I am trying for electrical resistivity and Hall Effect measurements of Physical Vapour deposited ZnO thin films (of about 300 nm) by Van der Pauw Method with 1500 Gauss of magnetic field, Keithley 6220 current source and Keithley 2000 DMM. I have made the electrical contact with silver paste. I was not giving any heat treatments for silver paste contact. But I am not getting consistent and repetitive readings for voltages for the sample. Even If I am trying for direct resistance measurement with Keithley 2000 DMM, I am getting continuously, exponentially increasing resistance as time progresses. I am not getting the reason for these fluctuations.
Whether the problem is with the silver paste ohmic contact? Please anybody can help me the procedure of making ohmic contact with silver paste. What are the optimum drying condition/ temperature for silver paste contact? How to check the certainty of ohmic contact? Is it by linear IV curve?
How much gauss of Magnetic field is suitable?
Whether the problem with the thin film sample? Why the resistance exponentially increases continuously?
1) First of all!
Ohmic contact from silver paste is not easy, because you can't control the thickness of paste as well as other parameters. You must use metal whose work function is comparable to that of ZnO to achieve better electrical contact. Rapid thermal annealing of contact is also better option for making good contact.
2) Increase in resistance can be due to variety of reasons, involving some contact limited conduction mechanisms.
3) Usually when slop~ 1 in double logarithmic plot of IV curve, dominant conduction mechanism is Ohmic.
Regards
1) First of all!
Ohmic contact from silver paste is not easy, because you can't control the thickness of paste as well as other parameters. You must use metal whose work function is comparable to that of ZnO to achieve better electrical contact. Rapid thermal annealing of contact is also better option for making good contact.
2) Increase in resistance can be due to variety of reasons, involving some contact limited conduction mechanisms.
3) Usually when slop~ 1 in double logarithmic plot of IV curve, dominant conduction mechanism is Ohmic.
Regards
Kamran is correct in his advices. First think that what you are measuring is difficult: ZnO of 300 nm is not a good conductor. So do not expect a fantastic curve as ITO or AsGa.
Try to be systematically: first use a well known conductor film reference, then modify the contacts (I use SnIn contacts for Au, while better in principle, has a very different work function from my material). Think if you can put the contacts before deposit the film so that later the point probes touch the contacts perfectly (it's only 300 nm). Care about what Van der Pauw says about the periphery position of the contacts. Using a solding station you can heat up the contacts and improve a diffusion with the ZnO film (but be careful not to evaporate locally the film itself!)
Normally a way to check the ohmic contact is perform a Voltage-Current curve, you should obtain a linear region (usually below 1 microAmps) in which you can have good statistics. Within that region plotting in log scale the resistivity, carrier and Hall effect.
Hall effect is very sensitive to carrier concentration as is the mobility, if your films are very conducting you will have no problems, but your material is hard to measure I guess. Think about doping the ZnO or increase the defects in order to improve the resistivity (if that is what you want).
Good luck!
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