Thermotransport phenomena in quantum point contacts and quantum dots

Abstract

Electrostatically-defined nanostructures in bilayer graphene (BLG), known for its tunable bandgap, have promising applications in spintronics and valleytronics, however, its thermo- transport phenomena have not yet been investigated. This thesis aims to fabricate a BLG field-effect transistor (FET) device and characterize the thermotransport phenomena (See- beck coefficient) in electrostatically defined quantum point contacts (QPCs) and quantum dots (QDs). For this purpose, a bilayer graphene flake is encapsulated in hexagonal boron nitride (hBN) with Ti/Au heaters, top gates and 100-nm-separated split gates placed on top of the upper hBN flake. To define a QD, the 100 nm wide finger gates were separated from the top gates by a 30 nm Al2O3 dielectric. However, the electric field induced by the back gate was being screened by a layer of charges somewhere between the back gate and the bilayer graphene. The origin of this layer of charges remains unknown. As a result, the channel could not be fully depleted (unless when B = 5 T) and showed features indicating an unintended charging and discharging effect somewhere in the sample. As a consequence, the formation of a QD or a QPC at B = 0 T was not possible. Despite that, thermal voltages were measured in the two-dimensional BLG, applying currents up to 50 μA to the aforementioned heaters. The estimated Seebeck coefficient (based on resistance characterizations) was in the range of μV/K (corresponding with theoretical predictions) and enabled an estimate of an induced temperature gradient of 0.5 ± 0.2 K.