Resistive-like Behavior of Ferroelectric p–n Bilayer Structures Based on Epitaxial Pb(Zr_0.2Ti_0.8)O₃ Thin Films

The p–n junctions are the building blocks of nowadays electronic devices. The n- or p-type conductivity is obtained in classic semiconductors, like Si, by doping with atoms acting as donors or acceptors, respectively. Doping was used in ferroelectrics to influence the transition temperature, magnitude of some physical properties, but not necessarily conduction type. Therefore, comprehensive studies to obtain true ferroelectric p–n junctions by controlled doping are missing. Recently, it has been shown that Pb(Zr_0.2Ti_0.8)O₃ films doped with ≈1% atomic Nb (n-type doping) or Fe (p-type doping) have different orientations of polarization in the as-grown state. Knowing that polarization orientation depends on doping type, the next step is to build ferroelectric p–n homojunctions and to study their properties in relation to ferroelectric polarization. p–n and n–p structures were grown for this purpose by successive deposition of Nb-doped and Fe-doped Pb(Zr,Ti)O₃ layers with different thicknesses. We find that these p–n homojunctions are ferroelectric, but the magnitude of the polarization and coercive field, as well as the dominant polarization orientation in the as-grown state, depend on the conduction type of the first grown layer. The I–V characteristics are quasi-linear, although the interfaces with the electrodes behaves as Schottky contacts. The resistance extracted from the I–V characteristics displays an exponential dependence on temperature, with an activation energy in the range of 0.14–0.17 eV. These results are explained assuming that the total current in the junction is the total of electron and hole injections at the electrode interfaces. It is shown that for relatively low doping concentrations, the current density contains a dominant term with a linear voltage dependence and an exponential temperature dependence, as observed experimentally, and a secondary (correction) term that is dependent on the free carrier density and can induce non-linear voltage dependence when this density is significant.