New electric functionality of lattice matched ferroelectric semiconductor heterostructures

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1) Current transport through ferroelectric heterostructuresCurrent through (Pb, La)(Zr, Ti)0_3 ferroelectrics on perovskite semiconductors is found to exhibit diode characteristics of which polarity is universally deternuned by the carrier conduction type of the semiconductors. The observed current voltage (IV) characteristics, the polarity dependence, the relaxation, and the modulation are explicable, if we assume a pn or a pp junction at the ferroelectric semiconductor interface. A persisting highly reproducible resistance modulation by a dc voltage, which has a short retention, is observed and is ascribed to a band bending of the ferroelectric by the formation of charged states. A reproducible resistance modulation by a pulse voltage, which has a long retention, is observed in (Pb, La)(Zr, Ti)0_3/SrTiO_3 : Nb due to a possible band bending by the spontaneous polarization (P) switching. (p : hole conduction type, a : electron conduction type).Additionally, an anomalous increase of el … More ectric conduction with decreasing temperature was found in the current through Pb(Ti, Zr)0_3 ferroelectric/SrTiO_3 epitaxial heterostructures. Comparison with current-voltage characteristics of other ferroelectric epitaxial heterostructures and the analysis using the band diagram indicate that the current is tunneling through the Rn lunction formed by Pb(Ti, Zr)O_3 and SrTiO_3.Furthermore, properties of nm-scale electrical contact on single domained part of ferroelectric is investigated.2) Construction of theoretical frame workIdeal ferroelectrics have mostly been modeled as insulators, i.e., infinite band gap materials, having a spontaneous polarization. Based on this assumption, theories for the finite size effect, the domain configuration, and the depolarization field instability have been proposed. However, most of oxide perovskite ferroelectrics have finite band gaps of 3 to 4 eV.We show that the inclusion of the finite band gap effect changes drastically conventional understanding of the finite size effect, domain structares and others. The conclusions extracted from the present approach are consistent with recent experimental results. Less