“So, for example, if you have a thin metal layer on top of the sample and a piezoelectric material underneath, you can see what’s happening to the piezo material through the metal sheet,” Kabos says.
It may seem counterintuitive that wavelengths on the order of a centimeter down to a few millimeters can provide nanometer-scale resolution. “It’s something of a paradox,” Kabos says, “until you look at the dimensions involved. The sharp AFM tip is only about 100 nanometers wide, and it is positioned only a few nanometers from the surface or is in a soft contact with the sample. It is the tip elevation that governs the resolution. The tip distance is so much smaller than the signal wavelength that ‘near-field’ effects predominate, permitting resolution in nanometers and highly accurate, quantitative measurements.”
The researchers can alter both the tip voltage and the microwave frequency to examine different aspects of the sample. For example, in one 2012 paper, Atif Imtiaz and PML colleagues, along with researchers at instrument-maker Agilent Technologies, Inc., showed how changing the tip bias reveals the local carrier density in a semiconductor, and hence the dopant profile of a region. “Depending on the sign of the applied bias to the tip, the charge carriers in the semiconductor are attracted or depleted from the surface, forming a space charge region,” the authors write.
Because the width of that region is also a function of the carrier density, it provides a sensitive measure of dopant composition. “Very recently,” Kabos says, “we have been able to look into a 3µm wide area and see how the doping is distributed at the P-N junction, as well as locate the junction precisely.”
Alternatively, changing the microwave frequency reveals other properties. “The same material may look very different at 5, 7, or 18 GHz,” Kabos says. “So we can not only determine the doping profile locally, but also look at performance for a given frequency in a given region.”
The new device, slated for a fall arrival, will provide many new capabilities. The existing instruments have a single tip that is open to the air. The new NSMM has four tips, permitting simultaneous comparisons of materials, and it is enclosed in an ultra-high vacuum chamber to minimize signal interference and sample contamination. It also features a vacuum sample-preparation chamber and temperature control from about 30 K to room temperature.
Among other uses, the new instrument will allow the researchers to explore in much greater detail the properties that affect the suitability of various materials, operating at microwave frequencies, for use in nanoscale electronics and bioelectronics, as well as emerging technologies for spintronic or phasetronic applications in single- and multi-layer configurations.
Background: PML “homemade” NSMM, shown with tip poised above the surface of a 2 cm wide sample. © NIST Cover: Schematic diagram of NSMM apparatus. © NIST
http://www.nist.gov/
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