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.

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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.

Temperature ͑T͒ dependent current-voltage (IV) characteristics of epitaxial Pb(Ti,Zr)O 3 /SrTiO 3 heterojunctions exhibiting the characteristics of Zener tunneling are measured in the dark and in the light. Ultraviolet light is found to induce a prominent photovoltaic effect at all T down to at least 50 K, while the open circuit voltage increases with decreasing T. In the dark the reverse bias current increases with decreasing T. In the light the reverse bias current at low T is a superposition of a photovoltaic current and a small component that is identical to the reverse bias current in the dark. These observations indicate that the anomalous T dependence of the reverse bias current in the dark is attributable to the tunneling from the Pb(Ti,Zr)O 3 band to the SrTiO 3 band. Additionally, short pulse voltages modulate the tunneling current, which is retained.

A substantial photovoltaic effect is found in heterostructures of typical ferroelectric oxides. Pb͑Ti, Zr͒O 3 /Nb-doped SrTiO 3 , especially, exhibits current-voltage characteristics of the photovoltaic effect of a typical pn junction ͑p: hole carrier type, n: electron carrier type͒. A preliminary nonoptimized device shows high performance such as open circuit voltage of 0.7-0.8 V, external conversion efficiency of 0.6%-0.8%, and response time faster than 20 s for ultraviolet light at room temperature, suggesting the potential of this diode as a new class of photodiode. The results support the formation of a pn like junction by ferroelectric oxides. Additionally, the photovoltaic characteristics are tuned by the application of short pulse voltages and retained.

BaTiO3 (BTO) films are grown on the bottom electrodes of Nb-doped SrTiO3 (STON) and La2CuO4. The current through the BTO films exhibits diode properties having a reproducible hysteresis at a forward bias with polarity determined by the bottom electrode. In BTO/La2CuO4 the current intensity is almost independent of metal top-electrode materials, which is explicable by regarding BTO/La2CuO4 as a pn junction [p: hole carrier, n: electron carrier]. The current through BTO/STON is limited by the work functions of the metal and BTO, suggesting that it is suppressed by an electrode metal with a high work function, e.g., Pt, in an n-type ferroelectric such as BTO. In a p-type ferroelectric such as PLZT, it is suppressed by an electrode metal with a low work function. The relaxation current, the large leakage current in epitaxial ferroelectric films, and the effect of the ferroelectric carrier type on fatigue are discussed.