Magnetic properties of Er-doped ZnO films prepared by reactive magnetron sputtering

All Zn1−xErxO (x = 0.04, 0.05, and 0.17) films deposited on glass substrates by radio-frequency reactive magnetron sputtering exhibit the mixture of ferromagnetic and paramagnetic phases at room temperature. The estimated magnetic moment per Er ion decreases with the increase of Er concentration. The temperature dependence of the magnetization indicates that there is no intermetallic ErZn buried in the films. The ferromagnetism is attributed to the Er ions substitution for Zn2+ in ZnO lattices, and it can be interpreted by the bound-magnetic-polaron model. In semiconductor devices, one usually takes advantage of the charge of electrons. In contrast, magnetic materials are utilized on the basis of electron spin. In order to develop new electronics, it is necessary to combine both features. Diluted magnetic semiconductor (DMS) can serve as an avenue to utilize both charge and spin functions of electrons for various applications [1]. Many group IV [2], III–V [3], and II– VI [3] DMS materials have been obtained by doping magnetic impurities into semiconductors. Nevertheless, most of them have a low Curie temperature (TC ), which limits their J. Qi ( ) · D. Gao · J. Liu · W. Yang The Key Laboratory for Magnetism and Magnetic Materials of MOE, Lanzhou University, Lanzhou 730000, China e-mail: [email protected] Fax: 86-931-8913554 J. Qi · Q. Wang · J. Zhou · Y. Yang Department of Physics, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China J. Qi · J. Liu Quantum Structures Laboratory, Department of Electrical Engineering, University of California at Riverside, Riverside, CA 92521, USA practical applications. In the quest for materials with high TC , the wide-gap material, ZnO, has emerged as an attractive candidate based on both theoretical and experimental studies for integrating the optical, electronic, and magnetic properties into a single substance [4, 5]. So far, room temperature (RT) ferromagnetism (FM) has been reported in many transition metal doped ZnO such as those doped with Mn [6], Co [7], Cr [8], Ni [9], V [10], Fe [11], and Gd [12]. On the other hand, characteristics of erbium doped ZnO thin films have been widely studied, such as the structural characteristics [13], optical properties [14], and electrical properties [15]. Nevertheless, the magnetic properties of Er-doped ZnO materials have seldom been reported. In this paper, the magnetic properties of a series of Er-doped ZnO thin films are studied and the origin of FM is also discussed. The transparent Er-doped ZnO films were grown on glass substrates by radio-frequency (RF) reactive magnetron sputtering using Zn (purity: 99.99%) and Er (purity: 99.996%) together as targets. The sputtering chamber was evacuated by a molecular pump to a base pressure below 2 × 10−5 Pa. During sputtering, the substrate temperature was kept at RT. Oxygen was introduced into the chamber as the reactive gas and the flow rate was controlled at 20, 25, 30, and 35 sccm, respectively. Additionally, argon was introduced into the chamber as working gas and its flow rate was regulated at 15 sccm. During the deposition, the chamber pressure and sputter power were fixed at 2.0 Pa and 5.3 W/cm2, respectively. The as-grown samples are denoted as Zn1−xErxO. Magnetic measurements were performed by vibrating sample magnetometer (VSM) and superconducting quantum interference device (SQUID), where the magnetic fields were parallel to the film planes. X-ray photoelectron spectroscopy (XPS) was used to analyze chemical valences and chemical compositions of the films. Author's personal copy