current affiliation: ]QuTech and Kavli Institute of Nanoscience, Delft
University of Technology, 2600 GA Delft, The Netherlands †current affiliation:
]CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751013,
India.

# Enhancement of concentration of XeV and GeV centers in nanocrystalline
diamond through $He^{+}$ irradiation

T. Chakraborty1,2,∗, K. J. Sankaran1,2,†, K. Srinivasu3, R. Nongjai4, K.
Asokan4, C. H. Chen3, H. Niu3, K. Haenen1,2 1Institute for Materials Research
(IMO), Hasselt University,3590 Diepenbeek, Belgium.  2IMOMEC, IMEC vzw, 3590
Diepenbeek, Belgium.  3Accelerator Laboratory, Nuclear Science and Technology
Development Center, National Tsing Hua University, Hsinchu 30013, Taiwan.
4Materials Science Group, Inter-University Accelerator Centre, New Delhi 110
067, India.  [  [

###### Abstract

Atomic defect centers in diamond have been widely exploited in numerous
quantum applications like quantum information, sensing, quantum photonics and
so on. In this context, there is always a requirement to improve and optimize
the preparation procedure to generate the defect centers in controlled
fashion, and to explore new defect centers which can have the potential to
overcome the current technological challenges. Through this letter we report
enhancing the concentration of Ge and Xe vacancy centers in nanocrystalline
diamond (NCD) by means of $He^{+}$ irradiation. We have demonstrated
controlled growth of NCD by chemical vapor deposition (CVD) and implantation
of Ge and Xe ions into the CVD-grown samples. NCDs were irradiated with
$He^{+}$ ions and characterized through optical spectroscopy measurements.
Recorded photoluminescence results revealed a clear signature of enhancement
of the Xe-related and Ge vacancies in NCDs.

In the last few decades, there have been notable advancements in the direction
of synthesizing and growing diamond crystals in a controlled way which on one
hand has boosted up the progress in material science and solid state physics
(Rudolph, 2014; Mildren and Rabeau, 2013), and on the other hand, has
privileged the quantum revolution which exploits the atomic defect centers in
diamond (Doherty  _et al._ , 2013; Jelezko and Wrachtrup, 2006). The Nitrogen
vacancy (NV) center in diamond is established as a promising atomic defect
center in diamond which has exhibited promising applications in quantum
information processing (Bernien and et al., 2012; Neumann  _et al._ , 2008),
magnetometry (Rondin  _et al._ , 2014; Wolf  _et al._ , 2015), bio-sensing
(Schirhagl  _et al._ , 2014; Haziza  _et al._ , 2017), thermometry (Wang  _et
al._ , 2015; Neumann  _et al._ , 2013) and so on. However, the NV center has
certain disadvantages: the photoluminescence (PL) signal from the zero phonon
line (ZPL) has only a contribution of $\sim 4\%$ to the total spectrum where
the rest of the signal is in phononic sidebands. Moreover, the NV center
possesses a permanent dipole moment which makes it sensitive to external
electrostatic fields. This is responsible for the spectral diffusion of the NV
optical transitions and hence that affects the efficiency of generating single
photons (Schmidgall  _et al._ , 2018; Acosta  _et al._ , 2012).

Figure 1: (a) SEM micrograph with inset of Raman spectrum of as-deposited NCD
films. (b) and (c) Raman spectra of (I) as-implanted and (II) He-ion
irradiated and annealed Ge and Xe-ion implanted NCD films.  Figure 2: SRIM
results of (a) Ge ion implantation and (b) SRIM results of Xe ion implantation
on NCD films.

Elements of group IV of the periodic table, namely the SiV (Neu  _et al._ ,
2011; Hepp  _et al._ , 2014; Becker  _et al._ , 2016) and GeV (Ekimov  _et
al._ , 2015; Iwasaki  _et al._ , 2015; Zhou  _et al._ , 2018) centers in
diamond have the potential to overcome the above-mentioned issues. The
structure of these vacancy centers is inversion symmetric which makes them
less susceptible to electrostatic fields. This results in a bright and
narrowband emission into the ZPL and indistinguishability of the single
photons emitted from multiple centers (Iwasaki  _et al._ , 2015).
Specifically, the GeV center has certain advantages over the SiV center:
because there is considerable probability that the excited state of the SiV
center decays through certain nonradiative pathways, the SiV center has lesser
quantum efficiency than that of the GeV center (Neu  _et al._ , 2013). These
excellent spectral and spin properties make the GeV center a suitable system
for quantum photonic and plasmonic applications (Bray  _et al._ , 2018;
Siampour  _et al._ , 2018). In this context, a constant experimental progress
on the preparation procedure and better optimization of the growth parameters
are necessary to grow GeV centers in diamond in a deterministic way, which is
one of the main focuses of the present study.

In certain optical applications, it is advantageous to find out single photon
sources which emit in the far-infrared region. For instance, in fiber optic
based communication the attenuation through silica glass fibers is
considerably less in the infrared and far-infrared region in comparison to
that in the visible range (Keiser, 2003). The Xe-related vacancy (XeV) center
in diamond, which exhibits a strong and a weak ZPL $\sim 811$ nm and $\sim
795$ nm, fulfills the above criterion. The XeV center is less investigated
defect center in diamond (Sandstrom  _et al._ , 2018; Zaitsev  _et al._ ,
2006; Bergman  _et al._ , 2009), whose exact crystallographic structure and
spin properties are still to be explored. Considering its possible application
in optical technology, we have focused on preparation and spectroscopic study
of the XeV center.

A widely used technique to create NV centers in diamond is through implanting
high energy ($\sim$ MeV) nitrogen ions into the diamond substrate (Meijer  _et
al._ , 2005; Pezzagna  _et al._ , 2010) and subsequent annealing around 800 ∘C
(Chakraborty  _et al._ , 2019) in ultra high vacuum (air pressure $\sim$
$10^{-10}$ mbar). However, such implantation technique requires a highly
sophisticated ion accelerator, ion-focusing technique and supporting
experimental set-up (Pezzagna  _et al._ , 2010; Chakraborty  _et al._ , 2019)
that are expensive and is not easy to assemble, which hinders the easy
production of NV centers. An alternative efficient procedure to make large
scale production of vacancies is by irradiating the diamond substrate by low
energy ions like protons and $He^{+}$ ions, which requires substantially less
incident beam energy ($<$ 1 keV)(Fu  _et al._ , 2007). $He$ being a chemically
inert gas, neutralization of the ions and embedment of $He$ atoms in diamond
hardly affects the photo-physical properties of the NV center(Chang  _et al._
, 2008). $He^{+}$ ions are proven to be more efficient than protons in terms
of creating larger number of vacancies (Fu  _et al._ , 2007). With $He^{+}$
irradiation, the larger number of vacancies can be created with an
implantation energy two orders of magnitude less than what is required for
electron (Lawson  _et al._ , 1998; Wee  _et al._ , 2007; Ziegler and Biersack,
1985) and $H^{+}$ implantation. $He^{+}$ beams with sufficient power required
for creating vacancies can be generated using an RF ion source which can
produce a current of two order magnitude higher than what can be generated for
$H^{+}$ ions from an accelerator (Ziegler and Biersack, 1985).

Earlier reports have demonstrated large scale preparation of stable NV centers
in synthetic Nanocrystalline diamond (NCD) (Chang  _et al._ , 2008) with
$He^{+}$ irradiation and the efficiency of the process has been investigated
as a function of ion beam energy and dose (Kumar  _et al._ , 2018). We have
investigated the effectiveness of $He^{+}$ irradiation on enhancing the
concentration of the GeV and XeV centers through this work. We demonstrate
controlled growth of NCD, creation of GeV and XeV centers through ion
implantation, spectroscopic investigation of the created centers and the study
of the effect of $He^{+}$ irradiation on the optical properties of the
centers. We chose to create the GeV and XeV centers in NCDs as the preparation
of color centers in NCD thin films is much cheaper compared to the single-
crystalline diamond (SCD) layers. NCD films are grown on cheap materials like
Si substrates whereas to grow an SCD layer, one needs SCD substrates which are
much expensive. As a host of the color centers, NCDs are excellent candidate
for applications in bioimaging, targeted drug delivery, sensing of
temperature, spatial orientation, strain, pressure, magnetic and electric
fields (Schirhagl  _et al._ , 2014; Merson  _et al._ , 2013; Chang  _et al._ ,
2008).

NCD films about 250 nm thick were deposited onto Si (100) substrates using
linear antenna microwave plasma enhanced chemical vapor deposition. The
deposition conditions of the NCD films are reported elsewhere (Drijkoningen
_et al._ , 2017; Kumar  _et al._ , 2019). The pre-seeding process on the Si
substrates was performed by spin coating a water-based state-of-the-art
colloidal suspension of ultradispersed 6–7 nm sized detonation diamond
particles. The scanning electron microscopy (FEI Quanta 200 FEG scanning
electron microscope operated at 15 kV) image shown in Figure 1a illustrates
that the as-deposited NCD films have uniform coverage of 1 $\mu$m sized
faceted diamond grains. The inset of Fig. 1 (a) shows the Raman spectrum (a
Horiba Jobin Yvon T64000 Raman spectrometer equipped with a BXFM Olympus 9/128
microscope in combination with a Horiba JY Symphony CCD detector with a 488 nm
Lexell SHG laser) of as-deposited NCD films, revealing a sharp peak at 1332
cm-1, representing sp3-diamond and the broad bands at ~1390 cm-1 and ~1450
cm-1 representing the disordered carbon (D) and graphitic (G) phases,
respectively.

Ge and Xe ion implantations into NCD films were then carried out using the low
energy ion beam facilities available at Inter University Accelerator Centre,
(IUAC), New Delhi. We used 100 keV and 300 keV implantation energy for Ge and
Xe ions, respectively, whereas the ion fluence was $1\times{}10^{15}$
ions/cm2, for both ions. The penetration depth of implanted ions was obtained
by SRIM. The Ge ions showed a penetration depth of 32 nm with a straggle of 8
nm for implantation energy of 100 keV [Fig. 2 (a)], whereas penetration 67 nm
with a straggle of 11 nm was observed for the Xe-ions with an implantation
energy of 300 keV [Fig.2 (b)]. The Raman spectrum I in Fig. 1 (b) and (c)
illustrates the amorphization induced in the NCD film implanted with the ion
fluence of $1\times{}10^{15}$ ions/cm2. The surface amorphization induced by
implantation is converted into a graphitic phase because of the post-annealing
process [spectrum II in Fig. 1 (b) and (c)]. Such a phenomenon agrees with
literature reports (Kalish, 1999; Prawer and Kalish, 1995). The shift of the
Raman peaks from spectra I to II is obvious due to the stress created by the
implantated ions substituted at the carbon sites of the diamond lattice. In
addition, the implantation and annealing induced amorphization and the size
effect of newly formed nano-sized graphite are responsible for shifting the
Raman peaks.

Figure 3: (a) The schematic representation of the home-built confocal
microscope which is integrated with a spectrometer. The components of the set-
up are mentioned in the main text. (b) Schematic diagram of the energy levels
of the XeV center. ’G’ represent the ground state, E1 and E2 are the excited
states. The arrows depict the transitions between the energy levels as
described in the text. (c) Photoluminescence spectra of the implanted XeV
centers which were (blue curve) and were not (red curve) subjected to He
irradiation. The inset shows Gaussian fit to the ZPL at 811 nm for the blue
curve. (d) Intensity mapping image of the XeV centers in diamond.  Figure 4:
(a) Schematic diagram of the energy levels of the GeV center. (b) Intensity
mapping image of the GeV centers measured using the confocal microscope. (c)
Photoluminescence spectrum of the GeV centers marked by a circle in (b). The
inset shows the background corrected data, with the ZPL and its fit to
Gaussian peak function in an enlarged scale.

After the ion-implantation, the samples were He-ion irradiated in a High
Voltage Engineering Europa B.V. (HVE) particle accelerator. The energy of the
$He^{+}$ ion beam was 100 keV with a fluence of $1\times{}10^{14}$ ions/cm2.
Subsequently, annealing was done at 900 ∘ C for 2 h.

To check the formation of the XeV and GeV centers, we performed optical
characterization of the diamond samples in a home-built experimental setup
where standard confocal microscopy was combined with optical spectroscopy. Our
setup allows to capture photoluminescence (PL) mapping images on the XY plane
of the diamond substrate as a function of depth and to perform optical
spectroscopy on desired spots of the diamond substrate. Fig. 3(a) shows the
schematic diagram of the optical setup. The optical beam from a diode pumped
532 nm laser is used for optical excitation. A high numerical aperture
(NA=1.3) microscope objective (MO) focuses the laser beam onto the diamond
sample and also collects the PL signal which is separated from the reflected
laser by a dichroic mirror (DM) and a 550 nm long pass filter, and reaches the
detection part of the set-up where a Silicon Avalanche Photodiode measures the
signal. For creating mapping images of the sample, the MO is mounted on a
nano-positioning piezo stage which has a traveling range of $100\mu m\times
100\mu m$ in XY plane and $20\mu m$ along the Z axis. In order to measure
spectra, we use a flip mirror after the DM to couple the signal to the input
slit of a Andor Kymera 328i spectrograph combined with an Electron Multiplying
charge-coupled device (EMCCD) camera, which has the sensitivity to detect
optical signals at single photon level.

Fig. 3(b) exhibits a schematic representation of the possible energy states of
the XeV system which has been determined through group theoretical
calculations (Bergman  _et al._ , 2007). It consists of a ground state G and
two excited states E1 and E2. The non-resonant 532 nm excitation triggers the
transition from the state G to A, a virtual absorption state. Subsequently,
nonradiative transitions take place from the states A to the upper and lower
excited states E2 and E1. The zero phonon lines $\thicksim$794 nm and
$\thicksim$811 nm are associated with the transition E2 to G and E1to G,
respectively. In order to examine the distribution of created XeV centers in
the diamond nanocrystals through ion-implantation, we measured the PL signal
and generated PL mapping images. The excitation power of the laser was 0.75
mW, which was measured right before the MO. The PL mapping images revealed
bright spots distributed over certain locations of the diamond substrate. To
ensure the formation of XeV centers, we measured wavelength spectra of these
bright spots. Fig. 3(c)(the red curve) exhibits the 811 nm ZPL in such a
spectrum measured for a certain bright spot. Keeping all the experimental
parameters like laser power, measurement time, number of accumulation etc
fixed for the spectroscopy measurements, we captured the spectra at different
spots. Although signal intensity varied in the range $\thicksim$ $\pm$50$\%$,
we were able to capture the $\thicksim$ 811 nm ZPL consistently at these
spots. Thus the spectroscopy data ensures successful formation of the XeV
center in ion-implanted samples. However, we could not observe the ZPL
$\thicksim$794 nm between E1 and G states. The 794 nm line being considerably
weaker, its observation is hindered by the scattering and reflection of the
signal through grain boundaries and rough surfaces of the nanodiamond crystals
and the existence of a strong background possibly originated from the
amorphous carbon and graphite. Moreover, there is a high probability that the
signal gets absorbed by $sp^{2}$ hybridized carbon atoms (Zaitsev, 2013).
Although the 811 nm signal has to encounter the above mentioned losses, the
signal being comparatively stronger (Deshko and Gorokhovsky, 2010), we were
able to capture it. Next, after irradiating the sample with $He^{+}$ ions, we
performed the optical measurements again keeping the experimental parameters
the same. We captured PL data for a large number of positions throughout both
the samples which was and was not irradiated by $He^{+}$ ions. We observed a
significant enhancement in the average PL intensity for the $He^{+}$
irradiated sample in the mapping image and in the spectroscopy data
accumulated through these measurements. Fig. 3(d) exhibits such a mapping
image from an area of 100 $\mu$m $\times$ 100 $\mu$m. The optical spectrum was
measured for the spot encircled in Fig. 3(d), and is shown by the blue curve
in Fig. 3(d). The intensity of the spectra can be considered as approximately
the mean intensity of several spectra measured at different spots. The
spectrum evidently shows the ZPL peak $\thicksim$ 811 nm with almost 3 times
enhanced intensity compared to the non-irradiated sample. The inset of Fig.
3(c) shows the fit of the XeV ZPL data to Lorentzian line-shape function and
we observe a full width half maximum (FWHM) of 12.9 nm which is consistent
with earlier reports(Sandstrom  _et al._ , 2018; Zaitsev  _et al._ , 2006). In
general, NCD films grown on non-diamond substrates lead to the growth of
polycrystalline diamond films with intrinsic strain, which results in the
broadening of ZPLs of color centers.

Incorporation of GeV centers in the diamond nanocrystals was also investigated
by combining confocal microscopy and optical spectroscopy using the optical
set-up described above. Similar to SiV center (Hepp  _et al._ , 2014), the
energy spectrum of the GeV center also consists of an excited state
${}^{2}{E}_{g}$ and ground state ${}^{2}{E}_{u}$, with both levels having two-
fold degeneracy. One can assign level 1, 2 to the ground and 3, 4 to the
excited states, respectively as shown in Fig. 4(a). Although at low
temperature it is possible to spectrally resolve the 4 to 2, 4 to 1, 3 to 2
and 3 to 1 transitions, at RT one can only observe a single ZPL $\thicksim$602
nm. First, we performed measurements on the ion implanted sample. Although the
mapping images showed bright spots distributed over the diamond substrate, the
spots were not considerably bright compared to the background. The spectra
measured at these spots showed either very weak or no peak close to 602 nm,
which hardly confirms the creation of GeV centers. Next, to observe any change
in the optical data, we measured the $He^{+}$ irradiated sample under the same
experimental conditions. The PL mapping images depicted bright spots with high
contrast (SNR $>$ 5) being distributed inhomogeneously with varying
concentration throughout the sample. Fig. 4(b) shows such a mapping image. To
ensure the formation of the GeV centers, we performed optical spectroscopy
measurements at different bright spots. The optical spectra from different
positions consistently exhibit a distinct peak around 603 nm which can clearly
be attributed to the room temperature (RT) ZPL of the GeV center. We measured
all the spectra at same excitation laser power of 2 mW. The RT PL spectrum of
the encircled spot shown in Fig. 4(b), is exhibited in Fig. 4(c). Earlier
observations reported that at RT the GeV ZPL appears at 602.5 nm (Ekimov  _et
al._ , 2015), $\thicksim$602 nm (Iwasaki  _et al._ , 2015), 602.7 nm
(Ralchenko  _et al._ , 2015) etc. However in our case we see the ZPL
$\thicksim$603 nm. ZPL being highly sensitive to local crystal strains
(Batalov  _et al._ , 2009), the appearance of the peak position differs in
different samples. The inset shows an enlarged view of the ZPL peak and its
fit to Lorentzian line-shape, which has a FWHM of 4.5 nm which is consistent
with earlier findings (Ralchenko  _et al._ , 2015; Iwasaki  _et al._ , 2015).
A significant enhancement in the PL intensity and consistence appearance of
603 nm peak at different positions throughout the sample suggest that $He^{+}$
irradiation and subsequent annealing has successfully created GeV centers in
our diamond sample. To conclude, we have demonstrated an engineered process of
$He^{+}$ irradiation for creating XeV and GeV centers in nanocrystalline
diamond and enhancing their concentration. $He^{+}$ irradiation and subsequent
annealing at 900ºC were performed on the Ge and Xe ion implanted diamond
samples. We captured the spectroscopic characteristics of both without and
with $He^{+}$ irradiated samples in confocal microscope integrated with a
spectrometer. We observed clear signature of increased PL signal of XeV and
GeV centers for the irradiated samples, which evidently signifies an effect of
$He^{+}$ irradiation. These results open the possibility for large scale
creation of GeV and XeV centers in single crystal diamonds which can have
potential application in quantum technology. Further optimization of the
preparation and process parameters will allow us to maximize the production of
GeV and XeV centers with reduced cost, which will favour their applicability
in quantum photonics platforms.

###### Acknowledgements.

This work was financially supported by the Methusalem NANO network and the
Research Foundation – Flanders (FWO) via SBO-project S004018N and the Special
Research Fund (BOF) Postdoctoral Fellowship of KJS. We thank Milos Nesladek
for his insightful suggestions. We also thank IUAC staff for their support in
ion implantations.

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