In-situ study of CVD graphene growth on liquid metal catalysts by radiation-mode optical microscopy

• Κύριος συγγραφέας: Σφουγκάρης, Ηλίας
• Άλλοι συγγραφείς: Sfougkaris, Ilias
• Γλώσσα: English
• Έκδοση: 2021
• Θέματα: Graphene; Chemical vapor deposition (CVD); LMCat; Γραφένιο; Χημική εναπόθεση ατμών

Graphene is a perfect two-dimensional crystal consisting of covalently bonded carbon atoms, ar- ranged in a honeycomb lattice. Due to its extraordinary physical properties, it is a strong candidate material for a variety of electrical, thermal, and mechanical applications. In fact, it is very likely that graphene will play a key role in overcoming the fundamental challenge that the electronics industry will face in the next 20 years, the further miniaturization of technology. Graphene has already been successfully applied in lab-scale for the construction of many electronic devices, such as light-emitting diodes (LED) and field-effect transistors (FET). This has been achieved using high-quality graphene, which has been mechanically exfoliated from bulk graphite. However, mechanical exfoliation of gra- phene is too labor-intensive to be applied commercially. The lack of an industrially scalable method to produce centimeter-sized, defect-free graphene films is the missing piece for the commercial pro- duction of graphene-based electronics. In 2009, chemical vapor deposition (CVD) of graphene on copper foil was first reported, marking an important step towards large-scale production of graphene films. The obtained graphene film was ~95% monolayer with some few-layer regions, and polycrystalline with a grain size of a few microm- eters. Since then, the CVD process on copper foil has been extensively studied and improved. Later, in 2012, CVD growth of graphene on liquid copper was reported. It has since been observed that liquid metal catalysts (LMCat), such as liquid copper, offer several advantages compared to traditional solid metal catalysts (SMCat), such as copper foil. Recently, facile growth of millimeter-sized hexago- nal graphene crystals was demonstrated on liquid copper, taking graphene electronics one step closer to reality. However, CVD growth of graphene is a delicate process with a huge parameter space. A key take- away from past research work is that it is immaterial to look for the perfect CVD "recipe", since small variations in the experimental setup, such as in gas flow, temperature distribution and reactor geom- etry can lead to very different results. Therefore, industrial adoption of the CVD process for graphene growth will inevitably require the development of in situ metrology for real-time monitoring and dy- namic control of the growth process. A feedback loop which dynamically adjusts process parameters based on real-time metrology data will enable the high yield and high throughput necessary for in- dustrial production. In this thesis, we demonstrate the use of in situ radiation-mode optical microscopy (Rad-OM) to monitor CVD growth of graphene on liquid metal catalysts. We show that CVD growth of graphene can be dynamically controlled using real-time Rad-OM data. We probe the effect of the feed gas composition on the growth process of graphene flakes and obtain insights about the dynamics of flake growth. Under appropriate conditions, graphene flakes undergo a self-assembly process on LMCat. Additionally, we transfer LMCat graphene from copper to other substrates and employ ex situ characterization methods, to assess the quality of LMCat-grown graphene. The graphene is mono- layer, has a low defect-density, and is under compressive stress, as highlighted by Raman spectros- copy on SiO2/Si-supported graphene. Tapping-mode atomic force microscopy (AFM) reveals the pres- ence of contaminants on the graphene film, which result in a relatively high electrical sheet resistance (~3.6 kΩ/sq), as probed via van der Pauw measurements. Finally, LMCat graphene can withstand high strain, as highlighted by a uniaxial tensile test combined with Raman spectroscopy performed on PMMA-supported graphene.