Fast pick up technique for high quality heterostructures of bilayer graphene and hexagonal boron nitride

Abstract

We present a fast method to fabricate high quality heterostructure devices by picking up crystals of arbitrary sizes. Bilayer graphene is encapsulated with hexagonal boron nitride to demonstrate this approach, showing good electronic quality with mobilities ranging from 17 000 cm 2 V −1 s −1 at room temperature to 49 000 cm 2 V −1 s −1 at 4.2 K, and entering the quantum Hall regime below 0.5 T. This method provides a strong and useful tool for the fabrication of future high quality layered crystal devices.A critical step for high mobility graphene device fabrication and the rising field of van der Waals heterostructures[1] is marked by the development of polymer based dry transfer methods for two-dimensional (2D) crystals. [2][3][4][5] With these methods, high quality graphene devices on hexagonal boron nitride (h-BN) and more complicated stacks have become generally accessible, but the early methods face a major setback. The method of stacking the crystals one by one typically leaves each transferred crystal contaminated by polymer. To obtain a high quality device, thorough cleaning is required before proceeding with device fabrication or measurement. This cleaning step typically involves several hours of annealing [3][4][5] or it may go as far as nanobrooming the entire graphene flake using contact mode atomic force microscopy (AFM). [6,7] Altogether this makes the fabrication of a multilayer heterostructure not only very time consuming, but also risky as each step may again introduce contaminants to the stack. This issue has recently been overcome by L. Wang et al. [8], introducing a method that allows for polymer free assembly of layered materials based on van der Waals force. Instead of depositing a 2D crystal, e.g. h-BN, directly on top of another crystal, e.g. graphene, one can use the h-BN to pick up the graphene from the substrate. This can be done because the van der Waals force between the atomically flat h-BN and graphene is stronger than between the graphene or the h-BN and the rough SiO 2 substrate. The power of this method lies in the fact that now the interface between the two crystals has not been contaminated by polymer, and one can directly pick up the next crystal. This way the materials inside the stack not only remain much cleaner, but a stack can also be fabricated considerably faster. One problem when using this method is the reduced capability to pick up graphene flakes larger than the used top h-BN crystal. Therefore one has to etch through the stack before making one-dimensional (1D) contacts to the graphene. While resulting in very high quality devices with electron mean free paths up to 21 µm and good electrical * pj.zomer@gmail.com contact, [8] this limitation can be problematic for certain device types for which 1D contacts are not desirable, e.g. spintronic devices that include tunnel barriers at the contact interface. [9] In this letter we present a method which allows for fabrication of high quality graphene devices encapsulated in h-BN by successively picking up crystals. The a...

Helical edge states and fractional quantum Hall effect in a graphene electron–hole bilayer

“…All studied devices consist of two monolayer graphene flakes stacked together using a dry transfer process (15,16). The stacking results in a rotational misalignment, or twist, between the layers.…”

“…2. Flakes are picked up and transferred using a transparent polymer stamp made from either polypropylene carbonate (PPC) (15) or polycarbonate (PC) (16). The top hBN crystal is picked up first using the stamp, and then subsequently the hBN is used to pick up two graphene flakes and then the bottom hBN.…”

“…In a moderate magnetic field, this electronhole bilayer will develop quantum Hall edge states with opposite chiralities in each layer. Here, we demonstrate a graphene electron-hole bilayer, which we use to realize a helical 1-dimensional conductor made from quantum Hall edge states.All studied devices consist of two monolayer graphene flakes stacked together using a dry transfer process (15,16). The stacking results in a rotational misalignment, or twist, between the layers.…”

“…Our preliminary results with new fabrication techniques indicate that similar fractional quantum Hall states can be realized in twisted bilayer graphene at much lower magnetic fields ( < 9 T), which bodes well for future attempts to achieve a fractional quantum spin Hall state in this system (6-8). Correspondence to: pjarillo@mit.edu Fabrication Methods All twisted bilayer graphene devices were made using a dry-transfer process (15,16) to create a van der Waals heterostructure consisting of hBN-graphene-graphene-hBN layers. The devices all are dual-gated with a top and bottom gate electrode.…”

Nanosecond spin relaxation times in single layer graphene spin valves with hexagonal boron nitride tunnel barriers

“…al. 24 In short, a polydimethylsiloxane (PDMS) stamp coated with polycarbonate (PC) polymer is attached to a glass slide and is brought in contact with an exfoliated h-BN flake. Next, the PC film is heated to 70 °C to pick-up the h-BN flake from the SiO 2 surface.…”