Citation: Berzins, A.; Grube, H.;
Sprugis, E.; Vaivars, G.; Fescenko, I.
Impact of Helium Ion Implantation
Dose and Annealing on Dense
Near-Surface Layers of NV Centers.
Nanomaterials 2022, 12, 2234. https://
doi.org/10.3390/nano12132234
Academic Editor: Deepak Kukkar
Received: 26 May 2022
Accepted: 27 June 2022
Published: 29 June 2022
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Article
Impact of Helium Ion Implantation Dose and Annealing on
Dense Near-Surface Layers of NV Centers
Andris Berzins
1,
* , Hugo Grube
1
, Einars Sprugis
2
, Guntars Vaivars
2
and Ilja Fescenko
1
1
Laser Center, University of Latvia, LV-1004 Riga, Latvia; grube.hugo@gmail.com (H.G.);
iliafes@gmail.com (I.F.)
2
Institute of Solid State Physics, University of Latvia, LV-1063 Riga, Latvia; einars.sprugis@lu.lv (E.S.);
guntars.vaivars@lu.lv (G.V.)
* Correspondence: andris.berzins@lu.lv
Abstract:
The implantation of diamonds with helium ions has become a common method to create
hundreds-nanometers-thick near-surface layers of NV centers for high-sensitivity sensing and imag-
ing applications; however, optimal implantation dose and annealing temperature are still a matter
of discussion. In this study, we irradiated HPHT diamonds with an initial nitrogen concentration
of 100 ppm using different implantation doses of helium ions to create 200-nm thick NV layers. We
compare a previously considered optimal implantation dose of
∼
10
12
He
+
/
cm
2
to double and triple
doses by measuring fluorescence intensity, contrast, and linewidth of magnetic resonances, as well as
longitudinal and transversal relaxation times
T
1
and
T
2
. From these direct measurements, we also
estimate concentrations of P1 and NV centers. In addition, we compare the three diamond samples
that underwent three consequent annealing steps to quantify the impact of processing at 1100
◦
C,
which follows initial annealing at 800
◦
C. By tripling the implantation dose, we have increased the
magnetic sensitivity of our sensors by 28
±
5%. By projecting our results to higher implantation doses,
we demonstrate that it is possible to achieve a further improvement of up to 70%. At the same time,
additional annealing steps at 1100
◦
C improve the sensitivity only by 6.6 ± 2.7%.
Keywords: nitrogen-vacancy centers; He ion implantation; diamond annealing; dense NV layers
1. Introduction
The nitrogen-vacancy (NV) centers in diamonds are point defects consisting of a
vacancy in the diamond lattice adjacent to a substitutional nitrogen atom [
1
]. Negatively
charged NV
−
centers, which acquire an additional electron from other substitutional
nitrogen atoms, possess long coherence times of their electron and nuclear spins and can be
initialized and read optically [
2
]. These properties made them widely studied as potential
qubits and quantum sensors. Intensive studies of NV centers in the last decade have led
to a large variety of sensing applications [
3
–
7
], which benefit from nanometer resolution
and room-temperature operation of the NV-based devices, as well as from low toxicity
and mechanical or chemical durability of their diamond matrix. Mainly, these applications
exploit the high sensitivity of NV centers to magnetic fields via ground state Zeeman effect
by using optically detected magnetic resonance (ODMR) detection [2,8–10].
There are several methods to create NV centers in the diamond. Nitrogen ion implan-
tation is used in crystals with low initial nitrogen concentration [
11
–
15
], and the advantage
of this method is the control of nitrogen distribution within the diamond; the disadvantage
is the relatively high damage to the crystal during the implantation, thus introducing
undesirable defects and impurities that might create charge traps, paramagnetic centers,
and vacancy chains, leading to increased spectral diffusion and degraded spin coherence
properties [
16
–
18
]. In addition, this method suffers from electron donor deficit leading
to lower NV
0
to NV
−
charge-state conversion efficiency [
19
], since it is usually applied
to diamonds with a low initial concentration of nitrogen. Another widely used method
Nanomaterials 2022, 12, 2234. https://doi.org/10.3390/nano12132234 https://www.mdpi.com/journal/nanomaterials
