Date of Award

12-15-2014

Document Type

Open Access Dissertation

Department

Electrical Engineering

First Advisor

MVS Chandrashekhar

Abstract

Since its invention in 2004, Graphene, a two dimensional array of SP2 bonded carbon atoms has received significant interest by the scientific community due to its unique electrical, optical and mechanical properties. A promising route to the synthesis of large-area graphene, is epitaxial graphene formed by sublimation of silicon atoms from Silicon carbide at elevated temperatures (>1200oC). Although the electronic and mechanical properties of graphene with perfect atomic lattice are outstanding, the structural defects, which may appear during graphene growth, can influence the growth mechanism and material properties. However, deviations from perfection, i.e. introducing dopants in semiconductors often considered as engineered defects, can be very useful in some applications, as they make it possible to achieve new functionalities. In this thesis, a quantitative study is presented to investigate the role of structural defects on the growth of multilayer epitaxial graphene on polar(c plane Si and C face) and non-polar (a and m plane) 6H-SiC faces, with distinctly different defect profile and provide an insight for optimizing the EG growth. For Si-face with point defects, multilayer EG growth is influenced by diffusion of Si atoms to these defects as well as desorption through these defects. However, the growth on C-face and non-polar ( a and m plane) faces, the growth is limited by the lateral diffusion of the Si atoms to the line defects/grain boundaries.

Graphene is the ideal active material for gas detection owing to its physically stable surface, practically achievable thin form, and potentially fast response time. The structural defects inherent in EG grown on C-face allows diffusion and adsorption of gas molecules extending the remarkable surface sensitivity of EG to bulk multilayer films. The carrier transport phenomenon for three different gases (N2, NH3 and NO2) in EG on C-face is investigated by Fourier Transform Infrared (FTIR) reflection spectroscopy and the 3 gases were clearly distinguished, enabling a new paradigm for multi-modal gas sensing using optical interrogation of EG surfaces towards EG electronic or optical noses. Lastly, a novel technique is established to grow defect engineered thick multilayer (> 200 MLs) graphene on Si face 4H SiC substrates (0, 4 and 8 deg off cuts) than possible with solid-state decomposition at atmospheric pressure in Argon alone (~2ML). This method exploits the thermodynamic advantages of SiF4 to increase the Si-removal from the SiC surface, thereby increasing the graphene growth rate. The defect density for these EG layers varies from ~1 at 1400°C to <0.2 at 1600°C, enabling temperature controlled engineering of the defect profile of the material. A novel approach is also presented to estimate large number of graphene layers based on Raman and Infrared spectroscopy. This is critical for enabling defect-controlled applications in electrochemistry such as batteries and biosensors that require thick layers of activated graphitic carbon.

Rights

© 2014, Shamaita Shithi Shetu

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