Deposition and characterisation of sputtered nickel manganate thin films

This work investigates the structural and electrical properties of both bulk and rf magnetron sputtered thin films of spinel structured NixMn3-xO4+δ material system. The distribution of the LDOS of the thin films is also studied using STS. A rf magnetron sputtering system capable of reactive sputtering in a range of argon/oxygen ambients was designed, constructed and commissioned in the first phase of this work. The system was optimised in terms of the effect of various process parameters on the growth rate using factorial experimental design technique. Incident power, substrate to target distance and oxygen percentage in the ambient was found to be the most significant. The effect of different sintering temperatures was investigated for five different compositions of the NixMn3-xO4+δ material system. Monophase material could not be prepared without prolonged annealing at 800 deg C after sintering at higher temperatures. This was in contradiction with the published phase diagram of the material and hence a modified scheme was proposed. The lattice parameter of the spinel phase increased with decreasing nickel content. Grain growth was found to be exponentially dependent on the sintering temperature. The R-T characteristics below 300K followed the Shklovskii and Efros VRH model (T0 ∼2 x 105 K) and a change to the NNH model (ΔE ∼330 meV) was observed above 300K. The resistivity of the material was dependent on both the ratio of Ni:Mn and the oxygen stoichiometry (varying from 1.2 Kohm-cm up to 30 Kohm-cm). The as-deposited films showed poor crystallinity, hence post deposition annealing at 800 deg C was required. The microstructure and the degree of preferred orientation were found to be dependent on the substrate temperature and post deposition annealing. The lattice parameter of the films was lower than the target. The NNH model best described the R-T characteristics of the films deposited at low oxygen content <2.5% (ΔE ∼360 meV) whereas films deposited at higher oxygen content could be better described by the Shklovskii and Efros VRH model (T0 ∼2.4 x 105 K). The resistivity of the films decreased with increasing oxygen in the ambient in the as-deposited state, however after annealing the resistivity of all the films became similar and much lower than the target. The distribution of the LDOS of the films, using STS, was found to be parabolic and in agreement with the assumption in the Shklovskii and Efros VRH model. Additional features were observed in the LDOS with increasing temperatures (∼±0.15 eV and ∼+1.6 eV) however the changes were completely reversible with temperature. (author)