I am measuring the mobility of organic single crystal semiconductor by using Hall effect technique. But at every time there is a drastic change in mobility values for the same crystal.
Is there any reproduceable or systematic shift in the mobility observable?
How does the carrier density change?
Is your sample well within the setups specifications? (mainly the resistivity of your crystal)
Do you always have the same conditions?
Same illumination level, temperature, air humidity,...
The layers may also degrade with time due to thermal degradation, oxidation, light induced degradation,...
Another thing in Hall measurements, usually ruled out during the experiment by four point measurements and cyclic permutations are the ohmic contacts.
Please also have a look to:
http://www.nist.gov/pml/div683/hall_intro.cfm
Best regards!
Dear Arne Ludwig,
The crystal resistance is very high nearly 12 ohm. the crystal shape is well suitable for setup specifications. silver paste is used as a ohmic contact. the mobility valve shows 1cm/vs after few seconds 50cm/vs there is no change in the current and magnetic field. the current also few na=ano amp only
Hall mobility is a good parameter to describe the quality of your material.
I think ambient conditions are affecting your measurements. I will suggest you to do measurements at regular intervals of time with the same sample to verify this. Also, the mobility of organic semiconductors is around 1-10. 50 seems to be a high value to me.
Dear Satender Kataria
i agree your suggestion, in some times i got mobility of 1Cm/VS and some times 2 Cm/Vs... and its vary some times higher ... thats why i confused... can i choose the value which is related to earlier reports.
Test lenearity and stability of contact resistance.
Use amplifier with high input resistance.
For voltage leads, instrumentation amplifier INA111BP ( with input bias current ~ 1 pA) could be used.
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