Structural and optical properties of Ni doped ZnO thin films using sol–gel dip coating technique

Structural and optical properties of Ni doped ZnO thin films using sol–gel dip coating technique

Optik 124 (2013) 2023–2027 Contents lists available at SciVerse ScienceDirect Optik journal homepage: www.elsevier.de/ijleo Structural and optical ...

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Optik 124 (2013) 2023–2027

Contents lists available at SciVerse ScienceDirect

Optik journal homepage: www.elsevier.de/ijleo

Structural and optical properties of Ni doped ZnO thin films using sol–gel dip coating technique J. Ramesh a,∗ , G. Pasupathi b , R. Mariappan c , V. Senthil Kumar a , V. Ponnuswamy c a b c

Department of Physics, Karpagam University, Coimbatore 641 021, Tamil Nadu, India A.V.V.M Sri Pushpam College (Autonomous), Poondi 613 503, Tamil Nadu, India Department of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641 020, Tamil Nadu, India

a r t i c l e

i n f o

Article history: Received 12 January 2012 Accepted 14 June 2012

Keywords: Ni doped zinc oxide thin films XRD SEM EDAX Optical properties

a b s t r a c t Ni-doped zinc oxide (Ni:ZnO) thin films were deposited on glass substrates by sol–gel dip-coating technique with different pH values. The films were characterized structural, morphological, compositional and optical properties respectively. X-ray diffraction revels that all samples have a polycrystalline in nature with hexagonal wurtzite structure. The structural parameters such as crystallite size, dislocation density and micro strain were calculated from XRD studies. Scanning electron microscopy shows irregular grains with average grains size found to be ∼150 nm at 400 ◦ C. Energy dispersive X-ray analysis indicates that the presence of Ni, Zn and O elements with average atomic percentage of Ni:ZnO was 04.19:44.99:50.82 at 400 ◦ C. Optical absorption spectra show that the band gap energy is decreases with the increase in pH values. © 2012 Elsevier GmbH. All rights reserved.

1. Introduction Nickel doped zinc oxide (Ni:ZnO) films have been actively investigated by many research groups because they have potential applications in opto-electronic devices such as solar cells, flat panel displays and transparent heat mirrors. ZnO films doped with transitional metals, for example, Co, Ni, Fe, have been widely studied as diluted magnetic semiconductors [1–3]. A variety of methods have been used to fabricating nickel doped ZnO, such as sol–gel technique [4], pulsed laser deposition (PLD) [5], sol–gel spin coating [6], atom beam sputtering technique [7], and co-precipitation method [8]. A very few report is available on sol–gel dip-coating Ni-doped ZnO thin films. Particularly, there is no report on the preparation and characterization of sol–gel dip-coating Ni-doped ZnO thin films. ZnO material with bandgap energy (3.3 eV) is especially attractive for use in photodiodes. In the present work preparation and characterization of Ni doped ZnO thin films by simple and low cost sol–gel dip coating technique has been reported. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), and optical measurements. The results are discussed and reported.

(1.09 g), Nickel nitrate hexahydrate (1.45 g), was first dissolved in a mixture of 10 ml of 2-methoxy ethanol and 0.1 ml of monoethanolamine at room temperature. The concentration of zinc acetate was 0.5 mol/litre and the molar ratio of mono ethanolamine to zinc acetate was kept as 1:0. The resultant solution was stirred at 60 ◦ C for 30 min to yield a clear and homogeneous solution, which served as a coating solution. The film deposition was carried out in air at room temperature with controlled withdrawal speed of 1 cm min−1 . For each layer, the film were preheated at 275 ◦ C for 10 min and post heated with different pH values of (6, 8 and 10) and annealed at 400 ◦ C and 500 ◦ C for 30 min. The deposition was repeated five times to obtain five-layered films. X-ray diffraction data of the Ni doped ZnO films were recorded with the help of Philips Model PW 1710 diffractometer with Cu K␣ radiation ( = 0.1542 nm). Surface morphological studies and compositional analysis were carried out using a scanning electron microscope and energy dispersive X-ray analysis setup (EDAX) which is attached with SEM (Philips Model XL 30), respectively. Optical absorption spectrum was recorded using a JASCO-V-570 spectrophotometer. 3. Results and discussion

2. Experimental details 3.1. Structural analysis Ni-doped ZnO thin films deposited on glass substrates were prepared by sol–gel dip-coating method. Zinc acetate dehydrate

∗ Corresponding author. E-mail address: [email protected] (J. Ramesh). 0030-4026/$ – see front matter © 2012 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2012.06.035

Figs. 1 and 2 show the X-ray diffraction (XRD) patterns of Nidoped ZnO thin films deposited on glass substrate, annealed at 400 and 500 ◦ C with different pH values of (6, 8and 10). The peaks at 31.86, 36.87 and 66.38 correspond to (100), (002) and (101) planes respectively. XRD indicates that the films are polycrystalline in

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Fig. 1. XRD patterns of Ni doped ZnO thin films annealed at 400 ◦ C with different pH values: (a) pH = 6, (b) pH = 8 and (c) pH = 10.

nature with the hexagonal wurtzite structure [1]. Ni-doped ZnO thin films annealed at 400 ◦ C with pH values 10 shows poor crystallinity with amorphous nature, whereas pH value 6 and 8 shows good crystalline nature. But in case of Ni-doped ZnO thin films annealed at 500 ◦ C shows amorphous nature for all the 3 pH values.

Fig. 3. SEM images of Ni doped ZnO thin films annealed at 400 ◦ C with different pH values: (a) pH = 6, (b) pH = 8 and (c) pH = 10.

The average grain size of crystallite size was calculated by Scherer’s equation as follows: D=

0.94 ˇ cos 

(1)

where D is grain size, ˇ the full width at half maxima and  the ˚ The variations of lattice conwavelength of X-ray used (1.5402 A). stant (a), crystallite size (D), strain (ε), and dislocation density (ı) are calculated shown in Table 3. On increasing the annealing temperature as 400 ◦ C and 500 ◦ C, the crystallite size found to be increases with the increase in pH values. It is observed that (Table 3) the Ni:ZnO films, the lower dislocation density and micro strain improves of stoichiometry of the films. 3.2. Morphology analysis The surface morphology of the Ni-doped ZnO thin films is studied using SEM. Figs. 3 and 4 show the SEM images of Ni-doped ZnO films were annealed at 400 and 500 ◦ C with different pH values from 6 to 10 in step 2. The SEM image of Ni-doped ZnO films

Table 1 Atomic percentage of Ni doped ZnO thin films annealed at 400 ◦ C from EDAX analysis.

Fig. 2. XRD patterns of Ni doped ZnO thin films annealed at 500 ◦ C with different pH values: (a) pH = 6, (b) pH = 8 and (c) pH = 10.

pH values

Elements

Atomic %

pH = 6

O Zn Ni

49.75 48.16 01.63

pH = 8

O Zn Ni

65.93 32.09 01.98

pH = 10

O Zn Ni

50.82 44.99 04.19

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Fig. 4. SEM images of Ni doped ZnO thin films annealed at 500 ◦ C with different pH values: (a) pH = 6, (b) pH = 8 and (c) pH = 10.

on glass substrate revealed that the surface of the films did not contain voids, cracks or pinholes and any other serious defects. Fig. 3c shows fine hexagonal shaped grains for pH 10, the average particle size of microcrystalline was calculated from the intercept method is given by [9] and size determined from SEM in the range of 80–120 nm. SEM sample annealed at 500 ◦ C for different pH values of (6, 8 and 10) with different magnifications are shown in Fig. 4a–c. It is clear that the surface morphology was modified with doping. Fig. 4a shows uniform distribution of spherical grains over total coverage of the substrate with compact morphology. Agglomeration observed in Fig. 4c with smooth background that may correspond to some amorphous phase of Ni-doped ZnO thin films. 3.3. Energy dispersive X-ray analysis The EDAX spectrum of Ni-doped ZnO thin films annealed at 400 and 500 ◦ C with different pH values of (6, 8 and 10) are shown in Figs. 5 and 6 to identify the composition. Spectra were recorded in line scan mode and the characteristic peaks for Zn, O and Ni were clearly observed [10]. Ni content in the films increased from 1.63 to 4.19% with the increase in the pH values. The Ni/Zn ratio computed from the EDAX analysis found to increase and are given in Tables 1 and 2. 3.4. Optical properties The spectral normal transmittance (T) was measured by UV–visNIR Spectrometer over the wavelength range (200–2500 nm). The experimental accuracy of the transmittance is ±0.005 and the wavelength is ±0.05 nm. The observed transmittance data were corrected relative to optically identical uncoated glass substrate. The transmittance spectra of the Ni:ZnO films annealed at 400 ◦ C and 500 ◦ C with different pH values are shown in Figs. 7 and 9.

Fig. 5. EDAX spectrum of Ni doped ZnO thin films annealed at 400 ◦ C with different pH values: (a) pH = 6, (b) pH = 8 and (c) pH = 10.

The optical measurements have shows an increase in transmission T (%) with the decrease in pH values and decrease in thickness. The optical band gap energy (Eg ) was calculated from transmission spectra. The variation of (˛h)2 with photon energy (h) was found to given this relation: 2

(˛h) = A(h − Eg )

1/2

(2)

Table 2 Atomic percentage of Ni doped ZnO thin films annealed at 500 ◦ C from EDAX analysis. pH values

Elements

Atomic percentage

pH = 6

O Zn Ni

50.69 49.31 00.10

pH = 8

O Zn Ni

51.37 48.37 00.26

pH = 10

O Zn Ni

50.69 49.31 01.53

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Table 3 Structural parameters of Ni-doped ZnO thin films. Annealing temperature (◦ C)

pH values

d-spacing

400 ◦ C

6 8 10

2.80530 2.80853 2.83704

500 ◦ C

6 8 10

2.80820 2.81577 2.82318

Lattice constant (a × 10−10 m)

Crystallite size, D (nm)

Dislocation density, ı (1014 lin/m2 )

Strain, ε (10−4 lin−2 m−4 )

100 100 100

3.2392 3.2430 3.2759

117 132 147

7.30 5.70 1.03

2.960 2.610 2.350

100 100 100

3.2426 3.2513 3.2599

41 52 69

5.83 4.20 3.05

1.452 1.352 1.202

hkl

where A is a constant, ˛ is a absorption coefficient and Eg is the optical band gap energy. The variation of (˛h)2 versus h for the Nidoped ZnO thin films prepared at different pH values of (6, 8 and 10), annealed at 400 and 500 ◦ C. The optical bandgap energies are 3.25, 2.95 and 2.90 eV for Ni-doped ZnO thin films annealed at 400 ◦ C and are shown in Fig. 8. Similarly Fig. 10 shows bandgap energies is 3.25, 3.20 and 3.16 eV for Ni-doped ZnO thin films annealed at 500 ◦ C. The bandgap values decrease with the increase in pH values [11].

Fig. 7. Transmittance spectrum of Ni doped ZnO thin films annealed at 400 ◦ C with different pH values: (a) pH = 6, (b) pH = 8 and (c) pH = 10.

Fig. 8. The plots of (˛h)2 versus h for Ni doped ZnO thin films annealed at 400 ◦ C with different pH values: (a) pH = 6, (b) pH = 8 and (c) pH = 10.

Fig. 6. EDAX spectrum of Ni doped ZnO thin films annealed at 500 ◦ C with different pH values: (a) pH = 6, (b) pH = 8 and (c) pH = 10.

Fig. 9. Transmittance spectrum of Ni doped ZnO thin films annealed at 500 ◦ C with different pH values: (a) pH = 6, (b) pH = 8 and (c) pH = 10.

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with different temperatures and different pH values. X-ray diffraction indicates that the Ni:ZnO films are crystalline in nature have preferential orientation (1 0 0) planes with the hexagonal structure. SEM image show that the Ni:ZnO film are irregular growth and hexagonal shaped grains annealed at 400 ◦ C with pH value 10. The average grain size is found to be 90 nm. The presence of elemental constituents was confirmed from EDAX analysis. The average atomic percentage of nickel, zinc and oxygen was 04.19, 50.82 and 44.99 respectively. The optical studies shows an decrease in the transmission T (%) with a increase in the pH values. The optical bandgap energies are calculated as 3.25, 2.95 and 2.90 eV for Ni-doped ZnO thin films annealed at 400 ◦ C with different pH values. References

Fig. 10. The plots of (˛h)2 versus h for Ni doped ZnO thin films annealed at 500 ◦ C with different pH values: (a) pH = 6, (b) pH = 8 and (c) pH = 10.

4. Conclusion Nickel doped zinc oxide (Ni:ZnO) thin films were deposited by simple, low cost and effective sol–gel dip coating technique

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