Direct Measurement of Dirac Point Energy at the Graphene/Oxide Interface

Abstract

ABSTRACT:We report the direct measurement of the Dirac point, the Fermi level, and the work function of graphene by performing internal photoemission measurements on a graphene/SiO 2 /Si structure with a unique optical-cavity enhanced test structure. A complete electronic band alignment at the graphene/SiO 2 /Si interfaces is accurately established. The observation of enhanced photoemission from a one-atom thick graphene layer was possible by taking advantage of the constructive optical interference in the SiO 2 cavity. The photoemission yield was found to follow the well-known linear density-of-states dispersion in the vicinity of the Dirac point. At the flat band condition, the Fermi level was extracted and found to reside 3.3 eV ± 0.05 eV below the bottom of the SiO 2 conduction band. When combined with the shift of the Fermi level from the Dirac point, we are able to ascertain the position of the Dirac point at 3.6 eV ± 0.05 eV with respect to the bottom of the SiO 2 conduction band edge, yielding a work function of 4.5 eV ± 0.05 eV which is in an excellent agreement with theory. The accurate determination of the work function of graphene is of significant importance to the engineering of graphene-based devices, and the measurement technique we have advanced in this Letter will have significant impact on numerous applications for emerging graphene-like 2-dimensional material systems. KEYWORDS: Graphene, work function, internal photoemission, band alignment, graphene−insulator−semiconductor S ince the pioneering work of Novoselov et al. in 2004, 1 graphene has attracted an immense amount of interest from many related disciplines.2,3 Fundamental knowledge of the physical properties of graphene and the physical mechanisms governing the electrical operation of graphenebased devices has grown dramatically. 4 With the recent success of large area chemical vapor deposition (CVD) growth of graphene, 5 industrial applications such as transparent electrodes, 6 field-effect transistors (FETs), 7 and quantum well devices 8 are becoming more promising. Many studies have been conducted to characterize the various physical properties of graphene, including the work function, which is one of the most important electronic parameters. Among the numerous investigations by techniques such as Kelvin probe measurements, 9−11 ab initio calculations, 9,12 and recently by capacitance−voltage measurements, 13 the values of work function scatter in a rather wide range from 4.2 to 5.0 eV. Experimentally, the work function may be found to vary depending on the type of metal contact due to interactions between the graphene and the metal, which may result in pinning of the work function.9−13 Surprisingly, there is little information on the intrinsic electronic band alignment of the graphene/oxide interface to date, despite its important role in the design, fabrication, and characterization of graphene-based devices. For example, the accurate band alignment between graphene and another material determines how effectively to turn on an...

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