Electrical properties of paraelectric (Pb_0.72La_0.28)TiO₃ thin films with high linear dielectric permittivity: Schottky and Ohmic contacts

Linear, paraelectric (Pb_0.72La_0.28)TiO₃ or PLT(28) thin films with a bandgap > 3 eV were deposited on Pt/Ti/SiO₂/Si substrates by the sol-gel technique. Specific top-contact metals from two distinct groups (i.e., non-noble or M_T and noble or M_N; the former being oxidizable transition metals) were selected to understand the electrical nature of the interfaces in terms of electrode dependent energy band diagrams and equivalent circuit models. Using a high sensitivity high-pass filter circuit to evaluate the charging and discharging behavior coupled with results of the thickness and voltage dependence of capacitance, it was determined that M_T (Ni, Cr, Ti) and M_N (Pt, Au, Ag) metals form Ohmic and Schottky contacts, respectively. Supported by thermochemical data and calculations, the ohmic M_T-PLT interfaces are envisioned to be of the form: M_T-M_TO_x-n⁺PLT-nPLT. In contrast, the M_N-PLT interfaces may be characterized by a metal work function independent Schottky diode; the surface Fermi level being pinned at the mid-gap. For example, a Schottky barrier height of 1.83 eV and a built-in voltage of 1.3 eV at the Pt-PLT interface were estimated. From low field capacitance measurements, the ratio of interfacial to bulk resistance, R_i/R_b, was estimated to be 23.