Growth process approaches for improved properties of tunable ferroelectric thin films

Two approaches are shown for property control of tunable BaxSr1−xTiO3 (BST) films: (i) two-step growth and (ii) self-selective epitaxial growth. Both processes are based on the deposition of an amorphous ultra thin BST layer prior to the growth of an epitaxial BST film. In the first approach, the mechanical strain in the epitaxial films is controlled by employing a two-step growth process, where the very thin amorphous layer deposited first acts as an absorption layer for the misfit strain through its crystallization. Two-step grown films on LaAlO3 substrates showed effective relaxation of the compressive misfit strain, which resulted in higher in-plane permittivity and tunability of the film. In the second approach, self-selective epitaxial growth, epitaxial/amorphous BST composite structures are achieved where the two phases are electrically inter-connected in parallel. Having a low permittivity, the amorphous component efficiently suppressed the effective permittivity of the composite capacitor, while the epitaxial component helps keep the tunability as large as that of a purely epitaxial BST film. The resultant composite film demonstrated a low permittivity having a high tunability, which is the contradictory phenomenon for a pure tunable ferroelectric.