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

Abstract

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 (atmospheric pressure high temperature, high pressure high temperature, pressure controlled furnace) and scalable BN films deposited by either chemical or physical vapor deposition (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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mn>1</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:mi>ns</mml:mi> </mml:mrow> </mml:mrow> </mml:math> measured on as-grown hBN crystals by TRCL is sufficient to achieve high graphene room temperature charge carrier mobilities of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mn>80</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:mn>000</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:msup> <mml:mi>cm</mml:mi> <mml:mn>2</mml:mn> </mml:msup> <mml:mstyle scriptlevel="0"/> <mml:mo stretchy="false">(</mml:mo> <mml:mi>Vs</mml:mi> <mml:msup> <mml:mo stretchy="false">)</mml:mo> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> at a carrier density of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mo stretchy="false">|</mml:mo> <mml:mi mathvariant="normal">n</mml:mi> <mml:mrow> <mml:mo stretchy="false">|</mml:mo> </mml:mrow> <mml:mo>=</mml:mo> <mml:mn>1</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>12</mml:mn> </mml:mrow> </mml:msup> <mml:mstyle scriptlevel="0"/> <mml:msup> <mml:mi>cm</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> , while respective exciton lifetimes around <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mn>100</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:mi>ps</mml:mi> </mml:mrow> </mml:mrow> </mml:math> yield mobilities up to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mn>30</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:mn>000</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:msup> <mml:mi>cm</mml:mi> <mml:mn>2</mml:mn> </mml:msup> <mml:mstyle scriptlevel="0"/> <mml:mo stretchy="false">(</mml:mo> <mml:mi>Vs</mml:mi> <mml:msup> <mml:mo stretchy="false">)</mml:mo> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> . For scalable PVD-grown BN films, we measure carrier mobilities exceeding <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mn>10</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:mn>000</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:msup> <mml:mi>cm</mml:mi> <mml:mn>2</mml:mn> </mml:msup> <mml:mstyle scriptlevel="0"/> <mml:mo stretchy="false">(</mml:mo> <mml:mi>Vs</mml:mi> <mml:msup> <mml:mo stretchy="false">)</mml:mo> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> which correlates with a graphene Raman 2D peak linewidth of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mn>22</mml:mn> <mml:mstyle scriptlevel="0"/> <mml:msup> <mml:mi>cm</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:mrow> </mml:math> . 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.