Nonstoichiometry, structure, and properties of Ba1−xTiOy thin films

Abstract

The effects of growth conditions on the chemistry, structure, electrical leakage, dielectric response, and ferroelectric behavior of Ba_1−xTiO_y thin films are explored. Although single-phase, coherently-strained films are produced in all cases, small variations in the laser fluence during pulsed-laser deposition growth result in films with chemistries ranging from BaTiO₃ to Ba_0.93TiO_2.87. As the laser fluence increases, the films become more barium deficient and the out-of-plane lattice parameter expands (as much as 5.4% beyond the expected value for Ba_0.93TiO_2.87 films). Stoichiometric BaTiO₃ films are found to be three orders of magnitude more conducting than Ba_0.93TiO_2.87 films and the barium-deficient films exhibit smaller low-field permittivity, lower loss tangents, and higher dielectric maximum temperatures. Although large polarization is observed in all cases, large built-in potentials (shifted loops) and hysteresis-loop pinching are present in barium-deficient films – suggesting the presence of defect dipoles. The effects of these defect dipoles on ferroelectric hysteresis are studied using first-order reversal curves. Temperature-dependent current–voltage and deep-level transient spectroscopy studies reveal at least two defect states, which grow in concentration with increasing deficiency of both barium and oxygen, at ∼0.4 eV and ∼1.2 eV above the valence band edge, which are attributed to defect–dipole complexes and defect states, respectively. The defect states can also be removed via ex post facto processing. Such work to understand and control defects in this important material could provide a pathway to enable better control over its properties and highlight new avenues to manipulate functions in these complex materials.