Semester

Spring

Date of Graduation

2020

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Chemical and Biomedical Engineering

Committee Chair

Charter D. Stinespring

Committee Member

Robin S. Hissam

Committee Member

John W. Zondlo

Committee Member

Edward M. Sabolsky

Committee Member

Jeremy M. Dawson

Abstract

Having been theorized in 1947, it was not until 2004 that graphene was first isolated. In the years since its isolation, graphene has been the subject of intense, world-wide study due to its incredibly diverse array of useful properties. Even though many billions of dollars have been spent on its development, graphene has yet to break out of the laboratory and penetrate mainstream industrial applications markets. This is because graphene faces a ‘grand challenge.’ Simply put, there is currently no method of manufacturing high-quality graphene on the industrial scale. This grand challenge looms particularly large for electronic applications where the synthesis process must be scaled to wafer dimensions and, for the most part, must utilize existing semiconductor processing tools. Two widely researched methods, selective thermal sublimation and chemical vapor deposition, have the potential to meet this grand challenge; yet despite a global development effort, each method continues to face significant challenges at this time.

The work detailed in this dissertation approaches the problem from a fundamentally different direction. To that end, a novel plasma assisted growth method for producing graphene on silicon carbide (SiC) wafers has been investigated. This process has been under investigation in the Stinespring laboratory since 2012. In this two-step process, a halogen (Cl2 or CF4) based inductively coupled plasma (ICP) combined with a reactive ion etch (RIE) was used to selectively etch silicon from the surface layers of a SiC wafer to produce a carbon rich film in the first step. In the second step, the film and substrate underwent a rapid thermal anneal (RTA) under ultra-high vacuum (UHV) to crystalize the carbon film into graphene layers. The primary goals of the research described in this dissertation were to increase the manufacturability of the process, improve the quality of the graphene films and to develop and characterize simple sensors using these films.

To improve manufacturability of the graphene films, this work developed an atmospheric pressure (AP) RTA under an inert gas to replace the UHV process. Films produced by this AP-RTA method were characterized by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. It was found that the standard process yielded good quality, two to three layer thick graphene films with low levels of oxygen defects. These analyses also provided strong evidence that the graphene films were tightly coupled to the SiC substrate by way of a buffer-layer. This buffer-layer was structurally and compositionally similar to graphene, but a substantial fraction of the carbon atoms were covalently tethered to silicon atoms of the SiC substrate surface. This result was confirmed by electrical characterization measurements of the films up to 800 oC in which a variety of electrical properties were measured. The temperature dependence of these electrical properties also yielded valuable insight into fundamental aspects of the films, such as the charge injection mechanisms, carrier scattering processes, and other phenomena which are only reveal at elevated temperatures.

To improve film quality, a parametric investigation was conducted to determine the influence of each processing parameter on the film properties. In total, nine different parameters (SiC polytype, crystal face, doping type, ICP power, RIE power, and plasma pressure as well as RTA temperature, heating rate and cooling rate) were investigated. While it was found that each parameter effected the film properties to varying degrees, the most important parameters were the RIE power, the anneal temperature, and the anneal time.

Despite optimizing theses key parameters, the process still produced a buffer-layer coupled film. To remedy this and enhance film quality, high temperature annealing of the graphene films in hydrogen was investigated. It was found that repeated high temperature thermal cycling of the samples in hydrogen resulted in the largest degree of buffer-layer decoupling. Ultimately, however, full film decoupling was not achieved. This indicates a fundamental difference between these buffer-layers and those produced by the thermal sublimation process.

In pursuit of the device development goal, simple chemoresistive gas sensors and photo detectors were fabricated and tested. The results of these studies both demonstrate the utility of the graphene films produced here and provide additional insight into the structural and electrical properties of the films.

The sensitivity of the chemoresistive sensors was investigated for a variety of gases including H2, CO, and CH4 gas mixtures in Ar and He at temperatures ranging from 25°C to >800°C. The devices were found to function effectively as gas sensors by a thermal and chemical response mechanisms. With regard to photodetection, the devices were found to display a strong response to visible laser irradiation in both a power and unpowered state, including 650 nm (red or 461 THz), 532 nm (green or 564 THz) and 405nm (purple or 740 THz) wavelengths. Overall, these results demonstrate the utility of the films produced here in application areas including gas sensors, flow measurement, temperature measurement, and photodetection.

In conclusion, this work has significantly advanced the development of the plasma assisted graphene synthesis process by improving the manufacturability of the process and quality of the resulting films. Moreover, our scientific understanding of many fundamental aspects of the synthesis process, the film structure, and the electrical behavior of these tethered graphene films has been advanced in a significant way. These results, particularly as they relate to the electrical properties, charge injection and transport processes, will be of interest to researchers pursuing graphene growth by selective thermal sublimation and the related research community who must deal, in some cases, with tethered graphene films or who look to utilize tethered films for device applications. Finally, this work has uncovered a plethora of future research directions in both the synthesis and application arenas.

Embargo Reason

Patent Pending

Available for download on Tuesday, April 27, 2021

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