Chemical vapor deposition of graphene: A route to device integration

Abstract

In this thesis, I have shown that the quality of synthetic graphene can be as high as mechanically exfoliated graphene if we can get rid of the wrinkles. Any defects, impurities and grain boundaries will induce scattering, preventing ballistic transport. The experiment described in chapter 4 was the first to demonstrate ballistic transport in synthetic graphene, and it will pave the way for the mass production of high quality monolayer graphene. In 2008, monolayer graphene was difficult to make and it was ranked as the most expensive material in the world. A piece of mechanically exfoliated monolayer graphene with size smaller than the diameter of a human hair will cost more than $1,000 from Graphene Research Ltd in Manchester. Currently (2014), Monolayer graphene single crystals with size up to millimeters in diameter have already been routinely produced worldwide, and the price per cm2 sample has dropped to less than $200. In chapter 5 of this thesis, a parameterless equation was provided to calculate the number of graphene layers by measuring the transmittance of graphene films. The search for novel transparent electrode materials with good stability, high transparency and excellent conductivity is driven by the required trade-off between transparency and conductivity: Metals are very conductive but not transparent; plastics are quite transparent but not conductive. Graphene is a transparent and conductive material. However, the conductivity of monolayer graphene might not be sufficient for fabricating a highly conductive electrode. The dilemma is that the transmittance of a graphene film decreases as the number of layers increases. It therefore is of great importance to have a fast and reliable method to determine the number of layers in the fabrication and measurement of multilayer graphene. Having a simple relation to determine how many graphene layers can meet the transmittance requirement and provide good conductivity at the same time is a valuable tool. I have also successfully demonstrated the integration of graphene into silicon chips, and the pressure sensor devices can potentially be produced in wafer scale. A sensitivity of ?8.5?mV/bar was obtained, and the standard deviation was less than 15?mbar at 700?mbar full scale. This chip scale integration of graphene devices is a key process to link the current semiconductor technology and the future applications of high quality graphene films. The further development of wafer scale automation transfer process is one of the directions to realize the valorisation of graphene. In chapter 7 of this thesis, I have shown the potential application of graphene for optoelectronics devices. The speed of optoelectronics will outperform the current transistors especially in transmitting high-volume information on chip scale. Although lots of technical issues are still waiting to be solved and the delivery of high quality and large quantity of graphene based products is still a key issue, tremendous progresses has been made in graphene research. In the scope of 10-20 years, I believe that the price of large-scale monolayer graphene will drop below $1/cm2 comparable to the price of gate-oxide, which will enable graphene to enter potential markets, such as optoelectronics computing, flexible sensors and wearable devices, etc. There will be bright future for graphene technology.