[Paper Review] Realization of graphene logics in an exciton-enhanced insulating phase

Kaining Yang, Xiang Gao+16 more|arXiv (Cornell University)|Oct 6, 2021

Graphene research and applications37 references4 citations

TL;DR

This study demonstrates a robust excitonic insulating phase in bilayer graphene by interfacing it with the anti-ferromagnetic insulator CrOCl, enabling a controllable band gap with high mobility in the 'ON' state. The system achieves ON-OFF ratios exceeding 10⁷ and realizes a functional CMOS-like graphene inverter, marking a critical advance toward carbon-based logic computing.

ABSTRACT

For two decades, two-dimensional carbon species, including graphene, have been the core of research in pursuing next-generation logic applications beyond the silicon technology. Yet the opening of a gap in a controllable range of doping, whilst keeping high conductance outside of this gapped state, has remained a grand challenge in them thus far. Here we show that, by bringing Bernal-stacked bilayer graphene in contact with an anti-ferromagnetic insulator CrOCl, a strong insulating behavior is observed in a wide range of positive total electron doping $n_\mathrm{tot}$ and effective displacement field $D_\mathrm{eff}$ at low temperatures. Transport measurements further prove that such an insulating phase can be well described by the picture of an inter-layer excitonic state in bilayer graphene owing to electron-hole interactions. The consequential over 1 $\mathrm{GΩ}$ excitonic insulator can be readily killed by tuning $D_\mathrm{eff}$ and/or $n_\mathrm{tot}$, and the system recovers to a high mobility graphene with a sheet resistance of less than 100 $\mathrmΩ$. It thus yields transistors with "ON-OFF" ratios reaching 10$^{7}$, and a CMOS-like graphene logic inverter is demonstrated. Our findings of the robust insulating phase in bilayer graphene may be a leap forward to fertilize the future carbon computing.

Motivation & Objective

• To overcome the fundamental challenge of achieving a controllable, wide-range band gap in graphene while preserving high mobility in the conducting state.
• To realize a stable, tunable excitonic insulating phase in bilayer graphene through strong electron-hole interactions induced by proximity to CrOCl.
• To demonstrate practical graphene-based logic devices, including a CMOS-like inverter, by leveraging the sharp transition between metallic and insulating states.
• To establish a new platform for carbon-based electronics by exploiting excitonic correlations in 2D van der Waals heterostructures.

Proposed method

• Fabricated h-BN/BLG/CrOCl heterostructures using mechanical exfoliation and van der Waals assembly techniques.
• Employed dual-gate field-effect measurements at 1.5 K to tune total electron doping (n_tot) and effective displacement field (D_eff), enabling control over the insulating phase.
• Used AC lock-in and DC Ohm meter techniques to map electrical transport properties in the D_eff–n_tot parameter space.
• Applied Landau level quantization analysis and theoretical modeling to extract effective mass (m* ≈ 0.037mₑ) and Coulomb interaction parameters.
• Developed a theoretical model based on inter-layer excitonic condensation, incorporating chemical potential (μ), charge density (n_I), and capacitance (C_0, C_eff).
• Derived key equations (e.g., Eq. 8) linking gate voltages (V₁, V₂) to chemical potential and effective field, explaining the observed phase boundary and built-in field enhancement.

Experimental results

Research questions

• RQ1 Can an excitonic insulating phase be stabilized in bilayer graphene via proximity coupling to a magnetic insulator?
• RQ2 Does the resulting insulating state exhibit a tunable, wide-range band gap while preserving high carrier mobility in the metallic phase?
• RQ3 Can the transition between metallic and insulating states be exploited to achieve high ON-OFF ratios in graphene transistors?
• RQ4 What is the role of inter-layer Coulomb interactions and electron-hole pairing in forming the observed excitonic insulator?
• RQ5 To what extent can the built-in electric field in the heterostructure enhance the effective band gap beyond conventional electrostatic gating?

Key findings

• A robust excitonic insulating phase with resistivity exceeding 1 GΩ was observed over a wide range of positive n_tot and D_eff at 1.5 K.
• The insulating behavior is attributed to inter-layer excitonic condensation, confirmed by transport measurements and Landau level quantization analysis.
• The system exhibits a sharp transition from the insulating to high-mobility metallic state upon tuning D_eff or n_tot, enabling ON-OFF ratios exceeding 10⁷.
• The effective mass of charge carriers in bilayer graphene was estimated as m* ≈ 0.037mₑ from cyclotron resonance analysis.
• A built-in electric field of ~10 V/nm was inferred from the gate-voltage dependence, far exceeding conventional dielectric limits.
• A functional CMOS-like graphene inverter was experimentally demonstrated, validating the feasibility of graphene-based logic circuits.