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4.1 Local conductivity of SmNiO3 thin films 4.1 Local conductivity of SmNiO3 thin films Local conductivity measurements were performed between room temperature 293- 305 K and 453 K, which is well above the transition temperature for bulk SmNiO3 compound (TMI ≈ 400 K). For each temperature several images of surface mor- phology as well as local current maps were collected. On top of that in specific areas I-V curves were collected in order to further investigate the electrical char- acter of the area. In certain areas also I-V curve maps were collected, that is a map of regularly placed points in which I-V curves were measured and then saved together with the location on the surface for further processing. The first decision to make when measuring local conductivity is the value of bias voltage that is applied between the sample and the scanning tip. If the bias is too small, the current does not flow through the sample or is below detection limit. In such case only a ’topographic’ current map is obtained. The image is then only made of charges that are formed on the surface and strongly resembles the surface topography. The bias voltage cannot also be too large, so that the detector is not saturated and the response is possible to be measured. This is a difficult task for some of the materials, either strong insulators or for example topological insulators. The electrical behaviour will also change with temperature, which cannot always be taken into account especially for unknown materials. In order to successfully visualize changes in conductivity at the surface region, for as-deposited sample S-30AOn, a bias voltage of 2.5 V was used. When using smaller voltages, the image was only mimicking surface morphology. Selected im- ages of local current distribution on the surface of sample S-30AOn for various temperatures is depicted in figure 4.1. Although the surface is not very homo- geneous in terms of electrical behaviour, it is evident that despite temperature increase there is no clear change of electrical behaviour. Randomly picked images show a very similar range of current values. No distinct features with tempera- ture change are observed. The images also depict some artefacts such as gradual current increase or stripe-like current pattern that are discussed later. The current measured after applying certain bias voltage was averaged over each whole image, and further those averaged values were averaged over several images taken at a certain temperature. The idea was to detect any changes of the conductivity as a function of temperature. The dependence was plotted in figure 4.2 a. Due to locality of the measurement the scatter of the current values is quite large. There is however a very minor trend visible - the value of the current increases slightly with temperature. 89PDF Image | Investigation of metal-insulator transition in magnetron sputtered samarium nickelate thin films
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