Towards graphene-based devices: Fabrication and characterization

Licentiatavhandling, 2012

Graphene is a new material with a large set of impressive properties, interesting both for fundamental studies and applications. Reliable synthesis of large-scale, high-quality graphene is key to its future success. This thesis is focused on the development of such fabrication techniques and experimental studies on three different graphene-based devices. The highest quality graphene is produced by mechanical exfoliation of graphite. While this technique is not scalable, it provides high quality graphene for scientific purposes and proof of principle devices. Catalytic chemical vapor deposition of graphene on copper is the most promising scalable method for graphene synthesis. Techniques for high temperature growth of graphene from methane as a precursor gas on high-purity copper foils are developed. Also, techniques for transferring the as‑grown graphene to insulating substrates are presented. Large-scale graphene with high uniformity and a mobility ~3000 cm2/Vs at room temperature is obtained. The transfer process needed for catalytically grown graphene on copper introduces issues with process reliability and future integration in semiconductor manufacturing. A non-catalytic chemical vapor deposition technique is shown to give uniform large-area graphene directly on insulating substrates like SiO2 and Si3N4, avoiding transfer. Raman spectroscopy, transmission electron microscopy, and transport measurements show that graphene grown this way is nanocrystalline and its electronic properties are inferior to those of catalytically grown graphene. Contamination and unintentional doping are difficult to avoid in graphene processing. A mechanical cleaning technique using an atomic force microscope is shown to efficiently remove contaminants and improve the electronic properties of graphene. The technique is easy and can be applied to substrates that cannot sustain standard graphene cleaning procedures. Graphene devices are realized on ferroelectric barium strontium titanate. The strong field effect in graphene is utilized as a read-out of the ferroelectric state, thus realizing a hybrid graphene-ferroelectric memory device. A graphene-based cold-electron bolometer is fabricated and characterized at cryogenic temperature. The low volume of graphene makes it an interesting absorber material for high-sensitivity bolometers. The Aharonov-Bohm effect is studied in graphene rings having metallic mirrors. The mirrors confine electrons to the Aharonov-Bohm ring, improving the visibility of higher-order Aharonov-Bohm oscillations.