In-situ terahertz optical Hall effect measurements of ambient effects on free charge carrier properties of epitaxial graphene

Sean Knight,Vallery Stanishev,Vanya Darakchieva,Philipp Kühne,Tino Hofmann,Nerijus Armakavicius,Rositsa Yakimova,Ivan G Ivanov,Mathias Schubert,Shawn Wimer,Chamseddine Bouhafs

doi:10.1038/s41598-017-05333-w

Abstract

Unraveling the doping-related charge carrier scattering mechanisms in two-dimensional materials such as graphene is vital for limiting parasitic electrical conductivity losses in future electronic applications. While electric field doping is well understood, assessment of mobility and density as a function of chemical doping remained a challenge thus far. In this work, we investigate the effects of cyclically exposing epitaxial graphene to controlled inert gases and ambient humidity conditions, while measuring the Lorentz force-induced birefringence in graphene at Terahertz frequencies in magnetic fields. This technique, previously identified as the optical analogue of the electrical Hall effect, permits here measurement of charge carrier type, density, and mobility in epitaxial graphene on silicon-face silicon carbide. We observe a distinct, nearly linear relationship between mobility and electron charge density, similar to field-effect induced changes measured in electrical Hall bar devices previously. The observed doping process is completely reversible and independent of the type of inert gas exposure.

Highlights

Two dimensional materials are a new class of materials that attracted significant interest due to their unique electronic, optical, and mechanical properties
We report on the first in-situ, contactless determination of the majority free charge carrier type, Ns, and μ using terahertz-frequency optical Hall effect measurements (THz-OHE)[18,19,20]
Ellipsometric data are measured at oblique incidence as a function of time for various gas exposure phases

Summary

Introduction

Two dimensional materials are a new class of materials that attracted significant interest due to their unique electronic, optical, and mechanical properties. Ambient effects on the free charge carrier density and mobility of epitaxial graphene grown by Si-sublimation on the Si-face (0001) of 4H-SiC is studied as an example[12]. This type of graphene typically exhibits n-type conductivity due to a complex interaction with the Si-face SiC substrates via the buffer layer[13, 14]. Previous studies of the electrical properties of epitaxial graphene on SiC as a function of the ambient conditions employ contact-based techniques or Kelvin probe approaches and typically do not report changes in Ns and μ simultaneously.

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Results

Conclusion