International Journal of Magnetics and Electromagnetism
Volume 4, Issue 1
Research Article DOI: 10.35840/2631-5068/6513
Hydrogen Annealing on the Structural, Optical and Magnetic Properties of Yb-Doped Zno Diluted Magnetic Semiconductor Thin Films
Weibin Chen1,2, Xuechao Liu1*, Shiyi Zhuo1, Jun Chai1,2, Tingxiang Xu1,2 and Erwei Shi1
Table of Content
Figure 1: a) Magnetization versus magnetic field (M-H) curves for...
a) Magnetization versus magnetic field (M-H) curves for Zn0.985Yb0.015O thin films annealed at 500 ℃, 600 ℃ and 700 ℃ in H2 atmosphere for 1 h; b) The dependence of saturation magnetization on annealing temperature for Zn0.985Yb0.015O thin films.
Figure 2: XRD patterns of Zn0.985Yb0.015O thin films...
XRD patterns of Zn0.985Yb0.015O thin films after annealed at 500 ℃, 600 ℃ and 700 ℃ in H2 atmosphere for 1 h.
Figure 3: Room-temperature PL spectra of Zn0.985Yb0.015O thin films...
Room-temperature PL spectra of Zn0.985Yb0.015O thin films after annealed at 500 ℃, 600 ℃ and 700 ℃ in H2 atmosphere for 1 h.
Figure 4: Room-temperature Raman spectra of Zn0.985Yb0.015O...
Room-temperature Raman spectra of Zn0.985Yb0.015O thin films after annealed at 500 ℃, 600 ℃ and 700 ℃ in H2 atmosphere for 1 h.
Figure 5: a) XPS spectra; b) Zn-2p3/2 high-resolution XPS spectrum;...
a) XPS spectra; b) Zn-2p3/2 high-resolution XPS spectrum; c) O-1s high-resolution XPS spectrum; d) Oa and Ob for Zn0.985Yb0.015O thin films annealed at 500 ℃, 600 ℃ and 700 ℃ in H2 atmosphere for 1 h.
Figure 6: The schematic illustration of ferromagnetism mechanism in...
The schematic illustration of ferromagnetism mechanism in Yb-doped ZnO system.
Table 1: The distance (d) between the (002) lattice plane and the average grain size (D) of the films calculated from XRD pattern.
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Weibin Chen1,2, Xuechao Liu1*, Shiyi Zhuo1, Jun Chai1,2, Tingxiang Xu1,2 and Erwei Shi1
1Shanghai Institute of Ceramics, Chinese Academy of Sciences, People's Republic of China
2Graduate School of the Chinese Academy of Sciences, People's Republic of China
Xuechao Liu, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, People's Republic of China, Tel: +86-2169987663, Fax: +86-2169987661.
Accepted: June 25, 2018 | Published Online: June 27, 2018
Citation: Chen W, Liu X, Zhuo S, Chai J, Xu T, et al. (2018) Hydrogen Annealing on the Structural, Optical and Magnetic Properties of Yb-Doped Zno Diluted Magnetic Semiconductor Thin Films. Int J Magnetics Electromagnetism 4:013.
Copyright: © 2018 Chen W, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Zn0.985Yb0.015O thin films have been prepared by using inductively coupled plasma enhanced physical vapor deposition method. The as-deposited Zn0.985Yb0.015O thin films are annealed in hydrogen atmosphere at different temperatures from 500 ℃ to 700 ℃. The structural, optical and magnetic properties are characterized by using different measurement techniques. All the annealed Zn0.985Yb0.015O thin films show room-temperature ferromagnetism and the saturation magnetization exhibits a similar dependency on the annealing temperature. The photoluminescence spectra imply that oxygen vacancy defects obviously rise with the increase in annealing temperature. It is found that oxygen vacancies can act as bound magnetic polaron and play an important role in the ferromagnetic coupling in Yb-doped ZnO thin films.
Diluted magnetic semiconductor (DMS) is considered as a promising material for spintronic devices [1,2]. DMS can simultaneously take advantage of electronic charge and spin and operate with faster speed, less energy consumption and higher efficiency. Zinc oxide (ZnO) is regarded as a suitable DMS material when doping with transition metals or rare earth metals. ZnO-based DMS can exhibit room-temperature ferromagnetism, which has been theoretically predicted by T. Ditel . However, the origin of spin coupling still remains under debate. Many factors such as electrons , holes , intrinsic defects and stress  can play important roles in the magnetic properties. Coey, et al.  proposed a theoretical model of bound magnetic polaron (BMP) in connection with semi-insulating materials where ferromagnetic exchange is mediated by defects and form BMPs. Up to present, many experiments have proven that intrinsic point defects, such as oxygen vacancies (VO) [8,9], zinc vacancies (VZn) [10,11], zinc interstitial (Zni) , and oxygen interstitial (Oi)  are indeed related to ferromagnetism in DMS materials. But how the defects function in DMS materials remains a complex puzzle.
Ytterbium (Yb) can be used as the dopant for ZnO-based DMS due to its matchable ion radius with Zn2+. Previous research  claimed that the ferromagnetism in Yb-doped ZnO was induced by the coexistence of oxygen vacancies and Yb point defects. In order to understand the role of oxygen vacancies in Yb-doped ZnO, we perform a series of annealing experiments at different temperatures since oxygen easily escapes from ZnO lattice in reducing atmosphere. In this paper, an in-depth study on the influence of annealing temperature on structural, optical and magnetic properties of Yb-doped ZnO thin films is performed. The origin of room-temperature ferromagnetism in Yb-doped ZnO thin films is discussed. We specially focus on the influence of oxygen vacancies on the ferromagnetism.
Yb-doped ZnO thin ﬁlms (Zn0.985Yb0.015O) are deposited on sapphire substrates by using inductively coupled plasma enhanced physical vapor deposition method. The deposition temperature, sputtering power, and the argon partial pressure is 300 ℃, 120 W and 1.2 Pa, respectively. The thickness of Zn0.985Yb0.015O thin ﬁlms is controlled at 400 nm by adjusting the deposition time. After the Zn0.985Yb0.015O thin ﬁlms are prepared, they are loaded in the heating zone of a tube furnace for annealing treatment. The furnace is refilled with high purity H2 until achieving atmospheric pressure, followed by increasing the temperature to 500 ℃, 600 ℃ and 700 ℃ at a rate of 10 ℃/min, then keep at the annealing temperature for 1 h. The as-deposited Zn0.985Yb0.015O thin film is marked as blank in the following figures.
The magnetic properties of Yb-doped ZnO thin ﬁlms were measured by using a superconducting quantum interference device (SQUID, Quantum Design MPMS) magnetometer. The phase composition and crystal structure were characterized by using a X-ray diffractometer (XRD, Rigaku Ultima IV) with Cu Ka radiation (λ = 1.5406 Å). The Raman and photoluminescence (PL) spectra were performed on a confocal Raman spectroscope (Renishaw, Invia Raman microscope) with 325 nm wavelength He-Cd laser. The chemical bonding formation was analyzed by X-ray photoelectron spectroscopy (XPS) measurement (XPS, Thermo Scientiﬁc ESCALAB 250) with Al Ka X-ray (1486.6 eV).
Results and Discussion
Figure 1a shows the magnetization versus magnetic field (M-H) curves measured at room temperature for Yb-doped ZnO thin films. The diamagnetic background signal of sapphire substrate has been subtracted. It can be seen that all the samples exhibit clear ferromagnetic hysteresis loops. The saturation magnetization for different Zn0.985Yb0.015O thin ﬁlms, as shown in Figure 1b, is 0.230, 0.260, 0.283 and 0.321 emu/cm3, respectively. The ferromagnetism of Yb-doped ZnO thin films exhibits an obvious enhancement after annealing in H2 atmosphere. That is to say, the higher annealing temperature, the stronger magnetization, meaning a correlation between the ferromagnetism and annealing temperature.
XRD are used to study the crystal structure of Yb-doped ZnO thin films. Figure 2 shows the XRD patterns of Zn0.985Yb0.015O thin films after annealed at different temperatures. The comparison of peak positions with standard Joint Committee for powder diffraction standard files suggests that all the Zn0.985Yb0.015O thin films are polycrystalline. The XRD results reveal a single hexagonal wurtzite structure of Zn0.985Yb0.015O thin film without Yb cluster and Yb-related oxides. With the increase in annealing temperature, the intensity of ZnO (002) diffraction peak slightly moves to higher level and the FWHM value of ZnO (002) become smaller, which jointly indicates that high annealing temperature can improve the crystallization quality and the orientation of crystalline grains converts to ZnO (002) on the c-sapphire substrate.
To further characterize the crystal quality, the average grain size in Zn0.985Yb0.015O thin films is calculated by Scherrer formula  and shown in Table 1.
Wherein λ, β, and θ are the X-ray wavelength (1.5406 Å), diffraction angle, and FWHM of Zn0.985Yb0.015O (002) peak. It can be seen that the grain size gradually rises with the increase in annealing temperature and reaches a maximum value when the annealing temperature is 700 ℃. Larger grain size means less proportion of grain boundary and better crystallization quality.
Photoluminescence (PL) spectra can be used to detect defects in thin film materials. Figure 3 shows the room-temperature PL spectra of Zn0.985Yb0.015O thin ﬁlms after annealed at different temperatures. It can be seen from Figure 3, four peaks located at 380 nm, 460 nm, 580 nm and 980 nm are excited by 325 nm wavelength laser. The origin of these four PL peaks will be discussed in detail in the follow text. The ultraviolet emission at 380 nm is the characteristic of a near-band-edge (NBE) transition of wide-band-gap intrinsic ZnO. With the increase in annealing temperature, the intensity of NBE increases to higher level, meaning that elevating annealing temperature can make the crystal structure quality perfect with fewer non-irradiation recombination defects. This is consistent with XRD analysis. The explicit mechanism of blue PL emission centering at 460 nm is not clear so far. Most of the reports attributed the blue emission due to the presence of oxygen vacancies occurring between 420 and 480 nm. Chen  synthesized ZnO quantum dots (QDs) with strong blue emission at 460 nm by gel-sol method. Moreover, Jana  prepared ZnO nanorods exhibiting blue emission at same wavelength. They both concluded that the blue emission is due to the transition from deep donor level by singly ionized oxygen vacancies to the valence band. A broad defect-related visible emission centering at 580 nm is commonly attributed to the doubly charged oxygen vacancies . These states are created when the hole is captured by the center in a depletion region. The yellow emission (580 nm) is observed in ZnO nanorods  and un-doped ZnO thin films  which is induced by double ionized oxygen vacancies as well. Therefore, it can be concluded that the broad visible emission stems from combination of two emissions with different oxygen vacancy states. Ning, et al.  also found that and coexisted in one system, where can trap electrons and become . The intensity of both peaks related to oxygen vacancies is obviously enhanced with increasing the annealing temperature, meaning an increase in the concentration of oxygen vacancies. The fourth peak around 980 nm is unique in Yb-doped ZnO, which is attributed to up conversion luminescence of Yb3+ from 2F5/2 to 2F7/2 .
Raman spectrum is a powerful tool to identify the extrinsic dopant contamination and the type of defects for ZnO thin films. As for wurtzite structure ZnO that belongs to the space group , A1 (LO) (574 cm-1) Raman peak is one of the fundamental phonon modes which is associated with oxygen vacancies, Zn interstitials, or their complexes [22-24]. As Figure 4 shows, both A1 (LO) phonon mode and second-order A1 (LO) phonon mode (1164 cm-1) exist in Yb-doped ZnO thin films. Based on the preparation condition and atmosphere of annealing treatment, the A1 (LO) phonon mode is possible to be induced by oxygen vacancies.
X-ray photoelectron spectroscopy
Figure 5a shows the typical XPS survey scan for Zn0.985Yb0.015O thin films. Not any impurity element except carbon (C) is detected within the detection limit of XPS. Two Strong ZnO 2p3/2 peaks and ZnO 2p1/2 peaks, located at 1022 eV and 1045 eV are observed, which corresponds to Zn2+ in the ZnO lattice. To further investigate the state of Zn atom, we try to fit high-resolution Zn 2p3/2 XPS spectra into two or three peaks, as shown in Figure 5b. However, the fitted peaks cannot match the initial peaks well, indicating the initial peaks are of good symmetry. Therefore, Zn elements in Zn0.985Yb0.015O thin films only occupy Zn2+ state and bound to oxygen. Zn vacancies and Zn interstitials can be neglected. The inset in Figure 5a shows high-resolution XPS spectra of Yb-4d core-level. Each of Yb-4d XPS spectrum consists of four peaks in the range of 180 eV~210 eV. These peaks are ascribed to Yb3+, thus the existence of Yb cluster and Yb2+ can be excluded [25,26]. High-resolution XPS spectra of O-1s core-level, as shown in Figure 5c, are recorded to investigate the state of 1s electrons of oxygen. Contrary to other peaks, these peaks exhibit clear asymmetry, which is an indication that different O states coexist in Zn0.985Yb0.015O thin films. All O-1s core-level XPS spectra can be fitted into two different peaks located at 530.0 eV and 531.5 eV, as shown in Figure 5d. The former peak (Oa) is originated from O2- ions in the normal lattice sites surrounded by Zn2+ and Yb3+. However, explanation for the latter peak (Ob) is controversial. Some researchers claimed that the peak around 532.0 eV originated from near-surface oxygen or absorbed oxygen, while other researchers believed this peak came from O2- ions in the oxygen-deficient regions [27,28]. In this experiment, both the film preparation process and annealing treatment are performed at oxygen-deficient environment. Thus, the Ob peak is probably attributed to latter explanation. Besides, it is found that the position of O 1s peaks for annealed samples shifts to higher binding energy. There are researchers [29,30] reporting that the existence of oxygen vacancies can cause the shift to higher binding energy. Therefore, more oxygen vacancies are formed during the annealing process. Based on the Raman, PL and XPS results, it can be concluded that the main defect in our Yb-doped ZnO thin films is oxygen vacancies.
Based on above XRD, Raman, XPS, PL and magnetization results, it can be concluded that the bridge between annealing temperature and magnetism is oxygen vacancy. Oxygen vacancies can carry different amount of charge, such as , and VO. Among them, only has net magnetic moments and can contribute to the ferromagnetism . BMP model is a proposed theory to explain the origin of room-temperature ferromagnetism for defect-rich material. This is also favors Yb-doped ZnO system. In this system, can capture an electron from the conduction band, turning into . The electron trapped locally around the oxygen vacancies 5678 occupies an orbit overlapping with 4f-shell electrons of YbZn neighbors, because of quantum effect, the orientation of magnetic moments of oxygen vacancies is coincident with that of Yb atom, forming BMPs. The formed BMPs randomly associate with oxygen vacancies overlap, leading to long-range ferromagnetic ordering, as Figure 6 illustrates.
In conclusion, Zn0.985Yb0.015O thin films have been prepared by using inductively coupled plasma enhanced physical vapor deposition method and then annealed in hydrogen atmosphere at 500 ℃, 600 ℃ and 700 ℃ for 1h. We perform an in-depth study on the influence of annealing temperature on structural, optical and magnetic properties. The crystallization quality of Zn0.985Yb0.015O thin films is obviously improved after annealing process. The amount of oxygen vacancies rises with increasing the annealing temperature. Single and double ionized oxygen vacancies coexist in Zn0.985Yb0.015O thin films, but only single ionized oxygen vacancies make contribution to ferromagnetism. BMP model is well used to explain the ferromagnetic mechanism of Yb-doped ZnO thin films.
This work is sponsored by the National Key R&D Program of China (Grant Nos. 2016YFB0400401 & 2017YFB0405700) and Young Scientists Fund of the National Natural Science Foundation of China (Grant no. 51602331).