Effect of thermal annealing on the properties of ZnO thin films

Highlights

• Modulation of Stress in ZnO Thin Films with Thermal Annealing.

• Decrease of Oxygen Vacancies with Annealing as Observed by X-ray absorption Spectroscopy and Photoluminescence Spectroscopy.

• Evidences of O(2p) States Contribution to The d^o Ferromagnetism as Investigated from X-ray Magnetic Circular Dichroism.

Abstract

This work reports the effect of thermal annealing ranging from 0 to 800 °C on the various properties of zinc oxide thin film grown using radio-frequency sputtering. X-ray diffraction studies reveal the relaxation of stress up to thermal annealing of 400 °C and induction of residual stress thereafter. Zn K-edge X-ray absorption spectroscopic measurements reveal the variation of Zn–O and Zn–Zn bond distances with annealing temperature. Variation of coordination number with annealing temperature shows onset of oxygen vacancies at lower annealing temperatures. These vacancies and defects are also supported by the photoluminescence measurements. The optical band-gap of these films exhibit a significant variation with annealing temperature, which is in-line with the variation of crystallite size. Further, the magnetic behavior of these films is observed to follow the behavior of O 2p states along with defects as investigated from the X-ray magnetic circular dichroism.

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.