Abstract:

CaBi₂Nb₂O₉ (CBN) thin films with a bismuth layered structure are important precursors for ferroelectric memories, due to their high fatigue resistance, and good ferroelectric properties. The epitaxial growth of CBN along the a-axis is desirable for the integration and application. However, the structural anisotropic makes it challenging to regulate the polarization with respect to the crystallography. In this paper, a new strategy is proposed for the growth of a-axis oriented CBN thin films on MgO(100) single crystal substrate. This strategy is realized by changing the deposition temperature using the pulsed laser deposition technique. The CBN thin films are grown along (115)-, (200)-, and (00l)-crystallographic direction, when the films are deposited at 500, 600 and 700 ℃, respectively. As the deposition temperature increases, the CBN films undergo (115)-(200)-(00l) orientation transition. Meanwhile, SEM results show that 600 ℃ is the optimal deposition temperature for high-quality a-axis epitaxial growth of CBN films on MgO substrates. Under this condition, the films are well bonded to the substrate with a low roughness. HRXRD and HRTEM analyses show that the (200)-oriented CBN films are heterogeneous epitaxial growth, forming a semiconformal lattice interface between the (200)-oriented CBN film and the MgO substrate, also revealing a crystallographic relationship between the as-grown thin film and the substrate, i.e., (100)[001]CBN//(100)[001]MgO. The average (200) spacing at the CBN/MgO interface was measured to be 0.5312 nm. Based on the lattice-matching relationship, a theoretical model is proposed that four CBN unit cells matching five MgO unit cells. Moreover, the nanodomain structure of the (115)-oriented CBN thin films and the out-of-plane polarization switching of the (200)-oriented CBN thin films are demonstrated by PFM.

Key words: epitaxial film, growth mechanism, orientation regulation, calcium bismuth niobate