Orienting high Curie point CaBi2Nb2O9 ferroelectric films on Si at 500 C

Abstract

With the highest Curie temperature in all known ferroelectrics and a lead-free composition, CaBi2Nb2O9 (CBNO) has a good potential in electromechanical applications demanding a good temperature stability and environmental friendliness. However, due to its highly anisotropic crystalline lattice (a = 5.435 Å, b = 5.466 Å, c = 24.970 Å, Pa = Pb, Pc ≈ 0), a/b-axis oriented CaBi2Nb2O9 films with excellent ferroelectric and piezoelectric properties are difficult to grow on Si substrates (a ≈5.431 Å) at 500 °C or lower, temperatures compatible with the existing CMOS-Si technology. It is also difficult to grow them a/b-axis oriented on those commonly used semiconductor substrates (such as SrTiO3 and MgO), whose lattice parameters well match with the diagonal length of the CBNO (00l)-plane (a ≈4 Å ≈ aCBNO/√2). In this work, CaBi2Nb2O9 thin films with a SrRuO3 (a ≈3.96 Å) buffer layer, which promoted the growth of grains tilting away from the weakly polarized c-axis, were successfully prepared on platinum-coated (100)Si substrates at 500 °C via a radio-frequency magnetron sputtering process. These films exhibited a (117) texture with enhanced electrical properties, including a high dielectric constant (ɛr ∼ 480 @ 1 kHz), large electric polarizations (Pr ∼ 12 μC/cm2, Ps ∼ 63 μC/cm2 @ Emax = 1800 kV/cm), and a good transverse piezoelectric coefficient e31,f (∼1.25 C/m2). As a consequence of its high Curie temperature (>600 °C), the dielectric constant of the CBNO film showed a slow declination with temperature (∼0.1%/oC before 300 °C, and less than 0.03%/oC from 300 °C to 600 °C). Its dielectric loss is less than 5% at temperatures lower than 200 °C under a low working frequency (5 kHz/10 kHz), and it stays lower than 5% up to 500 °C when a high working frequency (500 kHz/1 MHz) is used. These enhanced electrical properties will greatly promote applications of CBNO thin films in lead-free ferroelectric devices, especially in those demanding a good high temperature stability.