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\begin{document}

\title{Power effect on the properties of copper nitride films as solar absorber
deposited in pure nitrogen atmosphere}



\author[1]{M.I Rodríguez-Tapiador}%
\author[2]{J. Merino}%
\author[3]{T. Jawhari}%
\author[4]{A.L. Muñoz-Rosas}%
\author[4]{J. Bertomeu}%
\author[1]{S. Fernández}%
\affil[1]{Centro de Investigaciones Energeticas Medioambientales y Tecnologicas}%
\affil[2]{Universidad Rey Juan Carlos - Campus de Madrid}%
\affil[3]{Universitat de Barcelona Centres Cientifics i Tecnologics}%
\affil[4]{Universitat de Barcelona Departament de Fisica Quantica i Astrofisica}%


\vspace{-1em}



  
  \date{May 28, 2024}


\begingroup
\let\center\flushleft
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\selectlanguage{english}
\begin{abstract}
Nowadays, the copper nitride (Cu \textsubscript{3}N) is of great
interest as a new solar absorber material, flexible and lightweight thin
film solar cells. This material is a metastable semiconductor,
non-toxic, composed of earth-abundant elements, and its band gap energy
can be easily tunable in the range 1.4 to 1.8 eV. For this reason, it
has been proposed for many applications in the solar energy conversion
field. The main aim of this work is to evaluate the properties of the Cu
\textsubscript{3}N thin films fabricated by reactive radio-frequency
(RF) magnetron sputtering at different RF power values to determine its
potential as light absorber. The Cu \textsubscript{3}N films were
fabricated at room temperature (RT) from a Cu metallic target at the RF
power ranged from 25 to 200 W on different substrates (silicon and
glass). The pure nitrogen flux was set to 20 sccm, and the working
pressures were set to 3.5 Pa and 5 Pa. The XRD results showed a
transition from (100) to (111) preferred orientations when RF power
increased; the AFM images revealed a granular morphology, while FTIR and
Raman spectra exhibited the characteristics peaks related to Cu-N bonds,
which became narrower when the RF power increased. Finally, to stablish
the suitability of these films as absorber, the band gap energy was
calculated from transmission spectra.%
\end{abstract}%



\sloppy


\textbf{Power effect on the properties of copper nitride films as solar
absorber deposited in pure nitrogen atmosphere}

M.I Rodr\selectlanguage{ngerman}íguez-Tapiador\textsuperscript{1,*}, J.
Merino\textsuperscript{2}, T. Jawhari\textsuperscript{3}, A.L.
Muñoz-Rosas\textsuperscript{4}, J. Bertomeu\textsuperscript{4}, S.
Fernández\textsuperscript{1 ,*}

\textsuperscript{1}Energy Department, CIEMAT, Av. Complutense 40, Madrid
28040, Spain.

\textsuperscript{2}University Rey Juan Carlos, Technology Support Center
CAT, Tulipán, s/n, Móstoles 28039, Madrid, Spain

\textsuperscript{3}Centres Científics i Tecnològics (CCiTUB),
Universitat de Barcelona, Spain

\textsuperscript{4}Departament de Física Aplicada, Universitat de
Barcelona, Barcelona, Spain

\emph{\textsuperscript{*}Correspondence:MariaIsabel.Rodriguez@ciemat.es}
;susanamaria.fernandez@ciemat.es

\textbf{Abstract}

Nowadays, the copper nitride (Cu\textsubscript{3}N) is of great interest
as a new solar absorber material, flexible and lightweight thin film
solar cells. This material is a metastable semiconductor, non-toxic,
composed of earth-abundant elements, and its band gap energy can be
easily tunable in the range 1.4 to 1.8 eV. For this reason, it has been
proposed for many applications in the solar energy conversion field. The
main aim of this work is to evaluate the properties of the
Cu\textsubscript{3}N thin films fabricated by reactive radio-frequency
(RF) magnetron sputtering at different RF power values to determine its
potential as light absorber. The Cu\textsubscript{3}N films were
fabricated at room temperature (RT) from a Cu metallic target at the RF
power ranged from 25 to 200 W on different substrates (silicon and
glass). The pure nitrogen flux was set to 20 sccm, and the working
pressures were set to 3.5 Pa and 5 Pa. The XRD results showed a
transition from (100) to (111) preferred orientations when RF power
increased; the AFM images revealed a granular morphology, while FTIR and
Raman spectra exhibited the characteristics peaks related to Cu-N bonds,
which became narrower when the RF power increased. Finally, to stablish
the suitability of these films as absorber, the band gap energy was
calculated from transmission spectra.

\textbf{Keywords:} Copper nitride, reactive magnetron sputtering, pure
nitrogen atmosphere, absorber, photovoltaic devices.

\textbf{Introduction}

The Cu\textsubscript{3}N is of great interest as a new solar absorber
material, flexible and lightweight thin film solar cells {[}1, 2{]}.
This increasing interest as absorber for photovoltaic (PV) applications
is due to the fact that is a metastable, non-toxic semiconductor,
composed of earth-abundant elements, and its band gap energy can be
easily tunable in the range 1.4 to 1.8 eV {[}3{]} . These values mean
that the Cu\textsubscript{3}N film can be considered as a possible
substitute for the conventional silicon. Furthermore, this material can
show both n and p-type character, which can be obtained by doping it
with different elements that placed in interstitial position, such as
fluorine {[}3{]}.

Cu\textsubscript{3}N is a metastable semiconductor with an
anti-ReO\textsubscript{3} cubic crystal structure{[}3{]}. The structure
exhibits nitrogen (N) atoms at the corners of the cube and copper (Cu)
atoms in the middle of each edge. The material is thermally and
structurally stable at temperatures up to 200°C {[}4{]}. However, its
thermal decomposition occurs in the temperature range of 100--470 °C
{[}5{]}. In this sense, it has been reported that at high temperatures,
visible structural changes can be appreciated and they may be related to
the release of pure Cu {[}6{]}.

The Cu-N family of bonding compounds can offer different optoelectronic
properties, i.e. Cu\textsubscript{4}N that shows metallic behaviour,
while Cu\textsubscript{3}N presents a semiconductor one, where there is
a strong relationship between its chemical bonds and its electronic
properties. These different behaviours can be achieved depending on the
fabrication technique employed and modifying the deposition conditions.
Such different behaviours are basically attributed to the vacant
interstitial sites, because Cu atoms do not occupy the face-centered
cubic (FCC) close packing sites{[}7{]}. Thanks to this, it is possible
to tune the optoelectronic properties by incorporating different atoms
in such vacant sites. In addition, the films can show different
properties depending on the technique used. Among the main methods used
for the Cu\textsubscript{3}N deposition, we can find molecular beam
epitaxy (MBE) {[}8{]}, pulsed magnetron sputtering (PMS) {[}9{]}, pulsed
laser deposition (RPLD) {[}10{]}, and reactive direct current (DC)
magnetron sputtering {[}11{]} or reactive radio-frequency (RF) magnetron
sputtering {[}12-15{]}. In this last case, the film stoichiometry and
the properties can be easily changed. This can be achieved by modifying
the main deposition parameters: (i) the gas pressure due to the
interaction between N and Cu that could modify the band gap {[}6{]},
(ii) the substrate temperature and the power that can affect the
structural, electrical and optical properties of the films {[}16{]},
(iii) the type of the substrate {[}13{]}, and (iv) the substrate to
target distance {[}8{]}. Among these parameters, the plasma gas
composition and the gas pressure control the N fraction in the gas
mixture, which has a strong impact on the film structure, i.e. the
lattice constant ranged from 0.3815 to 0.3885 nm, and consequently, on
their optoelectronic properties {[}17{]}.

In view of the above, in this work we evaluate the effect of the RF
power on the structural and optoelectronic properties of
Cu\textsubscript{3}N thin films sputtered at room temperature (RT) in a
pure N atmosphere, without introducing argon, to obtain a suitable
material for being used as light absorber. It is expected to determine
the optimized RF values to achieve the Cu\textsubscript{3}N films with
appropriated morphologies, structures and band gap energies suitable for
the chosen application.

\textbf{Materials and Methods}

The deposition of the Cu\textsubscript{3}N thin films were carried out
with a commercial MVSystem INC. monochamber sputtering system, where
there is only one gun that can be vertically adjustable and is RF
operated. The target used is a pure commercial Cu target (99.99\%) with
a 3-inch diameter, from Lesker Company. In this work, we used two types
of substrates: 1737 Corning glass (Corning Inc., New York, USA) and
\textless{}100\textgreater{} polished n-type floating zone crystalline
silicon (c-Si) wafers. Prior to be loaded into the sputtering chamber,
both substrates were prepared in a different way. The native silicon
dioxide on the surface silicon wafer is removed with a 1\% HF solution
in deionized water and ethanol for 5 minutes. In the case of the glass,
it was cleaned for 10 minutes in ultrasound, with ethanol and deionized
water, and finally immersed in isopropyl alcohol. Subsequently, the
substrates were dried by blowing nitrogen over them. After loading the
substrates, the chamber was pumped to a pressure of
10\textsuperscript{-5} Pa. The working pressure was set to 3.5 and 5.0
Pa, adjusting it with a ``butterfly'' valve. The process gas was pure
N\textsubscript{2} (purity 99.999\%) at a flow rate of 20 sccm,
controlled by a mass flow controller (MFC). The distance between the
target and the substrate was set to 10 cm. Cu\textsubscript{3}N thin
film depositions were performed at different powers ranged from 25 to
200W, and the deposition time was modified from 420 to 1800 s, depending
the RF power value, to counteract the effect of the power on deposition
rate.

The structure of the thin film was determined by X-ray diffraction
(XRD), using a commercial Panalytical powder diffractometer model X'Pert
MPD/MRD, with a Cu anode and secondary monochromator. The radiation used
was Cu-k\selectlanguage{greek}α \selectlanguage{english}radiation (\selectlanguage{greek}λ\selectlanguage{english}=0.15406nm), and the scanned 2\selectlanguage{greek}θ \selectlanguage{english}range was 10\selectlanguage{ngerman}°-60°
at a step size of 0.01°. To determine the chemical composition and
molecular structure, Raman and Fourier transform infrared spectroscopy
(FTIR) were used. Raman measurements were carried out with the
dispersive spectrometer Horiba Jobin-Yvon LabRam HR 800 coupled, to an
optical microscope Olympus BXFM, using a solid-state laser as an
excitation source emitting at 532 nm. The Raman spectra were obtained
with a laser power at the sample of about 5 mW and using a 100×
microscope objectives. FT-IR measurements were carried out with a Perkin
Elmer Spectrum 100 FT-IR in range of 400-4000 cm\textsuperscript{-1}.
The morphology of the films was studied by atomic force microscopy (AFM)
using an AFM model III A multimode nanoscope (Bruker). The roughness of
the samples was determined from the root mean square (RMS) calculated
with the commercial Gwyddion software. Finally, the optical properties
were calculated from the transmittance spectra in the range 300-2000 nm,
measured at RT and normal incidence with a Perkin Elmer Lambda 1050
spectrophotometer.

\textbf{Results and discussion}

Figure 1 shows the XRD spectra of the Cu\textsubscript{3}N films
deposited on glass at different RF powers and at the different working
pressures of 3.5 and 5.0 Pa. Regardless the pressure used, the XRD
patterns show Cu\textsubscript{3}N crystallites with an
anti-ReO\textsubscript{3} structure {[}18{]}. The Cu\textsubscript{3}N
films deposited at low RF power values (below 150 W) present as
preferential orientation the (100) plane that corresponds to the N-rich
planes. As the RF power increases above 100 W, the intensity of the main
(100) diffraction peak begins to decrease, while the (111) diffraction
peak, corresponding to a Cu-rich plane, starts to appear as preferred
from the sample deposited at 150 W {[}5{]}. In addition, this sample
also presents several diffraction peaks of weaker intensity such as the
(110) and (210) planes, referenced from the XRD card number 00-047-1088
(The Joint Committee on Powder Diffraction Standards, 47--1088). This
change observed in the phase transition orientation from the preferred
(100) plane to the (111) one as the RF power increases can be attributed
to the presence of a larger number of Cu atoms in the plasma. The high
presence of Cu atoms could cause the formation of a Cu\textsubscript{3}N
material saturated with Cu, which retains a cubic structure that begins
to disappear and/or change at very high RF powers.

On the other hand, the main effect that the total pressure exerts on the
films is the amorphous character, observed in the sample deposited at
the highest pressure and the highest RF power of 200 W. Upon increasing
the deposition pressure from 3.5 Pa to 5.0 Pa produces more N ions, are
bombarding the Cu target, leading to a higher amount of Cu ions or atoms
in the plasma, with the Cu\textsuperscript{2+} and
Cu\textsuperscript{+}ionization states.The sample deposited at 200 W and
5.0 Pa shows amorphous structure, in comparison with that deposited at
200 W and 3.5 Pa. This can be attributed to a high presence of Cu atoms,
because of the high RF power, that would reach the substrate surface
with less energy due to the increase of the collisions between the
particles within the plasma at the increased pressure The saturation
effect of the Cu in the film at higher pressures is more evident. This
is the reason why the Cu-rich phase disappears. In addition, under these
extreme conditions, there is an increase in ion bombardment, resulting
in more defects produced in the sputtered films. This indicates that the
crystallinity of the film is seriously deteriorated due to the excessive
RF power and subsequently, excessive amount of Cu atoms.

Conversely, at low RF power values, more N\textsuperscript{+2} and
N\textsuperscript{+} free radicals are generated due to the high
proportion of low-energy electrons that are formed in the
plasma{[}15{]}. Hence, the samples deposited at low RF powers show the
preferential N-rich planes, and a better crystal quality.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Figure 1. X-ray diffraction pattern of Cu\textsubscript{3}N films
deposited at different RF Power and the working pressure of 3.5 Pa
(left) and 5.0 Pa (right).

The stoichiometry of Cu\textsubscript{3}N is studied in the films
deposited on silicon by using FTIR and Raman techniques. These two
techniques give information on the vibrational state of the molecular
bonds that are found in the films. Both techniques are considered as
complementary. As shown the Figure 2 and Figure 3, the FTIR and Raman
peaks reveal the formation of Cu\textsubscript{3}N. The Figure 2 shows
the FTIR spectra of the films as function of the RF power values, where
a band around the wavenumber of 650 cm\textsuperscript{-1} was observed,
being the typical peak of the stretching vibrations attributed to Cu-N
bonds. The peaks at 5.0 Pa are wider than the ones obtained at 3.5 Pa
and closer to 650 cm\textsuperscript{-1}; while the band of the samples
deposited at 150 and 200 W is quite far from the [?] 650
cm\textsuperscript{-1}, revealing the change in their structure. On the
other hand, the Figure 3 depicts the Raman spectra where the typical
bands of Cu\textsubscript{3}N thin films, found at the wavelength of [?]
634cm\textsuperscript{-1} {[}19{]}, are the characteristic ones of Cu-N
bonds {[}20{]} . A slight shift to lower wavenumbers is observed when
the RF power increases, attributed to deviation from the stoichiometric
level more evident at high RF powers where the Cu\textsubscript{3}N
structure is Cu-rich {[}21{]}. Whereas, the sample deposited at 200 W
and 5.0 Pa presents a Raman spectrum completely different, which can be
attributed to the appearance of CO\textsubscript{2} and/or CO absorption
bands {[}22{]}.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Figure 2. FTIR spectra of Cu\textsubscript{3}N films prepared by
magnetron sputtering at different RF power and the working pressures of
3.5 Pa and 5.0 Pa.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

Figure 3. The Raman spectra of Cu\textsubscript{3}N films prepared by
magnetron sputtering at different RF power and the working pressures of
3.5 Pa and 5.0 Pa.

The surface of the samples was analysed by AFM. The scanning area was
1x1\selectlanguage{greek}μ\selectlanguage{english}m\textsuperscript{2}. Figure 4 shows the surface roughness obtained
from the root-mean-square (rms) as function of the RF power. As it can
be observed, the higher the rms value, the rougher the surface of the
thin film. It was also observed that the smoothest films with
close-grained surface morphology were the surface of the samples made at
5.0 Pa and lower RF power values.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

Figure 4. Rms values (nm) calculated vs RF power at different working
pressures of 3.5 Pa and 5.0 Pa.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

Finally, the optical properties of Cu\textsubscript{3}N thin films
deposited on glass substrates were analysed by UV-VIS-NIR spectroscopy
in the range of 300-2000nm. Figure 5 pictures the transmittance spectra
of the deposited thin films at 3.5 Pa and 5.0 Pa, where a greater number
of fringes appear as RF power increases, attributed to the thicker films
obtained by increasing the deposition rate with the RF power. These
spectra clearly show a low transparency of the films in the visible
region, and a maximum optical transmission close to 80\%, showed for the
samples deposited at low values of RF power, in the near infrared
region.

Figure 5. Transmittance spectra of the Cu\textsubscript{3}N in study. In
the inset, representative (h\selectlanguage{greek}να\selectlanguage{english})\textsuperscript{2} and
(h\selectlanguage{greek}να\selectlanguage{english})\textsuperscript{1/2} vs h\selectlanguage{greek}ν \selectlanguage{english}plots of the sample deposited at 150 W,
for the determination of the direct and indirect band gap.

From the transmittance spectra and the Tauc equation {[}23{]}, the band
gap of the Cu\textsubscript{3}N films were calculated, with the aim of
determining their potential as solar absorbers.

\(\left(\selectlanguage{greek}\text{hυα}\selectlanguage{english}\right)^{\frac{1}{p}}=A\left(h\nu-Eg\right)\)Eq.1

where A is the band tailing parameter and p is the parameter associated
with type of transition, i.e. p=2 for an indirect allowed transition and
p=1/2 is direct allowed transition, \selectlanguage{greek}α \selectlanguage{english}is the absorption coefficient, and
h\selectlanguage{greek}ν \selectlanguage{english}is photon energy. In the inset of Figure 5, the
(h\selectlanguage{greek}να\selectlanguage{english})\textsuperscript{2} and (h\selectlanguage{greek}να\selectlanguage{english})\textsuperscript{1/2} vs h\selectlanguage{greek}ν \selectlanguage{english}plots for
the sample deposited at 150 W are pictured as example. Table 1
summarised the values of the direct and indirect band energies obtained
varying the RF power. They are very close to those found in the
literature {[}9, 23{]}. In the case of the samples deposited at 3.5 Pa,
we observe a slightly blue-shift in the band gap energy as the RF power
increases. This may be attributed to the increasing presence of unbonded
Cu atoms, which would form a defect level, leading to a reduction in the
band gap. We would be faced with the effect of Cu supersaturation.
However, in the case of the samples deposited at 5.0 Pa, that tendency
is not so well appreciated. This leads us to believe that the Cu
oversaturation effect would not be occurring due to the increase of the
gas pressure {[}24{]}.

Table 1. Direct and indirect band gap calculated by using Tauc plot for
the samples deposited at different RF powers and a total pressure of 3.5
Pa and 5.0 Pa, respectively.\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
& & & \textbf{3.5 Pa} & & &\tabularnewline
\midrule
\endhead
\textbf{RF}~\textbf{Power (W)} & 25 & 50 & 75 & 100 & 150 &
200\tabularnewline
\textbf{Band Gap Eg (eV)} & & & & & &\tabularnewline
\selectlanguage{greek}\textbf{Direct (αhν)\textsuperscript{2}}\selectlanguage{english} & 2.16 & 2.12 & 2.04 & 2.08 &
2.04 & 2.04\tabularnewline
\selectlanguage{greek}\textbf{Indirect (αhν)\textsuperscript{1/2}}\selectlanguage{english} & 1.90 & 1.80 & 1.70 & 1.76
& 1.61 & 1.68\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
& & & \textbf{5.0 Pa} & & &\tabularnewline
\midrule
\endhead
\textbf{RF~Power (W)} & 25 & 50 & 75 & 100 & 150 & 200\tabularnewline
\textbf{Band Gap Eg (eV)} & & & & & &\tabularnewline
\selectlanguage{greek}\textbf{Direct (αhν)\textsuperscript{2}}\selectlanguage{english} & 2.12 & 2.12 & 2.04 & 2.04 &
2.04 & 2.04\tabularnewline
\selectlanguage{greek}\textbf{Indirect (αhν)\textsuperscript{1/2}}\selectlanguage{english} & 1.68 & 1.80 & 1.68 & 1.68
& 1.68 & 1.72\tabularnewline
\bottomrule
\end{longtable}

\textbf{Conclusions}

This work presents the optimization of the sputtering deposition of
Cu\textsubscript{3}N thin films using a N\textsubscript{2} pure
environment, at RT and at different RF power values with the aim to
determine its suitability as optical absorber. XRD patterns reveal a
change in the preferred orientation from (100) to (111) as increasing
the RF power. Moreover, at RF power of 100 W and above, a
polycrystalline structure with the presence of several weak diffraction
peaks are obtained. From the chemical point of view, FTIR and Raman
measurements confirmed the presence of Cu-N bonds. From AFM images, it
is observed that thin films prepared at low RF power and 3.5 Pa show
very smooth surfaces. Finally, the direct and indirect band gap values
calculated are suitable for solar absorbers. Hence, these results
indicate that (i) the magnetron sputtering is an appropriate method to
fabricate this kind of material, and (ii) the depositions at low RF
power (up to 100 W) and N\textsubscript{2} pressures of 3.5 Pa are
mainly preferred due to its smoother surfaces, its stoichiometry closer
to the required one, and suitable band gap energies that permit to use
this material as a potential absorber substitute of silicon.

\textbf{Acknowledgements}

This research has been supported by Grants PID2019-109215RB-C42 and
PID2019-109215RB-C43 funded by MCIN/AEI/ 10.13039/501100011033. M.A.
Rodr\selectlanguage{ngerman}íguez-Tapiador also acknowledge partial funding through MEDIDA
C17.I2G: CIEMAT. Nuevas tecnologías renovables híbridas, Ministerio de
Ciencia e Innovación, Componente 17 ``Reforma Institucional y
Fortalecimiento de las Capacidades del Sistema Nacional de Ciencia e
Innovación''. Medidas del plan de inversiones y reformas para la
recuperación económica funded by the European Union -- NextGenerationEU.
The authors would also like to thank A. Soubrie from Centro de
Microscopía Electrónica ``Luis Bru'' for her advice and AFM
measurements. Finally, A.L. Muñoz-Rosas is grateful to SECTEI México for
granted postdoctoral fellowship.

\textbf{6. References}

{[}1{]} H. Noh, H. An, J. Lee, J. Song, H.J. Hong, S. Seo, S.Y. Jeong,
B.-J. Kim, S. Ryu, S. Lee, Large enhancement of the photocurrent density
in N-doped Cu3N films through bandgap reduction, Journal of the Korean
Ceramic Society 57(3) (2020) 345-351.

{[}2{]} A. Tilemachou, M. Zervos, A. Othonos, T. Pavloudis, J.
Kioseoglou, p-Type Iodine-Doping of Cu3N and Its Conversion to \selectlanguage{greek}γ-\selectlanguage{english}CuI for
the Fabrication of \selectlanguage{greek}γ-\selectlanguage{english}CuI/Cu3N pn Heterojunctions, Electronic Materials
3(1) (2022) 15-26.

{[}3{]} C.M. Caskey, R.M. Richards, D.S. Ginley, A. Zakutayev, Thin film
synthesis and properties of copper nitride, a metastable semiconductor,
Mater. Horiz. 1(4) (2014) 424-430.

{[}4{]} S. Okrasa, M. Wilczopolska, G. Strzelecki, K.
Nowakowska-Langier, R. Chodun, R. Minikayev, K. Kr\selectlanguage{ngerman}ól, L. Skowronski, K.
Namy\selectlanguage{polish}ś\selectlanguage{english}lak, B. Wicher, The influence of thermal stability on the
properties of Cu3N layers synthesized by pulsed magnetron sputtering
method, Thin Solid Films 735 (2021) 138889.

{[}5{]} D.M. Borsa, D.O. Boerma, Growth, structural and optical
properties of Cu3N films, Surface Science 548(1-3) (2004) 95-105.

{[}6{]} \textless{}Design of nitride semiconductors for solar energy
conversion.pdf\textgreater{}.

{[}7{]} G. Sahoo, Site selective Ag doping in Cu3N and its consequences
on structural and electronic properties: A DFT study, Physica B:
Condensed Matter 619 (2021) 413238.

{[}8{]} M. Asano, K. Umeda, A. Tasaki, Cu3N thin film for a new light
recording media, Japanese Journal of Applied Physics 29(10R) (1990)
1985.

{[}9{]} M. Wilczopolska, K. Nowakowska-Langier, S. Okrasa, L.
Skowronski, R. Minikayev, G.W. Strzelecki, R. Chodun, K. Zdunek,
Synthesis of copper nitride layers by the pulsed magnetron sputtering
method carried out under various operating conditions, Materials 14(10)
(2021) 2694.

{[}10{]} C. Gallardo-Vega, W. De la Cruz, Study of the structure and
electrical properties of the copper nitride thin films deposited by
pulsed laser deposition, Applied surface science 252(22) (2006)
8001-8004.

{[}11{]} A. Majumdar, S. Drache, H. Wulff, A.K. Mukhopadhyay, S.
Bhattacharyya, C.A. Helm, R. Hippler, Strain effects by surface
oxidation of Cu3N thin films deposited by DC magnetron sputtering,
Coatings 7(5) (2017) 64.

{[}12{]} M.R. Zamani Meymian, A. Delavari Heravi, A. Kosari Mehr,
Influence of bias voltage on optical and structural characteristics of
Cu3N films deposited by reactive RF magnetron sputtering in a pure
nitrogen atmosphere, Materials Science in Semiconductor Processing 112
(2020) 104995.

{[}13{]} A. Razeghizadeh, M. Mahmoudi Ghalvandi, F. Sohillian, V. Rafee,
The Effect of Substrate on Structural and Electrical Properties of Cu3N
Thin Film by DC Reactive Magnetron Sputtering, Physical Chemistry
Research 5(3) (2017) 497-504.

{[}14{]} S.-C. Chen, S.-Y. Huang, S. Sakalley, A. Paliwal, Y.-H. Chen,
M.-H. Liao, H. Sun, S. Biring, Optoelectronic properties of Cu3N thin
films deposited by reactive magnetron sputtering and its diode
rectification characteristics, Journal of Alloys and Compounds 789
(2019) 428-434.

{[}15{]} H. Park, H. Seo, S.E. Kim, Anti-oxidant copper layer by remote
mode N2 plasma for low temperature copper--copper bonding, Scientific
reports 10(1) (2020) 1-13.

{[}16{]} A.N. Fioretti, C.P. Schwartz, J. Vinson, D. Nordlund, D.
Prendergast, A.C. Tamboli, C.M. Caskey, F. Tuomisto, F. Linez, S.T.
Christensen, E.S. Toberer, S. Lany, A. Zakutayev, Understanding and
Control of Bipolar Self-Doping in Copper Nitride, J Appl Phys 119(18)
(2016).

{[}17{]} J. Xiao, M. Qi, Y. Cheng, A. Jiang, Y. Zeng, J. Ma, Influences
of nitrogen partial pressure on the optical properties of copper nitride
films, RSC Advances 6(47) (2016) 40895-40899.

{[}18{]} X. Zhou, D. Gall, S.V. Khare, Mechanical properties and
electronic structure of anti-ReO3 structured cubic nitrides, M3N, of d
block transition metals M: An ab initio study, Journal of alloys and
compounds 595 (2014) 80-86.

{[}19{]} A. Fallberg, M. Ottosson, J.-O. Carlsson, Phase stability and
oxygen doping in the Cu--N--O system, Journal of crystal growth 312(10)
(2010) 5.

{[}20{]} A. Sajeev, A.M. Paul, R. Nivetha, K. Gothandapani, T.S. Gopal,
G. Jacob, M. Muthuramamoorty, S. Pandiaraj, A. Alodhayb, S.Y. Kim, Q.
Van Le, P.L. Show, S.K. Jeong, A.N. Grace, Development of Cu3N
electrocatalyst for hydrogen evolution reaction in alkaline medium, Sci
Rep 12(1) (2022) 2004.

{[}21{]} K. Nowakowska-Langier, R. Chodun, R. Minikayev, S. Okrasa, G.W.
Strzelecki, B. Wicher, K. Zdunek, Phase composition of copper nitride
coatings examined by the use of X-ray diffraction and Raman
spectroscopy, Journal of Molecular Structure 1165 (2018) 79-83.

{[}22{]} M. Zervos, A. Othonos, T. Pavloudis, S. Giaremis, J.
Kioseoglou, K. Mavridou, M. Katsikini, F. Pinakidou, E.C. Paloura,
Impact of Oxygen on the Properties of Cu3N and Cu3--x N1--x O x, The
Journal of Physical Chemistry C 125(7) (2021) 3680-3688.

{[}23{]} A. Jiang, M. Qi, J. Xiao, Preparation, structure, properties,
and application of copper nitride (Cu 3 N) thin films: A review, Journal
of Materials Science \& Technology 34(9) (2018) 1467-1473.

{[}24{]} Y.-H. Chen, P.-I. Lee, S. Sakalley, C.-K. Wen, W.-C. Cheng, H.
Sun, S.-C. Chen, Enhanced Electrical Properties of Copper Nitride Films
Deposited via High Power Impulse Magnetron Sputtering, Nanomaterials
12(16) (2022) 2814.

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