Terahertz time-domain spectroscopy to characterize graphene nanostructures for new optoelectronic applications

Resumen

Terahertz electromagnetic waves from 0.1 to 30 THz, bounded between the infrared and microwave regions of the spectrum, has been intensively attracted to explore scientific and engineering phenomena for the materials. The graphene, a single layer of carbon atoms in a hexagonal lattice with zero energy band gap, and the carbon nanotube, sheet of graphene rolled into a cylinder, have been widely recognized as the perfect options for next generation ultrafast high performance optoelectronic applications to operate at the sub-THz and THz frequencies. Measuring the electrical conductivity and carriers' responses of such very thin conductance materials in devices could be rather difficult because of the contact issue. The terahertz time-domain spectroscopy (THz-TDS) is a noncontact tool to measure the optical and electrical parameters of the nanometrics semiconductors/semimetals. This work analysis the THz-TDS signals from the references and samples measurement. It studies the THz frequency-dependent of the electrical and optical properties of the nanostructure graphene-like materials from the transmission and reflection of a home-made THz-TDS, mainly in the frequency range of 0.1-2 THz. The DC conductivity could not be directly determined, as the available THz power decreases sharply and falls below the noise level at lower frequencies. The electrical and optical frequency-dependent parameters are rather noisy due to thickness of thin films compared to the thick substrates and also the limited sensitivity of the THz-TDS set-up over 1.5 THz. In this thesis results obtained, for the first time, on the characterization and spectroscopic analysis of the graphene-like samples deposited on the top of the quartz substrate, not only at the THz frequencies but also in the zero-frequency, so-achieved DC level, and far-infrared regime are presented. The conductivity values of such fragile materials are extracted from noncontact measurements at THz frequencies using elaborated conductivity Drude and non-Drude models which nicely fit the experimental data, giving information on the physics of electrical conductivity in these materials. The carrier’s transport, scattering and density near the Dirac point of graphene and carbon nanostructure are extracted by combining the measures THZ with the Drude models/non-Drude conductivity. This procedure is validated by the good agreement between the extracted DC conductivity from the THz measurements and the micrometer classical four-point probe in-line contact ones. The extrapolated characteristic length from THz measurement enables us to predict the cut-off frequency of such materials before applying to the optoelectronic devices. This thesis presents a commercially relevant application of noncontact THz-TDS techniques and analysis for the electrical characterization of nanoscale semiconductors and polymers with high mobility in the new generation of optoelectronic devices.