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

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Control of electrical conductivity in diamond by boron-implantation
using an ECR ion source—application of high-temperature andhigh-pressure annealing
H. Yamazaki,∗1 T. Minamidate,∗2 M. Kidera,∗1 A. Yamamoto,∗3 R. Kato,∗4 and H. Ueno∗1Diamond is an excellent electrical insulator with alarge band-gap of 5.5 eV. It becomes a semiconductor
when doped with a small amount of boron (for p-type)or phosphorus (for n-type). Ekimov et al. reported that
B-doped diamond, when doped beyond the metal-toinsulator transition at nB ∼ 3 × 1020 B/cm3 , exhibits
superconductivity in samples grown by the high-pressureand high-temperature synthesis.1) Theoretically, the superconducting critical temperature Tc can be signiﬁcantly increased by reducing the eﬀects of the disorderin the B-doping processes.2) For a higher Tc , more subtle control of doping using CVD and/or MBE methods
is required. However, a diﬀerent method based on ion
implantation is also worth investigating, since it enables
selective ion-doping in a controlled manner. This has
great potential for future device applications.
We attempted to control the electrical conductivity indiamond by using the ion-implantation technique, utilizing RILAC at the RIBF facility. For n- and p-type semiconductors (and possibly superconductors), nitrogen andboron ions were implanted into diamonds, respectively.
By varying the beam intensity and irradiation time, theconcentration of nitrogen or boron was controlled. Notethat it is challenging to obtain the n-type semiconductor,
as well as the n-type superconductor, by nitrogen-dopingof diamond, since nitrogen behaves as a deep donor indiamond and does not contribute to conductivity.3)

Fig. 1. The process of high-temperature and high-pressure
annealing.
∗1
∗2
∗3
∗4

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

Fig. 2. Temperature dependence of electrical resistivity before and after annealing for B concentrations of (a)1.8 × 1021 , (b) 1.3 × 1022 , and (c) 6.8 × 1022 B/cm3 . The
possibility of the hopping conductivity in carrier-doped
semiconductor cannot be denied for these samples.

Fig. 3. Typical appearance of Ib- and IIa-type diamonds.

In this ﬁscal year, we continued studying boronimplanted diamonds. Boron ions were implanted into
diamond crystals (each size is 1 × 1 × 0.3 mm3 ) at 5 keV(implantation depth: ∼10 nm) by using an ECR ionsource.4) ) Ten samples of diﬀerent concentrations fromnB ∼ 4.9 × 1020 to 6.8 × 1022 B/cm3 were studied.
The magnetization and electrical resistivity measurements showed that the as-implanted diamonds do notexhibit superconducting transitions, even though nB ’s
are nominally beyond the metal-to-insulator transition
at 3 × 1020 B/cm3 . To reduce the lattice damage produced during the implantation, we performed annealing
treatments after implantation. The phase diagram ofcarbon shows that diamond is not stable at low pressures; we annealed the samples at 800◦ C and 4 GPa for
1 hour. The process of the annealing is shown in Fig. 1.
Contrary to our expectations, the annealed samples exhibited no sign of superconductivity (see Fig. 2).
The laser Raman spectra with 632.8 nm excitation
indicated that the annealing promoted the NV− center formation in diamond. To reduce this eﬀect, weare preparing for the boron implantation of IIa-type diamonds (Fig. 3) with nitrogen concentration less than
8 ppm.
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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