UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

634 IMMC 2018 | 19th International Metallurgy & Materials Congress

Characterization of Dual Cation Doped ZnO Films (DAZO) Sytnhesis by Sol-Gel Method

Semra Kurama, Atilla Yazal

¹Anadolu University, Faculty of Engineering, Department of Materials Science and Engineering, Iki Eylul Campus, 26555, Eskisehir/TURKEY²Anadolu University, Faculty of Aeronautics and Astronautics, Iki Eylul Campus, 26555,Eskisehir/TURKEY

Abstract

Use of thin film technology has opened the field to the fabrication of high quality material for applications such as UV light emitters, gas sensing elements, detectors and catalysts. By reduction in particle size, novel chemical, electrical, mechanical and optical properties could be improved.

In this study, Zn films, doped with dual and/or single cation (Dy and/or Al) have been obtained by sol-gel process. Prepared films were deposited from precursor solutions by spin-coating on quartz substrates. Using heat treatment at different temperature films were transformed into dual (Dy and Al) or single (Dy or Al) cation doped films. Characterization of microstructure and phase analyze of film was investigated by scanning electron microscope (SEM) and X-ray diffraction technique (XRD), respectively. The optical properties of AZO, D3AZO and DA3ZO samples showed very good performance with a transmittance value above 90% with Al+3, Dy+3 Zn+2 exchange. In thin films, as the amount of Al increases and accordingly the amount of Dy decreases, the result is that the electrical conductivity increases.

1. Introduction

One of the most important issues in material science today is the application and basic properties of semi-conductor transparent thin films. The characteristic feature of such coatings is their low electrical resistance and high permeability in the visible region. For semiconductor transparent coatings, many materials such as indium oxide, tin oxide, zinc oxide and cadmium stannate (cadmium stannate) are used. The most important basic features of these films for practical applications are; structure, morphology, electrical resistance and optical permeability. The coating technique plays an important role as the electrical and optical properties strongly depend on the microstructure, stoichiometry and pollution of the structure. As devices based on semiconductor transparent coatings become more complex, there is a need to recognize and improve the quality of these films.

Transparent conducting oxide layers have attracted much more attention due to their broad range of application such as transparent electrodes in solar cells and in photovoltaic devices [1, 2]. The demand of low cost and high performance optoelectronic devices lead to the development of more

mainly for applications such as solar cells [3], liquid crystal displays [4], heat mirrors [5], photothermal conversion system [6], gas sensors [7], optical position sensors [8] and acoustic wave transducers [9], etc. ZnO is an alternative material to tin oxide and indium tin oxide (ITO), which has so far been widely used [10]. In addition, zinc oxide is not poisonous, it is very common and cheap because this area has increased its use. ZnO has a very high transmittance in the range of 0.4-(Eg 3.3 eV) n-type semiconductors [11].From all the TCO materials studied, in last years, zinc oxide (ZnO) has emerged as one of the most promising materials due to its optical and electrical properties, high chemical and mechanical stability together with its abundance in nature, which makes it a lower cost material when compared with the most currently used TCO materials (ITO, SnO2). On the otherside, at high temperatures, doping the zinc oxide can reduce this disadvantage. The ZnO doping is achieved by replacing Zn2+

atoms with atoms of elements of higher valence such as indium [12], aluminium [13] and gallium [14]. The

the ionic radius of zinc. Dysprosium (Dy), a rare earth element, is a candidate element to be used as an additive element in terms of electrical resistance and optical properties. However, despite the preparation by the PLD method in the literature [15, 16], no single (DZO) and dual-cation (DAZO) oxide films doped with dysprosium element have been found using the sol-gel method

This study is very important in that it is the first time in the literature that the untested single and double cation ZnO-Al-Dy systems are prepared by sol-gel method and coated with glass surfaces by spin coating method. Thus, the effects of left and additive concentrations on the optical, structural and electrical properties of films were investigated by adding Al,

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Dy and their binary compositions at different ratios to the solids prepared in different compositions.

2. Experimental Procedure

Thin films were prepared by sol-gel method. As a starting solution, solvent zinc acetate dehydrates and stabilizer (99.5% Merck), 2-Ethoxyethanol (C4H10O2, 99% Merck) and monoethanolamide (MEA) (C2H7NO, %98+, Alfa Aesar) were used, respectively. The molarity of solution is 0.5M and the ratio of solution to stabilizer was maintained at 1. Aluminum chloride hexahydrate (AlCl3.6H2O, %99, Alfa Aesar) and Dysprosium (III) acetate tetrahydrate (Dy(OOCCH3)3.4H2O, %99.99, Alfa Aesar) were used as additive in system. The amount of additive was arranged about 2 % in system. Therefore ratio of Dy:Al were changed as 0:1, 1:3, 1:1, 3:1 and 1:0. The prepared solution was stirred at 60 C for 1h to obtain a clear and homogenous solution. This solution was cooled to room temperature and coating was made after 24 h.

Before coating process, glass substrates were cleaned by using special method which is given in Figure 1.

Figure 1. Preparation process of glass substrate

Prepared solution was dropped onto glass substrate which were rotated at 750-1500 rpm for 30s. After that the thin films were dried at 300 C for 10 min. for removing organic compound from gel. Coating was repeated 10 times for each composition and samples to obtain enough thickness of film. Obtained samples were annealed at 500 C for 2 h in furnace. The process of preparation thin film is given in Figure 2.Samples are called according to amount of dopant and it is given in Table 1.

Table 1. Sample codes and atomic composition.

Code Al (at. %)

Dy(at. %)

AZO 2 0DA3ZO 1.5 0.5 D3AZO 0.5 1.5

Figure 2. The process of preparation thin film

3. Results and Discussion

3.1. Phase analysis of ZnO thin film with different dopants

XRD patterns of films coated with different rate presented in Figures 3-5. Characteristic peaks of ZnO thin film were obtained at all rotational speeds and in all samples. This indicates that zinc oxide crystallizes on the surface of the bed, and these crystals are in hexagonal structure. The peaks (002) in the graphs correspond to the reflection plane and are consistent with the results in the literature [17-19]. The optimum rotational speed can be determined by examining the x-ray diffraction patterns of the films. It is understood that the obtained peaks show that generally there is better crystallization in samples prepared at a rotation speed of 1000 rpm. For this reason, when coating at a rotation rate of 750 rpm, the excess amount of the liquid evaporated rapidly in the remaining amount on the surface of the substrate, increasing the amount of void in the structure. On the other hand, when coating at a rotation speed of 1500 rpm, the gel structure is further dispersed by centrifugal force and this resulted with fine grained film formation. As it is seen from figures, the intensity of peaks for all samples are very close to each other.

UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

636 IMMC 2018 | 19th International Metallurgy & Materials Congress

Figure 3. XRD pattern of AZO film for different coating rate

Figure 4. XRD pattern of DA3ZO film for different coating rate

Figure 5. XRD pattern of D3AZO film for different coating rate

3.2. Microstructural characterization

Microstructural evolution of samples was investigated by scanning electron microscope (SEM). Results are given in Figure 6-9. When it is looked to SEM photos, surface of all samples are very homogenous and there is some empty area due to collection of particles. This results showed that, motion ability of electrons in system is limited. Therefore, this significantly affect electrical properties of films.

3.3. Optical properties of films

Optical properties of films were investigated in visual range (400-800 nm). To measure of optical transmittance of

films silicon diode and gallium phosphate light detector were used. Results are given in Figures 10-12.

Figure 6. Microstructure of AZO film coated with 750 rpm.

Figure 7. Microstructure of AZO film coated with 1000 rpm.

Figure 8. Microstructure of AZO film coated with 1500 rpm.

Figure 9. Microstructure of D3AZO film coated with 1000 rpm.

Results showed that all samples have above 90% transmittance. By change Al:Dy ratio results was not much effected. Thus, these results are in good agreement with SEM analyses.

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Figure 10. Change of optical transmittance behavior of AZOthin films with coating rate.

Figure 11. Change of optical transmittance behavior of DA3ZO thin films with coating rate.

Figure 12. Change of optical transmittance behavior of D3AZO thin films with coating rate.

3.4. Electrical conductivity of films

Electrical properties of thin films were measured with four probe method. In this method, voltage was measured by sending electrical current to surface of thin film. After that, resistance and values are used to calculation of resistivity of films.

Table 2. Electrical resistivity of thin films

Code Rotating(rpm)

Resistivity ( cm)

AZO1500 2.6 1000 2.2 750 3.6

DA3ZO 1500 35.3 1000 16.9 750 53.7

D3AZO 1500 609.2 1000 295.0 750 281.1

The best results for AZO specimens prepared at rotational speeds of 750, 100 and 1500 rpm were found to be 2.2 cm. Some of research from literature mentioned that resistivity of AZO film is 10-2-10-4 cm [20-21]. But there are also some results with high resistivity values in literature [19]. The reason of low electrical conductivity is explained with dissolution of Al2O3 in grain boundary and this resulted with lower mobility.

4. Conclusion

In this study, it was aimed that production of alternative system for ITO thin film. Therefore, Al3+ and Dy3+ atoms were added to ZnO system by substitutional.

It was understood that the films were crystallized in the plane of Al, Dy, ZnO thin film (002), which is formed by x-ray diffraction patterns, the structure is hexagonal and the optimum rotation speed is 1000 rpm. SEM images were examined and results showed that the film surfaces were highly porous. It was thought that the low electrical conductivity was related to the film in pore quantity.

Optical properties of thin films were obtained above 90% for all compositions. By increase of Al atoms in composition electrical properties of thin films were positively affected. In addition, the electrical conductivity can be improved by improving the surface properties of the thin films.

UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

638 IMMC 2018 | 19th International Metallurgy & Materials Congress

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