Sapphire/silicon-substrate dependent magnetism of Er-substituted ZnO (ZnO: Er) thin film: x-ray absorption near edge structure (XANES) study

Hexagonal wurtzite structure Er-substituted ZnO dilute magnetic semiconductors (DMSs) thin films have been deposited on sapphire/silicon substrate [(ZnO: Er)_{sapphire/silicon}] using DC sputtering technique at room temperature in oxygen-plasma atmosphere and studied their electronic/local atomic-structure and correlated with their magnetic behaviors. Incorporation of Er⁺³-ion in ZnO-lattice [(ZnO: Er)_{sapphire/silicon}] and formation of Zn⁺²/O^- vacancies (V_Zn/V₀) at the bonding between Zn and O₂ may induces room temperature ferromagnetism. Magnetic M-H loops of pure-ZnO shows diamagnetic behaviour, whereas (ZnO: Er)_{sapphire/silicon} are ferromagnetic in nature. Moreover, (ZnO: Er)_sapphire is higher ferromagnetic (saturation magnetization, M_S ≈2.3 emu/g, coercivity, H_C ≈63 Oe) than (ZnO: Er)_silicon (M_S ≈0.5 emu/g, H_C ≈15 Oe). Incorporation of Er⁺³-ion and formation of V_Zn/V₀ vacancies were observed from x-ray photoelectron spectroscopy (XPS) and x-ray absorption near edge structure (XANES) spectroscopy analysis. The peak positions of Zn 3p and O 1s in XPS spectrum of (ZnO: Er)_{sapphire/silicon} thin films are shifted at higher energy levels compared to pure ZnO, indicating that the behaviors of (ZnO: Er)_{sapphire/silicon} thin films are substrate dependent. XANES study of Zn K-edge/Zn L₃-edge shows the transition of Zn 1s →4p / Zn 2p → Zn 4s3d in antibonding states of Zn; whereas Er L₃-edge is the transition of 2p→5d states of Er upon x-ray absorption that demonstrates the local atomic structure around Er in the (ZnO: Er)_{sapphire/silicon} thin films. These transitions reveal the host-lattice in Zn-sites is substituted with Er³⁺-ions to preserve the symmetry with negligible distortion. The O K-edge shows the transition of O 2p - Zn 4s hybridized states through charge transfer the oxygen bands (Er-O-Zn) that reduced slightly atomic radius of Zn-O/Er-O/Zn-Zn found from extended x-ray absorption fine structure study, implies (ZnO: Er)_sapphire is higher ferromagnetic behaviour than (ZnO: Er)_silicon. The defect emission bands also identified through Er-substitution indicated that the emissions changed the diamagnetic-ZnO into room temperature ferromagnetic DMSs (ZnO: Er)_{sapphire/silicon} thin film that could be useful for optoelectronic/magnetic applications.