[Paper Review] Transport spectroscopy of ultraclean tunable band gaps in bilayer graphene

E. Icking, Luca Banszerus+12 more|arXiv (Cornell University)|Jun 4, 2022

Graphene research and applications4 citations

TL;DR

This study demonstrates ultraclean, electrically tunable band gaps in bilayer graphene (BLG) using graphite-gated van der Waals heterostructures, achieving up to 120 meV band gaps with resistances exceeding 100 GΩ due to minimal disorder. Finite-bias transport spectroscopy confirms near-ideal semiconducting behavior, outperforming gold- and silicon-gated devices, enabling applications in tunable transistors, THz detectors, and spintronic devices.

ABSTRACT

The importance of controlling both the charge carrier density and the band gap of a semiconductor cannot be overstated, as it opens the doors to a wide range of applications, including, e.g., highly-tunable transistors, photodetectors, and lasers. Bernal-stacked bilayer graphene is a unique van-der-Waals material that allows tuning the band gap by an out-of-plane electric field. Although the first evidence of the tunable gap was already found ten years ago, it took until recent to fabricate sufficiently clean heterostructures where the electrically induced gap could be used to fully suppress transport or confine charge carriers. Here, we present a detailed study of the tunable band gap in gated bilayer graphene characterized by temperature-activated transport and finite-bias spectroscopy measurements. The latter method allows comparing different gate materials and device technologies, which directly affects the disorder potential in bilayer graphene. We show that graphite-gated bilayer graphene exhibits extremely low disorder and as good as no subgap states resulting in ultraclean tunable band gaps up to 120 meV. The size of the band gaps are in good agreement with theory and allow complete current suppression making a wide range of semiconductor applications possible.

Motivation & Objective

• To achieve and characterize ultraclean, electrostatically tunable band gaps in bilayer graphene (BLG) by minimizing disorder.
• To compare the impact of different gate materials—graphite, gold, and doped silicon—on band gap quality and transport properties.
• To demonstrate that graphite-gated BLG/hBN heterostructures exhibit near-ideal semiconducting behavior with minimal subgap states.
• To validate the tunable band gap against theoretical predictions using finite-bias transport spectroscopy.
• To enable high-performance semiconductor applications in BLG-based devices such as tunnel FETs, diodes, and spintronic devices.

Proposed method

• Fabricated double-gated bilayer graphene/hexagonal boron nitride (hBN) van der Waals heterostructures using graphite, gold, or doped silicon as bottom gates.
• Employed finite-bias transport spectroscopy to probe subgap states and disorder potentials, sensitive to hopping-transport through localized states.
• Conducted temperature-activated transport measurements to extract band gap energies and assess thermal activation behavior.
• Used the displacement field (D-field) to tune the band gap and correlate it with measured resistance and conductance suppression.
• Compared device performance across different gate materials by analyzing conductance diamonds and resistance maxima in the gapped regime.
• Validated results against theoretical predictions for ideal BLG, focusing on band gap scaling with D-field and absence of tail states.

Experimental results

Research questions

• RQ1 How does the choice of bottom gate material (graphite, gold, Si/SiO2) affect the quality of the tunable band gap in bilayer graphene?
• RQ2 To what extent can the band gap in bilayer graphene be tuned and suppressed to achieve a true insulating state in transport?
• RQ3 What is the role of disorder and subgap states in limiting the performance of gated bilayer graphene devices?
• RQ4 How does finite-bias spectroscopy enable the detection and quantification of potential disorder and impurity states in BLG?
• RQ5 To what degree do graphite-gated BLG/hBN heterostructures match theoretical predictions for ideal semiconducting behavior?

Key findings

• Graphite-gated bilayer graphene/hBN heterostructures achieve band gaps of up to 120 meV, in excellent agreement with theoretical predictions.
• Maximum resistance values exceeding 100 GΩ were measured within the band gap, limited only by the experimental setup, indicating near-complete current suppression.
• No appreciable subgap states or trap states were observed in graphite-gated devices, confirming ultraclean transport and minimal disorder.
• Finite-bias spectroscopy revealed strong conductance suppression diamonds in graphite-gated devices, indicating robust band insulating behavior.
• Gold- and silicon-gated devices exhibited signatures of disorder-induced tail states, as evidenced by conductance anomalies and reduced resistance contrast.
• The graphite-gated technology outperforms other gate materials, enabling device performance closest to ideal theoretical BLG behavior.