Novel Electrical Conduction of Insulators under Examination of Defects and Injection and Relationship to Theories of Fer
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Novel Electrical Conduction of Insulators under Examination of Defects and Injection and Relationship to Theories of Ferroelectric Domains
Y. Watanabe,1 Y. Urakami,1,2 D. Matsumoto,1,2 S. Kaku,1,3 S.-W. Cheong4, G. A. Thomas5, and S. Miyauchi1,6 1 Kyushu University, Fukuoka 812-8581, Japan 2 Hitachi Central lab., Kokubunji 185-0014, Japan 3 Tokyo Institute of Technology, Tokyo 152-8551, Japan 4 Rutgers University, Piscataway, NJ 08854, USA 5 New Jersey Institute of Technology, Newark, NJ 07102, USA 6 Murata Co., Izumo 699-0696, Japan
ABSTRACT Electrical conductions in insulators such as resistance switching, conduction at interfaces, and conduction at domain boundaries and free surface of ferroelectrics are of interest. These conductions are often attributed to novel mechanism such as ferroelectric polarization. On the other hand, these interpretations appear not fully accepted, because the recent advanced theories of ferroelectric domains disregard screening indicated by these conduction phenomena. That is, these conduction phenomena are quietly regarded as the classical conduction originating from defects. In this paper, we examine these conductions in pure wide bandgap insulators in view of defects, using the direct-accessibility (tangibility) of conduction at free surfaces. Although most of these conductions in ferroelectrics may not be useful in large-scale applications, we show that they have fundamental implications on renovations of ferroelectric basics. INTRODUCTION Electrical conductions in ferroelectrics and insulators are interested because of possible applications in high-density nonvolatile memory (Fig.1). The examples are the resistance switching [1,2] that occur also in nanometer-scale [3] and tunneling junctions [4], where some are attributed to ferroelectricity [5,6], the conduction at the interface of SrTiO3 and LaAlO3 [7,8], the conduction at domain boundaries [9,10] and free surfaces of ferroelectrics [9]. In addition, ferroelectric field effect has been interested since 1957 and was demonstrated with epitaxial perovskites by several groups [11-13]. These conductions are often attributed to novel mechanisms, meaning that the mechanisms are different from established ones such as impurities and defects, especially oxygen vacancies (VO). On the other hand, Augsburg and Stanford groups explain clearly the conduction at the interface of SrTiO3 and LaAlO3 by interdiffusion and defects [7,8], where high mobility of bulk SrTiO3 ~104 cm2/Vs and modulation doping were known [14,15]. Here, in many metal oxides, interdiffusion and formation of VO, which creates other defects, are more difficult to reduce in thin film than in bulk single crystals.
Fig. 1 Schematic overview of electrically formed conduction in insulators [10]. (a) Ordinary resistance switching, (b) resistance switching possibly due to PS, (c) conduction along domain boundaries, (d) ferroelectric field-effect transconductance, (e) ferroelectric field-effect transconductance with use of conventional semiconductors, (f) conduction of free s
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