Structural and Optical Dispersion Characterisation of Sprayed Nickel Oxide Thin Films

Crystalline and non-crystalline nickel oxide (NiO) thin films were obtained by spray pyrolysis technique (SPT) using nickel acetate tetrahydrate solutions onto glass substrates at different temperatures from 225 to 350 ̊C. Structure of the as-deposited NiO thin films have been examined by X-ray diffraction (XRD) and atomic force microscope (AFM). The results showed that an amorphous structure of the films at low substrate temperature (Ts = 225 ̊C), while at higher Ts ≥ 275 ̊C, a cubic single phase structure of NiO film is formed. The refractive index (n) and the extinction coefficient (k) have been calculated from the corrected transmittance and reflectance measurements over the spectral range from 250 to 2400 nm. Some of the optical absorption parameters, such as optical dispersion energies, Eo and Ed, dielectric constant, ε, the average values of oscillator strength, So, wavelength of single oscillator λo and plasma frequency, ωp, have been evaluated.


Introduction
The optical properties of thin films are very important for many applications, including interference devices, such as antireflection coatings, laser mirrors and monochromatic filters, as well as optoelectronics, integrated optics, solar power engineering, microelectronics and optical sensor technology depending on the reflectance and transmittance properties of the films during their preparation.
Dispersion models parameterize the spectral dependencies on the dielectric functions of films or corresponding functions, such the band gap, optical dispersion energies, E o , E d , dielectric constant, ε, ratio between the number of charge carriers and the effective mass, N/m * , wavelength of the single oscillator, λ o , plasma fre-quency…etc., can be determined by data treatment.The optical constants of thin films provide us with information concerning with microscopic characteristics of the material.
Transition metal oxides like nickel oxides have found wide applications due to these anti-ferromagnetic semiconductor with wide gab ≈ 3.6 eV and cubic rock saltlike crystal structure [1,2].It offers promising candidature for many application such as electrocatalysis [3], positive electrode in batteries [4], fuel cell [5], electrochromic devices [6], solar thermal absorber [7], catalyst for oxygen evolution [8] and photo electrolysis [9].Several physical and chemical methods, such as sputtering [10], pulsed laser deposition [11], chemical bath deposition [10,12] and sol-gel [13] have been used to obtain nickel oxide films.All these methods offer different advantages depending on the application of interest and many efforts have been conduced to obtain films with the desirable physical and/or chemical properties.Among the different methods for film deposition, the relative simplicity of the spray pyrolysis method and its potential application for large area deposition make it very attractive, low cost and feasible for mass production processes.This paper reports the influence of substrate temperature (T s ) as an important parameter on the preparation of nickel oxide (NiO) thin films by spray pyrolysis tech-nique (SPT) using nickel acetate tetrahydrate solutions onto glass substrates.The accurate determination of the optical constants of these materials is important, not only in order to know the basic mechanisms underlying these phenomena, but also to exploit and develop their interesting technological applications.Therefore, the optical absorption parameters, such as optical dispersion energies, E o , and E d , dielectric constant, ε, the average values of oscillator strength S o , wavelength of single oscillator λ o , and plasma frequency, ω p , have been evaluated under the effect of substrate temperature.

Film Preparation
) in a concentration of 5 × 10 -2 M in ethanol was used as a starting solution for deposition of nickel oxide films.Nickel acetate tetrahydrate decomposes in a two stepprocess, firstly dehydration from 95˚C to 150˚C and secondly decomposition of the acetate between 300˚C to 350˚C [14,15].The solution was passed through a pneumatic nebulizer with a nozzle diameter of 0.7 mm.The spraying process lasted for about 15 s.The period between spraying processes was about 3 min; this period is enough to avoid excessive cooling of glass substrate.The overall reaction process can be expressed as heat decomposition of nickel acetate to clusters of nickel oxide in the presence of water and air oxygen.
In order to explore the influence of the ultrasonically cleaned preheated glass substrate temperature (Ts), different deposition temperatures ranged between 225 to 350˚C were used to prepare these films.In order to get uniform thin films, the height of spraying nozzle, solution molarity and the rate of spray process were kept constant during the deposition process at 35 cm, 5 × 10 -2 M and 15 cm 3 /min, respectively.A thermocouple was fixed to the substrate surface using silver past and the temperature was measured at the four corners of the glass substrate surface, then the results were averaged and the standard deviation was calculated (± 5˚C).

Thickness Measurements
Thickness of the as-deposited films prepared at different substrate temperatures was determined by multiple-beam Fizeau fringes at reflection using either white light or monochromatic light (Hg, λg = 546 nm).A glass substrate was partially masked during deposition of the film to obtain a sharp edge.Then, a thermally evaporated aluminum layer coated the total surface of the substrate.The colored interference fringes enabled the determina-tion of the order of magnitude of the fringes shift, while the monochromatic fringe shift, as a fraction of the order separation was measured using an eyepiece micrometer.

Structural Investigation
Crystallinity and the phases of the as-deposited films were characterized using JEOL X-ray diffractometer (XRD) (Model JSDX-60PA) with monochromatic highintensity Cu K α radiation (λ = 0.154184 nm).Continuous scanning was applied with a slow scanning speed (1˚/min) and a small time constant (1 s).

Microstructure Analysis
Atomic force microscope (AFM) was used to analyze microstructures by molecular imaging in two-and three-dimensions.The atomic cross-section and surface roughness were determined.Each sample before AFM investigation was washed carefully with distilled water and dried.A Nanoscope III instrument with a 100 μm long silicon tip with repulsive and constant interaction force less than a few nano-Newtons were used [16].

Optical Measurements
Optical transmission and reflection of the prepared films were recorded over the wavelength range from 250 -2400 nm using Shimadzu UV 3101 PC; UV-Vis-NIR double-beam spectrophotometer with reflection attachment based on V-N geometry (incident angle 5˚).The spectral variation of transmission and reflection obtained in this work are used to provide a qualitative guide to the film quality.A laboratory developed computer program [16] was used to calculate the absorption coefficient, energy gaps, optical constants and dielectric constants.

Variation of the Film Thicknesses with Substrate Temperature
Thickness of the obtained films was investigated as a function of the substrate temperatures ranged between 225˚C and 350˚C and the data are graphically represented in Figure 1.It was found that the film thickness decreases with increasing the substrate temperature despite the fact that, the kinetics of the NiO forming reaction should increase with temperature.The observed dependence can be explained by the diminished mass transport to the substrate at higher temperature due to gas convection from bath pushing the droplets of the precursor, as well as the rate of re-evaporation was increased at higher temperatures.In addition, the decrease in the film thickness may be attributed to water loss [17] or removal of interlayer water with consequent formation of compact NiO films.

Structural Characterization
Figure 2 shows the variation of the diffraction patterns as a function of the substrate temperature.The Bragg geometry is employed for analysis and the results of XRD for the as-deposited NiO films at T s = 225˚C -250˚C show that the film structure is non-crystalline structure.Further increasing in the substrate temperature up to ≥ 275˚C, the films were fully transformed to polycrystalline phase of NiO (JCPDS 47-1049) with cubic structural [9,18,19].Three peaks are observed in the pattern at 2θ = 37.1˚, 43.3˚ and 62.8˚ assigned to the (111), ( 200) and (220) crystal planes, respectively.The estimated lattice parameters was found to be a = b = c = 0.417 nm.The increase in the amount and degree of crystallinity of the phase is indicated by the enhancement of the peak intensity.Furthermore, no other peaks corresponding to other phases have emerged.This suggested that, NiO phase is stable and its formation is independent of substrate temperature.The films show prefential growth along the [111] direction.The increase in the intensity of the peaks may be attributed to the increase in the degree of crystallinity caused by increasing the substrate temperature.
Scherre's formula on the bases of full-wave at Half-Maximum (FWHM) was used to calculate the grain size and the average grain size was found to increase from 38 to 45 nm with increasing the substrate temperature from 275˚C to 350˚C.

Microstructural Characterization
Figure 3 shows a NiO crystal surface imaged with the AFM in air and the three-dimensional plot (Figure 3(a)) atomic corrugations are visible as well as grains can be seen on the thin film (Figure 3(b)).The relative smooth surface could be seen with 45 nm thick.A grain size analysis of the thicker NiO film gives a mean grain size area of 3.9 μm 2 .This means that an average radius of the grains of ≈ 35 nm could be determined.These data are found to be in a good agreement with XRD measurements.
The roughness analysis and the cross section profile shown in Figure 3(c) give an impressive insight of the excellent capability of the method to measure surface topography.The surface roughness, (R) is the mean roughness of the surface relative to the center plane of the glass substrate ranged between 0.5 and 1.5 nm.However, the thicker film was found to have a greater roughness than that of the thinner one.

Optical Characterization
Optical measurements of transmittance and reflectance of the films deposited at different substrate temperatures are shown in Figure 4.These measurements have been taken in the wavelength range from 250 to 2400 nm.The optical properties of the investigated films were studied by the method of the spectral optical absorption and reflection.The spectral data of normal transmittance, T exp (λ), and reflectance, R exp (λ), were corrected due to substrate effect according to the following formula [20].
Copyright © 2011 SciRes.  where T sub and R sub are the transmittance and the reflectance of the glass substrate respectively.As shown in Figure 4, the general increase in the transmittance is observed as the film thickness decrease.This behavior may be attributed to perfection and stoichiometry of the films.Anomalous behaviour is observed at low substrate temperature that disappearing in the absorption edge and lowering in the transmittance.This probably is due to the following reasons.
1) Transformation of the phase from amorphous to crystalline considering the sample prepared at T s = 250˚C as a critical case.
2) Absorption edge disappeared in the films that deposited in low temperature; this is due to the fact that, as Ts decreases, bigger clusters are formed before the film creation and the scattered radiation became remarkable due to the surface roughness.
3) Adsorption of water in films deposited at low temperatures during the handling of the film.

Optical Energy Gap
From the transmittance data and according to Tauc [21] relation, (αE) 2 versus incident photon energy, (E), plots were obtained.The graphs are represented in Figure 5.The band gap energy values are found to be 3.83 and 3.14 eV for the NiO thin films with increasing the substrate temperature from 225˚C to 350˚C, which are in a good agreement with that obtained for normal bulk NiO (3.6 -4.0 eV) [22].
Furthermore, it is well known that the energy gap of the semiconductor increases with the decrease of the grain size.NiO nanotubes embedded in porous anodic alumina template also demonstrated a strong absorption in the UV region and showed a band gap of 3.9 eV which is quite close to our result [23].The decrease in E g with T s can be attributed to crystallinity improvement of films deposited at high substrate temperatures.

Determination of Refractive Index
Refractive index is one of the fundamental properties for an optical material because it is closely related to the electronic polarization of ions and the local field inside materials.The complex optical constant (refractive index, (n)) of the prepared samples has been calculated from the corrected T(λ) and R(λ) using a developed computation program [24].
where n is the refractive index of the thin film.The reflectance, R, at the vacuum-film interface has to take into consideration with the value of extinction coefficient, k, so that R can be written as Karmers-Kronig relation.
The behaviour of refractive index has a higher value at very low wavelength (strong absorption).This is due to the quality between the frequency of incident electromagnetic radiation and the plasma frequency.Therefore, there is an anomalous dispersion of refractive index in the region of the plasma frequency.But when λ crosses λ p (plasma wavelength) and the corresponding dielectric constant becomes negative, the refractive index becomes largely imaginary.Hence, there is no propagation of electromagnetic radiation through the medium and gets reflection as mentioned before.
Figure 6 shows the spectral variation of the refractive index as a function of wavelength at different substrate temperatures.It can be seen that, the maximum value of refractive index (n = 2.8 at T s = 350˚C) at very low wavelength, λ = 300 nm (strong-absorption region), was observed in all samples.This is due to the equality between the frequency of incident electromagnetic radiation and the frequency of electrons.This leads to the coupling of electrons in NiO films to the oscillating electric field.At longer wavelengths, λ > 500 nm, the refractive index is sharply decreasing for all patterns reaching to the lowest value of 1.8 at λ ≥ 900 nm and then the n value remains slightly changed for the whole wavelength.Furthermore, the behaviors of refractive index of all films are similar, which is due to the normal dispersion.In the NIR region, λ ≥ 1000 nm, the n values differ slightly with substrate temperature, which diverge remarkably in the visible region.These results are in good agreement with the data obtained by others [9] in the same phase.

Determination of Dielectric Constant
Analyses of the obtained data of refractive index can be used to obtain the high frequency dielectric constant through two procedures [25]: The first, describes the contribution of the free carriers and lattice vibration modes of the dispersion, while the second procedure is based upon the dispersion arising from the bound carriers in an empty lattice.However, both procedures were employed for the obtained value of lattice high frequency dielectric constant ε  .

The First Procedure
The following equation shows the relation between the optical dielectric constant ε, wavelength λ and refractive index [26]: as shown in Figure 7, at λ and n = 0, the values of ε  and ω p are listed in Table 1.

The Second Procedure
Determination of the dielectric constant could be defined using the dispersion relation of the incident photon.The refractive index was also fitted using a function for extrapolation towards shorter wavelength.The Moss model [32], which stated that: "the free carriers contribution to dispersion are relatively small".This means that data corresponding to the wavelength range lying below the absorption edge of the material has to be used [33].The properties of the investigated NiO could be treated as a single oscillator at wavelength λ o at high frequency.The following equation which calculates the high frequency dielectric constant ε ( 2) is: where c is the velocity of the light, ε o the permittivity of free space (8.854 × 10 -12 F/m), N the free carrier concentration and m * the effective mass of the charge carriers.
The nature of the dispersion of n 2 as a function of wavelength (λ 2 ) for different substrate temperatures is shown in Figure 7.As shown un this Figure, the refractive index is an anomalous dispersion in the region of the high frequency.Both the refractive index and increases and absorption of electromagnetic radiation were found to be increased as frequency increases.Furthermore, the refractive index becomes largely, when the frequency of the radiation crosses with the characteristic frequency of the electron.Hence, there is no propagation of electromagnetic radiation through the NiO films.From Figure 7, it can be seen that the dependence of n 2 is linear at longer wavelength.The values of lattice high frequency dielectric constant, ε (1) , is determined from the intersection of the straight line with λ 2 = 0.The values of ε (1) increased with increasing substrate temperature (subfigure).Table 1 shows the values of both ε (1) and the ratio, N/m * , of the investigated films at different substrate temperatures, which determined from the intercept and slopes of the line, respectively.The obtained values of the ration N/m * were in the range between 6.25 × 10 40 to 5.49 × 10 40 cm -3 g -1 with different substrate temperatures.These results are in a good agreement with Al-Ghamdi et al. [19] for NiO thin films and Hutchines et al. [27] and El-Nahass et al. [28] found (N/m * ) for WO 3 , nickel phthalocyanine films, and also for TiInS 2 single crystal [29], in the same order.It is known that in the range of transparency, when the electron damping parameter where S o is the average oscillator strength and λ o an average oscillator wavelength.Equation ( 7) can be written in the following former: where ω p is the plasma frequency and 2     and ε (2) are shown in insert graph and tabulated in Table 1.The values of ε (1) and ε (2) are closed with each other and small difference between them may be due to the lattice vibrations and bounded carriers in an empty lattice which are in the transparent region [34,35].The difference between ε (1) and ε (2) decreased with increasing substrate temperature.This means that the lattice vibration and bounded currieries decreased with increasing the of substrate temperature.Moreover, the values of average oscillator strength (S o ) were calculated from Figure 8.These values are in a good agreement with that obtained by Assim for TiO films [33], by Abdel-Aziz et al. for GeSe (9-x) Tl x thin-film [36] and also, by Caglar et al. for sulfonated poly-aniline film (SPAN) [34].Unfortunately, there are no data about high frequency dielectric constant of NiO thin films were found to compare.

Dispersion Energy Parameters of NiO Thin Films
The dispersion energy plays an important role in the research for optical materials because it is a significant factor in optical communication and in designing devices for spectral dispersion.The dispersion of refractive index in NiO films was analyzed using the concept of the single oscillator and can be expressed by Wemple and Di-Domenico relationship [35]: where E, E o and E d are the photon energy, the oscillator energy and the dispersion energy, respectively.The parameter E d , which is the measure of the intensity of the inter-band optical transition, does not depend significantly on the band gap.A plot of versus E 2 of NiO deposited films for different substrate temperatures as shown in Figure 9.It is clear that, the effect of substrate temperature on the refractive index and semiconductor dispersion profiles were exhibited a linear displacement in the shape of the dispersion profile with decreasing refractive index.The refractive index declines towards long wavelengths.This is due to the influence of lattice absorption.The values of E d and E o were obtained  E E for NiO thin films at different substrate temperatures is calculated as 1.87, which shows an agreement with this relation.A simple connection between the single-oscillator parameters of E o and E d and the imaginary part of the dielectric constant, ε (2) , spectrum can be expressed in terms of moments of the ε  (2) as follows [34] The oscillator energy E o , which was independent of the scale of ε 2 is consequently an ''average'' energy gap, whereas E d depends on the scale of ε 2 and thus serves as an inter band strength parameter.Since the M -1 and M -3 moments are involved in computation of E o and E d and the values of M -1 and M -3 are listed in Table 1.

Conclusions
1) Nickel oxide thin films are prepared by spray pyrolysis technique (SPT) using nickel tetrahydrate solutions onto glass substrates at different temperatures ranged from 225˚C to 350˚C.
2) XRD analyses show that an amorphous films have been obtained at low T s = 225˚C, while at higher T s ≥ 275˚C a cubic single phase structure of NiO film is formed and the films show prefential growth along the [111] direction.
3) AFM shows relatively smooth surface with 45 nm thick.A grain size analysis of the thicker NiO film gives a mean grain size area of 3.9 μm 2 .
4) The refractive index for the prepared films depends on the substrate temperature and film thickness.The higher value of the refractive index at very low temperature at λ > 400 nm was observed in all samples.
5) The values of ε ∞(1) and ε ∞(2) , which determined by two different procedures are increased with increasing T s , while the values of E o decrease with increasing T s .

Figure 1 .
Figure 1.Variation of film thicknesses with substrate temperatures.

Figure 2 .
Figure 2. The X-ray diffractograms of NiO films at different substrate temperatures.

Figure 3 .
Figure 3. Three-dimensional AFM images of atomic resolution of NiO films (a), AFM images with total number of grains, mean and standerd deviation of of defined grains (b), with corresponding atomic cross-section analysis of NiO thin films (c).

Figure 4 .
Figure 4.The transmittance spectrum of NiO films prepared at different substrate temperature.

Figure 6 .
Figure 6.The refractive index (n) of NiO films as a function of wavelength λ, at different substrate temperatures.
ω the incident light frequency[30,31].From the approximation of the linear part of the dependences n 2 (λ 2 ) of NiO films deposited at different substrate temperatures,

Figure 7 .
Figure 7.The relation between n 2 and λ 2 for NiO films.

Figure 8 2 
 at different substrate temperatures from 225˚C to 350˚C.In addition, the values of the high frequency dielectric constant ε (2) increased with an increasing in the substrate temperature.The values of ε (1)

Figure 8 .
Figure 8.The (n 2 -1) -1 as a function of reciprocal function of square wavelength for NiO films at different substrate temperature.

Figure 9 .
Figure 9.The (n 2 -1) -1 as a function of reciprocal function of square photon energy for NiO films at different substrate temperature.fromthe slope and the intercept, respectively, resulting from the extrapolation of the lines.The individual errors in the calculated E o and E d should be significantly small to make the proposed method significant.The values of the standard deviation of the parameters E o and E d presented in Table1were calculated by the fitting procedure.The variation of E o and E d with different substrate temperatures are shown in Figure9.The values of E o was found to be decreased with increasing the substrate temperature, while the values of E d increased with increasing the substrate temperature.This could be attributed to the increase in the number of scattering center due to the dissolving of iridium atoms in the film matrix.The relation between single oscillator energy E o and en- :

Table 1 . Dispersion parameters of NiO thin film prepared at different substrate temperature.
 is the refractive index at infinite wavelength.Table1shows the values of λ o and S o values were obtained from slope and intercept of versus 1where n