It is common to get resistance value from the I-V curve generated by two-point probe. However, if the two points are taken on the surface of the thin film layer, how can I get the resistivity? According to the resistivity formula expressed with resistance, the area is required. However, in this case, what is the 'area'?
- For good results the four point measurement on a test structure is better, because you can measure current and voltage separately.
- For thin films the van der Pauw method can be used
https://en.wikipedia.org/wiki/Van_der_Pauw_method
Two-point measurements do not work well for such small resistances, contact resistance will also be an issue. The area in the resistivity equation is the transversal area which will depend on your experimental setup, whereas the length in the equation is the distance between probes. In two-dimensional systems, it is more common to characterize the sheet resistivity Rs, since it can be directly measured and is independent of sample size (as long as the thickness is uniform). Rs is the resistance of a squared piece of sample and depends on the thickness and bulk resistivity of the material, so it is like the resistivity per unit of thickness.
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