Domain structure and phase transitions in epitaxial KNbO3 thin films studied by in situ second harmonic generation measurements

Epitaxial thin films of KNbO3 were deposited on SrTiO3 (100) substrate by laser ablation. In the orthorhombic phase, the four possible domain variants in the KNbO3 (110) film growth plane were determined to be KNbO3[1(1) over bar0$]parallel to SrTiO3[001], [00(1) over bar], [010], and [0(1) over bar0$] denoted as X+, X-, Y+, and Y-, respectively. Using a fundamental beam of 1064 nm transmitted normal to the film, the second harmonic generation (SHG) signal at 532 nm was measured and correlated to the area fractions A(X+), A(X-), A(Y+), and A(Y-) Of the four domain variants X+, X-, Y+, and Y-, respectively, in the growth plane of the film. At room temperature, the area fractions delta A(x)=A(X+)-A(X-) and delta A(y)=A(Y+)-A(Y-) were-determined to be similar to 3.3% and similar to 2.2%, respectively. In-situ SHG measurements revealed the phase transitions to be 210+/-10 degrees C for orthorhombictetragonal and 450+/-10 degrees C for tetragonal-cubic transitions. In the tetragonal phase (between 210 degrees C and 450 degrees C) the KNbO3 [100]parallel to SrTiO3 [100]. The Use of SHG as a sensitive, non-destructive and real-time probe of phase transitions and evolution of domains in ferroelectric thin films is demonstrated. (C) 1996 American Institute of Physics.