Scanning electron acoustic microscopy of electric domains in ferroelectric materials
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Scanning electron acoustic microscopy of electric domains in ferroelectric materials MengLu Qian and XianMei Wu Shanghai Institute of Acoustics, Tongji University, Shanghai 200092, People’s Republic of China
QingRui Yin and BingYing Zhang Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, People’s Republic of China
John H. Cantrell National Aeronautics and Space Administration, Langley Research Center, Mail Stop 231, Hampton, Virginia 23681-2199 (Received 8 July 1998; accepted 29 March 1999)
Electric domains in single-crystal and polycrystalline barium titanate (BaTiO3) have been observed by use of scanning electron acoustic microscopy (SEAM). A model is presented of the SEAM signal generation, spatial resolution, and contrast mechanism associated with the imaging of electric domains in ferroelectric materials. The SEAM signal is found to depend directly on the sum of the piezoelectric coupling coefficient and spontaneous polarization of the domain, on the charge density of the electron beam interaction volume, and inversely on both the permittivity and the elastic constants of the material. Application of the model to BaTiO3 yields a contrast of roughly 3.5% from 90° domain structures and 6.8% from 180° domain structures.
I. INTRODUCTION
Ferroelectrics are piezoelectric materials characterized by the presence of spatial regions called electric domains in which the permanent electric dipoles in the unit cells of the ferroelectric crystals are aligned along certain allowed directions. The array of parallel dipoles in a domain gives rise to a spontaneous polarization (dipole moment per unit volume) Ps along that direction. The spontaneous polarizations in adjacent domains have different allowed orientations. The observation of electric domains is important to the study of ferroelectric properties. Optical microscopy,1 electron microscopy,2 x-ray photography,3 atomic force microscopy4 using noncontact mode imaging,5 friction mode imaging,6 and piezoresponse imaging7 have been used to image electric domains. The first three techniques require special materials processing, and the observations are generally limited to providing surface information about the specimen. Atomic force piezoresponse imaging and scanning electron acoustic microscopy (SEAM) have been shown to yield not only surface but also subsurface information.4,8 SEAM was developed independently by Cargill9 and by Brandis and Rosencwaig10 in 1980. In SEAM, the focused electron beam in a scanning electron microscope (SEM) is chopped or modulated at a frequency f, typically in the range 10 kHz to several 5 MHz. The periodic heating near the irradiated region of a nonpeizoelectric specimen generates both thermal and acoustic waves of 3096
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J. Mater. Res., Vol. 14, No. 7, Jul 1999 Downloaded: 01 Apr 2015
the same frequency. If a surface or subsurface microstructural feature in the specimen is within a diffusion length or so of the generated thermal wave,
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