Surface conduction on a transparent BaTiO3 crystal suggesting a possible surface electron layer

The increasing demand to incorporate miniature ferroelectrics in large-scale integrated circuits (LSI) has triggered a renewed interest in their size effect [1-8]. The surface of the ferroelectric, which influences the size effect significantly, is unique because that is where the macroscopic electric charges caused by the ferroelectric spontaneous polarization appear. There has been speculation that such a field may form a special layer on the free ferroelectric surface [9-12]. If such a layer exists, it will change our understanding of the fundamental properties and of the limitations of a very small ferroelectric, because the layer can be regarded as a natural electrode. However, its direct observation has not been reported so far, and the surface electrons have been usually regarded as immobile, if they exist at all. Here, we report conduction measurements on the clean, free surface of insulating single crystals of a prototype ferroelectric system, BaTiO(3), in high vacuum. We find that the BaTiO(3) crystals exhibit surface conductance that is dependent on the spontaneous polarization and shows semimetallic temperature dependence. The observations provide evidence of a two-dimensional electron or hole layer on a clean, free ferroelectric surface that may be regarded as a ferroelectric metal.