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Comparative Study of the Structural Properties for Thin and Thick ZnO Films Deposited on PPC Plastic Substrates

July 2021
Journal of Physics Conference Series 1963(1):012074

DOI:10.1088/1742-6596/1963/1/012074

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Authors:

N.N. Jandow

Al-Mustansiriya University

Abbas Ali Abbas

Al-Mustansiriya University

Ghazwan Jabbar Mohammed

Al-Mustansiriya University

Nadir Fadhil Habubi

Alnukhba University College

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ZnO films with various thicknesses (0.4, 0.6, 0.8, 1 and 1.3 μm) had been prepared on PPC plastic bases by using DC sputtering. XRD results showed that all the films displayed principally ZnO (002) peak at 2θ = 34.115°, 34.01, 34.16, 34.07 and 34.12° with FWHM of 0.41°, 0.34, 0.27, 0.21 and 0.368° respectively, which is coincide with wurtzite hexagonal phase, indicated that films were preferentially grown along c-axis. XRD results also showed that the lattice constant and the crystallite size for the deposited thin films became larger than those for the thick film 1.3 μm; while the stress and microstrain increased for the thick films.

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Journal of Physics: Conference Series

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Comparative Study of the Structural Properties for Thin and Thick ZnO

Films Deposited on PPC Plastic Substrates

To cite this article: N. N. Jandow et al 2021 J. Phys.: Conf. Ser. 1963 012074

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2nd International Conference on Physics and Applied Sciences (ICPAS 2021)

Journal of Physics: Conference Series 1963 (2021) 012074

IOP Publishing

doi:10.1088/1742-6596/1963/1/012074

Comparative Study of the Structural Properties for Thin and

Thick ZnO Films Deposited on PPC Plastic Substrates

N. N. Jandow1*, A.A. Abbas1, Kh. G. Mohammed1, N. F. Habubi1, F.K. Yam2

1. Department of Physics/ College of Education/ Mustansiriyah University/Baghdad/Iraq.

2. Nano-Optoelectronics Research and Technology Laboratory School of Physics

Universiti Sains Malaysia, Minden, Penang/Malaysia

email: nidhaljandow@uomustansiriyah.edu.iq

Abstract ZnO films with various thicknesses (0.4, 0.6, 0.8, 1 and 1.3 Pm) had been prepared on

PPC plastic bases by using DC sputtering. XRD results showed that all the films displayed

principally ZnO (002) peak at 2T = 34.115o, 34.01, 34.16, 34.07 and 34.12o with FWHM of 0.41o,

0.34, 0.27, 0.21 and 0.368o respectively, which is coincide with wurtzite hexagonal phase,

indicated that films were preferentially grown along c-axis. XRD results also showed that the

lattice constant and the crystallite size for the deposited thin films became larger than those for the

thick film 1.3 Pm; while the stress and microstrain increased for the thick films.

Keywords:Thin Films; PPC Plastic; Physical properties; Structural Properties; Thick Films

1. Introduction

ZnO is belong to the 2nd and 6th groups of periodic table. ZnO has a good sensitivity in the UV

area and high photoconductivity [1]. ZnO has many applications like conducting electrodes, SAV filters,

gas sensors, LED, and laser diodes [2-5].

ZnO thin films was deposited using various methods like DC sputtering technique, RFMS, PLD,

MOCVD, CSP and Sol-Gel, metalorganic vapor phase epitaxy on various inorganic bases such as quartz,

silicon, glass, diamond, NaCl, and InP [6-16]. But, plastic own lightweight and small size in comparison

with inorganic bases [17], and employed in various applications like resilient sensors and curved detector

modes. Plastic bases tool up lighter, more resistant to injury, flexible, and strong devices [18]; these assign

make them appropriate as remote control, and circuits camcorders [19]. The base materials, were

Polycarbonate, Polyarylate , Polyestersulfone , Polyimide , Polyethylene terephthalate, Teflon,

poly(ethylene naphthalate), thermoplastic Perpex, Plexiglas, Polyethylene naphthalate, and cellulose

triacetate. [20].

Poly Propylene Carbonate (PPC), that employed as a base for deposition ZnO thin films, this may turn

on a novel mehtod for the manufacture of cheap optical devices [20].Residual stresses play a

considerable part in mechanical achievement and reliability of thin films. The presence of large residual

stresses results in film buckling or cracking, and even interface delamination like a exemplary telephone-

2nd International Conference on Physics and Applied Sciences (ICPAS 2021)

Journal of Physics: Conference Series 1963 (2021) 012074

IOP Publishing

doi:10.1088/1742-6596/1963/1/012074

cord peeling [21, 22]. The film is in a stressed state due to a difference in thermal expansion coefficients

between film and base [23].

In this work, thin films with various thicknesses (0.4, 0.6, 0.8 and 1Pm) and thick ZnO thin film with (1.3

Pm) thickness were grown on PPC plastic substrates using DC sputtering technique. A comparison study

of structural properties of ZnO films dependent on thickness had been carried out. The authors believe that

this comparison of the structural properties between thick and thin films is made for the first time.

2. Experimental work

ZnO films were grown on PPC plastic bases after cleaning by DC sputtering unit . The sputtering

unit operated with a base pressure of about 5x10-5 Torr; target material have a diameter of about 76.2 mm.

Distance between the target and bases was 7 cm.

PPC was subjected to clean procedure before deposition. The target was pre-sputtered for 20 min to take

off any pollution, from target surface. Thickness was evaluated employing surface optical system

Filmetric F20-VIS. Structural properties of films were studied by XRD. Fig.1 shows two deposited films,

at the left with dark brown colored is 1.3Pm thickness and the other is the film with 0.8Pm thickness.

Fig. 1: The deposited films with 1.3Pm and 0.8Pm thicknesses.

3. Results and discussions

Figure 2 offers XRD styles of the deposited ZnO films. All the films offer principally ZnO (002)

peak at 2T = 34.115o, 34.01o, 34.16o, 34.07o and 34.12o with FWHM of 0.41o, 0.34o, 0.27o, 0.21o and

0.368o respectively, that agree with wurtzite hexagonal phase of ZnO. The figure also displays other less

intense peaks assigned to the planes (102) and (110) respectively that emphasize the forming of

polycrystalline ZnO. This result agrees well with Lee et al. [24].

The crystallite size is obtained via Scherrer’s formula [25]:

2nd International Conference on Physics and Applied Sciences (ICPAS 2021)

Journal of Physics: Conference Series 1963 (2021) 012074

IOP Publishing

doi:10.1088/1742-6596/1963/1/012074

where B is the FWHM , λ is the X-ray wavelength, T is the Bragg diffraction angle [25]. The

results indicate that D increases with increasing film thickness from 0.4 -1 mm, while it decreases for the

thick film with 1.3 nm thickness as shown in fig 3. This means that the thin films have better crystal

quality and larger D than the thick ZnO film. Table 1 summarizes XRD data.

2nd International Conference on Physics and Applied Sciences (ICPAS 2021)

Journal of Physics: Conference Series 1963 (2021) 012074

IOP Publishing

doi:10.1088/1742-6596/1963/1/012074

Fig. 2 XRD patterns of the grown films

Fig. 3: FWHM and D of the intended films.

The stress (σ) was calculated by using Equation [26].

where c film is the lattice parameter of c axis, and cbulk is the lattice parameter of bulk. The

numerical values of cfilm is calculated by equation 3 [26]:

= 2dhkl sin

(3)

where dhkl is lattice spacing is obtained from the equation [26]:

2nd International Conference on Physics and Applied Sciences (ICPAS 2021)

Journal of Physics: Conference Series 1963 (2021) 012074

IOP Publishing

doi:10.1088/1742-6596/1963/1/012074

where a and c are lattice constants. The calculated cfilm of ZnO film is 0.5204 nm it is close to the

value from (JCDD) card data for ZnO cbulk 5.2066 Å.

Stress values is found with negative sign indicates that films are compressive stress.

The total stress consists of two components: one, is the intrinsic stress and the second is the extrinsic stress

[27]. In this paper, thickness varies from 0.4 to 1 Pm, thus, the total stress was prevailingly intrinsic.

Since substrate used in this research is plastic and this material bears a certain temperature, for this, the

high temperatures resulting from the long period of film evaporation for 1.3 Pm, led to the curvature of the

substrate, which caused high stress in the deposited film. For this reason, we believe that this film showed

less structural properties quality compared to the other films, also, we can expect that the stress in our

prepared thin films is intrinsic, while in the thick film it can be considered as an extrinsic because of the

long deposition time and the plastic substrate as offer in Fig. (1).

Figure 4 offers the difference of stress with film thickness and Table 1 lists the variance of lattice

parameters, D, σ and ezz with the film thickness.

Fig. 4 : the variation of stress with film thickness.

The strain (ezz) originates principally due to lattice mismatch between film and base can be

calculated from the lattice constant c value in the z-direction using the following equation [27]. Fig. 5

shows the strain against film thickness.

All the films exhibit tensile strain and from Table 1, the compressive stress decreases with

increasing film thickness from 0.4 to 1 Pm, while the sample prepared with the thick thickness (1.3 Pm)

shows an increase in stress, this may be due to the low structural properties as shown from the XRD result.

2nd International Conference on Physics and Applied Sciences (ICPAS 2021)

Journal of Physics: Conference Series 1963 (2021) 012074

IOP Publishing

doi:10.1088/1742-6596/1963/1/012074

Fig. 5: Strain as a function of film thickness.

The dislocation density was obtained from the equation 6 [28]:

Figure 6 displays the dislocation density against films thickness. As mentioned before, the

crystallite size increased gradually from 20.27 to 38.48 nm with the increase of thicknesses from 0.4 to 1

Pm for the deposited thin films, while it decreased for the deposited thick film to be 12.15 nm. The larger

D and smaller FMHM values mark to better crystallization. δ decreases with increasing thicknesses for

thin films, which may be due to a decrease in the content of lattice imperfections. [28].

Fig. 6: The dislocation density as a function of film thicknesses

2nd International Conference on Physics and Applied Sciences (ICPAS 2021)

Journal of Physics: Conference Series 1963 (2021) 012074

IOP Publishing

doi:10.1088/1742-6596/1963/1/012074

Table 1: XRD summary of the intended films

From all the obtained results and from the data in Table 1, for the thin films, that strain (ezz) in the

film is tensile, indicating the existence of more relaxed films for higher thickness as shown in Fig 5. As

thickness increases, the amount of ZnO reaching base surface increases to form film and therefore

electrostatic interaction between ZnO particles becomes larger thereby increasing the probability of more

ZnO particles to be gathered together forming a grain. But after one micron, as it’s known that the film

change to a thick film, we notice that the crystallite size decreases, this might be due to heat generated by

the sputtering process in the film of (1.3) micron which affected the order of crystallinity and increases the

compression stress. Fig. 7 shows the comparison between the stress and strain in both the thin and thick

deposited films as a function of thickness. These results are in agreement with Rao et al [27].

Fig. 7: The stress and the strain of the intended films.

Thickness

(μ m)

2θ

(

)

Relative

peak

intensity

FWHM

)

Crystallite

size (nm)

(nm)

film

(nm)

(%)

stress

(σ)

(GPa)

(nm

0.4

34.115

1153

0.41

20.270

0.2627

0.5254

0.0979

1.2677

0.0024

0.6

34.015

1346

0.34

24.448

0.2622

0.5245

0.0812

0.8688

0.00167

0.8

34.165

1624

0.27

30.784

0.2621

0.5243

0.0645

0.7892

0.00105

34.075

2425

0.21

38.488

0.2620

0.5240

0.0516

0.6302

0.00067

1.3

34.125

484

0.68

12.151

0.2629

0.5258

0.1634

1.4276

0.00677

2nd International Conference on Physics and Applied Sciences (ICPAS 2021)

Journal of Physics: Conference Series 1963 (2021) 012074

IOP Publishing

doi:10.1088/1742-6596/1963/1/012074

4. Conclusion

This work was a trial to calculate and compared the stress and strain for thin and thick films,

which was as the authors believe that it is achieved for the first time. It was noticed that the stress and

strain decreased as the thickness of thin film increase, but for a thick film, the situation was reversed. It

can be concluded that ZnO thin films deposited on PPC substrates had better crystalline structure than the

thick one; these results maybe give a good improvement to use a thin film for many applications better

than the thick one.

Acknowledgments

The authors are thankful to Department of Physics/ College of Education/ Mustansiriyah University for

their support and School of Physics /Universiti Sains Malaysia for XRD measurements.

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