Thickness dependence of piezoelectric property of ultrathin BiFeO3 films

Effects of film thickness on the piezoelectric property of the ultrathin BiFeO3 films (t=1-150 nm), grown on the (0 0 1)-SrTiO3 substrates with a La0.67Sr0.33MnO3 cover layer, have been studied by the technique of piezoresponse force microscope. Special attention has been paid to the evolution of the piezoelectric response with film thickness. Well ferroelectric property maintains in the BFO film with a thickness down to 6 nm, below which no obvious ferroelectric domains are observed. Based on careful analysis of the piezoelectric response images of the ferroelectric domains upwards or downwards poled by external field, a quantitative description of the piezoelectric coefficient d33 can be obtained. d33 is found to display a monotonic decrease with the decrease of film thickness, descending from ∼46 pm/V for t=150 nm to ∼8 pm/V for t=6 nm. A corresponding growth of the out-of-plane lattice parameter of the film from 4.011 Å to 4.077 Å is also observed. The effects of depolarization, lattice strains and substrate clamping on the piezoelectric property of the films are discussed.