Benchmarking the integration of hexagonal boron nitride crystals and thin films into graphene-based van der Waals heterostructures

TL;DR

This paper introduces a benchmarking protocol for evaluating boron nitride substrates in graphene heterostructures, linking optical and electronic properties to device performance, emphasizing the importance of Raman spectroscopy for interface quality assessment.

Contribution

It provides a comprehensive benchmarking method combining optical and electronic characterization of BN crystals and films, highlighting the role of Raman linewidth in predicting graphene mobility.

Findings

High graphene mobility (80,000 cm²/Vs) achieved with long exciton lifetime BN crystals.

Carrier mobility exceeds 10,000 cm²/Vs in PVD-grown BN films.

Raman 2D linewidth correlates with interface quality and charge mobility.

Abstract

We present a benchmarking protocol that combines the characterization of boron nitride (BN) crystals and films with the evaluation of the electronic properties of graphene on these substrates. Our study includes hBN crystals grown under different conditions and scalable BN films deposited by either chemical or physical vapor deposition (CVD or PVD). We explore the complete process from boron nitride growth, over its optical characterization by time-resolved cathodoluminescence (TRCL), to the optical and electronic characterization of graphene by Raman spectroscopy after encapsulation and Hall bar processing. Within our benchmarking protocol we achieve a homogeneous electronic performance within each Hall bar device through a fast and reproducible processing routine. We find that a free exciton lifetime of 1 ns measured on as-grown hBN crystals by TRCL is sufficient to achieve high graphene room temperature charge carrier mobilities of 80,000 cm$^2$/(Vs) at a carrier density of |n| = 10$^{12}$ cm$^{-2}$, while respective exciton lifetimes around 100 ps yield mobilities up to 30,000 cm$^2$/(Vs). For scalable PVD-grown BN films, we measure carrier mobilities exceeding 10,000 cm$^2$/(Vs) which correlates with a graphene Raman 2D peak linewidth of 22 cm$^{-1}$. Our work highlights the importance of the Raman 2D linewidth of graphene as a critical metric that effectively assesses the interface quality (i.e. surface roughness) to the BN substrate, which directly affects the charge carrier mobility of graphene. Graphene 2D linewidth analysis is suitable for all BN substrates and is particularly advantageous when TRCL or BN Raman spectroscopy cannot be applied to specific BN materials such as amorphous or thin films. This underlines the superior role of spatially-resolved spectroscopy in the evaluation of BN crystals and films for the use of high-mobility graphene devices.