The investigated samples are 20mm x 20mm with thicknesses in the range of 100-200nm. The measured resistance values are in the megaohm order of magnitude. The layers are ZnO, prepared using the CCVD (combustion CVD) method and measured using the van-der-Pauw geometry.
When changing the contacts of current and voltage outside the magnetic field, the resistance changes quite strong. This setting points to a rather inhomogeneous layer - right?
If the sample is brought to the magnetic field, the measured voltage of 80mV (at 100nA, 0.5T) changes to 40mV. When reversing the magnetic field for the same contact positions, one would expect a change in the voltage at about 120mV, but it is measured 40mV again. Does anyone have an explanation? Theoretically, it would indicate a simultaneously n-and p-type sample, thereby once electrons and once holes are deflected. However, this explanation lacks any basis to me.
What contacts are you using? Maybe they aren't as ohmic as you think - ZnO rarely is as resistive as you report. The discussion on the question "What are the reasons for incosistency in resistivity and Hall Effect measurements of ZnO thin films by Van der Pauw method?" might give hints on what to try to improve contacts.
Also, the surface conductivity of ZnO is very sensitive to ambient conditions and may change over time.