Ferroelectric domain study by piezoresponse force microscopy

Abstract

Ferroelectric domains in ferroelectric materials are micro-regions with the same oriented spontaneous polarizations. The ferroelectric domain structure and its evolution play a very important role for many applications of ferroelectrics such as non-volatile ferroelectric random-access-memory elements. A deep understanding of the nature of domain structures and switching behaviors in ferroelectric materials may lead to more applications of ferroelectrics. Among many methods of studying ferroelectric domains, piezoresponse force microscopy (PFM) is receiving more attention due to its relatively high resolution and ability of in-situ domain switching. The PFM is an extended function of atomic force microscope utilizing a conductive tip with tip radius in nanometer scale and a lock-in amplifier, therefore nanometer-sized domain imaging and polarization switching can be achieved through tip scanning across the sample surface with an ac and/or dc electric field applied on the tip. In this project, ferroelectric domain evolution in Pb(Zr0.4Ti0.6)O3 (PZT40/60) film and (PbMg1/3Nb2/3O3)0.75(PbTiO3)0.25(PMN-25PT) single crystals were studied utilizing the PFM technique. PZT4O/60 films were deposited on Pt(111)/Ti/SiO2/Si(l00) substrates by sol-gel process followed by thermal annealing at 650 C for 5 minutes. PFM observation revealed lamellar domain structure in the PZT40/60 grains and we attribute the lamellar domains to 90o ferroelectric domains. Polarization switching mechanism of the 90o domains in the PZT40/60 films under external electric fields has also been studied and it revealed that a large area polarization switching is usually accompanied by appearance of new direction of 90o domains in order to reduce the stress in the grains. By contrast, a nanometer-sized polarization switching is believed to be accomplished by generating 180o switching within a single lamellar domain. (PbMg1/3Nb2/3O3)1-x(PbTiO3)x(PMN-PT) single crystals are known as relaxor ferroelectrics, which are different from normal ferroelectrics such as PZT films. The relaxor ferroelectricity can be interpreted by composition non-uniformity induced polar nanometer-sized regions (PNRs) with different Curie temperatures (Tc) and their conversions to paraelectrics. The PMN-PT single crystals naturally have a morphotropic phase boundary (MPB) in the range of 28-36% of PT. Ferroelectric domain structures and their evolution in PMN-PT single crystals of 25% PT (PMN-25PT) grown by Bridgman technique with (110)-cut and (111)-cut were studied by means of temperature dependent PFM. It revealed that, during heating, the as-grown PMN-25PT single crystal exhibits transition from microdomain ferroelectric phase to paraelectric phase; while after being cooled down to room temperature the microdomain structure is rebuilt. Nanometer-sized domains were also observed embedded in the microdomains. By contrast, the poled (110)-cut sample exhibits transitions from macrodomain to microdomain structures at 90 C and from microdomain to paraelectric phase at 115 C, respectively. These direct observations are consistent with temperature-dependent relative permittivity measurements. In conclusion, PFM has been implemented in the study of ferroelectric domain structures and their evolution and some significant results have been achieved. However, due to the limited resolution of PFM (larger than 10 nm), more detailed domain structures are still not clear. The relatively small size of scanned area in PFM also limits the view area of macrodomain structure. Therefore, a whole picture of domain structure from micrometer to nanometer scale is highly desirable. In addition, PFM study of relaxor ferroelectric phase transitions, such as from rhombohedral to tetragonal and monoclinic structures deserves further studies.