Structural and Optical Properties of CuO Thin Films Synthesized Using Spray Pyrolysis Method

: Copper oxide thin ﬁlms were obtained using pulsating spray pyrolysis method. The morphological, structural, and optical properties of fabricated ﬁlms were studied. X-ray analysis revealed that the CuO thin ﬁlms are single-phase. The study of ﬁlms morphology by SEM and AFM methods showed that the obtained ﬁlms have a fairly high surface roughness and contain grains of different shapes and sizes. It was found that the obtained ﬁlms of copper oxide have high values of the absorption coefﬁcient, which conﬁrms the possibility of their use as absorbing layers for solar cells. The obtained values of the optical band gap of the material are in the range from 1.45 eV to 1.60 eV. Raman spectroscopy revealed three modes A 1g , B 1g , and B 2g , of the crystal structure of monoclinic CuO. The devices based on p -type copper oxide are promising for solar cells fabrication because they can reduce production costs, due to their low cost and inexpensive production methods compared to silicon solar cells fabrication.


Introduction
The rapid development of electronics requires using a variety of materials with the necessary physical, electrical, and optical characteristics to create modern devices. That is why scientists are paying more and more attention to studying new and already known chemical compounds that can exhibit semiconductor properties. Among such compounds, metal oxides can be considered the most promising [1]. These materials have several advantages over most other compounds when creating detectors, sensors, photovoltaic devices, and more. These advantages include environmental safety, non-toxicity, and chemical stability, allowing for obtaining materials directly in the environment. The composition of oxides, as a rule, contains elements that are widespread in the earth's crust, the methods of obtaining them are simple and require only low-temperature processing of the material. In addition, the oxide compounds themselves have a potentially lower cost due to inexpensive components and efficient and inexpensive production methods [2][3][4].
In recent years, a compound of copper oxide has been intensively studied as a semiconductor material. The most common and stable phases of this semiconductor are CuO (tenorite), which has a monoclinic crystal lattice, and Cu 2 O (cuprite), with a cubic structure. Both oxides are being studied as photovoltaic applications because of their reasonable optical properties. CuO is a p-type semiconductor, which according to various data, has a band gap of 1.2 to 2.1 eV, a high absorption coefficient (10 5 cm −1 , 300 K), good thermal conductivity (76.5 W mK −1 ), and electrical resistance, which can range from 10 to 10 5 Ohm·cm depending on the method of production [5][6][7][8][9][10][11].
Due to their physical properties, copper oxide (CuO) films have already been used in various fields of science and technology, such as the creation of lithium batteries [12], bio-and gas sensors [13], solar cells [14], capacitors [15], infrared photodetectors [16], catalysis [17], superconductivity [13], for the manufacture of various electrical and optical devices [18,19], etc. However, the most promising applications of CuO, as novel material, are gas sensors and photovoltaic solar cells [20,21].
The spray pyrolysis technique has the next attractive characteristics: cheapness, simplicity, high application rate, and ability to obtain films on large area substrates from various precursors, and therefore it is an effective method of fabricating oxide films with high quality, which allows to precipitate them of varying thickness and size. An essential advantage of this technique is that it does not require a vacuum. In addition, the obtained films are usually homogeneous and are time and temperature stable. The physical properties of the spray-formed CuO thin films can be precisely controlled by adjusting the deposition parameters such as gas flow rate, spray rate, substrate temperature, deposition time, etc. In addition, 3D printers can be equipped with a spray head, which allows, if necessary, the application of semiconductor films on complex three-dimensional surfaces.
It is well-known that functional properties of electronic devices depend on the fundamental characteristics of the applied films that can be changed by deposition regimes [16]. In this regard, we have studied structural and optical properties of CuO films deposited by spraying precursor solution based on copper acetate monohydrate onto glass substrates. The influence of substrate temperature on structural and optical properties of CuO layer films deposited by pulsed spray pyrolysis has been studied in this paper.

Synthesis of CuO Thin Films
CuO thin films were obtained by pulsed spray pyrolysis method. For their fabrication, a solution of copper acetate monohydrate (Cu(CH 3 CO 2 ) 2 ·H 2 O) with a concentration of 0.05 M in distilled water was used as a precursor. Then a few drops of 10% aqueous solution of hydrochloric acid (HCl) were added to the initial solution to increase the degree of solubility of the precursor, which allowed us to maintain the acidity of the pH solution within (6-7) and avoid the formation of copper molecular complexes.
The preparation time of the solution was 10 min under stirring. The resulting solution was sprayed using a jet nozzle onto heated amorphous glass substrates with dimensions of 10 × 5 × 1 mm 3 , which were pre-cleaned in an ultrasonic bath for 10 min and washed in ethyl alcohol and then in distilled water. The nozzle diameter was 0.2 mm. The work distance (from the nozzle to the substrate surface) was 17 cm. The spraying rate and the volume of sprayed solution were 2 cm 3 /min and 5 cm 3 per sample, respectively. The samples were obtained in the substrate temperature range from T s = 600 K to 725 K with a step of 25 K.
The spray droplets of precursor evaporate the during transport to the heated substrate. Then the solid precipitate melts and evaporates without decomposition, and the vapor diffuses to the substrate. This leads to the formation of layer of substance on the surface. The compound containing the copper is slowly degraded to form CuO thin film. At low temperature the droplet splashes onto the substrate and decomposes. At high substrate temperatures the powdery films were produced because of the vaporization and decomposition of the precursor prior to the substrate surface. The optimum temperature the solvent evaporates near substrate surface and adsorbed onto the surface, followed by decomposition to yield a dense film with good adhesion.

Measurements
The structural characteristics of the deposited films were analyzed by the X-ray diffraction (XRD) method (X-ray diffractometer Bruker D8 Advance, Fremont, CA, USA). The method for calculating structural and substructural properties is described in detail in [29,30]. Additionally, UnitCell (ver. Mar 2021) software was used to determine the lattice parameters, which allows calculations on the position of all diffraction lines from the material [31].
The morphological aspects and the structural quality of samples were probed by scanning electron microscopy (SEM, Jeol, Tokyo, Japan) and atomic force microscopy (AFM, Park XE-100, Park Systems).
Raman measurements were performed using a confocal microscope (Horiba-MTB Xplora, HORIBA France SAS, Kyoto, Japan) at room temperature. The optical properties of CuO films were investigated at room temperature using an Ocean Optics USB4000-UV-VIS fiber optic spectrometer in the wavelength range λ = (300-900) nm. The band gap energy of the obtained films was determined using the method described in detail in [32]. A probe profilometer Dektak XT (Bruker Corporation, Fremont, CA, USA) was used to determine the thickness of the samples.

XRD Analysis
To determine the structural features of the CuO thin films the method of XRD analysis was used. The comparing of the diffraction angles and relative intensities of lines from the fabricated thin films and the JCPDS standard were performed to phase analysis. The XRD patterns from CuO thin films are shown in Figure 1, including the reference spectrum from the pure CuO monoclinic phase (JCPDS card No. 00-048-1548) and Cu 2 O cubic phase (JCPDS No. 01-078-2076).
The spray droplets of precursor evaporate the during transport to the heated substrate. Then the solid precipitate melts and evaporates without decomposition, and the vapor diffuses to the substrate. This leads to the formation of layer of substance on the surface. The compound containing the copper is slowly degraded to form CuO thin film. At low temperature the droplet splashes onto the substrate and decomposes. At high substrate temperatures the powdery films were produced because of the vaporization and decomposition of the precursor prior to the substrate surface. The optimum temperature the solvent evaporates near substrate surface and adsorbed onto the surface, followed by decomposition to yield a dense film with good adhesion.

Measurements
The structural characteristics of the deposited films were analyzed by the X-ray diffraction (XRD) method (X-ray diffractometer Bruker D8 Advance, Fremont, CA, USA). The method for calculating structural and substructural properties is described in detail in [29,30]. Additionally, UnitCell (ver. Mar 2021) software was used to determine the lattice parameters, which allows calculations on the position of all diffraction lines from the material [31].
The morphological aspects and the structural quality of samples were probed by scanning electron microscopy (SEM, Jeol, Tokyo, Japan) and atomic force microscopy (AFM, Park XE-100, Park Systems).
Raman measurements were performed using a confocal microscope (Horiba-MTB Xplora, HORIBA France SAS, Kyoto, Japan) at room temperature. The optical properties of CuO films were investigated at room temperature using an Ocean Optics USB4000-UV-VIS fiber optic spectrometer in the wavelength range λ = (300-900) nm. The band gap energy of the obtained films was determined using the method described in detail in [32]. A probe profilometer Dektak XT (Bruker Corporation, Fremont, CA, USA) was used to determine the thickness of the samples.

XRD Analysis
To determine the structural features of the CuO thin films the method of XRD analysis was used. The comparing of the diffraction angles and relative intensities of lines from the fabricated thin films and the JCPDS standard were performed to phase analysis. The XRD patterns from CuO thin films are shown in Figure 1, including the reference spectrum from the pure CuO monoclinic phase (JCPDS card No. 00-048-1548) and Cu2O cubic phase (JCPDS No. 01-078-2076).   from the crystallographic planes [33]. These results are fully consistent with the data of the standard JCPDS card and confirm the monoclinic structure of the fabricated CuO thin film (spatial group C 2 /c). No additional peaks of secondary phases and uncontrolled impurities were detected on the XRD patterns. This indicates a reasonably high purity of the obtained thin films, which are single-phase and contain only the CuO phase.
The results of determining the lattice constants of CuO using UnitCell software and obtained by calculations (expressions for calculation are shown in supporting material) are shown in Figure 2a. The solid line shows calculations at a constant value of the angle β = 99 • 506 , taken from the reference data (JCPDS card No. 048-1548).
As shown in Figure 2a, the lattice constants a, b, and c of the CuO thin films obtained by calculations are quite close to the reference values. At the same time, the values of the constants obtained using the UnitCell program have much more significant differences with the reference ones and for all the investigated temperatures of the films exceed the calculated values. However, the use of this program allowed us to determine the angle β values of the unit cell of the material, which equals 103.7 • . Because of the above, only the calculated results are discussed below.  Figure 1 shows that all XRD patterns contain diffraction peaks located at angles of 32.5° (110), 35.5° (11-1), 38.7° (111), 48.7° (20-2), 53.4° (020), 58.2° (202), 61.5° , 66.2° (31-1), 68.0° (113), 72.2° (311), and 75.0° (004) that correspond to the specified reflections from the crystallographic planes [33]. These results are fully consistent with the data of the standard JCPDS card and confirm the monoclinic structure of the fabricated CuO thin film (spatial group C2/c). No additional peaks of secondary phases and uncontrolled impurities were detected on the XRD patterns. This indicates a reasonably high purity of the obtained thin films, which are single-phase and contain only the CuO phase.
The results of determining the lattice constants of CuO using UnitCell software and obtained by calculations (expressions for calculation are shown in supporting material) are shown in Figure 2a. The solid line shows calculations at a constant value of the angle β = 99°506', taken from the reference data (JCPDS card No. 048-1548).
As shown in Figure 2a, the lattice constants a, b, and c of the CuO thin films obtained by calculations are quite close to the reference values. At the same time, the values of the constants obtained using the UnitCell program have much more significant differences with the reference ones and for all the investigated temperatures of the films exceed the calculated values. However, the use of this program allowed us to determine the angle β values of the unit cell of the material, which equals 103.7°. Because of the above, only the calculated results are discussed below. It is established that when the substrate temperature increases above 625 K, the lattice constant a of the investigated films slightly increases. At the same time, the constant b initially slightly decreases, but the film deposited at Ts = 725 K increases significantly. The constant c, on the contrary, slightly increases with increasing substrate temperature, but for the sample obtained at Ts = 725 K decreases considerably in magnitude. In this the case, the a values were larger, and c values were smaller than those given in the reference JCPDS card.
Based on the results of measuring the lattice constants of the material, we calculated the volume of the unit cell of the obtained samples. The unit cell volume was in the range from 81.22 to 81.78 nm 3 .
Calculations of the substructural parameters of CuO thin films were performed in the direction  of the crystal lattice of the monoclinic phase (by reflection from the plane (11-1) and ). The coherent scattering domain (CSD) size calculations results by three different methods are summarized in Figure 2b. As can be seen from the Figure 2b It is established that when the substrate temperature increases above 625 K, the lattice constant a of the investigated films slightly increases. At the same time, the constant b initially slightly decreases, but the film deposited at T s = 725 K increases significantly. The constant c, on the contrary, slightly increases with increasing substrate temperature, but for the sample obtained at T s = 725 K decreases considerably in magnitude. In this the case, the a values were larger, and c values were smaller than those given in the reference JCPDS card.
Based on the results of measuring the lattice constants of the material, we calculated the volume of the unit cell of the obtained samples. The unit cell volume was in the range from 81.22 to 81.78 nm 3 .
Calculations of the substructural parameters of CuO thin films were performed in the direction  of the crystal lattice of the monoclinic phase (by reflection from the plane (11-1) and ). The coherent scattering domain (CSD) size calculations results by three different methods are summarized in Figure 2b. As can be seen from the Figure 2b, the values of the parameters of the film substructure obtained using different approximations correlate well with each other. This indicates the reliability of the results. However, the most accurate of these are the values obtained by the method of threefold convolution. That is why they will be discussed. The CSD size of the studied films increases from 23.1 nm (T s = 600 K) to 24.1 nm at 650 K with increasing substrate temperature, and then begins to decrease to 21 nm (T s = 725 K). Figure 2b (line 4) shows the effect of the substrate temperature on microstrain level in CuO thin films. Presented dependence Coatings 2021, 11, 1392 5 of 10 of microstrain level was obtained using threefold convolution. Microstrain values with increasing deposition temperature first increased and then decreased. We observed a similar phenomenon in the films of chalcogenides applied by vacuum methods [34].

Surface Morphology
SEM images of the surface of the synthesized CuO thin films are presented in Figure 3. It can be seen that the nanostructured thin CuO films are quite homogeneous, have a clear polycrystalline structure and a fairly high surface roughness, and also contain small pores. No macroscopic defects, whether exfoliation, voids, or holes, were detected. Figure 3 shows that the grains of the obtained thin films are not homogeneous in size and shape. They acquire spherical, cubic, lamellar, and other shapes. At the same time, those samples deposited at higher substrate temperatures had crystallites predominantly cubic-like cut. Increasing the temperature of the substrate leads to an increase in the size of the crystallites in the thin films, and the average grain diameter is about 90 nm.
to decrease to 21 nm (Ts = 725 K). Figure 2b (line 4) shows the effect of the substrate temperature on microstrain level in CuO thin films. Presented dependence of microstrain level was obtained using threefold convolution. Microstrain values with increasing deposition temperature first increased and then decreased. We observed a similar phenomenon in the films of chalcogenides applied by vacuum methods [34].

Surface Morphology
SEM images of the surface of the synthesized CuO thin films are presented in Figure  3. It can be seen that the nanostructured thin CuO films are quite homogeneous, have a clear polycrystalline structure and a fairly high surface roughness, and also contain small pores. No macroscopic defects, whether exfoliation, voids, or holes, were detected. Figure  3 shows that the grains of the obtained thin films are not homogeneous in size and shape. They acquire spherical, cubic, lamellar, and other shapes. At the same time, those samples deposited at higher substrate temperatures had crystallites predominantly cubic-like cut. Increasing the temperature of the substrate leads to an increase in the size of the crystallites in the thin films, and the average grain diameter is about 90 nm.
AFM images of the surface of CuO thin films and their 3D topology are shown in Figure  4. The resulting films have a high homogeneous surface with tightly packed grains. The figure shows that the increasing Ts during obtaining process leads to a grain size increase. The values of Rq the root mean square deviation of their profile from the mean line and Ra average arithmetic roughness were calculated to quantify the surface roughness of the films.  AFM images of the surface of CuO thin films and their 3D topology are shown in Figure 4. The resulting films have a high homogeneous surface with tightly packed grains. The figure shows that the increasing T s during obtaining process leads to a grain size increase. The values of R q the root mean square deviation of their profile from the mean line and R a average arithmetic roughness were calculated to quantify the surface roughness of the films.  Changing the temperature of the substrate affects the roughness of thin films. It was established that increasing of substrate temperature from Ts = 600 K to TS = 650 K leads to an increase in surface roughness of the thin films (Ra from 15.51 nm to 46.08 nm, Rq from 18.69 nm to 58.12 nm, respectively). Further increase in Ts leads to decrease in the values of Ra and Rq to 19.30 nm and 25.36 nm, respectively. Changing the temperature of the substrate affects the roughness of thin films. It was established that increasing of substrate temperature from T s = 600 K to T S = 650 K leads to an increase in surface roughness of the thin films (R a from 15.51 nm to 46.08 nm, R q from

Raman Measurements
CuO is a compound that crystallizes into a monoclinic crystal structure. It is described by the space group C 2h (C 2/c), where Cu 2+ ions have four bonds with oxygen [32]. The compound has a four-atom crystallographic cell but contains only two atoms in the primitive cell. Thus, the existence of 12 vibrational modes is possible in the connection. Furthermore, two copper atoms in a primitive cell are in states with symmetry, described as C 1 (2), and an oxygen atom in C 2 (2) [35].
The optical phonon modes of the compound relative to the center of the Brillouin zone are given by the expression Γ RA = 4A u + 5B u + A g + 2B g , where Γ is the degree of freedom of vibrations, A u and B u correspond to infrared modes, and A g and B g correspond to Raman modes. Thus, we have six infrared active modes (3A u + 3B u ), three of which belong to acoustic (A u + 2B u modes), and three to Raman scattering (A g + 2B g modes) [36][37][38]. Figure 5a shows the Raman spectra of CuO thin films deposited at different substrate temperatures in the frequency range from 180 cm −1 to 800 cm −1 . For the synthesized films, the Raman peaks at frequencies 268, 320, and 604 cm −1 correspond to the modes A 1g , B 1g , and B 2g of the crystal structure of monoclinic CuO, respectively. The obtained frequencies correlate with those given in the reference [39,40]. Peaks at frequencies of about 430 cm −1 and 505 cm −1 correspond to peaks in the spectra from the substrate.
It is known that the intensity of Raman peaks depends on the grain size. The most intense mode A 1g (268 cm −1 ), which characterizes the monoclinic structure of CuO, is present in the spectra of all deposited films. The shift of the position of the peaks in different samples (Figure 5b) indicates the presence in the films of microstresses, which may be due to the presence of one-dimensional defects associated with the presence of oxygen vacancies and two-dimensional defects (dislocations) that deform the lattice [41].  Changing the temperature of the substrate affects the roughness of thin films. It was established that increasing of substrate temperature from Ts = 600 K to TS = 650 K leads to an increase in surface roughness of the thin films (Ra from 15.51 nm to 46.08 nm, Rq from 18.69 nm to 58.12 nm, respectively). Further increase in Ts leads to decrease in the values of Ra and Rq to 19.30 nm and 25.36 nm, respectively.

Raman Measurements
CuO is a compound that crystallizes into a monoclinic crystal structure. It is described by the space group C2h (C 2/c), where Cu 2+ ions have four bonds with oxygen [32]. The compound has a four-atom crystallographic cell but contains only two atoms in the primitive cell. Thus, the existence of 12 vibrational modes is possible in the connection. Furthermore, two copper atoms in a primitive cell are in states with symmetry, described as C1 (2), and an oxygen atom in C2(2) [35].
The optical phonon modes of the compound relative to the center of the Brillouin zone are given by the expression ГRA = 4Au + 5Bu + Ag + 2Bg, where Г is the degree of freedom of vibrations, Au and Bu correspond to infrared modes, and Ag and Bg correspond to Raman modes. Thus, we have six infrared active modes (3Au + 3Bu), three of which belong to acoustic (Au + 2Bu modes), and three to Raman scattering (Ag + 2Bg modes) [36][37][38]. Figure 5a shows the Raman spectra of CuO thin films deposited at different substrate temperatures in the frequency range from 180 cm −1 to 800 cm −1 . For the synthesized films, the Raman peaks at frequencies 268, 320, and 604 cm −1 correspond to the modes A1g, B1g, and B2g of the crystal structure of monoclinic CuO, respectively. The obtained frequencies correlate with those given in the reference [39,40]. Peaks at frequencies of about 430 cm −1 and 505 cm −1 correspond to peaks in the spectra from the substrate.
It is known that the intensity of Raman peaks depends on the grain size. The most intense mode A1g (268 cm −1 ), which characterizes the monoclinic structure of CuO, is present in the spectra of all deposited films. The shift of the position of the peaks in different samples (Figure 5b) indicates the presence in the films of microstresses, which may be due to the presence of one-dimensional defects associated with the presence of oxygen vacancies and two-dimensional defects (dislocations) that deform the lattice [41].  Figure 6 shows the transmittance and absorbance spectra of films taken in UV and the visible wavelength range λ = (300-900) nm. It is established that CuO thin films have a high absorbance in the visible spectrum (85-90%), which decreases with increasing wavelength. The highest values of the absorbance are observed for films deposited at a higher substrate temperature. Figure 6 shows the transmittance and absorbance spectra of films taken in UV and the visible wavelength range λ = (300-900) nm. It is established that CuO thin films have a high absorbance in the visible spectrum (85-90%), which decreases with increasing wavelength. The highest values of the absorbance are observed for films deposited at a higher substrate temperature.   It is known that the change in Eg of the film material may be due to the presence of defects, small grain size, semiconductor degeneracy, etc. [43,44]. Thus, the change of the material's band gap indicates a difference in the quality of the thin films applied at different deposition temperatures. The obtained band gap values are similar to the values determined for CuO films presented in the works of other researchers [45].

Optical Properties
The band gap energy increases with decreasing size of the nanostructured material, and discrete energy levels appear in the band. When photons fall on semiconductor material, they will be absorbed only when the minimum photon energy is sufficient to excite the electron. Of course, the band gap energy of nanostructured CuO thin films is greater than the energy of the bulk material [42,46].   Figure 6 shows the transmittance and absorbance spectra of films taken in UV and the visible wavelength range λ = (300-900) nm. It is established that CuO thin films have a high absorbance in the visible spectrum (85-90%), which decreases with increasing wavelength. The highest values of the absorbance are observed for films deposited at a higher substrate temperature.   It is known that the change in Eg of the film material may be due to the presence of defects, small grain size, semiconductor degeneracy, etc. [43,44]. Thus, the change of the material's band gap indicates a difference in the quality of the thin films applied at different deposition temperatures. The obtained band gap values are similar to the values determined for CuO films presented in the works of other researchers [45].

Optical Properties
The band gap energy increases with decreasing size of the nanostructured material, and discrete energy levels appear in the band. When photons fall on semiconductor material, they will be absorbed only when the minimum photon energy is sufficient to excite the electron. Of course, the band gap energy of nanostructured CuO thin films is greater than the energy of the bulk material [42,46]. It is known that the change in E g of the film material may be due to the presence of defects, small grain size, semiconductor degeneracy, etc. [43,44]. Thus, the change of the material's band gap indicates a difference in the quality of the thin films applied at different deposition temperatures. The obtained band gap values are similar to the values determined for CuO films presented in the works of other researchers [45].
The band gap energy increases with decreasing size of the nanostructured material, and discrete energy levels appear in the band. When photons fall on semiconductor material, they will be absorbed only when the minimum photon energy is sufficient to excite the electron. Of course, the band gap energy of nanostructured CuO thin films is greater than the energy of the bulk material [42,46].
In addition, thin film resistance was analyzed by Van Der Pauw using four small In contacts placed on the corners of the square on the top of the thin film. It has been found that resistance is reduced linearly from 12 Ohm·cm to 5.9 Ohm·cm for 600 to 725 K, respectively. For resistivity calculation an average thickness~1000 nm was used.

Conclusions
Thin films of CuO were deposited by pulsating spray pyrolysis method. The influence of substrate temperature on morphological, structural, and optical properties was determined.
Based on XRD analysis, it was determined that the CuO thin films are single-phase. AFM and SEM measurements showed that the obtained films have a reasonably high surface roughness. An increase in the substrate temperature leads to an increase in grain size in the obtained thin films.
Raman spectroscopy revealed three modes A 1g , B 1g , and B 2g of the crystal structure of monoclinic CuO. Peak shifts in different samples indicate the presence of microstresses and defects that violate the CuO lattice.
It is established that the obtained thin films of CuO have high values of the absorption coefficient, which confirms the possibility of using them as an absorbing layer in solar cells. Furthermore, the optical band gap of CuO thin films, ranging from 1.45 eV to 1.60 eV, was determined. The discrepancy between the values of the band gap relative to the values for the bulk material may be due to the presence of defects or changes in grain size in thin films, which can be considered the main factors of changing the band gap of deposited CuO thin films.