Examination of insulator regime conduction mechanisms in epitaxial and polycrystalline SmNiO3 thin films

Abstract

Correlated oxides that exhibit metal–insulator phase transitions are emerging as potential candidates for switching devices. One such material is SmNiO3, which has a transition temperature above room temperature (∼400 K in bulk crystals). In this work, we study temperature- and bias-dependent conduction mechanisms in epitaxial and polycrystalline SmNiO3 thin films. In both cases, at low electric field we observe thermally assisted hopping conduction through defect states with activation energies of ∼170 meV and ∼270 meV, respectively. At high electric field the conduction transitions to a space-charge limited regime controlled by an exponential trap distribution. The power law exponents are ∼3 in epitaxial films and ∼8–14 in polycrystalline films. The trap decay parameter in epitaxial films does not have the expected 1/T temperature dependence, which may be a signature of bandgap narrowing at high temperature because of the insulator-to-metal transition. The larger activation energy and power law dependency in polycrystalline films are consistent with additional defect density from extraneous phases. In polycrystalline films, current-voltage data measured perpendicular to the film surface are rectifying because of asymmetry in electrode work functions with a ratio of 104 at ± 1 V. We find that whereas the space-charge limited conduction for positive bias is bulk limited, the negative bias conduction is injection limited.