[Paper Review] Method for Transferring High-Mobility CVD-Grown Graphene with Perfluoropolymers

Jianan Li, Jen‐Feng Hsu+11 more|arXiv (Cornell University)|Jun 28, 2016

Fiber-reinforced polymer composites3 citations

TL;DR

This paper presents a novel transfer method for high-mobility CVD-grown graphene using Hyflon, a perfluoropolymer, as a sacrificial handle and contamination barrier. The method enables graphene transfer onto LaAlO3/SrTiO3 heterostructures with atomically clean interfaces, achieving mobilities up to 30,000 cm²V⁻¹s⁻¹ at 2 K and clear quantized Hall and magneto-resistance features, while preserving local control of the interfacial metal-insulator transition.

ABSTRACT

The transfer of graphene grown by chemical vapor deposition (CVD) using amorphous polymers represents a widely implemented method for graphene-based electronic device fabrication. However, the most commonly used polymer, poly(methyl methacrylate) (PMMA), leaves a residue on the graphene that limits the mobility. Here we report a method for graphene transfer and patterning that employs a perfluoropolymer---Hyflon---as a transfer handle and to protect graphene against contamination from photoresists or other polymers. CVD-grown graphene transferred this way onto LaAlO$_3$/SrTiO$_3$ heterostructures is atomically clean, with high mobility (~30,000 cm$^2$V$^{-1}$s$^{-1}$) near the Dirac point at 2 K and clear, quantized Hall and magneto-resistance. Local control of the LaAlO$_3$/SrTiO$_3$ interfacial metal-insulator transition---through the graphene---is preserved with this transfer method. The use of perfluoropolymers such as Hyflon with CVD-grown graphene and other 2D materials can readily be implemented with other polymers or photoresists.

Motivation & Objective

• To address the persistent issue of polymer residue from PMMA during CVD graphene transfer, which degrades electron mobility.
• To develop a transfer method that minimizes contamination and preserves the electronic quality of graphene on complex oxide substrates.
• To enable high-mobility graphene devices on oxide heterostructures like LaAlO3/SrTiO3 while retaining local electrical control.
• To demonstrate compatibility of the Hyflon transfer method with standard lithography and patterning processes.
• To achieve atomically clean, high-mobility graphene for studying strong electronic correlations in 2D systems.

Proposed method

• A perfluoropolymer, Hyflon AD 60, is spin-coated onto CVD-grown graphene on copper substrates as a transfer handle and protective layer.
• Copper is dissolved in ammonium persulfate solution, releasing the Hyflon-graphene stack, which floats on water.
• The floating Hyflon-graphene is transferred onto a pre-patterned LaAlO3/SrTiO3 substrate, where it is carefully lifted and positioned.
• The Hyflon layer acts as a physical barrier, preventing contamination from photoresists or other polymers during subsequent processing.
• Post-transfer cleaning is achieved via contact-mode atomic force microscopy (AFM), which removes residual Hyflon and particles without damaging the graphene.
• Low-temperature transport measurements and piezoforce microscopy (PFM) are used to characterize mobility, quantum Hall effects, and local carrier density.

Experimental results

Research questions

• RQ1 Can a perfluoropolymer like Hyflon effectively replace PMMA as a transfer medium to reduce contamination and improve graphene mobility?
• RQ2 Does the Hyflon transfer method preserve the integrity of the LaAlO3/SrTiO3 interfacial 2D electron gas and its tunable metal-insulator transition?
• RQ3 Can high-mobility, atomically clean CVD graphene be achieved on complex oxide substrates without defect-inducing annealing?
• RQ4 To what extent does the Hyflon layer shield graphene from electrical and chemical contamination during lithography?
• RQ5 Can the combination of high-mobility graphene and tunable 2D electron gas in LaAlO3/SrTiO3 enable new quantum phenomena?

Key findings

• Graphene transferred using Hyflon achieved a mobility of up to 30,000 cm²V⁻¹s⁻¹ at 2 K, significantly higher than most wet-transferred CVD graphene.
• Clear quantization of Hall resistance and magnetoresistance was observed under a 5 T magnetic field, confirming the presence of quantum Hall states with filling factors ν = 4(N + 1/2).
• The Dirac point was located at +7 V at 2 K, indicating slight p-type doping, with hole carrier density reaching 6 × 10¹² cm⁻² at -15 V backgate voltage.
• Electrical shielding by graphene reduced the piezoelectric response of underlying nanowires by ~10–15%, confirming local carrier density modulation via PFM.
• Contact-mode AFM cleaning effectively removed Hyflon residues and particles, improving mobility by a factor of two at low temperature.
• The method preserved the ability to locally write conductive nanostructures in the LaAlO3/SrTiO3 interface using c-AFM, even with graphene on top.