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Growth of wafer-scale MoS2 monolayer by magnetron sputtering† Junguang Tao,*a,c Jianwei Chai,*a Xin Lu,b Lai Mun Wong,a Ten It Wong,a Jisheng Pan,a Qihua Xiong,b,d Dongzhi Chia and Shijie Wang*a The two-dimensional layer of molybdenum disulfide (MoS2) exhibits promising prospects in the applications of optoelectronics and valleytronics. Herein, we report a successful new process for synthesizing wafer-scale MoS2 atomic layers on diverse substrates via magnetron sputtering. Spectroscopic and microscopic results reveal that these synthesized MoS2 layers are highly homogeneous and crystallized; moreover, uniform monolayers at wafer scale can be achieved. Raman and photoluminescence spectroscopy
Received 31st October 2014, Accepted 13th December 2014 DOI: 10.1039/c4nr06411a www.rsc.org/nanoscale
indicate comparable optical qualities of these as-grown MoS2 with other methods. The transistors composed of the MoS2 film exhibit p-type performance with an on/off current ratio of ∼103 and hole mobility of up to ∼12.2 cm2 V−1 s−1. The strategy reported herein paves new ways towards the large scale growth of various two-dimensional semiconductors with the feasibility of controllable doping to realize desired p- or n-type devices.
Introduction Recently, atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs), MX2 (M = Mo, W; X = S, Se, Te), were demonstrated to offer rich collection of physical properties and functionalities in the area of nanoelectronics,1–3 optoelectronics,4 catalysis,5 photo-detection,6 photovoltaics7 and photocatalysis.8 The lattice of bulk TMDs is formed by covalently bonded X-M-X 2D hexagonal trilayers, which weakly bonds with neighboring layers via van der Waals forces.9,10 In each layer, the electrons and holes are vertically confined, giving rise to numerous exotic physics phenomena, particularly at the monolayer limit. When exfoliated down to a monolayer, the band gap of TMDs crosses over from indirect to direct due to quantum confinement, which entails rather efficient light absorption and emission. As a prototype of TMDs, molybdenum disulfide (MoS2) has been demonstrated to exhibit high current on/off ratio (1 × 108),11,12 high mobility
a Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), 3, Research Link, Singapore 117602. E-mail: [email protected], [email protected], [email protected] b Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371 c School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China d NOVITAS, Nanoelectronics Centre of Excellence, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4nr06411a
This journal is © The Royal Society of Chemistry 2015
(∼200 cm2 V−1 s−1) and negligible off current. This indicates that the sensitivity can be significantly improved with MoS2based field effect transistors (FETs).3,4,13 In addition to its good electronic properties, the inherent band gap (1.8 eV for monolayer and 1.2 eV for bulk),4,14,15 excellent mechanical and optical properties permit its applications in large-area flexible optoelectronics.3 To facilitate the integration of this fascinating material into macroscopic electronic applications, it is essential to develop a large-area growth technique that is compatible with current micro- or nano-fabrication processes. Single and few-layered MoS2 were first obtained by a topdown mechanical exfoliation technique,11 which is commonly used for graphene. Although this exfoliation method has the potential to achieve high quality materials, it is unsuitable for large scale commercially viable devices. More recently, thinning of the TMD few-layers via laser,16 plasma,17 patterning method18 or thermal annealing19 was reported; however, these methods require thin layers and thus are not feasible for largescale and massive production. Subsequently, bottom-up growth methods have been introduced to develop more scalable techniques such as physical vapor deposition (PVD),20,21 chemical vapor deposition (CVD),22,23 sulfurization of molybdenum oxides,24 hydrothermal synthesis,25 and electrochemical lithiation processes.26 Several approaches based on CVD have been reported by using Mo,27 MoO3,23,24 MoCl5,22 or (NH4)2MoS4 28 as an Mo precursor followed by the second-step thermal sulfurization. More recently, a seeding promoter has been used to improve the CVD growth quality.29 Although the CVD method has demonstrated its success in synthesis of large scale, high quality TMDs on various substrates, the
Nanoscale, 2015, 7, 2497–2503 | 2497
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Published on 16 December 2014. Downloaded by University of California - San Diego on 12/07/2017 13:44:21.
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control of thickness, purity and uniformity is still a challenge. For practical application, a scalable and controlled synthesis technique is required. Therefore, alternative approaches, either complementary or advanced, are highly desired. To date, the majority of the synthesized MoS2 films based on the abovementioned methods are n-type doped.13,28,30 Very recently, it has been reported that p-type-doped MoS2 can be realized by efficient plasma treatment.31 Moreover, the formation of MoS2 p–n junction devices was exhibited by using electric double-layer gating.32,33 To meet the digital logic application demands, the research for an intrinsically p-type TMD monolayer has become necessary. However, the large-scale device applications still rely on the breakthrough in the growth of large-sized, controlled-doping thin layers. In this work, we report a one-step process that can grow good quality monolayers or few-layer MoS2 films at wafer scale on various substrates using magnetron sputtering. The detailed growth procedure is given in the ESI.† As is well known, the magnetron sputtering technique is capable of large scale massive production, which is compatible with current industrial processes with low cost and easy controllability. Previously, the magnetron sputtering method is infrequently used for 2D material growth, particularly for the TMDs. The main obstacle is possibly the difficulty of controlling the S or Se sources. In contrast to previous CVD methods,22–24,27,28 Mo was sputtered to produce energized molecular-sized reactive Mo atoms or clusters that are more reactive than Mo or MoO3 films in the CVD method. Mo atoms react with vaporized S atoms before being placed on hot substrates to form MoS2 layers. Our spectroscopic, microscopic and electrical measurements suggest that this synthetic process can lead to the growth of monolayers, bilayers, trilayers and thicker MoS2 sheets. These MoS2 films are highly homogeneous, and their size is up to several centimeters, which is currently limited by the size of our sample stage. They exhibit optical and electrical qualities comparable to that of the MoS2 synthesized by other methods.
Experimental methods Characterization The surface morphology of the samples was examined with a commercial atomic force microscope (AFM, Bruker ICON-PKG). Raman spectra were obtained on a single-gating micro-Raman spectrometer (Horiba-JY T64000) excited with 532 nm laser. The signal was collected through a 100× objective lens, dispersed with a grating of 1800 g mm−1, and detected by a liquid nitrogen-cooled charge-coupled device. Photoluminescence (PL) was obtained from the same microRaman spectrometer. Note that the Si peak at 520 cm−1 was used for calibration in the experiments. The samples were in situ transferred to an X-ray photoelectron spectroscopy (XPS) chamber for analysis. XPS measurements were performed in a VG ESCALAB 220i-XL system using a monochromatic Al Kα source. The pass energy
2498 | Nanoscale, 2015, 7, 2497–2503
Nanoscale
of the analyzer was set to 10 eV for high measurement resolution. Transmission electron microscopy (TEM) (JEOL 2100) was used to obtain information of the microstructures. For sample transfer, poly(methyl methacrylate) (PMMA, from MicroChem) was spin-coated on the top of the sample. After baking at 180 °C for 2 min, the PMMA-coated sample was immersed into a 60 °C 2 M NaOH solution to etch the SiO2 until the PMMA with MoS2 film floated on the surface. Subsequently, a lacey carbon TEM grid was used to identify PMMA and the MoS2 film. Finally, the PMMA film was removed by acetone and the MoS2 layers were cleaned by DI water. Field-effect transistors fabrication and measurements To fabricate back-gated MoS2 FETs, the metallic drain/source contacts (5 nm Cr/100 nm Au) were fabricated by photolithography, followed by metal deposition and lift-off. Finally, another Au coating layer was made onto the backside of the Si substrate, which served as a back gate contact. The device characteristic curves were measured using a Keithley 4200 semiconductor parameter analyzer.
Results and discussion Fig. S1 (see the ESI†) schematically illustrates our experimental setup for growing MoS2. The MoS2 layers are first grown on c-plane sapphire (Al2O3) and SiO2/Si. The as-grown MoS2 thin films show uniformity and continuity across an area of centimeters under an optical microscope or by the naked eye. Fig. 1(a) shows an image of MoS2 thin layer grown on sapphire. The as-grown MoS2 layer is in faint yellow color and found to have specular reflection without grain boundaries. In addition, the absence of grain boundaries was confirmed via atomic force microscopy (AFM) measurement at the micrometer scale. Combining these measurements, large-area continuous and uniform MoS2 film is indicated. The size of the synthesized films is limited by the dimensions of our sample heating holder. The surface morphology is further characterized by AFM at different areas on the film, which exhibit similar surface morphology across the film. A typical AFM measurement (5 µm × 5 µm) is presented in Fig. 1(b), which demonstrated an atomically flat surface. The AFM images confirm that the synthesized film possesses a continuous and smooth surface (root-mean-square (RMS) roughness
Published on 16 December 2014. Downloaded by University of California - San Diego on 12/07/2017 13:44:21.
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Cite this: Nanoscale, 2015, 7, 2497
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Growth of wafer-scale MoS2 monolayer by magnetron sputtering† Junguang Tao,*a,c Jianwei Chai,*a Xin Lu,b Lai Mun Wong,a Ten It Wong,a Jisheng Pan,a Qihua Xiong,b,d Dongzhi Chia and Shijie Wang*a The two-dimensional layer of molybdenum disulfide (MoS2) exhibits promising prospects in the applications of optoelectronics and valleytronics. Herein, we report a successful new process for synthesizing wafer-scale MoS2 atomic layers on diverse substrates via magnetron sputtering. Spectroscopic and microscopic results reveal that these synthesized MoS2 layers are highly homogeneous and crystallized; moreover, uniform monolayers at wafer scale can be achieved. Raman and photoluminescence spectroscopy
Received 31st October 2014, Accepted 13th December 2014 DOI: 10.1039/c4nr06411a www.rsc.org/nanoscale
indicate comparable optical qualities of these as-grown MoS2 with other methods. The transistors composed of the MoS2 film exhibit p-type performance with an on/off current ratio of ∼103 and hole mobility of up to ∼12.2 cm2 V−1 s−1. The strategy reported herein paves new ways towards the large scale growth of various two-dimensional semiconductors with the feasibility of controllable doping to realize desired p- or n-type devices.
Introduction Recently, atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs), MX2 (M = Mo, W; X = S, Se, Te), were demonstrated to offer rich collection of physical properties and functionalities in the area of nanoelectronics,1–3 optoelectronics,4 catalysis,5 photo-detection,6 photovoltaics7 and photocatalysis.8 The lattice of bulk TMDs is formed by covalently bonded X-M-X 2D hexagonal trilayers, which weakly bonds with neighboring layers via van der Waals forces.9,10 In each layer, the electrons and holes are vertically confined, giving rise to numerous exotic physics phenomena, particularly at the monolayer limit. When exfoliated down to a monolayer, the band gap of TMDs crosses over from indirect to direct due to quantum confinement, which entails rather efficient light absorption and emission. As a prototype of TMDs, molybdenum disulfide (MoS2) has been demonstrated to exhibit high current on/off ratio (1 × 108),11,12 high mobility
a Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), 3, Research Link, Singapore 117602. E-mail: [email protected], [email protected], [email protected] b Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371 c School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China d NOVITAS, Nanoelectronics Centre of Excellence, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4nr06411a
This journal is © The Royal Society of Chemistry 2015
(∼200 cm2 V−1 s−1) and negligible off current. This indicates that the sensitivity can be significantly improved with MoS2based field effect transistors (FETs).3,4,13 In addition to its good electronic properties, the inherent band gap (1.8 eV for monolayer and 1.2 eV for bulk),4,14,15 excellent mechanical and optical properties permit its applications in large-area flexible optoelectronics.3 To facilitate the integration of this fascinating material into macroscopic electronic applications, it is essential to develop a large-area growth technique that is compatible with current micro- or nano-fabrication processes. Single and few-layered MoS2 were first obtained by a topdown mechanical exfoliation technique,11 which is commonly used for graphene. Although this exfoliation method has the potential to achieve high quality materials, it is unsuitable for large scale commercially viable devices. More recently, thinning of the TMD few-layers via laser,16 plasma,17 patterning method18 or thermal annealing19 was reported; however, these methods require thin layers and thus are not feasible for largescale and massive production. Subsequently, bottom-up growth methods have been introduced to develop more scalable techniques such as physical vapor deposition (PVD),20,21 chemical vapor deposition (CVD),22,23 sulfurization of molybdenum oxides,24 hydrothermal synthesis,25 and electrochemical lithiation processes.26 Several approaches based on CVD have been reported by using Mo,27 MoO3,23,24 MoCl5,22 or (NH4)2MoS4 28 as an Mo precursor followed by the second-step thermal sulfurization. More recently, a seeding promoter has been used to improve the CVD growth quality.29 Although the CVD method has demonstrated its success in synthesis of large scale, high quality TMDs on various substrates, the
Nanoscale, 2015, 7, 2497–2503 | 2497
View Article Online
Published on 16 December 2014. Downloaded by University of California - San Diego on 12/07/2017 13:44:21.
Paper
control of thickness, purity and uniformity is still a challenge. For practical application, a scalable and controlled synthesis technique is required. Therefore, alternative approaches, either complementary or advanced, are highly desired. To date, the majority of the synthesized MoS2 films based on the abovementioned methods are n-type doped.13,28,30 Very recently, it has been reported that p-type-doped MoS2 can be realized by efficient plasma treatment.31 Moreover, the formation of MoS2 p–n junction devices was exhibited by using electric double-layer gating.32,33 To meet the digital logic application demands, the research for an intrinsically p-type TMD monolayer has become necessary. However, the large-scale device applications still rely on the breakthrough in the growth of large-sized, controlled-doping thin layers. In this work, we report a one-step process that can grow good quality monolayers or few-layer MoS2 films at wafer scale on various substrates using magnetron sputtering. The detailed growth procedure is given in the ESI.† As is well known, the magnetron sputtering technique is capable of large scale massive production, which is compatible with current industrial processes with low cost and easy controllability. Previously, the magnetron sputtering method is infrequently used for 2D material growth, particularly for the TMDs. The main obstacle is possibly the difficulty of controlling the S or Se sources. In contrast to previous CVD methods,22–24,27,28 Mo was sputtered to produce energized molecular-sized reactive Mo atoms or clusters that are more reactive than Mo or MoO3 films in the CVD method. Mo atoms react with vaporized S atoms before being placed on hot substrates to form MoS2 layers. Our spectroscopic, microscopic and electrical measurements suggest that this synthetic process can lead to the growth of monolayers, bilayers, trilayers and thicker MoS2 sheets. These MoS2 films are highly homogeneous, and their size is up to several centimeters, which is currently limited by the size of our sample stage. They exhibit optical and electrical qualities comparable to that of the MoS2 synthesized by other methods.
Experimental methods Characterization The surface morphology of the samples was examined with a commercial atomic force microscope (AFM, Bruker ICON-PKG). Raman spectra were obtained on a single-gating micro-Raman spectrometer (Horiba-JY T64000) excited with 532 nm laser. The signal was collected through a 100× objective lens, dispersed with a grating of 1800 g mm−1, and detected by a liquid nitrogen-cooled charge-coupled device. Photoluminescence (PL) was obtained from the same microRaman spectrometer. Note that the Si peak at 520 cm−1 was used for calibration in the experiments. The samples were in situ transferred to an X-ray photoelectron spectroscopy (XPS) chamber for analysis. XPS measurements were performed in a VG ESCALAB 220i-XL system using a monochromatic Al Kα source. The pass energy
2498 | Nanoscale, 2015, 7, 2497–2503
Nanoscale
of the analyzer was set to 10 eV for high measurement resolution. Transmission electron microscopy (TEM) (JEOL 2100) was used to obtain information of the microstructures. For sample transfer, poly(methyl methacrylate) (PMMA, from MicroChem) was spin-coated on the top of the sample. After baking at 180 °C for 2 min, the PMMA-coated sample was immersed into a 60 °C 2 M NaOH solution to etch the SiO2 until the PMMA with MoS2 film floated on the surface. Subsequently, a lacey carbon TEM grid was used to identify PMMA and the MoS2 film. Finally, the PMMA film was removed by acetone and the MoS2 layers were cleaned by DI water. Field-effect transistors fabrication and measurements To fabricate back-gated MoS2 FETs, the metallic drain/source contacts (5 nm Cr/100 nm Au) were fabricated by photolithography, followed by metal deposition and lift-off. Finally, another Au coating layer was made onto the backside of the Si substrate, which served as a back gate contact. The device characteristic curves were measured using a Keithley 4200 semiconductor parameter analyzer.
Results and discussion Fig. S1 (see the ESI†) schematically illustrates our experimental setup for growing MoS2. The MoS2 layers are first grown on c-plane sapphire (Al2O3) and SiO2/Si. The as-grown MoS2 thin films show uniformity and continuity across an area of centimeters under an optical microscope or by the naked eye. Fig. 1(a) shows an image of MoS2 thin layer grown on sapphire. The as-grown MoS2 layer is in faint yellow color and found to have specular reflection without grain boundaries. In addition, the absence of grain boundaries was confirmed via atomic force microscopy (AFM) measurement at the micrometer scale. Combining these measurements, large-area continuous and uniform MoS2 film is indicated. The size of the synthesized films is limited by the dimensions of our sample heating holder. The surface morphology is further characterized by AFM at different areas on the film, which exhibit similar surface morphology across the film. A typical AFM measurement (5 µm × 5 µm) is presented in Fig. 1(b), which demonstrated an atomically flat surface. The AFM images confirm that the synthesized film possesses a continuous and smooth surface (root-mean-square (RMS) roughness
