Terahertz transport dynamics of graphene charge carriers

The electronic transport dynamics of graphene charge carriers at femtosecond (10-15 s) to picosecond (10-12 s) time scales are investigated using terahertz (1012 Hz) time-domain spectroscopy (THz-TDS). The technique uses sub-picosecond pulses of electromagnetic radiation to gauge the electrodynamic response of thin conducting films at up to multi-terahertz frequencies.

In this thesis THz-TDS is applied towards two main goals; (1) investigation of the fundamental carrier transport dynamics in graphene at femtosecond to picosecond timescales and (2) application of terahertz time-domain spectroscopy to rapid and non-contact electrical characterization of large-area graphene, relevant for industrial integration.

We show that THz-TDS is an accurate and reliable probe of graphene sheet conductance, and that the technique provides insight into fundamental aspects of the nanoscopic nature of conduction in graphene films. This is demonstrated by experimental observation of diffusive transport as well as signatures of preferential back-scattering of carriers on a nanoscopic scale in poly-crystalline graphene, which may be related to reflections at crystal domain boundaries.

This is the first observation of preferential back-scattering of graphene charge carriers in THz-TDS measurements, and the results are expected to have a significant impact on the graphene and THz- TDS communities, as they may provide insight into the impact of nanoscopic morphology on the electrical conduction in poly-crystalline graphene.

Through THz-TDS measurements with accurate carrier density control by careful electrical back-gating, we find that terahertz conductance scales linearly with carrier density, consistent with charge transport limited by long-range scattering on charged impurities, which is also observed in most contact-based transport measurements.

By demonstrations of wafer-scale sheet conductance mapping and large-area field-effect mobility mapping, it is shown that the non-contact nature of THz-TDS measurements facilitates the rapid and reliable large-scale characterization of graphene electronic properties and their uniformity, that might be viewed as a vital requirement for industrial implementation of the material.

We find significant spatial variations in carrier mobility of a factor of 2-3 on a scale of just few millimeters, which highlights the importance of techniques that provide highly statistical or spatially resolved approaches to electronic characterization of large-area graphene. In a comparative study, we observe significant suppression of DC micrometer-scale transport, probed using micro four-point probe conductance mapping, relative to AC nanoscopic transport, probed by THz-TDS conductance mapping.

A detailed analysis of micro four-point probe, THz-TDS and Raman spectroscopy data reveals that the suppression of micrometer-scale conductance is a signature of electrical defects on the scale of 10 μm, giving rise to 1D-like micrometer-scale transport.