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Sub-10 nm Tunable Hybrid Dielectric Engineering on MoS2 for Two-Dimensional Material-Based Devices Lanxia Cheng,*,† Jaebeom Lee,† Hui Zhu,† Arul Vigneswar Ravichandran,† Qingxiao Wang,† Antonio T. Lucero,† Moon J. Kim,† Robert M. Wallace,† Luigi Colombo,‡ and Jiyoung Kim*,† †
Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States Texas Instruments, Dallas, Texas 75243, United States
‡
S Supporting Information *
ABSTRACT: The successful realization of high-performance 2D-materials-based nanoelectronics requires integration of high-quality dielectric films as a gate insulator. In this work, we explore the integration of organic and inorganic hybrid dielectrics on MoS2 and study the chemical and electrical properties of these hybrid films. Our atomic force microscopy, X-ray photoelectron spectroscopy (XPS), Raman, and photoluminescence results show that, aside from the excellent film uniformity and thickness scalability down to 2.5 nm, the molecular layer deposition of octenyltrichlorosilane (OTS) and Al2O3 hybrid films preserves the chemical and structural integrity of the MoS2 surface. The XPS band alignment analysis and electrical characterization reveal that through the inclusion of an organic layer in the dielectric film, the band gap and dielectric constant can be tuned from ∼7.00 to 6.09 eV and ∼9.0 to 4.5, respectively. Furthermore, the hybrid films show promising dielectric properties, including a high breakdown field of ∼7.8 MV/cm, a low leakage current density of ∼1 × 10−6 A/cm2 at 1 MV/cm, a small hysteresis of ∼50 mV, and a topgate subthreshold voltage swing of ∼79 mV/dec. Our experimental findings provide a facile way of fabricating scalable hybrid gate dielectrics on transition metal dichalcogenides for 2D-material-based flexible electronics applications. KEYWORDS: MLD, hybrid dielectric, MoS2 FETs, Raman, photoluminescence, dielectric scaling ince the pioneering work on the first single-layer MoS2 switchable transistor,1 2D transition metal dichalcogenides (TMDs) have ignited significant interest in pursuing them for electronic applications, including tunnel field transistors (TFETs),2,3 heterojunction p−n photodiodes,4 transparent flexible optoelectronics,5,6 and memory devices.7,8 However, achieving the physical limits of these 2D semiconductors has met many integration challenges, such as growth of wafer-scale channel materials,9,10 high Schottky barrier,11 doping strategies,12−14 and scaling dielectrics,15,16 each of which requires sustained efforts before the full technological potential of these 2D materials can be implemented.17,18 Earlier studies on graphene18,19 and MoS21,20−22 transistors have shown a considerable mobility enhancement by having the 2D channel encapsulated with high-k dielectrics to screen charged impurities.23 Moreover, vertically stacked 2D heterojunctions, being physically isolated by a few layers of hexagonal boron nitride (h-BN),2,3,24 have exhibited interband tunneling and negative differential resistance, allowing for further
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© 2017 American Chemical Society
reduction of the subthreshold slope below 60 mV/decade and supply voltages less than 0.5 V. In all cases, conformal pinhole-free and ultrathin dielectrics are desired to minimize the OFF-state power dissipation. However, direct atomic layer deposition (ALD) growth of thin dielectrics on clean untreated 2D channels exhibits poor nucleation due to the dearth of dangling bonds.15,16,20 To date, most reported conformal ALD dielectrics were grown on 2D surfaces pretreated with oxygen species,15,16,25 seeding layers, e.g., organic TiOPc,26,27 or metal oxide buffer layers,28,29 showing performance trade-offs between dielectric thickness and the electrical metrics. Whereas h-BN, an intrinsic 2D insulator, exhibits nearly ideal dielectric properties,2,30 such as wide band gap (6 eV), high breakdown field (7 V/cm), and low interface-trapped charges, the unavailability of high-quality large-area material has been a major roadblock for process integration.9,10 Alternatively, the Received: July 9, 2017 Accepted: August 23, 2017 Published: August 23, 2017 10243
DOI: 10.1021/acsnano.7b04813 ACS Nano 2017, 11, 10243−10252
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Cite This: ACS Nano 2017, 11, 10243-10252
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Figure 1. (a) Schematic illustration of MLD of an OTS-Al2O3 supercycle on the MoS2 substrate at a deposition temperature of 100 °C. (i, ii) As-exfoliated and ozone-pretreated MoS2 surface, (iii−vi) one MLD supercycle starting with the formation of a monolayer OTS film followed by ozone oxidation of the OTS tail group (CC) and ALD-TMA/H2O to form an Al2O3 linker between the alternating OTS layers. AFM images and contact angle measurements of MoS2 surfaces showing surface morphology and wettability of MoS2 surfaces of (b) as-exfoliated, (c) O-functionalized, (d) with monolayer OTS, and (e) 10 MLD supercycles of an OTS-Al2O3 hybrid film.
hybrid gate dielectrics on TMDs for 2D-material-based electronics.
use of hybrid dielectrics, a family of self-assembled organic and inorganic nanolaminate films, on MoS2 has not yet been explored. Their low process temperature, large-area deposition capability, and great mechanical flexibility, combined with the excellent in-plane stiffness of MoS2,31 could provide a great step forward to the practical application of TMDs for large-area 2Dmaterials-based flexible electronics.6,17 Herein, we explored the integration of an organic and inorganic hybrid film on MoS2 using molecular layer deposition (MLD) and investigated the dielectric properties using dualgated MoS2 devices. Since MLD utilizes a self-limiting surface reaction mechanism similar to ALD, it not only enables film deposition with precise thickness control but also permits the “tuning” of the film’s chemical and electrical properties by modulating the functionality of the relevant organic and inorganic components and their relative composition ratio.32,33 Here, we employed the MLD of octenyltrichlorosilane (OTS) to form the self-assembled organic layer and used ALD of trimethylaluminum (TMA)/H2O to deposit the Al2O3 linker between the neighboring OTS layers. By combining MLD and ALD, a hybrid dielectric composed of an alternatively arranged OTS−Al2O3−OTS film is achievable on the MoS2 substrate. Our results show the formation of a pinhole-free and uniform hybrid dielectrics on the MoS2 surface with a sub-nanometer growth rate of 0.5 nm/cycle and excellent thickness scalability down to 2.5 nm. The interface chemistry studies using X-ray photoelectron spectroscopy (XPS), Raman, and photoluminescence (PL) suggest that MoS2 maintains excellent structural and composition integrity after deposition of OTS-Al2O3 films. Electrical results of topgated MoS2 devices showed that these films exhibit promising transistor dielectric properties, such as low leakage current (10−6 A/cm2 at 1 MV/cm), minimal doping, small hysteresis, and subthreshold voltage at zero back-gate bias. Our experimental findings provide a way of fabricating scalable
RESULTS AND DISCUSSION MLD Dielectric Development and Characterization. Figure 1a shows the schematic integration process of the MLD OTS-Al 2 O 3 hybrid films on the MoS 2 substrate (see Experimental and Methods for details). Since the as-exfoliated MoS2 surface is relatively inert due to the low dangling bond density, which limits its reactions with the incoming precursors, we employ an in situ ozone pretreatment to functionalize the MoS2 with oxygen adsorbates that are necessary to anchor the alkyltrichlorosilane (R-SiCl3), forming densely packed selfassembled alkyl-chains.34,35 Ozone or oxygen plasma enhanced ALD dielectrics have been demonstrated in several recent studies,15,16,25 in which a reactive oxygen species exposure prior to ALD can significantly improve the dielectric nucleation originating from the resultant weak sulfur−oxygen interactions. After ozone pre-exposure at 100 °C, a monolayer of OTS organic layer is deposited via five repeated OTS/H2O subcycles to promote the hydrolysis of the trichlorosilane (−-SiCl3) to form (CH2 ) n−Si−O−S (Mo) chains until a saturated monolayer OTS is reached, as shown in Figure 1a(i−iii). To initiate the MLD supercycle for a continuous hybrid film growth, a second ozone exposure of 10 s is performed immediately on the self-assembled OTS film to chemically convert the OTS terminal CC bond into a carboxyl (−COOH) group.34,36 The resultant hydrophilic carboxyl tail groups then serve as nucleation sites for the subsequent TMA/ H2O surface chemical adsorption, leading to the formation of an atomic layer of inorganic linkers (C−O−Al(OH)−OH) that connects with the successive OTS layer through an O−Si bond. By repeating the pulse of each precursor in such a sequence as shown in Figure 1a(iii−vi), that organic−inorganic OTS-Al2O3 hybrid film ranging from monolayer organic film to tens of 10244
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Figure 2. (a, b) XPS spectra of Mo 3d and S 2p, Al 2p, and Si 2p binding states of OTS-Al2O3 films on MoS2 as a function of the number of MLD supercycles. (c) OTS-Al2O3 film thickness derived from XPS signal attenuation in reference to the ellipsometer thickness on a H-Si substrate. (d) High-resolution cross-sectional HAADF-STEM image of a 2.5 nm uniform OTS-Al2O3 film deposited with five MLD supercycles.
nanometers of hybrid films is achievable by controlling the MLD growth supercycles. The surface morphology and energy (wettability) changes of MoS2 are monitored using atomic force microscopy (AFM) and DI water contact angle measurement. As shown in Figure 1b,c, the pristine MoS2 surface is smooth with a root mean squared (RMS) roughness of 0.06 nm and a contact angle of ∼55°, suggesting that the MoS2 surface is hydrophobic. Ozone exposure under static vacuum conditions (10 s pulse with 100 s “static vacuum”; see Experimental and Methods for details) results in a slight increase of the RMS roughness to 0.09 nm and a decrease of the contact angle to less than ∼10° due to the oxygen adsorption, indicative of the formation of a completely oxygen- or hydroxyl-covered MoS2 surface. Adsorption of oxygen species on MoS2 is also confirmed by the appearance of the O 1s peak in the corresponding XPS results shown in Figure S1a,b. Notably, the relevant XPS spectra of Mo 3d and S 2p chemical binding states reveal neither the detection of Mo oxidation states (232.7 eV) nor the formation of a weak S−O binding state (164.8 eV) after brief ozone exposure,37 where the latter is claimed to stem from the UV light exposure16 or longer oxygen radical treatment.37 Since all of the MoS2 surface characterization after ozone exposure was carried outside of the deposition chamber, contact angle measurements and XPS characterization were completed within 5 min in order to minimize the ambient air exposure. In accordance with the phenomena reported on the short-time oxygen species pretreated MoS2 surface,15,25 it is likely that only physisorbed oxygen species are introduced onto the MoS2 surface during the brief ozone exposure, which merely functionalizes the MoS2 surface with O atoms but does not introduce noticeable oxidation states (i.e., etching) in the MoS2 lattice structure. Our findings, together with other studies,16,38,39 provide substantial evidence that ozone, rather than an oxygen plasma, provides a gentle and efficient functionalization of the MoS2 surface, given the presence of relatively stronger Mo−S covalent bonds.15
The MoS2 surface contact angle changes to ∼78° after five successive subcycles of OTS/H2O exposures at 100 °C, signifying the formation of saturated OTS self-assembled layers, which increases the MoS2 surface RMS roughness to 0.12 nm as shown in Figure 1d. In contrast to the rough surface prepared in wet solution self-assembled OTS film shown in Figure S1c, the MLD monolayer OTS film tends to be clean and smooth without formation of any island aggregates arising from the trichlorosilane intermolecular cross-link reactions or solvent residues.35 Figure 2d shows the AFM surface morphology of an MLD-prepared OTS-Al2O3 film after 10 supercycles; the low RMS roughness of 0.15 nm suggests that a very smooth and uniform hybrid dielectric film can be achieved on the MoS2 surfaces, enabling the formation of a hydrophobic surface with a high contact angle of ∼80°. Hydrophobic surfaces of low roughness have led to several beneficial impacts on the electrical performance of graphene40,41 and MoS242,43 transistors in terms of enhanced carrier mobility, controllable doping, and improved device stability. The excellent film uniformity and smoothness of the MLD-prepared OTS-Al2O3 suggest these materials hold great prospects as a dielectric film for improving the electrical performance of 2D-material-based electronics. To ascertain the integration of OTS-Al2O3 films on the MoS2 surface using the MLD process, XPS characterization of the film composition and interfacial chemical states of MoS2 was conducted. Figure 2a shows the peak evolution of Mo 3d and S 2p binding states as a function of MLD supercycles in reference to that of the as-exfoliated MoS2 surface. The prominent doublet peaks located at 229.5 and 232.6 eV are ascribed to the Mo 3d5/2 and Mo 3d3/2 components of the Mo 3d chemical state in MoS2, whereas similar doublet peaks appearing at 162.3 and 163.5 eV correspond to the S 2p3/2 and S 2p1/2 signals of the S 2p binding state, respectively. Close examination of the XPS binding states of the as-exfoliated MoS2 does not reveal any detectable additional chemical states at ca. 232.7 eV (Mo 6+) or 229.2 eV (Mo 4+) and S−O (164.8 eV) associated with 10245
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Figure 3. (a) XPS valence band spectra of the as-exfoliated bulk MoS2 and OTS-Al2O3 (∼15.7 nm) film showing the relevant valence band maximum (VBM) offset. (b) XPS O 1s energy loss feature of OTS-Al2O3 films with an extracted band gap of ∼6.09 eV. (c) Proposed energyband alignments between OTS-Al2O3 hybrid films in reference to the bulk MoS2 surface. The error bar for XPS spectra linear fitting is defined to be ±0.1 eV.
and conformal deposition of OTS-Al2O3 ultrathin hybrid films can also be transferred onto the WSe2 surface with excellent film uniformity and thickness scalability down to 2.3 nm, in addition to the preservation of the surface structural and chemical integrity of the WSe2 substrate. To gain further insight into the energy band alignment at the interface of MoS2 and OTS-Al2O3 hybrid dielectrics, XPS analysis of the valence band-edge spectra of the bulk MoS2 and OTS-Al2O3 film (∼15.7 nm prepared by 30 MLD supercycles) is performed according to the photoelectron spectroscopic method elaborated in previous studies.45,46 As shown in Figure 3a, the relevant valence band maximum (VBM) is extracted by linearly extrapolating the leading valence band edge of an experimental VB spectrum to the baselines collected on the bulk MoS2 and OTS-Al2O3 dielectric, which is determined to be ∼1.04 eV for MoS2 and ∼3.08 eV for the thick OTS-Al2O3 dielectric. Since XPS core level binding energies of the interface Mo 3d and Al 2p remain the same, the valence band offset (VBO) is therefore directly determined from the VBM difference and is ∼2.04 eV at the MoS2 and hybrid dielectric interface. The XPS binding energy accuracy we defined here is ±0.1 eV37,62 and is due to the linear extrapolation errors associated with the scatter of the VB spectrum and O 1s loss feature. Figure 3b shows the energy separation between the O 1s core level of OTS-Al2O3 films and the onset of the O 1s loss feature that correlates to the excitation of electrons from the valence band maximum to the conduction band minimum, which enables a direct determination of a band gap of ∼6.09 eV for the OTS-Al2O3 hybrid film.47,48 On the basis of these experimentally determined values in reference to the band gap of ∼1.2 eV for bulk MoS2, the schematic energy band diagram for MoS2 and the OTS-Al2O3 dielectric is proposed in Figure 3c, where the related conduction band offset (CBO) is inferred to be ∼2.85 eV. In contrast to the relatively wider VBO and CBO found for the MoS2/Al2O3 interface,45 our results suggest that the inclusion of the organic OTS component in ALD highk dielectric films can lower the inorganic dielectric band gap, but still render a sufficient barrier to be used as a dielectric. In addition to the preservation of the surface chemical and structural integrity, Raman and photoluminescence spectroscopic studies of the electronic band structure of MoS2 after OTS-Al2O3 film deposition are also important. Recently, there have been several studies reporting the charge carrier doping of TMD channels utilizing self-assembled organic monolayers, e.g., octadecyltrichlorosilane49,50 or alkanethiol,51 by virtue of their inherent molecular polarity and functionality. However, since in all of these studies the monolayer self-assembling process
either molybdenum or sulfur oxidization.37 In addition, the peak intensities of both the Mo 3d and S 2p states display continuous attenuation as a result of the deposition of thicker hybrid dielectrics. The XPS intensity attenuation of the Mo 3d peak is estimated to be 25% after five successive OTS/H2O subcycles, which is complementary to the appearance of the asymmetric Si 2p binding states at 102.8 (Si−O) and 101.5 eV (Si−C),35 providing convincing evidence for the formation of a monolayer OTS hydrocarbon layer on MoS2. As indicated in Figure 2b, the lower binding energy peak at 101.5 eV is assigned to the Si−C bond from the alkane−silicon backbone, and the higher binding energy peak corresponds to the Si−O bond resulting from the hydrolysis interaction of the Si-OH group with the oxygen adsorbates on the MoS2 interface. It is noted that our XPS survey spectra and Cl 2p core level shown in Figure S2a,c do not reveal any detectable chlorine, which is indicative of complete hydrolysis of chlorosilane and the removal of inorganic byproducts during the purging process. After five OTS-Al 2 O 3 supercycles, along with further attenuation of Mo 3d peak intensity, the Al 2p chemical binding state at 74.6 eV emerges and is attributed to the formation of the Al−O bond as a result of the chemical reaction between the carboxyl (COOH) tail group and TMA precursor. As depicted in Figure S2b, the conversion of the CC tail group of OTS into a carboxylate species (−COOH) is also clearly identified from the presence of multiple C 1s chemical binding states deconvoluted at 288.6 eV (CO), 285.8 eV (C−O), and 284.6 eV that is attributed to the −CH 2 component of the OTS hydrocarbon backbone.44 On the basis of the Mo 3d peak intensity attenuation (detailed in the Supporting Information), the overlayer thickness is calculated to be 0.63 ± 0.1 nm for monolayer, 2.3 ± 0.2 nm for five, and 5.3 ± 0.5 nm for 10 MLD OTSAl2O3 supercycles, in good agreement with the corresponding thickness measured on hydrogen-terminated Si as shown in Figure 2c. This growth rate indicates a further tilted OTS molecular orientation of ∼60° from the surface normal.36 The constant growth rate of approximately 0.5 nm/supercycle is further verified with the cross-sectional high-angle annular dark field (HAAF)-scanning transmission electron microscopy (STEM) thickness measured on five MLD supercycles of OTS-Al2O3 films. Figure 2d shows the STEM image from a uniform pinhole-free organic−inorganic hybrid layer of 2.5 nm on the surface of MoS2 with an atomically sharp interface. The corresponding EELS result shown in Figure S3 confirms the composition of the hybrid film is dominant with C and O elements. Notably, as demonstrated in Figure S4, this uniform 10246
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Figure 4. Raman and photoluminescence characterizations of MoS2 flakes of various thickness prior to and after OTS-Al2O3 dielectric deposition. (a−c) Raman spectra of 1 L, 2 L, and 4 L of MoS2 and corresponding high-resolution Raman (E12g) peak shift mapping images. (d−f) Relevant PL response spectra and A exciton peak mapping of MoS2 flakes showing dielectric tensile strain induced peak intensity and position changes.
and split the in-plane E12g resonance mode due to the crystal symmetry breaking. However, since the tensile strain imposes a negligible effect on the A1g mode, the observed minor red-shift of the A1g resonance frequency may be related to the carrier doping effect that alters the strength of electron−phonon interactions contributing to mostly the shift of the out-of-plane vibration mode. In contrast to the significant doping phenomena discovered with solution-based self-assembled monolayers49,50 or chemical doping (≥1.0 × 1013/cm2),13,54 the observed A1g shift after MLD dielectric deposition is nearly negligible, within the error of Raman spectra resolution (0.5 cm−1). Moreover, both the strain and doping effects become completely suppressed as the MoS2 thickness increases to more than two layers, as seen from the related Raman mapping images (Figure 4b,c). This suggests that any electronic structure variations introduced from the OTS-Al2O3 film deposition exist only in the topmost layer of the MoS2 flake. Similar minor red-shifts (∼0.3 cm−1) of MoS2 vibration modes are also visible with the ALD-Al2O3 covered monolayer MoS2 (Figure S5e), indicating that the strain effect is the main reason accounting for the observed Raman shifts. Consistent with the Raman spectra (Figure 4a), the slight A1g Raman resonance softening merely happens to the monolayer region and is nearly suppressed entirely as the film thickness increases (Figure S5c,d). In agreement with the Raman results, the PL signal of monolayer MoS2 exhibits a tensile strain associated red-shift and intensity attenuation after OTS-Al2O3 deposition.53 As shown in Figure 4d, as-exfoliated MoS2 shows two characteristic PL peaks at 1.82 eV (A exciton) and 1.98 eV (B exciton), respectively, which are associated with the direct-gap transition from the spin−orbit valence band splitting to the conduction band edge. The intensity of the A exciton peak decreases as the
involves the dipping of TMDs in solution for varying times, the doping impact arising from the physisorbed polar solvent molecule and reaction byproducts cannot be excluded, and this may account for the inconsistent doping behaviors observed with the OTS-treated MoS2.42,50 In contrast, the vapor-based MLD process employed here provides a very clean film growth environment (vacuum chamber with base pressure ∼4 × 10−2 Torr) for deposition of self-assembled monolayers or hybrid dielectric films on the MoS2 surface, which gives the least concerns over the film reproducibility arising from any uncontrollable preparation conditions. Raman and PL Response of MoS2 with a Hybrid Dielectric. Figure 4 displays the Raman and PL spectra, along with the corresponding peak position mapping images, of a MoS2 flake of varied thicknesses prior to and after MLD of OTS-Al2O3 films. The layer thickness of the MoS2 flake is verified by a combination of optical contrast, AFM, and spectroscopic measurement, as shown in Figure S5a,b. It should be noted that the SiTO Raman mode at 520.5 cm−1 is calibrated before each Raman and PL measurement to ensure the consistency in monitoring the MoS2 Raman and PL shifts. Figure 4a shows the typical Raman spectra of MoS2 flakes with two dominant peaks at 385.6 and 404.4 cm−1 that are assigned to the S−Mo−S in-plane (E12g) and out-of-plane (A1g) mode, respectively. As the MoS2 thickness increases, the peak separation also increases from 18.8 to 20.5 and 24.5 cm−1, corresponding to 1, 2, and 4 L of MoS2 according to the published literature.52 After MLD, both the E12g and A1g modes of 1 L MoS2 exhibit a slight red-shift of 1.2 ± 0.1 and 0.4 ± 0.1 cm−1, respectively. The red-shifted E12g most likely originates from the strain-induced phonon softening that has been discussed by Conley et al.,53 in which an increase of the uniaxial tensile strain on monolayer MoS2 is found to downshift 10247
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Figure 5. Electrical characterization of dual-gated MoS2 FETs fabricated using 7.5 nm OTS-Al2O3 as a top-gate dielectric. (a) Schematic diagram of the dual-gated MoS2. (b, c) Transfer and output curves of a back-gate MoS2 FET before and after dielectric deposition (channel and gate dimension: Lch = 6 μm, Ltg = 3 μm, Wch = 14 μm). (d, e) Ids−Vtg plots and related 2D current density map of a dual-gate MoS2 FET as a function of top-gate voltage (Vtg sweeping from −1 to 1 at 0.1 V step) at varied back-gate voltages (Vbg = −60 to 60 at 10 V steps). (f) Topgate leakage current density of OTS-Al2O3 dielectric films scaling from 7.5 nm down to 2.5 nm in reference to four layers of hBN on graphene.2
followed by electron-beam-deposited Cr/Au (15/65 nm) as contact metals. Figure 5b,c compare the transfer and output characteristics of the back-gated MoS2 transistor before and after dielectric deposition. The transfer curve of as-fabricated MoS2 exhibits a typical n-type transistor behavior with an on/ off ratio of ∼106 and hysteresis of ∼ΔVth = 15 V as the backgate voltage sweeps back and forth from −30 to 30 V at a fixed Vds of 0.5 V. Upon encapsulation with an OTS-Al2O3 dielectric, the threshold voltage (Vth) shifts positively ∼4.1 V with hysteresis reduced to 5.3 ± 0.5 V. The positive Vth is indicative of a p-doping effect, in contrast to the negative Vth shifts reported on most ALD Al2O320 or HfO222,56 oxides. This opposite doping behavior may be explained by the presence of interface trapped charges from H2O or O2 adsorbates during MLD OTS layer deposition, whereas ALD oxides tend to introduce positive fixed charges.20 Using the formula ne = Q/e = CBG|ΔVth|/q, where q is the elementary charge, the doping carrier concentration is estimated to be ∼3 × 1011/cm2, which is smaller than the doping concentrations (∼7 × 1012/cm2) introduced by solution-based self-assembled organic films.42,51 In addition, using the equation μ = gmL/WCbgVds where L and
thickness of MoS2 increases from 1, 2 to 4 L layers, in accordance with the published results,14,55 due to the thicknessrelated direct to indirect band gap transition. Alongside the relatively minimal energy shift occurring to the A exciton of the bilayer and multilayer MoS2, the growth of the OTS-Al2O3 dielectric red-shifts only the monolayer A peak by 20 meV. This energy shift, in addition to the decreased PL intensity, suggests that only a small amount of tensile strain, ca. less than 1%,53 is introduced into the topmost layer of MoS2 flakes. Like the Raman mapping results, the PL mapping images (Figure 4e,f) suggest that this A peak energy shift is noticeable only in the monolayer MoS2 region and is attenuated considerably as the MoS2 thickness increases. It is important to note that an unambiguous determination of the reasons causing the redshifts of MoS2 Raman and PL phonon emission modes is nontrivial when considering the complex phonon−electron interaction scenarios in carrier-doped, strained, and oxygen radical degraded MoS2 structures.14,39,51,53 Nevertheless, on the basis of our Raman and PL observation of the minimal peak shifts in comparison to aforementioned studies, MLD deposition of OTS-Al2O3 on MoS2 tends to maintain the electronic band structure of underlying MoS2 resembling Raman spectra changes caused by ALD high-k dielectric process.15,25,38 Electrical Characterization of MoS2 FETs Using MLD Dielectrics. Figure 5a displays a schematic diagram of a representative dual-gated MoS2 field effect transistor with 7.5 nm OTS-Al2O3 as a hybrid top-gate dielectric to evaluate the electrical performance of MoS2 devices. The dual-gated MoS2 FETs are fabricated with few-layer MoS2 (∼6−10 layers) exfoliated on a 285 nm SiO2/n+Si substrate. Source/drain and top-gate regions are patterned using e-beam lithography
W are the channel length and width, respectively, and g =
dIds dVg
is the transconductance, the effective carrier mobility of the pristine MoS2 back-gated devices prior to MLD is around 28.7 cm2/(V s), which increases to 39.1 cm2/(V s) after OTS-Al2O3 dielectric deposition. The Ids−Vbg curve shows a slight increase in current density (∼10%) from 0.95 to 1.1 μA/μm measured at Vds = 0.5 and Vbg = 10 V and saturates at 3.4 μA/μm at a high drain bias region; this is likely owing to the pinch-off of the conducting channel. In comparison to the considerable charge carrier mobility improvement reported in ALD high-k dielectric 10248
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Article
ACS Nano encapsulated MoS2 devices,20,28,57 our electrical characteristics of OTS-Al2O3-capped MoS2 show no significant charge carrier mobility enhancements. However, the back-gate hysteresis, 5.3 ± 0.5 V (Figure S6a,b), and the interfacial trapped charge density, according to the subthreshold swing of ∼1.95 × 1012/ cm2, are close to the the reported results (∼1012−1013/cm2) using UV-ozone-functionalized ALD-oxides38,57,58 or a lowtemperature ALD process,20,59 which is likely due to the trapped H2O or O2 interfacial charge scattering. Figure 5d shows the current density as a function of top-gate voltage (Vtg = −1 to +1 V) with the back-gate bias sweeping from Vbg = −60 to +60 V at steps of 10 V. As the Vbg is varied from negative to positive voltage, the carrier density in the MoS2 channel region is changed from p- to n-type doping, therefore shifting the threshold voltages negatively and leading to an increase in current density.60 At zero back-gate voltage, a small hysteresis of less than ∼50 mV is identified as the topgate bias sweeps from −1 to 1 V forward and backward, which, using the geometrical capacitance of the gate stack, yields a topgate trap charge density of ∼1.8 × 1011/cm2 at the interface of MoS2 and the OTS-Al2O3 dielectric. The field effect mobility of this top-gate device is extracted to be ∼25 cm2/(V s) under Vds = 0.5 V with a subthreshold voltage of ∼79 mV/dec, which is similar to the reported values for thin top-gate ALD-Al2O3 on MoS2.20,25 Figure 5e shows a corresponding two-dimensional current density map of a dual-gate MoS2 FET with a 7.5 nm OTSAl2O3 film as top-gate dielectric and 285 nm SiO2 as a bottomgate dielectric, representing the combined dual-gate (Vtg and Vbg) modulation of the channel current density (Ids) at Vds = 0.5 V. By calculating the slope of the current maxima in the 2D current density plot, the OTS-Al2O3 top-gate dielectric constant and capacitance can be extracted using the following equation: Cbg/Ctg = 1/slope, where Ctg and Cbg refer to the topand back-gate capacitances, respectively. The back-gate capacitance is calculated to be 12 nF/cm2, which gives a topgate capacitance of 563 nF/cm2 and a dielectric constant of 4.5 ± 0.5 for OTS-Al2O3 hybrid films, in close agreement with the value extracted from the Au/OTS-Al2O3/Si metal−insulator− metal structures shown in Figure S6c.60 This dielectric constant is lower than that of ALD-Al2O3 (∼9.0) because of the incorporation of the OTS component with a lower dielectric constant (2.5),60 which, hence, offers a way to tune the dielectric constant of the stack by incorporation of proper organic moieties. Furthermore, the breakdown voltage of our hybrid films is determined to be 7.8 ± 0.2 MV/cm, close to the prior reported breakdown field of high-k ALD oxides.25,57 Figure 5f displays the top-gate leakage current density of a hybrid OTS-Al2O3 film scaling down from 7.5 to 2.5 nm using Ru as a top metal contact to prevent leakage current reduction from metal oxidation. The leakage current density through a 7.5 nm thick OTS-Al2O3 film is ∼1 × 10−6 A/cm2 at an electric field of ±1 MV/cm, and it increases to ∼2.3 × 10−4 and ∼10 A/cm2 as the dielectric thickness scales down to 5 and 2.5 nm. The leakage current density of the 7.5 nm thick hybrid dielectric resembles the minimal leakage level measured for thicker OTS-Al2O3 dielectrics and is comparable to the reported high-k ALD dielectrics of similar thickness.25−27 In light of the electrical characterization, along with the other interesting properties including low integration temperature, film flexibility, and ultralow water permeability,61 the OTSAl2O3 hybrid film presents a promising dielectric candidate for diverse applications in 2D-material-based flexible electronics.
CONCLUSION In summary, we have explored the deposition of hybrid dielectrics on the MoS2 surface by combining the molecular layer deposition of an organic OTS layer and atomic layer deposition of an inorganic Al2O3 linker. The self-limiting surface reactions of MLD OTS-Al2O3 demonstrate a constant sub-nanometer deposition rate of 0.5 nm/supercycle, allowing for a scalable dielectric film thickness down to 2.5 nm with excellent preservation of the surface chemical and structural properties. XPS and electrical characterization suggest a reduction of the inorganic metal oxide band gap to 6.09 eV and dielectric constant to 4.5 by incorporation of an organic OTS component in the dielectric films, suggesting possible band structure tunability by a proper selection of organics. Additionally, the Raman and PL spectroscopic characterization reveal minimal alteration of the interfacial electronic structure of few-layer MoS2 flakes after OTS-Al2O3 dielectric integration. The electrical results of a dual-gate MoS2 transistor exhibit promising dielectric properties including a small subthreshold swing (∼79 mV/dec), small hysteresis (∼50 mV), and low leakage current density (
