Jury :

• Annick LOISEAU, Research Director - LEM, ONERA / CNRS, Rapporteur
• Catherine JOURNET GAUTIER, Professor - LMI Laboratory - UMR 5615 - University of Lyon, Rapporteur
• Bernard PLAÇAIS, Research Director - LPENS, Examiner
• David BRUNEL, Senior Lecturer - Geeps - CNRS-UMR8507, Examiner
• Dominique VIGNAUD, Research Fellow - IEMN, Examiner
• Emiliano PALLECCHI, Senior Lecturer - University of Lille, Examiner
• Henri HAPPY, Professor - University of Lille, Thesis supervisor

Summary:

The excellent mobility of graphene makes it a material of choice for radiofrequency applications. However, this mobility is experimentally limited by structural and environmental defects introduced by the growth of the material on a metal substrate, the method of transfer to a host support, the interaction of the graphene with the host substrate to which it is transferred, and by the component manufacturing processes. The aim of this thesis is to remedy these problems in order to make graphene virtually insensitive to its environment. It consists of two main parts: (i) The transfer by electrochemical exfoliation (wet transfer) of millimetre-sized (~5mm) graphene single crystals synthesised by CVD, and their physical and electrical characterisation; this study is being carried out as part of an exchange programme between the IEMN and the University of Irvine in California (PUF-Partner University Funding programme focusing on the development of flexible electronics). (ii) Fabrication and characterisation of hBN/Graphene/hBN heterostructures by dry transfer of exfoliated materials.
Although the CVD (Chemical Vapour Deposition) method has recently made it possible to obtain large graphene single crystals of very high quality on copper, transfer to an SiO2 substrate generally introduces defects and contamination into the graphene, resulting in low-performance devices. The first part of the work carried out in this thesis enabled the laboratory to develop and perfect a reliable CVD graphene transfer system. The method used is based on an electrochemical exfoliation approach exploiting the effect of bubbles generated at the graphene/Cu interface. Optimisation of this approach enabled us to transfer graphene crystals while preserving their quality. Finally, electrical characterisation of devices fabricated on graphene crystals enabled us to obtain a relatively low contact resistance, attesting to the good quality of the transferred graphene.
In order to limit the interaction of graphene with its environment and thus preserve its high mobility, encapsulation by hexagonal boron nitride hBN makes it possible to satisfy this need. As the growth of large surfaces of hBN is still a major scientific challenge, mechanical exfoliation is a necessary synthesis approach for producing these Van der Waals-type heterotructures. The second part of the work carried out in this thesis involved the development (from design to production) and implementation within the laboratory of a 'Stamping set-up' nano-manipulation platform dedicated to the stacking of 2D materials, as well as the development of a process for encapsulating graphene by dry transfer. Various samples were successfully fabricated using monolayer and bilayer graphene. Morphological and structural characterisations have shown that graphene after encapsulation exhibits very low doping values and stress variations at the nanometric scale. This promises high mobility values.
This work provides a route to obtaining high-quality graphene, which is an essential building block for the development of electronic devices based on 2D material heterostructures.

Abstract:

The high theoretical mobility of graphene makes it an excellent material for radio frequency applications. However, this mobility is limited by structural defects introduced by material growth techniques, the transfer method from metallic substrates to hosting semiconductor substrates, the fabrication processes of devices as well as the interaction of graphene with hosting substrate. This thesis aims to address these issues in order to make graphene practically insensitive to its environment. There are mainly two parts involved in this work: (i) Transfer by electrochemical exfoliation (wet transfer) of millimetre size single domains of graphene (~ 5mm) synthesized by CVD as well as their physical and electrical characterization; this study is part of an exchange program between the IEMN and the University of Irvine-California (PUF-Partner University Funding Program-on the development of flexible electronics). (ii) Fabrication and characterization of hBN/Graphene/hBN heterostructures by dry transfer of exfoliated materials.
Although the CVD (Chemical Vapor Deposition) method made it possible to obtain large single crystals of graphene on copper; the mandatory transfer to SiO2 substrate generally introduces defects and contaminations in graphene resulting in low performance devices. A reliable transfer system for CVD graphene is developed and optimized for cleanroom use. The method used is based on an electrochemical exfoliation approach known as Bubble transfer. By optimizing this approach, we were able to transfer graphene single domains without structural defects. Finally, the electrical characterization of devices based on the transferred graphene crystal made it possible to obtain a relatively low contact resistance owing to the good quality of the transferred graphene.
In order to limit the interaction of graphene with its environment and thus preserve its high mobility, encapsulation with hexagonal boron nitride hBN makes it possible to satisfy this need. The fabrication of these Van der Waals heterostructures is performed using mechanically exfoliated materials because the growth of large areas hBN is still considered a great scientific challenge. An experimental nano-manipulation platform "Stamping set-up" dedicated to the stacking of 2D materials is developed (from design to realization) as well as a process for graphene encapsulation by dry transfer. Different samples have been successfully fabricated using monolayer and bilayer graphene. Morphological and structural characterizations have shown that graphene after encapsulation shows very low doping values and uniform strain at the nanometre scale; which promises high mobility values.
This work paves the way towards obtaining high quality graphene which is an important part for the development of electronic devices based on heterostructures of 2D materials.