Ⅲ-1. Atomic & Solid State Physics (Ion)
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Control of electrical conductivity in diamondby boron-implantation using an ECR ion source
H. Yamazaki,∗1 T. Minamidate,∗2 M. Kidera,∗1 A. Yamamoto,∗3 R. Kato,∗4 and H. Ueno∗1
Diamond 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 (forp-type) or phosphorus (for n-type). When doped beyond the metal-to-insulator transition at a boron density of nB ∼ 3 × 1020 B/cm3 , boron-doped diamondexhibits superconductivity in the samples grown by thehigh-pressure and high-temperature synthesis.1) Theoretically, the superconducting critical temperature Tc
can be signiﬁcantly increased by reducing the eﬀects
of disorder in boron-doping processes.2) For a higher
Tc , more subtle control of doping using CVD and/or
MBE methods is highly required. However, a diﬀerent method based on ion implantation is also worth
investigating as it enables selective ion doping in a controlled manner. This has great potential for future device applications. Successful inclusion of boron ions by
the implantation technique has been conﬁrmed using
SEM/EDS measurements. The surface image of oursample and its EDS spectrum are shown in Fig. 1. At
0.19 keV, we can see an energy peak from the boron
ions embedded in the diamond.

Fig. 1. Left: SEM image of our sample with a boron concentration of nB ∼ 6.8 × 1022 B/cm3 ; right: EDS spectrum obtained with an acceleration voltage of 7.0 kV.
The strongest peak comes from the carbon atoms in thediamond.

For the past few years, we have been trying to control
the electrical conductivity in Ib-type diamonds usingboron-ion implantation. To reduce the lattice damage
produced during the implantation, we performed annealing treatments after implantation. As the phase
diagram of carbon shows, the diamond is not stable
at low pressures; therefore, we annealed the samples
at 800◦ C and 4 GPa for 1 hour. Contrary to our expectations, the annealed samples exhibited no sign ofsuperconductivity. The laser Raman spectra (632.8 nm∗1∗2∗3∗4RIKEN Nishina Center
Faculty of Science, Tokyo University of Science
Graduate School of Engineering and Science, Shibaura Institute of Technology
Condensed Molecular Materials Laboratory, RIKENexcitation) indicated that the annealing promoted theformation of NV− centers, which behave as carrier traps
in the diamond.
In this ﬁscal year, to decrease the NV− defect density, we used single-crystals of IIa-type diamond witha nitrogen concentration of less than 8 ppm (Fig. 2,right). We also investigated the dependence of boronimplantation yield on the crystallographic orientationof diamond surface: (100) and (111). Using an ECR
ion source,3) boron ions were implanted into the diamond crystals at 5 keV corresponding to an implantation depth of ∼10 nm. Five samples of diﬀerent concentrations from nB ∼ 5 × 1020 to 2 × 1022 B/cm3 wereprepared. These samples were annealed under the same
conditions as noted above.

Fig. 2. Pristine single-crystals of IIa-type diamond (right)
and Ib-type diamond (left, for comparison). The size ofeach crystal is approximately 1 × 1 × 0.3 mm3 .

Figure 3 shows a sample and some parts of highpressure media after removing the sample. For comparison, the implanted samples without annealing werealso prepared. We are now ready to measure the samples with diﬀerent combinations of the conditions: (a)ﬁve diﬀerent nB ’s, (b) two types of surface orientations:(100) and (111), and (c) with/without annealing treatment after implantation.

Fig. 3. The boron-implanted diamond (arrowhead) after theprocess of high-temperature and high-pressure annealing. Parts of high-pressure media are also shown.

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

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