Fabrication, functionalisation and characterisation of epitaxial graphene devices

Abstract

Graphene has attracted considerable attention in recent years due to its remarkable material properties and its potential for applications in next-generation nanoelectronics. In particular, the high specific surface area, extremely high electrical conductivity and exceptionally low electrical noise make graphene an ideal material for surface-sensitive applications such as chemical sensing, biological sensing and DNA sequencing. The surface cleanliness of graphene devices is critical for these applications, along with low contact resistance at metal/graphene interfaces. In addition, having pristine surface is also essential to carry out controlled functionalisation of graphene to target its chemical reactions with designated analyte species. However, it was found that conventional lithography processing techniques used for graphene device fabrication significantly contaminates the graphene surface with resist residues, which cannot be removed by any known organic solvents. The presence of such chemical contamination degrades the intrinsic properties of graphene and also significantly affects the performance of graphene based electronic devices. In this thesis, two methods were developed to address this issue, where, for the first method, rapid thermal annealing of graphene devices was performed in N2/H2 atmosphere, whilst for the second method, a metal sacrificial layer was used to prevent graphene from coming into direct with photoresist during the lithography process. Chemical, electrical, structural and surface morphological analysis showed that clean graphene surfaces can be achieved by both these methods, which allowed the intrinsic properties of graphene to be measured. In addition to surface contamination, the performance of graphene devices is also limited by contact resistance associated with the metal-graphene interface, where an unique challenge arise as charge carriers are injected from a three-dimensional metal film into a two-dimensional graphene sheet. The quantitative analysis of the data demonstrates that highly reactive metals such as Ti destroys the graphene lattice and results in high contact resistance, whereas metals with higher work functions and lower lattice mismatch to graphene (such as Ni) was found to result in significantly lower contact resistance. The work function, binding energy, diffusion energy and the lattice mismatch of the deposited metals were used to explain the electrical and structural characteristics of different types of metal/graphene interfaces. ABSTRACT vi In order to enhance the chemical reactivity of graphene surfaces, controlled functionalisation of epitaxial graphene films using electron-beam generated oxygen plasma has been demonstrated at room temperature. It was found that oxygen functionalisation not only introduces different oxygen functional groups onto the graphene surface, but also results in strain relaxation, in which the intrinsic compressive strain present in epitaxial graphene film decreased progressively with the increasing plasma pressure. A detailed study on the effect of e-beam plasma treatment on the chemical, electrical, structural and morphological characteristics of epitaxial graphene films have been investigated from initial to advanced oxidation stages. Finally, the effectiveness of oxygen functionalised graphene as a chemical sensor for detecting a wide range of polar chemical vapours in the ambient atmosphere has been demonstrated. The sensing characteristics of oxygen functionalised graphene devices showed ultra-fast response (10 s) and recovery times (100 s) to different chemical vapours, whilst unfunctionalised graphene sensors showed considerably weaker sensitivity and extremely slow recovery time in the range of ∼1.5 to 2 hours. A strong correlation between the dipole moment of the chemical and the magnitude of the response was observed, in which oxygen functionalised sensors displayed a twofold increase in the sensitivity over un-functionalised sensors with the increasing dipole moment from 2.0 D to 4.1 D. The sensing properties of graphene and the effect of oxygen functionalization on sensor responses were critically examined in an effort to provide a detailed understanding on the graphene sensing mechanism and provide a pathway for future advancements in the graphene sensor research.