19 research outputs found
Chemically Resolved Interface Structure of Epitaxial Graphene on SiC(0001)
Atomic-layer 2D crystals have unique properties that can be significantly modified through interaction with an underlying support. For epitaxial graphene on SiC(0001), the interface strongly influences the electronic properties of the overlaying graphene. We demonstrate a novel combination of x-ray scattering and spectroscopy for studying the complexities of such a buried interface structure. This approach employs x-ray standing wave-excited photoelectron spectroscopy in conjunction with x-ray reflectivity to produce a highly resolved chemically sensitive atomic profile for the terminal substrate bilayers, interface, and graphene layers along the SiC[0001] direction. DOI: 10.1103/PhysRevLett.111.215501 PACS numbers: 61.48.Gh, 61.05.cm, 68.49.Uv, 79.60.Ài Epitaxial graphene (EG) grown on the Si-terminated face of silicon carbide [SiC Early studies revealed that EG/SiC(0001) possesses a complex 6 p 3 Â 6 p 3R30 (6R3) reconstructed interfacial layer [10], referred to herein as the interfacial, or EG 0 , layer. This layer has significant influence on the growth, morphology, and electronic behavior of the overlaying graphene Because of the importance of the interfacial layer to the behavior of EG/SiC(0001), there have been numerous efforts to characterize its structure, including low-energy electron diffraction In this Letter we detail the structure of the interface by employing a suite of x-ray characterization techniques, including depth-sensitive XPS, x-ray standing waveenhanced XPS (XSW-XPS), and x-ray reflectivity (XRR). These tools, when employed collectively, provide the chemically specific structural information necessary to clarify previously unknown details of the EG/SiC(0001) interface. This approach ultimately enables the construction of a chemically resolved interfacial map with sub-Å resolution along the SiC[0001] direction. The XSW technique affords conventional photoelectron spectroscopy with high spatial resolution due to the influence of the XSW [here produced by the SiC(0006) Bragg reflection] on the photoabsorption process. A depiction of this phenomenon is shown i
Probing the Structure and Chemistry of Perylenetetracarboxylic Dianhydride on Graphene Before and After Atomic Layer Deposition of Alumina
The superlative electronic properties of graphene suggest
its use
as the foundation of next-generation integrated circuits. However,
this application requires precise control of the interface between
graphene and other materials, especially the metal oxides that are
commonly used as gate dielectrics. Toward that end, organic seeding
layers have been empirically shown to seed ultrathin dielectric growth
on graphene via atomic layer deposition (ALD), although the underlying
chemical mechanisms and structural details of the molecule/dielectric
interface remain unknown. Here, confocal resonance Raman spectroscopy
is employed to quantify the structure and chemistry of monolayers
of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on graphene
before and after deposition of alumina with the ALD precursors trimethyl
aluminum (TMA) and water. Photoluminescence measurements provide further
insight into the details of the growth mechanism, including the transition
between layer-by-layer growth and island formation. Overall, these
results reveal that PTCDA is not consumed during ALD, thereby preserving
a well-defined and passivating organic interface between graphene
and deposited dielectric thin films
Self-Assembled Two-Dimensional Heteromolecular Nanoporous Molecular Arrays on Epitaxial Graphene
The
development of graphene functionalization strategies that simultaneously
achieve two-dimensional (2D) spatial periodicity and substrate registry
is of critical importance for graphene-based nanoelectronics and related
technologies. Here, we demonstrate the generation of a hydrogen-bonded
molecularly thin organic heteromolecular nanoporous network on epitaxial
graphene on SiC(0001) using room-temperature ultrahigh vacuum scanning
tunneling microscopy. In particular, perylenetetracarboxylic diimide
(PTCDI) and melamine are intermixed to form a spatially periodic 2D
nanoporous network architecture with hexagonal symmetry and a lattice
parameter of 3.45 ± 0.10 nm. The resulting adlayer is in registry
with the underlying graphene substrate and possesses a characteristic
domain size of 40–50 nm. This molecularly defined nanoporous
network holds promise as a template for 2D ordered chemical modification
of graphene at lengths scales relevant for graphene band structure
engineering
Quantitatively Enhanced Reliability and Uniformity of High‑κ Dielectrics on Graphene Enabled by Self-Assembled Seeding Layers
The full potential of graphene in integrated circuits
can only
be realized with a reliable ultrathin high-κ top-gate dielectric.
Here, we report the first statistical analysis of the breakdown characteristics
of dielectrics on graphene, which allows the simultaneous optimization
of gate capacitance and the key parameters that describe large-area
uniformity and dielectric strength. In particular, vertically heterogeneous
and laterally homogeneous Al<sub>2</sub>O<sub>3</sub> and HfO<sub>2</sub> stacks grown via atomic-layer deposition and seeded by a
molecularly thin perylene-3,4,9,10-tetracarboxylic dianhydride organic
monolayer exhibit high uniformities (Weibull shape parameter β
> 25) and large breakdown strengths (Weibull scale parameter, <i>E</i><sub>BD</sub> > 7 MV/cm) that are comparable to control
dielectrics grown on Si substrates
Ambient-Processable High Capacitance Hafnia-Organic Self-Assembled Nanodielectrics
Ambient and solution-processable,
low-leakage, high capacitance
gate dielectrics are of great interest for advances in low-cost, flexible,
thin-film transistor circuitry. Here we report a new hafnium oxide-organic
self-assembled nanodielectric (Hf-SAND) material consisting of regular,
alternating Ï€-electron layers of 4-[[4-[bisÂ(2-hydroxyethyl)Âamino]Âphenyl]Âdiazenyl]-1-[4-(diethoxyphosphoryl)
benzyl]Âpyridinium bromide) (PAE) and HfO<sub>2</sub> nanolayers. These
Hf-SAND multilayers are grown from solution in ambient with processing
temperatures ≤150 °C and are characterized by AFM, XPS,
X-ray reflectivity (2.3 nm repeat spacing), X-ray fluorescence, cross-sectional
TEM, and capacitance measurements. The latter yield the largest capacitance
to date (1.1 μF/cm<sup>2</sup>) for a solid-state solution-processed
hybrid inorganic–organic gate dielectric, with effective oxide
thickness values as low as 3.1 nm and have gate leakage <10<sup>–7</sup> A/cm<sup>2</sup> at ±2 MV/cm using photolithographically
patterned contacts (0.04 mm<sup>2</sup>). The sizable Hf-SAND capacitances
are attributed to relatively large PAE coverages on the HfO<sub>2</sub> layers, confirmed by X-ray reflectivity and X-ray fluorescence.
Random network semiconductor-enriched single-walled carbon nanotube
transistors were used to test Hf-SAND utility in electronics and afforded
record on-state transconductances (5.5 mS) at large on:off current
ratios (<i>I</i><sub>ON</sub>:<i>I</i><sub>OFF</sub>) of ∼10<sup>5</sup> with steep 150 mV/dec subthreshold swings
and intrinsic field-effect mobilities up to 137 cm<sup>2</sup>/(V
s). Large-area devices (>0.2 mm<sup>2</sup>) on Hf-SAND (6.5 nm
thick)
achieve mA on currents at ultralow gate voltages (<1 V) with low
gate leakage (<2 nA), highlighting the defect-free and conformal
nature of this nanodielectric. High-temperature annealing in ambient
(400 °C) has limited impact on Hf-SAND leakage densities (<10<sup>–6</sup> A/cm<sup>2</sup> at ±2 V) and enhances Hf-SAND
multilayer capacitance densities to nearly 1 μF/cm<sup>2</sup>, demonstrating excellent compatibility with device postprocessing
methodologies. These results represent a significant advance in hybrid
organic–inorganic dielectric materials and suggest synthetic
routes to even higher capacitance materials useful for unconventional
electronics