The Impact of Hydrogen and Oxidizing Impurities in Chemical Vapor Deposition of Graphene on Copper

Le graphene, un materiau bidimensionnel constitue d'une monocouche d’atomes de carbone, a attire l’attention des scientifiques et des technologues pour ses proprietes mecaniques, electriques et optoelectroniques remarquables. L'utilisation du graphene dans des applications pratiques necessite toutefois le developpement de methodes de synthese fiables et rentables pouvant produire de larges films de graphene avec une faible densite de defauts et une morphologie bien controlee. Une des methodes les plus significatives est la croissance catalytique de graphene par depot chimique en phase vapeur (CVD) sur substrats metalliques. La croissance a haute temperature (850-1100°C) a partir d’un melange d’hydrocarbures et d’hydrogene permet de produire de larges feuillets de graphene polycristallin et d’excellente qualite. Cette voie de synthese est prometteuse car elle ouvre la voie a une production de graphene a grande echelle et une commercialisation massive de produits a base de graphene. Cette these porte sur la synthese du graphene sur des feuillets de cuivre polycristallin par la voie CVD a basse pression (LP-CVD) avec du methane comme source de carbone et de l’hydrogene pour assurer un environnement reducteur. Plus precisement, elle porte sur la determination du role de l'hydrogene et des impuretes oxydantes pendant la formation de graphene. Le but ultime de la these est d’elaborer les bases fondamentales pouvant mener a un procede de croissance commercialement viable et reproductible de graphene de haute qualite pour d’eventuels procedes de fabrication industriels. La premiere partie de la these traite du role de l'hydrogene moleculaire dans la croissance CVD de graphene. A partir d’experiences de recuits, certaines etudes ont affirme que l’hydrogene moleculaire vient graver le graphene sur cuivre. Nous avons verifie que cette gravure du graphene vienne plutot de la presence de traces d'oxygene dans l'atmosphere du four ou dans la bombonne d’hydrogene. Pour cela, nous avons adopte une approche basee sur des experiences de recuits systematiques de graphene sur des feuilles de cuivre en presence d'hydrogene de differentes puretes. ----------Abstract Graphene, the single-atom layer of carbon, has attracted scientists and technologists due to its outstanding physical and opto/electronic properties. The use of graphene in practical applications requires a reliable and cost-effective method to produce large area graphene films with low defects and controlled thicknesses. Direct growth of graphene using chemical vapor deposition (CVD) on copper, in which carbonaceous gaseous species react with the metal substrate in the presence of hydrogen at high temperatures (850-1100° C), led to high coverage of high quality graphene, opening up a promising future for methods of this type and a large step towards commercial realization of graphene products. The present thesis deals with the synthesis of graphene via low pressure CVD (LP-CVD) on copper catalyst using methane as the carbon precursor. The focus is mainly on the determination of the role of hydrogen and oxidizing impurities during graphene formation with an ultimate purpose: to elucidate a viable and reproducible method for the production of high quality graphene films compatible with industrial manufacturing processes. The role of molecular hydrogen in graphene CVD is explored in the first part of the thesis. Few studies claimed that molecular hydrogen etches graphene films on copper by conducting annealing experiments. On the other hand, we speculated that this graphene etching reaction is due to the presence of trace amount of oxygen in the furnace atmosphere. Thus, we took another approach and designed systematic annealing experiments to investigate the role of hydrogen in the etching reaction of graphene on copper foils. No evidence of graphene etching on copper was observed when purified ultra high purity (UHP) hydrogen was used at 825 °C and 500 mTorr. Nevertheless, graphene films exposed to the unpurified UHP hydrogen were etched due to the presence of oxidizing impurities. Our results show that hydrogen is not responsible for graphene etching reaction and oxygen impurities are the main cause of this etching reaction. We have also determined that graphene etching reaction is catalyzed by the copper surface. Next, we systematically investigated the role that hydrogen plays during the growth and coolingdown stage of LP-CVD of graphene on copper.