  Diamond has outstanding physical properties: the hardest known material, a
wide band gap, the highest thermal conductivity, and a very high Debye
temperature. In 2004, Ekimov et al. discovered that heavily boron-doped
(B-doped) diamond becomes a superconductor around 4 K. Our group successfully
controlled the boron concentration and synthesized homoepitaxially grown
superconducting diamond films by a CVD method. By CVD method, we found that
superconductivity appears when the boron concentration (nB) exceeds a
metal-insulator transition concentration of 3.0x10^20 cm^-3 and its Tczero
increases up to 7.4 K with increasing nB. We additionally elucidated that the
holes formed at the valence band are responsible for the metallic states
leading to superconductivity. The calculations predicted that the hole doping
into the valence band induces strong attractive interaction and a rapid
increase in Tc with increasing boron concentration. According to the
calculations, if substitutional doped boron could be arranged periodically or
the degree of disorder is reduced, a Tc of approximately 100 K could be
achieved via minimal percent doping. In this work, we have successfully
observed zero resistivity above 10 K and an onset of resistivity reduction at
25.2 K in heavily B-doped diamond film. However, the effective carrier
concentration is similar to that of superconducting diamond with a lower Tc. We
found that the carrier has a longer mean free path and lifetime than previously
reported, indicating that this highest Tc diamond has better crystallinity
compared to that of other superconducting diamond films. In addition, the
susceptibility shows a small transition above 20 K in the high quality diamond,
suggesting a signature of superconductivity above 20 K. These results strongly
suggest that heavier carrier doped defect-free crystalline diamond could give
rise to high Tc diamond.
