Giant Strain and Induced Ferroelectricity in Amorphous BaTiO₃ Films under Poling

We report an effect of giant surface modification of a 5.6 nm thick BaTiO₃ film grown on Si (100) substrate under poling by conductive tip of a scanning probe microscope (SPM). The surface can be locally elevated by about 9 nm under -20 V applied during scanning, resulting in the maximum strain of 160%. The threshold voltage for the surface modification is about 12 V. The modified topography is stable enough with time and slowly decays after poling with the rate ~0.02 nm/min. Strong vertical piezoresponse after poling is observed, too. Combined measurements by SPM and piezoresponse force microscopy (PFM) prove that the poled material develops high ferroelectric polarization that cannot be switched back even under an oppositely oriented electric field. The topography modification is hypothesized to be due to a strong Joule heating and concomitant interface reaction between underlying Si and BaTiO₃. The top layer is supposed to become ferroelectric as a result of local crystallization of amorphous BaTiO₃. This work opens up new possibilities to form nanoscale ferroelectric structures useful for various applications.