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Title: CVD graphene : growth, transfer, properties and device applications
Author: Goniszewski, Stefan
ISNI:       0000 0004 6348 310X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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Large scale synthesis of graphene is essential for its exceptional properties to benefit an end user. Chemical vapour deposition (CVD) is the most promising method to achieve this but graphene produced this way has received little research interest when compared to pristine graphene derived from bulk graphite. In this thesis I address and investigate the consistent growth of graphene via CVD, its mechanical and electrical properties and its applicability into field effect transistor and mechanical resonator devices. Copper foil treated with acetic acid and annealed at 1000oC proved an ideal substrate for graphene CVD. Graphene nucleation begins at temperatures > 700 oC and reaches a continuous sheet at 1000oC. Pressure variation below 500 mbar was found to have little effect on the growth properties. An ideal precursor ratio of CH4/H2 = 0.0125 was found for high continuity monolayer growth. A Langmuir adsorbative model is derived from a non-catalytic deposition hypothesis and gives a fundamental rate limiting growth factor proportional to the surface concentration of CH species which I relate to precursor partial pressures. An idealised CH surface coverage of 0.27% is calculated from empirical literature values for unbroken graphene growth and gives a statistically significant parameter for repeatable CVD of high quality monolayer graphene using methane and hydrogen precursors. An optimised transfer process of graphene from the CVD growth substrates to a target substrate based on a wet polymethyl-methacryalate (PMMA) support layer method is derived. Adhered and freestanding graphene membranes are transferred to flat and patterned target substrates and show minimal continuity degradation or induced contaminants. The properties of CVD graphene are measured to be comparable to pristine graphite exfoliated monolayers with mobilities of 16000 cm2V-1s-1 and intrinsic carrier densities of 1011 - 1012 cm-2 on untreated substrates. Freestanding graphene is measured to have a negligible intrinsic carrier density with negligible strain even though its architecture indicates competing compressive and tensile stresses caused by graphene unwrinkling and substrate van der Waals bonding. From Raman spectra analysis a model relating the D peak wavenumber and full width half maximum to the carrier concentration is proposed. CVD graphene shows no substrate induced disorder effects when using SiO2 substrates but does show a carrier density dependency. The surface treatment of SiO2 dielectric layers in graphene field effect transistors with common cleaning techniques is shown to have a p-doping effect proportional to treatment aggressiveness, with hole carrier densities up to 6∙1012 cm-2 measured. Counter intuitively no cleaning of substrates prior to graphene transfer is recommended to preserve graphene's intrinsic electrical properties. From this a contact angle model is proposed for non-destructive calculation of the charge carrier density of graphene on SiO2. Moderate yield of freestanding CVD graphene membranes with maximum lateral dimensions of ≈ 3 µm is achieved using the optimised transfer technique over arrays of well and hole patterned substrates and PMMA annealing at 350 oC. The Young's modulus of CVD graphene membranes is measured and has an average value of 0.40 TPa. The mechanical resonance of circular freestanding graphene membranes (drums) is accomplished using an ultra-sensitive non-contact microwave method (limited to 1 MHz in this study) and is proven on cantilevers. Graphene shows limited resonant quality factors (Q) and unexpectedly low frequencies compared to model predictions. The method and model show promise of higher Q fundamental frequency excitation and readout if a higher frequency range of 1-100 MHz is used for drum diameters ≤ 3 µm and thus the development of mass and force sensors with zg and fN resolution can possibly be realised. This work provides a comprehensive insight into CVD graphene and its growth, properties and possible integration into devices for research and commercial purposes.
Supervisor: Klein, Norbert ; Alford, Neil Sponsor: National Physical Laboratory ; Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral