Conductive AFM Imaging Using the MFP-3D™ AFM
Conductive AFM is a powerful current sensing technique for electrical characterization of conductivity variations in resistive samples. It allows current measurements in the range of hundreds of femtoamps to ten microamps. Conductive AFM can simultaneously map the topography and current distribution of a sample. It is a measurement useful in a wide variety of material characterization applications including thin dielectric films, ferroelectric films, nanotubes, conductive polymers, and others.
How It Works
Figure 2 shows an example image made at a 1.5 volt bias. The sample is a 10nm thick film of Europium-doped ZnO. This is a relatively high resistivity sample, particularly challenging for conductive AFM measurements. The contact mode topographic image “A” shows a relatively uniform grainy structure. The current image “B”, however, shows patches of high conductivity surrounded by very low conductivity regions (see Figure 3 for a higher resolution scan). The NPS™ nanopositioning closed loop sensors on the MFP-3D make it possible to reproducibly position the cantilever at a point of interest as shown by the colored circles in Figure 2B. The tip was positioned in the center of the colored circles using the MFP-3D’s “pick a point” force curve interface. The bias voltage was then swept from -5 to 5 volts and the response current measured. Figure 2C shows the resulting current-voltage (IV) curves. The conductivity curves are consistent with the contrast observed in Figure 2B. Specifically, the conductivity is highest at the position marked with the black circle, in between at the red, and lowest at the blue.
Current as a Function of Loading Force
Combined Force and Current Measurements
ARC2, MFP-3D, NPS and ORCA are trademarks of Asylum Research. Specifications subject to change without notice.
ORCA Sample Mount
Figure 1: Gain selection chart. Johnson noise and the relevant current ranges for a transimpedance amplifier digitized at 16 bits. At a gain of nearly 1010 volts/amp, Johnson noise is equivalent to the best resolution of a 16-bit ADC. At smaller gains, the main limitation is the resolution of the ADC. At higher gains, Johnson noise dominates.
Figure 2: Topography (top left) and current (top right) image of a Europium-doped ZnO sample at a bias of 1.5 volts, 2µm scan sample courtesy of the Krishnan Lab, Univ. of Washington. Corresponding IV curves (bottom) recorded at three specific positions indicated in B. The curves are consistent with the current contrast observed in 2B. Specifically, the conductance is highest at the black location, medium at the red and lowest at the blue.
Figure 3: High resolution topography (l) and current (r) at a bias of 1.5 volts, 50pA scale, 2µm scan. Sample courtesy K. Krishnan Lab, University of Washington.
Figure 4: IV curves as a function of loading force on a Europium doped ZnO sample. Each curve was recorded at the same X-Y position on the sample as a function of cantilever load.
Figure 5: Current (above) and force (below) as the cantilever extends towards the ZnO sample surface and retracts away. The blue curves show the positively biased response while the red shows the negatively biased response.