Optimized Fabrication Procedure for High-Quality Graphene-based Moiré Superlattice Devices

Summary

This article presents an optimized, experience-informed protocol for fabricating high-quality graphene-based moiré superlattice devices with precise twist angles, utilizing a modified dry transfer technique based on a highly tunable, custom-built transfer setup.

Abstract

Moiré superlattices constitute a versatile platform to investigate emergent phenomena arising from the interplay of strong correlations and topology, while offering flexible in situ tunability. However, the fabrication of such moiré superlattices is challenging. It is difficult to achieve highly uniform devices with a precise twist angle because of the unintentional introduction of heterostrain, twist angle disorder, and angle/lattice relaxation during the nanofabrication process. This article introduces an optimized, experience-informed protocol for fabricating high-quality graphene-based moiré superlattice devices, focusing on a modified dry transfer technique. The transfer process is performed in a highly tunable, custom-built transfer setup that enables precise position, angle, and temperature control. By combining rigorous flake selection criteria, pre-cleaned bubble-free bottom gates, and graphene laser ablation, the moiré superlattice is constructed by deliberately overlaying twisted graphene flakes at a submicron speed at room temperature. Through precise control of the transfer process, the resulting graphene moiré superlattice devices exhibit high uniformity and desired twist angles. This optimized protocol addresses existing challenges in the fabrication of graphene-based moiré superlattice devices and paves the way for further advances in the rapidly evolving field of moiré materials.

Introduction

A key driving force of modern condensed matter physics is the combination of strong correlations and topology into a single tunable physical system. Since the discovery of correlated insulator states¹ and superconductivity² in magic-angle twisted bilayer graphene (MATBG), its remarkable properties have stimulated a growing number of studies on moiré materials³. In recent years, a variety of graphene-based moiré superlattices have been constructed and investigated. This rapidly growing field has revealed a wealth of emergent phenomena, including but not limited to ex....

Protocol

The materials and instruments used throughout the protocol can be found in the Table of Materials.

1. Preparation of two-dimensional (2D) material flakes

1. Graphene exfoliation
   1. Cut a Si/SiO₂ wafer into 2 cm × 2 cm pieces using a tungsten carbide scriber. Clean the wafer surface afterward with a nitrogen spray gun to remove silicon dust.
      NOTE: Wafers with 285 nm thick SiO₂ are typically used for graphene and hBN exfoliation, as this thickness provides a good color contrast for visual identification of 2D materials.
   2. Preheat the hot plate to 250 °C, ....

Results

Low temperature transport measurements were performed to characterize the graphene moiré superlattice devices. Starting with MATBG as an example, the typical Landau fan diagram of a high-quality magic-angle device is shown in Figure 8A²². The filling factor of the flat bands ν = 4n/n_s describes the carrier density of the system, where n is the carrier density and n_s =

Discussion

Imposing a small twist angle between graphene layers can introduce significant disorder within the heterostructure. Therefore, to maximize the success rate in fabricating high-quality graphene moiré superlattice devices, a highly tunable transfer setup with precise control over position, angle, and temperature is essential. Additionally, several critical aspects during the fabrication process are identified: rigorous flake selection criteria, pre-cleaned bubble-free bottom gates, graphene laser ablation, and, most i.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Bench Top Vibration Isolation Platform	Minus K Technology, Inc	100BM-8	Vibration isolation for the custom-built transfer setup
Color camera	Teledyne FLIR LLC	BFS-U3-200S6C-C	CMOS camera for the custom-built transfer setup
Diaphragm pump	Pfeiffer Vacuum	MVP 015-2	Provide vacuum suction power to secure the wafer on the sample stage
Direct drive rotation stage	Thorlabs, Inc	DDR 100	Rotation stage for the custom-built transfer setup
Ergonomic high-power microscope	Mitutoyo	FS70	Microscope body for the custom-built transfer setup
Goniometer stage	Thorlabs, Inc	GNL 18 and GNL 10	Control the tilt angle of the glass slide
Graphite crystals	NGS Trading & Consulting GmbH	Flaggy Flakes	Flat and shiny graphite crystals for graphene exfoliation
High power supercontinuum fiber laser	Fianium, Inc	SC-400	For graphene laser ablation
Micro linear actuators with built-in controllers	Zaber Technologies	X-NA08A25	Control the X-Y sample stage
Mitutoyo 10x M Plan APO Objective	Mitutoyo	378-803-3	10x objective used on the transfer stage
Mitutoyo 2x M Plan APO Objective	Mitutoyo	378-801-12	2x objective used on the transfer stage
Mitutoyo 50x M Plan APO Objective	Mitutoyo	378-805-3	50x objective used on the transfer stage
Motorized actuator	Zaber Technologies	TRB12CC	Control the X-Y-Z glass slide holder stage
Poly(Bisphenol A carbonate)	MilliporeSigma	181641-25G	PC particles for making glass slides with PC/PDMS stamps
Prime Grade Si wafers	NOVA Electronic Materials	FP02-61160-DO	P-doped Si wafer for 2D material exfoliation. Detailed description: 6" P <100> .001-.005 ohm-cm 650-700μm Thick Prime Grade SSP Si wafers w/Primary Flat Only & 2850 A°±5% Dry Thermal Oxide
Silicone-free blue adhesive plastic film	Ultron Systems, Inc	1005R/1009R	Blue tape for exfoliating 2D materials
SYLGARD 184 Silicone Encapsulant Clear	Dow Inc.	4019862	Make PDMS stamp
Tapping Mode AFM Probe with Aluminum Reflective Coating	Innovative Solutions Bulgaria Ltd	Tap300Al-G	Tapping mode tip used for tip-cleaning
XY manual linear stage	Newport Corporation	M-462-XY	X-Y sample stage for the custom-built transfer setup
XYZ manual linear stage	Newport Corporation	M-562-XYZ	X-Y-Z glass slide holder stage for the custom-built transfer setup