Chair of Nanoelectronics

Multiferroic Tunnel Junctions

Schema eines ferroelektrischen Tunnelkontaktes sowie drei mögliche Effekte, die den Tunnelstrom durch die ferroelektrischen Eigenschaften der Barriere beeinflussen können
Figure 1: Schematic of a Ferroelectric Tunnel Junctions and three possibilities how the electron tunneling current might be modified by the ferroelectric nature of the barrier

THROUGH THE BARRICADES - SPANDAU BALLET - 1986

Abstract

Due to breakthroughs in various disciplines in past view years, the research on complex oxides has become one of the most exciting areas in solid state physics and electronics. The manifold properties of oxides and their interfaces form a nearly infinite research area. 

The current transport through a nanometer thin ferroelectric tunnel barrier is a novel, fundamental research topic. It combines ferroelectricity on the nanometer scale and direct quantum mechanical electron tunneling. Epitaxial complex oxides such as LaxSr1-xMnO3, SrRuO3, BaTiO3, PbZrxTi1-xO3, YBa2Cu3O7 , SrBiTaO9, LiNbO3, LiTaO3 etc.. are typical of the complex oxides applied in our research. With the advent of multiferroic materials, e.g. BiFeO3, numerous types of new tunnel junctions can be explored. Before studying these novel tunnel junctions, multiferroic materials have to be prepared in thin film form by means of high-pressure sputtering or pulsed Laser deposition (PLD). For the successful realization of ferroelectric and multiferroic tunnel junctions, a controlled layer deposition of complex oxide heterostructures on a unit cell level is a necessity. Novel applications in the field of non-volatile data storage (Non-volatile Random Access Memories), ultra-sensitive sensors and a new class of gate dielectrics for advanced MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors) are only a few opportunities. The magnetic and ferroelectric performance of such films will be studied using a magnetometer as well as piezoelectric and dielectric characterization techniques. 

Our group started; research on ferroelectric tunnel junctions in 1999, and chose this name for the first time. In figure 1, a simplified schematic of a FTJ is shown. In an FTJ, a nm thin ferroelectric barrier is sandwiched between two metal electrodes. The quantum mechanical tunneling current might depend on several properties of the ferroelectric nature of the barrier. Most likely the tunneling current depends on the depolarization field inside the barrier, the inverse piezo electric effect and interfacial effects. Indeed FTJs are considered as resistive switching devices for non-volatile data storage and since 2007, the FTJ is a “member” of the International Roadmap of Semiconductors. 

Neue Materialkombination für Tunnelkontakte
Fig. 2: The “zoo” of Multiferroic tunnel junctions: Ferroelectric tunnel barriers, multiferroic tunnel barriers with superconducting and/or magnetic electrodes offer new opportunities as sensors or non-volatile data storage devices.