Terahertz imaging of inhomogeneous electrodynamics in single-layer graphene embedded in dielectrics

Abstract

We investigate electron transport properties in large-area, single-layer graphene embedded in dielectric media, using free-space terahertz (THz) imaging and time-domain spectroscopy. Sandwiched between a thin polymethyl methacrylate (PMMA) layer and a Si substrate, graphene layers of different growth recipes exhibit distinctive spatial inhomogeneity of sheet conductivity. The non-contacting, non-destructive THz probe reveals that the PMMA layer induces a small, yet noticeable reduction in conductivity. V

High-field terahertz response of graphene

“…A universal optical conductivity of πe 2 /(2h) due to interband transitions is predicted and observed for photon energies less than ∼1 eV and greater than twice the Fermi level E F (Fermi level measured relative to the charge neutrality point) [11]. The optical conductivity of graphene demonstrates a Drude-like frequency dependence in THz frequency range [13][14][15][16].…”

“…The graphene sheet was subsequently covered with a thin poly(methyl methacrylate) (PMMA) layer (thickness ∼100 nm) and transferred to a sample holder that allows the graphene-on-PMMA film to be suspended freely over 2 mm diameter holes. The free-standing graphene-PMMA thin film sample structure reduces parasitic substrate effects in two ways: (i) THz absorption and interference caused by the substrate are negligible and (ii) the single graphene-PMMA interface induces only small changes in the THz properties of graphene [16]. The PMMA is an uncharged polymer and contains much fewer charge traps than substrates like SiO 2 .…”

“…The large linear THz absorption by graphene (∼30%) at low intensities indicates that intraband transitions dominate the interactions of THz waves with graphene. From this, we obtained the sheet conductivity of the graphene sample (σ S ≈ 1.0 × 10 −3 −1 ) using the thin-film transmission coefficient based on the Drude model, where the graphene-PMMA layer was treated as an infinitely thin film [15,16]. Z 0 (376.7 ) is the vacuum impedance.…”

Robust mapping of electrical properties of graphene from terahertz time-domain spectroscopy with timing jitter correction

“…The non-contact approach is advantageous for in-line characterization and quality-control for industrial integration of graphene, especially when compared to standard field-effect measurements on graphene that require additional and intrusive fabrication steps for device processing. Non-gated rapid spatial mapping of the carrier drift mobility (µ drift ) and carrier density (N s ) of graphene by THz-TDS was recently reported [5], which presents a step forward compared to earlier reports that mainly gauges the conductivity [6,7] or extracts the mobility through back-gated THz-TDS measurements requiring special substrates [8,9].…”

Terahertz wafer-scale mobility mapping of graphene on insulating substrates without a gate

“…Even so, no viable means for assessing consistency in electronic properties across these length scales or area production rates are currently offered by the prevailing electronic characterization methods, typically based on field-effect or Hall mobility measurements in lithographically defined devices. Over the past five years, several demonstrations of non-contact techniques for characterization of graphene electrical properties have been reported [10][11][12][13][14][15]. Most techniques, however, gauge the graphene conductance rather than the other primary parameters of interest, which are the carrier mobility, µ, and sheet carrier density, Ns.…”