First-Principle Study Of Superconductivity Of P-doped Diamond Thin Film

Diamond is well known as hardest material with notable mechanical property in the world. It also has wide applications in the industry because of its outstanding acoustical, optical, thermal, electrical properties. E. A. Ekimov and V. A. Sidorov found superconductivity in boron-doped diamond sintered at high pressure in 2004. Y. Takano and M. Nagao found superconductivity in CVD boron-doped diamond thin film in the same year. And the Tc of the thin film is higher than that of the bulk samples. The finding of superconductivity in diamond thin film creates a new attractive research field.The superconductivity of diamond thin film originates from the coupling of electrons and phonons which locate at the surface. First principle calculations make it possible to study the contribution from the coupling of every phonon and the electron. Through Density Functional Theory and Density Functional Perturbation Theory, we have simulated the electron and phonon properties of (001) and (111) oriented boron-doped diamond thin film. We analyzed the vibrations and electron-phonon couplings located on the surface and the effect to the transition temperature. Besides, the concentration of boron is the crucial factor which determines the DOS at fermi energy, the intensity of electron-phonon coupling and Tc. We simulated doping boron through Virtual Crystal Approximation, and found that Tc is enhanced with boron doping concentration increasing.First of all, we built up virtual pseudo-potentials to simulate doping level of 2%, 4%, 6% and 8%. Then we fitted the Marnaghan state function and got the balance volume. The volume expanded with the increase of doping level. We used the volumes to built up slabs with two free-surfaces. Each slab has 10 atom layers and 10 ? vacuum layer. We calculated the electron-phonon coupling constants atΓpoint through linear response theory, and found that bounce mode and sub-bounce mode contributed the largest amount ofλ, which also increased with the concentration of holes. Through the analysis to PDOS and density of electrons, we found that the electrons near fermi surface is localized in the area under the dimer, which resulted in intensively coupling between electrons and phonons.Next we built up diamond(111)(2×1)Pandey chains reconstruction surfaces, and then calculated the phonon modes and electron-phonon coupling constants atΓpoint. Then we found that the mode of symmetric bounce contributed more on the electron-phonon coupling. Meanwhile the vibrations of the atoms under the reconstructed surface coupled strongly with the electrons, and the electron-phonon coupling was enhanced.λof (111) oriented film was as two times large as that of (001) oriented film and was higher than doped bulk diamond.More effective electron-phonon coupling resulted in thatλof (111) oriented film is higher than that of (001) oriented film. Higher DOS at fermi energy and vibrations of atoms under surface layer are another two important reasons. We used McMillan equation to estimate Tc of the two systems, and Tc of (111) oriented film was higher than that of (001) oriented film in different concentrations.