Phase Transitions in Free-Standing (K,Na)NbO3 Membranes

Ferroelectric perovskite oxides with the chemical formula ABO3 typically pass through a series of structural phase transitions upon cooling from a paraelectric, cubic high temperature phase to various ferroelectric phases with lower crystal symmetry. However, phase transitions in ferroelectric thin films could exhibit significantly different characteristics than their corresponding bulk material depending on effects like boundary conditions, (hetero-) epitaxial strain or growth-related defects. Phase formation and phase transitions can be specifically tuned by strain and defect engineering, although this is restricted to specific film-substrate combinations. In contrast, free-standing membranes which are released from a stiff substrate provide higher flexibility. This is of particular interest because in perovskite oxides the functional properties (e.g. dielectric permittivity or piezoelectric coefficients) are often strongly enhanced in the vicinity of the phase transitions.<br/>In this contribution, we have investigated the phase transitions of heteroepitaxially grown films and free-standing (K,Na)NbO3 membranes. (K,Na)NbO3 is regarded as one of the most promising lead-free piezoelectric materials in respect to its electromechanical, electrooptical and piezoelectric properties. In order to gain fundamental understanding of phase formation, stability and transitions, films with high structural quality and chemical homogeneity are essential. For that purpose, we have applied metal-organic vapor phase epitaxy (MOVPE) as an ideal growth method since it works nearby thermodynamic equilibrium and provides well-ordered films with smooth surfaces and interfaces. Furthermore, it enables the growth of intentionally stoichiometric as well as off-stoichiometric films. For the preparation of (K,Na)NbO3 free-standing membranes (K,Na)NbO3 films have been grown on rare-earth scandate substrates with SrRuO3 as intermediate layer serving as sacrificial layer for the subsequent detachment process. Significant differences in phase transition behavior have been observed for free-standing and epitaxially strained films.