Resolving mobility anisotropy in quasi-free-standing epitaxial graphene by terahertz optical Hall effect

Nerijus Armakavicius,Philipp Kühne,Jens Eriksson,Chamseddine Bouhafs,Vallery Stanishev,Ivan G Ivanov,Rositsa Yakimova,Alexei A Zakharov,Ameer Al-Temimy,Camilla Coletti,Mathias Schubert,Vanya Darakchieva

doi:10.1016/j.carbon.2020.09.035

Abstract

In this work, we demonstrate the application of terahertz-optical Hall effect (THz-OHE) to determine directionally dependent free charge carrier properties of ambient-doped monolayer and quasi-free-standing-bilayer epitaxial graphene on 4H–SiC(0001). Directionally independent free hole mobility parameters are found for the monolayer graphene. In contrast, anisotropic hole mobility parameters with a lower mobility in direction perpendicular to the SiC surface steps and higher along the steps in quasi-free-standing-bilayer graphene are determined for the first time. A combination of THz-OHE, nanoscale microscopy and optical spectroscopy techniques are used to investigate the origin of the anisotropy. Different defect densities and different number of graphene layers on the step edges and terraces are ruled out as possible causes. Scattering mechanisms related to doping variations at the step edges and terraces as a result of different interaction with the substrate and environment are discussed and also excluded. It is suggested that the step edges introduce intrinsic scattering in quasi-free-standing-bilayer graphene, that is manifested as a result of the higher ratio between mean free path and average terrace width parameters. The suggested scenario allows to reconcile existing differences in the literature regarding the anisotropic electrical transport in epitaxial graphene.

Highlights

Since its first experimental demonstration, graphene continues to attract vast attention as a consequence of its exceptional properties and rich physics that arise from the reduced dimensionality [1,2]
We demonstrate the application of terahertz-optical Hall effect (THz-OHE) to determine directionally dependent free charge carrier properties of ambient-doped monolayer and quasi-freestanding-bilayer epitaxial graphene on 4HeSiC(0001)
Despite the increased free charge carrier density in QFS-BL epitaxial graphene (EG), the hole mobility parameters are larger by a factor of two compared to those in ML EG as a result of reduced interaction with the substrate

Summary

Introduction

Since its first experimental demonstration, graphene continues to attract vast attention as a consequence of its exceptional properties and rich physics that arise from the reduced dimensionality [1,2]. The technology has free charge carrier mobility parameters as compared to exfoliated graphene [14,15]. The interaction with the underlying substrate renders SiC surface morphology to be an important factor for the electronic and transport properties of EG. It was reported that the step-like surface morphology of SiC can cause anisotropic carrier scattering in ML EG [17]. Such anisotropic transport in EG may significantly limit its applications in electronic devices

Methods

Results

Conclusion