INFLUENCE OF STRESSES ON THE PROPERTIES OF FERROELECTRIC BATIO3 THIN-FILMS

Stresses in rf sputtered polycrystalline BaTiO3 thin films, deposited at 300-degrees-700-degrees-C and at pressures from 0.1 to 100 Pa, on both Si and sapphire single crystals, were investigated. From the measured total stress values at room temperature, both the intrinsic stresses in the film and biaxial modulus of the film, E(t)/(1 - v(f)), were calculated. At lower deposition temperatures and pressures compressive intrinsic stresses were obtained whereas, at high deposition temperatures and pressures intrinsic stresses were tensile in nature. The biaxial modulus value approached the single crystal value for films deposited at low pressures and decreased significantly with increasing deposition pressure. BaTiO3 films with high compressive stresses (greater-than-or-equal-to 400 MPa) showed higher refractive indexes, higher extinction coefficients and lower optical bandgaps compared to those for films with low compressive stresses (almost-equal-to 40 MPa). Furthermore, Curie point and coercive fields were increased while remanent polarization decreased with increasing compressive stress. In addition, films in a state of high compression showed a broad peak in dielectric constant vs. 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 were obtained. For example, some of the properties of BaTiO3 films deposited at 650-degrees-C and at 2 Pa pressure on single substrate are: n = 2.37 at 700 nm, E(g) = 3.13 eV, P(r) = 15.9 muC/cm2, E(c) = 10.2 kV/cm, Curie point 129-degrees-C and sigma(i) = -47 MPa.