Evolution of the memory effect of the current through ferroelectric p/p and p/n heterostructures

https://doi.org/10.1016/S0167-2738(98)00027-7Get rights and content

Abstract

Diode effects in BaTiO3/La2CuO4, BaTiO3/SrTiO3:Nb (Nb-doped SrTiO3), (Pb,La)(Zr,Ti)O3/Nd2CuO4, and Pb(Zr,Ti)O3/LaNiO3 heterostructures are reported. La2CuO4 and LaNiO3 are p-type (hole conduction-type) semiconductors, and SrTiO3:Nb and Nd2CuO4 are n-type (electron conduction-type). The heterostructures were epitaxially grown to form pp, nn, and pn heterojunctions. At low applied bias, the leakage current through the heterostructures was symmetric with regard to bias polarity. With increasing applied bias, it became asymmetric and exhibited a reproducible memory effect at a forward bias. This behavior was observed for many combinations of ferroelectrics and perovskite semiconductors. A model is presented to explain the polarity of the current characteristics and its temperature dependence.

Introduction

The leakage current through ferroelectric and high-dielectric-constant (high-ε) oxide films is observed to increase rapidly as their thickness decreases. The suppression of the leakage current is one of the central issues for incorporating these films into commercial applications. The origin of the leakage current is likely the decrease of the grain boundaries and the increase of the defects which act as carrier donors and acceptors. In high-quality epitaxial films, both grain boundaries and defects are reduced.

However, a space charge can exist even in defect-free ferroelectric/high-ε perovskite films, when a large external field is applied, or when it contacts an insulating film, with a finite component of its polarization being perpendicular to the interface. This is because they are semi-insulators having a band gap of 3 eV. Moreover, elimination of the defects, for example oxygen vacancies, is limited thermodynamically. This limitation is severe in ultrathin films and at the surface, which implies that the leakage current can be reduced by reducing the defects, especially oxygen vacancies, which has been demonstrated experimentally.

The above consideration indicates that a certain density of space charge or charge carrier is always present even in a very-high-quality epitaxial film and that the ferroelectric tends to exhibit intrinsically the property of a semiconductor with decreasing thickness. Therefore, understanding of the carrier transport becomes important for clarifying the physics of these films. Moreover, study of the carrier transport can reveal the electronic energy states of ferroelectric/high-ε perovskites and also open a way for novel applications.

The mechanisms of carrier transport, especially in a very thin film, are obtained from the study of high-quality all-perovskite epitaxial ferroelectric structures. In the course of such a study, we found a novel phenomenon: a highly reproducible memory effect 1, 2, 3which can be used for a neural device. Moreover, this effect and other leakage current properties elucidate the electronic state at the interface. In this paper, we report the bias-amplitude dependence of the leakage current through all-perovskite epitaxial ferroelectric heterostructures, with special emphasis on its polarity which is the most importance basis of the memory effect.

Section snippets

Experimental

All ferroelectric films were grown on perovskite single crystal substrates, and all the substrates reported in this paper are (001) SrTiO3 which are insulating (STO) and conducting, i.e. 0.5 wt% Nb-doped (STON).

Pb1−yLayZr1−xTixOδ (x=0, 0.1, 0.2, 0.3, 0.4, 0.5; y=0, 0.05, 0.1) (PLZT) (200–300 nm thick) and BaTiO3 (BTO) (100–200 nm thick) films were grown epitaxially on STON and STO substrates directly and also via La1.99Sr0.01CuO4 (LSCO), Nd1.99Ce0.01CuO4 (NCO), or LaNiO3 (LNO) films. STON and

Pb-based ferroelectric

We have reported IV hystereses of various Pb-based ABO3 ferroelectrics and Bi4Ti3O12−δ having STON and LSCO as the bottom electrode and Au as the top electrode 1, 3, 7, 8. Similar results were confirmed by using Al as the top electrode on PbZr0.2Ti0.8Oδ/STON. In these heterostructures, the polarity of the forward bias was determined by the conduction type of the oxide semiconductors [3]. Namely, the forward bias was positive for n-type, while it was negative for p-type.

As another example of

Discussion

The IV characteristics are limited by the least conductive part in the current path. The overall conduction consists of carrier injection at the metal/ferroelectric interface (M/F injection), conduction through the ferroelectric, and carrier injection at the ferroelectric/perovskite-semiconductor interface (F/S injection). The dominant conduction mechanism is dependent on the applied bias and the ferroelectric film thickness. The observed polarity dependence can be explained by the band

Conclusion

The diode-like IV curves of Au(Al)/ferroelectric heterostructures showed a memory effect at forward bias. The IV curves for Pb0.95La0.05Zr0.5Ti0.5Oδ/NCO, PbZr0.5Ti0.5Oδ/LNO, BTO/STON, and BTO/LSCO are presented as new examples. The polarity of the forward bias was positive for heterostructures with an n-type electrode and negative for those with a p-type electrode. These observations are explained by regarding the ferroelectrics as p- and n-type semiconductors.

Acknowledgements

This work was performed through the support of a Grant-in-aid for Scientific Research from the Ministry of Education, Science and Sports and the Ogasawara Foundation.

References (10)

  • Y. Watanabe et al.

    Physica C

    (1994)
  • P. Blom et al.

    Phys. Rev. Lett.

    (1994)
  • Y. Watanabe

    Appl. Phys. Lett.

    (1995)
  • Y. Watanabe et al.

    Jpn. J. Appl. Phys.

    (1994)
  • Y. Watanabe et al.

    Appl. Phys. Lett.

    (1995)
There are more references available in the full text version of this article.

Cited by (0)

View full text