New Journal of Physics
The open–access journal for physics

Creation efficiency of nitrogen-vacancy centres
in diamond
S Pezzagna1,2,4 , B Naydenov3 , F Jelezko3 , J Wrachtrup3
and J Meijer1
1
RUBION, Ruhr-Universität Bochum, Universitätsstrasse 150, 44780 Bochum,
Germany
2
Research Department IS3 /HTM, Ruhr-University Bochum, 44780 Bochum,
Germany3
3rd Physical Institute, University of Stuttgart, Stuttgart, Germany
E-mail: sebastien.pezzagna@rub.de
New Journal of Physics 12 (2010) 065017 (8pp)

Received 29 November 2009
Published 28 June 2010
Online at http://www.njp.org/
doi:10.1088/1367-2630/12/6/065017

Abstract. Nitrogen-vacancy (NV) colour centres in diamond are attracting
growing attention due to potential applications in solid-state quantum information processing and magnetometry. Although proof-of-principle experiments
have been demonstrated, further development requires the controllable production of defects with a high yield. In this paper, we experimentally show that the
production efficiency of NV defects strongly depends on the ion implantationenergy. This can be explained in terms of the number of vacancies produced per
implanted ion and surface proximity. The dependence on ion fluence is also underlined, revealing a nonlinear regime and showing how the diamond lattice isdamaged at higher fluences.

Quantum computers based on solid state quantum bits (qubits) attracted the attention of many
research groups in the last decade [1, 2] because they promise good possibilities for scaleup. Tools to manipulate and implant single atoms, clusters or molecules with nanometer
resolution have been successfully developed [3]–[7]. Today, colour centres in diamond andmore particularly the nitrogen-vacancy (NV) centre are one of the most promising systems
as they fulfil most of the requirements for solid-state qubits (the so-called Di Vincenzo
criteria) [8]–[10]. Whether or not scalable quantum computers based on NV can be created4

Author to whom any correspondence should be addressed.

New Journal of Physics 12 (2010) 065017
1367-2630/10/065017+08$30.00© IOP Publishing Ltd and Deutsche Physikalische Gesellschaft

2remains a major question directly related to the creation efficiency of single NV centres. These
colour centres possess a strong optical transition from the triplet ground state to the triplet
excited state. It has been demonstrated that the electron spin of a single NV centre can becoherently manipulated and optically read out at room temperature [11]–[13]. Moreover, it
has been shown that the electron spin coherence time can reach 2 ms [13] in specially growndiamond crystals by using nuclear spin free carbon sources (12 C) and by removing otherparamagnetic impurities. Recently, the potential of application of single NV centres has been
extended to ultrasensitive, nanoscale magnetic field sensors [14]–[16].
In most of the reported experiments, the NV centres were created during the diamond
growth. For scalable quantum computer architecture and for magnetometry applications, one
needs an accurate positioning of the NV centres as well as a high probability of creation. These
two conditions have not yet been simultaneously fulfilled. On the one hand, high yield for NV
centres creation (the ratio between active NV centres and the number of implanted N atoms) has
already been demonstrated to be up to 50% for high-energy (2 MeV) implantation of nitrogenions [17], but with an intrinsic straggling of several tens of nanometers. On the other hand,
only low-energy implantation (a few keV) can ensure a spatial resolution in the nm range butwith much lower yield. For example, Rabeau et al [18] reported a 2.5% yield of single NV
centres. Usually after implantation, the samples are annealed at high temperature in vacuum inorder to induce diffusion of the vacancies, which can then be trapped by the implanted nitrogenatoms [19, 20]. The yield could also be influenced by other factors, such as the background
doping or impurity level [21], and, in the case of shallow implanted N, by the proximity to thesurface where the vacancies can disappear.
Here, we report on the creation efficiency of NV centres in diamond by direct implantationof nitrogen ions. A large range of energies has been used (between 1.5 keV and 18 MeV) inorder to give a complete picture of how the yield of NV creation depends on the ion energy. The
samples were characterized using a home-built confocal microscope. The influence of the ion
fluence on the NV creation efficiency was also studied and will be discussed below. The idea
behind these experiments is to obtain a better understanding of the NV centres formation andhow to find conditions to improve their production yield.
Before the implantation, the samples are cleaned in an ultrasonic bath of warm solvent(acetone followed by methanol). After the implantation, the diamonds are annealed for 2 hours
at 800 ◦ C in vacuum (pressure 10−7 mbar). Then the samples are cleaned in a boiling acidmixture (nitric acid/sulfuric acid/perchloric acid, 1 : 1 : 1) in order to remove graphitic residues
and other contaminations from the surface. The implantations in the MeV range are performedin the 4 MV Dynamitron-Tandem accelerator at the University of Bochum. The ion beam can befocused down to diameters 61 µm by using a superconducting lens [22]. For the implantationsin the keV range, a focused ion gun with a gas source is used in a vacuum chamber (10−7 mbar).
Either molecular or atomic species can easily be selected using a mass filter. The ion beam
is focused by a single electrostatic lens, and the beam diameter (between 10 and 40 µm) ismeasured on a test sample covered with photoresist (PMMA), which changes its colour underion impact. This effect is directly observed with an in situ optical microscope. Finally, the ion
current is measured both with a Faraday cup having 0.01 pA precision and with an electron
multiplier (in ion counting mode) placed after a 1 µm aperture.
In a first experiment, 15 N+ and 15 N+2 stripes are implanted at acceleration voltages of 3, 4 and5 kV. Figure 1(a) shows a map of fluorescence intensity from the created NV− centres (bandpass optical filters were tuned for selective detection of negatively charged NV defects). It can
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Figure 1. (a) Fluorescence image of an area of a diamond sample presenting

vertical stripes implanted with different nitrogen ions (atoms or molecules) and atdifferent acceleration voltages. The narrow lines were implanted using a 10 µm
aperture and the wide lines using a 30 µm aperture. The colour scale gives
the recorded photon counts per second. The horizontal line was implanted asa position reference. (b) 15 × 15 µm2 zoom scan in a focused spot implantedwith 5 keV 15 N+ ions (diameter 25 µm, fluence of 5 × 1010 cm−2 ). The density of
NV is estimated around 4 NVµm−2 in the centre. Outside this spot, almost no
NV is present, illustrating the high purity of the diamond sample.
be directly seen that, even within these relatively small energy differences, the fluorescenceintensity increases significantly with the ion energy, for both the atomic and the molecularions. Moreover, all the 15 N+ stripes are brighter than the 15 N+2 stripes, which have neverthelessbeen implanted with a higher nitrogen atom density than the 15 N+ ones (5 × 1013 cm−2 and2.5 × 1013 cm−2 , respectively). Similar implantations have been repeated with a lower and equal
fluence of 5 × 1012 nitrogen atoms per cm2 . Again, the same behaviour is observed. For such
experiments, the yield Y is given by the following equation:
Istripe 1
Y=
· ,
(1)
INV F
where Istripe is the averaged fluorescence intensity (counts/s and per unit area) of each stripe fromwhich the background fluorescence level has been subtracted, INV is the fluorescence intensityof a single NV at the same excitation power (calibrated by measuring a saturation curve), and F
is the ion fluence. A yield of 0.75% is found for 15 N+ ions at 5 keV and fluence of 5 × 1012 cm−2 .
In the case of lower fluences, one has the possibility of counting directly the numberof NV per unit area. This is illustrated in figure 1(b). A 5 keV 15 N+ focused beam (25 µm
diameter) is used to implant a fluence of 5 × 1010 ions cm−2 . Single NV centres can be seen in
the 15 × 15 µm2 zoom and an average density of 4 NV µm−2 can be determined at the centreof the implanted spot. This gives a yield of 0.8%. Note that such a density corresponds to thesuperior limit of what is measurable by this technique, as regards the size of a confocal spot
∼250–300 nm.
For MeV implantations, patterns such as the one shown in figure 2(a) are implanted witha beam focused below 1 µm. The created NVs cannot be resolved within a confocal spot.
New Journal of Physics 12 (2010) 065017 (http://www.njp.org/)

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Figure 2. (a) Fluorescence image of an NV pattern implanted by 2 MeV nitrogenions. (b) Second-order auto-correlation function measurement showing a single(top) and a double (bottom) NV centre. The dip in both graphs occurs at zerotime delay.
Nevertheless, if the spots contain only a few NV centres, it is still possible to determine their
number by measuring the emitted photon statistics. A single two-level system cannot emit two
photons at the same instant and one can determine the number of emitters in a confocal spot bymeasuring the second-order auto-correlation function:g (2) (τ ) =hI (t)I (t + τ )i,hI (t)i2

where the fluorescence intensities at time t and t + τ are denoted by I(t) and I (t + τ ),respectively. Figure 2 shows g (2) for a single and a double NV, where at zero delay (τ = 0)
a dip is observed (anti-bunching) dropping to zero (figure 2(b), top, single NV) or one half
(figure 2(b), bottom, double NV). By performing this experiment for all implanted NVs, one
could precisely determine the creation efficiency.
In figure 3(a), the yield of NV creation as a function of the ion energy is plotted. The
points corresponding to 1.5, 2, and 2.5 keV were measured by using 15 N+2 molecular ions having
the energy of 3, 4 and 5 keV, respectively. All other points were determined by implanting
15 +N . It is assumed that when a 15 N+2 ion hits the diamond surface, it splits into two atoms
having half of the initial energy. Figure 3(a) shows that the creation efficiency of NV centres
increases continuously with increasing the ion energy. The slope is strong in the keV region andit becomes smoother at MeV energies. A high yield of 45% is reached at 18 MeV, while forenergies lower than 5 keV it is under 1%. As already mentioned, the implantation of low energy
ions (<10 keV) is desirable for improving the positioning of the NV. Indeed, the ion straggling
is in the range of a few nanometers only for such low energies, as shown in figure 3(c) (SRIM
calculation [23]).
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5yield (%)

(a)

energy per nitrogen atom (keV)(c)

energy per atom (keV)

ion straggling (nm)average depth (nm)vacancies created per lon(b)energy per atom (keV)

Figure 3. (a) Plot of the measured yield for NV centre creation as a function of

the energy per nitrogen atom. (b) Number of vacancies created per implanted
15 +N ion as a function of the ion energy (simulated with SRIM). (c) On the left
axis: average depth of the implanted 15 N+ ions as a function of the ion energy.
Right axis: longitudinal and transverse ion stragglings as a function of the ion
energy (simulated with SRIM).
It is well known that every implanted ion induces along its path in the crystal a certain
number of vacancies. The probability of obtaining an NV centre from an implanted nitrogen
atom is thus expected to increase with the amount of available vacancies (as long as the crystal
lattice is not too damaged). Using SRIM simulations [23], one can estimate the number ofvacancies and their distribution along the implantation axis. Figure 3(b) shows the number ofcreated vacancies per ion as a function of the ion energy. The higher the energy, the morevacancies are created around the substitutional nitrogen atom, giving rise to a higher yield asshown by the experimental data in figure 3(a).
Moreover, the curve in figure 3(b) shows a similar behaviour, namely a strong slope at low
energy, which becomes smoother at high energy. The yield and number of vacancies appear tobe well correlated. Furthermore, if the two curves are superimposed, they start to diverge forenergies lower than 10 keV. Note that, in this case, the nitrogen atoms and their surrounding
New Journal of Physics 12 (2010) 065017 (http://www.njp.org/)fluorescence intensity (Cts/s)

yield (%)

6

fluence (nitrogen atoms / cm2)

average N-N distance (nm)fluence (N / cm2)

fluence (N / cm2)

Figure 4. (a) Fluorescence intensity versus fluence for atomic and molecular ionsaccelerated at 5 kV. The fluence is the number of nitrogen atoms implanted per
cm2 , i.e. a 15 N+2 ion gives two atoms. (b) Yield versus ion fluence for atomic
ions at 5 keV. The dotted curve is a guide for the eye. (c) Red: average in-plane
distance between implanted N atoms as a function of the ion fluence. Green:vacancy volume concentration with respect to the carbon atoms in the 10 nm
layer under the surface. At 5 keV, the implanted nitrogen atoms have an average
depth of 8 nm with a 2 nm straggling, and all the vacancies are produced within
10 nm depth.
vacancy clouds are very close to the surface. This can be seen in figure 3(c). For example, themean depth of a 15 N+ ion implanted at 5 keV is approximately 8 nm and most of the vacancies
are distributed between the surface and the nitrogen atom. The proximity of the surface is likely
to explain the drop in the yield towards low energy as the few vacancies created can easily bedestroyed on the surface during the annealing step. Although the diffusion of vacancies is notwell understood, this has to be taken into account, especially for such shallow implantations.
A possible way of studying this effect would be to overgrow the diamond samples, inorder to bury deeper the N atoms. One should also consider the presence of a thincontamination/adsorption layer at the diamond surface, which may also lower the apparent yieldfor low-energy ions (if some of the latter do not reach the diamond). Nevertheless, simulations
show that it can be neglected for energies higher than 1 keV.
The yield strongly depends on the ion energy, mainly through the number of vacancies
available to form NV centres. We have also studied the NV creation efficiency as a functionof the ion fluence at the energy of 5 keV, of particular interest for the low ion straggling. For
this purpose, 15 different areas have been implanted (eight with 15 N+ and five with 15 N+2 ) withfluence from 6.9 × 1010 cm−2 to 3.5 × 1014 cm−2 . The yield is determined by measuring thefluorescence intensity of each implanted zone. The results are plotted in figure 4(a), for both theatomic (blue stars) and the molecular (red triangles) species.
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It can be seen that, as expected from figure 3(a), at an equivalent fluence, more NVs
are produced in the case of atomic ions. For 15 N+ ions the fluorescence increases almost
linearly with fluence, up to 2 × 1012 cm−2 . If the fluence is further increased, the curve bends;
then the fluorescence intensity reaches a maximum (at approximately 1.2 × 1014 cm−2 ) andthen starts to decrease. The same behaviour is observed for the 15 N+2 ions. This phenomenon
illustrates the ion-induced damage of the crystal lattice, which becomes more important andfinally leads to the amorphization of the diamond [24]–[26]. We estimate it to occur atabout 5 × 1021 vacancies cm−3 . This value is comparable with the damage density thresholdof 1022 vacancies cm−3 given by Uzan-Saguy et al [24] for diamond. According to them, thisthreshold does not depend on the energy or the type of ions used. Finally, we calculated theyield of NV for the atomic ions as a function of the fluence (figure 4(b)). For low fluences
up to 2 × 1012 cm−2 , the yield is constant around 0.7%. Then, surprisingly, it increases, almost
doubles, and strongly drops at high fluences exceeding the previously observed amorphizationthreshold. The fact that in the low implantation regime the yield is independent of fluence
indicates that, during the annealing, the vacancies created by one implanted N have no chance ofbeing trapped by another implanted N and form an NV centre with it. Indeed, the increase in theyield starts at ∼4 × 1012 cm−2 (i.e. when the average in-plane distance between the implanted N(figure 4(c)) becomes less than 5 nm), when the vacancy clouds around each N start to overlap,
resulting in a superior number of vacancies available in the very proximity of each N atom. This
positive effect on the yield cannot be further extended since a very high density of the vacancies
is reached (as illustrated by the second curve in figure 4(c), showing the vacancy concentration
with respect to the carbon atoms), and the diamond structure cannot be recovered during theannealing.
In summary, it has been shown that the yield for NV centres creation increases with theion energy on the whole studied range from less than 1% to almost 50%. The dependence on
the ion fluence also shows that the yield goes up with the number of vacancies available aroundthe implanted N, provided that the amorphization threshold is not reached. These results tend
to show that different post-implantation methods can be used to increase the yield by creating
more vacancies: carbon implantation, proton or electron irradiation, or even the use of hard
x-rays. It would be interesting to explore these methods at high temperatures, keeping thesample under the critical threshold of 5 × 1021 vacancies cm−3 . Such experiments could help
to determine whether high yields (such as the ones of ∼50% obtained for N implantations at
MeV energies) can be reached for shallow implanted N, which guarantee low straggling andhighly accurate positioning.
Acknowledgments

We thank G Balasubramanian for experimental support. We acknowledge the financial supportof the Volkswagen Foundation and the State of North Rhine-Westphalia through the Research
Department Integrity of Small-Scale Systems/High Temperature Materials.
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