Highly conductive n-type NiCo2O4-δ epitaxial thin films grown by RF sputtering

NiCo2O4-δ (NCO) epitaxial thin films have been grown on MgAl2O4(0 0 1) substrates at different Ar/O2(1:1) pressure by RF sputtering technique. HRXRD study indicates best crystalline quality and smallest unit cell volume are obtained in NCO thin films deposited at 75 Pa. Hall measurement reveals NCO thin films are of n-type conductivity with the electron mobility and electron density of NCO films in the range of 0.09-1.05 cm2/Vs and 1.06-5.94 × 1021 cm−3, respectively. Maximum conductivity of 178 S/cm is obtained in NCO thin films deposited at 75 Pa, which surpasses the conductivity of NCO films grown by using PLD method. XPS spectra confirm mixed valence states for nickel and cobalt ions, and existence of oxygen vacancies which triggers n-type conductivity. The deposition pressure dependent carrier density and unit cell volume are closely related to the concentration of oxygen vacancies.