Influence of injected charges on thin Ba0.6Sr0.4TiO3 films. Electric and dielectric characterization and simulation of the charge transport

In this study, the electric and dielectric properties of barium strontium titanate (BST) thin films were investigated by means of injected charge carriers in order to determine material properties like the field-dependent permittivity and charge carrier mobility. Starting point of this study is the band alignment of BST and tin-doped indium oxide (ITO) determined by X-Ray photoelectron spectroscopy, which reveals a negligibly small injection barrier for electrons from ITO into BST. It was shown that the resulting space charge limited currents (SCLC) cannot be fitted empirically, but need to be simulated numerically since in SCLC theory the electric field is not a constant within BST. Thus the field-dependent permittivity of BST exhibits a spatial variation. Additionally, it was pointed out that the measured current-voltage characteristics were not influenced by grain boundaries and static conditions could be applied so that for numerical simulations solely the Poisson and current density equation were necessary. On the basis of the derivative of the current-voltage characteristics in double logarithmic plot - the exponent of the voltage - the matching of measurement and simulation was possible. Three different voltage ranges were identified which could be correlated to defect states, a quasi-Fermi level dependent mobility, and a field-dependent permittivity. The basic concept for a quasi-Fermi level dependent mobility is formed by the existence of ''Urbach states'' resulting in a finite number of states within the band gap. Since their density decays exponentially with the distance from the conduction band minimum, this leads to a ''mobility edge''. Additionally, the influence of ''Urbach states'' on the charge carrier transport could be confirmed using complex nonlinear least squares fitting of impedance data.