Structural and optical properties of RF sputtered ZnO thin films: Annealing effect

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Abstract

In this study, the effects of annealing temperature on morphological, optical and structural properties of Radio Frequency (RF) magnetron sputtered Zinc oxide (ZnO) thin films were investigated. X-ray diffraction spectra confirmed the hexagonal wurtzite structure with preferential orientation along the c-axis. The annealing temperature had an important role on the crystallite size and it was calculated as 19.7 nm, 17.2 nm, 15.1 nm and 24.5 nm, depending on the increasing annealing temperature. The optical transmittance of ZnO thin films ranged from 85% to 95% at 550 nm wavelength, and this change occurred linearly with annealing temperature. The optical band gap (Eg) of pure ZnO was 3.25 eV; this first decreased to 3.23 eV at low annealing temperature and then increased linearly to 3.26 eV with increasing annealing temperature.

Introduction

In recent years, transparent conductive oxides (TCOs) such as Zinc Oxide (ZnO) have been playing an important role in nano-device technology due to their excellent optical transparency and electrical properties. ZnO based thin films are widely used in solar cells [[1], [2], [3], [4], [5]], organic light emitting diodes, gas sensors [[6], [7], [8], [9]] and photovoltaic devices [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19]]. ZnO stands out as a good alternative to indium tin oxide (ITO) thin films, a commercial product and widely used as photo anode material in solar cells [20]. Although ITO is an important material for these applications due to its low electrical resistivity and high optical transmittance in the visible region, researchers have recently turned to alternative materials due to the high cost and toxicity of its raw material [21,22]. The surface resistivity of ITO, which is widely used as electrode material in solar cell applications, increases due to the operating temperature of the solar cell, and therefore its use for these applications is negatively affected. Thus, recently, the use of zinc oxide and its composite derivatives has attracted more attention by researchers [23]. It is well known that the low cost and high transparency of zinc oxide thin films can be improved much more by various metal doping [24]. Some researchers have reported that ZnO has improved structural and optical properties by doping additional atoms such as Al [25] and Ni [26] in their studies to further improve device performance. However, superior optical properties are not sufficient for it to be used as an alternative electrode material, but it is also desirable for the solar cell to have a stable resistivity value depending on the operating temperature. It is reported that the resistivity of nanostructured zinc oxide thin films can vary from 104 to 10−3 Ω. m by adjusting the annealing conditions [27,28]. A number of studies have been reported on the efficiently production of important metal oxides such as manganese oxide [[29], [30], [31], [32], [33]], aluminum oxide [34,35] and ZnO [26,36]. Zinc oxide thin films are prepared using various techniques such as spray pyrolysis [[37], [38], [39]], molecular beam epitaxy (MBE) [40], pulsed laser deposition, RF magnetron sputtering [41], sol-gel [[42], [43], [44], [45]] and chemical vapor deposition (CVD) [46]. The RF magnetron sputtering technique is more ideal than other techniques for preparing both high quality and thin film of desired thickness. Thin films are likely to be produced by controlling the deposition parameters such as substrate temperature, RF power, and argon gas pressure. There are many studies [[47], [48], [49], [50], [51], [52], [53]] in which pure ZnO and various metal-added ZnOs are produced from the techniques mentioned above, especially using the RF sputtering technique. It is understood that the most important findings of these studies are that the crystal quality is quite high compared to pure or doped ZnO produced using other techniques, and their optical properties are mostly suitable for applications such as solar cells. ZnO, which is preferred as photo anode material in solar cells, especially in paint sensitive solar cell applications, is known to greatly transmit sunlight thanks to its high optical transmittance, thereby facilitating the stimulation of dye molecules. In a study conducted by Shin et al. [53], Yiitrium-added ZnO thin films were produced and how the optical, structural and morphological properties of these films changed depending on the annealing temperature of 400–550 °C was examined in detail. As a result of the annealing process, it was reported that the crystal quality of ZnOs increased, the average particle size varied between 24 and 26 nm, the optical transmittance for 550 nm wavelength increased to about 86% with the effect of increasing annealing temperature. On the other hand, there are also studies where ZnO thin films are produced by using the low-cost thin film production technique such as sol-gel. Zhang et al. [47], produced doped ZnO thin films using the sol-gel production technique and then exposed them to annealing at various temperatures. The average particle size of the produced ZnOs ranged from 23 nm to 33 nm depending on the annealing temperature. ZnOs thin films were determined to have 80% optical transmittance at a wavelength of about 550 nm and an optical band gap between 3.3 eV and 3.55 eV. Muchuweni et al. [51], investigated the structural, optical and electrical behavior of ZnO thin films produced using hydrothermal technique. Although thin films produced have a very excellent crystal quality, it is understood that they have low optical transmittance (approx. 70% optical transmittance at 550 nm wavelength) compared to those in the literature studies.

From the most expensive thin film production technique to the cheapest production technique mentioned above, there are many studies in the literature where ZnOs are produced using techniques other than these [[54], [55], [56]]. However, considering the crystal quality and optical properties, it is understood that ZnO thin films have similar properties to these studies. In this study, we aimed to produce ZnOs with excellent optical transmittance and high crystal quality, especially in terms of optical properties. For this purpose, we preferred to use RF magnetron sputtering technique in order to produce quality film as understood from the literature and we aimed to achieve the desired high optical behaviors for important applications such as solar cell and sensor by optimizing the coating parameters. The thickness of thin films produced in this study is similar for all samples. All thin films were produced under the same coating conditions and then the effect of annealing was investigated. As it is expected that thinner film production as much as possible is expected to improve the optical properties relatively, this was taken into account when determining the coating parameters. Since the annealing process is known to be effective for the stability of the surface resistivity of thin films, these nanostructures, which were later grown, were subjected to annealing.

In this study, the authors tried to investigate how annealing temperature affects the structural and optical properties of thin films produced by RF magnetron sputtering technique. The effect of annealing temperature on the structural and optical properties of thin films was investigated in detail using XRD, SEM, Raman and Uv–Visible spectroscopy.

Section snippets

Materials and methods

The ZnO target material, which was 99.99% pure, was obtained by purchase from sigma Aldrich. ZnO thin films were produced with the help of RF magnetron sputtering technique on glass substrates. Surface cleaning of the glass substrates prior to coating was carried out using various alcohols such as acetone and propanol. Immediately after this cleaning process, these substrates were washed in an ultrasonic bath using deionized water. Argon gas was used in the vacuum chamber to scatter the target

XRD analyses

The determination of the structural properties of ZnO thin films subjected to various annealing temperatures was investigated by XRD and the results are given in Fig. 1. The plane at about 31° (100) and the plane of reflection at 34° (002) were identified as characteristic peaks of all thin films. ZnO annealed at 500 °C was determined to have only (002) peak at 34.7°. However, for pure ZnO and ZnOs annealed at 400, 450 and 550 °C, another peak was detected at about 31.7° attributed to the (100)

Conclusion

ZnOs were successfully grown on glass substrates using RF magnetron sputtering technique and annealed at various temperatures. All analyzes showed that annealing had a significant effect on thin film quality. The crystallite size changed significantly with the annealing temperature, and the crystallite size, which was 14.16 nm for pure ZnO, was 24.51 nm for the ZNO4 thin film after annealing. Although the morphology of pure ZnO, composed of coarse-grained nanoparticles, was not initially

CRediT authorship contribution statement

Emre Sener: Investigation, Funding acquisition. Ozkan Bayram: Methodology, Investigation, Writing - review & editing, Funding acquisition, Writing - original draft, Data curation. Ugur Cem Hasar: Review & editing, Supervision. Onder Simsek: Writing - review & editing, Supervision, Funding acquisition.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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