Nanometer scale domain inversion and application to new devices

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The crystal of ferroelectric materials such as LiNbO_3 and LiTaO_3 have large spontaneous polarization and 180ﾟ domain structure at room temperature. Development of new opto-electronic devices such as ultra high capacity memory would be possible by realization of the nanometer scale domain inversion.In this study, basic research of the nanometer scale domain inversion by an irradiation of electron beam to LiTaO_3 single crystal. High capacity memory is also investigated as an application. An irradiation to the area of smaller than 2 mum square by modified electron beam patterning equipment produces an inversion domain as large as 140 mumphi. Theoretical analysis reveal (1) an effect of coulomb Interaction is very strong, and charge distribution is estimated to be a trigonal pyramid with constant vertical angle, named as "sand glass" model. (2) There are two kinds of threshold value, the threshold of domain nucleation and the threshold of domain expansion. We have succeeded to form an inversion domain as small as 670 nm phi in the crystal of 500mum thick. A new method for imaging the inversion domains, named coherent detection, is also proposed based on the piezoelectric effect. Non-destructive and direct observation of the domains is achieved. A measurement of P-E hysteresis curve reveals (1) Ps is as large as *51muC/cm^2, and the curve is ideal square shape. (2) The threshold values are 204kV/cm and 120kV/cm for threshold of domain nucleation and threshold of domain expansion, respectively. Therefore domain inversion of LiTaO_3 single crystal is suitable for memory application. The results shown above would be informative for establishing the nanometer scale domain inversion technique and the device application.