Sheet resitance with four-point probes

[aapirzado]

Hi All,
I am measuring the sheet resistance of few layer graphene (FLG) film grown on 2x2 cm glass substrate. I have deposited four gold electrodes on top of FLG film. After injecting the range of currents from 1*10e-5 to 1*10-1 A, I record the Voltage for each value of injected current. With my results, I draw the V/I*F graph with F is a geometric factor=4.53 and findout that Sheet resistance is linear for one part (1*10e-5 to 1*10-3 A) and is non-linear for the other part (2*10e-3 to 1*10-1 A) where the V measured starts to decrease gradually, implying that Sheet resistance is decreasing. These measurements are repeatable/reproducible as well.Image attached.

Why I get this behaviour?
Is it due to high current/temp?
or there is any other reason?

Thanks in anticpation!
Pirzado

 

[dick_freebird]

A rectifying contact will act that way on a 2-point
measurement. Depending on what you have for SMU
impedance, you might see some of that in a 4-point
(if the Vsense is not lossless).

Contacts could be changing their electrical properties
in short-time due to charging or contaminant migration
under field / current stress. Repeating the curve-pull
would show you about that.

Thermal behavior is also a possibility, you could have
a negative TCR in the film. Measure the same force
current and a couple of baseplate temps, and see.

 

[chuckey]

Carbon has a negative temperature coefficient, so this could be an indicator of the negative slope of the resistance. Can you re-run the experiment and give you graphene sample a good blast of freezer spray while it is conducting the higher current, should cool it down and the resistance should increase. Or use a pulsing technique to keep the mean power down.
Frank

 

[jeffrey samuel]

Hi All,
I am measuring the sheet resistance of few layer graphene (FLG) film grown on 2x2 cm glass substrate. I have deposited four gold electrodes on top of FLG film. After injecting the range of currents from 1*10e-5 to 1*10-1 A, I record the Voltage for each value of injected current. With my results, I draw the V/I*F graph with F is a geometric factor=4.53 and findout that Sheet resistance is linear for one part (1*10e-5 to 1*10-3 A) and is non-linear for the other part (2*10e-3 to 1*10-1 A) where the V measured starts to decrease gradually, implying that Sheet resistance is decreasing. These measurements are repeatable/reproducible as well.Image attached.

Why I get this behaviour?
Is it due to high current/temp?
or there is any other reason?

Thanks in anticpation!
Pirzado

I have limited Idea in this concept to help you out pal

But I would suggest you to upload images in jpeg format rather than in bmp format as it takes a longer time for the photo to load in slower connections

 

[aapirzado]

Thanks Dick_Freebird and chuckey! your responses helped me. I am trying to measure the temp dependece of resistance to verify this. However, up to now, these measurements are repeatable. I get more or less same pattern and values or resistance when I redo the measurement.

I will post jpeg image(s) in future if need be! Thanks Jeffrey Samuel!
Pirzado

 

[FvM]

I think, the presentation of results is at least suboptimal. A relation, that is expected linear should be plotted in double linear scale, or possible double log, but not with arbitrary varying scale as in your figure.

 

[aapirzado]

yeah, you're rite! That's what i usually do but yesterday, origin was not happy with me so i had little priblem with the scales! and i chose this way to fix it for the moment!