Characteristics of nanocomposites and semiconductor heterostructure wafers using THz spectroscopy

All optical, THz-Time Domain Spectroscopic (THz-TDS) methods were employed towards determining the electrical characteristics of Single Walled Carbon Nanotubes, Ion Implanted Si nanoclusters and Si1-xGex HFO2, SiO2 on p-type Si wafers. For the nanoscale composite materials, Visible Pump/THz Probe spectroscopy measurements were performed after observing that the samples were not sensitive to the THz radiation alone. The results suggest that the photoexcited nanotubes exhibit localized transport due to Lorentz-type photo-induced localized states from 0.2 to 0.7THz. The THz transmission is modeled through the photoexcited layer with an effective dielectric constant described by a Drude + Lorentz model and given by Maxwell-Garnett theory. Comparisons are made with other prevalent theories that describe electronic transport. Similar experiments were repeated for ion-implanted, 3-4nm Si nanoclusters in fused silica for which a similar behavior was observed. In addition, a change in reflection from Si1-xGex on Si, 200mm diameter semiconductor heterostructure wafers with 10% or 15% Ge content, was measured using THz-TDS methods. Drude model is utilized for the transmission/reflection measurements and from the reflection data the mobility of each wafer is estimated. Furthermore, the effect of high-K dielectric material (HfO2) on the electrical properties of p-type silicon wafers was characterized by utilizing non-contact, differential (pump-pump off) spectroscopic methods to differ between HfO2 and SiO2 on Si wafers. The measurements are analyzed in two distinct transmission models, where one is an exact representation of the layered structure for each wafer and the other assumed that the response observed from the differential THz transmission was solely due to effects from interfacial traps between the dielectric layer and the substrate. The latter gave a more accurate picture of the carrier dynamics. From these measurements the effect of interfacial defects on transmission and mobility are quantitatively discussed

Probing the momentum relaxation time of charge carriers in ultrathin semiconductor layers

We report on a terahertz time-domain technique for measuring the momentum relaxation time of charge carriers in ultrathin semiconductor layers. The phase sensitive modulation technique directly provides the relaxation time. Time-resolved THz experiments were performed on n-doped GaAs and show precise agreement with data obtained by electrical characterization. The technique is well suited for studying novel materials where parameters such as the charge carriers' effective mass or the carrier density are not known a priori

Dynamics of quantum materials at the nanoscale

Programming the properties of quantum materials on demand is a central goal of condensed matter physics with the potential to usher in a new technological era. Photoexcitation has proven to be an exceptionally capable means of resonant and non-resonant control over matter offering coveted routes to selectively control the electronic, lattice, interband or valley optical and excitonic properties of quantum materials. One major limitation of probing the rich class of phenomena enabled by photoexcitation is the diffraction limit. The properties of quantum materials are often sensitive to the microscopic details of the environment at phase transition boundaries: which naturally leads drastic inhomogeneity at the nanoscale. In other cases, the media may transiently support high-momentum “nano-light” or host topologically protected conductive channels that are localized to one-dimensional physical edges. All of these phenomena demand a probe with the spatial resolution that is commensurate with the emergent behavior. To address these demands the author contributed to the development of time-resolved scattering near-field optical microscopy (Tr-SNOM). Utilizing the principles developed as part of this thesis amplified laser technology was combined with a commercial near-field optical microscope to produce a state-of-the-art time-resolved nanoscope. The custom apparatus operates with twenty nanometer spatial resolution with unprecedented spectral coverage spanning visible to mid-infrared all with (30-300) femtosecond temporal resolution. The experimental apparatus was, first, applied to investigate the photo-induced insulator-to-metal transition in Vanadium Dioxide. We observe nanoscale inhomogeneity of the transient conductivity. Our data reveals that local nanoscopic variations of the strain exist in our particular VO2 thin film at equilibrium. Regions of compressive strain are, furthermore, found to correlate with regions where a high degree of transient conductivity is attained. Our systematic study of the local fluence dependence and dynamics reveal that the fluence threshold, Fc, for the monoclinic-insulator to rutile-metal transition is inhomogeneous in real-space. A second growth process is identified, even at excitations fluences well below Fc, which operates on a longer timescale with an inhomogeneous rise time, tau-1. Together Fc and tau-1 govern the inhomogeneous nano-texturing of the transient conductivity. Secondly, we uncover that crystals of van-der Waals (vdW) semiconductors behave as optical waveguides with broadly tunable properties at femto-second time scales. We detect giant optical phase shifts of waveguided photons under strong photo-excitation devoid of any unwanted added losses in the vdW crystal, WSe2. Our results firmly implicate bound excitons in the observed behavior. Our transient spatio-temporal maps reveal two concomitant effects: i) photo-generation of electron-hole plasma that drives the WSe2 crystal towards a Mott transition where excitons dissociate and ii) a coherent interaction between the waveguide material and pump light, known as the optical Stark effect, that alters the phase velocity of guided photons on the femtosecond timescale

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

Copyright © 2011 American Physical SocietyTime-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconductors and semiconductor structures over the past decades. Covering the energy range from a few to about 100 meV, terahertz radiation is sensitive to the response of charge quasiparticles, e.g., free carriers, polarons, and excitons. The distinct spectral signatures of these different quasiparticles in the THz range allow their discrimination and characterization using pulsed THz radiation. This frequency region is also well suited for the study of phonon resonances and intraband transitions in low-dimensional systems. Moreover, using a pump-probe scheme, it is possible to monitor the nonequilibrium time evolution of carriers and low-energy excitations with sub-ps time resolution. Being an all-optical technique, terahertz time-domain spectroscopy is contact-free and noninvasive and hence suited to probe the conductivity of, particularly, nanostructured materials that are difficult or impossible to access with other methods. The latest developments in the application of terahertz time-domain spectroscopy to bulk and nanostructured semiconductors are reviewed.Nederlandse organisatie voor Wetenschappelijk Onderzoek (NWO): Stichting voor Fundamenteel Onderzoek der Materie (FOM) research programmeNational Science Foundatio

Characterization of multi-wall carbon nanotubes and their applications

PhDCarbon nanotubes (CNT) and their applications is a field which has attract a lot of interest in the past two decades. Since the first invention of CNTs in 1991, and in view of utilising nanoantennas, the focus in many laboratories around the world has shifted to trying to lengthen nanotubes longer from nanometers to few centimeters. Eventually this could lead to CNTs’ use in sub-millimeter, millimiter wave and microwave antenna applications. In this thesis, fundamental properties of carbon nanotube films are investigated, and some applications such as the use of CNTs as absorbers or CNT doped liquid crystals are considered. The concept of frequency tunable patch antennas is also presented. Simulation and measurement results of the liquid crystal based antenna show that frequency tuning is possible, through the use of a liquid crystal cell as a substrate. Additionally, greater tuning can be achieved using liquid crystals with higher dielectric anisotropy at microwave frequencies. This can be achieved by using CNT doped liquid crystals. As mentioned, microwave and terahertz measurements of vertically aligned carbon nanotube arrays placed on the top surface of a rectangular silicon substrate are presented. The S-parameters are calculated allowing the extraction of the complex permittivity, permeability and conductivity of the samples. Theoretical models are being introduced delineating the behaviour of the multi-walled nanotube (MWNT) samples. The material properties of this film provide useful data for potential microwave and terahertz applications such as absorbers. Finally, finite-difference time-domain (FDTD) modelling of CNTs is introduced, verifying the measurements that have been performed, confirming that CNT arrays can be highly absorptive. A novel estimation of the permittivity and permeability of an individual carbon nanotube is presented and a periodic structure is simulated, under periodic boundary conditions, consisting of solid anisotropic cylinders. In addition, the optical properties of vertically aligned carbon nanotube (VACNT) arrays, when the periodicity is both within the sub-wavelength and wavelength iii regime are calculated. The effect of geometrical parameters of the tube such as length, diameter and inter-tube distance between two consecutive tubes are also examined