Epitaxial growth of Mn-Co-Ni-O thin films and thickness effects on the electrical properties

Mn1.56Co0.96Ni0.48O4 (MCN) thin films were grown on thermally oxidized Si(1 0 0) substrates using laser molecular beam epitaxy technique. The X-ray diffraction and high-resolution transmission electron microscopy analyses indicated an epitaxial structure with [1 0 0] growth direction and a lattice parameter of 8.28 Å. The resistivity-temperature relationships of the epitaxial MCN thin films exhibited negative temperature coefficient thermistor characteristics, and the electron conduction mechanism was found to be nearest-neighbor hopping. Interestingly, the room temperature resistivity ρ25, the characteristic temperature T0, the activation energy Ea, and the temperature coefficient of resistance αT were all highly dependent on the film thickness t, and as t increased all of them decreased. An approximately linear relation was further revealed between ρ25 and 1/t2, which could be approximately expressed as ρ25(t)=211.7+119.1×104/t2 (t in nm; ρ25 in Ω cm). The successful growth of epitaxial MCN thin films opens a door for studying the effects of thickness on the electrical properties of MCN thin films, and may consequently provide an alternative approach for controlling the properties.