Effect of thermal annealing on the properties of ZnO thin films
Introduction
Semiconductor devices have been mainly made using the concept of charge, one of the properties of electrons. On one hand, to achieve the fabrication of highly integrated semiconductor devices, a new concept of semiconductor, spintronics which controls not only the charge of electrons but also the spin of electrons, was introduced. There are three types of spintronic materials; magnetic metals and alloys, topological insulators, and magnetic semiconductors [1]. One of the properties of these materials is ferromagnetic (FM) with large spin polarization [1]. In particular, these materials should be room temperature ferromagnetic (RTFM) materials with ferromagnetism at or above room temperature [2,3]. The realization of this behavior in nonmagnetic semiconductors thought to be impossible due to the absence of unpaired electrons. Thus, doping with magnetic elements such as Mn, Fe, and Co is used frequently as a representative method to achieve RTFM in such semiconductors [[4], [5], [6], [7]]. It has been, however, shown that such behavior could be achieved without the use of magnetic elements [[8], [9], [10], [11], [12], [13], [14], [15], [16]]. For this reason, extensive research has been conducted on various nonmagnetic oxides to identify the origin of magnetism and to apply it for various nonmagnetic materials [8,10,17,18]. On one hand, ZnO is of increasing importance because it is recognized as an alternative candidate for electronic and optoelectronic devices due to direct wide bandgap and a large exciton energy (60 meV) [2,3,19]. ZnO has been used in different applications such as chemical sensors [20], solar cell [21], optoelectronic [22] and thermoelectric devices [23]. Moreover, induction of ferromagnetic behavior in ZnO thin films will open the pathway for its utilization in spintronic applications. Recently, Sundaresan et al. assumed that the origin of ferromagnetism may be the interaction between unpaired electron spins arising from the oxygen vacancy in pure ZnO nanoparticles upon annealing at range of 1000–1400 °C [10]. Banerjee et al. reported the enhancement of ferromagnetism in pure ZnO particles upon annealing at 900 °C for 2 h. They proposed that magnetization was caused from the formation of oxygen vacancy clusters [12]. Darma et al. reported that paramagnetic property changes to ferromagnetic property in the ZnO thin films upon the structural modification of nanostructure using annealing treatment at 600 °C for 10 min with O2. They observed that zinc vacancy increased and oxygen vacancy decreased simultaneously in photoluminescence (PL) data [24]. Ghosh et al. reported the defect induced RTFM for single crystal, poly-crystalline, and nanorod ZnO. They proposed that the increased RTFM caused from the population of defects and/or vacancies at the O sites [25]. However, despite numerous studies, demonstrating the FM behavior, the understanding of origin in this behavior remains unclear. Thus, the systematic investigation of defects, their nature, and their contributions to magnetism can provide an effective way to understand the origin of magnetism in this system. This can be achieved by understanding the defects with growth parameters. Thermal annealing is commonly employed parameters in such systems to control crystalline phase, however, this process also manipulates defects [[26], [27]]. Hence, a critical analysis of defects with annealing will pave up way to get insights of magnetic behavior of ZnO.
In this study, the properties of differently annealed ZnO thin films grown by radio frequency (RF) magnetron sputtering were examined with various optical and structural characterization tools such as Rutherford backscattering spectrometry (RBS), time of flight secondary ion mass spectrometry (TOF-SIMS), X-ray diffraction (XRD), UV–Vis., PL, X-ray absorption spectroscopy (XAS), superconducting quantum interference device (SQUID) magnetometery, and X-ray magnetic circular dichroism (XMCD). XAS can be used to characterize vacancies. On one hand, it is difficult for SQUID to distinguish whether the magnetism is due to the segregation of magnetic elements or ferromagnetic such as diluted magnetic semiconductor (DMS). Thus, the ferromagnetic properties were confirmed with XMCD. On the basis of the measured results, the effect of annealing temperatures on the magnetic properties of ZnO thin films is discussed and an attempt is made to correlate the presence of vacancies with the magnetism of ZnO thin films.
Section snippets
Preparation of ZnO thin films
ZnO thin films were deposited onto quartz and sapphire substrates at 100 °C by RF magnetron sputtering using a 2-inch ZnO target (99.99%, TASCO Co. Ltd.) (Section S1). The surfaces of the quartz and sapphire substrates were cleaned using an ultrasonicatior in acetone, methanol, and deionized water for 5 min per cleaning agent and dried with N2 gas before being loaded into the sputter chamber. The working pressure was maintained at 5.0 × 10−3 torr during the deposition of ZnO films. The mixture
Results and discussion
The thickness of the films obtained by RBS is illustrated in Fig. 1. The RBS spectra for films annealed at different temperatures are shown in Fig. 1(a). Regions showing Zn feature are presented in Fig. 1(b). In the spectra, no additional feature present, which shows absence of magnetic impurity. This has also been confirmed from SIMS measurements (Fig. S1). The feature in the RBS spectra (Fig. 1(b)) is slightly shrunk with increasing temperature (as marked by the arrow), which is associated
Conclusion
Thus, a systematic investigation on the nature of ZnO thin films with various annealing temperature is presented using various techniques. ZnO thin films grown using radio frequency sputtering exhibits slight decrease in film thickness with annealing. With the increase of annealing temperature, the structural behavior of films approaches towards the behavior of bulk ZnO. Extended X-ray absorption fine structure measurements revealed the presence of oxygen vacancies at lower annealing
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The work is supported in part by Korea Institute of Science and Technology (2V08170), Republic of Korea. Aslo, T-YS gratefully acknowledges the support from the National Research Foundation (NRF) of Korea (NRF-2017K1A1A2013160).
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