LaAlO3 ultrathin films, 10 unit cells in thickness, have been deposited epitaxially on TiO2-terminated (001) SrTiO3 substrates with various O2 pressures. Electromechanical response from the LaAlO3/SrTiO3 heterostructures is studied using combined piezoresponse force microscopy, electrostatic force microscopy, and scanning Kelvin probe microscopy. Oxygen vacancies are found to be responsible for the observed piezoelectric response but only for samples deposited with an oxygen pressure lower than 10(-5) mbar. However, ambient humidity is demonstrated to have a significant effect on the electromechanical response. The observations are discussed in terms of modulations on the intrinsic electrostriction in LAO/STO by an electric field induced by nonuniform distribution of either oxygen vacancies in the bulk or ionic adsorbates on the surface of LAO.
Keywords: electromechanical response; piezoresponse force microscopy; surface adsorbate; surface and interface; two-dimentional electron gas.