Calcium intercalation underneath epitaxial graphene on 6H-SiC(0001) were investigated by scanning tunneling microcopy (STM) and density functional theory (DFT) calculations. After the preparation of calcium-adsorbed graphene on 6H-SiC(0001), annealing the sample to 900 degrees C activated the intercalation of the adsorbed calcium into buffer layer (BL), monolayer (1L) and bilayer epitaxial (2L) graphene. Subsequent DFT modeling show the dependence of the most stable intercalation sites for calcium underneath BL, 1L and 2L graphene on the intercalation density. Further DFT calculations indicate charge transfer from the intercalant into graphene layers, leading to the n-type doping of the calcium-intercalated BL, 1L and 2L graphene. (C) 2018 Published by Elsevier B.V.