MoS2 Functionalization with a Sub-nm Thin SiO2 Layer for Atomic

the deposition of continuous 2 nm thin Al2O3 and HfO2 layers on MoS2 at 300°C. ... despite the sub-nm thickness and discontinuity of SiO2 nucleation ...
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MoS2 Functionalization with a Sub-nm Thin SiO2 Layer for Atomic Layer Deposition of High‑κ 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*,†,‡ †

Department of Chemistry, KU Leuven, 3001 Leuven, Belgium Imec, Kapeldreef 75, 3001 Leuven, Belgium § Department of Materials Engineering, KU Leuven, 3001 Leuven, Belgium ∥ Department of Physics and Astronomy, KU Leuven, 3001 Leuven, Belgium

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‡

ABSTRACT: Several applications of two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) in nanoelectronic devices require the deposition of ultrathin pinhole free high-κ dielectric films on 2D TMDs. However, deposition of nm-thin high-κ 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 × 1015 atoms/cm2 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 the SiO2 nucleation layer. On the basis of 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 × 10−6 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.

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INTRODUCTION Semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDs) have promising applications in nanoelectronic devices, tunnel field effect transistors (FETs), and monolithic integration because of the intrinsic band gap, 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-κ dielectric films on MoS2 is required to suppress Coulomb scattering in the channel and improve the electrostatic control.10 nm-thin high-κ 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 self-limiting chemical reactions between the precursors and reactive sites on the surface.11−13 However, due to the inherent 2D nature, the surface of TMDs © 2017 American Chemical Society

cannot provide active sites for the reaction with ALD precursors. Therefore, ALD of high-κ dielectrics on exfoliated and synthetic MoS2 proceeds via a three-dimensional (3D) growth mode, whereby layer closure is only obtained for rather thick layers.14−17 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 on-conductance.15,18 ThereReceived: April 25, 2017 Revised: July 14, 2017 Published: July 14, 2017 6772

DOI: 10.1021/acs.chemmater.7b01695 Chem. Mater. 2017, 29, 6772−6780

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Chemistry of Materials

corresponds to 5.0 × 1015 atoms/cm2. Then, the samples are exposed to a cleanroom atmosphere for 48 h 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 durations for the 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). Physical Characterizations. The surface topography of MoS2 before and after the deposition/oxidation of Si is examined by an atomic force microscope (AFM, Dimension 3100, Veeco). The chemical state of Si after air exposure as well as the high-κ/SiO2/MoS2 interface is determined by X-ray photoelectron spectroscopy (XPS, Theta300, Thermo Instruments) with a monochromatized Al Kα source (1486.6 eV). Note that the typical analysis depth of XPS is a few nanometers; 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 a 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 35 4+ Cl ion beam and a time-of-flight detector.31 The surface coverage evolution of SiO2 on MoS2 is investigated by monitoring the time-offlight 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 a 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, Titan3 G2 60-300, FEI). 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 capacitance−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 the 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 an HP4156 system to evaluate the EOT and leakage current of 5 nm Al2O3 with SiO2 functionalization on MoS2.

fore, milder treatments that induce only surface S−O bonds have been investigated, such as nondestructive ultraviolet-ozone and remote oxygen plasma treatment of MoS2.19−21 However, the O atoms desorb at high temperature, insufficient to enable the ALD at 300 °C. In addition, continuous Al2O3 films are grown on TMDs by employing different nucleation layers, such as AlN, metal oxide, and organic molecules.22−26 Although plasma enhanced ALD grown AlN on MoS2 facilitates 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 the MoS2 surface due to the different surface energy (1.27 × 103 mJ/m2 for Al vs 46.5 mJ/m2 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 Although different pretreatments and nucleation layers have been investigated, it remains challenging to integrate nm-thin and pinhole free high-κ dielectric films in 2D TMDs-based top gate devices. Therefore, in this work, we investigate ALD of nm-thin and pinhole 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, a thin interfacial SiO2 layer improves the electron mobility by screening the remote phonon scattering from highκ dielectrics (e.g., HfO2 and ZrO2) on the 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-κ/SiO2/MoS2 interfaces are also investigated. Finally, the dielectric properties of Al2O3 on the functionalized MoS2 are investigated in a topgated capacitor, to evaluate the leakage current and EOT of the gate stack.

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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 the initial Mo layer is 0.5 nm (Mo: 4.9 × 1015 atoms/ cm2), resulting in 4 monolayers of MoS2 (MoS2: 4.9 × 1015 molecules/ cm2) after sulfurization. Si is deposited on MoS2 by electron-beam evaporation under high vacuum conditions (10−6 mbar) at a deposition rate of 0.01 nm/s. The thickness of Si is monitored in situ by a quartz crystal microbalance, and it can be converted to the content (atoms/cm2) based on the density of Si, whereby 1 nm Si

RESULTS AND DISCUSSION Characterization of the SiO2 Functionalization. In principle, a SiO2 nucleation layer for ALD of high-κ 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-κ dielectric layers. Furthermore, it was previously shown that lateral growth of high-κ 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 6773

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Chemistry of Materials 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 < 2), and SiO2 depending on the Si content. The root-mean-squared (RMS) surface roughness of SiOx on MoS2 as a function of Si content is determined by AFM (Figure 1). Interestingly, AFM indicates little to no additional surface

surface is covered by compactly arranged islands, which results in a lower surface roughness. As the surface coverage of SiOx on MoS2 will impact the layer closure process of the subsequently deposited high-κ dielectric layer, we investigate the surface coverage of SiOx on MoS2 using TOF-SIMS methodology, whereby we analyze the Mo+ intensity (originating from the MoS2) as a function of the Si content.14,36 A SiOx layer with a Si content up to 1.0 × 1016 atoms/cm2, the equivalent of about 10 monolayers of Si or SiO2, is not completely closed (Figure 2a). This further

Figure 2. (a) TOF-SIMS Mo+ intensity decay of SiOx on MoS2 as a function of Si content. (b) XPS Si 2p spectra for Si after air exposure for 48 h as a function of Si content. Static water contact angle measurement of (c) blanket MoS2 (80.4°) and (d) SiO2 (37.9°, 1.5 × 1015 Si atoms/cm2) on MoS2.

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 × 1016 atoms/cm2, 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 highκ dielectrics. In order to investigate the natural oxidation process of Si on MoS2 in a cleanroom atmosphere, the chemical state of Si in SiOx (after air exposure for 48 h) is determined by XPS (Figure 2b). SiOx with a Si content of 2.5 × 1015 atoms/cm2 or less consists only of SiO2, as the Si 2p spectrum contains only one peak at 103.4 eV, corresponding to the 4+ oxidation state of Si.37 In contrast, SiOx with a Si content of 5.0 × 1015 atoms/ cm2 and more consists of SiO2 as well as Si. In this case, the Si 2p spectrum also contains a second peak at 99.5 eV, corresponding to elemental Si.31 These Si layers are too thick to be completely oxidized by air exposure. This is consistent with the existence of 4 nm high islands (Figure 1d), which are too large to be fully converted to SiO2 at room temperature by air exposure. Therefore, only layers with a Si content of 1.5 × 1015 and 2.5 × 1015 atoms/cm2 are interesting for further investigation. It is known that the MoS2 basal plane is hydrophobic and chemically inert due to the self-passivated surface.38 The SiO2 surface can be hydrophilic if it is prepared by a wet chemical process, which generates hydroxyl groups on the surface.27 Thus, it is interesting to study the influence of SiO 2 functionalization on the hydrophilicity of the MoS2 surface. Static WCA measurements are carried out for MoS2 before and after SiO2 functionalization. As grown MoS2 shows a WCA of

Figure 1. Surface topography of as-grown MoS2 and SiOx with different Si content on MoS2 after air exposure for 48 h. (a) As-grown MoS2. (b)−(f) SiOx on MoS2 with varied Si content. The Si content on MoS2 and related RMS roughness can be found on the images.

roughness for a Si content up to 2.5 × 1015 atoms/cm2. The RMS roughness then increases with the Si content and reaches a maximum at 5.0 × 1015 atoms/cm2; afterward, it starts to decrease. We presume that Si agglomerates upon deposition on MoS2, as the surface energy of Si (1.05 × 103 mJ/m2) is much higher than that of MoS2 (46.5 mJ/m2).35 At a low Si content of 2.5 × 1015 atoms/cm2 or less, the agglomeration is not observed by AFM, indicating the very small size of the islands. As the Si content increases to 5.0 × 1015 atoms/cm2, agglomeration is observed and 4 nm high islands are formed (Figure 1d and its inset). Then, for a Si content of 1.0 × 1016 atoms/cm2 and higher, we propose that most of the MoS2 6774

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Chemistry of Materials 80.4° (Figure 2c). This value is in line with that for the exfoliated MoS2, indicating the high crystal quality of the MoS2 used in this work.39 In contrast, the WCA reduces to 37.9° for MoS2 with the SiO2 functionalization converted from 1.5 × 1015 Si atoms/cm2 (Figure 2d) and 38.1° for 2.5 × 1015 Si atoms/cm2 (not shown here). The smaller WCA for MoS2 with the SiO2 functionalization suggests the formation of surface hydroxyl groups after air exposure, which can be attributed to the combined effect of H2O and O2 during the oxidation of Si. These surface hydroxyl groups are expected to enable the fast nucleation of high-κ dielectrics (e.g., Al2O3) based on the following surface reaction: −Si−OH * + Al(CH3)3 → −OAl(CH3)*2 + CH4

To summarize this section, a complete coverage of SiOx on the MoS2 surface is only achieved for a thick layer (2.5 × 1015 Si atoms/cm2, the equivalent of 25 SiO2 monolayers) that consists of both elemental Si and SiO2. It is not suitable to serve as nucleation layer for high-κ ALD on MoS2. In view of our aim to scale down the EOT of the gate stack, a noncontinuous SiO2 layer with a Si content of 1.5 × 1015 atoms/cm2 will be investigated. Note that this Si content nominally corresponds to a 0.6 nm thick SiO2 layer if continuous. Although this SiO2 film is not continuous, we will investigate its impact on the nucleation and growth of high-κ ALD, as it was previously shown that lateral growth of a high-κ dielectric can occur on the MoS2 surface, resulting in a closed film.16 Surface Morphology of Al2O3 and HfO2 with SiO2 on MoS2. The first insight into the impact of SiO2 functionalization on the nucleation and growth of high-κ ALD is gained by inspecting the surface morphology of deposited high-κ on MoS 2 with SEM. For Al 2 O 3 or HfO 2 ALD on the unfunctionalized MoS2 surface, the nucleation starts on the grain boundaries and line defects at the MoS2 top surface, consistent with our previous results.16 There is little to no contribution of direct nucleation on the basal planes of MoS2 due to the inertness of the basal plane while the grain boundaries at the top surface have a higher reactivity. As such, in the initial growth regime, we observe a network of Al2O3 or HfO2 nanoribbons on the MoS2 surface. During further deposition, the created Al2O3 nanoribbons grow in both vertical and lateral directions over the basal planes of MoS2 until a continuous Al2O3 layer is reached.16 The lateral growth rate was found to be close to the Al2O3 steady growth-per-cycle, and the point of layer closure depends on the size of crystal grains at the MoS2 top surface, which typically is several tens of nanometers or more.16 In contrast, with the SiO2 functionalization on MoS2, both the Al2O3 and the HfO2 films appear to be continuous according to SEM, as no obvious pinholes can be observed even at low ALD cycle numbers (23 cycles Al2O3 in Figure 3c and 43 cycles HfO2 in Figure 3d). The complete coverage is explained on the one hand by the higher amount of Al2O3 and HfO2 deposited, implying that the nucleation delay of ALD on MoS2 can be dramatically alleviated using SiO2 functionalization. The Al content increases from 0.9 × 1015 atoms/cm2 directly on the MoS2 surface to 6.8 × 1015 atoms/cm2 on the MoS2 surface with SiO2 functionalization. The Hf content increases from 1.6 × 1015 atoms/cm2 directly on the MoS2 surface to 5.7 × 1015 atoms/cm2 on the MoS2 surface with SiO2 functionalization. On the other hand, the growth mode and morphology of high-κ dielectrics are influenced by the SiO2 functionalization, as can be deduced by comparing samples with

Figure 3. SEM images of high-κ dielectrics ALD on MoS2. (a) 23 cycles ALD of Al2O3 on as-grown MoS2. (b) 43 cycles ALD of HfO2 on as-grown MoS2. (Note that 23 cycles Al2O3 and 43 cycles HfO2 ALD on a fully hydroxylated SiO2 surface will result in 2 nm Al2O3 and 2 nm HfO2, respectively.) (c) 23 cycles ALD of Al2O3 with SiO2 functionalization on MoS2. (d) 43 cycles ALD of HfO2 with SiO2 functionalization on MoS2. (e) 60 cycles ALD of Al2O3 on as-grown MoS2. (f) 107 cycles ALD of HfO2 on as-grown MoS2.

an equal amount of deposited material. 60 cycles of Al2O3 ALD (Figure 3e, Al content: 8.5 × 1015 atoms/cm2) and 107 cycles of HfO2 ALD (Figure 3f, Hf content: 1.3 × 1016 atoms/cm2) directly on MoS2 still result in films with pinholes, even though the amount of deposited material is higher. These results are consistent with the improved surface hydrophilicity after SiO2 functionalization, whereby the surface hydroxyl groups enable the faster nucleation and layer closure for both Al2O3 and HfO2 ALD. Growth Mode of Al2O3 ALD on MoS2 with SiO2 Functionalization. Our next aim is to systematically investigate the impact of SiO2 functionalization on the nucleation and growth mode of Al2O3 ALD on MoS2. The growth evolution of Al2O3 with SiO2 functionalization on MoS2 is analyzed by quantifying the Al content using ERD versus ALD cycles.31 The growth curves indicate no growth inhibition when we apply the SiO2 functionalization on MoS2 (Figure 4a). The growth curve of Al2O3 ALD with SiO2 functionalization on MoS2 is practically identical to that of Al2O3 ALD on a fully hydroxylated SiO2 surface, which is a known example of linear ALD growth.40,41 In contrast, strong growth inhibition is observed for Al2O3 ALD on the unfunctionalized MoS2, whereby the steady growth can only be reached at an Al content of 3.8 × 1016 atoms/cm2 (corresponding to 120 ALD cycles). The steady growth-per-cycle is 3.2 × 1014 atoms/(cm2· cycle), characteristic for Al2O3 ALD. These results indicate that the SiO2 functionalization enables a fast reaction of the Al2O3 ALD precursors with the initial surface. The Al2O3 layer closure process is investigated by observing the TOF-SIMS Mo+ signal decay as a function of the Al content. ALD of Al2O3 directly on MoS2 proceeds via a 3D growth mode and a slow coalescence process of Al2O3.16 In that case, only layers with an Al content of at least 7.0 × 1016 atoms/ cm2 are continuous, which nominally corresponds to Al2O3 with a thickness of 20 nm (Figure 4b). It was previously demonstrated that the point of layer closure depends on the size of the crystal grain at the MoS2 top surface, which is indeed in the order of 40 nm (=2 × 20 nm) for MoS2 prepared by 6775

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simple structural model. As the SiO2 layer on the MoS2 surface is discontinuous, we assume that it consists of islands. Al2O3 nucleation is likely to occur on the SiO2 islands owing to the presence of hydroxyl groups, as well as on grain boundaries and defects at the uncovered MoS2 surface. We presume that the MoS2 surface gets covered by the lateral growth of Al2O3, starting from the edges of SiO2 islands (Figure 5). The SiO2

Figure 5. Schematic mechanism for the ALD of the Al2O3 layer coalescence process with SiO2 functionalization on MoS2: (a) Polycrystalline MoS2 prepared by sulfurization approach. (b) MoS2 with SiO2 functionalization. (c) ALD of Al2O3 nucleates on SiO2 islands and grows in both lateral and vertical directions. (d) A continuous Al2O3 film is obtained on MoS2.

islands are assumed to be hemispheres with radius r. All islands have equal size and are homogeneously distributed on the MoS2 surface (Figure 5b). The volume of each SiO2 island can

Figure 4. Growth curve and surface coverage of Al2O3 ALD on MoS2. (a) Al content of Al2O3 ALD on MoS2 without and with SiO2 functionalization versus the cycle number. Al2O3 ALD on fully hydroxylated surface is also shown as a reference. (b) TOF-SIMS Mo+ signal decay as a function of Al content (The black line is the characteristic for an ideal two-dimensional growth).

2π r 3

be obtained as V = 3 . The amount of Si atoms n in each SiO2 island is calculated from the Si atom density of SiO2 (26.6 atoms/nm3): n = 26.6·V. Thus, the density of SiO2 islands can 1.5 × 1015 /cm 2 and the surface coverage of n Nπr 2 = . For the convenience of modeling, 1 cm 2

be expressed as N =

sulfurization.30 In contrast, the TOF-SIMS Mo+ signal decay for Al2O3 with SiO2 functionalization on MoS2 is much steeper and close to the event of the ideal 2D growth mode (Figure 4b). It implies that the SiO2 functionalization enables a faster layer closure process of Al2O3 ALD on MoS2. The Al2O3 film with an Al content of 6.8 × 1015 atoms/cm2 is completely continuous, which nominally corresponds to an Al2O3 thickness of 2 nm. It is quite remarkable that a not fully continuous SiO2 layer with a Si content of only 1.5 × 1015 atoms/cm2 can dramatically accelerate the nucleation and layer closure process of Al2O3 and HfO2 ALD. In the next section, we will propose a mechanism that can explain the fast layer closure process, based on the experimental results. Nucleation Mechanism for High-κ Dielectric ALD. The following mechanism for the SiO2 functionalization of MoS2 for Al2O3 ALD (or other dielectric films) can be proposed. The model is based on the combination of different physical characterization results, e.g., the low amount of deposited Si (1.5 × 1015 atoms/cm2, QCM), the nondetectable change in surface roughness (AFM), the incomplete surface coverage (TOF-SIMS), the fact that all Si oxidizes during air exposure (XPS), and finally the fast layer closure process of ALD (SEM/ TOF-SIMS). In order to roughly estimate how many cycles actually are needed to get a closed Al2O3 film, we propose the following

SiO2 on MoS2 is θ we assume a hexagonal arrangement of the SiO2 islands (Figure 5b). The side length of a hexagonal unit can be obtained as a=

2 3π 9θ

·r . Finally, the largest distance between the edge of

the SiO2 island and the hexagonal unit is expressed as d = a − r (Figure 5b). The lateral growth rate is equal to the ALD growth-per-cycle (Δc), as determined in our previous investigation for ALD on the unfunctionalized MoS2 surface.14 Then, the required ALD cycle number to get a closed film is d obtained as nALD = Δc . Now, the critical parameter is the radius of the islands. If the radius is assumed to be smaller than 0.85 nm, the corresponding surface coverage of SiO2 will be 100%. This disagrees with TOF-SIMS results which indicate a discontinuous film. On the other hand, the radius should not be much larger than 1 nm, as the islands are fully oxidized to SiO2 by air exposure. XPS confirms a fully oxidized SiO2 layer (Figure 2b). Thus, we propose that the radius of SiO2 islands is approximately 1 nm, in line with the formation by room temperature oxidation via air exposure. Then, the nucleation density of SiO2 islands on the MoS2 surface is 2.7 × 1013/cm2 for a Si content of 1.5 × 1015 atoms/cm2, and the surface coverage is 85%. The number of ALD cycles to achieve layer 6776

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Chemistry of Materials closure nALD is only 5 for Al2O3 ALD. Therefore, a fast layer closure process of ALD can indeed occur, consistent with the above experimental results. Note that additional nucleation on the uncovered grain boundaries at the MoS2 top surface can also occur. Characterization of the Interface Structure. The interfacial bonding of Al2O3/SiO2/MoS2 and HfO2/SiO2/ MoS2 is investigated by XPS. The Al 2p, Si 2p, and O 1s spectra of Al2O3/SiO2/MoS2 are shown in Figure 6a−c, and the

clearly resolved because of the low amount of SiO2, the projection effect of TEM characterization, and the small difference in contrast for SiO2 and Al2O3 (Figure 7a,b).43

Figure 7. TEM characterization of Al2O3/SiO2/MoS2 and HfO2/ SiO2/MoS2 interface. (a) TEM and (b) STEM images for the stack of nominal 5 nm Al2O3 with SiO2 functionalization on MoS2, whereby the substrate is sapphire. (c) TEM and (d) STEM images for the stack of 2 nm (43 ALD cycles) HfO2 with SiO2 functionalization on MoS2, whereby the substrate is SiO2.

Nevertheless, the thickness of the SiO2 layer should be lower than 1 nm, as the total thickness of the oxides is 5.7 nm, with a nominal 5 nm Al2O3 (56 ALD cycles). The top surface appears smooth. Also, the layered structure of MoS2 with 4 monolayers is demonstrated in both TEM and STEM images. In the HfO2/ SiO2/MoS2 case, the presence of SiO2 between MoS2 and HfO2 can be observed due to the larger difference in contrast. Again, due to the projection effect, many 1 nm small SiO2 islands should appear as a “continuous” layer in the TEM image. This is indeed what is observed at most locations, consistent with our structural model. Nevertheless, the interface is not completely homogeneous in TEM and STEM (Figure 7c,d). This might suggest that SiO2 islands with slightly different sizes and inhomogeneous distribution are present, whereas our simplified model presumed an equal size and homogeneous distribution of all islands. Dielectric Properties. Finally, we evaluate the dielectric properties of gate dielectrics formed on MoS2 with the SiO2 functionalization. The 5 nm Al2O3 with SiO2 functionalization are used as gate dielectrics in the RG-FET (Figure 8a). The low gate leakage current confirms that the gate dielectric layer is fully continuous: a leakage current of 3.8 × 10−6 A/cm2 is reached at a 1 V gate bias (Figure 8b). The capacitance dispersion as a function of frequency (1 kHz to 100 kHz) further confirms the finite leakage current (Figure 8c).44 The dual-frequency method is applied to extract the capacitance

Figure 6. XPS spectra of Al2O3/SiO2/MoS2 and HfO2/SiO2/MoS2 interface. (a−c) Al 2p, Si 2p, and O 1s spectra of the Al2O3/SiO2/ MoS2, respectively. (d−f) Hf 4f, Si 2p, and O 1s spectra of the HfO2/ SiO2/MoS2, respectively.

Al 2p from bulk Al2O3 (30 nm thick) and Si 2p from bulk SiO2 (90 nm thick) are also shown as references. The Al 2p peak shifts toward higher binding energy (BE) compared to that of bulk Al2O3, whereas the Si 2p peak shifts toward lower BE compared to that of bulk SiO2. The O 1s peak is located between that of the bulk Al2O3 and SiO2 (Figure 6c). The shift of these peaks indicates the formation of interfacial Al−O−Si bonds due to the reaction of surface hydroxyl groups with TMA:42 −Si−OH * + Al(CH3)3 → Si−O−Al(CH3)*2 + CH4

For the stack of HfO2/SiO2/MoS2, the Hf 4f peak shifts toward higher BE and Si 2p shifts toward lower BE compared to bulk HfO2 (30 nm thick) and SiO2, respectively (Figure 6d,e). An additional O 1s peak appears at 532.0 eV, which is correlated with the presence of Hf−O−Si bonds (Figure 6f).42 Similarly, the formation of Hf−O−Si bonds is attributed to the surface reaction: Si−OH * + HfCl4 → Si−O−HfCl3* + HCl

according to the formula: C =

f12 C1 − f22 C 2 f12 − f22

, where C1 and C2 refer

to the capacitance measured at frequencies of f1 and f 2, respectively.45 Then, a total capacitance of 1.05 μF/cm2 is

TEM confirms that the SiO2 layer is thinner than 1 nm. In the Al2O3/SiO2/MoS2 case, the SiO2 island structure cannot be 6777

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statistical range of the polycrystalline MoS2 in this work.49 Overall, the RG-FET demonstrates that the dielectric property of Al2O3 with SiO2 functionalization on MoS2 is comparable to that of the typical Al2O3 ALD on a Si substrate.13,46

■

CONCLUSIONS A SiO2 layer with a Si content as low as 1.5 × 1015 atoms/cm2 enables the ALD of 2 nm thin and fully continuous Al2O3 and HfO2 layers on synthetic polycrystalline MoS2. Fast nucleation and layer closure can be achieved despite the discontinuity of the SiO2 nucleation layer, which can as such be sub-nm thin. This is explained by the following nucleation and growth mechanism. Nucleation of Al2O3 and HfO2 ALD starts on the small nm-spaced SiO2 islands via the reactions between surface hydroxyl groups and ALD precursors (TMA, HfCl4), with the formation of Al−O−Si or Hf−O−Si bonds, respectively. The fast layer closure is achieved by lateral growth over the MoS2 surface starting from these small nm-spaced SiO2 islands. The mechanism for layer closure is similar to that proposed earlier for ALD on an unfunctionalized polycrystalline MoS2 surface.16 In that case, the layers also close by means of lateral growth with a similar lateral growth rate, the steady ALD growth-percycle. However, the point of layer closure is much later, as nucleation only occurs at grain boundaries on the MoS2 top surface while the grain size is several tens of nanometers and more. The dielectric properties of Al2O3 on the functionalized MoS2 are investigated in a top-gated capacitor, showing an EOT as low as 3.4 nm and a leakage current of 3.8 × 10−6 A/ cm2 at 1 V gate bias. These results suggest that SiO2 functionalization is a promising approach for the ultimate EOT scaling of gate dielectrics on TMDs. The combination of SiO2 functionalization with dielectrics of higher dielectric constant (e.g., HfO2 and ZrO2) and the impact of the formed interfacial layer on device performance will be the subject of further studies. 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.

Figure 8. (a) The schematic of top ring-gate field effect transistor (RG-FET). (b) Current−voltage characterization and (c) capacitance−voltage characterization of the 5 nm Al2O3/SiO2 stack on MoS2 using RG-FET.

obtained for the full stack (5 nm Al2O3 with SiO2). Assuming a parallel-plate capacitor geometry, the average dielectric constant and EOT of the gate stack are calculated to be 6.5 and 3.4 nm, respectively, based on eqs 1 and 2 C t ε = ox ox ε0A

EOT =

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

(1)

ORCID

ε0εSiO2A Cox

Haodong Zhang: 0000-0001-9572-9133 Annelies Delabie: 0000-0001-9739-7419

(2)

Author Contributions

where tox is the total thickness of the Al2O3/SiO2 stack, ε0 is the vacuum dielectric constant, and A is the area of the ring-gate electrode. If the thickness of SiO2 is assumed to be 0.6 nm with a dielectric constant of 3.9, then the dielectric constant of the Al2O3 layer is obtained as 7.3. This value is in line with that of Al2O3 deposited on a SiO2/Si substrate by ALD.13,46 Furthermore, the density of the interface trap is roughly estimated to be 5.0 × 1012 cm−2 eV−1 using the conductance method.47 This value is comparable to that of high-κ directly on a MoS2 flake, suggesting the SiO2 functionalization does not introduce too much additional interface defects.48 In addition to the dielectric properties of the gate stack, the drain current of the RG-FET is also collected. The RG-FET shows an Ion/Ioff ratio of more than 3 orders of magnitude with a mobility of MoS2 around 0.25 cm2/(V·s), which are in the

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors would like to thank the Imec Beyond CMOS program for financial support. REFERENCES

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