Li diffusion and surface segregation in K0.5Na0.5NbO3 films grown by Pulsed Laser Deposition

Potassium Sodium Niobate has recently raised particular attention in the field of piezoelectric perovskites thanks to its remarkable piezoelectric properties and high Curie temperature. Previous works have shown that slight Lithium (Li) doping can improve the piezoelectric properties and reduce leakage of stoichiometric K0.5Na0.5NbO3 (KNN). In this paper, evidence is provided that this is not the case for thin films synthesized by Pulsed Laser Deposition, at growth temperature high enough to induce the (001) orientation on a Pt(111) /TiO2/SiO2/Si template. The high mobility of alkali ions leads to Li surface segregation and high non-uniformity of Li doping over the film thickness. This is accompanied by a degradation of KNN thin film electrical properties upon Li doping, with an increase in leakage currents by up to one order of magnitude which impedes a macroscopic piezoelectric characterization. On the other hand, local piezoelectric properties investigated by Piezoresponse Force Microscopy are unchanged upon doping. The results clearly point out the critical issues regarding the growth method to obtain a uniform doping of KNN with light elements such as Li. Due to the high mobility of this species, sizable migration phenomena can be thermally activated in physical vapor deposition processes with high thermal budget, ultimately hampering the envisaged benefits of Li on piezoelectric response.