Materials for large-area electronics: characterization of pentacene and graphene thin films by ac transport, Raman spectroscopy, and optics

Abstract

This dissertation explores techniques for fabricating and characterizing two classes of novel materials which may be useful for large-area electronics applications: organic semiconductors and graphene.

Organic semiconductors show promise for large-area electronics because of their low cost, compatibility with a variety of substrates, and relative ease of fabricating and patterning thin-film transistors (TFTs). Nearly all published work has focused on the dc electronic transport properties of these materials, rather than their ac behavior, which could be affected by their polycrystalline, granular structure. To address this, I have constructed a model of organic TFTs based on lossy transmission lines, and determined the relationship between the film conductivity and the overall device behavior for a bottom-contacted TFT.

I apply this transmission-line framework to interpret my experiments on pentacene TFTs designed in a special long-channel geometry to hasten the onset of high-frequency effects. The experiments reveal an intrinsic frequency-dependent conductivity of polycrystalline pentacene, which can be understood within the context of the universal dielectric response model of ac conduction in disordered solids. The results are important for establishing practical limits on pentacene's ac performance.

Graphene is a two-dimensional crystalline form of carbon, with a remarkably simple structure. It is a gapless semiconductor with an extremely high mobility and very high optical transparency, attracting great interest both for its possible uses as a replacement for silicon and as a transparent conducting material.

I have synthesized large-area films of graphene via atmospheric-pressure chemical vapor deposition (CVD) on copper substrates, adapting a low-pressure CVD method previously reported to produce exclusively monolayer graphene films. I have transferred the graphene films to insulating SiO2, and characterized them using optical transparency, Raman spectroscopy, and atomic-force microscopy, observing significant differences from the measured properties of widely studied mechanically-exfoliated graphene. I analyze the strengths and weaknesses of these three techniques for distinguishing films of different layer number, and relate them to uncertainties in the known properties of one- and few-layer graphene. I conclude that atmospheric-pressure CVD of graphene on copper produces significant areas of multilayer, rotationally-misoriented graphene, in a significant departure from results on low-pressure CVD of graphene on copper.