JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

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NV Centers in Diamond Irradiated with High Energy
Nitrogen Ions and 2 MeV Electrons
J. Isoya a) , T. Umeda a) , N. Mizuochi a) , T. Ohshima b) , S. Onoda b) , S. Sato b) and N. Morishita b)
a) Graduate School of Library, Information and Media Studies, University of Tsukuba,
b) Environment and Industrial Materials Research Division, QuBS, JAEA
The NV (nitrogen-vacancy) center(S=1, C3v symmetry)
in diamond is a negatively charged state ([NV] ) of a pair of
substitutional nitrogen and adjacent vacancy. With the
remarkable properties such as extremely long coherence
time and the ability to initialize and readout individual spins
optically at room temperature, the NV center is a potential
candidate for solid state devices of quantum information
processing. In the quantum computing, scalability (i.e. the
increase of qubits) is attainable by fabrication of a chain-like
array of NV centers with a separation (50 nm) long enough
for individual addressing. With such a long separation, the
weak dipole-dipole interaction which is used for 2-qubit
CNOT gate necessitates a long coherence time (T2). The
long T2 is attained by the enrichment of 12 C isotope having
zero nuclear spin and lowering paramagnetic nitrogen
impurity (isolated substitutional nitrogen [Ns] 0 called P1).
In ion implantation, which is a key technology for position
controlled incorporation of NV centers, yield optimization to
convert nitrogen atoms implanted to NV and elimination of
residual unwanted defects having electron spins which
shorten T2 of implantation-produced NV centers are required.
For understanding defect behaviors, in the present work,
conversion from P1 to NV with minimizing residual
unwanted defects by using 2 MeV electron irradiation has
been also studied.
(1) 2 MeV electron irradiation
In HPHT (high pressure high temperature) synthesis of
diamond, the concentration of P1is controlled from 1 ppb
to 300 ppm by varying the composition of the metal
solvent. Electron irradiation creates vacancy. Vacancy
starts to diffuse in the lattice above 600 C. Thus, NV
centers are formed by trapping the vacancy by P1 during
thermal annealing (800 C). Since the negative charge of
NV centers is attained by donation of an electron from the P1
center, the P1 centers are converted to [NV] and [N s] + (S=0).
Since a part of vacancies created is used for
recombination with interstitials, a large dose is required for a
full conversion of P1 to [NV] if the initial concentration of
P1 is high. We used electron irradiation at 450 C to
eliminate excess accumulation of unwanted defects before
annealing. We have achieved various concentration of NV
from 0.1 ppm to 15 ppm by using various HPHT crystals
with various concentrations of P1. Under an excess dose
of electron irradiation, a fraction of nitrogen-vacancy pair in
the neutral charge state ([NV] 0 ) increases, since the negative
charge requires for an electron donated from P1. It has
been revealed that the conversion ratio from P1 to NV is
different for different P1 concentrations. Thus, for attaining
a full conversion of P1 to [NV] - without remaining P1, the
dose of electron irradiation has been carefully chosen.
Here, we demonstrate how the conversion ratio depends
on the P1 concentration by using a HPHT crystal which has
two growth sectors with different P1 concentrations. The
spins excited by the microwave pulses contribute to the
JAEA-Review 2010-065
- 16 -
instantaneous spectral diffusion. By using the instanta-
neous spectral diffusion effect in the 2-pulse echo decay, the
dipolar broadening among the P1 spins and that among the
[NV] spins were extracted. With two different
concentrations of P1/[NV] , the 2-pulse echo decay of
P1/[NV] exhibited to be biexponential. By plotting b (the
inverse of the time constant) of the 2-pulse decay as a
function of sin 2 (θ p/2) where θ p is the flipping angle of the
second pulse, the dipolar broadening of each sector is
obtained from the slope. From the dipolar broadening, the
concentration was estimated. It has been found that the
conversion ratio from P1 to NV is different for the two
sectors with different P1 concentrations.
b(10
GYAN_P1
2
2
y = 1.6059x + 0.2849
5 s ‐1 ) b(105 s ‐1 b(10
GYAN_P1
)
2
2
y = 1.6059x + 0.2849
5 s ‐1 ) b(105 s ‐1 )
b(10**5 sec**-1)
1.5
1
0.5
[P 1]=33 ppm
[P1]=5.2 ppm
y = 0.2542x + 0.1209
0
0 0.5
sin[(theta/2)**2]
1
sin2 sin (θp /2) 2 (θp /2)
b(10**5 sec**-1)
1.5
0.5
GYAN_NV
y = 0.1234x + 0.1735
y = 0.0432x + 0.0455
0
0 0.2 0.4 0.6 0.8 1
Fig. 1 Instantaneous diffusion of electron-irradiated
(5.6 × 10 18 e/cm 2 , 450 C) and annealed (800 C, 2 h)
type Ib crystal.
(2) High energy nitrogen ion implantation
To search a condition which optimizes the yield of [NV] -
and minimizes the remaining unwanted defects, nitrogen
implantation (1 × 10 13 cm -2 for each of 7 steps of energy
between 4 MeV and 13 MeV for both sides of 4.5× 4.5×
0.5 mm 3 substrate) at high temperatures (800 C, 1,000 C,
and 1,200 C) was carried out. High sensitivity ESR
(electron spin resonance) technique detects 1.1× 10 12 spins
of the P1 center in the high-purity ([P1]=0.6 ppb) CVD
single crystal substrate (Element6) before implantation.
After implantation of 2.4× 10 13 nitrogen ions, the increase of
the signal intensity of P1 was not detected. PL
measurements suggest that nitrogen ions are likely to be
incorporated dominantly as nitrogen-vacancy pair below
1,000 C. At the irradiation temperature of 1,200 C, it is
noticed that the production of the H3 center (N-V-N)
increases. For the identification of the charge state of
nitrogen-vacancy pairs produced by ion implantation,
further studies (ESR under light illumination, PL with
excitation laser with longer wavelength) are being carried
out.
1
[NV - [NV ] - ]
sin[(theta/2)**2]
sin2 sin (θp /2) 2 (θp /2)
6.5 ppm
2.3 ppm