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MoS2 Functionalization with a Sub-nm Thin SiO2 Layer for Atomic Layer Deposition of High-k Dielectrics Haodong Zhang, Goutham Arutchelvan, Johan Meersschaut, Abhinav Gaur, Thierry Conard, Hugo Bender, Dennis Lin, Inge Asselberghs, Marc Heyns, Iuliana Radu, Wilfried Vandervorst, and Annelies Delabie Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b01695 • Publication Date (Web): 14 Jul 2017 Downloaded from http://pubs.acs.org on July 16, 2017
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Chemistry of Materials
MoS2 Functionalization with a Sub-nm Thin SiO2 Layer for Atomic Layer Deposition of High-k Dielectrics Haodong Zhang,1,2 Goutham Arutchelvan,2,3 Johan Meersschaut,2 Abhinav Gaur,2,3 Thierry Conard,2 Hugo Bender,2 Dennis Lin,2 Inge Asselberghs,2 Marc Heyns,2,3 Iuliana Radu,2 Wilfried Vandervorst,2,4 Annelies Delabie1,2* 1
Department of Chemistry, KU Leuven, 3001 Leuven, Belgium
2
Imec, Kapeldreef 75, 3001 Leuven, Belgium
3
Department of Materials Engineering, KU Leuven, 3001 Leuven, Belgium
4
Department of Physics and Astronomy, KU Leuven, 3001 Leuven, Belgium
ABSTRACT: Several applications of two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) in nano-electronic devices require the deposition of ultrathin pin-hole free high-k dielectric films on 2D TMDs. However, deposition of nm-thin high-k dielectric films on 2D TMDs remains challenging due to the inert TMD surface. Here, we demonstrate that the surface of a synthetic polycrystalline 2D MoS2 film is functionalized with SiO2 to enable the atomic layer deposition (ALD) of thin and continuous Al2O3 and HfO2 layers. The origins of nucleation, the growth mode and layer coalescence process have been investigated by complementary physical characterization techniques, which can determine the chemical bonds, absolute amount and surface coverage of the deposited material. SiO2 is prepared by oxidizing physical vapor deposited Si in air. The surface hydrophilicity of MoS2 significantly increases after SiO2 functionalization owing to the presence of surface hydroxyl groups. SiO2 layers with a Si content of only 1.5 × 10 / enable the deposition of continuous 2 nm thin Al2O3 and HfO2 layers on MoS2 at 300°C. This fast layer closure can be achieved despite the sub-nm thickness and discontinuity of SiO2 nucleation layer. Based on the experimental results, we propose a nucleation mechanism that explains this fast layer closure. Nucleation of Al2O3 and HfO2 occurs on the SiO2 islands, and fast layer closure is achieved by the lateral growth starting from the many nm-spaced SiO2 islands. Finally, the dielectric properties of Al2O3 on the functionalized MoS2 are confirmed in a top gated capacitor that shows a leakage current of 3.8 × 10 A/cm2 at a 3.4 nm equivalent oxide thickness. To conclude, fast nucleation and layer closure in ALD can be achieved even for a sub-nm thin, discontinuous nucleation layer. We propose that this insight can also be applied to other ALD processes, materials or applications where thin and fully continuous layers are required.
INTRODUCTION Semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDs) have promising applications in nano-electronic devices, tunnel field effect transistor (FET) and monolithic integration because of the intrinsic bandgap, self-passivated surface, atomic level thickness and low dielectric constant.1-6 For instance, 2D molybdenum disulfide (MoS2) has been investigated as the channel material in metal oxide semiconductor field effect transistors (MOSFETs), which show a high current on/off ratio of up to 108 and a low subthreshold swing of 65~70 mV/dec.7-9 The deposition of nm-thin high-k dielectric films on MoS2 is required to suppress Coulomb scattering in the channel and improve the electrostatic control.10 nm-thin high-k dielectric films have been successfully integrated in Si-based FET using atomic layer deposition (ALD), a deposition technique that provides sub-nm thickness control because it is based on selflimiting chemical reactions between the precursors and reactive sites on the surface.11-13 However, due to inherent 2D nature, the surface of TMDs cannot provide active
sites for the reaction with ALD precursors. Therefore, ALD of high-k dielectrics on exfoliated and synthetic MoS2 proceed via a three-dimensional (3D) growth mode, whereby layer closure is only obtained for rather thick layers.14-16 For example, the nucleation of Al2O3 ALD occurs only on line defects and grain boundaries at the top surface of synthetic MoS2, of which the basal planes do not provide reactive sites for ALD.16 Different approaches have been attempted to adjust the surface chemistry to overcome the nucleation issues for ALD on the inert surface of 2D TMDs. The Al2O3 or HfO2 surface coverage can be improved by forming an ultrathin MoOx nucleation layer on the top surface of MoS2 using oxygen plasma, whereas the formed insulating MoO3-rich disordered domains degrade the mobility and onconductance.15,18 Therefore, milder treatments that induce only surface S-O bonds have been investigated, such as non-destructive ultraviolet-ozone and remote oxygen plasma treatment of MoS2.19-21 But the O atoms desorb at high temperature, insufficient to enable the ALD at 300°C. In addition, continuous Al2O3 films are grown on
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Chemistry of Materials
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TMDs by employing different nucleation layers, such as AlN, metal oxide and organic molecules.22-26 Although plasma enhanced ALD grown AlN on MoS2 facilitate the growth of subsequent Al2O3, it is not yet clear what extent plasma damage will be sufficient for the continuous dielectric deposition.22 Because large Al agglomerates are formed on MoS2 surface due to the different surface energy (1.27 × 10 ⁄ for Al versus 46.5 ⁄ for MoS2), 10 nm Al2O3 is needed to realize a uniform continuous film on MoS2 using an Al2O3 nucleation layer, obtained by oxidizing physical vapor deposited Al.21 Interestingly, 3 nm equivalent oxide thickness (EOT) is reached using a titanyl phthalocyanine monolayer, deposited by molecular beam epitaxy, as a nucleation layer for Al2O3 ALD on WSe2. In this approach, the ALD temperature is set to 120°C due to the limited thermal stability of titanyl phthalocyanine molecules.25
um conditions (10-6 mbar) at a deposition rate of 0.01 nm/s. The thickness of Si is monitored in-situ by quartz crystal microbalance, and it can be converted to the content ( / ) based on the density of Si, whereby 1 nm Si corresponding to 5.0 × 10 / . Then, the samples are exposed to cleanroom atmosphere for 48 hours to selectively oxidize the Si. The ALD of Al2O3 and HfO2 are performed at 300°C in a PULSAR 2000 ALD reactor of an ASM Polygon cluster. TMA and H2O are used as precursors for Al2O3 while HfCl4 and H2O are used for HfO2. Nitrogen (N2) is used as carrier and purging gas for both processes. One ALD cycle for Al2O3 (HfO2) consists of alternating pulsing of precursors between N2 purge in the sequence of TMA/N2/H2O/N2 (HfCl4/N2/H2O/N2). The corresponding duration for TMA/N2/H2O/N2 (HfCl4/N2/H2O/N2) ALD process are 0.1 s/0.4 s/0.4 s/1.2 s (0.15 s/3 s/0.3 s/3 s).
Although different pretreatments and nucleation layers have been investigated, it remains challenging to integrate nm-thin and pin-hole free high-k dielectric films in 2D TMDs-based top gate devices. Therefore, in this work, we investigate ALD of nm-thin and pin-hole free gate dielectrics on functionalized TMDs. We explore an ultrathin SiO2 functionalization layer obtained by oxidizing physical vapor deposited Si and its impact on the nucleation and growth of Al2O3 and HfO2 ALD on polycrystalline MoS2. SiO2 is a well-known nucleation layer for Al2O3 and HfO2 ALD because of the fast reaction of surface hydroxyl groups with ALD precursors, such as trimethylaluminum (TMA) and hafnium tetrachloride (HfCl4).27 In Si-based MOSFETs, thin interfacial SiO2 layer improves the electron mobility by screening the remote phonon scattering from high-k dielectrics (e.g. HfO2 and ZrO2) on Si channel.28 In 2D TMDs based devices, tunnel FETs with a ZrO2/SiO2 gate stack show excellent properties, yet with 20 nm thick ZrO2 on 5 nm SiO2.29 Therefore, in order to scale down the SiO2 thickness, we first study the topography, layer closure and oxidation process of Si on MoS2 as a function of the Si content. Then, the surface hydrophilicity of MoS2 without and with the SiO2 functionalization is examined. Next, we investigate the nucleation and growth behavior of ALD on the MoS2 after functionalization with SiO2 as well as how this enables the thickness scaling of gate dielectrics. The high-k/SiO2/MoS2 interfaces are also investigated. Finally, the dielectric properties of Al2O3 on the functionalized MoS2 are investigated in a top gated capacitor, to evaluate the leakage current and EOT of the gate stack.
Physical Characterizations. The surface topography of MoS2 before and after the deposition/oxidation of Si is examined by atomic force microscope (AFM, Dimension 3100, Veeco). The chemical state of Si after air exposure as well as the high-k/SiO2/MoS2 interface is determined by X-ray photoelectron spectroscopy (XPS, Theta300, Thermo Instruments) with a monochromatized Al ! source (1486.6 eV). Note that the typical analysis depth of XPS is a few nm, therefore, 2 nm Al2O3 and 2 nm HfO2 with SiO2 functionalization on MoS2 is measured to ensure the interface information can be fully collected. C 1s of adventitious carbon contamination at 284.8 eV is used as a charge reference for all the XPS spectra. The surface morphology of Al2O3 and HfO2 on MoS2 is inspected by scanning electron microscope (SEM, NOVA NanoSEM 200, FEI). The growth curves of Al2O3 on MoS2 without/with SiO2 functionalization are determined by measuring quantitatively the Al content with elastic recoil detection (ERD) analysis using an 8 MeV 35Cl4+ ion beam and a time-of-flight detector.31 The surface coverage evolution of SiO2 on MoS2 is investigated by monitoring the timeof-flight secondary ion mass spectrometry (TOF-SIMS) signal decay of the original surface element (Mo), as a function of the deposited Si content. In the same way, the surface coverage evolution of Al2O3/SiO2 on MoS2 is investigated. The TOF-SIMS measurements are performed in a TOF-SIMS IV (ION-TOF) using 25 keV Bi3+ analysis beam. The detailed principle of using TOF-SIMS to investigate the growth characteristics can be found elsewhere.16,32 The surface hydrophilicity of MoS2 before and after SiO2 deposition is characterized by static water contact angle (WCA) measurement using deionized water droplets at room temperature (Dataphysics OCAH230L system). The thickness as well as the interfacial structure of nominal 5 nm (56 ALD cycles) Al2O3 and 2 nm (43 ALD cycles) HfO2 with SiO2 functionalization on MoS2 is inspected by transmission electron microscopy (TEM, Titan3TM G2 60300, FEI).
EXPERIMENTAL DETAILS Materials Processes. The fabrication of polycrystalline MoS2 consists of evaporating a Mo thin film onto a 90 nm-SiO2/Si(100) (or c-plane sapphire substrate) followed by sulfurization in H2S at 800°C (or 1000°C). The detailed information regarding the fabrication and characterization of MoS2 can be found elsewhere.16,30 The thickness of initial Mo layer is 0.5 nm (Mo: 4.9 × 10 / ), resulting in 4 monolayers of MoS2 (MoS2: 4.9 × 10 / ) after sulfurization. Si is deposited on MoS2 by electron-beam evaporation under high vacu-
Devices Fabrication and Measurements. The properties of the dielectric layers formed on MoS2 with the SiO2 functionalization are evaluated in a ring-gated field effect transistor (RG-FET), which is suitable for capaci-
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Chemistry of Materials
tance-voltage (C-V) and current-voltage (I-V) measurements. 33 Due to the difficulty on wet etching of HfO2, we choose 5 nm Al2O3 with SiO2 functionalization as gate dielectrics on MoS2/sapphire to make the RG-FET. A 50 nm Al layer is deposited by thermal evaporation as top gate electrode. Then, in one photolithography step, part of the Al and dielectrics are etched to open the via for the source and drain contacts. Finally, Ti/Au (20 nm/30 nm) source and drain contacts are fabricated using thermal evaporation followed by lift-off. C-V and I-V measurements are performed with HP4156 system to evaluate the EOT and leakage current of 5 nm Al2O3 with SiO2 functionalization on MoS2.
tent up to 1.0 × 10 / , the equivalent of about 10 monolayers of Si or SiO2, is not completely closed (Figure 2a). This further confirms the formation of islands on the MoS2 surface due to the agglomeration of Si, originating from the different surface energy of Si and MoS2. A continuous SiOx layer is observed only at a Si content of 2.5 × 10 / , the equivalent of 25 SiO2 monolayers. A so thick SiOx layer is obviously not suitable to serve as nucleation layer for the EOT scaling of high-k dielectrics.
RESULTS AND DISCUSSION Characterization of the SiO2 Functionalization. In principle, a SiO2 nucleation layer for ALD of high-k dielectric layers on MoS2 should be as thin as possible for EOT scaling, as the dielectric constant of SiO2 is only 3.9. Also, an as high as possible surface coverage of SiO2 is needed to ensure the fast layer closure of high-k dielectric layers. Furthermore, it was previously shown that lateral growth of high-k dielectrics can occur on MoS2 and result in a closed film.16 We therefore propose that a not fully continuous thin SiO2 layer can be considered, providing the dimensions of the uncovered surface are sufficiently small. In addition, complete oxidation of the deposited Si layer to SiO2 needs verification, to avoid the presence of elemental Si and Si suboxide.34 Keeping these requirements in mind, we have characterized the surface topography, layer closure and chemical state of Si in SiOx layers on MoS2. Note that we will use the term SiOx to represent the air-exposed Si layers on MoS2, which can consist of Si, SiOx (x
