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• W655964836 abstract "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." @default.
• W655964836 created "2016-06-24" @default.
• W655964836 creator A5018356464 @default.
• W655964836 date "2010-04-20" @default.
• W655964836 modified "2023-09-27" @default.
• W655964836 title "Graphene on various substrates" @default.
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