Hydrogen Annealing on the Structural, Optical and Magnetic Properties of Yb-Doped Zno Diluted Magnetic Semiconductor Thin Films

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. *Corresponding author: Xuechao Liu, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, People’s Republic of China, Tel: +86-2169987663, Fax: +86-2169987661, E-mail: xcliu@mail.sic.ac.cn VIBGYOR ISSN: 2631-5068


Introduction
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 [3]. However, the origin of spin coupling still remains under debate. Many factors such as electrons [4], holes [5], intrinsic defects and stress [6] can play important roles in the magnetic properties. Coey, et al. [7] 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 (V O ) [8,9], zinc Abstract Zn 0.985 Yb 0.015 O thin films have been prepared by using inductively coupled plasma enhanced physical vapor deposition method. The as-deposited Zn 0.985 Yb 0.015 O thin films are annealed in hydrogen atmosphere at different temperatures from 500 °C to 700 °C. The structural, optical and magnetic properties are characterized by using different measurement techniques. All the annealed Zn 0.985 Yb 0.015 O 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.
vacancies (V Zn ) [10,11], zinc interstitial (Zn i ) [12], and oxygen interstitial (O i ) [13] 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 ZnObased DMS due to its matchable ion radius with Zn 2+ . Previous research [14] 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.

Experiment
Yb-doped ZnO thin films (Zn 0.985 Yb 0.015 O) 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 °C, 120 W and 1.2 Pa, respectively. The thickness of Zn 0.985 Yb 0.015 O thin films is controlled at 400 nm by adjusting the deposition time. After the Zn 0.985 Yb 0.015 O 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 H 2 until achieving atmospheric pressure, followed by increasing the temperature to 500 °C, 600 °C and 700 °C at a rate of 10 °C/min, then keep at the annealing temperature for 1 h. The as-deposited Zn 0.985 Yb 0.015 O 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). 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 hys-teresis loops. The saturation magnetization for different Zn 0.985 Yb 0.015 O thin films, as shown in Figure 1b, is 0.230, 0.260, 0.283 and 0.321 emu/cm 3 , respectively. The ferromagnetism of Yb-doped ZnO thin films exhibits an obvious enhancement after annealing in H 2 atmosphere. That is to say, the higher annealing temperature, the stronger magnetization, meaning a correlation between the ferromagnetism and annealing temperature.

Structural properties
XRD are used to study the crystal structure of Ybdoped ZnO thin films. Figure 2 shows the XRD patterns of Zn 0.985 Yb 0.015 O 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 Zn 0.985 Yb 0.015 O thin films are polycrystalline. The XRD results reveal a single hexagonal wurtzite structure of Zn 0.985 Yb 0.015 O 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  To further characterize the crystal quality, the average grain size in Zn 0.985 Yb 0.015 O thin films is calculated by Scherrer formula [15] and shown in Table 1.
0.9λ = cos θ D β Wherein λ, β, and θ are the X-ray wavelength (1.5406 Å), diffraction angle, and FWHM of Zn 0.985 Yb 0.015 O (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 °C. Larger grain size means less proportion of grain boundary and better crystallization quality.

Photoluminescence spectra
Photoluminescence (PL) spectra can be used to detect defects in thin film materials. Figure 3 shows the room-temperature PL spectra of Zn 0.985 Yb 0.015 O 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 [16] synthesized ZnO quantum dots (QDs) with strong blue emission at 460 nm by gel-sol method. Moreover, Jana [17] prepared ZnO nanorods exhibiting blue emission at same wavelength. They both concluded that the blue emission is due to the transition from deep 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 Yb 3+ from 2 F 5/2 to 2 F 7/2 [14].

Raman characterization
Raman spectrum is a powerful tool to identify the extrinsic dopant contamination and the type of defects C υ space group [21], A 1 (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][23][24]. As Figure 4 shows, both A 1 (LO) phonon mode and second-order A 1 (LO) phonon mode (1164 cm -1 ) exist in Yb-doped ZnO thin films. Based on the preparation condition and atmosphere of annealing treatment, the A 1 (LO) phonon mode is possible to be induced by oxygen vacancies. Figure 5a shows the typical XPS survey scan for Zn 0.985 Yb 0.015 O thin films. Not any impurity element except carbon (C) is detected within the detection limit of XPS. Two Strong ZnO 2p 3/2 peaks and ZnO 2p 1/2 peaks, located at 1022 eV and 1045 eV are observed, which corresponds to Zn 2+ in the ZnO lattice. To further investigate the state of Zn atom, we try to fit high-resolution Zn 2p 3/2 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 Zn 0.985 Yb 0.015 O thin films only occupy Zn 2+ 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 Yb 3+ , thus the existence of Yb cluster and Yb 2+ 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 Zn 0.985 Yb 0.015 O 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 (O a ) is originated from O 2ions in the normal lattice sites surrounded by Zn 2+ and Yb 3+ . However, explanation for the latter peak (O b ) 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 O 2ions 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 O b 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.

Discussion
Based on above XRD, Raman, XPS, PL and magne-   [20]. 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, o V + can capture an electron from the conduction band, turning into o V + . The electron trapped locally around the oxygen vacancies 5678 occupies an orbit overlapping with 4f-shell electrons of Yb Zn 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.

Conclusion
In conclusion, Zn 0.985 Yb 0.015 O thin films have been prepared by using inductively coupled plasma enhanced physical vapor deposition method and then annealed in hydrogen atmosphere at 500 °C, 600 °C and 700 °C for 1h. We perform an in-depth study on the influence of annealing temperature on structural, optical and magnetic properties. The crystallization quality of Zn 0.985 Yb 0.015 O 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 Zn 0.985 Yb 0.015 O 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.