preparing a substrate in a reaction chamber;

performing a first growth process of growing a plurality of graphene aggregates apart from each other on the substrate at a first growth rate, the first growth process using a reaction gas comprising a carbon source; and

performing a second growth process of forming a graphene layer by growing the plurality of graphene aggregates at a second growth rate slower than the first growth rate, the second growth process using the reaction gas comprising the carbon source,

wherein the plurality of graphene aggregates that are grown in the second growth process are grown on the plurality of graphene aggregates formed in the first growth process.