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Here, we present the fabrication details of our CNT devices with large scale CVD grown monolayer hBN films as tunnel barriers. Figure 3.8 shows the schematic cross section view of our devices with tunnel contacts. A CNT with direct clean metal contacts (e.g. Pd) forms QDs of the size determined by the contact separation, or, in disordered CNTs, by defects and potential fluctuations. By placing hBN film as a tunnel barrier to the CNT, we expect to observe a large QD that is not confined by the metal contacts.

We use wet-etching [103] and electrochemical bubbling methods [110] to transfer the films onto the target substrate to form a tunnel barrier to the CNT.

(a) (b)

Figure 3.8. Schematic cross section view of CNT devices with hBN tunnel barriers.

(a) Three-terminal tunnel contacts to a CNT. (b) A three-terminal device with two tunnel contacts and one normal contact.

Sample preparation

The sample fabrication starts with patterning the target substrate with 5 nm/45 nm thick Ti/Au alignment markers by EBL. We use a heavily p-doped Si wafer that is capped either by 300 nm thick SiO2 or by 200 nm thick Si3N4. We stamp the CNTs onto the target substrate. A small amount of catalyst particles are stamped onto the target substrate as well. The presence of these catalyst particles nearby a CNT is problematic for placing atomically thin hBN tunnel barriers. An alternative approach is the direct growth of CNTs on a wafer substrate, which is prepatterned with 40 nm thick Re alignment markers. In practice, any metal that is compatible with the CNT growth conditions, especially the high growth temperature of ∼ 1000^◦C, can be used for the alignment markers. We note that SEM contrast of Re is much weaker than that of Ti/Au and makes it difficult to locate the CNTs. After locating the CNTs with respect to the alignment makers using SEM imaging, we transfer the hBN film from its growth substrate Cu or Fe foil onto the target substrate to cover the CNTs. We employ the wet-etching [103] and the electrochemical bubbling methods [110] to transfer the film, as described in Fig. 3.9 and Fig. 3.10, respectively. We select CNTs of several micrometers long and fabricate tunnel contacts to the CNTs by thermal evaporation of

∼ 20 nm thick Pd on the hBN film.

Wet-etching method

We adopt the wet-etching technique to transfer the CVD grown hBN film from its growth substrate Cu foil onto the Si/SiO₂ substrate. The wet-etching method is further

divided into the wet and dry transfer processes, as shown in Fig. 3.9. The hBN film, which is commercially available, is grown on both sides of the Cu foil, and we first describe the wet transfer process. We spin-coat ∼ 1 µm thick PMMA on one side of the Cu foil to protect the film and etch away the hBN film on the other side of the foil in a reactive ion etcher using CHF₃/O₂ plasma. Afterwards, the Cu foil is etched away by chemical wet-etching in a ∼ 2 % ammonium persulfate solution. The remaining hBN/PMMA complex is rinsed with distilled water to wash off the etchant residues. We place the hBN/PMMA complex onto the Si/SiO₂ substrate by fishing it out directly from the distilled water and let it dry in ambient conditions. A heat treatment is carried out to soften the PMMA, thereby increasing the adhesion between the film and the substrate. At the end, we immerse the sample in acetone to remove the PMMA. A thermal annealing in a furnace at 250^◦C with forming gas (H₂ and N₂) for 3 h is carried out to remove the PMMA residues.

(a) Wet transfer

Figure 3.9. Schematic illustration of the wet-etching method used for transferring the CVD grown hBN film from the Cu foil. (a) Wet and (b) dry transfer process.

The wet transfer process can lead to water layers being trapped between the hBN film and the substrate. In order to avoid this, we introduce the dry transfer process, where we use PDMS frame to support the hBN/PMMA complex. It allows us to remove the hBN/PMMA/PDMS complex from the etchant with tweezers and rinse it with distilled

water. We place the complex and the target substrate inside a glove box. The target substrate is first heated at 200^◦C for 12 h to remove the solvent residues from its surface and then cooled down to the room temperature. Afterwards, we place the complex onto the target substrate and slowly increase the hotplate temperature to 140^◦C while pressing hard on the PDMS frame with tweezers. The sample is then removed from the glove box and immersed in acetone to remove the PMMA and PDMS. A complete removal of PMMA is difficult to achieve [111]. Acetone and chloroform treatments of PMMA lead to contaminated hBN surfaces with polymer residues. Thermal annealing of the sample at temperatures in the range of 300 − 400^◦C in vacuum [112] or in an Ar/H₂ flow [113] and the removal of PMMA by catalytic activity of platinum (Pt) metals [114] appear to help, but they do not lead to ultra-clean hBN films.

Electrochemical bubbling method

For hBN films grown on Fe foils, we employ the electrochemical bubbling method based on the electrolysis of water between the layers to separate the materials. Here, the hBN film is grown on a 100 µm thick Fe foil by our collaborator [51]. A schematic illustration of the electrochemical bubbling transfer process is shown in Fig. 3.10. First, the foil is spin-coated with ∼ 1 µm thick PMMA. Afterwards, the hBN/Fe/hBN/PMMA complex and a Pt metal plate are placed in a 1 M NaOH aqueous solution and used as the cathode and anode of an electrolysis cell, respectively. The reaction of water reduction takes place at the negatively charged Fe foil to produce a large amount of H₂ bubbles at the interface between the film and the foil. This leads to the detachment of the hBN/PMMA complex from the Fe foil in few minutes. After rinsing with distilled water, a target substrate is dipped into the distilled water to fish out the floating hBN/PMMA complex. The sample is heated to increase the adhesion between the film and the substrate. PMMA is removed by immersing the sample in acetone and rinsing in

PMMA Fe Monolayer hBN

+

-PMMA Fe H₂ bubbles

distilled water PMMA

PMMA PMMA SiO₂

Si p⁺⁺

1 M NaOH

Pt

-Figure 3.10. Schematic illustration of the electrochemical bubbling method used for transferring CVD grown hBN film from the Fe foil.

IPA. In comparison to the wet-etching method, the electrochemical bubbling is faster, and the hBN film transferred using this method is free of metal residues. However, we find that the electrochemical bubbling method is aggressive enough to break the atomically thin hBN film, thereby creating a large amount of structure defects, such as holes and wrinkles.