We report conduction measurements on the clean, free surface of transparent insulating BaTiO3 single crystals in high vacuum. We find that the insulating BaTiO3 crystals exhibit surface conductance that is dependent on the spontaneous polarization, whereas no conduction perpendicular to the surface is observed. The surface conduction shows semimetallic temperature dependence and persists down to at least 100 K. The observations suggest a two-dimensional electron on a clean, free ferroelectric surface that may be regarded as a ferroelectric metal. The results have important implications for understandings of the fundamental properties of ferroelectrics, the size effect, and ferroelectric devices.
Conductance decreasing with increasing temperature ͑T͒ above a characteristic T (T 0 ) is found in the reverse-diode characteristics of metal contacts on strained BaTiO 3 epitaxial films. The conduction mechanisms below and above T 0 near the Curie temperature of the bulk BaTiO 3 are distinctly different. Marked similarities to these characteristics are found in the surface conduction on BaTiO 3 single crystal in a high vacuum. By comparing the observations with the positive temperature coefficient of resistance ͑PTCR͒ effect in ceramics, we suggest that the anomaly is regarded as a PTCR effect at metal/ferroelectric contact, and discuss the origin of the effect in thin films and single crystals.
Leakage current through epitaxial BaTiO3 films was investigated to clarify the difference between the characteristics of nanometer and millimeter-size metal contacts. SrTiO3:Nb bottom electrode revealed genuine properties of a single metal/BaTiO3 contact and demonstrated that breakdown voltage and leakage current density at both nanometer and millimeter-size contacts were controlled by the Schottky barrier. However, in marked contrast with millimeter-size contacts, nanometer-size contacts conducted little current below breakdown voltage and repeatedly exhibited abrupt breakdowns having a giant current density >10 A mm−2. The breakdown field was as high as 0.45 MV cm−1 at the forward bias, while no breakdown occurred up to 0.5 MV cm−1 at the reverse bias.
The conductance of diodes formed by epitaxial (Pb, La)(Zr, Ti)O3 on SrTiO3 doped with Nb is programed using the relaxation semiconductor characteristics of a ferroelectric. Namely, a three-terminal device function is given to a two-terminal device by time-domain control. The conductance modulation programed by a short-voltage pulse is perfectly retained for ten days at room temperature and is nondestructively read. The relaxation current at very low bias is also programmable. When the current is regulated by a metal/ferroelectric contact, the pulse modulation is obscure and is not retained. This implies that the surface layer at the metal/ferroelectric contact is negligibly thin or thinner than the tunneling distance.
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