Growth and Correlation of the Physical and Structural Properties of Hexagonal Nanocrystalline Nickel Oxide Thin Films with Film Thickness

: This study investigated nonstoichiometric nickel oxide thin ﬁlms prepared via the DC-sputtering technique at di ﬀ erent ﬁlm thicknesses. The prepared ﬁlms were characterized by a surface proﬁler for thickness measurement, X-ray di ﬀ raction (XRD) for ﬁlm nature, atomic force microscopy (AFM) for ﬁlm morphology and roughness, UV-visible-near infrared (UV-vis.-NIR) spectroscopy for optical transmittance spectra of the ﬁlms, and the photoluminescence (PL) spectra of the prepared ﬁlms were obtained. The measured ﬁlm thickness increased from 150 to 503 nm as the deposition time increased. XRD detected the trigonal crystal system of NiO 0.96 . The crystallite sizes were mainly grown through (101) and (110) characteristic planes. NiO 0.96 ﬁlms have a spherical particle shape and their sizes decreases as the ﬁlm thickness increased. The optical band gap values decrease from 3.817 to 3.663 eV when the ﬁlm thickness increases. The refractive index was estimated from the Moss relation, while the high-frequency dielectric constant and the static dielectric constant were deduced from the empirical Adachi formula. The photoluminescence behavior of the studied ﬁlms conﬁrmed the photogeneration of an electron-hole in nickel and oxygen vacancies. Hence, this study conﬁrms the presence of nickel oxide lattice in the hexagonal structure containing the defects originated from the nickel vacancies or the excess of oxygen.


Introduction
Three-dimensional transition metal oxide materials such as nickel oxide are mostly antiferromagnetic materials as a result of their partially filled 3d orbitals [1,2]. The magnetic moment of Ni 2+ ions is ordered below the Néel point at 523 K, in which the spins are parallel through the (111) plane and antiparallel between the adjacent (111) plane [2]. Nickel oxide has different crystalline phases, such as the cubic phase (NiO) and the hexagonal phase (Ni 2 O 3 and NiO x ). However, nickel oxide crystals are predominately deficient due to the excess of oxygen atoms and the deficiency in the nickel structure. This causes nickel oxide crystals non-stoichiometries to show p-type semiconductor behavior [3,4]. Passerini and Scrosati [5] investigated non-stoichiometric nickel oxide thin films prepared by the DC-sputtering technique. They found that these films could be used as intercalation electrodes, and hence could be applied to the improvement of electrochromic displays [5]. The supercapacitor

Materials and Methods
Before depositing thin films on both the normal glass substrate and silicon wafer, they were washed in boiled distilled water. Then the substrates were transferred to an ultrasonic bath filled with a mixture of distilled water/ethanol for 1 h. Finally, they were dried by nitrogen gas at normal room temperature.
The sputtering technique (DC-sputtering, Syskey Technologies, Hsinch County, Taiwan) is used to deposit nickel oxide material on the glass substrates/silicon wafer by oxidation of the nickel target with a dimension of 3 × 0.6 inch at a power of 200 W at a constant oxygen flow rate of 10 SCCM and a flow of high-purity gas of argon (99.99%) at 20 SCCM on the substrate. The films are synthesized at an operating pressure of 5 mTorr and a substrate cycle of 15 rpm, and the Ni base targets to the substrate distance are fixed at 140 mm. Different thicknesses of nickel oxide films can be achieved at different deposition times of 1200, 1800, 2100, and 2400 s, as measured by a surface profiler, type Dektak XT, Bruker, Karlsruhe, Germany. Their values are presented in Table 1. The film nature was detected using an X-ray diffraction device (type Ultima-IV; Rigaku, Tokyo, Japan) worked at 40 kV/40 mA with Cu kα radiation (λ = 0.154056 nm) in the angle range 20-80 • . The surface film structure was investigated via atomic force microscopy type Omicron-UHV-VT-AFM XA through a contact mode under ultra-high vacuum conditions. The estimation of the grain size and the roughness were obtained by software described in detail in a previous article [10]. Optical transmittance was recorded via the spectrometer Perkin Elmer Lambda 750, Massachusetts, USA in the photon wavelength 190-2500 nm with a resolution of 1 nm. Photoluminescence (PL) RF-5301PC spectrofluorophotometer SHIMADZU, Kyoto, Japan was measured for investigation of the films' luminescence at an excitation wavelength of 325 nm and at room temperature.

X-Ray Diffraction Analysis
The prepared samples showed the formation of a trigonal crystal system NiO 0.96 of the card number (01-078-4383). The trigonal crystal system is in the hexagonal crystal family. Figure 1 represents the XRD patterns for different thicknesses of nickel oxide thin films. The characteristic planes are also included in the XRD chart. Five planes characterize the hexagonal structure of NiO 0.96 films; (101) plane at 2θ~36.7 • , (012) at 2θ~42.7 • , (110) at 2θ~62.1 • , (113) at 2θ~74.1 • , and (202) at 2θ~78.1 • . Indeed, plane (101) is the major plane in all samples due to the higher intensity than other planes. Additionally, plane (110) is next to plane (101) in intensity, instead of plane (012) for the prepared films at different oxygen flow rates as discussed previously [10]. Plane (110) was observed to become broader and the intensity was reduced as the deposition time increased. The film nature was detected using an X-ray diffraction device (type Ultima-IV; Rigaku, Tokyo, Japan) worked at 40 kV/40 mA with Cu kα radiation (λ = 0.154056 nm) in the angle range 20-80°. The surface film structure was investigated via atomic force microscopy type Omicron-UHV-VT-AFM XA through a contact mode under ultra-high vacuum conditions. The estimation of the grain size and the roughness were obtained by software described in detail in a previous article [10]. Optical transmittance was recorded via the spectrometer Perkin Elmer Lambda 750, Massachusetts, USA in the photon wavelength 190-2500 nm with a resolution of 1 nm. Photoluminescence (PL) RF-5301PC spectrofluorophotometer SHIMADZU, Kyoto, Japan was measured for investigation of the films' luminescence at an excitation wavelength of 325 nm and at room temperature.

X-Ray Diffraction Analysis
The prepared samples showed the formation of a trigonal crystal system NiO0.96 of the card number (01-078-4383). The trigonal crystal system is in the hexagonal crystal family. Figure 1 represents the XRD patterns for different thicknesses of nickel oxide thin films. The characteristic planes are also included in the XRD chart. Five planes characterize the hexagonal structure of NiO0.96 films; (101) plane at 2θ~36.7°, (012) at 2θ~42.7°, (110) at 2θ~62.1°, (113) at 2θ~74.1°, and (202) at 2θ~78.1°. Indeed, plane (101) is the major plane in all samples due to the higher intensity than other planes. Additionally, plane (110) is next to plane (101) in intensity, instead of plane (012) for the prepared films at different oxygen flow rates as discussed previously [10]. Plane (110) was observed to become broader and the intensity was reduced as the deposition time increased. The average crystallite size (D) of the crystals is determined from the Scherrer formula [23] for (101) and (110) planes, as follows: where β is the full width at half maximum of the peak, and θ is the diffraction angle. The size grew The average crystallite size (D) of the crystals is determined from the Scherrer formula [23] for (101) and (110) planes, as follows: where β is the full width at half maximum of the peak, and θ is the diffraction angle. The size grew through plane (101) (as seen in Figure 2) from 18.27 to 19.76 nm as the film thickness varied from 150 to 503 nm, while the crystallite size decreased slowly through plane (110). XRD confirmed the nonstoichiometric ratio in the nickel oxide lattice. The crystallites were largely grown on planes (101) and (110). Therefore, under the present experimental condition of sputtered nickel oxide films, the crystallites preferred to grow on plane (101). The texture coefficient Tc(hkl) examined the preferred growth/orientation according to the following equation [24,25]: where I(hkl) represents the experimentally measured intensity, Io(hkl) is the relative incident intensity given from the standard XRD card, and N denotes the diffraction peak numbers. Table 1 shows the texture coefficient values and the calculated crystallite size obtained from the XRD chart. The texture coefficient of (101) is higher than (110), which confirms the assumption of crystallite growth along (101). However, the texture coefficient values decreased when the film thickness increased, while the texture coefficient values for plane (110) increased when the thickness increased (except the nickel oxide sample with a thickness of 503 nm).

Surface Morphology Investigation
All samples have a spherical shape, as seen in Figure 3. Increasing the film thickness or the deposition time causes the grains to accumulate on the silicon wafer substrates. The grain size decreases from 48.85 to 30.68 nm as the film thickness increases. Nickel oxide films with a thickness of 503 nm show the highest surface roughness relative to the other samples. Table 2 indicates the values of both the grain size and the surface roughness.
Indeed, XRD gives the crystallite size and it must be lower than the grain size obtained from scanning electron microscopy or atomic force microscopy. The grain size represents groups of crystallites.
Therefore, it can be said that a set of crystallites are located in grains of spherical shapes depend on the film thickness. As the thickness of nickel oxide increases, the grains reduce in size and aggregated, leading to an increase film roughness (with the maximum film thickness being 503 nm). XRD confirmed the nonstoichiometric ratio in the nickel oxide lattice. The crystallites were largely grown on planes (101) and (110). Therefore, under the present experimental condition of sputtered nickel oxide films, the crystallites preferred to grow on plane (101). The texture coefficient T c (hkl) examined the preferred growth/orientation according to the following equation [24,25]: where I(hkl) represents the experimentally measured intensity, I o (hkl) is the relative incident intensity given from the standard XRD card, and N denotes the diffraction peak numbers. Table 1 shows the texture coefficient values and the calculated crystallite size obtained from the XRD chart. The texture coefficient of (101) is higher than (110), which confirms the assumption of crystallite growth along (101). However, the texture coefficient values decreased when the film thickness increased, while the texture coefficient values for plane (110) increased when the thickness increased (except the nickel oxide sample with a thickness of 503 nm).

Surface Morphology Investigation
All samples have a spherical shape, as seen in Figure 3. Increasing the film thickness or the deposition time causes the grains to accumulate on the silicon wafer substrates. The grain size decreases from 48.85 to 30.68 nm as the film thickness increases. Nickel oxide films with a thickness of 503 nm show the highest surface roughness relative to the other samples. Table 2 indicates the values of both the grain size and the surface roughness.
Indeed, XRD gives the crystallite size and it must be lower than the grain size obtained from scanning electron microscopy or atomic force microscopy. The grain size represents groups of crystallites.
Therefore, it can be said that a set of crystallites are located in grains of spherical shapes depend on the film thickness. As the thickness of nickel oxide increases, the grains reduce in size and aggregated, leading to an increase film roughness (with the maximum film thickness being 503 nm).   The optical transmittance, T, of nickel oxide thin films is shown in Figure 4. The optical transference is observed through all samples, indicating the good quality and homogeneity of the films. The inset figure shows the displacement of the absorption edge toward a higher wavelength. Therefore, the expected optical band gap values decreased as the thin film thickness increased.

Optical Transmittance and Optical Band Gap
The optical transmittance, T, of nickel oxide thin films is shown in Figure 4. The optical transference is observed through all samples, indicating the good quality and homogeneity of the films. The inset figure shows the displacement of the absorption edge toward a higher wavelength. Therefore, the expected optical band gap values decreased as the thin film thickness increased. For the estimation of the optical energy band gap, Eg, and the transition order, q, the following procedure is applied: The optical energy band gap depends on the absorption coefficient, α according to the following equation [26]: where β represents the energy independent constant, and α is deduced from the expression α = ln . The optical transition order, q, is then to be either ½ for direct transition or 2 for indirect transition. It is agreed that in the case of the crystalline materials, the optical transition is almost a direct transition; q = ½ in Equation (3). Therefore, the resulting plot of (αhυ) 2 vs. hυ is a curve with a straight-line portion [27][28][29]. The intersection of this linear portion with the hυ axis is analogous to the value of the energy band gap as seen in Figure 5 for the film thickness of 150 nm. For the estimation of the optical energy band gap, E g , and the transition order, q, the following procedure is applied: The optical energy band gap depends on the absorption coefficient, α according to the following equation [26]: where β represents the energy independent constant, and α is deduced from the expression α = 1 t ln 1 T . The optical transition order, q, is then to be either 1 2 for direct transition or 2 for indirect transition. It is agreed that in the case of the crystalline materials, the optical transition is almost a direct transition; q = 1 2 in Equation (3). Therefore, the resulting plot of (αhυ) 2 vs. hυ is a curve with a straight-line portion [27][28][29]. The intersection of this linear portion with the hυ axis is analogous to the value of the energy band gap as seen in Figure 5 for the film thickness of 150 nm. The estimated optical band gap values have a similar trend, as shown in Figure 5, which decreases as the thin film thickness increases, as shown in Figure 6. The values of the optical band gap changed from 3.817 to 3.663 eV as the film thickness increased from 150 to 503 nm (red shift). The previous investigation of nickel oxide thin films prepared at different oxygen flow rates by Hammad et al. [10] showed that at an oxygen gas rate of 10 SCCM with a thickness of 141 nm has a band gap value of 3.74 eV. Therefore, the thickness changed in the range of 141 to 150 nm and by increasing the deposition time/film thickness, a red shift occurred, up to a higher thickness of 503 nm.  The estimated optical band gap values have a similar trend, as shown in Figure 5, which decreases as the thin film thickness increases, as shown in Figure 6. The values of the optical band gap changed from 3.817 to 3.663 eV as the film thickness increased from 150 to 503 nm (red shift). The previous investigation of nickel oxide thin films prepared at different oxygen flow rates by Hammad et al. [10] showed that at an oxygen gas rate of 10 SCCM with a thickness of 141 nm has a band gap value of 3.74 eV. Therefore, the thickness changed in the range of 141 to 150 nm and by increasing the deposition time/film thickness, a red shift occurred, up to a higher thickness of 503 nm. The estimated optical band gap values have a similar trend, as shown in Figure 5, which decreases as the thin film thickness increases, as shown in Figure 6. The values of the optical band gap changed from 3.817 to 3.663 eV as the film thickness increased from 150 to 503 nm (red shift). The previous investigation of nickel oxide thin films prepared at different oxygen flow rates by Hammad et al. [10] showed that at an oxygen gas rate of 10 SCCM with a thickness of 141 nm has a band gap value of 3.74 eV. Therefore, the thickness changed in the range of 141 to 150 nm and by increasing the deposition time/film thickness, a red shift occurred, up to a higher thickness of 503 nm.  Thin Film Thickness (nm) Figure 6. Dependence the optical band gap on the film thickness.

Refractive Index and High-Frequency Dielectric Constant
The linear refractive index, n o , is directly proportional to the optical band gap, E g , from the Moss relation [30]: Moreover, the high-frequency dielectric constant, ε ∞ , and the static dielectric constant, ε o , are also correlated with the optical band gap according to Adachi as follows [31]: The presented data of the refractive index, n o , the high frequency dielectric constant, ε ∞ , and the static dielectric constant, ε o , are presented in Table 3. These data agree with the study obtained by El-Nahass et al. [32] for nickel oxide films obtained via electron beam evaporation.  Figure 7 shows the PL spectra of NiO x for a film thickness of 150 nm, while the inset figure shows the PL behavior for the other film thicknesses from 264 to 503 nm as well as the glass substrate. The photoluminescence intensity descends as the film thickness grows and is higher in the intensity than the glass substrate. Three characteristic peaks/shoulders are observed for all films, located at 365, 377, and 468 nm, respectively. Indeed, the PL of the glass substrate has two peaks, at 370 and 385 nm. No further peaks occur in the range of 450-500 nm. This means that the PL peak at 468 nm is for NiO x and not for the glass substrate. Moreover, the coating of NiO x on the glass substrate shifts the peak from 370 to 365 nm and from 385 to 377 nm, forming a broad peak with a shoulder instead of two distinct peaks, as observed for the glass substrate. This could be explained as the superposition of the PL peaks of NiO and the glass substrate.

Photoluminescence Properties
Hence, PL investigation confirmed the structural change in the NiO x films. The peaks at 365 and 377 nm suggest exciton-exciton scattering in the UV emission [10,33]. The peak at 468 nm is correlated with the recombination of electron-holes in the vacancies of the Ni and O lattice [10,[34][35][36]. The photoluminescence spectra are correlated with the particle size of the prepared films. As the particle size decreases, the PL intensity is also decreased. This assumption was suggested and confirmed by Musevi et al. [34] when they investigated the particle size distribution and correlated it with the PL spectra. As the film's thickness grows, and the presence of the non-stoichiometric ratio is realized, the defects or vacancies increase, leading to the decreasing value of the PL intensity, as well as the transmittance spectra.

Conclusions
Sputtered nickel oxide thin films at a fixed oxygen flow rate of 10 SCCM and at different deposition times proved the trigonal phase of nickel oxide due to the vacancies in the nickel lattice and the excess of oxygen gas. Therefore, the present sputtering process suggests the nonstoichiometric behavior of nickel oxide. The crystallites are grown through planes (101) and (110). However, the crystallite growth on plan (101) is predominant through the film thickness growing. Spherical particles are grown and their size decreases as the film thickness increases. The absorption edge of the prepared films is red shifted towards a higher wavelength (or at higher thicknesses), and the optical band gap values are then decreased as the film thickness increases. The refractive index and the dielectric constants are proportional to the optical band gap values. Photoluminescence spectra showed that defects are created in the nickel oxide lattice. Moreover, this is strongly dependent on the film's morphology and the particle size. The PL intensity decreases as the film thickness increases, as well as when the particle size decreases. This means that the PL behavior of NiOx is dominated by both the film morphology and the film thickness. Acknowledgments: The authors, therefore, gratefully acknowledge the DSR technical and financial support.

Conflicts of Interest:
The authors declare no conflict of interest.

Conclusions
Sputtered nickel oxide thin films at a fixed oxygen flow rate of 10 SCCM and at different deposition times proved the trigonal phase of nickel oxide due to the vacancies in the nickel lattice and the excess of oxygen gas. Therefore, the present sputtering process suggests the non-stoichiometric behavior of nickel oxide. The crystallites are grown through planes (101) and (110). However, the crystallite growth on plan (101) is predominant through the film thickness growing. Spherical particles are grown and their size decreases as the film thickness increases. The absorption edge of the prepared films is red shifted towards a higher wavelength (or at higher thicknesses), and the optical band gap values are then decreased as the film thickness increases. The refractive index and the dielectric constants are proportional to the optical band gap values. Photoluminescence spectra showed that defects are created in the nickel oxide lattice. Moreover, this is strongly dependent on the film's morphology and the particle size. The PL intensity decreases as the film thickness increases, as well as when the particle size decreases. This means that the PL behavior of NiO x is dominated by both the film morphology and the film thickness.