ZnO characterisation under constant current

[chpolyz]

As ZnO is a semiconductor with a typical resistivity of ρ ~ 10^5 Ωcm, I tried to find the settling time during measurements.

If I impose a low-level constant current (as a source), I expect a transient phenomenon (of few tens of seconds) and then a constant state while measuring Voltage to time.

Yet, what I see is a decrease in the measured voltage after its expected rise. Could somebody explain why a fall in Voltage happens and the change of its sign (from positive to negative and vice versa later on, like in the V-t plot)?

 

[.Scott]

I'm guessing that you're separating the oxygen out through electrolysis.

 

[chpolyz]

My sample is a thin film. I do not thing it has to do with electrolysis. I used 2 probes for the current and 2 other for voltage measurement. All of them on the periphery of the sample.