Fabrication of Nanostructured ZnO Thin Films By Home-made Spin Coater

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Fabrication of Nanostructured ZnO Thin Films By Home-made Spin Coater

Mohua Fardousi,¹ M.F. Hossain,^1,* Dilip Kumar Saha,²Mahidul Hasan¹

1Department of Electrical and Electronic Engineering, Rajshahi University of Engineering & Technology, Rajshahi-6204, Bangladesh

2Atomic Energy Centre, Dhaka.

[email protected], ^*[email protected]

Abstract— ZnO is a promising wide band gap semiconductor for various thin film deposition. In this paper, the nanostructured ZnO thin films have been deposited on the silica glass substrate by own-prepared homemade spin coating system. From scanning electron microscope image, it is cleared that the nanoflakes-like structure is grown uniformly. The hexagonal wurtzite crystallinity has confirmed by X-ray diffractometer. The structural, optical and surface morphological properties are correlated and discussed elaborately in this work.

Keywords- ZnO, Spin coating, Nanostructures, thin films.

I. INTRODUCTION

In most recent, ZnO has received considerable attention for the excellent emission properties. ZnO is a direct band-gap material which has a wide band-gap of 3.37 eV, and at room temperature it has a large exciton binding energy of 60 meV [1, 2]. Though some applications are still problematic, for its good qualities it is now widely used for the applications in the short wavelength region, UV light emitting diodes, emerging thin-film transistors, gas sensors, detectors, and of course in the solar cells. There are different methods have been used to grow high quality ZnO thin films, like spin coating, hydrothermal, chemical vapor deposition, molecular beam epitaxy, sol-gel, pulsed laser deposition, spray pyrolysis and sputtering [2]. Spin coating is a promising technique for the growth of high quality ZnO thin films. It is quite easy to control the chemical components, and fabrication of thin film at a low cost to investigate structure and the optical properties of ZnO thin films [1, 3]. In this research work we reported here the optical and electrical properties of ZnO thin films.

From the effective various deposition methods of thin films we choose the spin coating method for its fast and suitable and the better quality films than the others [1, 4]. It is the most reliable method to deposit thin film and the way of preparing uniform and homogeneous thin films is quite simple [3, 4].

In previous works, upper surface of the ZnO electrodes are crack, compact and maximum area of the surface remains smooth. To overcome the limitations, here the precursor solution was used with the concentration of 1.0 mol/L and 60 sec deposition time in 50 ml solvent. A very high purity has recently been shown to be crystalline to determine a crystal structure by x-ray diffraction, as surface modified [2]. The PL spectra, UV PL peak and visible emission band range, optical, and surface morphological [2] properties of the resulting ZnO photo electrode have been investigated and discussed. The

overall deposition process and the optical properties of ZnO thin film has been explained in this paper.

II. SPIN COATING

The method Spin coating is widely used in micro fabrication. This was first used extensively in the 1950s [4].

Though the most common application for thin films is used in the microelectronic industry, but the technique is also used in the manufacture of compact discs, flat panel displays, highly reflecting mirrors, nanofabrication and solar cells [5]. It is important to create good quality thin films in the microelectronics industry [1, 6]. Therefore many other techniques like spray pyrolysis, hydrothermal, sol-gel, laser assisted evaporation, molecular beam epitaxy, ion beam sputtering, thermal evaporation, vacuum deposition, chemical vapor deposition, sputtering, chemical bath deposition etc are being used but these process are energy intensive and involve with high temperature, vacuum and pressure [7]. Here we use spin coating because it is one of the most suitable, cost effective, fast and quite simple method of thin film deposition.

It is also a good process of preparing uniform and homogeneous thin films. The limitations of the films prepared by sol-gel can avoid by using this method to contain the better quality films. In previous work we discuss about the design and mechanism of a homemade spin coater, which is a low cost, simple but quite reliable to produce thin films. After some experimental results we assured the film quality and properties. Then we use it for producing a large number of thin films with different variations. The films were successfully made and after measuring the properties we got our expected values for each and every film for the large number of thin film application. Here in figure: 1 shows the simply designed homemade spin coater. To make a organic and uniform thin film in large area Spin Coating is a method is the perfect choice. The whole process can be stepped by four parte or steps like mixing the chemicals with actual wt% by stirring for 30 min or more, then deposition of thin film, spin up, and spin off. All these stages occur sequentially and while the evaporation stage occurs throughout the process it means that the process of thinning the film is near the end [1, 6].

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Figure 1: A Simple Spin Coater Design.

III. EXPERIMENTAL SECTION

In this work, porous ZnO films were prepared [8]. Here the films have been deposited on bare glass substrate by using spin coating method. In case of making the solution to produce films by spin coating, usually the ZnO precursor solution is used. Here in this case, to make the solution we dissolved Zinc Acetate Dehydrate (Zn (CH3COO)2·2H2O) maintained at 1.0 concentration in ethanol (C2H5OH), Diethylamine (CH3CH2NHCH2CH3) and distilled water at room temperature [4, 9]. Solution was mixed in glass beaker, cleaned nicely by using 99.9% absolute ethanol [8]. Though the films were deposited on a flat silica glass substrate, these were also cleaned up with 99.9% absolute ethanol by using the ultrasonic cleaner [3, 9]. It should mention that the glass beaker is perfectly cleaned and chemicals should stirred for 30 to 40 min by taking accurate amount of the needed chemicals.

Here the prepared deposition solution was kept at an elevated place in a glass beaker which should be covered to avoid any dust nano particles or evaporate by the flowing of natural air.

Now the flat substrate is accelerated up for 60 sec or less or more to reach its final desired rotation speed. After evaporation of the whole solvent the required solid thin film is generated. We have to store the solution in conical flask with air tight cork to maintain the actual properties of the solvent and other chemicals for making the good quality ZnO film [2, 9].

The machine used to prepare ZnO thin films is called the spin coater or spinner [10]. The whole spinning process is shown here in Figure: 2 and 3. Solution is stirred about 30 to 40 min till it become a clear and homogeneous. Films were prepared with four number of deposition layers. The double sided tape was used to place the clean substrates in the center of the spinning disk. Now the solution fluid was applied by a drop wise process in all over the substrate by the dropper. By turning on the switch of the spin coater and continue the spinning for a fixed time. It can be noticed that the wafer will spread all extra fluid by centrifugal force [1, 10]. After every 60 min we stopped the switch bar of the spinner, the coated film was dried at 300°C in air for 20 min. Then the above

process was repeated to make the multi layer films to obtain desired film thickness in nanometer [1, 7]. Finally the obtained films were annealed at 500°C for 1 hour, total act of applying a thin film to a substrate is known as the deposition technique [1, 10].

Figure 2: The Full Process Chart.

Figure 3: The Simple Spinning Process.

We have to avoid the sprinkling of the solution and isolate film deposition from undesired air dust it is enclosed with a glass made cover box [4, 10]. To make a uniform thin film the glass substrate should placed in the centre of a plane disk that supports the substrates is perfectly leveled with the horizontal [10,11]. The image of prepared thin films in our laboratory is shown here in Figure 4.

Figure 4: Image of A Prepared 4 layer ZnO Thin Film.

Substrate

switch Speed Control

chuck Glass cover

Spinning disk

Ethanol Mixed & Stirred for 40mins

Distilled Water

Diethylamine Zinc Acetate

Dihydrate

Put 2-3drops of coating solution

Spin coat the solution on glass substrate

Drying at 500°C for 20mins

Annealing at 500°C for 1hours

ZnO films

Repeat 4 times

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For the good film deposition the substrates is perfectly leveled horizontally and the flatness and/or roughness of the substrate are also the important parameter for film deposition [10]. The solution must be uniform and homogeneous, free of air bubbles and lumps and the substrate must be clean and free from any kind of dust particle [11]. During spinning, the viscosity does not become an impediment to the flow of the solution on the substrate. To make a full control on the rotational speed and rotational time, viscosity and concentration of the solution and the evaporation rate of the solvent must be mentioned to get the uniform thin films [4, 11].

Figure 5: SEM Image Showing 4 layers ZnO Thin Film, Post Heated At The Temperatures (300°C).

IV. R^{ESULT AND}D^ISCUSSION

The main objective of this research work is to observe the optical properties of ZnO thin film [12, 3]. By measuring the optical and electrical properties we got the expected results.

The surface of the substrate is the porous structure though the peaks can also be seen in the same positions for the compact.

From the PL spectra we got two exothermic picks are in 380nm and 560nm [4, 12]. The surface morphological property of ZnO thin film was investigate by Scanning Electron Microscope (SEM), shown in figure 5. The irregular pores of different nanometer scales were investigated over the whole ZnO films surface. Figure: 6 and 7 shows the difference of transmittance (%) and absorbance (a. u) of ZnO film shows the higher ΔA value [3]. Graf of with respect to the wave length(nm) of both type of samples can notice the difference of the nanostructured ZnO film where the both porous and compact ZnO thin film and for the both type of films optical transmittance keeps above 80% beyond the wavelength of 560 nm [12]. Here the optical band gap of direct measurement is 3.92 eV.

Figure: 8 shows the XRD spectra which confirm the good crystallinity of the films. It also confirms the hexagonal Wurtzite structure of ZnO films with (101), (002) and (100) peaks. Among this curve, (101) peak of the 4layer sample is strong than others. The crystal size has been estimated from the Debye-Scherer's equation: D= 0.94λ/β cos θ. By using peak (101), the calculated crystallite sizes for 4layer sample is, 3.84 nm. In the chemical reaction, due to presence the

hydrogen bonds, there will formed another bond in between ZnO nanoparticals [3,11]. Figure: 9 shows the Photoluminescence intensity versus wavelength curve. This broadened peak exists to confirm the formation of ZnO crystal structure, as similar as XRD analysis. This 4 layer sample has wide peak. The curve within 400-760 nm shows visible emission curve. This sample shows strong emission which indicates large oxygen deficiency.

300 400 500 600 700 800

0 20 40 60 80 100

Transmittance (%)

Wavelength(nm)

Figure 6: Optical Transmittance of ZnO Thin Film, Post Heated At The Temperatures (300°C).

300 400 500 600 700 800

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Absorbance (a.u.)

Wavelength (nm)

Figure 7: The UV-Visible Spectra Photocatalytic Degradation Under UV Irradiation On ZnO Film of 4 Layers.

25 30 35 40 45 50 55 60 65 70 75

Intensity (a.u.) (102)

(110) (103)

(101) (112)

(100)(002)

Deffraction Angle, 2 (deg)

Figure 8: The XRD Spectra of ZnO Film.

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Figure 9: PL Spectra of 4 Layer ZnO Thin Film.

From the electrical properties it is easily shown that the ZnO films are more photoactive than the polymer using ZnO films [12]. For the film thickness, which is depends on evaporation rate of the solvent can easily measured by, film thickness varies with spin speed and time: H~w^-1t^-1/2 , when there is no evaporation, H~w^-2/3 , when the evaporation rate will constant and H~w^-1/2 , when the evaporation rate varies with the square root of the spin speed. Here, H is the film thickness and N is dependent on solvent evaporation [6, 12].

V. CONCLUSION

The nanostructures of the porous ZnO thin films were prepared on bare glass substrates by spin coating method using the precursor solution. From this research work we noticed the absorbance and transmittance graph which showing impressive results and the transmittance spectrum gave a high transmittance value. We also noticed the continuous increase of the porosity and roughness but the band gap decreases with the decreasing number of layers. The ZnO nanostructures are widely used in DSC application for its Spin coating system is widely used in micro fabrication [4, 6]. In future we can repeat this process on conductive substrate for the commercial use in producing low cost DSSC in Bangladesh.

VI. ACKNOWLEDGEMENT

Authors thank to the Department of Electrical and Electronic Engineering, Rajshahi University of Engineering &

Technology (RUET) for giving the chance to use the different chemicals, equipment and other necessary things of our photovoltaic and nanotechnology laboratory.

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PL Intensity(a.u.)

Wavelength(nm) 4 layers

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