices. We show that the composition of interfacial transition region between semiconducting WSe2 atomic layer channels and metallic NbSe2 contact layers can be engineered through interfacial doping with Nb atoms. WxNb1-xSe2 interfacial regions considerably lower the potential barrier height of the junction, significantly improving the performance of the corresponding WSe2-based field-effect transistor devices. The creation of such alloyed 2D junctions between dissimilar atomic layer domains could be the most important factor in controlling the electronic properties of 2D junctions and the design and fabrication of 2D atomic layer devices. KEYWORDS. Transition metal dichalcogenide, WSe2, NbSe2, Heterostructure, Atomic-layered FET.
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Two-dimensional atomic layer building blocks such as graphene, hexagonal boron nitride (h-BN), and transition metal dichalcogenides (TMDs) have recently gained significant attention as unique components for next-generation miniaturized optoelectronic devices.1–7 In particular, atomically thin TMDs are excellent 2D semiconductors due to their finite bandgaps and various other interesting electronic properties.3,4,7 In addition to drawing interest due to their potential use in individual-atomic-layer devices, combinations of atomic layers with different compositions into van der Waals (vdW) heterostructures have gained interest because of the possibility of generating a large number of electronically variant systems.5,6,8–11 In these multipleatomic-layer devices, the Schottky barriers originating from Fermi level misalignment present a critical issue that must be addressed.12–15 In this study, we were the first to synthesize atomiclayered metallic NbSe2 as an electrical contact via chemical vapor deposition (CVD). Moreover, herein we propose a new means of realizing a 2D semiconductor–metal system with a low Schottky barrier height for use in TMD-based electronic devices, by creating a precisely controllable mixed-composition transition region between the 2D semiconductor and metallic regions of the device. This system is based entirely on 2D units, with the WSe2 semiconducting layer in contact with the NbSe2 metallic electrode layer via an interfacial transition region with mixed composition, facilitating charge transport through the contact region.12,14 To demonstrate the proposed approach, we first designed a simple bottom-gate fieldeffect transistor (FET) with a WSe2 channel and NbSe2 electrodes. Figure 1a presents the optical image and schematic drawing of the designed FET. The semiconducting WSe2 channel (the bottom layer) and metallic NbSe2 (the top layer) electrical contacts were synthesized on Si wafers with thermal oxide using thermal CVD. Tungsten trioxide (WO3) and niobium pentoxide (Nb2O5) thin films were directly selenized onto each substrate at 1000 °C by CVD, forming
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WSe2 and NbSe2 (see detailed methods, Figure S1, S2 in Supporting Information (SI) for more details). Figures 1b and 1d show high-resolution transmission electron microscopy (HR-TEM) images of the as-synthesized WSe2 and NbSe2, respectively. The HR-TEM images clearly indicate that the as-synthesized WSe2 and NbSe2 films have quasi-hexagonal honeycomb atomic structures. Figures 1c and 1e show the Raman spectra recorded from the surfaces of the WSe2 and NbSe2 films, respectively. The spectra exhibit two distinguishable phonon modes: A1g (corresponding to an out-of-plane mode) and E12g (corresponding to an in-plane mode).16,17 The X-ray photoelectron spectra (XPS) show the elemental compositions of the as-grown atomiclayered WSe2 and NbSe2 films (Figure S3). Figures 1f and 1h show the results of TEM-electron energy loss spectroscopy (EELS) analysis. Again they substantiate the chemical compositions of the as-synthesized WSe2 and NbSe2, respectively, with nanoscale spatial resolution. As shown in Figure 1g, the excitonic absorption peaks A and B of the optical absorbance spectrum recorded from the as-synthesized WSe2 correspond to direct gap transitions at the K point of the Brillouin zone.18 In addition, the overlap of the Se p orbitals with the W d orbitals yields additional features, A' and B', in the WSe2 absorbance spectrum.19 These peaks correspond to the splitting of the ground and excited states of the A and B transitions due to inter- and intra-layer perturbations of the d electron band by the Se p orbitals.20 The Hall measurement results show that the as-synthesized NbSe2 film has metallic characteristics and that the majority of carriers are holes (Figure 1i). The most interesting aspect of this work is the introduction of an interfacial transition region (WxNb1-xSe2) between semiconducting WSe2 and metallic NbSe2 to reduce the Schottky barrier height. The transition interfacial layers were designed systematically via a combination of thermal evaporation of WO3, plasma-enhanced atomic layer deposition (PEALD) of Nb2O5, and
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a sequential selenization process to form Nb-doped WSe2. The PEALD process was adopted to control precisely the Nb doping in the WxNb1-xSe2. The use of 1, 3, and 5 PEALD cycles for the Nb2O5 deposition led to thicknesses of 0.8 nm, 1.2 nm, and 1.9 nm, respectively (Figures S4 and S5 and Table S1). To elucidate the atomic structures and compositions corresponding to the different amounts of Nb doping in WSe2, atomic-resolution annular dark-field (ADF) scanning TEM (STEM) was conducted on the three WxNb1-xSe2 transition layers (top left of Figures 2a–c and Figure S6). The high-resolution ADF-STEM top-view images of the WxNb1-xSe2 transition layers indicate that Nb atoms are substituted for W sites in the atomic structure. Thus, thermally diffusing Nb atoms relocated to the W sites to form the complex W–Nb–Se atomic structures. Figures 2a–c (top right) show the intensity profiles along the normal to the interface passing through the W/Nb and Se sub-lattices, which are indicated by the green boxes in the raw ADFSTEM images (top left). These images demonstrate that the three different atoms can be distinguished by the pixel intensities, because they are approximately proportional to the square of the corresponding atomic numbers. Furthermore, the color-rendered images (bottom left of Figures 2a–c) show clear brightness differences between the W and Nb (red circles: Nb sites, green circles: W sites), suggesting that the intensity of W is stronger than that of Nb due to the relatively high atomic number of W. To investigate the crystal structure in depth, gray-scale ADF-STEM images were analyzed (Figure S10). As discussed before, the pixel intensities are used to classify atoms: the intensities of the W sites are greater than those of the Se and the Nb sites in the ADF-STEM images, as shown in Figures 2a–c (top left). Therefore, the pixel intensities can be used to classify atoms. However, regular image segmentation methods cannot be used, because the boundary between the atoms and the background cannot be determined precisely on the
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molecular scale, due to the uncertainty principle.21 Instead, we exploited the nearly periodic structure of the TMD layer to identify the regions associated with the atoms. The strictly periodic pattern was obtained by considering the six intensity peaks in the Fourier domain (Figure S10).22 The size-invariant circle detection method was used to determine the approximate positions of the atoms, which could then be corrected by the method described in Figures S10 and S11. The averaged intensities inside the detected circles were classified as W atoms or collections of Se and Nb atoms by Otsu’s method for three classes.23 The classification results for the three different cycles are shown in Figures 2a–c (bottom right). The XPS spectra (Figures S7) and EELS results (Figure 2d) reveal the chemical compositions (W, Nb, and Se) of the three mixed transition layers. While maintaining a consistent amount of Se, the atomic ratio of Nb to W was adjusted by varying the number of PEALD cycles used to introduce the Nb dopants, as shown in Figure S7 and Figure 2d. It is evident that the amount of Nb doping in WxNb1-xSe2 increases as number of cycles increases from 1 to 5 (Figures S7, S9, and 2d). On the other hand, the atomic ratio of W decreases since Nb atoms occupy W sites (Figure S7a). The Raman spectra also substantiate the existence of Nb–Se vibrational modes for the WxNb1-xSe2 transition layers. As shown in Figure 2e, in the spectra recorded from the transition layers, the vibrational modes of Nb–Se (A1g and E12g: dashed arrows 1 and 2, respectively) emerge with increasing Nb concentration, resulting in total band broadening. On the contrary, the vibrational modes of W–Se (E12g and A1g: dashed arrows 3 and 4, respectively) decrease and disappear as the composition changes from pure WSe2 to NbSe2.16,17 To elucidate the electrical behaviors of the WxNb1-xSe2 transition layers, Hall measurements were conducted (Figure S8). Figure 2f shows the variations of the Hall parameters (sheet carrier density, mobility, and conductivity) with increasing Nb content in the WxNb1-xSe2
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transition layer. The sheet carrier density is proportional to the amount of Nb doping, while the mobility shows the maximum near 6% Nb content. The conductivity increases as the PEALD cycle increases and eventually reaches to a level close to bare NbSe2. Thus, doping of the Nb atoms donate holes, which increases the hole carrier density. Next, we turned our attention to the roles of transition layers between the semiconducting and metallic TMDs in FET devices with global back gates. (Figure S12: (i) metal (Pd)–semiconducting WSe2 (MS) junction, (ii) NbSe2–WSe2 vdW junction, and (iii) mixed layer containing NbSe2–NbxW1-xSe2–WSe2 junction, more specifically denoted as m-vdW: 1 cycle, 3 cycles, and 5 cycles). The detailed fabrication methods and atomic force microscopy and optical images of each junction devices are shown in Figures S12-16. In the drain-source current vs. drain voltage (IDS–VDS) curves for each junction (Figure S17) were compared, indicating the m-vdW: 1 cycle junction devices exhibit relatively symmetric and linear IDS–VDS behaviors. Introduction of metallic NbSe2 and WxNb1-xSe2 atomic layers with NbSe2 increased the drain current, enhancing charge transport across the contacts. The statistical WSe2-based FET device data (10 devices each with MS, vdW, or m-vdW junctions, where the m-vdW junctions had 1, 3, or 5 cycle junctions) show the p-type behavior of WSe2(Figure 3a). Channel-length-dependent FET properties for each junction are also shown in Figure S18. The most remarkable observation is that the transfer curves of IDS vs. VBG (with VDS = -5 V) of the FET devices with the mixed transition layers (m-vdW: 1 cycle) exhibit the largest ON currents and transconductances. The superior contact property of the mixed transition layer of the m-vdW: 1 cycle device also improves the output characteristics (IDS–VDS modulated by VBG of 20 V to -60 V with step size of -20 V) (Figure S19). Specifically, the contact resistance values of the MS, vdW, and m-vdW: 1cycle devices, determined by transfer length method, were measured to be ~8.2, 3.1, and 1.6
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MΩ·µm under Vg = 0 V and 71.8, 37.2, and 14.7 kΩ·µm under Vg = –60 V (Figure S20 and Table S2). Thus, it is believed that the improvement of FET performance on the m-vdW: 1cycle device was ascribed to the reduced contact resistance. Improvement in subthreshold swing and mobility was also observed in the m-vdW: 1 cycle device (Figure S21). The average mobility values of MS, vdW, and m-vdW: 1cycle devices were calculated to be ~0.08, 0.11, and 0.38 cm2V-1s-1. On the other hand, the MS junction devices exhibit relatively low ON currents with significant fluctuations. Furthermore, the statistical data reveal that all of the 2D TMD junction devices (vdW and m-vdW: 1, 3, and 5 cycles) show relatively small variations of the ON and OFF states, indicating their atomically clean and dangling-bonding-free 2D heterojunction interfaces (Figure 3b). The electrical contact properties of all FET devices with the 2D TMD junction also remained stable over time (Figure S22). The histograms depicting the data for the ON (VBG = -60 V) and OFF states (VBG = 30 V) show that the employment of NbSe2 electrodes instead of Pd metal increases the charge injection efficiency and, moreover, that the insertion of an additional transition layer with a low Nb doping level (i.e., WxNb1-xSe2: 1 cycle film) most significantly improves the charge transport across the metal–semiconductor TMD interface. The number of hole carriers increases as the Nb ratio in WxNb1-xSe2 increases, which corresponds to the Hall measurement results (Figure S23). The high carrier concentration at the mixed transition layer interface reduces the effective Schottky barrier height, as in the case of charge transport enhancement via degenerate doping in conventional bulk Si metal contacts.24 As discussed above, three mixed transition layers were formed by controlled doping of Nb into WSe2. Obviously, their electrical characteristics were closely related to the atomic ratio between Nb and W, as shown in the Hall measurement results (Figure S23). Interestingly, the performance of the m-vdW: 1 cycle FET device was the best among all of the devices, including
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those with MS and vdW FET junctions; however, overdose of Nb atoms in the transition region did not improve the electrical performance and even degrade it as in the cases of m-vdW: 3 and 5 cycles (Figure 3b). There are considerable differences between the temperature-dependent mobility obtained by Hall measurement of the WxNb1-xSe2: 1 cycle device and those of the other TMDs (WxNb1-xSe2: 3 and 5 cycles and NbSe2) (Figure S23). The WxNb1-xSe2: 1 cycle film only shows a positive dµ/dT, while the other films exhibit negative slopes (i.e., dµ/dT < 0) or negligible temperature dependences. The sheet carrier densities of all of the films were found to be almost temperature-independent, showing high hole carrier densities of 1014–1015 cm-2 (Figure S23). These results imply that the conduction mechanism switches from semiconducting behavior (the case of the WxNb1-xSe2: 1 cycle film) to metallic behaviors (the cases of the WxNb1-xSe2: 3 and 5 cycles and NbSe2 film) with increasing Nb doping in WSe2 (Figure S23).25 Thus, the best transition layer should maintain its semiconducting behavior. Furthermore, the crystal structures of the mixed transition layers are distorted by substitution of Nb atoms onto W sites of WSe2 due to the different binding lengths of W–Se and Nb–Se. This non-negligible structural distortion adversely affects the charge transport throughout the vertical heterojuctions.26,27 As the amount of Nb atoms increases for the transition WxNb1-xSe2 layers, the degree of disorder changes more rapidly, which eventually might disturb the charge transport due to the irregular lattice structure. This is the reason why the transition layers with relatively large amount of Nb dosage did not improve the FET performance anymore and even degrade it (as the cases of m-vdW: 3 and 5 cycles). Thus, the PEALD-based light-doping approach to 2D TMD materials is a unique method of systematically manipulating their physical properties. Since even the transition layers have high conductivities of 102–104 Scm-1, it is important to elucidate the exact role of the metallic NbSe2 placed on the transition layers. Thus,
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we fabricated 10 devices without and 10 with final NbSe2 top electrodes on the transition channel layers of WxNb1-xSe2/WSe2 and then compared the statistical VBG–IDS data of the two types of devices. The charge collection efficiency clearly increased by introducing the NbSe2 electrode, as shown in Figure S24. The high contact resistances of the devices without NbSe2 induced low ON currents, indicating that the use of a transition layer alone is not sufficient to collect the hole charges that accumulate throughout channel/SiO2 interfaces. To demonstrate the electrical uniformity of an 8 × 10 array of integrated m-vdW: 1 cycle FET devices with channel lengths varying from 10 µm to 50 µm, we measured all 80 devices and then mapped their OFF (IDS @ Vg = 30 V) and ON (IDS @ Vg = -60 V) states (Figures 3c and S25). The yield of devices with ON/OFF ratios of over 103 is ~85% (68 functioning devices out of 80 FET), indicating that the p-type transistors are spatially uniform. To elucidate the electrical contact properties of the different junctions, we evaluated the drain currents at different temperatures and extracted the barrier heights using the 2D thermionic emission model (Figure SS26 and S27, and Table S3). The slopes of the Arrhenius plots of ln(IDS/T3/2) vs. 1000/T at a fixed VDS of 0.5 V for the MS, vdW, and m-vdW: 1 cycle devices are -3.6, -2.3, and -2.0, respectively (Figure S27). The negative slopes indicate the existence of Schottky barriers in all of the tested FET devices. The Schottky barrier heights (@ VDS = 0 V) for the MS, vdW, and m-vdW: 1cycle devices, estimated from the extrapolation curves of the slopes vs. VDS, are 326, 199, and 176 meV, respectively (Figure 3d). Compared with the vdW and m-vdW: 1cycle, relatively large barriers for m-vdW: 3 and 5 cycles are responsible for the reduced transistor performance (Figure S28). The m-vdW junction was most effective at lowering the electronic contact barrier height. The energy band diagrams of the WSe2 channel between the source and drain illustrate the interface-induced lowering of the Schottky barrier height; for the m-vdW: 1 cycle junction, the tunnelling effect
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can be also considered (Figure 3e). In all of the considered junctions (MS, vdW, and m-vdW: 1 cycle junctions), Schottky barriers exist for the holes, as experimentally observed and shown in Figure 3d. It is very likely that interfacial traps in metal-semiconductor junctions cause Fermilevel pinning, suppressing barrier height modulation by metal electrode materials with varying work functions. This behavior explains why the MS junction shows an unexpectedly large Schottky barrier height corresponding to p-type WSe2 (Figure 3e(i)). The metallic NbSe2 abruptly lowers the Schottky barrier height (Figure 3e(ii)), because the work function of NbSe2 aligns more closely with the electron affinity of WSe2. The dangling-bonding-free and atomically sharp interface in the vdW junction results in the low contact barrier.12 The barrier height and thickness are significantly reduced for the m-vdW: 1 cycle junction, and a fraction of the holes would quantum-mechanically tunnel through the barrier (Figure 3e(iii)). Thus, using a mixed-composition m-vdW interface between 2D heterolayers is likely to become a new strategy for reducing the barrier height and increasing the charge injection efficiency across metal– semiconductor interfaces. Therefore, the strategy of creating novel 2D vdW structures by stacking could be carefully and precisely controlled via the PEALD-CVD hybridization technique suggested in this study, ultimately enabling atomic-scale lego devices with the necessary functionality. In summary, we demonstrated a unique and effective vdW stacking approach that can result in significant 2D TMD FET performance improvement by using mixed-composition atomic layers between semiconducting WSe2 and metallic NbSe2. The electrical characteristics of the WxNb1-xSe2 mixed transition layers strongly depend on the Nb dosage, which can be precisely controlled via PEALD-CVD. The presence of a WxNb1-xSe2 mixed-transition atomic layer in between the semiconducting WSe2 and metallic NbSe2 layers considerably reduces the
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Schottky barrier height, leading to a high ON current for the WSe2 FET. The Schottky barrier lowering and tunnelling are considered to contribute simultaneously to the enhanced charge transport throughout such novel mixed 2D atomic layer heterojunctions. The described composition-engineered junctions in 2D atomic layer heterostructures promise to open new avenues in research on the fabrication of atomic-layer-based electronic devices.
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FIGURES.
Figure 1. (a) Optical image and schematic drawing of WSe2-based bottom-gate FET with NbSe2 electrode. Scale bar is 10 µm. (b) HR-TEM images of as-grown WSe2 film. Scale bar is 5 nm. (c) Raman spectrum recorded from as-synthesized WSe2 film. Spectrum shows two major phonon modes: out-of-plane mode (A1g) and in-plane mode (E12g). (d) Top-view HR-TEM image of asgrown NbSe2, showing quasi-hexagonal honeycomb atomic structure. Scale bar is 5 nm. (e) Raman spectrum recorded from as-synthesized NbSe2 film, revealing two major vibrational modes (A1g and E12g). (f) EELS analysis substantiates chemical composition of as-synthesized WSe2. (g) Excitonic absorption peaks A and B of optical absorbance spectrum recorded from assynthesized WSe2. (h) EELS analysis substantiates chemical composition of as-grown NbSe2. (i) Hall measurement results, showing metallic characteristics of as-synthesized NbSe2 film. 14 Environment ACS Paragon Plus
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Figure 2. (a), (b), and (c) Sets of high-resolution atomic-resolution ADF-STEM images and image analysis of WxNb1-xSe2 transition layers with Nb2O5 1, 3, and 5 PEALD cycles, respectively. Magnified STEM and color-rendered images, illustrating W and Nb sites (green: W sites and red: Nb sites). Intensity profile along normal to interface passing through W/Nb and Se sub-lattices corresponding to green boxes in raw ADF-STEM images. Results of atomic classifications (bottom right of a, b, and c) for W (gray), Nb (hunter green), and Se (yellow) atoms. (d) EELS analysis substantiates increased Nb doping on transition layers. (e) Raman spectra also validate existence of Nb–Se bonding on WxNb1-xSe2 transition layers. Vibrational modes of Nb–Se (A1g and E12g) emerge and increase as amount of Nb doping increases. (f) Hall measurements show that carrier densities and conductivities increase with increasing Nb doping. The thicknesses of approximately 4, 4.5, and 7 nm for WxNb1-xSe2: 1, 3, and 5 cycle transition layers were considered for the Hall measurement.
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Figure 3. (a) Transfer characteristics (IDS–VBG) of MS, vdW, and m-vdW: 1, 3, and 5 cycle junction devices. Data were fitted by averages and standard deviations of 10 devices with each junction type. IDS was measured at VDS of -5 V in devices with channel length of 50 µm and width of 100 µm. (b) Histograms of OFF (IDS @ VBG = 30 V and VDS = -5 V) and ON (IDS @ VBG = -60 and VDS = -5 V) states in MS, vdW, and m-vdW junction devices. OFF and ON parameters were extracted from VBG–IDS curves of Figure 3a. (c) Top: optical image of 8 × 10 array of integrated m-vdW: 1 cycle FET devices on SiO2/Si wafer. Top inset: Zoomed-in image of devices with channel lengths varying from 10 µm to 50 µm depending on row line. Bottom 16 Environment ACS Paragon Plus
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images: mapping images of OFF (IDS @ VBG = 30 V and VDS of -5 V) and ON (IDS @ VBG = -60 and VDS of -5 V) states obtained from 80 devices. Device yield (defined as devices with ON/OFF ratio of over 103) is ~85%, indicating high structural and electrical uniformity of stacked 2D layers. (d) Schottky barrier heights of MS, vdW, and m-vdW junction devices. In each case, Schottky barrier height was calculated from Y-intercept of extrapolated slope vs. VDS curve. (e) Energy band diagrams for (i) MS, (ii) vdW, and (iii) m-vdW junctions. qΦMS, qΦvdW, and qΦmvdW
indicate Schottky barrier heights of MS, vdW, and m-vdW junctions, respectively. Dashed
green curves of junctions represent thermionic emission, while dashed red line for m-vdW junction indicates tunneling.
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ASSOCIATED CONTENT Supporting Information. Detailed methods for synthesis and fabrication, detailed process of image analysis, supporting figures S1 –S28, and supporting table S1 – S2. This material is available free of charge via the Internet at http://pubs.acs.org
AUTHOR INFORMATION Corresponding Author *To whom correspondence should be addressed: e-mail: [email protected] (M.G.H.); [email protected] (P.M.A.); [email protected] (B.C.)
ACKNOWLEDGMENT This study was supported by the Fundamental Research Program (PNK4580 & PNK4890) of the Korean Institute of Materials Science (KIMS). M. G. H. and B. C. are grateful for the support from the Basic Science Research Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2014R1A1A1006214 and NRF-2014R1A1A1036139). J. N. was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013R1A1A1004986, NRF-2015M1A8A1053681). H.-S. Chung acknowledges support of Korea Basic Science Institute (KBSI-C35928).
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