J. Bangladesh Electron.16 (1-2); 33-39, 2016
Effect of thickness variation on structural and morphological properties of ZnO thin films prepared by sol-gel method Md. Nasrul Haque Mia1,*, Md. Firoz Pervez1, Md. Khalid Hossain1** Mohammad Reefaz Rahman2, Md. Shah Alam1, Mohammod Abu Sayid Haque1 Himangshu Kumar Ghosh1 and Mahbubut Hoq1 1lnstitute of Electronics, Atomic Energy Research Establishment Bangladesh Atomic Energy Commission, Savar, Dhaka, Bangladesh 2 University of Liberal Arts Bangladesh (ULAB) Corresponding authors: *
[email protected], **
[email protected]
Abstract Physical properties of ZnO thin film depend on crystal structure and surface morphology. Again, the crystal structure of ZnO thin film is determined by fabrication procedure and thickness of the fabricated film. In this paper, ZnO thin films of three different thicknesses (100, 200, and 300 nm), measured by a surface profilometer, were fabricated by sol-gel spin coating technique on glass substrates. To form a better view of the influences of film thickness, we investigated surface morphology and crystalline structure parameters of ZnO thin films prepared by sol-gel method, in details. Scanning electron microscopy (SEM) and X-ray diffractometric (XRD) analyses revealed that ZnO thin films were of wurtzite structure shape. Grain size, dislocation density, interplanar distance, micro and lattice strains, bond length and bond angles were calculated by analyzing XRD data. The 100 nm and 200 nm thickness films show better crystallinity in comparison with 300 nm thickness film whereas 200 nm film has the least grain size. Keywords: ZnO thin film, solgel, spin coating, ZnO structural parameter, thickness variation 1. INTRODUCTION Oxide semiconductors, e.g. indium-doped tin oxide (ITO) and fluorine-doped tin oxide (FTO), are being used in various industrial applications. Because of production procedure complexity and cost, there has been substantial interest in finding a suitable and sustainable alternative material to replace these materials. ZnO is one of those promising materials. From the past decade, the global research interest in wide band gap semiconductors has been significantly focused to zinc oxide (ZnO) owing to its excellent properties as a semiconductor material. It has high electron mobility, high thermal conductivity, good transparency, wide and direct band gap (~3.37 eV), large exciton binding energy (~ 60meV), along with easy growth process in various nanostructured form by many different methods. Thus, ZnO thin film has potential application on window layer of solar cell, transparent thin film transistor, transparent conductive layer of a display, and other optoelectronics, piezoelectric, laser and sensing devices [1-3]. Different methods of ZnO thin film deposition result in different surface morphology, crystallinity, and other lattice parameters. Moreover, piezoelectric, mechanical, optical and electrical properties are dependent on surface morphology and lattice parameters. For example, electron mobility and carrier lifetime highly depend on defects, dislocation density, and grain size. Surface morphology has a significant effect on transmittance and diffuse reflectance. Similarly, interplanar distance, bond angle, strain, etc. have an effect on mechanical and piezoelectric properties [4-8]. Among various thin film deposition process, the sol-gel spin coating method of ZnO thin film deposition attained popularity owing to excellent control of the stoichiometry, requirement of lowcost instruments without a vacuum chamber, the possibility of using various precursor
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solution, etc. [2, 4, 5, 9]. Different thickness ZnO films can be attained varying precursor solution, rotation speed, and duration of a spin coater. Pre-heat treatment and post-heat treatment also play a crucial role in surface morphology and crystal structure. However, different thicknesses produce different surface morphology with different structural parameters value [4, 7, 14]. Therefore, there is a need for investigating surface morphology and other structural parameters in details to insight into thickness effect on optical, mechanical, piezoelectric, electrical properties as well as possible application related to these physical properties. In our work, we deposited ZnO thin film on a glass substrate of three different thicknesses by low-cost sol-gel spin coating technique. Morphology of the films has been investigated by Scanning Electron Microscopy (SEM) and various structural parameters e.g. interplanar distance, grain size, dislocation density, micro and lattice strain, bond length, bond angle, etc. have been evaluated by X-ray Diffractometer data analysis. The analysis indicates that the grown ZnO thin films were uniformly distributed with smooth grain boundaries. Comparison of these parameters of three samples of different thicknesses leads to in-depth understanding of various physical properties that depends on surface morphology and structural parameters. 2. Experimental details 2.1 ZnO thin film preparation:
Fig. 1: Flowchart of ZnO Thin Film preparation. Cleaning of glass: Some glass substrates of about 1 mm thickness were cut into pieces by 2 cm×2 cm. Afterward, they were cleaned using deionized water for five minutes each, dried by a hot-plate and stored in a dirt free place. Base solution preparation: At first, 1.31706 gm Zinc acetate dihydrate Zn(CH3COO)2·2H2O was dissolved in 20 ml of 2-methoxyethanol (2ME) (CH3OCH2CH2OH) ethanol to prepare 0.3M ZnO solution. The solution was stirred rigorously by a magnetic stirrer placed over a hot plate keeping the temperature at 75 °C for 30 minutes until the solution become cloudy. Afterward, stabilizer Mono-ethanolamine (NH2CH2CH2OH) has been added to make the solution homogeneous and transparent keeping molar ratio of Zinc acetate to Mono-ethanolamine in the solution 1:1. The solution was stirred for another 2 hours and then kept it at room temperature for 24 hours to have the precursor solution of spin coating to produce ZnO thin film. Spin coating: The next stage is to use a spin coater at 2500 rpm for 30 seconds with precursor ZnO solution on cleaned glass substrate to deposit ZnO on a glass substrate. After spin coating, the samples were preheated by placing on a hot plate at 200 °C for 10 minutes. The spin coating process and preheat treatment process were repeated several times to obtain three different thickness films namely 100 nm, 200 nm and 300 nm. Finally, the samples were placed inside a conventional maple furnace for post heat treatment at 500 degree Celsius for two hours. 3. RESULT AND DISCUSSION 3.1. Surface profilometer Dektak 150 surface profilometer was used to assess the surface and to determine the thicknesses of the films. Among the ZnO thin film samples, 100 nm, 200 nm, and 300 nm films were separated for further spectroscopic characterization 34
3.2. Scanning electron microscopy (SEM) Surface morphology of different thicknesses thin films, as shown in Figure-2, were investigated by JEOL JSM-7600F scanning electron microscope in vacuum chamber at pressure 9.6 105Pa10-6 mbar with electron gun voltage 5kV to have magnification x50,000. 100 nm sample provides uniform porous like surface with dense small spherical nanostructures (Fig.-2(a)). In 200 nm thin film (Fig.2(b)), nanostructures with uneven surface and a certain degree of inhomogeneity are observed. However, in case of 300 nm thin film, highly rough surface with irregular clustered nano structures are noticed.
(a)
(b)
(c)
Fig. 2: SEM images of fabricated ZnO thin film (a) 100 nm (b) 200 nm (c) 300 nm. 3.3. X-ray diffractometer (XRD) Comparison of XRD curves, as shown in Figure-3 (strong (002) peaks having sharp intensity and narrower spectral width as compared to the other peaks obtained in the XRD pattern and weak (004) peaks), and standard XRD card JCPDS 36-1451 confirms that deposited ZnO thin film is in the form of wurtzite hexagonal ZnO phase along the c-axis. Peaks become sharper along with film thickness, e.g. the 300 nm thin film shows the strongest peak whereas 100 nm thin film shows the least strong peaks, indicating the dependence of crystallinity of the film with thickness [10].
Fig. 3: The 2θ X-Ray Diffractometer spectra of ZnO thin films of different thicknesses.
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3.3.1. Determination of crystal structure parameters Average grain size (D) of the crystalline structure can be obtained from Scherrer’s relation [11] as: 𝐷 = 𝑘 ⁄ cos , where, λ is wavelength of the incident X-ray CuKα radiation, k is a constant (0.9), β is the full width at half maximum value and θ is the Bragg’s angle of respective plane (002) peak. Figure-4 and Table-1 show that grain size swings with film thickness. 100 nm thin film provides highest grain size whereas 200 nm has the least grain size among the samples, and 300 nm film finds its place between these samples. This changing trends also predicted by the SEM images. The distance between planes can be found by the following formula [12]: d= ⁄2 sin 𝜃 Where, d is the interplanar distance. Moreover, the lattice constants have been calculated by the following formula [13]: 𝑎 = 𝜆⁄√3𝑠𝑖𝑛𝜃 and 𝑐 = 𝜆/𝑠𝑖𝑛𝜃 Where, a and c are lattice constants. Table 1. Different structural parameters of ZNO thin films deposited on glass substrates Thickness (nm) 100 200 300
Positions 2θ (degree) 35.2 36.64 36.72
(a)
Interplanar distance (nm) 0.254753 0.245064129 0.244548561
hkl
FWHM
002 002 002
0.03295 0.08552 0.07154
(c)
(b)
(d)
Grain size(nm) 229.8141 88.1851 105.3934
(e)
(f)
Fig. 4: Change of crystal structure parameters with thickness. Strain towards c-axis [14] and towards a- axis [11] can be calculated from the formula as below as: 𝜀𝑐 = ((𝑐 − 𝑐∘ )⁄𝑐∘ ) × 100%
and 𝜀𝑎 = ((𝑎 − 𝑎∘ )⁄𝑎∘ ) × 100%
Where ℰ𝑐 and ℰ𝑎 are strain towards c axis and a axis respectively. The negative value indicates that the fabricated ZnO thin films have tensile strain towards the c-axis. Dislocation density is calculated by the following formula [15]: 𝛿 = 1⁄𝐷 2 lines/m2
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Bond length [17] of ZnO NR thin film is calculated by the relation as: √𝑎2 ⁄3 + (1⁄2 − 𝑢)2 𝑐 2 Where, a and c are lattice parameters, and u is defined as positional parameter of the wurtzite structure which is given [15] by 𝑢 = 𝑎2 ⁄3𝑐 2 + 0.25 Table 2: Morphological parameters of ZNO thin films with respect to structural and thickness variation Concentration
Thickness(nm)
a(nm)
c(nm)
c/a ratio
Dislocation density, (nm)-2
0.3M
100
0.3557242
0.509505097
1.432303723
1.89E-05
0.3M
200
0.32506714
0.490128258
1.507775473
0.000128591
0.3M
300
0.323479776
0.489097122
1.51198671
9.00271E-05
Mol/Lit
Nearest neighbor bond length towards c-direction (termed as b) and off c-axis (termed as b1) can by the calculated by the relations as [16]: 𝑏 = 𝑐𝑢 and 𝑏1 = √𝑎2 ⁄3 + (1⁄2 − 𝑢)2 𝑐 2 Table 3: Micro and Lattice strains of ZnO Thin Films Calculated from XRD Data Thickness (nm)
Micro strain towards a-axis
Micro strain towards caxis
Lattice strain
100
0.094704418
-0.021480926
0.000453225
200
0.00036048
-0.058694698
0.001126986
300
-0.004524462
-0.060675023
0.000940556
In addition to the nearest neighbors, there are three types of second-nearest neighbors
b designated as b1 (one along the direction), b2 (six of them) and 3 (three of them) with the bond lengths, 𝑏1′ = 𝑐(1 − 𝑢), 𝑏2′ = √𝑎2 + (𝑢𝑐)2 , and 𝑏3′ = √4𝑎2 ⁄3 + (1⁄2 − 𝑢)2 𝑐 2 The bond angles 𝛼 and 𝛽 are given by 𝛼=
and 2𝑠𝑖𝑛
−1
2
2
−1
𝜋 𝑐 1 + 𝑐𝑜𝑠 −1 [(√1 + 3 ( ) ( − 𝑢) ) ] 2 𝑎 2 2
2
−1
4 𝑐 1 [(√3 + 4 (𝑎) (2 − 𝑢) ) ]
Table 4: Neighbor Bond Length Parameters Thickness
Nearestneighbor bond length along cdirection, b (nm)
Nearestneighbor bond length off c-axis, b1 (nm)
Second Nearestneighbor bond length one along cdirection, bʹ1 (nm)
Second Nearestneighbor bond length six along c-direction, bʹ2 (nm)
Second Nearestneighbor bond length three along c-direction, bʹ3 (nm)
100
0.210162
0.254752548
0.299342796
0.413168125
0.412743216
200
0.194396685
0.245064129
0.295731572
0.378759443
0.377922389
300
0.193588791
0.244548561
0.295508331
0.376982474
0.37611917
(nm)
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Table 5: Positional Parameter, Both the Bond Angles and the Bond Length Thickness (nm)
Positional parameter, u
Bond angle, α (degree)
Bond angle, β (degree)
Bond length, L (nm)
100
0.412483215
102.2487852
110.4343793
0.2101623
200
0.396624113
105.1072114
106.0311328
0.194396685
300
0.395808485
105.2614365
105.7776988
0.193588791
4. CONCLUSION Single layer ZnO thin films were successfully deposited onto glass substrates by sol-gel spin coating method. SEM images show the structural properties whereas XRD data reveals that all the samples have hexagonal wurtzite structure. Moreover, using the XRD data, grain size and interplanar distance were found in the range of 105-229 nm and 0.24-0.25 nm respectively. The lattice parameter c/a ratio is in the range of 1.43-1.51 which indicates that the shape of ZnO wurtzite structures is reasonably perfect. Overall, ZnO thin film produced in this experiment showed good structural and morphological characteristics that can be used in various applications. REFERENCES [1] [2] [3] [4]
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