Martin Bjergfelt. Supervisors: Bjarke S. Jessen, Johanna Zultak, José M. Caridad, Peter Bøggild
Nanocarbon group, DTU Nanotech, Technical University of Denmark, Kongens Lyngby, Denmark
In this master thesis study, a van der Waals heterostructure with partially a clean and partially a contaminated interface was assembled. This heterostructure was used for studying the contamination's influence on the electronic properties of the encapsulated graphene. By applying the van der Pauw strategy for 4-terminal electrical characterization of sheet resistance, two square shaped devices with corner terminals were fabricated using Electron Beam Lithography. The devices were subsequently characterized by Raman spectroscopy, which indicated a difference in extrinsic doping of the graphene in the two device regions. A system with lock-in amplification, allowed accurate measurements of the device resistance while the devices were cooled in a cryostat. Measurements were performed from 300K to 2K in 9 steps. In the 2-50K range, the measurements showed negative resistances and large retraceable fluctuations at high carrier densities, indicating the onset of quasi-ballistic transport. Measurements at 50K exhibited carrier mobilities at n ± 10^(12) cm^(-2) up to 40,000 cm^(2)/Vs for the cleaner device and up to 22,000 cm^(2)/Vs for the more contaminated device. Analysis of the carrier mobility temperature dependence showed a temperature dependence for the clean device down to 150K, while the carrier concentration of the more contaminated exhibited temperature independence.
Martin Bjergfelt is a recent graduate (June 2016) from the Technical University of Denmark. He performed his thesis study at the Nanocarbon Group at DTU Nanotech on van der Waals heterostructure devices. He has primarily had experimental work with 2D materials, and has an interest in bringing the technological advancements in the field closer to industrial applications. Within 2D material technology, he has experience with mechanical exfoliation, van der Waals heterostructure assembly, Electron Beam Lithography, dry etching, metalization, as well as characterization experience with 4-terminal measurements, Raman spectroscopy, AFM, SEM and TEM.
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