Annealing modified surface morphology and electrical transport behavior of nebulized spray pyrolysis deposited LaNiO₃ and NdNiO₃ thin films

Abstract

LaNiO₃ (LNO) and NdNiO₃ (NNO) thin films have been prepared by nebulized spray pyrolysis on (001)-oriented LaAlO₃ (LAO) and SrTiO₃ (STO) substrates. The stoichiometric nitrate precursor in deionized water was spray pyrolyzed at ~ 300 °C. All films were given annealing treatment at different temperatures to tune the electrical transport behavior. The surface morphology of these films, in general, consists of granular and locally epitaxial regions. Films with electrical transport properties resembling those of similar films prepared by more sophisticated methods like pulsed laser deposition and sputtering are obtained at the annealing temperature range ~ 900–950 °C. High-resolution X-ray diffraction (HRXRD) and glancing incidence XRD bring out the single-phase nature. The films on STO show a small amount of in-plane tensile strain, while those on LAO are under small compressive strain. Annealing at a higher temperature (~ 1000 °C) enhances the tensile strain slightly while the compressive strain is lowered. Such behavior has been attributed to oxygen deficiency. The LNO films on STO and LAO show fairly good metallic behavior, and the electrical transport in the former is described in terms of a model represented by $\rho \left( T \right) = \rho_{0} - aT^{0.5} + bT$. In contrast, the electrical resistivity of the LNO/LAO film is described by $\rho \left( T \right) = \rho_{0} - aT^{0.5} + bT + cT^{2}$. Annealing at high temperatures promotes the insulating state in all the LNO films. The NNO films on LAO substrates show a paramagnetic metallic state which is followed by a metal insulator transition (MIT) on lowering the temperature. The MIT shifts to higher temperatures in 1000 °C annealed films. In contrast, the NNO/STO films show insulating behavior throughout, and annealing at higher temperatures further strengthens the insulating state. The insulator state shows a small polaron hopping kind of transport represented by $\rho \left( T \right) = ATe^{{{\raise0.7ex\hbox{${E_{{\text{A}}} }$} \!\mathord{\left/ {\vphantom {{E_{{\text{A}}} } {k_{{\text{B}}} T}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${k_{{\text{B}}} T}$}}}}$ with temperature-dependent activation energies in the range of 100–55 meV.