Electronic Structures of Heavily Boron-Doped Superconducting Diamond Films
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0956-J03-01
Electronic Structures of Heavily Boron-Doped Superconducting Diamond Films Takayoshi Yokoya1, Hiroyuki Okazaki1, Tetsuya Nakamura2, Tomohiro Matsushita2, Takayuki Muro2, Eiji Ikenaga2, Masaaki Kobata2, Keisuke Kobayashi2, Akihisa Takeuchi2, Akihiro Awaji2, Yoshihiko Takano3, Masanori Nagao3, Tomohiro Takenouchi4, Hiroshi Kawarada4, and Tamio Oguchi5 1 The Graduate School of Natural Science and Technology, Okayama University, 3-11,Tsushima-naka, Okayama, 700-8530, Japan 2 Japan Synchrotron Radiation Research Institute(JASRI)/Spring-8, 1-1-1 Kouto, Sayo, 679-5198, Japan 3 National Institute for Material Science, 1-2-1 Sengen, Tsukuba, 305-0047, Japan 4 School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, 169-8555, Japan 5 Department of Quantum Matter, Graduate School of Advanced Sciences of Matter (ADSM), Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, 739-8530, Japan
ABSTRACT Recent photoemission studies on heavily boron-doped superconducting diamond films, reporting the electronic structure evolution as a function of boron concentrations, are reviewed. From soft X-ray angle-resolved photoemission spectroscopy, which directly measures electronic band dispersions, depopulation of electrons (or formation of hole pockets) at the top of the valence band were clearly observed. This indicates that the holes at the top of the valence bands are responsible for the metallic properties and hence superconductivity at lower temperatures. Hard X-ray photoemission spectroscopy observed shift of the main C 1s core level and intensity evolution of a lower binding energy additional structure, suggesting chemical potential shift, carrier doping efficiency by boron doping, and possibility of boron-related cluster formations. INTRODUCTION Diamond has outstanding properties, including hardness, high thermal conductivity, and a wide band gap, and is regarded as a very promising semiconductor [1]. In 2004, Ekimov et al. have reported that heavily boron-doped diamond, made from a high pressure and high temperature synthesis, exhibits superconductivity below a transition temperature of ~ 4 K [2]. Reproducibility was confirmed with films made by microwave plasma assisted chemical vapor deposition (MPCVD) [3,4] and a transition temperature (Tc) of 11.4 K was achieved in a MPCVD film [5]. Thus, superconductivity adds another functionality to diamond, which might lead to new diamond-related devices when combined with other properties. For developing new diamond-based devices and/or to understand their characteristics, electronic structure diagrams provide fundamental information, as in lightly doped semiconductor devices [6]. The electronic structure of heavily boron-doped diamond is important for the development of new diamond devices. Scientifically, how an insulator with a wide band gap becomes a metal by doping has not been experimentally proven to date and therefore is a fundamental question [7]. Indeed,
depending on the origin of the metallic states (diamond band or impurity band), differe
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