Effect of Deposition Conditions on Structural, Optical, and Electrical Properties of NiO Thin Films Prepared by Spray Pyrolysis

INTRODUCTION
Since transparent conducting oxides (TCOs) have high transmittance and low resistivity, they are widely used in research, especially in the optoelectronics industry [1-3]. Most of them have n-type conductivity [4, 5]. However, nickel oxide is a transparent conductive oxide material with p-type conductivity [6]. The reason is that although pure stoichiometric NiO is an insulator with a resistivity at room temperature in the order of 1013 Ω cm, nonstoichiometric NiO exhibits p-type conductivity because of the presence of Ni3+ ions resulting from Ni2+ vacancies and/or interstitial oxygen in NiO crystallites [7-9]. In other words, Ni vacancies form easily and act as acceptors [10]. Among other properties, it has a wide band gap of 3.6 to 4 eV, NaCl crystal structure, high chemical ductility and stability, excellent durability, and low material cost [11-15]. These properties give NiO good optical, electrical, thermoelectric, and catalytic properties [4, 16]. The assets of NiO depend solely on the ratio of nickel to oxygen atoms [12]. The properties of nickel oxide make it a promising candidate for numerous applications such as supercapacitors [17], antireflection coatings [4], solar thermal absorber [18], the catalyst for the evaluation of oxygen [19], gas sensors [20], fuel cell electrodes [21], energy-efficient smart windows [22], random access memory devices [23], organic light-emitting diodes [8], electrochromic device [24], surface acoustic devices [25], UV photodetectors [26], thermoelectric devices [27], and so on.
NiO thin films have been deposited by various methods, e.g. by thermal evaporation [28], electron beam evaporation [29], physical-vapor [30], pulsed laser deposition [31], sputtering [32], atomic layer [33],  electrochemical [34], spin coating [35], dip-coating method [36], successive ionic layer adsorption and reaction (SILAR) [37], chemical vapor deposition [38], bath deposition (CBD) [39], and spray pyrolysis [40]. Chemical spray pyrolysis is a non-complicated, fast, and low-cost process for producing thin films with the capacity to uniformly cover larger areas [7, 13]. The substrate temperature, spray rate, and solution volume are preparation conditions that can greatly affect the film properties. The behavior of NiO thin films strongly depends on their stoichiometric ratio of nickel to oxygen atoms, which can be controlled by the thin film preparation conditions [8]. The effect of substrate temperature [13, 19], solution volume [41, 42], and spray rate [43] on the physical properties of nickel oxide thin films prepared by thermal spraying have been studied separately by researchers. Nevertheless, the examination of the conducted research shows that a systematic review of all the mentioned factors on the properties of nickel oxide thin films has not been reported. Therefore, NiO thin films were prepared here by spray pyrolysis, and the structural, optical, and electrical properties of the films were investigated as a function of substrate temperature, solution volume and spray rate.

MATERIALS AND METHODS
Film preparation
For each thin film deposition, the molarity of nickel in the spray solution was 0.1 M. For example, 100 ml of spray solution was prepared by dissolving 2.3769 g of nickel chloride [NiCl2.6H2O] in 100 ml of deionized water. Each solution was stirred well and heated at room temperature for approximately 45 minutes until it was bright and ready for spraying. The solutions were applied to a soda lime substrate under similar conditions: 30 cm distance between the nozzle and the substrate, 30 rpm rotation speed of the hot plate, and 3 atm pressure of the carrier gas, but at different substrate temperatures, solution volumes, and spray rates as shown in Table 1. To investigate the effect of substrate temperature, the temperature of the substrate was changed in a range of 440-500 °C with a step of 20 °C and an accuracy of ± 2 °C and a digital temperature controller was used to control it. Second, the effect of the solution volume on the physical properties of the thin films was investigated. The solution volumes were chosen to be 50, 100, 150, and 200 ml. To investigate the effects of the spray rate on the thin films, 1, 2.5, 5, and 7.5 mL/min were chosen. The glass substrates had been cleaned with ethanol in an ultrasonic bath for 15 minutes and placed on a hot plate, before the films were prepared. At the end, the system was allowed to cool to room temperature for approximately 80 minutes. In order to prepare the thin film a typical spray pyrolysis coating system was used [44].

Sample characterization
The crystal structures of the thin films were characterized using the XRD technique. The XRD instrument was a D8 Advance Bruker X-ray diffractometer with a Cu tube producing Cu-Ka radiation (k = 0.154056 nm) in a 2θ range of 10o-90o. The surface morphology of the samples was observed via FE-SEM using MIRA3 TESCAN. The cross-sectional FE-SEM image was also used to estimate the thickness of the thin films. The topology of the films was observed using an ARA AFM Ara Research Co. (Model No: 0201A) atomic force microscope (AFM). The transmittance and reflectance spectra of the thin films were measured using the UV-Vis. (300-1100 nm) spectrophotometer, Shimadzo-1800. An Ecopia HMS-7000 Photonic Hall Effect Measurement System was used for the electrical measurement.

RESULTS AND DISCUSSION
Structural characterization
Figs. 1, 2, and 3 show the XRD patterns of the films deposited at different substrate temperatures, solution volumes, and spray rates, respectively. As you can see, plentiful diffraction peaks in the diffraction patterns show a polycrystalline nature with a cubic phase and preferential orientation along the (1 1 1) direction of the pure NiO phase, and no other phase could be noticed [45-49].  Some weak reflection peaks were also detected, such as (2 0 0) and (2 2 0), which could be related to poor crystallization [46]. Peaks (1 1 1), (2 0 0) and (2 2 0) are located at 37.29o, 43.67o, and 6373o for all thin films, respectively. The obtained diffraction peaks agree well with the JCPDS Card Number 14-0481 [12]. All samples show the same structural pattern with only the intensity differences. This could be a good indicator of the crystallinity of the film [12]. This indicates that the NiO phase is stable, and its formation is independent of the deposition parameters of the thin films [50].
The XRD thin films deposited at substrate temperatures of 440, 460, 480, and 500 °C are shown in Fig. 1. The figure shows that as the substrate temperature increases, the intensity of the peaks decreases [12]. This can be due to, at a low substrate temperature, a suitable chemical reaction takes place to form a NiO thin film on the substrate. Moreover, at a substrate temperature of 440 °C, enlargement and densification of the grains occur [12]. In contrast, increasing the substrate temperature above 440 oC would result in the spray solution receiving more thermal energy than required to form the crystalline structure of the nickel oxide phase, which could be because of homogeneous reaction and incomplete thermal decomposition, reducing the crystallinity of the thin films [12]. The apparent decrease in the intensity of the diffraction peaks at higher temperatures is also related to the deterioration of the crystallinity, and the strain produced [51]. As a result, a small number of nucleation centers begin to grow, resulting in larger grains as the temperature increases. The thickness of the film decreases, which is typical of the spraying process where the spray droplets are driven away from the reaction zone, and only a few precursors reach the substrate [12, 52].
Fig. 2 shows the XRD of thin films prepared with 50, 100, 150, and 200 ml solution volumes. The size of the main XRD peak corresponding to the (1 1 1) plane and the value of the area under the peak improve slightly as film thickness increases [50, 53, 54]. From the general observations, it is clear that the solution volume plays a crucial role in the crystallinity of the nickel oxide film [50, 53, 54]. A closer look at Fig. 3 shows that as the spray rate increases, the intensity of the peaks indicates lower crystallinity. At a low spray rate, there is less grain fluctuation, and the thin film has the opportunity to improve its crystal structure.
The NiO crystallizes in the cubic structure. The interplanar spacings of the given Miller indices h, k, and l, dhkl  values of the NiO thin films were also calculated using the Bragg equation:

where n is the diffraction order (usually n = 1) and λ is the x-ray wavelength. The lattice constant (a) was calculated using the following equation [47]:
                                                     

Lattice parameter a for the cubic phase was calculated by (1 1 1) orientations. The average crystal grain size (D) was estimated by [55]: 

and the dislocation density (δ) and strain (ε) were calculated using the following relations [13]:

where θ is the usual Bragg angle, and β is the full width at half maxima of the diffraction peaks. The structural parameters for (111) the crystallographic orientation of the deposited NiO thin films is summarized in Table 2. The observed ‘d’ and ‘a’ values agree well with the standard ‘d’ and ‘a’ values Fig. 4 shows the differences in average crystal grain size as a function of substrate temperatures, solution volumes, and spray rates. It can be seen from Fig. 4 that the average crystal grain size decreases as the substrate temperature increases. It also shows that as the solution volume increases, so does the average crystal grain size increase. These results indicate that increasing the solution volume improves the crystallinity and possibly the rearrangement of the atoms, and removal of defects becomes more accurate with increasing the solution volume [54]. As the spray rate increases, the values for average crystal grain size and dislocation density change, but these values are similar at spray rates of 1 ml/min and 7.5 ml/min.
FE-SEM is a suitable method to study the shape, size, and grain growth mechanism of particles. Figs. 5, 6, and 7 show the surface morphology of the NiO thin films prepared at different substrate temperatures, solution volumes, and spray rates, respectively. The FE-SEM morphologies of the NiO thin films are similar and exhibit spherical grains on their entire surface. The surface morphology of the deposited NiO film seems quasi-smooth, with excellent adhesion and uniform surface with densely packed spherical grains and utterly devoid of pinholes and major cracks on the film surface. A similar morphology is observed for NiO thin films reported in [56-59]. Moreover, the surface of the thin films is covered with inhomogeneous, randomly oriented, clustered particles. However, the morphology of the NiO thin films rests on the deposition parameters. Fig. 5 shows that the grains are condensed and large, and the surface uniformity improves when the substrate temperature increases from 440 to 500 oC[12, 13]. Fig. 6 shows the morphological inflections in thin films at a different solution volume. From the figure, it can be seen that the surface morphology of the thin films improves as the solution volume increases. Fig. 7 shows that the films are relatively porous, and no significant morphological change is observed as the spray rate increases. As shown in Figs. 5, 6, and 7, a smooth film with surface morphology is a basic requirement for applications in electronics, optoelectronics, and electrochromics [12, 57]. 
Figs. 8, 9, and 10 show two- and three-dimensional AFM microscopic images of thin films deposited at different substrate temperatures, solution volumes, and spray rates. The surface description results of the films are summarized in Table 2. The AFM images and the results of the average roughness values of the thin films facilitated our analysis of the physical properties of the thin films. These results show that the morphology of the thin films depends on the deposition conditions. Table 2 shows that an increase in substrate temperature leads to a decrease in the average roughness values of the thin films. On the other hand, increasing the solution volume leads to an increase in the average roughness values of the thin films. Increasing the spray rate from 1 ml/min to 7.5 ml/min leads first to a decrease and then to an increase in the roughness values of the thin films.

Optical study
The exceptional optical properties of thin nickel oxide films make them suitable for a wide range of optoelectronic applications. Many of these applications require high transparency in the UV-vis region. This can be achieved by optimizing the parameters of the deposition layer, which play an important role in the optical properties of NiO thin films. These properties are related to the oxygen vacancy, surface roughness, structure, crystalline quality, grain size, and thickness of the deposited thin films [12, 18, 22, 43, 60-62]. Figs. 11, 12, and 13 show the optical transmission and absorption spectra of NiO thin films deposited at different substrate temperatures, solution volumes, and spray rates, respectively. All films were examined in the wavelength range of 300-1100 nm at room temperature. It can be seen that all films have an average transparency between 8% and 40% in the visible region. It was found that all the deposited samples have good transparency in the visible and near-infrared regions. In addition, the samples exhibit strong absorption in the UV range around 380 nm, which is consistent with previous studies [43]. At wavelengths below 380 nm, all samples show fundamental absorption related to the shift of electrons from the valence band to the conduction band [12, 27, 63-65]. 
Fig. 11 shows that the optical transmittance of the thin films increases as the substrate temperature increases [47, 66]. This is because the thickness and surface roughness of the thin films decrease with an increase in the substrate temperature [22, 43]. Thus, the decreasing thickness and surface roughness of the films display less scattering of the incident light, followed with increased transmittance [11, 18, 42, 43, 67]. 
The absorption edge shifts slightly to the higher wavelength region as the solution volume increases (see Fig. 12). This is attributed to the photoexcitation from the valence band to the conduction band. The film thickness increases as a function of the solution volume, leading to an increase in grain size and thus improved film coverage. Accordingly, the thicker films show more scattering of the incident light, resulting in lower transmittance [43]. Also, the increasing surface roughness with an increasing solution volume leads to decreasing transmittance. Thin film absorbance tends to decrease as wavelength gets higher. From the absorbance spectra, the absorption edge is found in the UV region at around 380 nm wavelength for all samples.
Optical absorption coefficients are calculated using the following formula [68]:

where α, d, R, and T are the absorbance coefficient, thickness, reflectance, and transmittance of the thin films, respectively. Figs. 14, 15, and 16 show the absorbance coefficient spectra of the NiO thin films deposited at different substrate temperatures, solution volumes, and spray rates. Examination of these figures shows that the deposition parameters affect the absorbance coefficient. The thin films show a low absorbance coefficient in the visible region and a strong absorbance coefficient in the regions of lower wavelength. Moreover, an absorption edge near (∼380 nm) is observed in the thin films. The onset of remarkable absorption is in the wavelength region below 380 nm, which provides excellent transparency for thin films in the visible region. In general, the absorbance coefficient changes depending on the crystalline properties, band gap, surface roughness, and density of the defects [18, 23]. When the substrate temperature is lowered, or the the solution volume is increased, the absorption edges of thin films show a significant shift to longer wavelengths (red shift). This is due to the improved crystalline structure of the films that results from a decrease in the substrate temperature or an increase in the solution volume.
The direct optical band gap can be determined via optical absorption measurements by plotting (αhυ)2 against the photon energy hϑ and using the Tauc equation [69]:

where α, β, and Eg define the absorption coefficient, constant, and direct band gap, respectively, as shown in Figs. 17, 18, and 19 for the thin films prepared at different substrate temperatures, solution volumes, and spray rates, respectively. The band gap values are estimated to be in the range of 2.64 - 3.48 eV. The obtained band gap values agree well with previously reported data [12, 18, 21, 22, 43, 68, 70]. Increasing the thickness of the film causes a decrease in the band gap of the thin films as it makes the structure of the film better, as stated in the XRD analysis [24, 26, 39, 47]. Furthermore, an increase in the crystal grain size can decrease the band gap of the thin films because of the quantum size effect [26, 39, 63]. 
We can study the disorder of the thin films by the changes in absorption coefficients [68]. The variation of the absorption coefficient with photon energy is given by the empirical Urbach low [71]:

where α0 is a constant, hϑ is the photon energy, and EU is the Urbach energy. The Urbach energy is an important parameter characterizing the disorder and the width of the localized energy states in the material’s band gap [25, 60, 65, 66]. The Urbach tail appears in poorly crystalline, disordered, and amorphous materials due to the localized states in these materials that widen or narrow the band gap [66]. Moreover, the interaction between charge carriers and phonons is responsible for the Urbach band tail in semiconductor materials [71]. The Urbach energy of the NiO thin films was estimated from the slopes of Ln(α) versus photon energy (hυ) plots. The inverted slope of Ln(α) versus photon energy (hυ) plots determine the values of the Urbach energy. Figs. 20, 21, and 22 show Ln(α) versus photon energy (hυ) plots of the thin films prepared at different substrate temperatures, solution volumes, and spray rates, respectively. We used these figures, determined the thin films’ Urbach energy, and summarized them in Table 3. Furthermore, an increase in the crystal grain size can decrease the band gap of the thin films because of the quantum size effect [16, 60, 63]. 
The refractive index of the thin films is of great importance in designing optical components for spectral dispersion applications. The refractive index which is one of the most notable constants depends on the wavelength of the electromagnetic wave by dispersion. The refractive index delivers information about electronic polarization, local fields, and phase velocity of light propagating in the material [13]. It is a typical characteristic and fundamental property of any optical material. The optical characteristics such as the extinction coefficient (k) and refractive index (n) provide information about the polarization and structure of the thin films, which were papered at different parameter depositions. The extinction coefficient was calculated according to the relation [72]:

The refractive index was calculated according to the relation [72]: 

From the equations, we can calculate the values of n and k. Figs. 23, 24, and 25 show the variations of the extinction coefficient and refractive index with the wavelength of the thin films deposited at different substrate temperatures, solution volumes, and spray rates. The extinction coefficient is a measurement of the portion of light loss as a result of scattering and absorption per unit distance of the medium penetration of the material [13]. The electronic polarization of ions and the local field within optical materials are closely connected to the refractive index [72]. Fig. 23 shows that the value of the extinction coefficient decreases with the substrate temperature as a result of  the absorbance in NiO thin films decreases at higher temperatures [13]. The increase in (n) values may be because of the improvement in crystallinity and packing density of the NiO films as the substrate temperature or solution volume increases. The low values of k suggest that the prepared films have high optical transparency and improved surface homogeneity [62, 73, 74]. They lead to a decrease in the roughness of the film as the substrate temperature increases or the solution volume increases [62]. K and n values are expressed as significant constraints to study the electronic polarizability of the ions and the local field with respect to λ [75]. Such values of k for thin films make them favorites for optoelectronic devices. From the figure, it can be concluded that the value of the refractive index decreases as the solution volume increases in the low wavelength region [62]. Moreover, the extinction coefficient increases as the solution volume increases or the substrate temperature decreases, which agrees with the surface roughness measurement. This proposes that the surface roughness may take part in the change in the extinction coefficient of the deposited films. Generally, the refractive index of a semiconductor is inversely proportional to the energy band gap [18]. This is also true in our case, where we observed lower values for the band gap and higher refractive indices as the solution volume increased [18]. The change in the refractive index with an increase in the substrate temperature may be the result of grain growth while the thin film deposits and a change in morphology, related to the change in extinction coefficient. The extinction coefficient measures the fraction of light loss caused by scattering and absorption per unit distance of the medium penetration of the material. The lowest value of (k) shows the lowest surface roughness, which results in decrease in scattering losses and increase in transmittance. Furthermore, a slight increase in the k value in the near-infrared region can be clarified by the metallic character of such thin oxide films [72].
The complex electronic dielectric constant describes the electron excitation spectrum of the thin films. It gives us information on the permittivity of the material and the polarizability of a material which is related to the density of states in the forbidden gap. The imaginary part of the dielectric constant is necessary to understand the interaction of photons and electrons in thin films [70, 72]. The real and imaginary parts of the dielectric constant are defined as follows [1]: 

Where ε1 and ε2 are the real and imaginary parts of the dielectric constant, n is the refractive index, and k is the extinction coefficient. 
The variation of the real and imaginary dielectric constants with wavelength is shown in Figs. 26, 27, and 28 for the thin films deposited at various substrate temperatures, solution volumes, and spray rates, respectively. Inspection of these figures shows that the variation of the dielectric constants with the incident photon energy proves an exchange between the incident photons and the free electrons in the studied wavelength range [13]. Moreover, these constants show similar behavior to k and n. The figures also show that the values of the real part are higher than those of the imaginary part ε1>ε2 [13, 68, 70, 72, 74]. The real part of the dielectric constant increased as the substrate temperature increased, while the imaginary part of the dielectric constant decreased as the substrate temperature increased. This behavior is reversed for thin films with an increased solution volume. 
The calculation of optical conductivity can help us study the optical resonance of the material and identify the allowed interband optical transitions in it [13, 16]. This is useful since it makes the electronic density of states available in the materials, the conductivity of a material in the presence of incident photons, and the optical response of the material [13, 62, 68]. By using the following formula the optical conductivity of the thin films is calculated [68]:

where, α, n and c are the absorption coefficient, refractive index, and velocity of light, respectively. The behavior of the optical conductivities as a function of the photon energy of the thin films deposited at different substrate temperatures, solution volumes, and spray rates, respectively, is shown in Figs. 29, 30, and 31. By looking at the figures, it can be understood that the optical conductivity of all samples increases as a function of photon energy [68].
The excitation of electrons could explain this by photon energy [68]. The thin films have a very high and stable value of σp in the order of 1014 s-1 in the energy range of 2.6 - 3.8 eV. The high and stable value of σp in the mentioned energy range confirms that the prepared films have good photoreaction [16, 62]. It was discovered that the optical conductivity increases with the substrate temperature, which is the result of the electrons movement from the valence band to the conduction band that occurs when photons of certain wavelengths strike the NiO thin films [13].

Electrical properties
The electrical properties at room temperature were measured and are summarized in Table 3. The compositional change, particularly the formation of Ni+2 vacancies, leads to a p-type conductivity of the thin films [5, 76, 77]. This is due to two Ni2+ ions must be converted to Ni3+ to sustain the crystal’s overall electrical neutrality. Thus, Ni2+ vacancies are at the origin of the hole conductivity of NiO since each Ni2+ vacancy is replaced by two Ni3+ ions [5, 24]. 
The electrical resistivity of the thin films is in the range of 1.33×103 to 2.96×103 Ω cm, as reported by many researchers [50]. The resistivity of the NiO films decreases as the substrate temperature increases and the solution volume increases. In contrast, it decreases when the rate increases. This is probably because of the formation of a nonstoichiometric film with excess or less oxygen [50]. The mechanism of electrical conductivity of NiO can be related to the existence of localized Ni3+ in the NiO lattice. So, the increase in the resistivity of the thin films can be the result of an increase in ions due to the added oxygen, which might lead to the formation of nickel vacancies or interstitial oxygen in NiO crystallites [24].
Therefore, microstructural defects such as Ni vacancies and interstitial O affect resistivity by acting as scattering centers in the NiO lattice Moreover, an increase in the grain size causes an improved crystalline structure and consequently a decrease in the resistivity of the NiO thin films [50, 65]. The carrier concentration is generally increased by increasing the substrate temperature, solution volume, and spray rate. These improvements lead to a decrease in ionized impurities, which in turn leads to an increase in hole carrier concentration [24]. The values of carrier mobility are presented in Table 3. The carrier mobility (μ) was affected by the grain boundary scattering (μGB) and the scattering of ionized impurities (μGB). This was derived using the equation 1/μ=1/μGB +1/μIS. This indicates that the carrier mobility decreases due to an increase in the grain boundary and impurities (Ni vacancies and relatively larger interstitial O) [77]. In addition, carrier mobility increases due to a decrease in the scattering centers of the impurities [24].

CONCLUSION
We prepared NiO thin films by spray pyrolysis on glass substrates with different deposition conditions in the present work. The effect of the substrate temperature, solution volume, and spray rate on the structural, optical, and electrical properties of the films was investigated using XRD, FE-SEM, AFM, UV-vis spectroscopy and Hall effect analysis. The structural analysis showed that the films have a cubic structure. The refractive index, extinction coefficient, and dielectric constants were calculated, and the values agreed well with the literature. Increasing the substrate temperature decreased the average roughness, Urbach energy, electrical resistivity, and carrier mobility on the one hand, and increased the optical cap and carrier concentration on the other. In general, increasing the solution volume increased the average roughness, Urbach energy, carrier concentration, and carrier mobility on the one hand, and decreased the optical cap and electrical resistivity on the other. In general, increasing the spray rate decreased the average roughness, optical gap, and carrier mobility and increased Urbach energy, electrical resistivity, and carrier concentration. The thin films exhibited p-type conduction, making them suitable for solar cells. The thin films exhibited p-type conduction, making them suitable for solar cells. We have shown that the physical properties of NiO thin films can be developed for use in optoelectronic devices.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.