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

Summary

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.

Abstract

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.

Introduction

One of the most common approaches to obtain 2D crystal films is using mechanical exfoliation from bulk materials^1,2,3,4,5. 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 prepared^6,7,8,9. 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 technique^10,11. 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 TMDs^13,14. 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 prepared^15,16. 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.

Protocol

1. Growth of Individual 2D material (MoS₂ and WS₂)

1. Transition metal deposition using an RF sputtering system
   1. 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's chemical stability at high temperatures and atomic-flat surfaces.
   2. Pump down the sputtering chamber to 3 x 10^-6 torr sequentially using a mechanical pump followed by a diffusion pump.
   3. Inject the Ar gas into the sputtering system and keep the gas flow at 40 mL/min using a mass flow controller (MFC).
   4. Keep the chamber pressure at 5 x 10^-2 torr using a manual pressure control valve and ignite the Ar plasma. Keep the output power at 40 W.
   5. Decrease the chamber pressure to 5 x 10^-3 torr using a manual wheel Angle poppet valve.
   6. Manually open the shutter between the sapphire substrate and the 2-inch metal target and start the metal deposition. During the deposition procedure, keep the sputtering power at 40 W for both Mo and W. The background pressure is kept at 5 × 10^-3 torr with 40 mL/min Ar gas flow.
   7. Control the sputtering time to deposit transition metal films with different thicknesses. Due to the thin metal thickness, sputtering times will provide better control over the film thickness than the readings from the quartz crystal resonator.
      NOTE: The layer numbers of MoS₂ and WS₂ grown using the method discussed in the current manuscript are proportional to the sputtering times of the pre-deposited Mo and W films. The determination of sputtering times to obtain MoS₂ and WS₂ with required layer numbers is based on the cross-sectional high-resolution transmission electron microscopy (HRTEM) images for samples with different sputtering times. However, if the pre-deposited Mo and W films are too thick, Mo and W oxide segregation will become the dominant growth mechanism, instead of planar MoS₂ and WS₂ film growth. Therefore, the proportionality of layer numbers with the sputtering times is limited to few-layer TMDs. With the growth condition of MoS₂ in the current manuscript, the layer numbers will be proportional to the sputtering times when the MoS₂ film is fewer than 10 layers. The sputtering time is 30 s for the growth of 5-layer MoS₂.
2. The sulfurization of the transition metal film
   1. Place the sapphire substrates with pre-deposited transition metal films in the center of a hot furnace for sulfurization.
   2. Place the sulfur (S) powder upstream of the gas flow, 2 cm away from the heating zone of the furnace. In this position, the evaporation temperature for the S powder will be 120 °C when the substrate temperature increases to 800 °C. Precisely control the S powder weight for different transition metals for sulfurization. In the work, the S powder weight is 1.5 g for Mo and 1.0 g for W.
      NOTE: We determine sulfur powder amounts for the preparation of MoS₂ and WS₂ films based on the results obtained for each material prepared using different amounts of sulfur powder.
   3. Keep the furnace pressure at 0.7 torr. During the sulfurization procedure, 130 mL/min Ar gas was used as carrier gas.
   4. Ramp the temperature of the furnace from room temperature to 800 °C in 40 min with a temperature ramping rate of 20 °C/min. Keep the temperature at 800 °C until the sulfur powder is fully evaporated. After that, the heater power is switched off to lower the furnace temperature. It takes about 30 to 40 min for the furnace to reach room temperature from 800 °C.
3. Perform Raman spectrum measurements using a 488 nm laser^15,16,17. Obtain the cross-sectional HRTEM images to verify the layer numbers of the 2D crystals^15,16,17.

2. The Growth of the WS₂/MoS₂ Vertical Single Hetero-structure

NOTE: This section is used to create a single hetero-structure consisting of a sapphire layer with 5 layers of MoS₂ and 4 layers of WS₂.

1. Follow the same procedure as step 1.1. Deposit the Mo film on the sapphire substrate using the RF sputtering system with a 30 s sputtering time.
2. Sulfurize the Mo film following the same sulfurization procedures as step 1.2 for the growth of MoS₂. Five layers of MoS₂ will be obtained after the sulfurization procedure.
3. Follow the same procedures as step 1.1. Deposit the W film on the MoS₂/sapphire substrate using the RF sputtering system with a 30 s sputtering time.
4. Sulfurize the W film following the same sulfurization procedure of step 1.2 for the growth of WS₂. Four layers of WS₂ will be obtained on top of the MoS₂ after the sulfurization procedure.
   NOTE: The metal deposition and sulfurization procedure is the same as the individual material. The sputtering times of the Mo and W films are determined depending on the required layer numbers of MoS₂ and WS₂ layers. Double- or multi-hetero-structures can be established by repeating the same growth procedure. The sequence of the TMDs in the vertical hetero-structures can also be changed depending on the sample structure.

3. The Film Transferring and Device Fabrication Procedures

1. The film transferring procedure of the 2D crystal films
   1. Spin coat three drops of poly(methyl methacrylate) (PMMA) on the TMD film to cover the whole film at room temperature. The two-stage rotation speeds of the spinner are 500 rpm for 10 s and 800 rpm for 10 s. After curing at 120 °C for 5 min, the PMMA thickness is around 3 µm.
   2. Place the PMMA/TMD/Sapphire sample in a Petri dish which is filled with deionized (DI) water.
   3. Peel off one corner of the PMMA/TMD film from the sapphire substrate using tweezers in DI water.
   4. Heat 250 mL of 1 M KOH aqueous solution (14 g KOH pellets mixed with 250 mL water) in a beaker to 100 °C. Move the sample in the heated KOH aqueous solution and continue peeling the PMMA/TMD film until the film is completely peeled off from the substrate. Peeling requires about 1 min to complete.
   5. Use a separate sapphire substrate to scoop up the PMMA/TMD film from the KOH solution. Move the film to a 250 mL beaker filled with DI water to wash off residue KOH on the film. At this stage, the adhesion between the PMMA/TMD film and the sapphire substrate to scoop the film is weak. Therefore, the film will de-attach from the sapphire substrate after immersion into the DI water.
   6. Repeat steps 3.1.4 - 3.1.5 three times using new DI water to make sure that most of the residue KOH is removed from the film.
      NOTE: The adhesion between each TMD layer is much stronger than TMDs with the sapphire substrate. Therefore, the same transferring procedure can be applied to either individual MoS₂/WS₂ materials or their hetero-structures. The 2D crystal films will be completely peeled off from the substrate, which is similar to the peeling of the MoS₂/graphene hetero-structure discussed in a previous publication¹⁸. Depending on the purpose of transferring the film, the substrate mentioned in this step can either be a sapphire substrate or a SiO₂/Si substrate with pre-deposited electrodes, as described in step 3.2. Other substrates can also be used for this purpose.
2. The fabrication of 2D crystal transistors.
   1. Use standard photolithography to define electrode patterns on SiO₂/Si substrates^15,16,17. Source and drain electrodes made of 10 nm titanium (Ti) or 100 nm gold (Au) are fabricated on a 300 nm SiO₂/p-type Si substrate.
   2. Immerse the SiO₂/Si substrate with pre-patterned source/drain electrodes into the beaker filled with DI water and attach to the TMD side of the PMMA/TMD film as prepared in step 3.1.
   3. Bake the sample at 100 °C for 3 min, after the film is attached to the SiO₂/Si substrate, to remove water residue.
   4. Drip three drops of PMMA on the sample with the PMMA/TMD film to cover the whole surface and make the film more firmly attached to the substrate.
   5. Place the sample in an electronic dry cabinet for at least 8 h before moving to the next step.
   6. Fill two different 250 mL beakers with acetone. Immerse the sample attached with the PMMA/TMD film sequentially into the two different beakers filled with acetone for 50 and 10 min, respectively, to remove the top PMMA layer.
   7. Define the transistor channel using standard photo-lithography and reactive-ion etching^15,16,17. Back-gate MoS₂ and WS₂/MoS₂ hetero-structure transistors are fabricated^15,16,17. The channel length and width of the devices are 5 and 150 μm, respectively.
   8. Use a dual-channel system sourcemeter instrument to measure the current-voltage characteristics of the transistors^15,16,17.

Results

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 and out-of-plane A<...

Discussion

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 <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.

Materials

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