Fabrication and characterization of graphene-superconductor devices
Graphene is the first single-atom-thick two-dimensional material and exhibits a large set of interesting properties. This thesis consists of two parts. The first regards the growth of large-area graphene using chemical vapor deposition (CVD). Graphene is grown using CVD on copper catalyst showing high quality with charge carrier mobility exceeding 3000 cm2/Vs.
Wet chemical etching is used to transfer graphene to insulating substrates. Cu is removed using either diluted HNO3 or diluted HCl with a small amount of added H2O2. To allow for faster transfer and avoid consuming copper, a hydrogen-bubbling method is developed to delaminate graphene from Cu. Graphene transferred this way shows properties similar to those of graphene transferred using wet etching.
To avoid transfer-related issues, graphene is grown non-catalytically directly on insulating substrates such as SiO2, Al2O3, and Si3N4. The grain size is only ~10 nm due to the lack of catalytic activity during growth. Such graphene shows inferior electronic properties with mobility in the order of ~tens of cm2/Vs. Despite that, sheet resistance around kΩ, the possibility to grow several layer thick films, and optical properties similar to those of pristine graphene make it an interesting material.
A method for cleaning graphene mechanically using atomic force microscopy (AFM) is developed. By appropriate choice of the applied force, atomically smooth (roughness < 0.2 nm) graphene with improved mobility and reduced doping is achieved.
The second part of this thesis considers experiments combining graphene and superconductors. A graphene-based cold-electron bolometer is realized using graphene as absorber material. It shows response to 100 GHz radiation at 300 mK and a temperature responsivity of ~0.4 µV/mK at 300 mK.
The Aharonov-Bohm effect is studied in graphene having superconducting or normal metal mirrors. The mirrors improve the visibility of the Aharonov-Bohm oscillations, and up to third order oscillations are observed.
Weak localization in inhomogeneous magnetic fields is studied in graphene by putting it in close proximity to a type-II superconductor. A deviation from the homogeneous result is observed for fields smaller than the characteristic field Bφ.
Atomic force microscopy
Chemical vapor deposition