Integrated Catalytic Fabrication Approaches for Graphene-Based Electronic Devices
Creators: name: Van Veldhoven, Zenas August
Publisher: name: Apollo - University of Cambridge Repository
Year: 2019
Subjects: graphene
encapsulation
passivation
ALD
Al2O3
GFET
CVD
transfer-free
interface
SiO2/Si
sapphire
nickel
copper
amorphous carbon
aC
growth mechanism
kinetics
thermodynamics
graphitization
solid-source
interface roughening

description: Catalytic Chemical Vapour Deposition (CCVD) of graphene is the most promising production method of large-area, high-quality graphene. However, the required transfer step from the metal growth substrate to the desired dielectric substrate is a major bottleneck for the integrated manufacturing of graphene-based electronic devices. The performance of a typical graphene field effect transistor (GFET) is marred by the presence of graphene grain boundaries, atmospheric doping and polymer residues from the transfer and fabrication steps. We here show that these three factors all interact with each other. The grain boundaries acts as a preferential adsorption site for polymer residues from the transfer step, which reduce the electronic properties of graphene and help to retain atmospheric doping. These polymer residues become embedded below the Al$_2$O$_3$ ALD layer during the encapsulation process, which prevents the passivation of the graphene surface and the restoration of its intrinsic electronic properties. The transfer-free catalytic graphene growth, directly at the interface between the metal catalyst and the desired dielectric substrate, is an interesting alternative for the integrated manufacturing process. However, the growth mechanism is poorly understood so far. We here present a detailed study of the feasibility, challenges and mechanism of growing these interface graphene films (IGF). Firstly, we find that with typical metal catalysts -such as nickel or copper-, the commonly used SiO$_2$/Si substrate roughens at relevant growth temperatures due to the capillary action of the overlaying metal catalyst grain boundaries and its high interface diffusion, which makes this substrate less attractive for IGF growth. Secondly, we investigate both the thermodynamics and kinetics of IGF growth. By modelling the different carbon fluxes through the metal catalyst thin film during the growth process, we find that the limited carbon bulk reservoir plays an important role in the IGF growth mechanism and that supersaturation at the interface is difficult to achieve. For stable catalysts -with no significant mass loss or dewetting occuring at elevated temperatures-, IGF growth is thus limited to precipitation during cooling, while for unstable catalysts isothermal growth is possible. The proposed mechanism is corroborated by systematic experimental results on IGF growth at the metal/sapphire interface. Additionally, our proposed growth mechanism allows for the first time to critically reinterpret the published IGF growth reports in the literature. We highlight the fundamental trade-offs found between growing full coverage, but low-quality, inhomogeneous IGF films by precipitation with stable catalyst and obtaining high-quality, homogeneous IGF films but with no full coverage when using unstable catalysts. Based on our findings and developed understanding of the IGF growth mechanism, we propose a novel engineered catalyst structure for high-quality, homogeneous, full coverage and controllable IGF growth, consisting of a Cu/ Ni/aC/dielectric stack. The copper both blocks the carbon from forming graphene at the free metal surface as well as gradually reduces the carbon solubility in the nickel layer. This pushes the dissolved carbon towards the interface and causes isothermally supersaturation and IGF growth to occur. Various challenges and solutions in the form of copper diffusion barriers are discussed to control the kinetics of this system and improve the resulting IGF quality. This work represents an important milestone for the IGF growth by systematically analysing the fundamental interactions, thermodynamics and kinetics and provides a stepping stone for further research in the area.