Microstructures, surface bonding states and room temperature ferromagnetisms of Zn_0.95Co_0.05O thin films doped with copper

Abstract

Zn_0.95−xCo_0.05Cu_xO (ZCCO, where x = 0, 0.005, 0.01 and 0.015) thin films were deposited on Si (1 0 0) substrates by pulsed laser deposition technique. Crystal structures, surface morphologies, chemical compositions, bonding states and chemical valences of the corresponding elements for ZCCO films were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and X-ray photoelectron spectroscopy (XPS). XRD and FESEM results indicate that crystallite sizes of the highly (0 0 2)-oriented ZCCO films slightly decrease with increasing Cu content. When the Cu content increases from 0 to 0.015, Zn 2p_3/2, Co 2p, Cu 2p_3/2 and O 1s peaks of the ZCCO film shift towards higher or lower binding energy regions, and the reasons for these chemical shifts are investigated by fitting the corresponding XPS narrow-scan spectra. Both in-plane and out-of-plane magnetization-magnetic field hysteresis loops of the ZCCO films reveal that all the films have room temperature ferromagnetisms (RTFMs). The conceivable origin of the RTFM is ascribed to the combined effects of the local structural disorder resulted from (Co²⁺, Cu²⁺, Cu¹⁺)-cations which substitute Zn²⁺ ions in the ZnO matrices, ferromagnetic coupling between coupled dopant atoms caused by Co²⁺ (3d⁷4s⁰) and Cu²⁺ (3d⁹4s⁰) spin states, and exchange interactions between the unpaired electron spins originating from lattice defects induced by Cu doping in the Zn_0.95Co_0.05O matrices.

Introduction

ZnO is attracting considerable attention for its potential applications in ultraviolet light emitters, nonlinear optical devices, gas sensors, spin functional devices, piezoelectric transducers and surface acoustic wave devices [1], [2], [3], [4]. Recently, ZnO-based thin films have been recognized as promising candidates for diluted magnetic semiconductors (DMSs) by virtues of its nontoxicity, low cost and material abundance, when they are doped or codoped with transition metal elements showing room temperature ferromagnetisms (RTFMs), in contrast to the GaMnAs-based materials for which the highest reported Curie temperatures (T_C = −101 °C) are still well below 27 °C [4], [5], [6], [7], [8]. A special interest in ZnO-related DMS was sparked by a theoretical prediction raised by Dietl et al., who predicted RTFM in Mn-doped p-type ZnO [9]. Subsequently, several research groups reported the RTFMs of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Sn-doped ZnO films and (P, Mn), (Al, Mn), (Mn, Cu), (Mn, Sn), (Fe, Cu), (Cu, Ga), (Li, Co), (N, Co), (Al, Co), (Fe, Co) and (Co, Cu)-codoped ZnO films [3], [4], [5], [6], [7], [8], [10], [11]. Among these doped ZnO films, Zn_0.95Co_0.05O films are particularly intriguing because Co provides an opportunity to realize RTFM in ZnO via high-spin Co²⁺ (3d⁷4s⁰, ⁴F_9/2, S = 3/2) and/or Co³⁺ (3d⁶4s⁰, ⁵D₄, S = 2) for which the net superexchange coupling is ferromagnetic at room temperature, based on its high T_C of up to 427 °C, the reversible cycling of ferromagnetic ordering and excellent ferromagnetic transport properties [3], [4], [6], [8], [12]. Nevertheless, Sluiter et al. reported that except the ferromagnetic exchange interaction between substituted Co²⁺ for Zn²⁺ in the ZnO matrix, there exists an antiferromagnetic exchange interaction between some of neighbouring Co²⁺ cations, and the RTFMs of Co-doped ZnO films depend strongly on dopants, experimental methods and conditions used in the preparation processes [4], [7]. Meanwhile, issues of the ferromagnetisms still remain controversial. Additionally, p-type Cu-doped ZnO films are also attractive because the Cu-doped ZnO matrix possesses a ferromagnetic ground state deduced from first principles, and Cu cations have no clustering tendency to overcome the problem of magnetic precipitates in the ZnO-based DMSs [10], [13], [14]. Therefore, it is conceivable that Co and Cu are promising candidates as codopants for ZnO films based on matching ionic radii [4]. According to first principles calculations based upon the density functional theory with Perdew–Burke–Enzerh generalized gradient approximation, the electrons with energies close to the Fermi level effectively transfer only between Cu and Co cations which substitute Zn²⁺ in ZnO matrices, and are located in the neighbour sites connected by O²⁻ [8]. The simulation results are consistent with the experimental observations that addition of Cu helps achieve ferromagnetism of Co-doped ZnO reported by Xu et al. [8]. Similarly, Lin et al. studied the enhanced ferromagnetism of bulk Zn_0.97Co_0.02Cu_0.01O in comparison with that of the Zn_0.98Co_0.02O sample [15]. Furthermore, Chakraborti et al. found that adding appropriate Cu to the Zn_0.95Co_0.05O matrix can stabilize and enhance ferromagnetic responses of the films [7]. However, the origin of RTFMs for the (Co, Cu)-codoped ZnO films is still not well understood.

In this work, crystal structures, surface morphologies, chemical compositions, bonding states and valence states of the corresponding elements for Zn_0.95−xCo_0.05Cu_xO (ZCCO, where x = 0, 0.005, 0.01 and 0.015) thin films grown on Si substrates by pulsed laser deposition (PLD) technique, were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and X-ray photoelectron spectroscopy (XPS). Chemical shifts of Zn 2p_3/2, Co 2p_3/2, Co 2p_1/2, Cu 2p_3/2 and O 1s peaks for the ZCCO films were quantitatively analyzed by the fitted XPS narrow-scan spectra. Room temperature magnetization versus magnetic field (M–H) hysteresis loops for the ZCCO films were measured by physical property measurement system (PPMS) with magnetic field applied in the in-plane and out-of-plane directions, respectively. Both in-plane and out-of-plane M–H hysteresis loops of the ZCCO films indicate that all the films have RTFMs, and then the conceivable origin of RTFMs for the ZCCO films were also discussed.