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Quantitative Electrical Measurements


Figure 3 : Image of a soft polymer that compares oscillating and contact mode imaging to Soft ResiScope mode. The area defi ned by the blue square was imaged using contact mode. The red square outlines the region imaged using Soft ResiScope and compares well to the area outlined in green for oscillating mode imaging. Final image width = 50 µm.


corresponds to data taken at the same locations in the standard ResiScope mode. Figure 3 illustrates the diff erences between contact, intermittent, and Soſt ResiScope modes for a compliant polymer sample. A small area of the sample was fi rst imaged in contact mode. T en a larger area was imaged, initially in oscillating mode and switched to Soſt ResiScope mode for imaging the remainder of the scan area. Damage from the original contact mode is evident, but no damage and no image diff erences are observed between the oscillating and Soſt ResiScope modes.


Results


Organic solar cells . New research developments seeking inexpensive renewable energy sources oſt en employ polymer materials [ 5 ]. Electrical characterization of conjugated polymer- based organic solar cells made from poly(3-hexylthiophene), referred to as P3HT, is of interest for this application. Conductive AFM would damage the organic polymer. Because these samples are soſt , the Soſt ResiScope mode was used to obtain resistance data for these samples. Figure 4 shows topography and resistance data for a P3HT organic solar cell. T ere is no AFM imaging-induced damage to the sample. T e resistance image indicates variation across the polymer sample and is a great demonstration of the sensitivity of the Soſt ResiScope mode. T e resistance image shows the conductivity across the P3HT layer and is independent of topography. Understanding diff erences in resistance and polymer distribution may help to improve solar cell effi ciency. T e images in Figure 4 are also an excellent illustration of why there are diff erent AFM imaging modes. For soſt samples, contact mode imaging is not possible without damaging the surface. Topography imaging alone does not always provide the answers that researchers need. Further, topography data oſt en does not correlate to the material properties. For these reasons, advanced imaging and Soſt ResiScope modes have been developed to provide quantitative data on a variety of surfaces. Semiconductor SRAM . Static RAM for the semiconductor industry is commonly used for memory. T e p-n junctions of the SRAM are clearly defi ned by resistance measurements as shown in Figure 5 . T e color scale for the resistance image (right) indicates that resistance in this sample ranges from


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Figure 4 : Topography (a) and resistance (b) data using Soft ResiScope mode for an organic solar cell sample of poly(3-hexylthiophene). Image width = 3 µm.


10 6 to 10 12 ohms. T ese images were acquired using a doped diamond probe with a sample bias of -1.0 V. T e regions in yellow and green have lower resistivity and correspond to the n-regions that are rich in electrons and have greater conduc- tivity. T e p-regions bordering n-regions have higher resistance, as indicated by the discrete blue regions in the resistance image. Multiple imaging modes . An advantage of using the ResiScope module with the Nano-Observer AFM is the ability to obtain data from multiple modes at the same sample


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