Name: preparation of large-area vertical 2d crystal hetero-structures through the sulfurization of transition metal films for device fabrication
Uploaded: 2017-11-28T13:00:00
Description: Watch this Scientific Journal Video about Preparation of Large-area Vertical 2D Crystal Hetero-structures Through the Sulfurization of Transition Metal Films for Device Fabrication at JoVE.com

This work was supported in part by projects MOST 105-2221-E-001-011-MY3 and MOST 105-2622-8-002-001 funded by the Ministry of Science and Technology, Taiwan, and in part by the focused project funded by the Research Center for Applied Sciences, Academia Sinica, Taiwan.

1. Growth of Individual 2D material (MoS2 and WS2)
<ol>
	<li>Transition metal deposition using an RF sputtering system
	<ol>
		<li>A clean 2 x 2 cm2 sapphire substrate is placed on the sample holder with the polished side toward the targets of the sputtering system for the transition metal deposition. Sapphire substrates are chosen due to sapphire&#39;s chemical stability at high temperatures and atomic-flat surfaces.</li>
		<li>Pump down the sputtering chamber to 3 x 1...

<ol>
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Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Epitaxial+growth+of+a+monolayer+WSe2-MoS2+lateral+p-n+junction+with+an+atomically+sharp+interface.">Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface.</a> Science. 349, 524-528 (2015).</li><li>Zhan, Y., Liu, Z., Najmaei, S., Ajayan, M. P., Lou, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Large-area+vapor-phase+growth+and+characterization+of+MoS2+atomic+layers+on+a+SiO2+substrate.">Large-area vapor-phase growth and characterization of MoS2 atomic layers on a SiO2 substrate.</a> Small. 8, 966 (2012).</li><li>Woods, J. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=One-Step+Synthesis+of+MoS2/WS2+Layered+Heterostructures+and+Catalytic+Activity+of+Defective+Transition+Metal+Dichalcogenide+Films.">One-Step Synthesis of MoS2/WS2 Layered Heterostructures and Catalytic Activity of Defective Transition Metal Dichalcogenide Films.</a> ACS Nano. 10, 2004-2009 (2016).</li><li>Wu, C. R., Chang, X. R., Wu, C. H., Lin, S. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Growth+Mechanism+of+Transition+Metal+Dichalcogenides+using+Sulfurization+of+Pre-deposited+Transition+Metals+and+the+2D+Crystal+Hetero-structure+Establishment.">The Growth Mechanism of Transition Metal Dichalcogenides using Sulfurization of Pre-deposited Transition Metals and the 2D Crystal Hetero-structure Establishment.</a> Sci. 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Compared with conventional semiconductor materials such as Si and GaAs, the advantage of 2D materials for device applications lies in the possibility of device fabrication with very thin bodies down to several atomic layers. When the Si industry advances into the &#60;10 nm technology node, the high aspect ratio of Si fin FET will make the device architecture unsuitable for practical applications. Thus, 2D materials have emerged due to their potential to replace Si for electronic device applications.
<p class="jove_c...

One of the most common approaches to obtain 2D crystal films is using mechanical exfoliation from bulk materials1,2,3,4,5. Although 2D crystal films with high crystalline quality can be easily obtained using this method, scalable 2D crystal films are not available through this approach, which is disadvantageous for practical applications. It has been demonstrated in previous publications that using chemical vapor deposition (CVD), large-area and uniform 2D crystal films can be prepared6,7,8,9. Direct growth of graphene on sapphire substrates and layer-number-controllable MoS2 films prepared by repeating the same growth cycle are also demonstrated using the CVD growth technique10,11. In one recent publication, in-plane WSe2/MoS2 hetero-structure flakes are also fabricated using the CVD growth technique12. Although the CVD growth technique is promising in providing scalable 2D crystal films, the major disadvantage of this growth technique is that different precursors have to be located for different 2D crystals. The growth conditions also vary between different 2D crystals. In this case, the growth procedures will become more complicated when demand grows for 2D crystal hetero-structures.
Compared with the CVD growth technique, the sulfurization of pre-deposited transition metal films has provided a similar but much simpler growth approach for TMDs13,14. Since the growth procedure involves only metal deposition and the following sulfurization procedure, it is possible to grow different TMDs through the same growth procedures. On the other hand, the layer number controllability of the 2D crystals may also be achieved by changing the pre-deposited transition metal thicknesses. In this case, growth optimization and layer number control down to a single layer are required for different TMDs. Understanding growth mechanisms is also very important for the establishment of complicated TMD hetero-structures using this method.
In this paper, MoS2 and WS2 films are prepared under similar growth procedures of the metal deposition followed by the sulfurization procedure. With the results obtained from the sulfurization of Mo films under sulfur sufficient and deficient conditions, two growth mechanisms are observed during the sulfurization procedure15. Under the sulfur sufficient condition, a uniform and layer-number-controllable MoS2 film can be obtained after the sulfurization procedure. When the sample is sulfurized under the sulfur deficient condition, the background sulfur is not sufficient to form a complete MoS2 film such that the Mo oxide segregation and coalescence will be the dominant mechanism at the early growth stage. A sample with Mo oxide clusters covered by few layers of MoS2 will be obtained after the sulfurization procedure15. Through sequential metal deposition and following sulfurization procedures, WS2/MoS2 vertical hetero-structures with layer number controllability down to a single layer can be prepared15,16. Using this technique, a sample is obtained on a single sapphire substrate with four regions: (I) blank sapphire substrate, (II) standalone MoS2, (III) WS2/MoS2 hetero-structure, and (IV) standalone WS217. The results demonstrate that the growth technique is advantageous for the establishment of vertical 2D crystal hetero-structure and is capable of selective growth. The enhanced device performances of 2D crystal hetero-structures will mark the first step toward practical applications for 2D crystals.

<table>
	<tbody>
		<tr>
			<td>RF sputtering system</td>
			<td>Kao Duen Technology</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>Furnace for sulfurization</td>
			<td>Creating Nano Technologies</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>Polymethyl methacrylate (PMMA)</td>
			<td>Microchem</td>
			<td>8110788</td>
			<td>Flammable</td>
		</tr>
		<tr>
			<td>KOH, &#62; 85%</td>
			<td>Sigma-Aldrich</td>
			<td>30603</td>
			<td></td>
		</tr>
		<tr>
			<td>Acetone, 99.5%</td>
			<td>Echo Chemical</td>
			<td>CMOS110</td>
			<td></td>
		</tr>
		<tr>
			<td>Sulfur (S), 99.5%</td>
			<td>Sigma-Aldrich</td>
			<td>13803</td>
			<td></td>
		</tr>
		<tr>
			<td>Molybdenum (Mo), 99.95%</td>
			<td>Summit-Tech</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>Tungsten (W), 99.95%</td>
			<td>Summit-Tech</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>C-plane Sapphire substrate</td>
			<td>Summit-Tech</td>
			<td>X171999</td>
			<td>(0001) &#177; 0.2 &#176; one side polished</td>
		</tr>
		<tr>
			<td>300 nm SiO2/Si substrate</td>
			<td>Summit-Tech</td>
			<td>2YCDDM</td>
			<td>P-type Si substrate, resistivity: 1-10 &#937; &#183; cm.</td>
		</tr>
		<tr>
			<td>Sample holder (sputtering system)</td>
			<td>Kao Duen Technology</td>
			<td>N/A</td>
			<td>Ceramic material</td>
		</tr>
		<tr>
			<td>Mechanical pump (sputtering system)</td>
			<td>Ulvac</td>
			<td>D-330DK</td>
			<td></td>
		</tr>
		<tr>
			<td>Diffusion pump (sputtering system)</td>
			<td>Ulvac</td>
			<td>ULK-06A</td>
			<td></td>
		</tr>
		<tr>
			<td>Mass flow controller</td>
			<td>Brooks</td>
			<td>5850E</td>
			<td>The maximum Argon flow is 400 mL/min</td>
		</tr>
		<tr>
			<td>Manual wheel Angle poppet valve</td>
			<td>King Lai</td>
			<td>N/A</td>
			<td>Vacuum range from 2500 ~1 &#215; 10-8 torr</td>
		</tr>
		<tr>
			<td>Raman measurement system</td>
			<td>Horiba</td>
			<td>Jobin Yvon LabRAM HR800</td>
			<td></td>
		</tr>
		<tr>
			<td>Transmission electron microscopy</td>
			<td>Fei</td>
			<td>Tecnai G2 F20</td>
			<td></td>
		</tr>
		<tr>
			<td>Petri dish</td>
			<td>Kwo Yi</td>
			<td>N/A</td>
			<td></td>
		</tr>
		<tr>
			<td>Tweezer</td>
			<td>Venus</td>
			<td>2A</td>
			<td></td>
		</tr>
		<tr>
			<td>Digital dry cabinet</td>
			<td>Jwo Ruey Technical</td>
			<td>DRY-60</td>
			<td></td>
		</tr>
		<tr>
			<td>Dual-channel system sourcemeter</td>
			<td>Keithley</td>
			<td>2636B</td>
			<td></td>
		</tr>
	</tbody>
</table>

preparation of large-area vertical 2d crystal hetero-structures through the sulfurization of transition metal films for device fabrication

We have demonstrated that through the sulfurization of transition metal films such as molybdenum (Mo) and tungsten (W), large-area and uniform transition metal dichalcogenides (TMDs) MoS2 and WS2 can be prepared on sapphire substrates. By controlling the metal film thicknesses, good layer number controllability, down to a single layer of TMDs, can be obtained using this growth technique. Based on the results obtained from the Mo film sulfurized under the sulfur deficient condition, there are two mechanisms of (a) planar MoS2 growth and (b) Mo oxide segregation observed during the sulfurization procedure. When the background sulfur is sufficient, planar TMD growth is the dominant growth mechanism, which will result in a uniform MoS2 film after the sulfurization procedure. If the background sulfur is deficient, Mo oxide segregation will be the dominant growth mechanism at the initial stage of the sulfurization procedure. In this case, the sample with Mo oxide clusters covered with few-layer MoS2 will be obtained. After sequential Mo deposition/sulfurization and W deposition/sulfurization procedures, vertical WS2/MoS2 hetero-structures are established using this growth technique. Raman peaks corresponding to WS2 and MoS2, respectively, and the identical layer number of the hetero-structure with the summation of individual 2D materials have confirmed the successful establishment of the vertical 2D crystal hetero-structure. After transferring the WS2/MoS2 film onto a SiO2/Si substrate with pre-patterned source/drain electrodes, a bottom-gate transistor is fabricated. Compared with the transistor with only MoS2 channels, the higher drain currents of the device with the WS2/MoS2 hetero-structure have exhibited that with the introduction of 2D crystal hetero-structures, superior device performance can be obtained. The results have revealed the potential of this growth technique for the practical application of 2D crystals.

The Raman spectrum and the cross-sectional HRTEM images of individual MoS2 and WS2 fabricated using the sulfurization of pre-deposited transition metals are shown in Figure 1a-b17, respectively. Two characteristic Raman peaks are observed for both MoS2 and WS2, which correspond to in-plane <img alt="Equation 1" src="/files/ftp_upload/56494/56494eq1.jpg" /> and out-of-plane A<...

Watch this Scientific Journal Video about Preparation of Large-area Vertical 2D Crystal Hetero-structures Through the Sulfurization of Transition Metal Films for Device Fabrication at JoVE.com

Preparation of Large-area Vertical 2D Crystal Hetero-structures Through the Sulfurization of Transition Metal Films for Device Fabrication

Through the sulfurization of pre-deposited transition metals, large-area and vertical 2D crystal hetero-structures can be fabricated. The film transferring and device fabrication procedures are also demonstrated in this report.

We have demonstrated that through the sulfurization of transition metal films such as molybdenum (Mo) and tungsten (W), large-area and uniform transition ...

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