Ⅲ-1. Atomic & Solid State Physics (Ion)

RIKEN Accel. Prog. Rep. 53

Control of electrical conductivity in diamond by boron implantation—application of high-temperature and high-pressure annealing
H. Yamazaki,∗1 T. Minamidate,∗2 M. Kidera,∗1 A. Yamamoto,∗3 R. Kato,∗2 and H. Ueno∗1Diamond is an excellent electrical insulator with alarge band gap of 5.5 eV. Interestingly, it becomes a semiconductor when doped with a small amount of boron(for p-type) or phosphorus (for n-type). Ekimov etal. reported that B-doped diamond, when doped beyond the metal-to-insulator transition at nB ∼ 3 ×1020 B/cm3 , shows superconductivity in samples grownby high-pressure and high-temperature synthesis.1) Theoretically, the superconducting critical temperature Tc
can be increased substantially by reducing the eﬀects ofdisorder in the B-doping processes.2) For a higher Tc ,more subtle control of doping using CVD and/or MBE
methods is required, whereas a diﬀerent method basedon ion implantation is also worth investigating, since itenables selective ion doping in a controlled manner.
We attempted to control the electrical conductivity
in diamond by means of the ion-implantation techniquewith the use of RILAC at RIKEN. In our study, for nand p-type semiconductors (and possibly superconductors), nitrogen and boron ions are implanted into diamond, respectively. By changing the beam intensity andirradiation time, the concentration of nitrogen or boron
was controlled. Note that achieving an n-type semiconductor, and needless to say an n-type superconductor,
by the nitrogen-doping of diamond is challenging, sincenitrogen behaves as a deep donor in diamond and does
not contribute to conductivity.3) The electrical conductivity observed in the nitrogen-implanted diamonds (e.g.,at nN ∼ 7.5 × 1021 N/cm3 ) is, therefore, most likely explained in terms of the carbon atoms connected via thesp2 bonding produced by radiation damage. The Raman
spectra support this scenario.
In this ﬁscal year, we mainly investigated boron-Fig. 1. Phase diagram of carbon. The annealing condition inthis work is indicated by a red circle.
∗1
∗2
∗3

RIKEN Nishina Center
Condensed Molecular Materials Laboratory, RIKEN
Graduate School of Engineering and Science, Shibaura Institute of Technology

Fig. 2. Laser Raman spectra (632.8 nm excitation) of the
as-implanted and annealed samples for nB ∼ 6.8 ×
1022 B/cm3 . The broken lines indicate the fluorescence emission of the zero-phonon line (ZPL) and nphonon lines (nPLs; “phonon side band”) due to negatively charged nitrogen-vacancy (NV− ) defect centers.

implanted diamonds. Boron ions were implanted into
diamond crystals (each size is 1 × 1 × 0.3 mm3 ) at
5 keV (implantation depth: ∼10 nm) using an ECR ionsource.4) We prepared ten samples of diﬀerent concentrations ranging from nB ∼ 4.9 × 1020 to 6.8 × 1022 B/cm3 .
Measurements of the magnetization and electrical resistivity show that the as-implanted diamond samples do
not exhibit superconducting transitions, even though nBvalues are nominally beyond the metal-to-insulator transition at 3 × 1020 B/cm3 . In order to reduce the lattice damage produced during the implantation, we attempted annealing treatments after implantation. As
the phase diagram of carbon (Fig. 1) shows, diamond is
not stable at low pressures; we annealed the samples at
800◦ C and 4 GPa (in the diamond-stable region) for one
hour. The annealed samples, however, indicate no sign ofsuperconductivity. Rather, the annealing treatment degraded the diamond crystals: the (222) peak at a higher
angle in x-ray diﬀraction measurement disappeared after
annealing, whereas the (111) peak remained. Figure 2
shows a typical change in the Raman spectra after annealing. The ﬂuorescence emission due to NV− defectcenters is clear, suggesting that the annealing treatment
promotes the NV formation process, where nitrogen ionshave been embedded in the Ib-type diamond as impurities. Hereafter, we must also consider the eﬀect of theNV− centers on the electrical conductivity in the implanted diamonds.
References

1) E. A. Ekimov et al., Nature 428, 542 (2004).
2) T. Shirakawa et al., J. Phys. Soc. Jpn. 76, 014711 (2007).
3) S. J. Sque et al., Phys. Rev. Lett. 92, 017402 (2004).
4) M. Kidera et al., Eur. J. Mass Spectrom. 13, 239 (2007).

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