84 research outputs found
Standardization of surface potential measurements of graphene domains
We compare the three most commonly used scanning probe techniques to obtain a
reliable value of the work function in graphene domains of different thickness.
The surface potential (SP) of graphene is directly measured in Hall bar
geometry via a combination of electrical functional microscopy and spectroscopy
techniques, which enables calibrated work function measurements of graphene
domains with values ~4.55+/-0.02 eV and ~4.44+/-0.02eV for single- and
bi-layer, respectively. We demonstrate that frequency-modulated Kelvin probe
force microscopy (FM-KPFM) provides more accurate measurement of the SP than
amplitude-modulated (AM)-KPFM. The discrepancy between experimental results
obtained by different techniques is discussed. In addition, we use FM-KPFM for
contactless measurements of the specific components of the device resistance.
We show a strong non-Ohmic behavior of the electrode-graphene contact
resistance and extract the graphene channel resistivity
A prototype of RK/200 quantum Hall array resistance standard on epitaxial graphene
Epitaxial graphene on silicon carbide is a promising material for the next generation of quantum Hall re-
sistance standards. Single Hall bars made of graphene have already surpassed their state-of-the-art GaAs
based counterparts as an RK/2 (RK = h/e^2) standard, showing at least the same precision and higher break-
down current density. Compared to single devices, quantum Hall arrays using parallel or series connection
of multiple Hall bars can offer resistance values spanning several orders of magnitude and (in case of parallel
connection) significantly larger measurement currents, but impose strict requirements on uniformity of the
material. To evaluate the quality of the available material, we have fabricated arrays of 100 Hall bars con-
nected in parallel on epitaxial graphene. One out of four devices has shown quantized resistance that matched
the correct value of RK/200 within the measurement precision of 1e-4 at magnetic fields between 7 and 9
Tesla. The defective behaviour of other arrays is attributed mainly to non-uniform doping. This result con-
firms the acceptable quality of epitaxial graphene, pointing towards the feasibility of well above 90% yield
of working Hall bars
Nanoscale structural characterization of epitaxial graphene grown on off-axis 4H-SiC (0001)
In this work, we present a nanometer resolution structural characterization of epitaxial graphene (EG) layers grown on 4H-SiC (0001) 8° off-axis, by annealing in inert gas ambient (Ar) in a wide temperature range (Tgr from 1600 to 2000°C). For all the considered growth temperatures, few layers of graphene (FLG) conformally covering the 100 to 200-nm wide terraces of the SiC surface have been observed by high-resolution cross-sectional transmission electron microscopy (HR-XTEM). Tapping mode atomic force microscopy (t-AFM) showed the formation of wrinkles with approx. 1 to 2 nm height and 10 to 20 nm width in the FLG film, as a result of the release of the compressive strain, which builds up in FLG during the sample cooling due to the thermal expansion coefficients mismatch between graphene and SiC. While for EG grown on on-axis 4H-SiC an isotropic mesh-like network of wrinkles interconnected into nodes is commonly reported, in the present case of a vicinal SiC surface, wrinkles are preferentially oriented in the direction perpendicular to the step edges of the SiC terraces. For each Tgr, the number of graphene layers was determined on very small sample areas by HR-XTEM and, with high statistics and on several sample positions, by measuring the depth of selectively etched trenches in FLG by t-AFM. Both the density of wrinkles and the number of graphene layers are found to increase almost linearly as a function of the growth temperature in the considered temperature range
Influence of impurity spin dynamics on quantum transport in epitaxial graphene
Experimental evidence from both spin-valve and quantum transport measurements
points towards unexpectedly fast spin relaxation in graphene. We report
magnetotransport studies of epitaxial graphene on SiC in a vector magnetic
field showing that spin relaxation, detected using weak-localisation analysis,
is suppressed by an in-plane magnetic field, , and thereby
proving that it is caused at least in part by spinful scatterers. A
non-monotonic dependence of effective decoherence rate on
reveals the intricate role of scatterers' spin dynamics in forming the
interference correction to conductivity, an effect that has gone unnoticed in
earlier weak localisation studie
Graphoepitaxy of High-Quality GaN Layers on Graphene/6H–SiC
The implementation of graphene layers in gallium nitride (GaN) heterostructure
growth can solve self-heating problems in nitride-based high-power electronic
and light-emitting optoelectronic devices. In the present study, high-quality GaN
layers are grown on patterned graphene layers and 6H–SiC by metalorganic
chemical vapor deposition. A periodic pattern of graphene layers is fabricated
on 6H–SiC by using polymethyl methacrylate deposition and electron beam
lithography, followed by etching using an Ar/O
2
gas atmosphere. Prior to GaN
growth, an AlN buffer layer and an Al
0.2
Ga
0.8
N transition layer are deposited.
The atomic structures of the interfaces between the 6H–SiC and graphene, as
well as between the graphene and AlN, are studied using scanning transmission
electron microscopy. Phase separation of the Al
0.2
Ga
0.8
N transition layer into an
AlN and GaN superlattice is observed. Above the continuous graphene layers,
polycrystalline defective GaN is rapidly overgrown by better quality single-crys-
talline GaN from the etched regions. The lateral overgrowth of GaN results in
the presence of a low density of dislocations (
≈
10
9
cm
−
2
) and inversion domains
and the formation of a smooth GaN surface
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