INTRODUCTION:

Transparent metal oxide (TMOs) are materials that had high transparency in the visible region and good electrical conductivity, they had a very vital place in micro and optoelectronic devices.  TMO films such as TiO₂ ZnO, SnO₂, CdO, and CuO had been widely studied for their used in semiconductor device tools. Among these TMOs, Nickel oxide (NiO) material is one of few p-types semiconducting with high band gap width ranged from (3.5–4.0 eV). There are many new studies and research related to Nickel oxide applications that are involved in solar energy applications and solar cells development.

To quantify the notable properties of those matters such as: good chemical stability, low cost with significant optoelectronic and magnetic properties many spectral studies had been carried by several workers on the uses of Nickel oxide. Chia-Cheng Huang and al.¹ had presented a study related by developing high-transmittance hetrojunction diodes based on NiO / TZO bilayer thin films. Whereas C. Smokes a, et al. had used nickel oxide in a diode connection Ni-NiO-Ni for detection and mixing of 30 THZ radiation². R.K. Gupta et al. Had reported the fabrication and characterization of NiO/ZnO p-n junctions by pulsed laser deposition³. Tonghui Guo et al. Also had made work about Synthesis of well-dispersed NiO ink for efficient perovskite solar cells⁴. The change of NiO films properties are extremely dependent on the method of formation⁵. Several techniques had been employed for the preparation of NiO films, these methods can be classified as a physical and chemical coating process. These techniques include vacuum evaporation, electron  beam evaporation, sol-gel, and  the  spray  pyrolysis technique (SPT)...etc. NiO  thin  films find  several  applications such  as: solar cells^6,7, gas collectors⁸, piezoelectric collectors^9,10, flat screens¹¹, devices  acoustic¹², laser  diodes¹³,... etc.

We focused in this work on the effect of a preparative parameter such source nature of Nickel on optical and electrical properties of NiO thin films to optimize the  deposition conditions using the SPT method deposited on heated glass substrates at 500°C.

MATERIALS AND METHODS:

Materials:

Undoped Nickel oxide films were synthesized by using the spray pyrolysis process. Nickel Nitrate [Ni(NO₃)₂.6H₂O] and Nickel Chloride [NiCl₂.6H₂O] were used as a nickel source. The starting solution was obtained by dissolving one nickel source in 30 ml of ultra-pure water (H₂O), the solution concentration was fixed at 0.1M. The mixture solution was stirred carefully for 30 min by a magnetic mixer, leading to the formation of clear green and homogeneous solution. Before starting, the glass substrates were chemically cleaned using acetone, rinsed with ultra-pure water, and well dried in air. Where the substrates were (0382 - 0004) Cito glass slide in size of (75 × 25 × 1.1mm³) and then we sprayed the solution at a rate of 3ml/min on a glass substrate which was heated to a temperature of 500°C. The impurities are evaporated by the action of the heat to leave the aperture of the fan and the nickel atoms bond with the oxygen to form the nickel oxide on the substrate as a layer. The distance nozzle substrate was maintained at 25cm. The device is turned off and the substrate remains to bring its temperature down at room temperature, and then it is taken for study after it has cooled so as not to break by thermal shocks.

Thin films characterization:

The optical properties of the films are determined using a UV-visible spectrophotometer (Shimadzu, Model 1800); the scanning measurements were ranged from 300-900nm. The measurement of the electrical conductivity of the films was performed with the two-point probe technique. Film thickness was estimated by the gravimetric technique considering the density of the bulk nickel oxide. All measurements were carried out at room temperature, all spectral had been designated by origin program.

RESULTS AND DISCUSSIONS:

Optical Properties:

Figure 1 shows the transmittance spectral of the sprayed NiO thin films prepared at 500°C with various Ni sources recorded in the wavelength ranged from 300 to 800 nm. All the obtained films show good transparency in the visible region. The averaged transmission value is over 52% for the film prepared with Nickel Chloride source, however, the transparency reaches the value of 61% for the film prepared with Nickel Nitrate source. This variation of the optical transmittance may be referred to change in the film thickness given by Lambert’s law and photon scattering in the films. Strong absorption had been observed between 300-360 nm, corresponding to the onset fundamental absorption edge of NiO due to the direct transition from the valence band to the conduction band, as presented in Figure 1.

Figure 1: Transmittance spectral of NiO thin films using different nickel sources.

The absorption coefficient α can be estimated using the transmittance T values and thickness t at the absorption edge of Lambert's law¹⁴:

The band gap energy Eg can be calculated using the transmission spectral from the Tauc’s relation:

Where C is a constant, hν is the photon energy, α is the absorption coefficient, and Egis the band gap energy. For directly allowed transitions the Eg values can be evaluated from the plot of (αhν)2 versus hν. Extrapolating the linear portion of the graph with energy axis gives the band gap, as shown in Figure 2. The calculated Egvalues are listed from Table 1.

Table .1: Values of optical parameters of NiO thin films using different Ni sources.

Nickel

source

Tmoy

(%)

Eg(eV)

(meV)

nmoy

Kmoy

Nickel nitrate

3.84

318

1.20

0.030

Nickel chloride

3.59

290

1.14

0.032

As can be seen that Eg values are in the range 3.59-3.84 eV obtained by; Nickel Chloride and Nickel Nitrate sources respectively. These results are very compatible with the Eg of intrinsic NiO and are in good correspondence with the literature reports. Variation in the Eg values may be related to the existence of defects or vacancies in the NiO material and can also affect by variation of Ni sources¹⁵.

Figure 2: Estimation of band gap energy (Eg) from Tauc’s relation of NiO thin films using different Ni sources.

The Urbach tail of the films can be identified by the following relation:

d Eu is the Urbach energy linked to the width of the exponential absorption edge. Eu values can be determined from the plot of ln(a) versus (hν) and its straight-line slopes, as observed in Figure 3.

Figure 3: Estimation of urbach energy (Eu) of NiO thin films using different Ni sources.

The evaluated Eu values are given in Table 1. It can be seen, the Euvalues change from 290 to 318 meV with a variation of Ni source nature. This variation is ascribed to the decrease of defects, which is related to the disorder in the film caused by the creation of new localized energy states near the band edges. Furthermore, the Eu values change inversely with the optical band gap of NiO films, as observed in Table 1.

Refractive index and extinction coefficient:

The index of Refraction (n) is a significant optical parameter for the characterization of optical materials. In this work, the n values can be defined from this equation¹⁶:

Where R is the reflectance of films. The variation of refractive index as a function of wavelength in the range 300-800 nm with different Nickel sources of NiO thin films is shown in Figure 4, the mean values are presented in Table 1. It can be seen that the refractive index values of NiO thin films are relatively constant in the visible region.

Figure 4: variation of refractive index of NiO thin films using different Ni sources.

The extinction coefficient k was calculated from the absorption coefficient a, which is defined by [16]:

𝑘 = 𝛼𝜆

4𝜋

Figure 5 shows the changed in the extinction coefficient (k) with wavelength. The average values of the extinction coefficient (k) are listed in Table 1.

Figure 5: variation of extinction coefficient of NiO thin films using different Ni sources.

Electrical Properties:

Table 2 shows the variation in the resistivity of un doped NiO thin films with the variation of Nickel sources. The resistivity of these films are varied from 0.106 to 5.065 (Ω). This behavior can be reported by the access of Ni ion vacancies and/or interstitial oxygen in NiO crystallites leading to the formation of Ni3+ ¹⁷.

Table .2: Values of square resistivity and resistivity of NiO thin films using different Ni sources.

Nickel source	Square resistivity (Ω/□)	Resistivity (Ω)
Nickel nitrate	1.075	0.106
Nickel chloride	51.1	5.065

CONCLUSION:

In this study, Undoped Nickel oxide thin films were synthesized by using the spray pyrolysis technique. Nickel Nitrate [Ni(NO₃)₂.6H₂O] and Nickel Chloride [NiCl₂.6H₂O] were used as the sources of Nickel the effect of Nickel sources variation on the optical and electrical properties of prepared thin films had been studied. The results show that all films exhibit averages optical transparency between 52% and 61% in the visible range and their band gap values were ranged in 3.59 – 3.84 eV. Refractive index and extinction coefficient values were decreased with a variation of Nickel sources. The electrical measurement shows that the minimum resistivity of the NiO films is 0.106Ω from film prepared with Nickel Nitrate source.  The prepared NiO thin films lead to highly optical transparency and good electrical properties layers. Through these results, the optical and electrical layers of NiO and their conformity with the results of many research and studies, we conclude that more film is thin, The electrons, which are present at it's surface are easily move, leaving a band gap, thus changing the crystal structure, which leads to the emergence of crystalline defects whose increase does not increase the characteristic of metathesis. This proves that changing the nickel source leads to a change in the optical and electrical properties. we found that the nickel nitrate thin film corresponds to these results and that its energy gap value is higher than that of nickel chloride, so the semi-conductor of nickel nitrate thin films is better for use in nickel oxide for optoelectronic applications.

CONFLICT OF INTEREST:

The authors have no conflicts of interest regarding this investigation.

ACKNOWLEDGEMENT:

The authors would like to thank the radiation, plasma and surface physics laboratory workers, University of Ouargla-Algeria for their kindness and all the help that they have given us.

REFERENCE:

1. Huang, C.C., Wang, F.H., Wu, C.C., Huang, H.H. and Yang, C.F., “Developing high-transmittance heterojunction diodes based on NiO/TZO bilayer thin films”, Nanoscale Research Letters, 8(1), 206(2013). doi: 10.1186/1556-276X-8-206.

2. Fumeaux, C., Herrmann, W., Kneubuhl, F.K. and Rothuizen,H., “Nanometer thin-film Ni-NiO-Ni diodes for detection and mixing of 30 THZ radiation”, Infrared Physics and Technology, 39(3), 123-183(1998). https://doi.org/10.1016/S1350-4495(98)00004-8

3. Gupta, R.K., Ghosh, K., and Kahol, P.K., “fabrication and characterization of NiO/ ZnO p-n junctions by pulsed laser deposition”, Physica E: Low-dimensional Systems and Nanostructures, 41(4), 617-620(2009). https://doi.org/10.1016/j.physe.2008.10.013

4. Guo, T., Zhang, Z., Yu, L., Yuan, H., Zhang, J., Liu, X., Hu, Z., and Zhu, Y., “Synthesis of well dispersed NiO ink for efficient perovskite solar cells”, Journal of Allys and Compounds, 860,157889 (2021). https://doi.org/10.1016/j.jallcom.2020.157889

5. Ukoba, K.O., Eloka-Eboka, A.C and Inambao, F.L.,“Review of nanostructured NiO thin film deposition using the spray pyrolysis technique”, Renewable and Sustainable Energy Reviews, 82(3), 2900-2915(2018). https://doi.org/10.1016/j.rser.2017.10.041

6. Ding, L., Boccard, M., Bugnon, G., Benkhaira, M., Nicolay, S., Despeisse, M., Meillaud, F., and Ballif, C., “Hightly transparent ZnO bilayers by LP-MOCVD as front electrodes for thin-film micromorph silicon solar cells ”, Solar Energy Materials and Solar Cells 98, 331-336(2012). https://doi.org/10.1016/j.solmat.2011.11.033

7. Hüpkes, Z.H., Bunte, J.E., and Huang, S.M.,“Study of ZnO: All films for silicon thin film solar cells, Applied Surface Science”, 261, 268-275(2012).

8. Mariappan, R., Ponnuswamy, V., Suresh, P., Ashok, N., Jayamurugan, P., andChandra Bose, A. Influence of film thickness on the properties of sprayed ZnO thin films for gas sensor applications, Superlattices and Microstructures. 71, 238-249(2014). https://doi.org/10.1016/j.spmi.2014.03.029

9. Atanas, J.P., Asmar, R.A., Khoury, A., and Foucaran, A., “Optical and structural characterization of ZnO thin films and fabrication of bulk acoustic wave resonator (BAW) for the realization of gas sensors by stacking ZnO thin layers fabricated by e-beam evaporation and r.f. magnetron sputtering technique s, Sensors and Actuators” A 127(2006), 49–55(2006). https://doi.org/10.1016/j.sna.2005.11.065

10. Fortunato, E., Barquinha, P., Pimentel, A., Gonçalves Marques, A., Pereira, L., and Martins, R., “Recent advances in ZnO transparent thin film transistors”. Thin Solid Films, 487(1–2), 205-211(2005). https://doi.org/10.1016/j.tsf.2005.01.066

11. Minami, T., Jun-ichi, O., Jun-ichi, N., and Miyata, T., “Effect of target properties on transparent conducting impurity-doped ZnO thin films deposited by DC magnetron sputtering”. Thin Solid Films”, 519(1), 385- 390(2010). https://doi.org/10.1016/j.tsf.2010.08.007

12. Serhane, R., Abdelli-Messaci, S., Lafane, S., Khales, H., Aouimeur, W., Hassein-Bey, A., and Boutkedjirt, T., “Pulsed laser deposition of piezoelectric ZnO thin films for bulk acoustic wave devices”.Applied Surface Science, 288, 572-578(2014). https://doi.org/10.1016/j.apsusc.2013.10.075

13. Lee, S.Y., Shim, E.S., Kang, H.S., Pang, S.S., and Kang, J.S., “Fabrication of ZnO thin film diode using laser annealing”. Thin Solid Films, 473(1), 31-34(2005). https://doi.org/10.1016/j.tsf.2004.06.194

14. Barir, R., Benhaoua, B., Benhamida, S., Rahal, A., Sahraoui, T., and Gheriani, R., “Effect of Precursor Concentration on Structural Optical and Electrical Properties of NiO Thin Films Prepared by Spray Pyrolysis”, Nanomaterials 10, 5204639(2017). https://doi.org/10.1155/2017/5204639

15. Sajilal, K., and Moses Ezhil Raj, A., “Effect of thickness on physico-chemical properties of p-NiO (bunsenite) thin films prepared by the chemical spray pyrolysis (CSP) technique”, optik, 127(3)2016, 1442-1449(2015). https://doi.org/10.1016/j.ijleo.2015.11.026

16. Mrabet, C., Ben Amor, M., Boukhachem, A., Amlouk, M., and Manoubi, T., “Physical properties of La-doped NiO sprayed thin films for optoelectronic and sensor applications” 42(5), 5963-5978(2016). https://doi.org/10.1016/j.ceramint.2015.12.144

17. Benhamida, S., Benhaoua, B., and Barir, R.,“Effect of Sprayed Solution Volume on Structural and Optical Properties of Nickel Oxide Thin Films”, nano- and electronic physics 9: 03001-03005(2017). http://doi.org/10.21272/jnep.9(3).03004

Received on 31.05.2021 Modified on 20.08.2021

Asian J. Research Chem. 2022; 15(2):159-162.

DOI: 10.52711/0974-4150.2022.00026