Mott Lecture

Chair: Safa Kasap, University of Saskatchewan

Monday 9:30–10:30

Amorphous Electride: A Novel Class Oxide Semiconductor

Hideo Hosono

Frontier Research Center & Materials and Structure Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama, JAPAN

In ICAMS-16 (1995), we proposed a material design concept for transparent oxide semiconductors (TAOS) with high electron mobility along with several examples [1]. TAOS represented by a-In-Ga-Zn-O(IGZO) [2] is attracting much attention as the channel material for TFTs to drive next generation FPDs. These materials have a distinct difference from conventional amorphous semiconductors, i.e. high ionic nature in chemical bonds. Here I present a novel class amorphous semiconductor, amorphous electride.

Electride is a crystal in which electrons serve as anions. The first electride material was synthesized in 1983 by James Dye along its material concept [3]. He obtained an electride by precipitation from metal cesium-dissolved crown ether solution. The resulting organic electride has attention because of its novel material concept and expectation for novel properties. However, materials research has not advanced so much due to their extreme instability to temperature and moisture or oxygen. Thus, realization of air & RT stable electride materials had been a long standing issue. In 2003, we overcame this difficulty by employing a refractory oxide, 12CaO·7Al2O3 (C12A7) composed of sub-nanometer-sized cages entrapping O2– as the counter anion, and by exchanging these O2– with electrons [4]. The resulting C12A7:e- is the first air &RT stable electride and has unique properties such as high electronic conductivity (~1500 Scm–1) at RT and superconductivity at 0.4K.

When C12A7:e- was melted above 1600K under low oxygen pressure, we found the resulting melt shows metallic conductivity [5]. When such a metallic conducting C12A7:e- melt was quenched rapidly, black colored glassy materials were obtained. These materials have electron concentration of ~1021 cm–3, which is almost the same as that in the starting polycrystalline C12A7:e-, and exhibit semiconducting properties. Amorphous C12A7:e- thin films were also fabricated by sputtering technique using polycrystalline C12A7:e- target. Amorphous electride is characterized by the presence of interstitial anionic electrons and has several unique properties. In this talk, I present the structure, properties and application of amorphous C12A7 electride in comparison with conventional amorphous semiconducting materials.

[1] H. Hosono, N. Kukuchi, N. Ueda, and H. Kawazoe, JNCS 198-200,165(1996)

[2] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, Nature 432, 488 (2004)

[3] A. Ellaboudy, J. L. Dye, and P. B. Smith, JACS, 105, 6490 (1983)

[4] S. Matsuishi, Y. Toda, M. Miyakawa, K. Hayashi, T. Kamiya, and H. Hosono, Science 301, 626 (2003)

[5] S. W. Kim, T. Shimoyama, and H.Hosono, Science 333, 71 (2011)

Keywords: amorphous oxide, semiconductors, electride, low work function