Structural and optical properties of RF sputtered ZnO thin films: Annealing effect
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:
References (76)
Effect of annealing temperature on the structural and optical properties of ZnO thin films prepared by RF magnetron sputtering
Phys. B Condens. Matter
(2010)Cu2O growth characteristics on an array of ZnO nanorods for the nano-structured solar cells
Surf. Coating. Technol.
(2013)Improving the efficiency of organic light emitting devices by using co-host electron transport layer
Thin Solid Films
(2006)Surface plasmon resonance optical gas sensing of nanostructured ZnO films
Sensor. Actuator. B Chem.
(2008)ZnO thin film on side polished optical fiber for gas sensing applications
Appl. Surf. Sci.
(2007)Highly transparent and conducting fluorine-doped ZnO thin films prepared by pulsed laser deposition
Sol. Energy Mater. Sol. Cell.
(2011)Investigations of the structural, morphological and electrical properties of multilayer ZnO/TiO2 thin films, deposited by sol–gel technique
Results in physics
(2016)- et al.
Hydrothermal synthesis of ZnO nanowires on rf sputtered Ga and Al co-doped ZnO thin films for solar cell application
J. Alloys Compd.
(2017) - et al.
Low temperature synthesis of ZnO nanowires on GAZO thin films annealed at different temperatures for solar cell application
Mater. Sci. Semicond. Process.
(2017) Surface characterization and microstructure of ITO thin films at different annealing temperatures
Appl. Surf. Sci.
(2007)
Microstructures and optical properties of Cu-doped ZnO films prepared by radio frequency reactive magnetron sputtering
Appl. Surf. Sci.
The effect of heat treatment on the physical properties of sol–gel derived ZnO thin films
Appl. Surf. Sci.
Effects of ZnAl2O4 segregation in high temperature sintered Al-doped ZnO sputtering target on optical and electrical properties of deposited thin films
Surf. Coating. Technol.
The influence of annealing on the properties of ZnO: Al layers obtained by RF magnetron sputtering
Opto-Electron. Rev.
Studying of transparent conductive ZnO:Al thin films by RF reactive magnetron sputtering
J. Cryst. Growth
The role of cobalt doping on the optical and structural properties of Mn3O4 nanostructured thin films obtained by SILAR technique
Superlattice. Microst.
Effect of doping concentration on the structural and optical properties of pure and tin doped zinc oxide thin films by nebulizer spray pyrolysis (NSP) technique
Superlattice. Microst.
Conjugated polythiophene/Ni doped ZnO hetero bilayer nanocomposite thin films: its structural, optical and photoluminescence properties
Ceram. Int.
Physical properties of spray deposited Ni-doped zinc oxide thin films
Ceram. Int.
Characterization and studying of ZnO thin films deposited by spray pyrolysis: effect of annealing temperature
Optik
Photoluminescence of ZnO films grown by plasma-assisted molecular beam epitaxy
J. Cryst. Growth
Characterization and optimization of zinc oxide films by rf magnetron sputtering
Thin Solid Films
Electrical and optical properties of ZnO transparent conducting films by the sol–gel method
J. Cryst. Growth
High quality zinc oxide films by pulsed laser ablation
Thin Solid Films
Sol-gel preparation of ZnO films with extremely preferred orientation along (002) plane from zinc acetate solution
Thin Solid Films
Effect of preheating and annealing temperatures on quality characteristics of ZnO thin film prepared by sol–gel method
Mater. Chem. Phys.
Highly conductive and transparent thin ZnO films prepared in situ in a low pressure system
J. Cryst. Growth
Effect of thermal annealing on the structural, electrical and optical properties of Al–Ni co-doped ZnO thin films prepared using a sol–gel method
Surf. Coating. Technol.
Fabrication of preferential orientation ZnO thin films with exposed holes by high temperature annealing low-temperature-grown ZnO thin films on different substrates
Superlattice. Microst.
Effects of rapid thermal annealing on structural, magnetic and optical properties of Ni-doped ZnO thin films
Curr. Appl. Phys.
Effect of annealing on the microstructural, optical and electrical properties of ZnO nanowires by hydrothermal synthesis for transparent electrode fabrication
Mater. Sci. Eng., B
Detailed investigation of photoluminescence, structural, and elemental properties of ZnO thin films under various annealing ambient
Superlattice. Microst.
Structural characterization of ZnO films grown on SiO2 by the RF magnetron sputtering
Phys. B Condens. Matter
Control of preferred orientation for ZnOx films: control of self-texture
J. Cryst. Growth
Sol-gel-derived c-axis oriented ZnO thin films
Thin Solid Films
Low temperature growth of highly transparent c-axis oriented ZnO thin films by pulsed laser deposition
Microelectron. Eng.
Photoluminescence and Raman scattering of Cu-doped ZnO films prepared by magnetron sputtering
Appl. Surf. Sci.
The structural and optical properties of ZnO thin films prepared at different RF sputtering power
J. King Saud Univ. Sci.
Cited by (18)
Structural and optical characterization of RF sputtered CdMgZnO thin film with different Cd concentrations
2023, Materials Chemistry and PhysicsZnO:Ca MSM ultraviolet photodetectors
2022, Optical MaterialsCitation Excerpt :However, the search for high performance and low-cost materials in optoelectronic devices led to the development of a new class of UV photodetectors. Among them, zinc oxide (ZnO) has gained considerable attention of researchers because of its high exciton binding energy, wide band-gap, environmental stability, high transparency in the visible region, non-toxicity, biocompatibility, low-cost and abundance [7–12]. The characteristics of ZnO based MSM devices were studied by S.I. Inamdar et al. [13] and found the suitability of these devices in UV detection.