Abstract
Nowadays, the copper nitride (Cu3N) 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
Cu3N thin films fabricated by reactive radio-frequency
(RF) magnetron sputtering at different RF power values to determine its
potential as light absorber. The Cu3N 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.
Keywords: Copper nitride, reactive magnetron sputtering, pure
nitrogen atmosphere, absorber, photovoltaic devices.
Introduction
The Cu3N 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 Cu3N 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].
Cu3N is a metastable semiconductor with an
anti-ReO3 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. Cu4N that shows metallic behaviour,
while Cu3N 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 Cu3N 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
Cu3N 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 Cu3N films with
appropriated morphologies, structures and band gap energies suitable for
the chosen application.
Materials and Methods
The deposition of the Cu3N 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
<100> 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
10-5 Pa. The working pressure was set to 3.5 and 5.0
Pa, adjusting it with a “butterfly” valve. The process gas was pure
N2 (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. Cu3N 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-kα radiation (λ=0.15406nm), and the scanned 2θ range was 10°-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-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.
Results and discussion
Figure 1 shows the XRD spectra of the Cu3N 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 Cu3N crystallites with an
anti-ReO3 structure [18]. The Cu3N
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 Cu3N
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 Cu2+ and Cu+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+2 and
N+ 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.