Date of Award

1-1-2012

Document Type

Campus Access Dissertation

Department

Electrical Engineering

First Advisor

Goutam Koley

Abstract

Since its invention in 2004, graphene, a two-dimensional (2D) monolayer of sp2 bonded carbon atoms, has attracted huge interest among researchers, due to its distinctive mechanical, thermal, and electrical properties. In particular, graphene exhibits remarkably high carrier mobility up to 200,000 cm2 V-1s-1 in suspended form, as the charge carriers resemble Dirac fermions; and the carrier transport can remain ballistic up to 0.3 μm in ambient conditions. However, due to different sources of scattering, this value is highly reduced in supported graphene. Due to the atomically thin nature of the graphene films, their electronic and transport properties are readily affected by adsorbed impurities, which can open up applications of these films in a wide range of sensor devices. Indeed, a wide variety of graphene based chemical sensors devices have been reported for sensing toxic gases, chemicals, explosives, and radiation, taking advantage of the unique material properties of graphene. For successful development of graphene based sensors and electronics, availability of high quality and large area graphene films is necessary, and epitaxial graphene grown on SiC substrates by graphitization has been shown to be quite satisfactory both in terms of quality and reliability. Graphene on SiC substrate offers the added advantage of integrating sensors and readout circuits on the same chip, that are also suitable for harsh environment operation, taking advantage of the wide bandgap of SiC.

In this thesis we examine the growth, applications in chemical and infrared (IR) sensing, and charge carrier transport of epitaxial graphene grown on different faces of doped and undoped SiC substrate. Our experimental results demonstrate that graphene layers grown on both the Si and C-faces of semi-insulating 6H-SiC can offer very high NO2 detection sensitivity and selectivity, as well as fast response time. Exposure to only 500 ppb NO2 reduced the conductivity by 2.25%, while 18 ppm caused a reduction of 10%. In contrast, high concentrations of commonly interfering gases, namely, CO2 (20%), H2O (saturated vapor), NH3 (550 ppm), and pure O2 increased the conductivity by a maximum of 2%. Graphene on the C-face of SiC resulted in somewhat lower sensitivity for the test gases, with the conductivity changing in an opposite direction compared to the Si-face for any particular gas. Measurements conducted at higher temperature showed significantly higher changes in conductivity and shorter response times. From a device perspective, charge transport properties and type of carrier in 2DEG (Two dimensional electron gas) is required to gain an understanding of the electric field effect for our next generations ballistic devices based on Nano patterned Si-face FLG (Few layer graphene) and C-face MLG (Multilayer graphene). Our results indicate that in C-face MLG p-type charge carriers are dominant, while in Si-face FLG, charge carriers are mostly n-type. An analytical model correlating the change in conductivity and work function with molecular doping, impurity density has been developed utilizing Boltzmann transport theory. Our results further indicate that for epitaxial graphene grown on both the faces of SiC, the charge interaction by the adsorbed molecules and related work function changes can be strongly influenced by the substrate, gas flow rate and temperature.

An intriguing and potentially revolutionary application for suspended atomically thin graphene sheets which remains largely unexplored is as an Infrared sensor pixel. The sizes of the IR pixels were chosen after a compromise between the sensor sensitivity and processing complexity. Our analytical and simulation results indicate that polymer/graphene composite film can offer very low Noise Equivalent Temperature Difference (NETD) of a few mK (2 mK), uncooled broadband operation (30 Hz) due to IR absorption over a large wavelength range, and very fast response time in the μs range (~100 μs) due to extremely low thermal capacity of the atomically thin graphene film which can results in a novel uncooled IR sensor with much superior capabilities than the state-of-the-art, utilizing these extraordinary properties of epitaxial FLG film.

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