Control of Electronic Properties of Diamond byBoron / Nitrogen Injection Using High Energy Ion Source
Takaaki Minamidate1, Hiroki Yamazaki2, Masanori Kidera3, Hiroshi Yamamoto4, Hideki Ueno2,and Reizo Kato1
1Condensed Molecular Materials Laboratory, RIKEN, 2 Nuclear Spectroscopy Laboratory, RIKEN,
3Ion Source Team, RIKEN, 4Institute for Molecular Science

Pure diamond is a good insulator with a wide bandgap of 5.5 eV, and it is known that it shows
semiconducting properties when the boron or phosphorus is induced as the carrier donor. Then thedoped-diamond is expected as a next generation semiconductor device. In addition, in 2004, it wasdiscovered by E. A. Ekimov et al. that the superconductivity appears in the boron-doped diamonds in a highconcentration region [1]. As long as it has been reported so far, with increasing the boron concentration, thesuperconducting transition temperature (TC) rises [2]. According to the theoretical calculations by T.
Shirakawa et al., we can increase TC when diamond is properly implanted with boron ions at its specificlattice positions of carbon. Thus the research on ion-doped-diamond using MBE (molecular beam epitaxy)
and CVD (Chemical Vapor Deposition) methods, which make subtle doping-control possible, has been
actively carried out. At present, TC exceeds 10 K for the CVD-grown diamond at the boron concentration of~1022 cm-1 [2].
In the present study, instead of MBE or CVD, we tried

to control the electronic states by the ion implantation using
ECR (Electron Cyclotron Resonance) ion sources. Thismethod provides extremely high-concentrations of ions, andvarious ion species can be injected. As is known in thecommercial semiconductors such as Si and GaAs, thedamage in the crystal and inhomogeneity of dopants areimproved by the thermal annealing process. In the case ofdiamond, since the diamond is the high-pressure phase ofFig. 1: Sample holder.

carbon, the conversion of diamond to graphite can occur bysimply heating the diamond. The thermal annealing
treatment of diamond is still a hot research field. In thisresearch, by suppressing extraction voltage, lattice damageis minimized.
This year, the boron-doped diamonds were prepared forthe hole-doped systems, and the experimental results werecompared with the literature data. The expected ion
concentration was calculated from the ion beam intensity

Fig. 2: Boron-doped diamond.

and exposure time. The ion concentration was systematicallyvaried among the samples by changing the exposure time. Figure 1 shows the samples placed on the sample
holder. A typical doped diamond is shown in Fig. 2. The right-hand side of the sample, which wastransparent before the irradiation, turned black.
First, in order to investigate the electric conductivity, we measured the electrical resistance by theconventional four-probe DC method. Figure 3 shows the electric resistivity for the samples with ion
concentrations (ρ) of 2×1021, 4×1021, and 2×1022 cm-3. The data of each sample is normalized with

respect to the room temperature value. The electricalconductivity systematically increases with increasing ρ .
The data points are very scattered. This behavior can be seen
when there is a problem in the electrode contact. We may
improve the situation by increasing the doping density on thesurface or by changing the method of the electrodepreparation.
Subsequently, in order to detect (a) the paramagnetic
moment due to the formation of lattice defects and (b) theappearance of superconductivity, the static magnetization
measurements were carried out using MPMS (QuantumDesign). The temperature dependence of the magnetization atFig.

the magnetic field of 0.1 T for various ρ is shown in Fig. 4.

resistivity of B-doped diamond for each doping

In these measurements, no Meissner effect associated withdensity.

3:

Temperaturedependenceofthethe superconducting transition was observed. Each curve ofthe magnetic susceptibility is composed of the Curie
component and a temperature-independent paramagnetic
component. We see that, as ρ increases. The T-independent
component

apparently

increases

in2122-3

thehigherion

concentrations of 8×10 and 2×10 cm . In general, the purediamond shows diamagnetism; however a paramagnetic
behavior is seen even in the sample without doping. The
diamond used here contains 0.1% nitrogen intrinsically, sothat the observed paramagnetic moment likely comes fromthe nitrogen-vacancy pairs, and/or the deterioration at thesections that were heated up in the laser-cutting process. We
are now investigating the origins of these paramagneticcomponents.

magnetization of B-doped diamond for eachAccording to the previous research, the doping amount of
21

10 ~1022cm

Fig. 4: Temperature dependence of the static-3

doping density.

is enough to drive the electronic state of

diamond into the metallic state and further into the superconducting state. Our results, however, show thatthe electric resistance exhibits the semiconducting behavior without any signs of superconductivity. It isprobable that effective ion concentrations were smaller than the calculated ones. As shown in Fig. 2, thedepth of black color varies across the specimen, suggesting that there is an inhomogeneous distribution of thedopants. Next year, we will attempt to carry out the thermal annealing to control the inhomogeneity of theion concentration and prepare the sample with more boron concentration to examine the relation between theelectronic state and ion-concentration.
Reference[1] E. A. Ekimov et al., Nature, 428, 542 (2004).
[2] H. Umezawa et al., arXiv:cond-mat/0503303 (unpublished).
[3] T. Shirakawa et al., J. Phys. Soc. Jpn., 76, 014711 (2007).