Graphene is a 2-dimensional material, with a unique combination of properties. Graphene's unprecedented high mobility makes it a uniquely suited material for many applications, including high-frequency transistors, or as a transparent conductive layer. One of the challenges nowadays is synthesizing graphene on a large scale, while retaining its excellent quality. A feasible method for producing large areas of single layer graphene, is growth by chemical vapor deposition (CVD). This deposition technique however, produces poly-crystalline graphene, with grain boundaries at the interface between crystals. These grain boundaries are known to have a negative effect on the electronic properties of graphene.This work describes the optimization of the quality of graphene, grown by CVD in the setup available at the Plasma & Materials Processing group of the Eindhoven University of Technology. In order to improve the quality, the amount of grain boundaries is reduced by decreasing the amount of nucleation sites.It is shown that the system pressure and the gas flow ratios can be used to control the nucleation density of graphene. The background pressure was increased by the addition of an argon gas flow to the precursor gas mixture. The increase in pressure caused by this inert gas resulted in a significant reduction in the nucleation density. The flows of the precursor gasses hydrogen and methane have also been optimized to further decrease the nucleation density. With the optimized growth condition, the nucleation density was decreased from (46.4 ± 15.4) ·103 mm-2 to (1.6 ± 0.3) ·103 mm-2.Furthermore, the effect of cleaning the copper substrate before it is used for the growth of graphene is investigated. It is found that the pre-cleaning of the copper foil with various cleaning agents reduces the nucleation density of graphene flakes. From the various solutions used, the best results have been obtained by pre-cleaning the copper sample for 30 s with 1 M nitric acid, before it is used for the synthesis of graphene. With this pre-cleaning step the nucleation density was reduced by a factor of ~2 to (1.1 ± 0.4) ·103 mm-2.The optimization of both the growth conditions and the pre-cleaning procedure have led to a decrease in the overall nucleation density by a factor of ~41, from (46.4 ± 15.4) ·103 mm-2 to (1.1 ± 0.4) ·103 mm-2. With Fourier transform infrared spectroscopy (FTIR) and Hall measurements, graphene grown with both the original and optimized growth processes was analyzed and compared. The mobility of the graphene grown with the optimized growth process was higher in all measurements. The optical mobility, obtained with FTIR, was 3.2·103 cm2/Vs. The mobility measured with the Hall setup was 2.7·103 cm2/Vs after annealing. The results of the comparative measurements are a strong indication that the amount of grain boundaries has been reduced, and that the quality of the graphene has improved by decreasing the nucleation density.
|Date of Award||31 Aug 2014|
|Supervisor||R.H.J. Vervuurt (Supervisor 1), J.W. Weber (Supervisor 2) & Ageeth A. Bol (Supervisor 2)|