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Interfacial Properties of Graphene for (Opto)Electronic Devices


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Abstract

The downscaling of field effect transistors (FETs) has empowered the development of the current (opto)electronic applications. However, the fundamental physical limitations of the current complementary metal-oxide-semiconductor (CMOS) FETs such as the short channel effect make it challenging to reduce the channel lengths further below 10 nm. The family of layered materials, which includes conductors, semiconductors, and insulators, has the potential to propel additional performance improvements, either by enabling further downscaling or by offering more functionalities. Hence, this thesis focuses on single-layer graphene (SLG) based (opto)electronic devices. As SLG has a high surface-to-volume ratio, its electrical properties are extremely sensitive to its substrate and environment. Therefore, I have optimized the interfacial properties of SLG according to the requirements for different applications. The first application that I have tackled is molecular communications, which uses molecules to transfer information and is one of the most promising techniques to connect nanoscale devices. Despite the extensive research on molecular communication presented in the literature, the consequences of the interface between real transceiver topologies and channels are usually ignored, creating a significant mismatch between theory and applications. To understand the role of the interface better, I have developed a micro/nanoscale molecular communication receiver based on SLG-based FETs (GFET) biosensors and carried out its information and communication technology-based evaluation in a specially designed microfluidic system with the information encoded into the concentration of DNAs. This experimental platform is the first real-world example of micro/nanoscale molecular communication and can be used as a testbed to create practical molecular communication techniques. The second application that I studied in this thesis is the realization of large area and high-quality GFETs, which are needed to make SLG-based (opto)electronic devices viable for industrial applications. Although the scalable growth and transfer methods of SLG are widely studied in the literature, the realization of SLG-based devices on a large scale is still lacking due to the need for a high-quality scalable dielectric interface for SLG. By focusing on both scalability of the processes involved and process integration constraints, I have studied the feasibility of high-quality SLG on large-scale amorphous boron nitride (aBN) and aluminum oxide (Al2O3) dielectric substrates. Raman spectroscopy was used to characterize the quality of SLG on these substrates and electronic transport measurements at both room and low temperatures were used to measure the performance of the FETs fabricated on these substrates. The quality of the scalable SLG/aBN and SLG/Al2O3 heterostructures were proven with the measured room temperature mobilities of ~ 9 kcm2/V s and ~ 14 kcm2/V s, respectively. The third application that I studied in this thesis is the controlled doping of SLG up to 2 eV using electrochemical doping. I transferred SLG on a conductive oxide and electrochemically gated SLG via an ionic liquid by applying a bias voltage between SLG and another conductive oxide. This doping technique paves the way to study the fundamental properties of SLG such as many body interactions at different doping levels up to its Van Hove singularity. This may lead to the observation of chiral topological superconductivity in SLG. The final application I studied is using SiO2 as a solid electrolyte to realize electrical double-layer gated solid state SLG and MoS2 transistors. I have achieved arrays of SLG and MoS2 FETs using SiO2 as solid electrolyte with operation gate voltages as low as 1.5 V. This is lower than ~50 V (~100 V) needed for SLG (MoS2) based FETs on 10 nm Al2O3 grown by atomic layer deposition on commercially available 285 nm SiO2/Si. This offers CMOS compatible and scalable platform for all applications of SLG and transition metal dichalcogenides-based electrolyte-gated devices such as in sensing applications, photodetectors, and long wavelength light modulators. This thesis demonstrates the importance of the interfacial properties of SLG for the fabrication of different (opto)electronic devices and paves the way for large-scale fabrication of these devices.

Description

Date

2022-09-01

Advisors

Ferrari, Andrea

Keywords

Biosensors, Charge carrier mobility, Electrochemical doping, Field-effect transistor based on graphene, GFET, Graphene, Layered materials, Microfluidics, Molecular communications, Receiver, Solid electrolyte

Qualification

Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Sponsorship
European Commission Horizon 2020 (H2020) Future and Emerging Technologies (FET) (881603)
European Commission Horizon 2020 (H2020) Future and Emerging Technologies (FET) (785219)
European Research Council (616922)