Stress induced modifications in ferroelectric films

Abstract

Stresses in rf sputtered polycrystalline BaTiO3 thin films, deposited at 300 to 700°C and at pressures from 0.1 to 100 Pa, on both Si and sapphire single crystals, are investigated. From the measured total stress values at room temperature, both the intrinsic stresses in the film and biaxial modulus of the film, Ef/(1–νf), are calculated. At lower deposition temperatures and pressures compressive intrinsic stresses are obtained, whereas at high deposition temperatures and pressures intrinsic stresses are tensile in nature. The biaxial modulus value approaches the single crystal value for films deposited at low pressures and decreases significantly with increasing deposition pressure. BaTiO3 films with high compressive stresses (≧ 400 MPa) show higher refractive indices, higher extinction coefficients, and lower optical band gaps compared to those for films with low compressive stresses (≈ 40 MPa). Furthermore, Curie point and coercive fields are increased, while remanant polarization decreases with increasing compressive stress. In addition, films in a state of high compression show a broad peak in the dielectric constant versus temperature curve which indicates a diffuse transition from ferroelectric to paraelectric state. By a judicious choice of deposition conditions, BaTiO3 films having low intrinsic stresses and properties close to those of single crystals are obtained. For example, some of the properties of BaTiO3 films deposited at 650°C and at 2 Pa pressure on silicon substrate are: n = 2.37 at 700 nm, Eg = 3.13 eV, Pr = 15.9 μC/cm2, Ec = 10.2kV/cm, Cure point 129°C, and σi = −47 MPa.