Effect of oxygen partial pressure on the NTCR characteristics of sputtered NixMn3−xO4+δ thin films

NixMn3−xO4+δ are ceramic materials with a spinel structure exhibiting a negative temperature coefficient of resistivity (NTCR). This inherent property makes this class of material ideally suited for temperature sensing applications. Films of NixMn3−xO4+δ were deposited on oxide coated <100> silicon (SiOx), using rf magnetron sputtering in an oxygen/argon atmosphere with different oxygen partial pressures (0.3, 1, 2.5, 5, 7.5, 10, 12.5, 15 percent oxygen). A first set of films were annealed in air at 800 °C for 1 h and quenched to room temperature. A second set of films were annealed in an atmosphere similar to that used during deposition (oxygen/argon), at 800 °C for 1 h and quench cooled to room temperature. The resistance versus temperature as measurements were done from room temperature up to 200 °C and the data fitted to a variable range hopping model where the resistivity can be expressed as ρ0Texp(T0/T). The characteristic temperature T0 (range ∼1.9×105–2.42×105 K) of the films was not noticeably different from the target material, with exception of the films deposited at low oxygen partial pressure, which showed higher values of T0 (∼3.1×105 K). The resistivity of the as-deposited films was dependent on the oxygen percentage in the ambient during sputtering, such that the resistance decreased with increasing oxygen. On annealing independent of the ambient, the resistivity of the films decreased considerably (∼300 ohm cm). The films produced in ambients with higher oxygen had a resistivity in the as-deposited state similar to that of the annealed films and showed least variation on annealing.