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Preparation and characterization of molybdenum disulfide films obtained by one-step atomic layer deposition method
Thin Solid Films 624 (2017) 101–105
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Preparation and characterization of molybdenum disulfide films obtained by one-step atomic layer deposition method Yazhou Huang, Lei Liu ⁎, Weiwei Zhao, Yunfei Chen ⁎ Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments, School of Mechanical Engineering, Southeast University, Nanjing 210096, People's Republic of China
a r t i c l e
i n f o
Article history: Received 5 September 2016 Received in revised form 20 December 2016 Accepted 9 January 2017 Available online 10 January 2017 Keywords: Atomic layer deposition MoS2 thin film Growth temperature Substrate Crystal structure
a b s t r a c t High crystalline MoS2 films are prepared by one-step ALD without followed high-temperature annealing. MoCl5 and H2S are used as precursors, while Si and Al2O3 are used as substrates respectively. The obtained MoS2 films are characterized by Atomic Force Microscopy (AFM), Raman spectroscopy, Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), indicating they possess structures in high quality. Experimental results demonstrate the film grain sizes can be tuned from ~20 nm to ~100 nm at various growth temperatures from 420 °C to 480 °C and excellent crystal performance can be guaranteed from 430 °C to 470 °C. Meanwhile, the growth temperature should not exceed 480 °C due to decomposition of the functional groups. Furthermore, Al2O3 can do better than Si as a substrate for the film building for more necessary hydroxyls during initial reaction on its surface. The average growth rate of the high crystallinity MoS2 film is ~4.3 Å/cycle for Al2O3 substrate and ~3.8 Å/cycle for Si substrate. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, MoS2 has attracted wide attention because of its distinctive properties. MoS2 is a two-dimensional (2D) layered transition-metal semiconductor material. Molybdenum and sulfur atoms form a hexagonal network with stable covalent bonds in the layer. Meanwhile, the layers are stacked by Van der Waals forces. Unlike graphene, the band gap of MoS2 varies with the number of layers. The indirect bandgap can gradually change to direct bandgap of MoS2 converted from bulk to a single layer material, making it have promising application values in many fields [1–7]. Generally, MoS2 films obtained from mechanical exfoliation are preferred as they possess perfect crystalline structures. However, the production efficiency from this method is extremely low. Furthermore, because of the limitations in size scale, films obtained from exfoliation cannot be adopted in large-scale applications such as integrated circuit fabrication. Fabricating ultra-thin MoS2 crystalline films (single layer or few layers) with high quality and large area is in great demands [8–11]. Till nowadays, considerable efforts have been made to obtain large-area MoS2 films from different methods [12–15]. Polycrystalline MoS2 can be grown on SiO2 substrate through chemical vapor deposition (CVD) using sulfur and MoO3, which demonstrates the possibility of band-gap engineering in polycrystalline films by tailoring the grain size [16]. Another promising and straightforward method is to prepare a Mo-containing thin film and then sulfurize it at high temperature, by ⁎ Corresponding authors. E-mail addresses: [email protected] (L. Liu), [email protected] (Y. Chen).
http://dx.doi.org/10.1016/j.tsf.2017.01.015 0040-6090/© 2017 Elsevier B.V. All rights reserved.
which large-area monocrystal MoS2 film can be obtained on sapphire [17,18]. At the same time, it is found that higher temperature is advantageous to improve the quality of MoS2 film [19,20]. For deposition on high aspect ratio structures such as a pore or a trench, CVD method cannot offer conformal deposition because more material is deposited at the opening [21]. Comparatively speaking, ALD can generate conformal film on the substrate with rather complex structure because of its selflimiting reaction [22–24]. Moreover, the thickness of film obtained from ALD can be easily and precisely controlled by defined ALD cycles [25,26]. According to the existed work, general precursors for MoS2 films by ALD method are MoCl5 and H2S, Mo(CO)6 and dimethyldisulfide (CH3SSCH3, DMDS), Mo(CO)6 and H2S, and the reported temperatures for film building are 300 °C, 100 °C and 155– 225 °C respectively [27–30]. Furthermore, it is difficult to get films with high crystallinity at these temperatures, and annealing process at high temperature (at least 800 °C) is usually followed [27,28]. Meanwhile, it indicates when the growth temperature increases, the crystallinity of the film will be significantly improved [25]. Unfortunately, decomposition will occur for Mo(CO)6 and DMDS above 200 °C [26]. However, MoCl5 and H2S can guarantee the stability at the 700 °C [31– 35], which makes them suitable for building high crystallinity MoS2 film. As has been said, substrate and growth temperature is very important to the quality of the grown MoS2 films. For one-step ALD, it means that the film can be directly grown on the substrate without any pre-treatment or post-treatment, such as spinning some graphene-like molecules onto the substrate to promote film growth and post-annealing at high temperature to improve the film properties. In this work, we report a one-step ALD for producing
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Fig. 1. Schematic illustration of ALD MoS2. MoS2 film can be obtained by Mo precursor (MoCl5 gas) and S precursor (H2S) alternately exposed onto the substrate.
MoS2 film with high crystallinity at moderate temperatures (420 °C– 490 °C). In this way, films with different grain structures can be directly formed on Si and Al2O3 substrates by using MoCl5 and H2S as precursors without followed high-temperature annealing. The structures of the obtained films were characterized. Moreover, influences of substrate and growth temperature on the films were also investigated. 2. Experimental 2.1. Preparation of MoS2 films In order to investigate the effect of substrate on the growth of MoS2 film, we prepared two different substrates. One was a single side polished bare Si wafer, and the other was an Al2O3 wafer. The Si wafer was cleaned by acetone and ethanol successively in an ultrasonic bath. A commercial ALD setup (SUNALETMR-100) from PICOSUN was employed for MoS2 films building. As precursors, MoCl5 (99.6%) and H2S (99.6%) were alternately vaporized into ALD chamber under N2 (99.999%) flow at a rate of 50 sccm. The temperature in ALD chamber was kept from 420 °C to 490 °C under the pressure of 5.3 hPa. The temperature of MoCl5 was kept at 200 °C to guarantee sufficient vapor pressure. The temperature of H2S was kept at room temperature under the base pressure of 8 hPa. Schematic illustration of ALD MoS2 is shown in Fig. 1. To the beginning, chamber and MoCl5 were heated to the preset temperatures and kept for half an hour. Afterward, MoCl5 and H2S were alternately interfused in the N2 carrier by gas switching valves (V1 and V2). Then they were alternately vented onto the substrate and purged by the N2 in the ALD chamber. Thus, one growth cycle contains four steps:
exposure to MoCl5, N2 purge, exposure to H2S and N2 purge again. By repeating these steps, MoS2 film with desired thickness can be obtained. 2.2. Characterization The thicknesses and microstructures of obtained films were measured by AFM (MFP-3D-SA, Asylum Research)using contact mode. Contact angle measurement (OCA-15, Dataphysics) was used to study the surface hydrophilicity of substrate. The structures of the obtained films were characterized by Raman spectroscopy (RAM-PRO-785E, Agiltron), SEM (Helios Nanolab 600i, FEI), TEM (G220,FEI) and XRD (Smartlab-3, Rigaku). Raman spectroscopy was carried out by a 514 nm laser. To observe the cross-section of the film using SEM, the film with an Al coating deposited by magnetron sputtering (MSP300C, KYKY) was etched by focused ion beam (Helios Nanolab 600i, FEI). TEM images were taken using an accelerating voltage of 300 kV. The MoS2 film was exfoliated from the substrate and transferred on a copper TEM grid. XRD was performed with Cu Kα radiation (λ = 1.54 Å) at 35 mA and 50 kV. 3. Results and discussion Mo(CO)6 and H2S reaction can be given as follows, 2MoCl5 + 5H2S → 2MoS2 + 10HCl + S. For atomic layer deposition, the binary reaction can be split into two half-reactions, (A) Mo\\SH⁎ + MoCl5 → Mo\\S\\MoCl4⁎ + HCl (B) MoCl⁎ + H2S → Mo\\SH⁎ + HCl + S where * denotes the surface species. Firstly, MoCl5 forms chemisorption (A). Then it has a chemical reaction with H2S (B), and MoS2 can be obtained. By repeating an ABAB⋯ reaction sequence, MoS2 film with desired thickness can be grown. Actually, first chemisorption of MoCl5 to the substrate is through hydroxyl functional group. Therefore, the hydroxyl is very important to the initial growth of MoS2. The reaction can be given as follows, |\\OH⁎ + MoCl5 → |\\O\\MoCl4⁎ + HCl where |\\ denotes surface.
Fig. 2. AFM images and height profiles of MoS2 films obtained by 100 ALD cycles on (a) Si substrate (b) Al2O3 substrate.
Elevated growth temperature is conducive to improve the crystallinity of the film. However, as an exothermic reaction, the high temperature is unfavorable for the chemical adsorption. Furthermore, excessive temperature may cause the precursor and functional group to be decomposed, which is fatal to the growth. In order to get a high crystallinity film, the influence of the growth temperature needs to be studied. Therefore, we prepared two different substrates for building films at different temperatures from 420 °C to 490 °C in a chamber at the same time. The one is bare Si (100), the other is amorphous Al2O3 covered with rich hydroxyl groups which is got by ALD 500 cycles Al2O3 on the Si wafer.
Y. Huang et al. / Thin Solid Films 624 (2017) 101–105
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Fig. 3. (a) Thicknesses of the MoS2 films against growth temperature. The ALD was performed for 100 cycles both on the Si substrate and the Al2O3 substrate. The bottom is the temperature window for ALD MoS2 films. (b) Thicknesses of MoS2 films against number of ALD cycles at 450 °C. Inserted images are Water contact angle measurements for the Si substrate (left) and the Al2O3 substrate (right).
Fig. 2(a) and (b) shows AFM images and height profiles of MoS2 films obtained by 100 ALD cycles at 450 °C on Si substrate and Al2O3 substrate respectively. The corresponding film thicknesses are 20.6 nm and 44.3 nm respectively. Fig. 3(a) shows the thickness changing tendency of films by 100 ALD cycles at different temperatures on Si substrate and Al2O3 substrate respectively. Fig. 3(b) shows the thicknesses of MoS2 films against the cycle number. Clearly, the film with same cycle number on Al2O3 substrate is thicker than that on Si substrate, which indicates that Al2O3 is more suitable for MoS2 film building than Si as a substrate. We attribute the reason for this indication to the enrichment hydroxyls on Al2O3 substrate surface, which is necessary for the initial reaction. Smaller water contact angle (inset of Fig. 3(b)) also demonstrates the above deductions. Moreover, unlike crystalline Si, amorphous structure of the Al2O3 may make the grown grains more disordered, which produces the thicker film. According to Fig. 3(a), the films growth can be negligible if the temperature is lower than 420 °C for both Al2O3 and Si substrate. However, with the temperature increasing, the film thickness increases and saturates till 450 °C. Furthermore, when the temperature is higher than 460 °C, a rapid decrease of the film thickness can be observed. The lower growth rate at the temperature of 420 °C attributes to incomplete surface reactions; the growth rate becomes slower when the temperature is higher than 460 °C due to increasing decomposition of the functional groups. The elevated temperature aggravates the dehydroxylation reaction, which makes hydroxyl groups to be desorbed as H2O. Meanwhile, desorption of the sulfydryl group becomes violent and decomposes into sulfur. The decomposition of functional groups is fatal to the two half-reactions. Therefore, the growth temperature cannot be blindly improved. According to our results, the growth temperature should not exceed 480 °C. Complying with the nature of ALD method, the thickness of film linearly increases when the cycle number increases, by which
the thickness of the film can be controlled. The average growth rate of per ALD cycle is about 4.3 Å for Al2O3 substrate and 3.8 Å for Si substrate at 450 °C (Fig. 3(b)). Grain structures of the films were observed by AFM. Fig. 4 shows AFM images for MoS2 films grown by 100 cycles on Si substrates at 420 °C (a), 450 °C (b) and 480 °C (c). Nanocrystalline films are obtained at these temperatures, and the grain sizes are ~ 20 nm, ~ 100 nm and ~ 50 nm respectively. Correspondingly, their surface roughnesses (RMS) are 3.6 nm, 8.8 nm and 5.1 nm. Obviously, grain structures of the films are different at various growth temperatures. For 420 °C, the grain size is the smallest and stacking is porous, which makes the film have promising application values in catalysis because of large specific surface area. For 450 °C, the grain size is the largest, which provides a possible of direct growth large area monolayer MoS2. For 480 °C, the uniform and compact film can be used for solid lubrication. As a valid nondestructive method for structural study, Raman spectroscopy has been widely used to research the structure and vibrational properties of MoS2. Fig. 5 shows the Raman spectra of MoS2 films obtained by 100 cycles on Si substrate (a) and Al2O3 substrate (b) at different temperatures. For the films obtained in the temperature range from 430 °C to 470 °C, out-of-plane A1g and in-plane E12g vibrational modes can be clearly observed in the Raman spectra. For the films obtained in the temperature higher than 470 °C or lower than 420 °C, corresponding Raman peaks become very weak. Although the films are built on two different substrates, their Raman absorption spectra are similar (Fig. 5(a) and (b), the intensely E12g and A1g peaks only appear in the temperature range from 430 °C to 470 °C). The Raman results show high crystalline MoS2 films can be got in the temperature range from 430 °C to 470 °C. When the temperature is higher than 470 °C or lower than 420 °C, crystallinities of the films will decrease. Fig. 5(c) shows the Raman spectra of MoS2 films grown by different ALD cycle
Fig. 4. AFM images for MoS2 films grown by 100 cycles on Si substrates at 420 °C (a), 450 °C (b) and 480 °C (c).
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Fig. 5. Raman spectra of MoS2 films were grown 100 cycles on (a) Si substrate (b) Al2O3 substrate at different temperatures (the temperature on each spectrum). (c) Raman spectra of MoS2 films grown for different numbers of ALD cycles (the number on each spectrum) on the Si substrates. (d) The full-width at half-maximum (FWHM) of the E12g and A1g peaks as a function of cycle number. The peak width gradually decreases with the increased cycle, which shows that the structural quality of MoS2 film is strongly improved with the increase of ALD cycles.
numbers on the Si substrates at 460 °C. The full width at half maximum (FWHM) of Raman peak can be correlated with the quality of the film. In our experiments, the FWHM of E12g and A1g peaks decreases with the number of ALD cycles increase (Fig. 5(d)), which indicates that the structural quality of MoS2 film is strongly improved with the increase of ALD cycles. Fig. 6(a) gives SEM image of MoS2 film by 100 ALD cycles at 450 °C on a Si substrate. As the cross-sectional SEM image for Fig. 6(a), (b) clearly shows a uniform thickness of the film. To investigate the structure of the MoS2 film by TEM, the grown MoS2 film was exfoliated from the Si substrate and transferred on a copper TEM grid. A typical TEM image of the MoS2 film with obvious layered structure is shown in Fig. 6(c). The (002) lattice plane paralleled to the substrate is shown in the cross-sectional TEM image (Fig. 6(d)). The d002 is around 0.61 nm from the TEM image. Fig. 6(e) shows the typical atomic spacing. It is clear that d100 and d110 for lattice planes (100) and (110) are
Fig. 6. (a) SEM image of 100 cycles ALD MoS2 film on a Si substrate. (b) Cross-sectional SEM image for (a). (c) TEM image of MoS2 film exfoliated from the Si substrate. (d) Cross-sectional TEM image of MoS2 film deposited on the Si Substrate. (e) TEM image of the MoS2 film shows a crystalline film. (f) Fourier-transformed election diffraction for (e).
0.27 nm and 0.16 nm respectively. A hexagonal lattice structure is shown in the corresponding selected area electron diffraction (SAED) pattern (Fig. 6(f)), which exhibits (100) and (110) lattice planes. The crystal structure of MoS2 film is also observed by XRD measurements. High resolution scans of the bare Si and 100 cycles MoS2 film grown on Si substrate at 450 °C are compared in Fig. 7. A noticeable diffraction peak at 14.2 degree relate to the (002) plane of MoS2 is observed. The presence of a unique intensely diffraction peak due to the (002) plane of MoS2 indicates a hexagonal MoS2 film is highly oriented. It can be inferred that the (002) basal planes of MoS2 film can be grown parallel to the substrate by the one-step ALD method. 4. Conclusion In summary, MoS2 films with high crystallinity have been directly built by one-step ALD method on Si and Al2O3 substrates without followed high temperature annealing. MoS2 film growth was observed starting from 420 °C up to 490 °C. The film grain sizes can be tuned from ~20 nm to ~100 nm at various growth temperatures from 420 °C to 480 °C and excellent crystal performance can be guaranteed from 430 °C to 470 °C. In addition, the film with same cycle number on Al2O3 substrate is thicker than that on Si substrate because of the
Fig. 7. XRD spectra for bare Si and 100 ALD cycles MoS2 film grown on Si substrate. MoS2 (002) diffraction peak is clearly shown.
Y. Huang et al. / Thin Solid Films 624 (2017) 101–105
enrichment hydroxyls on the amorphous Al2O3 surface. The average growth rate of the high crystallinity MoS2 film is ~ 4.3 Å/cycle for Al2O3 substrate and ~3.8 Å/cycle for Si substrate. Moreover, the (002) basal planes of MoS2 film can be grown parallel to the substrate by the one-step ALD method. However, one roadblock of this method is the small grain sizes due to the temperature restriction associated with the functional group decomposition. The present study provides a guide for getting monolayer or few-layer MoS2 films with big grain sizes by ALD. Acknowledgments This work is financially supported by the Natural Science Foundation of China (U1332134, 51675360, 51675502), the Natural Science Foundation of Suzhou (SYG201329), the Fundamental Research Funds for the Central Universities (3202006301, 3202006403), the Qing Lan Project (1102000192), the International Foundation for Science (F/4736-2), Stockholm, Sweden, and the Organisation for the Prohibition of Chemical Weapons, The Hague, Netherlands, through a grant to Lei Liu, the Natural Science Foundation of Jiangsu Province (BK20150505), the Tribology Science Fund of State Key Laboratory of Tribology (SKLTKF15A11), Open Research Fund of State Key Laboratory of High Performance Complex Manufacturing, Central South University (Kfkt2016-11), and Open Research Fund of State Key Laboratory of Solid Lubrication (LSL-1607). References [1] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Single-layer MoS2 transistors, Nat. Nanotechnol. 6 (2011) 147–150. [2] S. Ghatak, A.N. Pal, A. Ghosh, Nature of electronic states in atomically thin MoS2 field-effect transistors, ACS Nano 5 (2011) 7707–7712. [3] B.H. Baugher, H.H. Churchill, Y.F. Yang, H.P. Jarillo, Intrinsic electronic transport properties of high-quality monolayer and bilayer MoS2, Nano Lett. 13 (2013) 4212–4216. [4] M.Z. Arend, P.Y. Huang, A.C. Daniel, T.C. Berkelbach, Y. Meng, G.H. Lee, T.F. Heinz, D.R. Reichman, D.A. Muller, J.C. Hone, Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide, Nat. Mater. 12 (2013) 554–561. [5] Q.H. Wang, K. Kalantar-Zadeh, A. Kis, J.N. Coleman, M.S. Strano, Electronics and optoelectronics of two-dimensional transition metal dichalcogenides, Nat. Nanotechnol. 7 (2012) 699–712. [6] H. Nam, S. Wi, H. Rokni, M. Chen, G. Priessnitz, W. Lu, X. Liang, MoS2 transistors fabricated via plasma-assisted nanoprinting of few-layer MoS2 flakes into large-area arrays, ACS Nano 7 (2013) 5870–5881. [7] K. Kang, S. Xie, L. Huang, Y. Han, P.Y. Huang, K.F. Mak, C. Kim, D. Muller, J. Park, Highmobility three-atom-thick semiconducting films with wafer-scale homogeneity, Nature 520 (2015) 656–660. [8] A. Gurarslan, Y.F. Yu, L.Q. Su, Y.L. Yu, F. Suarez, S.S. Yao, Y. Zhu, M. Ozturk, Y. Zhang, L. Cao, Surface-energy-assisted perfect transfer of centimeter-scale monolayer and few-layer MoS2 films onto arbitrary substrates, ACS Nano 8 (2014) 11522–11528. [9] S. Das, H.Y. Chen, A.V. Penumatcha, J. Appenzeller, High performance multilayer MoS2 transistors with scandium contacts, Nano Lett. 13 (2013) 100–105. [10] R.L. Masihhur, L. Ma, S. Kannappan, P.S. Park, S. Krishnamoorthy, D.N. Nath, W. Lu, Y.Y. Wu, S. Rajan, Large area single crystal (0001) oriented MoS2, Appl. Phys. Lett. 102 (2013) 252108.
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Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Preparation and characterization of molybdenum disulfide films obtained by one-step atomic layer deposition method Yazhou Huang, Lei Liu ⁎, Weiwei Zhao, Yunfei Chen ⁎ Jiangsu Key Laboratory for Design and Manufacture of Micro-Nano Biomedical Instruments, School of Mechanical Engineering, Southeast University, Nanjing 210096, People's Republic of China
a r t i c l e
i n f o
Article history: Received 5 September 2016 Received in revised form 20 December 2016 Accepted 9 January 2017 Available online 10 January 2017 Keywords: Atomic layer deposition MoS2 thin film Growth temperature Substrate Crystal structure
a b s t r a c t High crystalline MoS2 films are prepared by one-step ALD without followed high-temperature annealing. MoCl5 and H2S are used as precursors, while Si and Al2O3 are used as substrates respectively. The obtained MoS2 films are characterized by Atomic Force Microscopy (AFM), Raman spectroscopy, Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), indicating they possess structures in high quality. Experimental results demonstrate the film grain sizes can be tuned from ~20 nm to ~100 nm at various growth temperatures from 420 °C to 480 °C and excellent crystal performance can be guaranteed from 430 °C to 470 °C. Meanwhile, the growth temperature should not exceed 480 °C due to decomposition of the functional groups. Furthermore, Al2O3 can do better than Si as a substrate for the film building for more necessary hydroxyls during initial reaction on its surface. The average growth rate of the high crystallinity MoS2 film is ~4.3 Å/cycle for Al2O3 substrate and ~3.8 Å/cycle for Si substrate. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, MoS2 has attracted wide attention because of its distinctive properties. MoS2 is a two-dimensional (2D) layered transition-metal semiconductor material. Molybdenum and sulfur atoms form a hexagonal network with stable covalent bonds in the layer. Meanwhile, the layers are stacked by Van der Waals forces. Unlike graphene, the band gap of MoS2 varies with the number of layers. The indirect bandgap can gradually change to direct bandgap of MoS2 converted from bulk to a single layer material, making it have promising application values in many fields [1–7]. Generally, MoS2 films obtained from mechanical exfoliation are preferred as they possess perfect crystalline structures. However, the production efficiency from this method is extremely low. Furthermore, because of the limitations in size scale, films obtained from exfoliation cannot be adopted in large-scale applications such as integrated circuit fabrication. Fabricating ultra-thin MoS2 crystalline films (single layer or few layers) with high quality and large area is in great demands [8–11]. Till nowadays, considerable efforts have been made to obtain large-area MoS2 films from different methods [12–15]. Polycrystalline MoS2 can be grown on SiO2 substrate through chemical vapor deposition (CVD) using sulfur and MoO3, which demonstrates the possibility of band-gap engineering in polycrystalline films by tailoring the grain size [16]. Another promising and straightforward method is to prepare a Mo-containing thin film and then sulfurize it at high temperature, by ⁎ Corresponding authors. E-mail addresses: [email protected] (L. Liu), [email protected] (Y. Chen).
http://dx.doi.org/10.1016/j.tsf.2017.01.015 0040-6090/© 2017 Elsevier B.V. All rights reserved.
which large-area monocrystal MoS2 film can be obtained on sapphire [17,18]. At the same time, it is found that higher temperature is advantageous to improve the quality of MoS2 film [19,20]. For deposition on high aspect ratio structures such as a pore or a trench, CVD method cannot offer conformal deposition because more material is deposited at the opening [21]. Comparatively speaking, ALD can generate conformal film on the substrate with rather complex structure because of its selflimiting reaction [22–24]. Moreover, the thickness of film obtained from ALD can be easily and precisely controlled by defined ALD cycles [25,26]. According to the existed work, general precursors for MoS2 films by ALD method are MoCl5 and H2S, Mo(CO)6 and dimethyldisulfide (CH3SSCH3, DMDS), Mo(CO)6 and H2S, and the reported temperatures for film building are 300 °C, 100 °C and 155– 225 °C respectively [27–30]. Furthermore, it is difficult to get films with high crystallinity at these temperatures, and annealing process at high temperature (at least 800 °C) is usually followed [27,28]. Meanwhile, it indicates when the growth temperature increases, the crystallinity of the film will be significantly improved [25]. Unfortunately, decomposition will occur for Mo(CO)6 and DMDS above 200 °C [26]. However, MoCl5 and H2S can guarantee the stability at the 700 °C [31– 35], which makes them suitable for building high crystallinity MoS2 film. As has been said, substrate and growth temperature is very important to the quality of the grown MoS2 films. For one-step ALD, it means that the film can be directly grown on the substrate without any pre-treatment or post-treatment, such as spinning some graphene-like molecules onto the substrate to promote film growth and post-annealing at high temperature to improve the film properties. In this work, we report a one-step ALD for producing
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Fig. 1. Schematic illustration of ALD MoS2. MoS2 film can be obtained by Mo precursor (MoCl5 gas) and S precursor (H2S) alternately exposed onto the substrate.
MoS2 film with high crystallinity at moderate temperatures (420 °C– 490 °C). In this way, films with different grain structures can be directly formed on Si and Al2O3 substrates by using MoCl5 and H2S as precursors without followed high-temperature annealing. The structures of the obtained films were characterized. Moreover, influences of substrate and growth temperature on the films were also investigated. 2. Experimental 2.1. Preparation of MoS2 films In order to investigate the effect of substrate on the growth of MoS2 film, we prepared two different substrates. One was a single side polished bare Si wafer, and the other was an Al2O3 wafer. The Si wafer was cleaned by acetone and ethanol successively in an ultrasonic bath. A commercial ALD setup (SUNALETMR-100) from PICOSUN was employed for MoS2 films building. As precursors, MoCl5 (99.6%) and H2S (99.6%) were alternately vaporized into ALD chamber under N2 (99.999%) flow at a rate of 50 sccm. The temperature in ALD chamber was kept from 420 °C to 490 °C under the pressure of 5.3 hPa. The temperature of MoCl5 was kept at 200 °C to guarantee sufficient vapor pressure. The temperature of H2S was kept at room temperature under the base pressure of 8 hPa. Schematic illustration of ALD MoS2 is shown in Fig. 1. To the beginning, chamber and MoCl5 were heated to the preset temperatures and kept for half an hour. Afterward, MoCl5 and H2S were alternately interfused in the N2 carrier by gas switching valves (V1 and V2). Then they were alternately vented onto the substrate and purged by the N2 in the ALD chamber. Thus, one growth cycle contains four steps:
exposure to MoCl5, N2 purge, exposure to H2S and N2 purge again. By repeating these steps, MoS2 film with desired thickness can be obtained. 2.2. Characterization The thicknesses and microstructures of obtained films were measured by AFM (MFP-3D-SA, Asylum Research)using contact mode. Contact angle measurement (OCA-15, Dataphysics) was used to study the surface hydrophilicity of substrate. The structures of the obtained films were characterized by Raman spectroscopy (RAM-PRO-785E, Agiltron), SEM (Helios Nanolab 600i, FEI), TEM (G220,FEI) and XRD (Smartlab-3, Rigaku). Raman spectroscopy was carried out by a 514 nm laser. To observe the cross-section of the film using SEM, the film with an Al coating deposited by magnetron sputtering (MSP300C, KYKY) was etched by focused ion beam (Helios Nanolab 600i, FEI). TEM images were taken using an accelerating voltage of 300 kV. The MoS2 film was exfoliated from the substrate and transferred on a copper TEM grid. XRD was performed with Cu Kα radiation (λ = 1.54 Å) at 35 mA and 50 kV. 3. Results and discussion Mo(CO)6 and H2S reaction can be given as follows, 2MoCl5 + 5H2S → 2MoS2 + 10HCl + S. For atomic layer deposition, the binary reaction can be split into two half-reactions, (A) Mo\\SH⁎ + MoCl5 → Mo\\S\\MoCl4⁎ + HCl (B) MoCl⁎ + H2S → Mo\\SH⁎ + HCl + S where * denotes the surface species. Firstly, MoCl5 forms chemisorption (A). Then it has a chemical reaction with H2S (B), and MoS2 can be obtained. By repeating an ABAB⋯ reaction sequence, MoS2 film with desired thickness can be grown. Actually, first chemisorption of MoCl5 to the substrate is through hydroxyl functional group. Therefore, the hydroxyl is very important to the initial growth of MoS2. The reaction can be given as follows, |\\OH⁎ + MoCl5 → |\\O\\MoCl4⁎ + HCl where |\\ denotes surface.
Fig. 2. AFM images and height profiles of MoS2 films obtained by 100 ALD cycles on (a) Si substrate (b) Al2O3 substrate.
Elevated growth temperature is conducive to improve the crystallinity of the film. However, as an exothermic reaction, the high temperature is unfavorable for the chemical adsorption. Furthermore, excessive temperature may cause the precursor and functional group to be decomposed, which is fatal to the growth. In order to get a high crystallinity film, the influence of the growth temperature needs to be studied. Therefore, we prepared two different substrates for building films at different temperatures from 420 °C to 490 °C in a chamber at the same time. The one is bare Si (100), the other is amorphous Al2O3 covered with rich hydroxyl groups which is got by ALD 500 cycles Al2O3 on the Si wafer.
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Fig. 3. (a) Thicknesses of the MoS2 films against growth temperature. The ALD was performed for 100 cycles both on the Si substrate and the Al2O3 substrate. The bottom is the temperature window for ALD MoS2 films. (b) Thicknesses of MoS2 films against number of ALD cycles at 450 °C. Inserted images are Water contact angle measurements for the Si substrate (left) and the Al2O3 substrate (right).
Fig. 2(a) and (b) shows AFM images and height profiles of MoS2 films obtained by 100 ALD cycles at 450 °C on Si substrate and Al2O3 substrate respectively. The corresponding film thicknesses are 20.6 nm and 44.3 nm respectively. Fig. 3(a) shows the thickness changing tendency of films by 100 ALD cycles at different temperatures on Si substrate and Al2O3 substrate respectively. Fig. 3(b) shows the thicknesses of MoS2 films against the cycle number. Clearly, the film with same cycle number on Al2O3 substrate is thicker than that on Si substrate, which indicates that Al2O3 is more suitable for MoS2 film building than Si as a substrate. We attribute the reason for this indication to the enrichment hydroxyls on Al2O3 substrate surface, which is necessary for the initial reaction. Smaller water contact angle (inset of Fig. 3(b)) also demonstrates the above deductions. Moreover, unlike crystalline Si, amorphous structure of the Al2O3 may make the grown grains more disordered, which produces the thicker film. According to Fig. 3(a), the films growth can be negligible if the temperature is lower than 420 °C for both Al2O3 and Si substrate. However, with the temperature increasing, the film thickness increases and saturates till 450 °C. Furthermore, when the temperature is higher than 460 °C, a rapid decrease of the film thickness can be observed. The lower growth rate at the temperature of 420 °C attributes to incomplete surface reactions; the growth rate becomes slower when the temperature is higher than 460 °C due to increasing decomposition of the functional groups. The elevated temperature aggravates the dehydroxylation reaction, which makes hydroxyl groups to be desorbed as H2O. Meanwhile, desorption of the sulfydryl group becomes violent and decomposes into sulfur. The decomposition of functional groups is fatal to the two half-reactions. Therefore, the growth temperature cannot be blindly improved. According to our results, the growth temperature should not exceed 480 °C. Complying with the nature of ALD method, the thickness of film linearly increases when the cycle number increases, by which
the thickness of the film can be controlled. The average growth rate of per ALD cycle is about 4.3 Å for Al2O3 substrate and 3.8 Å for Si substrate at 450 °C (Fig. 3(b)). Grain structures of the films were observed by AFM. Fig. 4 shows AFM images for MoS2 films grown by 100 cycles on Si substrates at 420 °C (a), 450 °C (b) and 480 °C (c). Nanocrystalline films are obtained at these temperatures, and the grain sizes are ~ 20 nm, ~ 100 nm and ~ 50 nm respectively. Correspondingly, their surface roughnesses (RMS) are 3.6 nm, 8.8 nm and 5.1 nm. Obviously, grain structures of the films are different at various growth temperatures. For 420 °C, the grain size is the smallest and stacking is porous, which makes the film have promising application values in catalysis because of large specific surface area. For 450 °C, the grain size is the largest, which provides a possible of direct growth large area monolayer MoS2. For 480 °C, the uniform and compact film can be used for solid lubrication. As a valid nondestructive method for structural study, Raman spectroscopy has been widely used to research the structure and vibrational properties of MoS2. Fig. 5 shows the Raman spectra of MoS2 films obtained by 100 cycles on Si substrate (a) and Al2O3 substrate (b) at different temperatures. For the films obtained in the temperature range from 430 °C to 470 °C, out-of-plane A1g and in-plane E12g vibrational modes can be clearly observed in the Raman spectra. For the films obtained in the temperature higher than 470 °C or lower than 420 °C, corresponding Raman peaks become very weak. Although the films are built on two different substrates, their Raman absorption spectra are similar (Fig. 5(a) and (b), the intensely E12g and A1g peaks only appear in the temperature range from 430 °C to 470 °C). The Raman results show high crystalline MoS2 films can be got in the temperature range from 430 °C to 470 °C. When the temperature is higher than 470 °C or lower than 420 °C, crystallinities of the films will decrease. Fig. 5(c) shows the Raman spectra of MoS2 films grown by different ALD cycle
Fig. 4. AFM images for MoS2 films grown by 100 cycles on Si substrates at 420 °C (a), 450 °C (b) and 480 °C (c).
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Fig. 5. Raman spectra of MoS2 films were grown 100 cycles on (a) Si substrate (b) Al2O3 substrate at different temperatures (the temperature on each spectrum). (c) Raman spectra of MoS2 films grown for different numbers of ALD cycles (the number on each spectrum) on the Si substrates. (d) The full-width at half-maximum (FWHM) of the E12g and A1g peaks as a function of cycle number. The peak width gradually decreases with the increased cycle, which shows that the structural quality of MoS2 film is strongly improved with the increase of ALD cycles.
numbers on the Si substrates at 460 °C. The full width at half maximum (FWHM) of Raman peak can be correlated with the quality of the film. In our experiments, the FWHM of E12g and A1g peaks decreases with the number of ALD cycles increase (Fig. 5(d)), which indicates that the structural quality of MoS2 film is strongly improved with the increase of ALD cycles. Fig. 6(a) gives SEM image of MoS2 film by 100 ALD cycles at 450 °C on a Si substrate. As the cross-sectional SEM image for Fig. 6(a), (b) clearly shows a uniform thickness of the film. To investigate the structure of the MoS2 film by TEM, the grown MoS2 film was exfoliated from the Si substrate and transferred on a copper TEM grid. A typical TEM image of the MoS2 film with obvious layered structure is shown in Fig. 6(c). The (002) lattice plane paralleled to the substrate is shown in the cross-sectional TEM image (Fig. 6(d)). The d002 is around 0.61 nm from the TEM image. Fig. 6(e) shows the typical atomic spacing. It is clear that d100 and d110 for lattice planes (100) and (110) are
Fig. 6. (a) SEM image of 100 cycles ALD MoS2 film on a Si substrate. (b) Cross-sectional SEM image for (a). (c) TEM image of MoS2 film exfoliated from the Si substrate. (d) Cross-sectional TEM image of MoS2 film deposited on the Si Substrate. (e) TEM image of the MoS2 film shows a crystalline film. (f) Fourier-transformed election diffraction for (e).
0.27 nm and 0.16 nm respectively. A hexagonal lattice structure is shown in the corresponding selected area electron diffraction (SAED) pattern (Fig. 6(f)), which exhibits (100) and (110) lattice planes. The crystal structure of MoS2 film is also observed by XRD measurements. High resolution scans of the bare Si and 100 cycles MoS2 film grown on Si substrate at 450 °C are compared in Fig. 7. A noticeable diffraction peak at 14.2 degree relate to the (002) plane of MoS2 is observed. The presence of a unique intensely diffraction peak due to the (002) plane of MoS2 indicates a hexagonal MoS2 film is highly oriented. It can be inferred that the (002) basal planes of MoS2 film can be grown parallel to the substrate by the one-step ALD method. 4. Conclusion In summary, MoS2 films with high crystallinity have been directly built by one-step ALD method on Si and Al2O3 substrates without followed high temperature annealing. MoS2 film growth was observed starting from 420 °C up to 490 °C. The film grain sizes can be tuned from ~20 nm to ~100 nm at various growth temperatures from 420 °C to 480 °C and excellent crystal performance can be guaranteed from 430 °C to 470 °C. In addition, the film with same cycle number on Al2O3 substrate is thicker than that on Si substrate because of the
Fig. 7. XRD spectra for bare Si and 100 ALD cycles MoS2 film grown on Si substrate. MoS2 (002) diffraction peak is clearly shown.
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enrichment hydroxyls on the amorphous Al2O3 surface. The average growth rate of the high crystallinity MoS2 film is ~ 4.3 Å/cycle for Al2O3 substrate and ~3.8 Å/cycle for Si substrate. Moreover, the (002) basal planes of MoS2 film can be grown parallel to the substrate by the one-step ALD method. However, one roadblock of this method is the small grain sizes due to the temperature restriction associated with the functional group decomposition. The present study provides a guide for getting monolayer or few-layer MoS2 films with big grain sizes by ALD. Acknowledgments This work is financially supported by the Natural Science Foundation of China (U1332134, 51675360, 51675502), the Natural Science Foundation of Suzhou (SYG201329), the Fundamental Research Funds for the Central Universities (3202006301, 3202006403), the Qing Lan Project (1102000192), the International Foundation for Science (F/4736-2), Stockholm, Sweden, and the Organisation for the Prohibition of Chemical Weapons, The Hague, Netherlands, through a grant to Lei Liu, the Natural Science Foundation of Jiangsu Province (BK20150505), the Tribology Science Fund of State Key Laboratory of Tribology (SKLTKF15A11), Open Research Fund of State Key Laboratory of High Performance Complex Manufacturing, Central South University (Kfkt2016-11), and Open Research Fund of State Key Laboratory of Solid Lubrication (LSL-1607). References [1] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Single-layer MoS2 transistors, Nat. Nanotechnol. 6 (2011) 147–150. [2] S. Ghatak, A.N. Pal, A. Ghosh, Nature of electronic states in atomically thin MoS2 field-effect transistors, ACS Nano 5 (2011) 7707–7712. [3] B.H. Baugher, H.H. Churchill, Y.F. Yang, H.P. Jarillo, Intrinsic electronic transport properties of high-quality monolayer and bilayer MoS2, Nano Lett. 13 (2013) 4212–4216. [4] M.Z. Arend, P.Y. Huang, A.C. Daniel, T.C. Berkelbach, Y. Meng, G.H. Lee, T.F. Heinz, D.R. Reichman, D.A. Muller, J.C. Hone, Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide, Nat. Mater. 12 (2013) 554–561. [5] Q.H. Wang, K. Kalantar-Zadeh, A. Kis, J.N. Coleman, M.S. Strano, Electronics and optoelectronics of two-dimensional transition metal dichalcogenides, Nat. Nanotechnol. 7 (2012) 699–712. [6] H. Nam, S. Wi, H. Rokni, M. Chen, G. Priessnitz, W. Lu, X. Liang, MoS2 transistors fabricated via plasma-assisted nanoprinting of few-layer MoS2 flakes into large-area arrays, ACS Nano 7 (2013) 5870–5881. [7] K. Kang, S. Xie, L. Huang, Y. Han, P.Y. Huang, K.F. Mak, C. Kim, D. Muller, J. Park, Highmobility three-atom-thick semiconducting films with wafer-scale homogeneity, Nature 520 (2015) 656–660. [8] A. Gurarslan, Y.F. Yu, L.Q. Su, Y.L. Yu, F. Suarez, S.S. Yao, Y. Zhu, M. Ozturk, Y. Zhang, L. Cao, Surface-energy-assisted perfect transfer of centimeter-scale monolayer and few-layer MoS2 films onto arbitrary substrates, ACS Nano 8 (2014) 11522–11528. [9] S. Das, H.Y. Chen, A.V. Penumatcha, J. Appenzeller, High performance multilayer MoS2 transistors with scandium contacts, Nano Lett. 13 (2013) 100–105. [10] R.L. Masihhur, L. Ma, S. Kannappan, P.S. Park, S. Krishnamoorthy, D.N. Nath, W. Lu, Y.Y. Wu, S. Rajan, Large area single crystal (0001) oriented MoS2, Appl. Phys. Lett. 102 (2013) 252108.
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