I am trying to measure an I-V curve utilizing a conductive contact tip in AFM. The more I try the less it works properly. I am applying a DC voltage to the tip and I sweep it between -5V to + 5V but the I-V characteristic is completely noisy with any specific shape. my sample is a 100 nm Nickel film coated on the surface of the SiO2. My AFM machine is a NT-MDT, Next solver. It seems that tip does not touch the substrate at all since changing the applied voltage has no meaningful reflection on the curve. How can I realize that tip is in contact with substrate or not? Does turning off the feedback system guarantee landing of the tip on the surface?
Hi Hossain,
I did tapping mode AFM for surface roughness measurement. But I did not do any contact mode. But the principle of operation should be straight forward. With my limited experience I can tell you that, whether the tip is touching the sample surface or not can be identified by the force-curve. Another possibilities could be use of proper conductive tip. And last but not the least, you may have to check whether the tip is broken or not.
Good luck with your experiment.
Hi Hossein, it depends on whether you want to probe photoconductive samples. If yes, then it may indeed be a crucial point. See for example our paper: Ledinský, Martin, Antonín Fejfar, Aliaksei Vetushka, Jiří Stuchlík, Bohuslav Rezek, and Jan Kočka. “Local Photoconductivity of Microcrystalline Silicon Thin Films Measured by Conductive Atomic Force Microscopy.” Physica Status Solidi (RRL) - Rapid Research Letters 5, no. 10‐11 (November 1, 2011): 373–375. doi:10.1002/pssr.201105413.
Hi Hossein. When you apply a voltage to the tip (our sample) for your I-V curve, you add an extra force to your system: the cantilever to sample force which can be considered as a capacitor...of high capacitance. So if you want to maintain a controlled contact between your tip and sample, you must keep your feedback loop activated. Note that even in conventional I-V measurement using tungsten tip, the tip to sample force is a critical parameter. So you must keep this parameter constant. So you must reach the tip to sample contact acting on the setpoint and then keep this contact constant. Good luck. Didier Tonneau
Thanks for all the responses.
Dear Didier, your completely right about the additional force while applying the bias voltage. But, when the feedback system is on an I am applying a DC voltage the is no significant current while when I'm applying an AC voltage current suddenly builds up. It show that tip has not intimate contact with sample and voltage is applied through a capacitor to the substrate (as you mentioned). This is the reason I asked about turning off the feedback system while measuring I-V characteristics of the sample. Do you still believe feedback system should be on?
Cheers,
Hossein
Hi Hossein
I just realized you worked on Ni thin films. The problem is the Ni native oxide NiO which has electronic properties which vary.... What I suggest tou you:
- for all electric measurement definitly you must keep on your feedback loop activated
- try to train on other smarter layer. A gold layer on SiO2 for example (do not forget a Ti or Cr flash before depositing Au). Test your electrical set-up on this sample. Try to vary the AFM tip force (via the setpoint) and look at the current variations with the force.
- then switch to your Ni sample. As far as I remember, you can change locally the conductivity of NiO under electric field. NiO, predicted as metallic by bandstructure theory, is in fact insulating. However, applying an electric field, you can switch to a conductive state. You must work in DC mode. You must first apply a high local voltage to the tip (I do not remember if it must be + or -, please try both). Example +10V (or -10V). According to your initial question +5V is probably too low and you do not switch to a conductive NiO state. . Then apply your ramp voltages 0/+5V (or 0/-5V), but never change the bias sign during your I-V curve. If you are in the right bias sign, NiO will be condcutive.
But do not worry, the low current signal is probably due to NiO native oxyde layer with special electronic properties. You will find easilly NiO properties in litterature. I guess you use AFM tip metallized with a platinum coating. Look at the properties of NiO/water/Pt structures... Good luck. Didier
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