11 research outputs found
Carrier Recombination and Generation Rates for Intravalley and Intervalley Phonon Scattering in Graphene
Electron-hole generation and recombination rates for intravalley and
intervalley phonon scattering in Graphene are presented. The transverse and the
longitudinal optical phonon modes (-modes) near the zone center
(-point) contribute to intravalley interband carrier scattering. At the
zone edge (-point), only the transverse optical phonon mode
(-mode) contributes significantly to intervalley interband scattering
with recombination rates faster than those due to zone center phonons. The
calculated recombination times range from less than a picosecond to more than
hundreds of picoseconds and are strong functions of temperature and electron
and hole densities. The theoretical calculations agree well with experimental
measurements of the recombination rates of photoexcited carriers in graphene.Comment: 6 pages, 9 figure
Measurement of the Optical Absorption Spectra of Epitaxial Graphene from Terahertz to Visible
We present experimental results on the optical absorption spectra of
epitaxial graphene from the visible to the terahertz (THz) frequency range. In
the THz range, the absorption is dominated by intraband processes with a
frequency dependence similar to the Drude model. In the near IR range, the
absorption is due to interband processes and the measured optical conductivity
is close to the theoretical value of . We extract values for the
carrier densities, the number of carbon atom layers, and the intraband
scattering times from the measurements
Measurement of the Optical Absorption Spectra of Epitaxial Graphene from Terahertz to Visible
We present experimental results on the optical absorption spectra of
epitaxial graphene from the visible to the terahertz (THz) frequency range. In
the THz range, the absorption is dominated by intraband processes with a
frequency dependence similar to the Drude model. In the near IR range, the
absorption is due to interband processes and the measured optical conductivity
is close to the theoretical value of . We extract values for the
carrier densities, the number of carbon atom layers, and the intraband
scattering times from the measurements
Ultrafast Relaxation Dynamics of Hot Optical Phonons in Graphene
Using ultrafast optical pump-probe spectroscopy, we study the relaxation
dynamics of hot optical phonons in few-layer and multi-layer graphene films
grown by epitaxy on silicon carbide substrates and by chemical vapor deposition
on nickel substrates. In the first few hundred femtoseconds after
photoexcitation, the hot carriers lose most of their energy to the generation
of hot optical phonons which then present the main bottleneck to subsequent
carrier cooling. Optical phonon cooling on short time scales is found to be
independent of the graphene growth technique, the number of layers, and the
type of the substrate. We find average phonon lifetimes in the 2.5-2.55 ps
range. We model the relaxation dynamics of the coupled carrier-phonon system
with rate equations and find a good agreement between the experimental data and
the theory. The extracted optical phonon lifetimes agree very well with the
theory based on anharmonic phonon interactions.Comment: 4 pages, 3 figure
Thickness Estimation of Epitaxial Graphene on SiC using Attenuation of Substrate Raman Intensity
A simple, non-invasive method using Raman spectroscopy for the estimation of
the thickness of graphene layers grown epitaxially on silicon carbide (SiC) is
presented, enabling simultaneous determination of thickness, grain size and
disorder using the spectra. The attenuation of the substrate Raman signal due
to the graphene overlayer is found to be dependent on the graphene film
thickness deduced from X-ray photoelectron spectroscopy and transmission
electron microscopy of the surfaces. We explain this dependence using an
absorbing overlayer model. This method can be used for mapping graphene
thickness over a region and is capable of estimating thickness of multilayer
graphene films beyond that possible by XPS and Auger electron spectroscopy
(AES).Comment: 14 pages, 9 figure
Free-Standing Epitaxial Graphene On Silicon Carbide And Transport Barriers In Layered Materials
This thesis is based on the topic of layered materials, in which different layers interact with each other via van der Waals forces. The majority of this thesis deals with epitaxial graphene (EG) obtained from silicon carbide (SiC). Free-standing epitaxial graphene (FSEG) structures are produced from EG using a photoelectrochemical (PEC) etching process developed for making suspended graphene structures on a large-scale. These structures are investigated for their mechanical and electrical properties. For doubly-clamped FSEG structures, a unique U-beam effect is observed which causes orders of magnitude increase in their mechanical resonance frequency compared to that expected using simple beam theory. Combined magnetotransport and Raman spectroscopy studies reveal that FSEG devices produced from nominally monolayer graphene on the Si-face of SiC exhibit properties of an inhomogeneously doped bilayer after becoming suspended. This suggests that the buffer layer which precedes graphene growth on the Si-face of SiC gets converted to a graphene layer after the PEC etching process. In the second theme of this thesis, transport barriers in layered materials are investigated. The EG-SiC interface is studied using a combination of electrical (I-V, C-V) and photocurrent spectroscopy techniques. It is shown that the interface may be described as having a Schottky barrier for electron transport with a Gaussian distribution of barrier heights. Another interface explored in this work is that between different layers of MoS2, a layered material belonging to the class of transition metal dichalcogenides. This interface maybe thought of as a one-dimensional junction. Fourpoint transport measurements indicate the presence of a barrier for electron transport at this interface. A simple model of the junction as a region with an increased threshold voltage and degraded mobility is suggested. The final chapter is a collection of works based on the topic of layered materials, which are not related to the main theme of the thesis. They include fabrication and characterization details of a dual-gated bilayer graphene device, an investigation of the graphene-Si interface and hexagonal boron nitride-based membranes. These are presented in the hope that they may be useful for further investigations along those directions
van der Waals Epitaxial Growth of Graphene on Sapphire by Chemical Vapor Deposition without a Metal Catalyst
van der Waals epitaxial growth of graphene on <i>c</i>-plane (0001) sapphire by CVD without a metal catalyst is presented. The effects of CH<sub>4</sub> partial pressure, growth temperature, and H<sub>2</sub>/CH<sub>4</sub> ratio were investigated and growth conditions optimized. The formation of monolayer graphene was shown by Raman spectroscopy, optical transmission, grazing incidence X-ray diffraction (GIXRD), and low voltage transmission electron microscopy (LVTEM). Electrical analysis revealed that a room temperature Hall mobility above 2000 cm<sup>2</sup>/V·s was achieved, and the mobility and carrier type were correlated to growth conditions. Both GIXRD and LVTEM studies confirm a dominant crystal orientation (principally graphene [10–10] || sapphire [11–20]) for about 80–90% of the material concomitant with epitaxial growth. The initial phase of the nucleation and the lateral growth from the nucleation seeds were observed using atomic force microscopy. The initial nuclei density was ∼24 μm<sup>–2</sup>, and a lateral growth rate of ∼82 nm/min was determined. Density functional theory calculations reveal that the binding between graphene and sapphire is dominated by weak dispersion interactions and indicate that the epitaxial relation as observed by GIXRD is due to preferential binding of small molecules on sapphire during early stages of graphene formation