Imaging ellipsometry of graphene

Imaging ellipsometry studies of graphene on SiO2/Si and crystalline GaAs are presented. We demonstrate that imaging ellipsometry is a powerful tool to detect and characterize graphene on any flat substrate. Variable angle spectroscopic ellipsometry is used to explore the dispersion of the optical constants of graphene in the visible range with high lateral resolution. In this way the influence of the substrate on graphene's optical properties can be investigatedComment: 3 pages, 3 figure

Counting molecular-beam grown graphene layers

Copyright © 2013 American Institute of PhysicsWe have used the ratio of the integrated intensity of graphene's Raman G peak to that of the silicon substrate's first-order optical phonon peak, accurately to determine the number of graphene layers across our molecular-beam (MB) grown graphene films. We find that these results agree well both, with those from our own exfoliated single and few-layer graphene flakes, and with the results of Koh et al. [ACS Nano 5, 269 (2011)]. We hence distinguish regions of single-, bi-, tri-, four-layer, etc., graphene, consecutively, as we scan coarsely across our MB-grown graphene. This is the first, but crucial, step to being able to grow, by such molecular-beam-techniques, a specified number of large-area graphene layers, to order.Work supported by ONR (N000140610138 and Graphene Muri), EFRC Center for Re-Defining Photovoltaic Efficiency through Molecule Scale Control (award DE-SC0001085), NSF (CHE-0641523), NYSTAR, CSIC-PIF (200950I154), Spanish CAM (Q&C Light (S2009ESP-1503), Numancia 2 (S2009/ENE-1477)), and Spanish MEC (ENE2009-14481-C02-02, TEC2011-29120-C05-04, MAT2011-26534)

MBE growth of Quantum nanostructures for optoelectronics

Ponencia presemtada en el Workshop on Frontier Photonic and Electronic Materials and Devices - German-Japanese-Spanish Joint Workshop, celebrado en Kyoto del 11 al 14 de julio de 2015.Molecular Beam Epitaxy (MBE) is a powerful technique for the fabrication of several self-assembled III-V nanostructures such as quantum rings, quantum dots, and quantum wires that can cover a wide range of the spectrum from 0.98 μm to 1.6 μm. The possibility of performing in-situ, real-time, measurements of accumulated stress (Σσ) during growth of these nanostructures enables to achieve a deep understanding of the growth processes. For example, whereas quantum rings (QR) formation is crucially linked to the presence of liquid indium on the surface, quantum wires (QWR) are produced as an effective way of relaxing a large asymmetrical accumulated stress present on the sample. This information allows a fine-tuning of the optoelectronic properties by controlling their size and shape. Furthermore, the capability of tracking Σσ during growth is used to engineer strain compensated structures like multilayer quantum dot solar cells.CHE-0641523, CSIC-PIF200950I154,S2009ESP-1503, S2009ENE-1477 and AIC-B-2011-0806, MAT2011-26534.Peer Reviewe

Graphene growth on h-BN by Molecular Beam Epitaxy

The growth of single layer graphene nanometer size domains by solid carbon source molecular beam epitaxy on hexagonal boron nitride (h-BN) flakes is demonstrated. Formation of single-layer graphene is clearly apparent in Raman spectra which display sharp optical phonon bands. Atomic-force microscope images and Raman maps reveal that the graphene grown depends on the surface morphology of the h-BN substrates. The growth is governed by the high mobility of the carbon atoms on the h-BN surface, in a manner that is consistent with van der Waals epitaxy. The successful growth of graphene layers depends on the substrate temperature, but is independent of the incident flux of carbon atoms.Comment: Solid State Communications, 201

Molecular beam growth of graphene nanocrystals on dielectric substrates

We demonstrate the growth of graphene nanocrystals by molecular beam methods that employ a solid carbon source, and that can be used on a diverse class of large area dielectric substrates. Characterization by Raman and Near Edge X-ray Absorption Fine Structure spectroscopies reveal a sp2 hybridized hexagonal carbon lattice in the nanocrystals. Lower growth rates favor the formation of higher quality, larger size multi-layer graphene crystallites on all investigated substrates. The surface morphology is determined by the roughness of the underlying substrate and graphitic monolayer steps are observed by ambient scanning tunneling microscopy.Comment: Accepted in Carbon; Discussion section added; 20 pages, 6 figures (1 updated

Graphene on various substrates

This thesis deals with the influence of the substrate materials on the mechanical, electrical and magnetotransport properties of exfoliated graphene. Therefore we prepared graphene transistor-like devices on top of crystalline molecular beam epitaxial grown GaAs-based substrates and compare the results with graphene on the commonly used silicon substrates with 300 nm silicon dioxide on top. We found that the various kind of surface configurations of our substrates ranging from amorphous silicon dioxide over polar very flat (001) GaAs to magnetic p-doped GaMnAs and rough cross-hatched InGaAs do not affect the formation or stability of graphene. The localization of the graphene flakes was done by a combination of scanning electron and atomic force microscopy (AFM). Investigations of the morphology by AFM attested graphene to follow a continuous substrate texture from approx. 8 nm to more than 1 micrometer. Extensive AFM studies including power spectral density analysis pointed out that a pattering step using electron beam lithography leaves some unwanted PMMA residues on top of the graphitic layer, even after careful lift-off. From extensive imaging ellipsometry studies, where a graphene monolayer on silicon dioxide was measured angle and wavelength dependent, the optical dispersion relation was modelled with a comprehensive algorithm basing on the Drude model and developed by Accurion company. Both, the values for extinction and refraction indices for incident wavelengths from 350 nm to 1000 nm increase with increasing wavelength. Furthermore, graphene flakes were detectable also on the crystalline GaAs-based substrates by imagine ellipsometry. The main focus of this thesis lies on the electrical and magnetotransport properties of graphene mono-, bi- and few-layer graphene on GaAs, on InGaAs and for comparison on silicon dioxide substrates. We found that position and absolute resistance value of the charge neutrality point and the charge carrier mobility exhibit an unexpected temperature dependence. The charge carrier mobility of our sample constitutes between 1660 and 3600 cm^2/(Vs), what is in the range also found for identically prepared graphene samples on silicon dioxide. Magnetotransport measurements in the high field region reveal signatures of quantized transport phenomena. From the Hall-slope and the 1/B periodicity of the Shubnikov-de Haas oscillations the charge carrier density was quantifiable. Interestingly, we found that at low temperatures the type of intrinsic doping was p-type for all graphene samples on GaAs and InGaAs substrates, whereas n- and p-type doping was found for graphene on silicon dioxide. Moreover, the magnetoresistance in whole magnetic field range up to temperatures of about T = 30 K is superimposed by highly reproducible universal conductance fluctuations. Additionally, the low field region is dominated by the signal of the weak localization phenomenon up to more than T = 65 K. From this, the phase coherence length, the elastic intravalley scattering length and the elastic intervalley scattering lengths were calculated. The coherence length reaches 317 nm and all determined lengths reveal values comparable to those reported in literature for graphene on silicon dioxide. In this context, an astonishing and highly reproducible aperiodical suppression of the weak localization signal by tuning the carrier density accompanied by lowering of all characteristic length was observed for all graphene samples on GaAs substrates. However, this feature was not observable for our graphene samples on silicon dioxide and the origin of this striking phenomenon is still unclear

In-plane anisotropy of tunneling magnetoresistance and spin polarization in lateral spin injection devices with (Ga,Mn)As/GaAs spin-Esaki diode contacts

We report here on in-plane anisotropy observed in the tunneling magnetoresistance of (Ga,Mn)As/n+-GaAs Esaki diode contacts and in the spin polarization generated in lateral all-semiconductor, all-electrical spin injection devices, employing such Esaki-diode structures as spin aligning contacts. The uniaxial component of the registered anisotropies, observed along [1 1 0] directions, does switch its sign as an effect of the applied bias, however the switching occurs at different bias values for magnetoresistance and for spin polarization cases

Single- and bi-layer graphene grown on sapphire by molecular beam epitaxy

Growth of nanocrystalline single- and bi-layer graphene on sapphire (0001) substrates is achieved by van der Waals molecular beam epitaxy (MBE) with a solid carbon source. Gradients in substrate temperature and incident carbon flux enable the exploration of growth parameters for fabrication of graphene layers. Raman spectroscopy reveals that fabrication of single layer, bilayer or multilayer graphene crucially depends on MBE growth conditions. Atomic force microscopy (AFM) images uncover the presence of etch pits in layers grown at higher substrate temperatures (around 1200 °C). The average spacing between etch pits (of about 100 nm) defines an upper limit of the nanocrystal size. Sharp Raman bands in the grown single-layer graphene nanocrystals indicate high crystallinity. Formation of etch pits during growth is regarded as evidence for a removal mechanism of carbon by reduction of sapphire. Growth of graphene on sapphire by MBE is driven by the interplay between carbon deposition and its removal. Tuning the easily controlled incident carbon flux and the markedly temperature dependent carbo-thermal reduction of sapphire should enable the growth of high quality graphene layers on large area sapphire substrates. © 2014 Elsevier Ltd.Work supported by ONR (N000140610138 and Graphene MURI), AFOSR (FA9550-11-1-0010), EFRC Center for Re-Defining Photovoltaic Efficiency through Molecule Scale Control (award DE-SC0001085), NSF (CHE-0641523), NYSTAR, CSIC-PIF (200950I154), Spanish CAM (Q&C Light (S2009ESP-1503), Numancia 2 (S2009/ENE-1477)) and Spanish MINECO (AIC-B-2011-0806, TEC2011-29120-C05-04, MAT2011-26534).Peer Reviewe

Exceptionally large migration length of carbon and topographically-facilitated self-limiting molecular beam epitaxial growth of graphene on hexagonal boron nitride

We demonstrate growth of single-layer graphene (SLG) on hexagonal boron nitride (h-BN) by molecular beam epitaxy (MBE), only limited in area by the finite size of the h-BN flakes. Using atomic force microscopy and micro-Raman spectroscopy, we show that for growth over a wide range of temperatures (500 C e 1000 C) the deposited carbon atoms spill off the edge of the h-BN flakes. We attribute this spillage to the very high mobility of the carbon atoms on the BN basal plane, consistent with van der Waals MBE. The h-BN flakes vary in size from 30 mm to 100 mm, thus demonstrating that the migration length of carbon atoms on h-BN is greater than 100 mm. When sufficient carbon is supplied to compensate for this loss, which is largely due to this fast migration of the carbon atoms to and off the edges of the h-BN flake, we find that the best growth temperature for MBE SLG on h-BN is ~950 C. Selflimiting graphene growth appears to be facilitated by topographic h-BN surface features: We have thereby grown MBE self-limited SLG on an h-BN ridge. This opens up future avenues for precisely tailored fabrication of nano- and hetero-structures on pre-patterned h-BN surfaces for device applications.This work is supported by ONR (N000140610138 and Graphene MURI), AFOSR (FA9550-11-1-0010), EFRC Center for Re-Defining Photovoltaic Efficiency through Molecule Scale Control (award DE-SC0001085), NSF (CHE-0641523), NYSTAR and Spanish Government (AIC-B-2011-0806, MAT2014-54231, MAT2015-67021-R). S.W. and A.P. were supported by the US Department of Energy Office of Science, Division of Materials Science and Engineering (award DE-SC0010695).Peer reviewe