Freestanding n-Doped Graphene via Intercalation of Calcium and Magnesium into the Buffer Layer–SiC(0001) Interface

Abstract

The intercalation of epitaxial graphene on SiC(0001) with Ca has been studied\nextensively, yet precisely where the Ca resides remains elusive. Furthermore,\nthe intercalation of Mg underneath epitaxial graphene on SiC(0001) has not been\nreported. Here, we use low energy electron diffraction, x-ray photoelectron\nspectroscopy, secondary electron cut-off photoemission and scanning tunneling\nmicroscopy to elucidate the physical and electronic structure of both Ca- and\nMg-intercalated epitaxial graphene on 6H-SiC(0001). We find that Ca\nintercalates underneath the buffer layer and bonds to the Si-terminated SiC\nsurface, breaking the C-Si bonds of the buffer layer i.e. 'freestanding' the\nbuffer layer to form Ca-intercalated quasi-freestanding bilayer graphene\n(Ca-QFSBLG). The situation is similar for the Mg-intercalation of epitaxial\ngraphene on SiC(0001), where an ordered Mg-terminated reconstruction at the SiC\nsurface and Mg bonds to the Si-terminated SiC surface are formed, resulting in\nMg-intercalated quasi-freestanding bilayer graphene (Mg-QFSBLG).\nCa-intercalation underneath the buffer layer has not been considered in\nprevious studies of Ca-intercalated epitaxial graphene. Furthermore, we find no\nevidence that either Ca or Mg intercalates between graphene layers. However, we\ndo find that both Ca-QFSBLG and Mg-QFSBLG exhibit very low workfunctions of\n3.68 and 3.78 eV, respectively, indicating high n-type doping. Upon exposure to\nambient conditions, we find Ca-QFSBLG degrades rapidly, whereas Mg-QFSBLG\nremains remarkably stable.\n