Defect-induced Fermi level pinning and suppression of ambipolar behaviour in graphene

Abstract

We report on systematic study of electronic transport in low-biased, disordered graphene nanowires. We reveal the emergence of unipolar transport as the defect concentration increases beyond 0.3% where an almost insulating behaviour is observed on n-type channels whilst a metallic behaviour is observed in p-type channels. The conductance shows a plateau that extends through the entire side above the Dirac point (n-type) and the conductivity coincides with the minimum conductivity at the Dirac point. The minimum conductivity decreases with increasing defect concentration pointing out towards the absence of zero energy modes in the disordered samples. Raman spectroscopy and X-ray photoemission spectroscopy were used to probe the nature of the defects created by helium ion irradiation and revealed the presence of oxygen-carbon bonds as well as the presence of sp 3 configuration uncovered from the C KLL Auger spectrum. The observed behaviour is attributed to the dangling bonds created by sputtering of carbon atoms in graphene lattice by bombarding helium ions. The dangling bonds act as charge traps, pinning the Fermi level to the Dirac point.

Electrical Conductance of Graphene with Point Defects

“…The concentration of point defects (c) is defined as the number of point defects in the scattering region divided by the width (w) of the scattering region perpendicular to the transport direction. The considered defect concentration in this work is 0.29-0.78 nm −1 , falling in the range of the experimental values (0.10-0.59 nm −1 ) 21 . We also examined graphene with even lower defect concentration c = 0.23 nm −1 , which gives very similar transmission spectra as that of c = 0.29 nm −1 (see Fig.…”

“…Atomic defects in graphene can lead to sublinear dependence of conductivity on carrier density, distinct from the linear dependency observed for charge impurities 20 . As the defect concentration increases, an almost insulating behavior was observed for n-type conduction while a metallic behavior was observed for p-type conduction, the conductance showed a plateau above the Dirac point and suppression of ambipolar behavior in graphene 21 . Lattice defects can also cause significant intervalley scattering, giving rise to constant mobility and insulating temperature dependence of conductivity, both are much lowered than those of graphene with charge impurities 22 .…”

Evidence of Fermi level pinning at the Dirac point in epitaxial multilayer graphene

“…On the other hand, several theoretical works have studied the electronic structure of graphene in the direct vicinity of the Dirac point and predicted the existence of a finite density of states at the Dirac point in the presence of vacancies [9,10]. Such localized states at zero energy, also called mid-gap states, can act as charge traps and play an important role on the Fermi level position [11].…”

Local hole doping concentration modulation on graphene probed by tip-enhanced Raman spectroscopy

“…Because of its excellent mechanical and electrical properties, graphene has been expected for various applications, such as nanoelectromechanical systems [3], flexible thin film transistors [4], and radio frequency transistors [5]. It is known that electrical characteristics of graphene are influenced by defects [6], strain [7,8], and doping concentration modulation [9][10][11][12]. As those could be induced either intrinsically in the growth of graphene or extrinsically during the device fabrication process, a method to evaluate the quality of the graphene with high spatial resolution down to nanoscale must be very useful for detailed analysis of graphene nanoelectronic devices.…”

“…Raman spectroscopy has been extensively used as a non-destructive method to obtain various properties in graphene [13], such as the number of layers [14], edge [15], disorder [16], defect [6,17,18], strain [19][20][21][22][23][24], and doping [24][25][26][27]. However, conventional Raman spectroscopy only gives the averaged signal over the microscale laser spot size and its spatial resolution is limited by laser diffraction.…”