Van der Waals Heterostructures with High Accuracy Rotational ...
Van der Waals Heterostructures with High Accuracy Rotational Alignment Kyounghwan Kim,1 Matthew Yankovitz,2 Babak Fallahazad,1 Sangwoo Kang1, Hema C. P. Movva,1 Shengqiang Huang,2 Stefano Larentis,1 Chris M. Corbet,1 Takashi Taniguchi,3 Kenji Watanabe,3 Sanjay K. Banerjee,1 Brian J. LeRoy,2 Emanuel Tutuc1 1
Microelectronics Research Center, Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, TX 78758, USA 2
3
Physics Department, University of Arizona, Tucson, Arizona 85721, United States
National Institute for Materials Science, 1-1 Namiki Tsukuba, Ibaraki 305-0044, Japan
*E-mail : [email protected]
Figure S1. Preparation of hemispherical handle substrates. (a), (b) Schematics of before and after placing a hemispherical M-bond 610 epoxy on a glass substrate using a pipette. (c) Image of the epoxy on the glass substrate after 24 hr curing. (d), (e) Schematics of before and after placing a hemispherical PDMS droplet on a planar PDMS mold. (f) Picture of the hemispherical PDMS on the planar PDMS mold that is attached on a 1 inch square glass substrate. We use commercially available epoxy (M-bond 610, SPI Supplies) to make a hemispherical epoxy on a glass substrate, the so called hemispherical handle substrate. To make the hemispherical shape of the epoxy, we use a toothpick or a narrow-end pipette. Figure S2 (a) and (b) show an epoxy droplet being placed on a glass substrate using a narrow-end pipette. The epoxy droplet is then cured for 24 hr. Figure S1 (c) shows an image of the hemispherical epoxy handle on a glass substrate. The picture was taken after 24 hour curing. For the second type of hemispherical handle substrate, we use a hemispherical PDMS on a premade planar PDMS mold or a glass substrate. The PDMS solution is made using a mixture of Sylgard
184 prepolymer, and curing agent with 10:1 molar % ratio. To form a hemispherical PDMS, a sharp tip is coated with the PDMS solution, followed by a soft contact with the pre-made planar PDMS or the glass slide [Figs. S1 (d), (e)]. Figure S1 (f) show an image of the hemispherical PDMS handle on planar PDMS, prepared on a glass substrate. Since PDMS and M-bond 610 epoxy have different hardness after curing, the use of these two types of handle substrates can also be varies. We find that the PDMS handle substrate is softer than the epoxy one when we try to make a contact during pick-up. As a result, the PDMS hemispherical handle substrate can have wider contact area with less contact force, compared to epoxy used one which results in smaller contact area with stronger contact force. Depends on the pick-up flake and exfoliated/transferred substrate conditions, the right handle substrate can be chosen to optimize the pickup and transfer processes.
Figure S2. Successive transfers with a controlled rotational alignment using metal alignment marks as reference. Step 1: Before the first pick up and transfer, two alignment marks are used to mark the reference angle. The separation between the metal alignment marks is 200 µm. Step 2: The handle substrate is brought in contact with the substrate for the first flake pick-up. Step 3: After a section of the flake is picked-up, the bottom substrate is rotated by a desired angle, while the transfer handle remains stationary. The above example shows a 15o rotation. A 1o rotation corresponds to a 3.5 µm displacement of the right-most alignment mark with respect to the initial position. Step 4: The handle substrate is brought in contact with the bottom substrate and the other section of the flake is picked-up. A 1o
Figure S3. Process flow for heterostructure transfer from the handle to a secondary substrate. Panel (a) corresponds to a transfer in which the last layer picked-up becomes the top-most on the secondary substrate (stacking order reversed). This process flow was used for the artificial Bernal stacked bilayer graphene fabrication. PVA, a water soluble polymer is used as adhesive polymer for individual layer pick-up. Step 1: Spin-coat PVA; Step 2: Layer pick-up; Step 3: Spin-coat PMMA; Step 4: Handle substrate placed in DI water, Step 5: PMMA membrane floated on the DI water surface; Inset schematic : cross-sectional view of PMMA/heterostructure on DI water, Step 6: PMMA membrane captured using a SiO2/Si substrate. (b) Process flow corresponding to a transfer in which the first layer becomes the topmost on the secondary substrate (stacking order maintained). PMMA is used as adhesive polymer for layers pick-up. Step 1: Spin-coat PVA; Step 2: Spin-coat PMMA; Step 3: Layer pick-up; Step 4: Handle substrate placed on the DI water surface; Step5: PMMA membrane on the DI water surface; Inset: cross
sectional view of heterostructure/PMMA on DI water. Step 6: PMMA membrane captured using a glass substrate. Step 7: Transfer to a secondary substrate using mask aligner. The double bilayer graphene tunneling field-effect transistor (TFET) was fabricated using panel (b) process flow.
Figure S4. Process flow for heterostructure fabrication using PPC as adhesive polymer. This process flow can also be used to fabricate double bilayer TFET. The final heterostructure maintains the stacking order. Step 1: Spin-coat PPC on the handle substrate and place it on mask aligner; Step 2: First layer pick-up; Step 3: Unload the bottom substrate and load the other exfoliated substrate; Step 4: Second layer pick-up, Step 5: Place a new secondary substrate for a transfer; Step 6: The new substrate makes a contact with heterostructure and PPC at 90oC; Step 7: PPC and heterostructure detached from handle substrate and transferred to a new substrate.
Microelectronics Research Center, Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, TX 78758, USA 2
3
Physics Department, University of Arizona, Tucson, Arizona 85721, United States
National Institute for Materials Science, 1-1 Namiki Tsukuba, Ibaraki 305-0044, Japan
*E-mail : [email protected]
Figure S1. Preparation of hemispherical handle substrates. (a), (b) Schematics of before and after placing a hemispherical M-bond 610 epoxy on a glass substrate using a pipette. (c) Image of the epoxy on the glass substrate after 24 hr curing. (d), (e) Schematics of before and after placing a hemispherical PDMS droplet on a planar PDMS mold. (f) Picture of the hemispherical PDMS on the planar PDMS mold that is attached on a 1 inch square glass substrate. We use commercially available epoxy (M-bond 610, SPI Supplies) to make a hemispherical epoxy on a glass substrate, the so called hemispherical handle substrate. To make the hemispherical shape of the epoxy, we use a toothpick or a narrow-end pipette. Figure S2 (a) and (b) show an epoxy droplet being placed on a glass substrate using a narrow-end pipette. The epoxy droplet is then cured for 24 hr. Figure S1 (c) shows an image of the hemispherical epoxy handle on a glass substrate. The picture was taken after 24 hour curing. For the second type of hemispherical handle substrate, we use a hemispherical PDMS on a premade planar PDMS mold or a glass substrate. The PDMS solution is made using a mixture of Sylgard
184 prepolymer, and curing agent with 10:1 molar % ratio. To form a hemispherical PDMS, a sharp tip is coated with the PDMS solution, followed by a soft contact with the pre-made planar PDMS or the glass slide [Figs. S1 (d), (e)]. Figure S1 (f) show an image of the hemispherical PDMS handle on planar PDMS, prepared on a glass substrate. Since PDMS and M-bond 610 epoxy have different hardness after curing, the use of these two types of handle substrates can also be varies. We find that the PDMS handle substrate is softer than the epoxy one when we try to make a contact during pick-up. As a result, the PDMS hemispherical handle substrate can have wider contact area with less contact force, compared to epoxy used one which results in smaller contact area with stronger contact force. Depends on the pick-up flake and exfoliated/transferred substrate conditions, the right handle substrate can be chosen to optimize the pickup and transfer processes.
Figure S2. Successive transfers with a controlled rotational alignment using metal alignment marks as reference. Step 1: Before the first pick up and transfer, two alignment marks are used to mark the reference angle. The separation between the metal alignment marks is 200 µm. Step 2: The handle substrate is brought in contact with the substrate for the first flake pick-up. Step 3: After a section of the flake is picked-up, the bottom substrate is rotated by a desired angle, while the transfer handle remains stationary. The above example shows a 15o rotation. A 1o rotation corresponds to a 3.5 µm displacement of the right-most alignment mark with respect to the initial position. Step 4: The handle substrate is brought in contact with the bottom substrate and the other section of the flake is picked-up. A 1o
Figure S3. Process flow for heterostructure transfer from the handle to a secondary substrate. Panel (a) corresponds to a transfer in which the last layer picked-up becomes the top-most on the secondary substrate (stacking order reversed). This process flow was used for the artificial Bernal stacked bilayer graphene fabrication. PVA, a water soluble polymer is used as adhesive polymer for individual layer pick-up. Step 1: Spin-coat PVA; Step 2: Layer pick-up; Step 3: Spin-coat PMMA; Step 4: Handle substrate placed in DI water, Step 5: PMMA membrane floated on the DI water surface; Inset schematic : cross-sectional view of PMMA/heterostructure on DI water, Step 6: PMMA membrane captured using a SiO2/Si substrate. (b) Process flow corresponding to a transfer in which the first layer becomes the topmost on the secondary substrate (stacking order maintained). PMMA is used as adhesive polymer for layers pick-up. Step 1: Spin-coat PVA; Step 2: Spin-coat PMMA; Step 3: Layer pick-up; Step 4: Handle substrate placed on the DI water surface; Step5: PMMA membrane on the DI water surface; Inset: cross
sectional view of heterostructure/PMMA on DI water. Step 6: PMMA membrane captured using a glass substrate. Step 7: Transfer to a secondary substrate using mask aligner. The double bilayer graphene tunneling field-effect transistor (TFET) was fabricated using panel (b) process flow.
Figure S4. Process flow for heterostructure fabrication using PPC as adhesive polymer. This process flow can also be used to fabricate double bilayer TFET. The final heterostructure maintains the stacking order. Step 1: Spin-coat PPC on the handle substrate and place it on mask aligner; Step 2: First layer pick-up; Step 3: Unload the bottom substrate and load the other exfoliated substrate; Step 4: Second layer pick-up, Step 5: Place a new secondary substrate for a transfer; Step 6: The new substrate makes a contact with heterostructure and PPC at 90oC; Step 7: PPC and heterostructure detached from handle substrate and transferred to a new substrate.
