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ELSHIN, A.; MISHINA, E.. Laser-induced crystallization in ferroelectric thin films. In: Materials Science and Condensed Matter Physics. Ediția a 9-a, 25-28 septembrie 2018, Chișinău. Chișinău, Republica Moldova: Institutul de Fizică Aplicată, 2018, p. 102.
|Materials Science and Condensed Matter Physics
Ediția a 9-a, 2018
Conferința "International Conference on Materials Science and Condensed Matter Physics" |
Chișinău, Moldova, 25-28 septembrie 2018
We investigate method of ferroelectric microstructure formation by femtosecond laser annealing. High-repetition-rate femtosecond (fs) pulsed lasers can induce local heating and crystallization of nonlinear optic phases inside bulk material, offering a means to introduce ferroelectric properties with three dimensional (3D) space selectivity . Advantage of such method is locality of heat treatment so it does not affect surrounding areas. Also due to threshold crystallization conditions it is possible to obtain sub-diffraction-limit structures by laser heating. In our research we use non-annealed quazi-amorphous Pb(Zr,Ti)O3 (PZT) precursor film deposited by RF magnetron sputtering on a platinized silicon substrate. Annealing was performed by femtosecond laser pulses at the wavelength falling into transparency band of PZT (800 nm) thus power was absorbed by platinum sublayer. We show that crystal growth started at the PZT surface instead of PZT/Pt interface. This counterintuitive phenomenon can occur due to thermal stress-induced modification of the crystallization activation energy . The heat propagates very fast and the temperature of both the top and the bottom interfaces are almost equal due to small thickness of the layer. At the same time, the film at the bottom interface undergoes high strain due to the difference in thermal expansion of Pt and PZT layers. The strain increase results in an increase of crystallization activation energy which suppresses crystallization at the bottom interface. This can be described in terms of theoretical model of explosive solid phase nucleation used in  if we take into account strain influence on activation energy value. Our theoretical description falls in a good agreement with experimental observations. One can obtain different types of microstructures: local circle structures, local ring structures and planar waveguides. Latter is possible due to refraction index of annealed area is higher than of surrounding non-annealed. Such waveguides can be used for short distance light transmission between planar optical elements. Most important research methods are second harmonic generation (SHG) microscopy and also transmission electron microscopy (TEM). Ex-situ SHG images allowed to distinguish ferroelectric phase and thus to find optimal conditions of processes (power density, annealing time for local structures and scan speed for waveguides). Cross-sections of annealed structures for TEM investigations were obtained by focused ion beam. Dependence of properties of obtained microstructures, such as their shape, width, homogeneity, nonlinear response on the conditions of annealing are discussed. Method of two-stage annealing process for annealing temperature decreasing and nonlinear optical respond increasing is offered.