Abstract

General Background: Thin-film technology is pivotal in advancing modern optoelectronic devices due to its ability to tailor material properties at the nanoscale. Specific Background: Zinc oxide (ZnO) and cobalt oxide (CoO) are prominent materials in this field, with doping techniques such as cobalt incorporation offering potential enhancements in film properties. Knowledge Gap: The impact of cobalt doping on the structural and optical properties of ZnO thin films remains inadequately explored, particularly concerning varying doping concentrations. Aims: Aims to elucidate the effects of cobalt doping on ZnO thin films' structural and optical characteristics, with concentrations of Zn0.6Co0.4O and Zn0.8Co0.2O compared to pure ZnO and CoO films. Results: Thin films were prepared via spin coating on glass substrates at 3000 rpm for 30 seconds. X-ray diffraction analysis revealed that pure ZnO films possess a hexagonal polycrystalline structure, whereas CoO films exhibit a cubic structure. Doping with cobalt resulted in a decrease in peak intensity at the (111) orientation and an increase in grain size. Optical measurements indicated enhanced transmittance and modified absorbance spectra with increased cobalt content, showing maximum absorbance at short wavelengths and diminished values in the visible region. Reflectivity increased with photon energy before decreasing at higher energies. Novelty: Cobalt doping significantly alters both structural and optical properties of ZnO films, with distinct changes in lattice constants, grain size, and optical behavior, suggesting a potential route for optimizing ZnO-based materials. Implications: Cobalt-doped ZnO films hold promise for improved performance in optoelectronic applications, such as solar cells and sensors, necessitating further research into diverse doping strategies to fully harness their technological potential.

Highlights:

1. Cobalt doping alters ZnO thin films' structural and optical properties.
2. Spin coating method used at 3000 rpm for 30 seconds.
3. Enhanced optical properties suitable for optoelectronic applications.

Keywords: Thin films, Zinc oxide, Cobalt doping, Optical properties, X-ray diffraction

Introduction

The term thin film is widely used in science and technology. A thin film can be defined as a layer or group of layers made up of atoms of matter, ranging in thickness from a few nanometers to one micrometer [1,2,3]. This composition refers to the condensation of atoms, molecules or ions. The researchers consider that the thickness of the film should be very small, less than one micrometer, as any thickness exceeding this limit is considered thick (thick film) [4,5].

Thin-film technology has attracted the attention of physical researchers since the eighteenth century [1,6] and has evolved over the years towards improving its preparation techniques, with the aim of obtaining films with excellent specifications in terms of thickness and homogeneity, making them suitable for different applications [7,8]. Due to the thickness of the thin film layer, it is usually deposited on different bases such as glass, silicon, quartz, aluminum [5,4,9] and some other materials such as salts and polymers [11,10,5]. The material to be prepared is deposited as a thin film on the base, which is a layer of atoms separated by voids. When atoms of matter fall on the surface of the base, these atoms occupy positions in the spaces, when atoms of matter fall on the surface of the base, these atoms occupy positions in the spaces. These atoms first reach the base surface and hold together in empty positions between the base top atoms and the first atoms that arrived. A base layer is formed that forms the separation between the next sedimentation layers and the base surface. Due to the separating layer, there is a significant impact on the properties of the material, whether it is as a thin film or as a material in its normal dimensions. After the sedimentation of the thin film is complete, an additional effect of its thickness appears in the determination of the physical properties of the film, which differ from those of its constituent substances [4,5].

Methods

2.1 Preparation the Glass Base

The preparation of glass bases on which solutions are deposited (substrates) involves their selection and cleaning. German-made glass substrates were used with a thickness ranging from (1 to 1.2) mm and dimensions (76.2 ×25.4) mm. They have been carefully cleaned to ensure they are free of impurities and plankton and prepared for sedimentation. The steps to clean them include:

1. Wash the samples with plain water and then clean them with diluted nitric acid. (HNO3).

2. Wash the samples with distilled water and then put them on the magnetic stirrer for 10 minutes.

3. Put the samples in acetone, which is expressed in the chemical formula (C3H6O), then take them out and dry them to get rid of traces, impurities and suspended materials.

4. Placing the samples in ethanol, which is expressed in the chemical formula (C2H5OH) for a period of (10 min) and drying them, so that they are ready to be deposited on them.

2.2 Preparation of zinc oxide and cobalt solution

To prepare the zinc oxide solution, a high-purity zinc nitrate (Natarat Zinc), a white powder solid with molecular weight (297.49 g/mol) and chemical formula Zn (No3)2 6H2O was used. The solution was prepared in different concentrations (0.2,0.4,0.6,0.8,1) molar by dissolving 17.8494g, 7.13976, 10.709, 14.2795, 3.569) in (100 ml) of distilled water and using a sensitive balance (410) the solution used for the precipitation of CoO films was prepared the solution was prepared in different concentrations (0.8,0.6,0.4,0.2,1) molar by dissolving Gram (17.4608,13.96872,10.4765,6.9844,3.4921)

From cobalt nitrate with the formula Co (NO3)2 6H2O, a substance in the form of a red powder with an equivalent weight (291.014 g/mol) and using a sensitive balance with sensitivity (10-4 grams). The solutions were prepared at a temperature of (60ºC) and using a magnetic stirrer at a speed of 6 cycles for a period of (1) hours and left for a period of (24) hours to ensure its homogeneity before sedimentation..

Result and Discussion

3.1 Results of Structural Measurements

The results of X-ray diffraction diagnosis of prepared undoped and zinc doped (Zn) films with different distortion ratios showed that they have a polycrystalline and cubic type structure, which is consistent with the results of published research [12,13,14,15]. While the results of the X-ray diffraction diagnosis of the zinc oxide film showed that it is a polycrystalline structure of the hexagonal type and these results are consistent with research [15] Figure (1) shows the X-ray diffraction curves of all prepared films, and by analyzing these that appear sharply when bundles of these (Peaks) curves are shed, the locations of the peaks were known.

Rays at different angles on the film so that it is allowed to overlap constructively when the Prague condition is available and we note that the prevailing trend of growth is (111) and there is no change in the prevailing trend by increasing the percentage of cobalt distortion of zinc oxide films and we note from the forms the emergence of diffraction peaks corresponding to levels (111) and (220) and (311) with a small displacement of some of these peaks and this may be attributed to mechanical microscopic stress resulting from various sources such as impurities, defects and voids inherent in the film even after Heat treatment, as the prevailing direction of the film depends on the sedimentation method used [16].

Figure 1.X-ray diffraction of cobalt- doped and undoped cobalt oxide films.

Calculated:

1. Distance between planes (d_hkl): The distance between crystal planes from Brack's law was calculated from

n𝝀=𝟐𝒅𝐬𝐢𝐧𝜽𝑩

as (n = 1, 2, 3) is an integer representing the order of interference (diffraction order) and (d) represents the distance between the crystal planes and (ϴ_B) represents the Braque angle. It was found that the values of (d) are consistent with the values of the card (JCPDS00-023-1390) for cobalt oxide shown in Table (1).

2. Lattice Constant: calculated as shown in Table 2. From the analysis of X-ray diffraction patterns according to the relationship

Figure 2.

Where:

• d is the distance between crystalline surfaces.

• A is the edge length of the unit cell.

• h, k, and l are Miller indexes that determine the directions of crystalline surfaces.

It was found that the lattice constant agrees with the card (JCPDS00-023-1390) and found that it changes slightly after the cobalt distortion in different proportions and this confirms that the cobalt distortion has an effect on the crystal structure of zinc oxide.

3. Average grain size: calculated using the Scherrer formula

Figure 3.

D = Crystal size

β = The greatest mid, which was converted to the radial angle by multiplying it π/180

For all prepared films, it has been found that it ranges from nm (22.21667-37.04667) and the grain size of the crystallized material plays an important role in determining the properties of the material.

2.3 Scanning Electron Microscopy (SEM) results

The undoped cobalt oxide (CoO) films and zinc oxide (ZnO) were examined by scanning electron microscopy (SEM) Figure (2) (a, b, c, d) shows images of scanning electron microscopy of cobalt oxide films and zinc smelter and undoped If we note that the nanoparticles are spherical in shape and lumpy and collected in different patterns and densities, it is possible to distinguish between the boundaries and this is evidence that the film is polycrystalline and the presence of zinc atoms in a replacement manner with cobalt atoms and this works on the increase collisions and thus the loss of molecular energy sufficient to form molecular assemblies [17].

Figure 4. (a) CoO(pure)

Figure 5. (b)Co0.4Zn0.6O

Figure 6. (C)Co0.2Zn0.8O

Figure 7. (d)Zn(pure)

Electron microscopy results for doped and undoped cobalt oxide and zinc films.

In the case of films doped with zinc and cobalt, the results of (EDS) showed the emergence of two dominant peaks dating back to (Zn) and the appearance of the cobalt element for all films smeared and this confirms the entry of cobalt as an impurity within the crystal structure of the zinc oxide films in the case of cobalt distortion, and peaks can be observed due to (Au) due to the thin gold layer that is deposited on the samples when prepared for examination of electron microscopy in order to increase the accuracy of images in all prepared films, so this type of analysis (EDS) It is necessary and important to know the quality of impurities that exist and whose presence cannot be known through the analysis and examination of (XRD), as in the figures below (a, b, c, d).

Figure 8.

Figure 9.

Figure 10.

Figure 11.Analysis results of (EDS) for cobalt oxide and zinc films doped and undoped.

2.4 Optical Measurements

1. Absorbance

Absorption measurements were made within the wavelength range (200-1200) nm for all undoped zinc oxide films doped with cobalt with different tinting ratios Figure (3) shows the variation of the absorption spectrum as a function of the wavelength, as the absorbance of all films is greatest at short wavelengths and then decreases with increasing wavelength to reach the lowest values in the visible region of the spectrum, so these films can be used as windows in solar cell applications.

Figure 12.Absorbency as a function of the wavelength of doped and undoped cobalt oxide films and of the pure zinc oxide film. 2. Transmittance

2. Transmittance

The transmittance spectrum behaves opposite to absorbance as in Figure (4) as the transmittance of undoped zinc oxide films doped with cobalt is as low as possible at the core absorption edge (short wavelengths), and increases with increasing wavelength and then shows a sudden increase to be established after the wavelength (400) in the visible and near infrared region, but when distortion, the transmittance increases with increasing distortion rates due to the high transmittance of cobalt in the visible spectrum region.

Figure 13.Transmittance as a function of the wavelength of the doped and undoped cobalt oxide films and the pure zinc oxide film.

3. Reflectance

The reflectivity of the absorption spectrum and permeability was calculated under the law of conservation of energy, which came in the relationship (in the reflectivity of the first relationship that connects n and K), Figure (5) shows the reflectivity as a function of the photon energy and for all the prepared films, as the behavior of the reflectivity curve of the unformed and distorted films gradually increases with the increase of photon energy and then begins to decrease in the range of high photon energies, and the explanation for this is that absorption is very little at photonic energies less than the value of a gap and at the energy equal to the value of Energy gap almost increases absorption due to (h_ν<Eg) energy Electronic transitions between the valence and conduction beams, which causes a decrease in the reflectivity values, but when gluing with zinc, the reflectivity decreases for the films with increasing distortion ratios.

Figure 14.Reflectivity as a function of photon energy for clouded and undoped cobalt oxide films and pure zinc oxide film.

4-Absorption Coefficient( α )

The absorption coefficient was calculated from the relationship (α t = 2.303 Log 10/1) and the figure shows the change in the absorption coefficient (7) as a function of photon energy for cobalt oxide films doped and undoped with zinc. It can be observed that the behavior of the absorption coefficient curve is similar for all prepared membranes, as it is small at low photon energies, where the possibility of electronic transitions is low, and the values ​​of the absorption coefficient increase at the basic absorption edge towards high photon energies, and the absorption coefficient at these energies has a value greater than (10⁴ cm^-1). This is due to the occurrence of permitted direct electronic transitions.

Figure 15.Change of absorption coefficient as a function of the incident photon energy for cobalt oxide films doped and undoped with cobalt, and for the pure zinc oxide film.

5-Extinction Coefficient

The extinction coefficient for all prepared membranes was calculated according to the relationship (𝒌 = ). Figure (7) shows the change in the extinction coefficient as a function of photon energy for cobalt oxide membranes doped and undoped with zinc and for pure zinc oxide membrane. We note from the figure that the extinction coefficient for the doped and undoped membranes increases gradually with the increase in photon energy is greatest at the energies corresponding to the basic absorption edge (high photon energies) as a result of electronic transfers between the valence and conduction bands.

Figure 16.

Figure (7): Extinction coefficient as a function of photon energy for zinc doped and undoped cobalt films and for pure zinc oxide film.

6- Refractive Index

The index of refraction was calculated according to the relationship (n = c/v ), and Figure (4.8) represents the change of the index of refraction as a function of photon energy for cobalt oxide films doped with and not doped with zinc. We notice from the figures that the nature of the refractive index curve is almost similar to the nature of the reflectivity curve, due to the correlation of the refractive index with reflectivity.

n = c/v

Where (c) represents the speed of light in a vacuum and (v) its speed in the material. The index of refraction depends on several factors, including the type of material and its crystal structure.

Figure 17.Index of refraction as a function of photon energy for zinc oxide films doped and undoped with cobalt and for pure zinc oxide films.

Conclusion

The investigation into the structural, morphological, and optical properties of zinc oxide (ZnO) and cobalt oxide (CoO) thin films, both doped and undoped with cobalt, has yielded significant findings. The X-ray diffraction results confirm that ZnO films exhibit a hexagonal polycrystalline structure, while CoO films display a cubic structure, with cobalt doping introducing slight changes in lattice constants and average grain sizes. Scanning electron microscopy revealed that the films are polycrystalline, with distinct nanoparticle formations, indicating effective doping. Optical measurements showed variations in absorbance, transmittance, reflectivity, absorption coefficient, extinction coefficient, and refractive index, highlighting the influence of cobalt doping on the optical properties of the films. These findings suggest that cobalt doping can enhance the optical characteristics of ZnO films, making them suitable for applications in optoelectronic devices and solar cells. Further research should explore the impact of different doping concentrations and other dopant materials on the structural and optical properties of ZnO thin films to optimize their performance for specific technological applications.

References

1. . G. Hass and R. E. Thun, Physics of Thin Films, Academic Press, New York, 1996.
2. . K. Seeger, Semiconductor Physics, Springer-Verlag, Wien-New York, 1978.
3. . O. S. Heavens, The Film Physics, Methuen Young Co. Ltd., London, 1970.
4. . G. I. Dr., General Inorganic Chemistry, translated by Dr. Habib Abdel Ahad, University Press, Mosul, 1986.
5. . K. L. Chopra, Thin Film Phenomena, McGraw-Hill, New York, 1985.
6. . H. A. Macleod, Thin Film Optical Filters, 3rd ed., Academic Press, New York, 2001.
7. . M. Ohring, The Materials Science of Thin Films, Academic Press, New York, 1992.
8. . L. Eckertova, Physics of Thin Films, Plenum Press, New York and London, 1977.
9. . J. R. Son, Thin Films Technologies, 2nd ed., McGraw-Hill, New York, 1986.
10. . B. Sasi, K. G. Gopchandran, P. K. Manoj, et al., "Preparation of transparent and semiconducting Nio films," Vacuum, vol. 68, pp. 149–154, 2003.
11. . S. Benhamida, "Caractérisation des Couches Minces D’oxyde De Nickel (Nio) Elaboré Par Spray Pyrolyse," Doctoral Thesis, Biskra University, Algeria, 2018.
12. . M. J. H. Amin Aziz, "Manufacturing and Study of the Properties of Hybrid Dividers (ZnO MgO Si)," Master Thesis, Faculty of Education, University of Tikrit, 2013.
13. . K. A. Aadim, A. A.-K. Hussain, and M. R. Abdulameer, "Effect of annealing temperature and laser pulse energy on the optical properties of CuO films prepared by pulsed laser deposition," Iraqi Journal of Physics, vol. 12, no. 23, 2014.
14. . F. I. Pires, E. Joanni, R. Savu, M. A. Zaghete, E. Longo, and J. A. Varela, "Microwave-assisted hydrothermal synthesis of nanocrystalline SnO powders," Materials Letters, vol. 62, pp. 431-434, Elsevier B.V., 2008.
15. . B. P. Singh, R. Kumar, J. Gaur, S. P. Singh, and R. C. Tyagi, "Effect of annealing on properties of transparent conducting tin oxide films deposited by thermal evaporation," Indian Journal of Pure and Applied Physics, vol. 51, pp. 65-68, 2013.
16. . M. H. H. Al-Qaisi, "Study of the Optical and Structural Properties of Indium-Doped Copper Oxide Films," Master Thesis, College of Education, Al-Mustansiriya University, 2005.
17. . A. S. Hassan, N. B. Hassan, and A. J. Haidar, "Study of the effect of manganese doping on topographic properties and some optical properties of the Zinc Sulfide film prepared by pulsed laser deposition technique," Journal of Karbala University, vol. 11, no. 4, 2013.