Physics Theses

Atomic force microscopy (AFM) based techniques have been used widely to study functional properties of ferroelectrics. In this thesis, we investigated the application of AFM tips loaded with different stress ranges (from ~nN to ~µN) on ferroelectric thin films at the nanoscale. AFM tips applied with tens of nN force were used for careful characterization of ferroelectric switching on an inhomogeneous Pb(Zr,Ti)O3 (PZT) thin film. Different switching loops and domains were obtained after performing band excitation piezoelectric spectroscopy (BEPS) on the film at adjacent nanoscale areas. By combining BEPS, piezoresponse force microscopy (PFM), transmission electron microscope (TEM) and machine learning, abnormal loops were clustered and proved to be induced from various mechanisms including electrostatic, ferroelastic, and charge injection, which was mediated by defect-led microstructural variations. Applied stress in a large force range up to ~1 µN on ferroelectric BiFeO3 (BFO) thin films showed significantly enhanced injection currents, much larger than typical switching currents, induced by polarization switching via conductive atomic force microscopy. This injected current can be effectively modulated by applying mechanical force. As the loading force increases from tens of nN to hundreds of nN, the magnitude of the injected current increases and the critical voltage to trigger the current injection decreases. Notably, changing the loading force by an order of magnitude increases the peak current by several orders of magnitude. The mechanically boosted injected current could be useful for the development of high-density FeRAM devices. The mechanical modulation of injected current may be attributed to the mechanical force-induced changes in barrier height and width of the interfacial layer. AFM tips with these forces can characterize or modulate ferroelectric related properties. We therefore expected that larger force (tens of µN) can further remove ferroelectric materials for nanomechanical machining as ferroelectrics are important candidate materials for a wide range of applications including data storage and actuators. AFM-based machining of ferroelectric nanostructures offers advantages such as low damage and low-cost modification for already-fabricated thin films. Through a systematic investigation of a broad range of AFM parameters, we demonstrate that AFM-based machining provides a low-cost option to rapidly modify local regions of the BFO thin film, as well as fabricate a range of different nanostructures, including a nanocapacitor array with individually addressable ferroelectric elements.