Ionic migrations during poling process in lanthanum aluminate investigated by time of flight-secondary ions mass spectrometry and piezoresponse force microscopy combined methodology

Time of flight-secondary ion mass spectrometry (ToF-SIMS) has been used to obtain the composition in depth of a few nanometers thick layer of lanthanum aluminate prepared by molecular beam epitaxy. The electrical properties of the films were probed by piezoresponse force microscopy (PFM) and Kelvin Force Microscopy (KFM). From PFM images, the sample under study behaves exactly as if it was ferroelectric although it is amorphous: this is a major artefact of this technique. This effect has often been related to the migration of oxygen vacancies within the sample, which modifies the molar volume under the AFM tip. ToF-SIMS results show that this effect may rather be due to the migration of lanthanum instead of oxygen vacancies. The authors also show that scanning with the tip of an atomic force microscope induces some chemical modifications at the surface. This combined methodology (ToF-SIMS, PFM, and KFM) allows one to better understand the contrast mechanisms at play during scanning probe experiments, especially when high electric fields and ionic migrations are involved.