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

The overall goal of this growth technique is to establish complex vertical 2D crystal heterostructures, with good control over the number of 2D layers, using similar growth procedures for each material. The main advantage of this simple technique is that it uses similar growth procedures for different 2D materials and it has good layer number controllability. This technique can help investigate practical applications of vertical 2D material heterostructures as it can produce larger vertical 2D crystal heterostructures with enhanced transitor properties compared to single material structures.

To begin the procedure, mount a clean C-plane two centimeter by two centimeter sapphire substrate on the sample stage of an RF sputtering system, with the polished side facing the molybdenum target. Pump down the sample chamber to three times 10 to the negative six tores. Inject argon gas into the system at 40 milliliters per minute and ensure that the chamber pressure is stable at five times 10 to the negative two tores.

Ignite the plasma, and set the power output to 40 watts. Reduce the chamber pressure to five times 10 to the negative three tores. Then, manually open the molybdenum target shutter and deposit the metal for 30 seconds.

Ensure that the power output remains constant throughout deposition. When sputtering has finished, place the substrate in a quart sample holder with the metal film facing up. Set the substrate in the center of the heating zone of a calibrated tube furnace.

Place 1.5 grams of sulfur powder in a alumina heating boat. Place the boat two centimeters upstream of the heating zone so that the sulfur temperature will be 120 degrees Celsius when the substrate reaches 800 degrees Celsius. Close and pump down the furnace to five times 10 to the negative three tores.

Then, flow argon gas through the furnace at 130 milliliters per minute. Ensure that the furnace pressure stabilizes at 0.7 tores. Ramp the furnace from room temperature to 800 degrees Celsius at 20 degrees Celsius per minute.

Hold the furnace at 800 degrees Celsius until the sulfur has fully evaporated. Then, turn off the furnace heat and allow the substrate to cool to room temperature under a flow of argon. Next, mount the substrate in the RF sputtering system with the molybdenum disulfide film facing the tungsten target.

Sputter tungsten on the substrate for 30 seconds using the same settings as for molybdenum. Place the tungsten-coated substrate in the center of the furnace heating zone and one gram of sulfur powder two centimeters upstream of the heating zone. Sulfurize the tungsten film using the same parameters as for the molybdenum film.

To begin the thin film transfer, spin coat three drops of polymethyl methacrylate on a prepared transition metal dichalcogenide film for 10 seconds each at 500 and 800 rotations per minute. Cure the PMMA at 120 degrees Celsius for five minutes. Then, place the PMMA-coated substrate in a Petri dish filled with deionized water.

Use tweezers with round, flat tips to carefully peel one corner of the PMMA with the attached TMD film from the substrate. Transfer the substrate to a one molar aqueous potassium hydroxide solution heated to 100 degrees Celsius. Slowly and carefully peel the entire PMMA TMD film from the substrate using tweezers.

Scoop up the PMMA TMD film from the potassium hydroxide solution with another clean sapphire substrate with the polished side facing the film. Transfer the film to a beaker of deionized water. Transfer the film to fresh deionized water three more times in the same way, to ensure that residual potassium hydroxide is completely removed.

Next, obtain a 300 nanometer thick P type silicon dioxide on silicon substrate patterned with titanium or gold electrodes. Immerse the patterned substrate in the final beaker of deionized water and carefully attach the TMD film to the substrate. Bake the substrate with the attached film at 100 degrees Celsius for three minutes to evaporate the water.

Cover the substrate surface with PMMA to ensure that the film is firmly attached to the substrate. Dry the substrate in an electronic drying cabinet for eight hours. Then, remove the PMMA layer and use photolithography and reactive ion etching to fabricate a TMD heterostructure transistor.

The molybdenum sulfurization mechanism was examined with HRTEM. Under sulfur rich conditions, the sulfur rapidly replaced the oxygen of molybdenum oxides, eventually forming a uniform film with a controllable number of layers. Molybdenum oxide segregation and coalescence was the dominant early growth mechanism in sulfur deficient conditions.

Raman spectroscopy of individual molybdenum disulfide and tungsten disulfide films each showed two characteristic peaks. HRTEM showed five molybdenum disulfide layers and four tungsten disulfide layers. Sequential metal deposition resulted in a tungsten disulfide molybdenum disulfide vertical heterostructure.

Both sets of characteristic peaks were apparent in the Ramen spectrum. The formation of a vertical TMD heterostructure was supported by repeated atomic etching of individual tungsten disulfide layers until only molybdenum disulfide remained. A transistor fabricated with a tungsten disulfide molybdenum disulfide single heterostructure showed a substantially greater drain current when compared to a transistor fabricated with only molybdenum disulfide.

The single heterostructure transistor had a higher field effect mobility value, which could have been the result of electron injection from tungsten disulfide to molybdenum disulfide and from channels with higher electron concentrations under thermal equilibrium. After watching this video, you should have a good understanding of how to use a equipment and procedures shown to easily establish vertical 2D material heterostructures for different device applications. When using this protocol, you should only need to optimize the sputtering powers, the position times and separation temperatures for your equipment to obtain high quality 2D materials and heterostructures.