Extrinsic contributions to the apparent thickness dependence of the dielectric constant in epitaxial Pb(Zr,Ti)O3 thin films

The problem of the thickness dependence of the dielectric constant, as well as the extrinsic contributions to its value, is analyzed for the case of epitaxial Pb(Zr,Ti)O-3 (PZT) thin films. It is shown that the frequency dependence of the measured capacitance is best simulated by an equivalent circuit incorporating the trap-containing capacitance of a Schottky contact. The thickness dependence of the dielectric constant, calculated using the formula of a plane capacitor, appears to be an extrinsic effect due to the interface phenomena in the metal-ferroelectric-metal structure. The intrinsic dielectric constant of the PZT material seems to be thickness independent and of low value of about 30-40. This is closer to the values estimated from Raman measurements or from quantum theories of ferroelectricity. The thickness independence is also proven by piezoresponse force microscopy measurements. The presence of traps is evidenced by the presence of a photovoltaic effect at subgap wavelengths.