CT-AFM is a significant departure from nearly 30 years of scanning probe microscopy. Instead of interrogating surface properties only, up to hundreds of images are acquired for a single location with intervening material removal. In this manner, resolution equivalent to conventional AFM methods is achieved laterally, and uniquely also into the depth of a specimen. Two examples are discussed, demonstrating nanoscale tomography for polycrystalline CdTe solar cells as well as BiFeO3 multiferroic thin films.
The novel volumetric maps of photovoltaic performance reveal order-of-magnitude enhancements in the short circuit current for distinct microstructural features such as grains and grain boundaries. These results are effectively independent of grain orientation, and instead mediated by percolation pathways for electrons along grain boundaries and holes along photoactive planar defects. The discovery of such orthogonal channels for carrier transport may provide opportunities to dramatically improve device efficiency and reliability.
With multiferroics, ferroelectric domain contrast is detected down to a critical thickness of less than 10 nm based on the local piezoresponse as a function of milled depth. The 3-dimensionally resolved domain walls also reveal both anticipated as well as unexpected feature geometries and properties. Switching studies as a function of film thickness are especially revealing for spatial and energy scaling investigations with respect to ever-diminishing future device dimensions and power budgets.
CTAFM literally provides a new perspective on nanoscale materials properties throughout the thickness of materials devices. The resulting insight suggests new pathways to improve design, efficiency, and reliability for 3-dimensionally engineered materials systems.