A study of the electronic states of NixMn3−xO4+δ thin films using scanning tunneling microscopy and current imaging tunneling spectroscopy

NixMn3−xO4+δ (0.4⩽x⩽1) are a series of spinel structured materials exhibiting negative temperature coefficient of resistance (NTCR) characteristics. Thin films were produced on <100> silicon substrates using rf magnetron sputtering in an oxygen/argon atmosphere (2.5% oxygen). Following deposition, the films were annealed at 800 °C in air and the crystalline structure studied using X-ray diffraction. Scanning tunneling microscopy (STM) was used to study the topology of the layers in both the as-deposited and annealed states. Annealing resulted in a more uniform and ordered structure. Tunneling conductance (dI/dV vs V) and normalised tunneling conductance {(dI/dV)/(1/V) vs V} characteristics were measured and used to generate conductance maps {dI/dV (x,y,eV)} and to study the local density of states (LDOS) of the film surface. The tunneling conductance maps showed islands of bright contrast in an otherwise relatively uniform background of dark contrast. There were substantially fewer bright contrast islands in annealed films and they appear to bear no relation to any topographical features. The distribution of the LDOS in the bright regions was similar to that normally associated with metallic behaviour, while in the regions of dark contrast, the LDOS was more characteristic of semiconductors (LDOS ∼0 at Fermi level). The resistance–temperature characteristics were measured and found to be consistent with conduction models based on electron hopping.