30 research outputs found
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Dielectric-graphene integration and electron transport in graphene hybrid structures
textDielectrics have been an integral part of the electron devices and will likely resume playing a significant role in the future of nanoelectronics. An important step in assessing graphene potential as an alternative channel material for future electron devices is to benchmark its transport characteristics when integrated with dielectrics. Using back-gated and dual gated graphene field-effect transistors with top high-k metal-oxide dielectric, we study the dielectric thickness dependence of the carrier mobility. We show the carrier mobility decreases after deposition of metal-oxide dielectrics by atomic layer deposition (ALD) thanks to the Coulomb scattering by charged point defects in the dielectric. We investigate a novel method for the ALD of metal-oxide dielectrics on graphene, using an ultrathin nucleation layer that enables the realization of graphene field-effect transistors with aggressively scaled gate dielectric thickness. We show the nucleation layer significantly affects the quality of the subsequently deposited dielectric. In addition, we study transport characteristics of double layer systems. We demonstrate heterostructures consisting of two rotationally aligned bilayer graphene with an ultra-thin hexagonal boron nitride dielectric in between fabricated using advanced layer-by-layer transfer as well as layer pickup techniques. We show that double bilayer graphene devices possess negative differential resistance and resonant tunneling in their interlayer current-voltage characteristics in a wide range of temperatures. We show the resonant tunneling occurs either when the charge neutrality points of the two bilayer graphene are energetically aligned or when the lower conduction sub-band of one layer is aligned with the upper conduction sub-band of the opposite layer. Finally, we study the Raman spectra and the magneto-transport characteristics of A-B stacked and rotationally misaligned bilayer graphene deposited by chemical-vapor-deposition (CVD) on Cu. We show that the quantum Hall states (QHSs) sequence of the CVD grown A-B stacked bilayer graphene is consistent with that of natural bilayer graphene, while the sequence of the QHSs in the CVD grown rotationally misaligned bilayer graphene is a superposition of monolayer graphene QHSs. From the magnetotransport measurements in rotationally misaligned CVD-grown bilayer we determine the layer densities and the interlayer capacitance.Electrical and Computer Engineerin
Dielectric Thickness Dependence of Carrier Mobility in Graphene with HfO2 Top Dielectric
We investigate the carrier mobility in mono- and bi-layer graphene with a top
HfO2 dielectric, as a function of the HfO2 film thickness and temperature. The
results show that the carrier mobility decreases during the deposition of the
first 2-4 nm of top dielectric and remains constant for thicker layers. The
carrier mobility shows a relatively weak dependence on temperature indicating
that phonon scattering does not play a dominant role in controlling the carrier
mobility. The data strongly suggest that fixed charged impurities located in
close proximity to the graphene are responsible for the mobility degradation.Comment: 3 pages, 4 figure
Quantum Hall Effect in Bernal Stacked and Twisted Bilayer Graphene Grown on Cu by Chemical Vapor Deposition
We examine the quantum Hall effect in bilayer graphene grown on Cu substrates
by chemical vapor deposition. Spatially resolved Raman spectroscopy suggests a
mixture of Bernal (A-B) stacked and rotationally faulted (twisted) domains.
Magnetotransport measurements performed on bilayer domains with a wide 2D band
reveal quantum Hall states (QHSs) at filling factors consistent
with a Bernal stacked bilayer, while magnetotransport measurements in bilayer
domains defined by a narrow 2D band show a superposition of QHSs of two
independent monolayers. The analysis of the Shubnikov-de Haas oscillations
measured in twisted graphene bilayers provides the carrier density in each
layer as a function of the gate bias and the inter-layer capacitance.Comment: 5 pages, 4 figure