Lattice defects formulated ferromagnetism and UV photo-response in pure, Nd, Sm substituted ZnO thin films
The induction of charge and spin in diluted magnetic semiconductor ZnO is explored for spintronic devices and its wide direct band gap (3.37 eV) and a large exciton binding energy (60 meV) have exhibit the potential in UV photodetectors. For this, we have reported ferromagnetic and optical properties of pure ZnO, Zn0.97Nd0.03O and Zn0.97Sm0.03O thin films. These thin films were synthesized by a metallo-organic decomposition method and annealed at 500oC/7 hrs. The Rietveld refinement of the XRD data results into wurtzite ZnO structure with Nd, Sm doping. The dopants and nanoparticles size are responsible into wurtzite structure deformation to induce lattice strain effect which may influence band gap energy and high-TC ferromagnetism of ZnO. The average size of ZnO nanoparticles with Nd, Sm doping is 10 nm that confirmed with atomic force microscopy. The Raman spectra measurement confirms the wurtzite structure of ZnO with crystalline quality and lattice defects formation with dopants Nd, Sm ions. A near-band-edge emission due to band gap energy is evaluated with photoluminescence spectra which also involved multiple visible emissions due to oxygen vacancies. The oxygen vacancies mediated magnetic interactions attributes room temperature ferromagnetism in pure ZnO which is enhanced with Nd, Sm doping. The electron paramagnetic resonance spectra revealed the effect of defects and unpaired electrons responsible into observed room temperature ferromagnetism. The zero field cooling and field cooling magnetic measurements includes antiferromagnetic interactions without any spin-glass formation. The observed ferromagnetism is also correlated with first principle calculation that reported for Nd, Sm doped ZnO and suggests long-range ferromagnetic ordering attributes defects carriers. The Nd, Sm doping into ZnO thin films exhibits significant enhancement in absorption in UV region and suggests its usability for UV detector. Under UV irradiation (λ = 325 nm), the value of photocurrent in Nd, Sm:ZnO thin films is highly enhanced to use in UV sensors.