Growth of Epitaxial Graphene on Single Crystal Copper Surfaces by Chemical Vapor Deposition

摘要

Graphene is of significant interest due to its unique properties, such as high carrier mobility, mechanical strength, and thermal conductivity. Potential applications include next generation transistors, interconnects, biological and chemical sensing devices, and super capacitors. The research presented here addresses unresolved questions regarding the nucleation and growth of graphene by chemical vapor deposition (CVD) on the high index surfaces of copper single crystals. While much CVD graphene growth has been performed on copper foils, the polycrystalline nature of the foils renders large-scale single domain growth of graphene difficult. For this reason, many groups seek to reduce the nucleation rate of graphene on copper foil to increase graphene grain size. Another approach to growing high quality graphene films is to use single crystal substrates. Under the proper conditions, using single crystals of copper has been shown to produce well-aligned epitaxial growth of graphene.;In this research project, CVD growth of graphene on Cu(100) and Cu(111) crystals polished on-axis to within 0.1°, Cu(111) polished off-axis by 5°, and Cu(110) polished on-axis to within 0.5° are compared. In addition, a detailed study of the graphene growth kinetics on the Cu(100) crystal polished to within 0.1° has been performed. To control for issues of surface contamination and ensure repeatability of the experiments, all growths were performed in an ultra-high vacuum chamber and characterized with in-situ low energy electron diffraction. In addition, ex-situ scanning electron microscopy and X-ray photoelectron spectroscopy was performed. It was found that well-ordered single-domain epitaxial growth is possible on the Cu(111) surfaces and that well-ordered two-domain epitaxial growth is possible on the Cu(100), surface. Although two-domain epitaxy was possible on the Cu(110) surface, it had considerable rotational disorder. Furthermore, it was found that the reactivity of the Cu(111) off-axis toward the catalytic decomposition of the ethylene precursor was the greatest, followed by Cu(110), then Cu(100), then finally Cu(111) on-axis. This indicates that step edges have the highest catalytic activity and that the catalytic activity of the terrace sites increased as the surface density of copper atoms decreased: (111) followed by (100) and (110). For growth on the Cu(111) on-axis surface, the reactivity was so low that competition between copper sublimation and graphene growth was important. An argon overpressure was needed to suppress copper sublimation in order to achieve the growth of epitaxial graphene overlayers. The graphene growth experiments performed on the Cu(100) surface showed that the decomposition of the ethylene precursor is initiated at about 700 °C but that to achieve well-ordered epitaxial growth, a temperature of approximately 900 °C is needed. The precursor pressure also had a strong affect on the ordering of the graphene. For precursor pressures of 5 mTorr or less, well-ordered two-domain epitaxy was achieved. At higher precursor pressures, the graphene showed considerable rotational disorder. At 50 mTorr, the in-phase epitaxy was lost.