Electron Affinity and Effect of Annealing on Heavily Boron-Doped Diamond Films

A series of heavily boron-doped polycrystalline diamond films was studied. It was found that the electron affinity of the diamond surfaces increased with increasing doping concentration and the Fermi level was pinned at ∼0.35 eV above the valence band maximum (VBM). After annealing at ∼500°C, the electron affinity of all samples decreased and the Fermi level increased to 0.87 eV. These changes were concurrent with a decrease in the concentration of both boron and oxygen, and an increase in that of B x O y relative to elemental boron on the diamond surfaces. A model was proposed to explain the dependence of electron affinity on boron concentration, and to explain other changes observed upon annealing.