Abigail Lee - University of Pennsylvania
Co-Author(s): Olivia Saouf, MIT, Cambridge MA Ramya Vishnubhotla, Carl Naylor, and Charlie Johnson, UPenn, Philadelphia, PA
My project is improving nanomaterial field-effect transistors with boron nitride (BN). The nanomaterials graphene and MoS2 are usually contaminated by the processes used to fabricate FETs. By using BN as a protective layer we hope to keep unwanted chemicals off of the graphene and MoS2, thereby increasing their mobility and improving FET performance. We started by fabricating devices on silicon dioxide wafers using standard cleanroom techniques. Using photolithography, chromium and gold are deposited into a 52-device pattern on a silicon dioxide wafer to create metal contacts. This creates areas in which our conductive material, graphene, will sit to complete the FET.
We then grow graphene through chemical vapor deposition. Graphene is grown on copper foil in a low pressure furnace. Methane gas is used as a source. Carbon from the methane deposits around defects in the copper in a monolayer sheet of graphene. Next the graphene is transferred to the silicon chips through the bubble transfer method. The graphene on the copper foil is spin coated with poly methyl methacrylate (PMMA) for structural support during transfer. Then a current is applied to the copper and to a bath of NaOH. The foil is dipped into the bath, and O2 and H2 bubbles generated by the voltage lift the graphene and PMMA layer off the foil. The floating graphene and PMMA layer is then cleaned in water baths and transferred onto the prepared chips. Finally, the PMMA is removed with acetone. The graphene must then be patterned to cover only the channels on of the device. This requires another round of photolithography. Photoresists greatly contaminate the graphene, so to prevent this we placed a layer of boron nitride on top of the graphene. Boron nitride has a similar structure to graphene in that both are two dimensional and hexagonal lattices. Due to this, boron nitride can sit well on top of graphene as a protective layer. To grow boron nitride we also use chemical vapor deposition. Then the boron nitride is transferred by bubble transfer on top of the graphene on the chip. The graphene and boron nitride are then patterned together to define the channels and no photoresist touches the graphene. With the completion of these steps, we have created a graphene field effect transistor covered by boron nitride. Testing of the devices has shown that devices fabricated with boron nitride have lower Dirac points than those without. Dirac voltage shifts occur due to charged molecules on the surface of the graphene interfering with current. This means that the graphene was kept far cleaner through our technique. By combining boron nitride and graphene, we have improved our field-effect transistors and made them even more conductive by decreasing contamination in the fabrication steps. To expand upon our findings, we tested the effects of boron nitride on top of molybdenum disulfide FETs. Molybdenum disulfide (MoS2) is a semiconductor and a transition metal dichalcogenide which can be used in FETs in a similar way to graphene. By fabricating devices in a similar way to the procedure described above, we created another type of functioning FET. The success of these two different materials, graphene and molybdenum disulfide, as FETs when covered with BN indicates that boron nitride is a strong protective layer to decrease contamination on conductive materials. Future research include the integration of further metal dichalcogenides in G-FETs.
Funder Acknowledgement(s): NSF, EFRI, UPenn
Faculty Advisor: Charlie Johnson, email@example.com
Role: I fabricated the Graphene Field-Effect Transistors through photolithography, plasma etch, and gold thermal evaporation. I also grew the graphene and boron nitride through chemical vapor deposition, and then annealed and transferred it onto the FETs. Finally, I helped my PhD student with the analysis of the devices with the added layer of boron nitride as opposed to G-FETs with only graphene.